The substrate and precursor effects in sol-gel derived Pb_0.90La_0.15TiO_3 thin films were studied at various annealing temperatures in the range of 350 - 650^oC. Films for this study were prepared using acetic acid and methoxyethanol routes on Pt/Si and Pt substrates. X-ray results indicate that films prepared by acetic acid route show texturing along (100) orientation of the films with increasing annealing temperature. These film also exhibit better ferroelctric properties on Pt compared to Pt/Si substrates. Raman spectra show the stress induced softening of the lowest frequency mode with increasing annealing temperature in the films prepared by acetic acid and methoxyethanol route. However, relatively less phonon damping was observed in the methoxyethanol route processed films. The phase transition behavior in these films will be studied by high temperature Raman measurements. The structural, micro-Raman and electrical data from these films will be presented in detail. This work is supported in part by DE-FG02-91ER75764, DEPSCOR DAAG55-98-1-0012 AND NSF-DMR9801759 grants.
[L36.002] Computer Modeling of the Metal-Insulator Transition in Disordered Systems
Dain Horning, Kendall Mallory (University of Northern Colorado)
We have developed a computer model to study the effects of
electron interactions on the metal-insulator transition in
disordered systems. We will present some early results from
this model. We have looked for the possibility of
delocalized electron states existing in systems that
normally exhibit only localized states.
[L36.003] Plasma Physics Calculations on a Parallel Macintosh Cluster
Viktor Decyk, Dean Dauger, Pieter Kokelaar (University of California, Los Angeles)
We have constructed a parallel cluster consisting of 16
Apple Macintosh G3 computers running the MacOS, and achieved
very good performance on numerically intensive, parallel
plasma particle-in-cell simulations. A subset of the MPI
message-passing library was implemented in Fortran77 and C.
This library enabled us to port code, without modification,
from other parallel processors to the Macintosh cluster. For
large problems where message packets are large and
relatively few in number, performance of 50-150 MFlops/node
is possible, depending on the problem. This is fast enough
that 3D calculations can be routinely done. Unlike
Unix-based clusters, no special expertise in operating
systems is required to build and run the cluster. Full
details are available on our web site:
http://exodus.physics.ucla.edu/appleseed/.
[L36.004] Spin Expectation Values in Fermion Gases
Lauren Ault (The College of Wooster, Wooster OH 44691), Don Colladay (Colby College, Waterville, ME 04901)
Recently, the possibility of spontaneously breaking Lorentz
invariance has been suggested in fundamental theories
underlying the standard model. Resulting terms can have
implications for a variety of low-energy experiments testing
Lorentz and CPT symmetry. Here we investigate the effect of
some of these terms on fermion gases. In some circumstances,
such as thermal equilibrium, a small net expectation value
on the spin of the fermion gas occurs. In addition, the
number densities of left- and right-handed particles are
found to differ. Some possible implications for early
universe physics are discussed. This research was conducted
at The College of Wooster with support from NSF-REU grant
DMR 9619406.
[L36.005] First-principles study of higher-energy phases in Cu and its relation to the atomic configurations of extended defects
L.G. Wang, M. Sob (Inst. of Physics of Materials, Acad. Sci. of the Czech Republic, Brno, Czech Republic)
We performed full-potential first-principles total energy calculations for three displacive phase transformation modes in Cu. The structural and elastic properties of the ground state (fcc) and higher-energy phases (bcc and 9R), as well as the energy barrier for sliding of \111\_fcc close-packed atomic planes and the stacking fault energy were obtained. Stability of higher-energy phases in the region of extended defects is discussed in detail. Examples presented are bcc and 9R Cu in grain boundaries and bcc Cu in pseudomorphic films at low temperatures. It is shown that the higher-energy phases, which are usually unstable, can be stabilized in the region of extended defects by certain imposed constraints.
[L36.006] Dipolar interactions in systems with uniaxial symmetry
Demetris Nicolaides (Bloomfield College)
It is well known how the critical behavior of a ferromagnet,
is altered considerably when one considers the effect of the
long-range interaction due to pairs of magnetic dipoles in
addition to the short-range spin-spin interactions. For
example, the work of Larkin and Khmel'nitskii, using
Feynman-graph expansions for d=3, showed that the critical
behavior of a uniaxial ferromagnet with both exchange and
dipolar interactions has logarithmic corrections not
expected classically. This was verified with exact
renormalization group equations and the \varepsilon
-expansion, as well as predicted that the critical behavior
of a d-dimensional uniaxial Ising ferromagnet with
dipole-dipole interactions belongs in the same universality
class as a (d+1)-dimensional, strictly short-range Ising
ferromagnet. In the present work, an alternative approach
will be used to confirm and generalize this result. It will
be shown that d-dimensional uniaxial systems with
isotropic short-range as well as dipole-dipole interactions
have d=3 as the upper marginal dimension above which
mean-field behavior fully sets in, and below which critical
behavior prevails. Unlike the \varepsilon -expansion where
the validity of the results depends on the smallness of
\varepsilon , the method used here derives the result for
any d without any constraints. The study of such systems
will be done by considering a model with reduced
interactions of fluctuations, allowing for the exact
calculation of the partition function.
[L36.007] Ground-state Properties of One-dimensional Symmetric Periodic Anderson Model Away From Half-filling
Yan Luo, Nicholas Kioussis (Department of Physics, California State University Northridge, Northridge CA 91330-8268)
The ground-state energy, the local moment, the effective
hybridization, and the projected f- and conduction density
of states of the one-dimensional periodic, symmetric
Anderson Lattice Model have been investigated as a function
of U, V and band filling using the local mean-field (LMF)
method. We have evaluated the magnetic phase diagram for the
symmetric case, which shows three distinct phases: the
antiferromagnetic, ferromagnetic, and Kondo phases. The
antiferromagnetic phase is found to be the ground state at
the half-filling and the quarter-filling cases with strong
Coulomb coupling.
[L36.008] X-ray Temperature Difference Absorption Spectra (XTDAS) Analysis of Phase Transitions in NiS2-xSex (x=0.62)
Quang Le, Professor Juana Acrivos Collaboration, Boichau Nguyen Collaboration, Rizwana Shaikh Collaboration, Charles Burch Collaboration
Transmission x-ray absorption spectra (XAS) for amorphous
powder NiS2-xSex (x=0.62) versus temperature (T) has been
measured at the Ni and Se K-edges in the 4K to 150K
temperature range to determine the presence of a temperature
and composition dependent metal to insulator transition. The
changes in the XAS over the temperature range support the
Mott-Hubbard model, which exists at Se compositions varying
between x~0.4 and x~0.6 and in the 0K to 115K temperature
range.
[L36.009] Spin-Echo Decay Rates of ^29Si in URu_2Si_2 at High Temperatures
C. Rodrigues, O. O. Bernal (California State University, Los Angeles), H. G. Lukefahr (Whittier College, Whittier)
We present a study of ^29Si spin-echo relaxation in URu_2Si_2. Our measurements reveal a difference between the Hahn-echo method [\pi/2--\tau--\pi--detection] and the Carr-Purcell-Meiboom-Gill method [(\pi/2)_90^\circ--\tau--\pi--\tau--\pi\ldots detection] even at high temperatures (T\ge40~K). These results are compared with Hahn-echo and Carr-Purcell-Meiboom-Gill measurements of spin-echo relaxation in insulator and simple-metal samples. Possible origins of the high-T difference between the two methods in URu_2Si_2 are explored. We discuss our results in view of published T_2-data on URu_2Si_2.
abstract.
format.
send as
[L36.010] Electronic properties of single crystalline UIrGe in high magnetic fields
H. Nakotte, S. Chang (Physics Department, New Mexico State University, Las Cruces NM 88003), K. Prokes (Hahn-Meitner-Institute, NE, Glienickerstrasse 100, D-141 09 Berlin, Germany), A. H. Lacerda (National High Magnetic Field Laboratory, Pulse Field Facility, Los Alamos National Laboratory, Los Alamos NM 87545), I. Hagmusa (Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat, 1018 XE Amsterdam, The Netherlands)
UIrGe crystallizes in the orthorhombic TiNiSi structure and
exhibits a huge magnetic anisotropy with the hard
magnetization axis along the a-axis. Previous magnetic,
transport and thermal studies indicated anti-ferromagnetic
ordering below 15.8 K with an additional transition at 14.1
K [1]. For the present work, we have performed
magnetization, magnetoresistance and specific heat
measurements in magnetic fields up to 18 T. The data were
taken in order to determine the magnetic phase diagrams for
UIrGe in fields applied along the principle axes. [1] K.
Prokes et al., Phys Rev B. 60 (1999) 9532
[L36.011] Ultrasonic measurements of UPt3 at high magnetic fields
Jeffrey R. Feller (Northwestern University, Physics Dept.), Chrong-Chu Tsai (Northwestern University), D. Dasgupta (University of Wisconsin-Milwaukee), D. G. Hinks (Argonne National Laboratory, Argonne, IL), J. B. Ketterson (Northwestern University), Bimal K. Sarma (University of Wisconsin-Milwaukee), Northwestern University Collaboration, University of Wisconsin-Milwaukee Collaboration, Argonne National Laboratory Collaboration
Ultrasound velocity and attenuation measurements have been
performed on the heavy fermion compound UPt3 in magnetic
fields up to 33 T and at temperatures down to 50 mK. With
the field applied in its basal plane, UPt3 is known to
undergo a metamagnetic transition near 20 T. This is
evidenced by a large velocity dip (Dv/v > 5% at the lowest
temperatures), accompanied by a peak in attenuation. The
temperature- and frequency-dependence of these features will
be discussed. Additional structure appears below \sim300 mK,
including quantum acoustic oscillations. The field
dependence of the period and amplitude of these oscillations
suggest a reconstruction of the Fermi surface at the
metamagnetic transition.
[L36.012] Microwave response of the heavy fermion superconductors UBe13 and U0.9725Th0.0275Be13
J. R. Feller, C. T. Lin, Chrong-Chu Tsai (Northwestern University), J. L. Smith (Los Alamos National Laboratory), J. B. Ketterson (Northwestern University), Bimal K. Sarma (University of Wisconsin-Milwaukee), Northwestern University Collaboration, University of Wisconsin-Milwaukee Collaboration, Los Alamos National Laboratory Collaboration
The superconducting transition temperature Tc of the heavy
fermion system U1-xThxBe13 depends strongly on the Thorium
concentration [Heffner, et al., Phys. Rev. Lett., 65, 2816
(1990)]. Tc is found to decrease monotonically from \sim0.86 K
for x = 0 to \sim0.48 K for x = 0.019. For 0.019 < x < 0.043,
however, two transitions are observed. These are visible as
anomalies in, for example, magnetization, specific heat, and
muon spin resonance data. It has been suggested that this
double transition is evidence of a coexisting
antiferromagnetic phase, or of a multi-component
superconducting order parameter. We have measured the
normalized surface impedance of two U1-xThxBe13 samples: the
first with a Thorium concentration of 2.75 %, the second
with no doping (UBe13). For these measurements a cylindrical
copper cavity, resonated at frequencies \sim9-30 GHz, was
employed. The cavity and sample were cooled using a dilution
refrigerator with a top-loading feature. The temperature was
varied from \sim85 mK to \sim0.9 K. Magnetic fields were also
applied. The predicted double transition, with Tc1 \sim 0.6 K
and Tc2 \sim 0.4 K, are observed in the doped sample. The
second transition is evidenced as a kink in the resistance
and a broad peak in the reactance.
[L36.013] Effects of p-p energy transfer on the pairing correlations in the one-dimensional d-p model
Zhongbing Huang, Haiqing Lin (Department of Physics, The Chinese University of Hong Kong, Hong Kong), James Gubernatis (Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545)
Using the constrained-path Monte Carlo (CPMC) method, we
have studied the one-dimensional d-p model for the cases
with finite Coulomb repulsion on d sites, and mainly
examined the effects of transfer energy t_pp between the
nearest-neighbor p sites on pairing correlations with
different symmetry. Of these pairing correlations, we
observed that only the tail of singlet pairing between the
nearest-neighbor p sites increases explicitly with the
increase of t_pp, while others decrease or increase
negligibly. But for all symmetries, the vertex contributions
fluctuate around zero at large distance, therefore it is
difficult to determine which pairing channel dominates when
the critical exponent of the correlation functions K_\rho
> 1.
[L36.014] Peierls instability in a polymer chain
C.Q. Wu, Y.Z. Zhang (Phys. Dept., Fudan Univ., Shanghai, China), H.Q. Lin (Phys. Dept., Chinese Univ. of Hong Kong, Hong Kong, China)
By using a Su-Schrieffer-Heeger-type Hamiltonian, we investigate the Peierls instability in the substituted polyacetylene with lateral radical R. It is found that the Peierls dimerization along the polyacetylene chain is suppressed by increasing \pi-electron hopping between the radicals and the associated carbons. The expected quadrimerization, which is required by the Peierls theorem, does not appear in this kind of one-dimensional systems. The physical reason behind the fact is that the electronic energy gap is not again proportional to the lattice distortion as usual and then the logarithm term, which is the driving force for the Peierls instability, does not exist in the electronic energy gain of the system.
[L36.015] Elastic moduli and Entropy Considerations at the delta-epsilon Phase Transition in Pu
Timothy Darling, Albert Migliori, Joseph Baiardo, Franz Freibert, Stuart Trugman (Los Alamos National Laboratory)
Recent elastic modulus measurements by us on Ga stabilized \delta-Pu, as well as a critical examination of latent heat, elastic modulus and crystal structure data on this unusual metal suggest that some of the many structural phase transitions of Pu may be understood within a simple thermodynamic model. We show that the free energy of Pu in the FCC \delta phase and the BCC \epsilon phase, as well as the negative volume expansion coefficient of \delta can be understood and almost entirely accounted for by the phonon contributions to the entropy and the structure. We suggest that the behavior and stability of Pu is driven by the low elastic shear moduli that this material possesses, and that electronic structure calculations need only account for the low temperature monoclinic phase. Variation of the number of itinerant f electrons is unnecessary for an accurate description of the higher temperature phases.
[L36.016] Ground-State of the Single Impurity Anderson Model (SIAM) with Correlated Conduction Electrons
William J. Massano (SUNY Maritime), Jay D. Mancini, Vassilios Fessatidis (Fordham University), Samuel P. Bowen (Chicago State University)
In this work we shall consider the Hamiltonian system given by
H=H_A+H_C where H_A is the usual SIAM Hamiltonian given
in second-quantized notation by
\[
H_A=\sum_k,sn_k,s\varepsilon _k,s
+\sum_sE_sN_s+UN_rN_l+\sum_k,sV(k)
\left[ c_k,sf_s+f_sc_k,s\right].
\]
The interaction H_c=U_c\sum_k
The interaction of a single bosonic
mode with a two-level fermion
system has been the subject of a large
number of articles in the physics
literature for quite some time.
For the condensed-matter physicist the
boson in question is typically a phonon
whereas the fermion system is
represented by a two-level electron structure.
The Hamiltonian which yields
the appropriate physics is given by the Rabi Hamiltonian.
In second-quantized
notation this may be written as
\[
\hatH=\frac12 ømega_0 \sigma_z+ømega b^b+
g\left(\sigma^+ b^\dagger+\sigma^+ b+
\sigma^- b^\dagger+\sigma^- b \right).
\]
Here the \sigma's are the usual Pauli matrices, while b
and b^\dagger are the Bose operators for the (quantized) field
modes. In general the natural transition frequency ømega_0 of
the atom need not coincide with the boson-mode frequency
ømega. In this work we shall use both the Connected Moments
Expansion (CMX) and the Alternate Moments Expansion (AMX) as
well as the non-perturbative Lanczos tridiagonal scheme to study
the ground-state spectrum of this system. Comparisons will be made with
other approximation schemes as well as ``exact" methods.
A number of years ago, Cioslowski
(Phys. Rev. Lett. 58, 83 (1987))
utilizing a t-expansion derived a novel expression to
estimate the expectation value of the
ground-state of quantum many-body systems.
His derivation of a Connected Moments Expansion (CMX)
was obtained through the use of a helper function.
Following this work, Mancini et
al.\ (Int. J. Quantum Chem. 50, 101 (1994) ) chose
a different helper function to derive
an Alternate Moments Expansion (AMX).
The algebra of the problem suggests
that there exists a set of helper functions and hence a set
of unique moments expansions.
In this work we present a Generalized Moments Expansion
(GMX) which has as a subset both the CMX and AMX,
and apply it to the harmonic oscillator.
We have examined the nonrelativistic retarded Green's function
for electron motion confined by an
anisotropic harmonic oscillator potential
and in the presence of constant magnetic and electric
fields. Here the electric field is at an arbitrary orientation
with respect to the magnetic field.
A closed form expression for the propagator is obtained
by employing Schwinger's equation of motion operator
technique.
An investigation of the liquid crystalline phases of DNA
fragments in different buffer solutions is reported.
Previous studies have shown that a strong correlation exists
between the critical concentration and the average length of
DNA fragments for the observed mesophases of DNA [1]. The
focus of our study is to understand the phase ordering of
DNA due to changes in concentration under various boundary
conditions. Previous studies have utilized evaporation as a
means for increasing the concentration of the DNA samples
[2]. The effects of evaporation on the development of
precholesteric and cholesteric phases will be examined.
[1] K. Merchant and R. Rill, Biophys. J. 73, 3154 (1997).
[2] F. Livolant, J. Physique 48, 1051 (1987). This research
was partially supported by the Copeland Fund, administered
by The College of Wooster.
In liquids, temperature gradients change the large scale
fluctuations due to mode coupling. For scales larger than
the mode coupling radius, nonequilibrium fluctuations are
stronger than equilibrium ones. Near the critical
temperature Tc the mode coupling radius decreases with
decreasing T-Tc, while the equilibrium correlation radius
increases, and at some temperature Tcc, both characteristic
lengths coincide. At Tcc>T>Tc, the fluctuations at large
scales in the critical range change, the temperature
gradient becomes a new relevant critical parameter; the
correlation radius and the mode coupling radius as functions
of this parameter yield new scaling laws. Suported by NASA
Grant NAG3-1932
Effect of heterogeneous matrix density on segregation and orientation
of short chain molecules (dimer, trimer, etc.), a model for polymer
dispersed liquid crystal (PDLC), is studied by a computer simulation.
The hybrid computational method involves discrete lattice and off-lattice
simulations to incorporate its strength in moving the system efficiently
and capturing the details; this method is also used for long chain polymer
systems. On increasing the matrix density (p_b), the size of molecular
aggregates is found to increase leading to a polymerization-induced
segregation and seems consistent with the recent experimental
observations. Orientational ordering is found to depend on the molecular
weight in which the ordering decreases monotonically with the matrix
density for low molecular weight liquid. For higher molecular weight
liquid, on the other hand, it depends non-monotonically on the
molecular weight. Similar study is also carried out at different
temperatures which play an important role in clustering of molecules
and their orientational ordering. Some of these results will be
presented.
We present a density functional theory approach to the study of main-chain
LC polymers.
These polymers are modeled to consist of M slender rod-like segments
joined together end-to-end.
A generic bending potential between successive segments
provides for a molecular flexibility mechanism.
The interaction between different polymers is described by segment-segment
excluded volume interactions.
The model allows the analytical determination of the isotropic-nematic (IN)
bifurcation (spinodal) densities.
Numerical solutions to the self-consistency equations are also obtained
under various conditions.
We determine the dependence of the order parameter jump
\langle P_2 \rangle at the transition and the coexistence (binodal)
densities as a function of
the chain flexibility for several values of the chain length.
One of the advantages with working with a segmented chain model is that
the generalization to other (more general) classes of systems is immediate.
We illustrate this point by the application of our model to
biaxial polymers and block copolymers for which we calculate the
IN bifurcation densities.
Nematic liquid crystal filled with aerosil particles has
been investigated by broadband dielectric spectroscopy (BDS)
and photon correlation spectroscopy (PCS). The aerosil
particles of diameter \simeq 10 nm in filled nematic
liquid crystals form a network structure with linear size of
LC domains about 250 nm and with random distribution of the
director orientation of each domain. This material has a
very developed liquid crystal-solid particle interface that
makes the role of the surface layers of LC important in the
determination of the properties of the material. BDS
provides information on reorientational motion of polar
molecules of liquid crystal while PCS probes dynamics of
collective modes associated with director fluctuations. We
found that the properties of 5CB are considerably affected
by the network. Two bulk-like dielectric modes due to the
rotation of molecules around short axes and the tumbling
motion were observed in filled 5CB. Additionally, a low
frequency relaxation process and dispersion of dielectric
permittivity due to conductivity were observed. The
treatment of the surface of filling particles has strongest
influence on the properties of the slow process and it is
less important for molecular modes. PCS experiment shows
that two new relaxation processes appear in filled 5CB in
addition to the director fluctuations process in bulk.
We have investigated the influence of layer thickness on
dynamical behavior of liquid crystal (LC) - 8CB - partially
filling cylindrical pores by dielectric and photon
correlation spectroscopies (PCS). Layers of different
thickness are formed on the pore walls as a result of
controlled impregnation of porous matrices (Anopore
membranes). Dielectric spectra of samples with different LC
layer thickness consist of the main peak (f_m \sim 5
MHz) due to the reorientation of molecules around short axis
and secondary peak (f_m \sim 50 MHz) - tumbling mode -
with much smaller amplitude. Since the probing electric
field was parallel to the pore axis the presence of the
tumbling mode suggests that the orientation of molecules in
partially filled pores is not perfectly axial. The relative
contribution of the tumbling process increases with the
decrease of the thickness of LC layer. The main relaxation
process broadens and its relaxation times decrease with
decreasing thickness. Results on structure and orientation
in surface layers obtained in PCS experiments, probing
collective dynamics of director fluctuations, were
consistent with dielectric data. These experiments allow
clarifying the role of the surface layers on the physical
properties of geometrically restricted LC.
We have investigated the electro-optical properties of
liquid crystals (LCs) that are confined in a regularly
arranged opal structure. It is known that periodic
structures of dielectric solids can exhibit photonic band
gaps. Three-dimensionally periodic structures made of
SiO_2 spheres with the diameter ranging from 100 to 400
nm are realized by a sedimentation process between two ITO
glass plates. The empty space is then infiltrated with a
liquid crystal. An external electric field applied on the
LC-opal composite changes the effective refractive index
and, as a consequence, alters the stop band wavelength.
Additionally we have made inverse opal structures by
replacing the liquid crystal with a polymer matrix, and then
by extracting the SiO_2 particles. Field induced
phenomena on the LC-inverse opal are discussed. This work
was supported by the NSF MRSEC Grant DMR 98-09555, the NSF
Grant DMR 96-14061 and the NASA Grant NAG3-1846.
A series of polynorbornenes (PNBEs) with
1,4-bis[(3´-fluoro-4´-n-alkoxyphenyl)ethynyl] benzene
mesogens (n = 9-12) laterally attached to the polymer
backbone through a one-carbon spacer was synthesized by
ring-opening metathesis polymerization of the corresponding
norbornene-based monomers. Wide angle X-ray diffraction
experiments demonstrate that the mesogens organize into the
tilted layer structure of a smectic C phase at room
temperature, and polarized light microscopy demonstrates
that the highest temperature phase is a nematic phase. Upon
heating above room temperature, the tilt angle of the
smectic C phase of all of the PNBEs (n = 9 - 12)
continuously decreases, especially at temperatures above
60°C. However, the smectic C phase of PNBE (n = 12)
transforms to a smectic A phase at 85 °C, whereas the
smectic C phase of the PNBEs (n = 9 - 11) transforms to a N
phase. The phase transformation between the smectic C and
the smectic A phases is characterized by a second-order
transition.
The experimental study of the onset of electrohydrodynamic
convection (EHC) through a dendritic growth is reported. If
a magnetic Freedericksz-distorted liquid crystal of negative
dielectric anisotropy is subjected to an electric field
parallel to the magnetic field, EHC sets in through the
nucleation of dendrites [1,2]. Measurements of tip speeds of
the dendrites as a function of applied voltage at a fixed
magnetic field are made. The goal is to explore the effect
of the magnetic and electric fields on the dendritic growth.
In addition, pattern dynamics is monitored once the final
state of spatio-temporal chaos is reached by the system. [1]
J. T. Gleeson, Nature 385, 511 (1997). [2] J. T. Gleeson,
Physica A 239, 211 (1997). This research was supported by
NSF grants DMR 9704579 and DMR 9619406.
It has recently been shown that in surface stabilized
ferroelectric liquid crystal (SSFLC) cells with high
spontaneous polarization the polarization charge
self-interaction leads to a so-called "V-shaped" optical
response to an applied voltage. The presence of ionic free
charges in the liquid crystal, however, changes the internal
electric field and therefore also the spatial dependence of
the polarization and optic axis orientation. We have
numerically solved the nonlinear ion diffusion equation in
the electric field due to external voltage, spontaneous
polarization and ions, and calculated the electrooptic
response to a triangular applied voltage. When the period of
the driving voltage is smaller than the zero-field diffusion
time of the ions across the cell and larger than the ion
transit time in the applied field, an inverse hysteresis in
the electrooptic response is obtained, in agreement with
experimental observations. At these time scales ions also
cause characteristic changes in the shape of the
electrooptic response. This work was supported by NSF MRSEC
Grant DMR 98-09555 .
Starburst Polyamidoamine (PAMAM) dendrimers represent ``dense star''
polymers that unlike classical polymers have specific size and shape
characteristics, a high degree of monodispersity and molecular
uniformity. Using optical microscopy, we have studied systems composed
of aqueous suspensions of charged polystyrene colloids and ``generation''
9.5 PAMAM dendrimers. Depletion-mediated crystallization of colloids is
observed. In addition, an entropy-driven reentrant phase behavior
(liquidlike to solidlike to liquidlike) of colloids is tentatively
observed with the systematic increase of dendrimer concentration and a
fixed colloid concentration.
In a selective solvent, diblock copolymers form micelles
consisting of colloid like cores and tethered polymer
"hairs". Liquid like order of these soft colloids gives rise
to a maximum in the structure factor S(q), which for high
polymeric molecular weight is accessible by dynamic light
scattering. While a pronounced slowing down of the
collective diffusion at the maximum position is well known
for hard spheres, which just reflects an enhanced lifetime
of the fluctuation with the largest amplitude, a similar
effect is not observed for micelles with long hairs (hairy
micelles). A coupling of translational and rotational motion
due to shearing forces is suggested as an explanation. On
going to micelles with shorter hair, the slowing down is
partly recovered. In addition to the diffusion dynamics, the
scattering data show two relaxation modes, which arise from
the motion of the hairs and are also detected by dynamic
mechanical experiments. They barely depend on q and - in
contrast to the viscosity - on the concentration.
The heat transfer has been studied in a Rayleigh-Bénard cell
filled with ^3He at the critical density over the reduced
temperature range 5\times10^-4 < \epsilon < 0.2 where
\epsilon = (T-T_c)/T_c with T_c = 3.316 K. The
experiment consisted in measuring the temperature difference
\Delta T(t) across the fluid layer as a function of time
after turning on a constant vertical heat current q. The
height of the fluid layer was 1 mm and the aspect ratio
\Gamma = 57. The thermal conductivity in the
non-convective state and the onset of convection were
determined, and measurements were made of \Delta T(t =
\infty) as a function of q up to reduced Rayleigh numbers
(Ra-Ra_c)/Ra_c of the order of 5 \times 10^3. The onset
of convection agreed well with predictions combining the
Schwarzschild and Rayleigh criteria. As q is increased
beyond the onset of convection, several qualitatively
different patterns in \Delta T(t) are observed until the
steady state is reached. These patterns, among them unusual
oscillatory ones, change as q and \epsilon are varied
and a map of the various features in \Delta T(t) is
presented in the [\Delta T(\infty), \epsilon] plane. The
power spectrum in the convective regime will also be
discussed. The transient measurements of \Delta T(t) in
the non-convective regime were in very good agreement with
predictions based on the theory by Onuki and Ferrell.
We report observations of two types of secondary flow
patterns in a channel flow subject to spanwise rotation:
twisted and braided vortices. These states have many
similarities to those patterns first observed in
Taylor-Couette flow (TVF) by Andereck, Dickman and Swinney
(Phys. Fluids 26, 1983) and can coexist at the
same Reynolds number. The streamwise wavenumber,
corresponding to the twists, or to the spacing of vortex
crossings, has broadband components in the power spectrum,
as in the case of TVF. The spectrum is determined from light
intensity measurements at a particular downstream point as
the vortices are swept by. A 1.75% by volume mixture of
Kalliroscope flakes in water is used for flow visualization
and reflectance measurements. A laser beam is transmitted
across the channel width and scattered light is detected
off-axis by a sensitive fast photodiode with wide angle
collection optics. With span-to-width aspect ratios as small
as 2, single braided roll pairs are observed. Spatial and
temporal characteristics are observed as a function of
Reynolds and Rossby number. Research supported by NSF grant
CTS-9422442.
A lattice gas model is used to study the flow of gas (methane) within marine
sediments with simplified pressure and temperature gradient conditions. The
porous sedimentary material is described by a matrix on a discrete lattice.
Geologic fault zones, in which the porosity is higher than the surrounding
material are imbedded in the model. The bottom layer of the matrix is
connected to a source of gas, which is modelled by particles with simplified
particle-particle, particle-pore, and particle-barrier interactions. The
Metropolis algorithm is used to move the gas particles, which are driven by
the density gradient at the source, temperature, and a pressure gradient,
from bottom to the top of the model where gas particles can escape the
system. In this non-conservative open system, a steady state is
accomplished for both the flow rate and the density distribution. Response
of the flow rate is studied in detail as a function of pressure and
temperature gradient. Both linear and nonlinear responses of the flow rate
are observed and the crossover regime is identified. We show that the
steady-state density profile depends on porosity, magnitude of the field,
and the temperature. Some specific profiles and flow predictions will be
presented.
Using a path-integral approach, we develop a self-consistent theory of
excitations at finite temperature for dilute Bose gases. The use of
pairing fields \langle \psi \psi \rangle,
\langle\psi^\psi \rangle ensures validity in a broader
temperature range than the traditional condensate-wavefunction
approach can offer. We derive Bogoliubov equations, dispersion
relations and critical temperatures for both homogeneous and
trapped systems.
We study the interaction effects on the condensates by
considering a model of one-dimensional bosons. The power-law
type external potential allows for the formation of a
condensate in these systems. Using a density-functional
theory type formalism we obtain an equation describing the
condensate wave function in the limit of very strong
interactions between the bosons. The properties of the
condensate in the model system with strong interactions are
investigated.
We solve the problem of an ideal boson or fermion gas in
d-dimensions trapped by \delta \leq d mutually
perpendicular harmonic oscillator potentials and obtain the
general density of states from which thermodynamic
properties such as internal energy, specific heat, etc. can
be determined. The trapped system is identical that of the
corresponding ideal quantum gas in d+\delta dimensions
but with an effective mass that vanishes in the
thermodynamic limit.
We study the stability of vortices in two-component Bose
condensates by means of Monte Carlo simulations. The total
energies of condensates with different vorticity have been
calculated. The global stability has been analyzed with and
without rotating fields. We study the local stability of
vortices prepared in the recent JILA experiment (M. R.
Andrews et al., Phys. Rev. Lett. 83, 2498 (1999)), where
there are two available configurations in which one specie
has vorticity m=1 and the other one has m=0. The
relationship between the stability with the interacting
strengths has been studied. We find both of configurations
are not locally stable when the interacting strengths are
weak, but when the interacting strengths are strong enough
both of configurations are locally stable. At a certain
condition, one configuration is locally stable and the other
one is not.
Experimental observations [1,2]of some of the modes of
lowest frequency above a Bose condensate in a harmonic trap
raises the question whether higher modes might also be
observed. A systematic study of various modes would provide
valuable diagnostics for the Bose condensate regime and
might also provide useful research tools, analogous to atom
Rydberg states. Using the Hartree-Fock Bogoliubov equations
and discrete variable representation methods [3], as well as
perturbation approaches, we have calculated energies as a
function of atom number, atom-atom interaction and
temperature. For a spherically symmetric trap, we find that
asymptotically, the energies approach the bare harmonic
oscillator values as 1/sqrt(n), where n is the number of
radial nodes. We have begun to address the crucial question
of widths, taking into account Landau and Beliaev processes.
Our first results, also for spherical symmetry, indicate
that as a function of energy, the widths vary somewhat
randomly about a slowly decreasing trend. We will discuss
means to excite these resonances selectively by
off-resonance modulated laser light or by stimulated Raman
scattering. 1. D. S. Jin et al. Phys. Rev. Lett. 78, 764
(1997). 2. D. Stamper-Kurn et al., Phys. Rev. Lett. 81,
500 (1998). 3. T. Bergeman et al., abstract this meeting.
A layer version of the tight-binding LMTO-GF method was used
to obtain exchange interaction parameters and spin wave
spectrum at the surface of elementary ferromagnets
(Fe,Ni,Co) with the local spin-density functional
approximation. Different ways of calculating of exchange
parameters in density functional approach were used and the
results compared. The stability and characteristic range of
surface spin wavecompare to bulk ones have been estimated
and possible connection with experimental data emphasized.
We present the bandstructure of YH_3, which exhibits a
metal insulator transition under continuous increase of
hydrogen concentration from 2 to 3. Although LDA
calculations suggest that the trivalent hydride has no band
gap unless we assume a complicated structure, the
quasiparticle band with the GW approximation reproduces
insulating behavior. The role of the self-energy correction
is to raise the unoccupied Y4d band rather than to
decrease the band-width of the occupied H1s band. The
results will be compared with other theories which are based
on model Hamiltonians. The effect of the self-consistency
will be also discussed.
We have built a database of Fermi surfaces in Virtual
Reality Modeling Language (VRML) for 45 elemental solids.
Running any of the free VRML browsers on an entry level
personal computer, a user can rotate and fly through the
3-dimensional Fermi surfaces in real time. The homepage of
the database contains a periodic table that links to the
Fermi surface pages of individual elements. For each of the
solids, the Fermi surface of a band that crosses the Fermi
level is contained in one of the 167 VRML files. The average
size of the files is less than 70KB. To obtain the Fermi
surfaces, the energy E(k) in k-space is calculated using
a nine-band tight-binding model. The energy data is then
passed through an isosurface generator which also clips the
surfaces with the boundary of the first Brillouin zone. The
website also provides a CGI script which returns Fermi
surfaces in VRML using user-submitted energy data. The
homepage of the database is
\underlinehttp://www.phys.ufl.edu/fermisurface .
We have performed a calculation of electronic structure of
(GaAs)_n/(Ge_2)_n Superlattice (2 \leq n \leq 4) with
Tight Binding Method. We have found that the band-gaps are
direct. Previous self-consistent pseudopotential
calculations also showed that in the range of ( 2 \leq n
\leq 4) the band-gaps of (GaAs)_n/(Ge_2)_n are direct.
The electronic excitation spectrum in baryllium, magnesium,
cadmium and mercury chalcogenides has been calculated by
means of the ab-initio GW approximation using
norm-conserving pseudopotentials, plane-waves or mixed-basis
expansions, and including the spin-orbit interaction. Both
real and imaginary parts of self-energy have been obtained,
thus giving insight into the dynamics of quasiparticles:
besides the correlated band structure, we present the
excitation-energy dependence of the lifetime and the
inelastic-mean-free path of hot electrons and holes.
The GW energy bands compare very well with ARPES data for
all materials studied except the mercury compounds, where
there are still significant differences. In addition to the
one-particle energy spectrum we present EELS. ellipsometry
and soft-x-ray resonant inelastic scattering spectra.
We have explored the efficiency of the generalized
simulated annealing (GSA) [1]
through a comparable study with the classical simulated annealing (CSA) [2]
and the fast simulated annealing (FSA) [3]. Our calculations on the Thomson
model and nickel clusters show that the relative efficiency of GSA to CSA
and FSA increases exponentially with the number of variables of the objective
function. We have
also observed a larger exponential factor in optimizing the structures of
Ni clusters than that in the Thomson model.
Thus, relative to CSA and FSA, the more complex the
system is, the more efficient the GSA method is.
The possible reason
for GSA with high probability in finding the global minimum is also
addressed through searching a two-dimensional phase space in a low temperature.
[1] C. Tsallis and D. A. Stariolo, Physica A 233, 395(1996).
[2] S. Kirkpatrick, C. D. Gelatt, Jr., and M. P. Vecchi, Science 220, 671(1983).
[3] H. Szu. and R. Hartley, Phys. Lett. A 122, 157(1987).
A number of years ago, Horn and Weinstein (Phys. Rev. D30,
1256(1984))
introduced a novel nonperturbative method for calculating ground-state
expectation values for Hamiltonian systems. Although close in spirit
to standard variational schemes this ``t-expansion" introduces a
fictional parameter t to the trial state
\exp(-\hatHt/2) |\Phi\rangle wherein the limit
t\rightarrow \infty yields convergence to the ground-state energy
E_0 for the expansion
\[
\lim _t\rightarrow \infty \frac\left\langle \Phi \right| \hatH
\exp \left( -\hatHt\right) \left|
\Phi \right\rangle \left\langle \Phi \right| \exp
\left( -\hatHt\right) \left| \Phi \right\rangle =E_0.
\]
Recently Samaj et al.\ (J. Phys. A30, 1471(1997)) have
generalized
the t-expansion technique and the related Connected Moments Expansion
to a more general canonical sequence.
They then apply this canonical series
to the quantum Ising model.
In the present work we have expounded upon the
work of Samaj et al.\ and have applied this
to a number of different many-particle Hamiltonian systems.
In this work a recently developed variational technique
(Phys. Rev. A51, 3337 (1995))
is applied in the estimation of the ground-state energy
of the Rabi Hamiltonian \hatH=\frac12
ømega_0 \sigma_z+ømega b^b+
g\left(\sigma^+ b^\dagger+\sigma^+ b+
\sigma^- b^\dagger+\sigma^- b \right).
For a trial wavefunction we use a single parameter
coherent state of the form
\left| \Psi _0\right\rangle =N_0\,e^\alpha b^\dagger
\left| 0\right\rangle \left( \left| \downarrow \right\rangle -
\left| \uparrow \right\rangle \right)
and a finite non-orthogonal basis is generated.
This is a generalization of the
Linear Variational Method
used by quantum chemists.
Our results are compared to those of Bishop et al.\
(Phys. Lett. A254, 215 (1999)),
where a large scale diagonalization
of the hamiltonian was performed.
The Piecewise Parabolic Method (PPM) hydrodynamics code and
other codes based on the PPM technique have been used
extensively for the simulation of astrophysical phenomena. A
new version of the PPM code under development at the the
University of Minnesota's Laboratory for Computational
Science and Engineering (LCSE) will be described. This new
code incorporates a version of dynamic local adaptive mesh
refinement (AMR) targeted specifically at improving the
treatment of flow around shocks and contact discontinuities.
Because the AMR is targeted at surfaces within a flow that
can develop complex shapes, a cell-by-cell approach to the
grid refinement is adopted in order to minimize the number
of fine grid cells, with the hope of controlling the
computational cost. Information is saved from the coarse
grid solution in order to reduce the difference stencil for
refined grids, also with the goal of reducing cost. The data
structures and parallization method are being carefully
designed to permit efficient implementation of the algorithm
on clusters of shared memory machines, with automatic
dynamic balancing of computational loads over the cluster
members. Sample results in 2-D will be presented.
In the Density Functional Formalism, the ground state energy
of any electron system is given by an unique universal
functional of the density. As a consequence, the same
functional must be able to describe systems confined in any
number of spatial dimensions. Concretely, the expression of
the functional for a 2D system should result from the 3D one
applied to a system where the density profile is a delta
function along one of the coordinates. In this work, we
present a study on the ability of several models for the
exchange-correlation (XC) energy to verify this property. We
obtain the behavior of various well known approximations in
the strict 2D limit. We show that the LDA and GGA
approximations diverge. However, the weighted density
approximation (WDA) gives an accurate XC energy for the 2D
electron gas as well as a good description of its pair
correlation function. The above theoretical study is
illustrated by applying these models to more realistic
quasi-2D electron systems.
Raman spectroscopy has long been used as a probe of the
atomic structure of chalcogenide glasses. Sharp features in
the spectra indicate the presence of characteristic
structural units, but the Raman measurements alone cannot
directly determine the atomic arrangements in these units.
In this presentation we discuss calculations based on the
density functional theory (DFT) that are aimed at
interpreting the spectral peaks in atomistic terms. The
calculations yield both the frequencies and Raman spectral
intensities for the vibrational modes of cluster models of
the materials. We create simulated spectra by combining
results of several cluster calculations. Here we focus on
SiS_2 and SiSe_2. The simulated spectra for these
materials are in good agreement with experiment. We contast
the results for these glasses with previous calculations for
the Ge-analogues, GeS_2 and GeSe_2 and discuss the
structural implications of the differences. Work supported
by Research Corporation and NSF DMR-9972333
A single photon with very high energy can form a tiny black
hole bound by its gravitational force, whose state is
referred as Space-Time Quantum of Action (STQA). The
Schwarzschild radius, energy and duration of STQA can be
calculated. It is very intriguing to find out that the
product of space interval, energy and time interval of STQA
is a constant (STQAC). In addition, STQAC is proven to be
the minimum of all particles, which means that the product
of space interval, energy and time interval for other
particles is integer times as much as STQAC. Thus it is
reasonable to hypothesize that the STQA is the basic unit of
all kinds of particles. We deduced that STQAs construct all
the particles in various fractal dimensions. The dimension
of the universe is calculated to be three. Since the product
of energy and time interval of the basic unit STQA is the
Plank constant, the real quantum of action can be found,
which leads to a new explanation to the in-determinant
principle of Quantum theory. We can foresee many practical
applications of this finding. One example is the novel
design of fractal antennas that might lead to revolution in
wireless communications.
In recent years arsenic selenide has shown a high potential
for application as an infrared fiber material. It transmits
in most of the 2-10 micron region, which is available at the
Vanderbilt Free Electron Laser (FEL). The ablation threshold
was measured for the bulk material at several different
wavelengths corresponding to impurity absorption, such as
Se-H and C-H vibrational modes. Using a time of flight
spectrometer to detect the ablated particles, individual
As/Se ions as well as bigger clusters of ~6 As/Se atoms were
observed. The Vanderbilt FEL delivers 3-5 microsecond long
macropulses which consist of 1 ps long micropulses separated
by 350 ps. Previous threshold measurements were made with
the entire length of the macropulse hitting the sample. In
the future a Pockel cell will be used to reduce the length
of the macropulse to investigate the pulse length dependence
of the ablation process. (Work funded by ONR)
Studies on a number of fragile glass formers has shown that
the non-exponential \alpha-relaxation at temperatures
close to T_g can be attributed to the existence of
spatially heterogeneous dynamics. We have applied a recently
developed multidimensional solid-state NMR method to measure
the length scale of dynamic heterogeneities for the first
time to a low molecular weight glass former. The length
scale in glycerol was determined to be about 0.6 nm, much
shorter than the length scale of 2-3 nm found in poly(vinyl
acetate). This difference may be related to the fact that
glycerol is a stronger glass former than poly(vinyl
acetate). Since the heterogeneity length is an upper bound
for the length scale associated with cooperative motion, our
result is an interesting test for current theories of the
glass transition.
Viscosity is a major factor contributing to the dynamical
behavior of a fluid system. Fluid viscosity is responsible
for the resistive force experienced by objects moving
through a fluid, and is directly proportional to the drag
force on an object. Since granular materials exhibit many
fluid-like properties, it is natural to assume the existence
of a granular viscosity analog. We measure the granular drag
force on cylindrical and spherical objects, and we compare
it to the cross-sectional area and the insertion depth of
the objects in granular materials to determine the
mathematical dependence of the drag force on these
parameters. From this relation, we determine a granular form
of Stokes' Law.
In 1987, Bak, Tang, and Wiesenfeld introduced a new paradigm
in the physics of large, complex, dynamical systems called
Self-Organized Criticality. One system that has previously
been used as a model for SOC is a sandpile. We
experimentally investigated smooth glass beads forming a
conical pile, where adding one bead at a time causes many
small, and a few large, avalanches. The size distribution of
avalanches was consistent with the predictions of
Self-Organized Criticality. In particular, the number of
avalanches of a given size was proportional to the size of
the avalanche to the power (-1.47±0.09), which is close to
the mean-field prediction of –1.40±0.03. This result was
independent of the size or shape of the base of the pile. We
were also able to determine the power spectral density,
which exhibited a power-law with slope -1.80±0.09 that was
also independent of base size and shape. This research was
supported by NSF-REU grant DMR 9619406.
We have studied axial segregation of mixtures of 2.0 mm and
0.90 mm soda-lime glass beads in a 5"-diameter, 22.5"-long
horizontal drum mixer. ``Reversible" axial segregation is
apparent on the surface down to concentrations of about
20/much of the segregation action occurs beneath the
avalanching surface and cannot be inferred from surface
observations. We have used a variety of mechanical
techniques to characterize the subsurface segregation.
Although mechanical probing is somewhat invasive, a
combination of "freezing" bead packs in gelatin, sieving
bead packs axially, and cutting into dry bead packs to
examine subsurface structure has allowed us to document the
time evolution of reversible axial segregation in a
50%-small, 50%-large bead mixture.
The shear induced deformation of polymeric droplets in an
immiscible polymeric matrix is studied using a transparent
rotating plate-plate device. We consider the case where the
viscosity ratio of the two phases is near unity, but the
elasticity ratio of the droplet to the matrix is of order
10^2. This is achieved by using a matrix of PDMS and a
droplet of a PIB based Boger fluid. In the limit of weak
shear and small droplets, the droplet alignment is along the
shear direction, whereas for strong shear and large
droplets, the alignment is along the vorticity direction.
There is a range of conditions for which alignment can be
along either axis. For droplets aligned along the vorticity
axis, the distribution of aspect ratios is broad. The
transformation from flow alignment to vorticity alignment
upon commencement of shear flow has been observed and
correlates with the time scale for development of normal
forces in the Boger fluid.
De Gennes (1992) suggested that loss of entanglement in the
interfacial region between two incompatible polymers causes
slip at interfaces. Goveas and Fredrickson (1998) developed
a quantitative model for the lowering of interfacial
viscosity. However, to date there appears to be no
quantitative experimental evidence for interfacial slip. We
coextruded polypropylene and polystyrene with closely
matched viscosities into multilayers with 2,32 and 128
layers. Pressure drop of the coextruded multilayer melts
through a slit die was measured. The data showed a 40reduction when the number of layers increased from 2 to 128,
which indicates interfacial slip. The steady shear viscosity
of the multilayer sample was also measured in parallel
plates. When the shear stress was higher than a critical
value, the viscosity of the multilayers was lower than
either of the components. The interfacial viscosity was
estimated, and 40 times reduction was observed. Diblock
copolymer which spanned the interfaces was shown to able to
suppress interfacial slip.
Electrospinning from polymer melts in a vacuum was studied.
Fibers of polypropylene, high-density polyethylene and
poly(ethylene terephthalate) were successfully electrospun.
The molten polymer was held in a glass tube. A metal wire
immersed in the molten polymer supplied charge, which
migrated through the melt to the surface exposed at the end
of the glass tube. The electrical forces overcame both
surface tension and the viscous forces within the polymer
melt, and ejected charged jets of molten polymer. The jets
were attracted to an aluminum sheet maintained at an
attractive electrical potential, where they solidified. The
motion of the larger jets could be followed by eye. The
diameters of the solidified jets ranged from 3 to 60 microns
in these preliminary experiments. The vacuum process is
being investigated because the magnitude of electric fields
produced in a vacuum are not limited by the low dielectric
breakdown strength of air.
The electrically charged surface of a polymer fluid becomes
unstable when the electrical forces overcome forces of
surface tension. A charged jet is then ejected from the
surface. The electrical charge carried with the ejected jet
can excite instabilities of the jet, which affect its path.
A bending instability [1] is frequently observed. In some
solutions of poly(2-hydroxyethyl methacrylate) (HEMA), high
speed videographic images show that a splitting instability
also occurs for the more concentrated solutions. In the
splitting instability, a smaller, straight jet is ejected
from the primary jet. The splitting instability may be
observed before or while the bending instability is growing.
In a 20instabilities are dominant. The bending instability became
observable when the concentration was reduced to 16more dominant when the concentration was further reduced to
16
1. D. H. Reneker, A. L. Yarin, H. Fong, S. Koombhongse, J.
App. Phys, to be published.
The dry nanofibers produced in a typical electrospinning
process are electrically charged. The nanofibers were
directed by an electrical field, a tensile force along the
axis of the fiber, and by the viscous drag force of moving
air. The looping and spiraling path of the nanofibers, which
resulted from bending and other kinds of instabilities that
occurred as the fiber was formed, also complicated the
collection process. Non-woven sheets of nanofibers were made
by attracting the nanofibers to a conducting sheet or
screen. The sheet or screen was flat and stationary, or
wrapped around a rotating drum. Nanofibers were also
collected in a liquid. The liquid removed charge or solvent.
Nanofibers were collected on the surface of a non-wetting
liquid, so that the patterns formed by the arriving
nanofibers were observed directly. Streams of air, and air
vortices were also used. These methods are being combined
with robotic manipulators to collect nanofibers in many
useful forms.
Liquid crystalline polymers (LCPs) have been the subject of
extensive studies because of potential commercial
applications and scientific challenges. The excellent
mechanical properties of LCPs arise from highly anisotropic
molecular structure, which develops as a complex interplay
between molecular dynamics and the applied flow field. We
study the behavior of model thermotropic main-chain LCP
(DHMS-7,9) under oscillatory shear flow using in situ X-ray
scattering techniques. Experiments were done in nematic
(140^o C) and x-phase (110^o C) to study the effects of
frequency (0.5 - 50 rps) and strain amplitude (50 – 200In nematic phase, strong alignment in the flow direction
(‘parallel’) was observed. The steady state was reached
quickly either at high strain amplitudes or high
frequencies. In x-phase, molecules aligned in flow direction
at high strain levels or oscillation frequency, while
alignment in vorticity (‘perpendicular’) direction was
observed at low strain amplitude or frequency. In addition,
we present the flipping of orientation from parallel to
perpendicular alignment as a result of step change in
temperature from 140^o C to 110^o C and oscillatory
motion from a pre-aligned parallel state in x-phase.
Amorphous, thermoplastic polymers are foamed using inert
gases such as carbon dioxide or nitrogen to create a
microcellular foam. The small cell size, smaller than
critical flaws already present in most polymers, allows the
microcellular foam to retain some critical mechanical
properties present in the bulk material. Microcellular foams
with a cell density greater than 10^8 cells/cm^3 and an
average cell size of order 10 microns or less, have been
succesfully produced in amorphous polymers such as
polystyrene, poly(methyl methacrylate) and polycarbonate.
Current processing methods make it difficult to achieve a
high cell nucleation density. In an effort to control the
cell nucleation density, heterogeneous nucleation sites,
both solid particle sites and in the form of block copolymer
micelles, have been added to polystyrene samples. Batch
experiments using carbon dioxide gas reveal anomalous trends
in nucleation behavior that can not be explained by the
classical nucleation theory, prompting further inquiry into
its validity.
Poly(meta-phenylene isophthalamide) was electrospun from
solution to form nanofibers. The resulting nanofibers were
birefringent and ranged in diameter from 30 to 300
nanometers. High-speed videographic observations showed that
the jet developed the bending, looping and spiraling path
characteristic of electrospinning. The thermal properties
and the structure of the as-spun and annealed nanofibers
were characterized by thermogravimetric analysis, X-ray
diffraction and transmission electron microscopy. Both
electron and X-ray diffraction patterns of the as-spun
fibers showed diffuse equatorial diffraction spots,
indicating that the molecules were oriented along the axis
of the nanofiber. After annealing, the diffraction spots
became much sharper, and other spots appeared, indicating
that the as-spun fibers were in a metastable state.
Nanofibers of this material were stable up to 200°C.
Previous studies in lamellar diblock copolymers have
determined the processing conditions necessary to create the
parallel and perpendicular orientations. In addition,
studies have shown a flipping behavior between the
perpendicular and parallel orientations. The purpose of this
study is to investigate the mechanisms by which the flipping
occurs. A low molecular weight styrene-isoprene diblock
copolymer was first aligned in the perpendicular orientation
through extensive large amplitude oscillatory shear.
Flipping from the perpendicular to parallel orientation was
then conducted and interrupted at intermediate stages. The
evolution of the parallel orientation and the morphological
development at these stages were characterized by the
electron microscopy. Simultaneous shearing and small angle
x-ray scattering were also performed to elucidate the
flipping mechanism.
The goal of this work is to investigate how the variables in
the hot-melt coating process affect the microstructure and
properties of pressure-sensitive adhesives based on a
styrene–isoprene–styrene triblock copolymer. This polymer is
a thermoplastic elastomer, able to be coated at high
temperatures and physically crosslinked at lower
temperatures. Adhesive tape samples have been made through
hot-melt and solvent coating methods. Hot-melt coatings are
prepared at speeds up to 110 feet/minute. Materials with the
same thermal history have been coated using both methods and
then tested for comparison of properties. PSA properties are
strongly dependent on the time scale of application and
debonding, as revealed by shear rheology data, and three
types of performance tests (tack, peel, and shear holding
power) are used to capture the various responses.
Solvent-coated tape has superior shear strength, while
hot-melt-coated tape performs better in peel tests. It is
expected that the varying flow and deformation histories of
the samples will lead to distinct chain orientations, while
the rate of cooling of hot-melt-coated samples may influence
the degree of phase separation achieved. These factors will
cause the adhesive coatings to have different
microstructures and therefore different properties.
An experiment is in progress to investigate the turbidity of
a liquid-liquid mixture very close to its critical
temperature. Temperature is controlled precisely through an
onion-layer design, with successive stages of control and
measurement. This process is automated using a program in
LabVIEW, an icon-based programming language. Control of the
temperature has been achieved to within a few microkelvin of
the critical temperature, which is near room temperature.
The techniques and methodology that allow such precise
temperature control and measurement will be presented along
with the results showing 10ppb control.
Acknowledgment is made to NSF-REU grant DMR 9619406 and to
NASA grant NAG8-1433 for support of this research.
A discrete lattice of size L_x \times L_y \times L_z is considered
with a large aspect ratio L_x/L_y(L_z). Polymer chains, each of
length L_c are released from one end (x=0) of the sample in
presence of a field (E) along x-direction. In addition to excluded
volume, a nearest neighbor repulsive interaction is considered among
the polymer nodes. Metropolis algorithm is used to move chain nodes:
kink-jump dynamics is primarily used in this study, however, attempts
are made to explore the effects of other dynamics such as crank-shaft
and reptation. As the polymer density grows at/near the wall (x=L_x),
an interface develops. Density and coformational profiles of the polymer
chains and the interface width are monitored as function of Monte Carlo
steps (MCS). Growth of the interface width (W) of the polymer density
with time, i.e., W \sim t^ \beta, is analyzed in detail and
the saturated width W_s = W, t \to \infty is evaluated as a
function of temperature, field strength, and chain length.
Using the scaling of the saturated width (W_s) with the transverse
dimension, i.e., W_s \sim L_y(z)^\alpha, the roughness exponent
\alpha is esimated. Variations of the interface width and the roughness
exponent with the smulation parameters (E, T, L_c) exhibit interesting
roghening and de-roughening phenomena, some of these results will be
presented.
We have investigated two dimensional pattern coarsening dynamics with a
block copolymer which microphase separates into spheres. The system
consisted of a single layer of polystyrene spheres in a
polyethylene-alt-polypropylene matrix (PS-PEP) on a substrate and was
studied by
hot stage atomic force microscopy (AFM). By performing Delaunay
triangulation, we quantitatively demonstrate a high density of
free dislocations and a relatively low density of free disclinations.
By obtaining sequential AFM images as a function of annealing time,
we have tracked the grain growth and defect motion. During annealing,
the correlation length saturates at temperature dependent values,
resulting
in pinned grain boundaries consisting of a high density of dislocations
and disclinations. Increasing the annealing temperature decreases
barriers
to motion, depinning defects from grain boundaries and resulting in a
better
ordered pattern with fewer grain boundaries.
Atomistic molecular dynamics simulations of the anchoring of a liquid crystal
at the surface of an amorphous polymer have been performed.
The particular materials modeled were 5CB in the nematic phase at the
surface of amorphous
polyethylene.
The inverted pendulum model of B. Lin and P. L. Taylor
(Physics Letters A 172) (1993) 281-284
indicates that
the dynamics of the substrate, in this case amorphous polyethylene, can
stabilize non-planar orientations of the liquid crystal molecules.
Simulations were performed to test this hypothesis.
In order to isolate the effects of the motion of the polymer substrate on
the liquid crystal, two sets of simulations were performed: in one set
the atoms comprising the polyethylene were allowed to move and in the other
set the polyethylene atoms were fixed.
Also, simulations were performed starting from both planar and homeotropic
initial conditions.
There is very keen interest from a thoretical as well as an experimental
point of view to determine the initial conditions which one can use to
predict the formation of a Wigner crystal in experiments.
The density at which solids can be formed from individual particles
is limited by the range of their interaction potential. Short-range
forces can only bind particles at high density, whereas long-range
forces, e.g. the Coulomb interaction, are capable of forming solids
even at very low densities. Wigner predicted the existence of such a
low-density Coulomb solid for electron systems, the so-called Wigner
crystal. Recently, the experimental realization of a Wigner crystal
was found in an electron system with charged calloidal particles in
an aqueous solution, and charged dust particles in plasmas.
Using numerical simulation, we demonstrate that there are several
densities at which a Wigner crystal exists for the two dimensional
electron gas.
Commensurability of chemical and
physical length scales between substrate
heterogeneity and copolymer morphology has been shown to induce control
over block copolymer ordering in thin films. In thicker films, free
surface effects compete with substrate incuded ordering and prevent
the oriented morphology from propagating throughout the entire film
thickness. Herein we examine the crossover region between substrate control
and free surface effects. Varying both film thickness and solvent casting
under different conditions, as well as spin casting and annealing, we probe
the effects of competing free surface and substrate control.
A broadening research effort on how cation species affects
association, structure, and packing of Copper
Phthalocyanines (CuPcTs) in Electrostatic Self-Assembly
(ESA) thin films is underway. Here we present molecular
dynamics simulations using Cerius2 software to examine the
CuPcT / Poly(1-lysine) molecular level deposition process.
The shape of the CuPcT molecule and the chain conformations
of the poly(1-lysine) molecule are examined. An amorphous
glass slide is simulated and on this molecular layers of
CuPcT and poly(1-lysine) are sequentially deposited. At each
level the depositing molecule atoms are allowed full motion
while the previously minimized layers atoms are fixed. A
grand canonical ensemble minimization at 800K is followed by
a minimization at 300K and finally one at 0K. After a
deposited monolayer global conformation is minimized, the
atoms in the top two layers are allowed to vary to obtain a
local minimum at 0K. We find this modeling scheme to be very
effective and final molecular spacings agree well with
ellipsometry data.
In the spinodal dewetting process, a thin liquid film breaks
up due to the unstable growth of capillary waves driven by
dispersion forces across the film. We have investigated the
dewetting of a polymer melt substrate by a polymer thin
film. The dewetting of polystyrene (PS) by a thin
poly(methyl methacrylate) (PMMA) film has been studied using
optical microscopy, atomic force microscopy and neutron
reflectivity. The characteristic length of the dewetted
morphology has been investigated by changing the thickness
of the bottom (PS) layer, which changes the effective
Hamaker constant of the system. Changing the substrate from
which the PMMA film was floated can control the degree of
order in the dewetted morphology. The early stage kinetics
of dewetting are followed by measuring the growth of the
interface and surface roughness with neutron reflection. The
kinetics are studied as a function of the molecular weight
of the polymers and of temperature.
We present a novel method for controling the wetting
properties of materials. Our technique is based on the
grafting reaction between w(CH_2)_xSiCl_3
molecules and the surface -OH functionalities. A thin film
made of a poly(dimethyl siloxane) (PDMS) network is first
stretched and then physically modified to produce surfaces
containing grafted -OH groups, which are used as attachment
points for the chlorosilane molecules. The grafting density
of the organic modifiers can be adjusted by controlling the
strain on the PDMS substrate, \Deltax. We test the
feasibility of the proposed grafting technique by attaching
semifluorinated (SF) self-assembled monolayers (SAMs),
F(CF_2)_y(CH_2)_xSiCl_3 (FyHx), to PDMS
substrates. After the SAM deposition, the stress from the
substrate is released and, as a result, the grafted FyHx
chains form well-organized SAMs with an exceptional order of
packing and alignment. The water contact angles (\theta)
are found to increase with increasing the strain on the
pristine PDMS substrate and reach values larger than
\theta=130^o for \Deltax=50resistance of the SF-SAMs against water exposure is
investigated. Experiments using scanning force microscopy on
samples exposed to water reveal significant variations in
the surface topography with the immersion time. However,
near-edge X-ray absorption structure and water contact angle
measurements indicate that regardless of the immersion time,
the SF chains stay oriented and maintain their great wetting
properties.
Relaxation of polymers from non-equilibrium conditions will
in general only occur if the polymeric system is above the
bulk glas temperature. The mobility of the chains near the
solid surface can be, depending on the interaction with the
surface, either enhanced or depressed. To examine the
influence of a free surface on the mobility of polymer
chains rough surfaces of thick polystyrene films (> 5
\mum) were prepared. Each peak or surface asperity of the
PS film presents a non-equilibrium system with a large
interfacial surface area. Depending on the mobility of
chains the peaks will decay in order to minimize the overall
surface area. Relaxation of the surface asperities at
temperatures well below the bulk glas temperature is
observed. One typically observes a fast decay of structures
with a high aspect ratio and small lateral extension,
leaving the surface with peaks of rather well defined aspect
ratio and a lateral extension above a critical value. A
second far slower process is observed with virtually no
change of the aspect ratio.
The morphology and structure of gradient copolymers of
poly[(styrene-r-maleic anhydride)-b-polystyrene] were
investigated. Reflection optical microscopy images showed
continuous color changes at the edge of the spin-coated thin
films indicating that a layering of the morphology parallel
to the substrate interface did not occur. Microphase
separation was observed by atomic force microscopy
measurements on reactive ion etched samples, which showed
cylindrical microdomains oriented normal to the surface. The
cylinder-cylinder separation distance was about 16nm,
consistent with bulk values determined by x-ray scattering.
No interference maxima were seen in neutron reflectivity
experiments over a temperature range from 130^oC to
190^oC, suggesting a vertical orientation of the
cylinders. The surface tension of the gradient copolymer was
slightly higher than that of PS. The combined control over
the orientation of the microdomains with the ability to
convert the anhydride to the corresponding acid makes the
materials interesting candidates for functional nanascopic
arrays.
We study rheologiacl behavior of di-end-functionalized
polymer confined between two flat walls by using Monte Carlo
simulation. In order to investigate the effect of the
associating structure induced by functional groups on the
rheological properties, we simulate three kind of telechelic
chains with different number of associating capacity, f=1, 2
and 3 and compared the shear responses of these polymers.
When the associating capacity of a functional group is
larger than 1, physically crosslinked network of associating
structure effectively blocks the extreme chain stretching
against shear deformation, resulting in decrease of limiting
shear stress.
The glass transition temperature of thin polymer film is
studied in this work. We have used ellipsometry to measure
the glass transition temperature(Tg) of poly(alpha-methyl
styrene)[PAMS] thin films as functions of film thickness and
molecular weight. When the films are thinner than a few
hundreds of angstroms, substantial reduction in Tg is
apparent and Tg of the film approaches the asymptotic value
with increasing film thickness. Tg depression pattern of
PAMS does not differ with molecular weights. To fit our
experimentally obtained thickness dependent Tg data, the
Michaelis-Menten equation, which is widely known model in
enzyme kinetics, is analogized. The observed Tg depression
data in thin film has the good agreement with our analogized
equation and two adjustable parameters are obtained from
fitting. The large surface to volume ratio in thin film
geometry is the main reason for the Tg reduction in thin
film.
We have investigated the structural properties of the
aromatic molecule p-quaterphenyl (p-4P) vapor deposited onto
a KCl (001) surface at a pressure of 10^-6 mbars. In a
series of AFM studies, thicknesses of p-quaterphenyl varying
from 1 to 20 monolayers exhibited unusual features not
previously imaged. Included in our observations are: i)
needle-like accumulations of the p-quaterphenyl around
surface defects, ii) a striped-phase region with a lateral
spacing of approximately 25 nm for a nominal monolayer of
p-4P, and iii) a thickness range where we found indications
for a transition from lying to standing orientation of the
molecules.
We investigated the interface of PS/PMMA and PS/ PVP as a
function of film thickness by X-ray scattering, SIMS and AFM
techniques. Although the neutron scattering of two
immiscible polymer/polymer interfaces has been investigated
by Sferrazza et.al( PRL, 78, 1997), the disadvantages of
small qz range(~0.15A^-1) and lower resolution of
neutron reflectivity brought difficulties to detect within
reasonable error range. By the way X-ray reflectivity can
overcome these limitation of neutron reflectivity because of
its much larger qz ranges and high resolution, but the small
contrast, which is that most of polymers have very similar
refractive index of X-ray, was only problem. Recently, we
found that X-ray reflectivity can be analyzed using Fourier
method (Seeck et al, APL, in preparation) to detect the
ultra-small density contrasts in thin film layer system.
Therefore with great advantages of X-ray scattering compared
to neutron scattering, this method can be applied specially
to polymer-polymer interfaces without any further contrast
alteration. The results are showing that interfacial energy
as well as film thickness are the most important factors to
modify the intrinsic interfacial width and additional
interfacial broadening by capillary wave. The results were
well matched to the theoretical calculation within very
small error ranges. Experiments are currently in progress
for bilayer systems with diblock copolymer. The existence of
diblock copolymer can reduce tremendously the interfacial
tension of immcible polymer/polymer interface and bring the
interfacial length broadening.
Design and control of polymer surface functionality is
desirable for numerous applications. It is widely known that
hydrophobic polymer surfaces can be produced on
block-copolymers due to surface segregation of hydrophobic
blocks. Less research has been performed on random copolymer
compositions. In this work, we investigate the surface
segregation of a series of random copolymers composed of
methyl methacrylate (MMA) and tetrahyrdroperflourooctyl
acrylate (TAN). Thin films (150-200 nm) were solution
spun-cast onto silicon substrates. As deposited and samples
annealed above the copolymer glass transition temperature
were studied to assess the degree of surface reorganization
that might occur. Surface analysis using water contact angle
measurements and angle resolved XPS indicate that the
concentration of fluoro groups increases near the surface of
the film in all compositions. The degree of surface
segregation was dependent on the PTAN to PMMA ratio of each
copolymer. The segregation was enhanced after annealing the
films at elevated temperatures.
We investigated the effect of chain architecture on surface
segregation and bulk thermodynamics of blends of
well-defined star-branched and linear polybutadiene. Bulk
thermodynamics were sensitively studied with Small Angle
Neutron Scattering. The thermodynamic interaction parameter
was found to increase with branching and also to vary
somewhat with composition. The surface properties were
studied with Neutron Reflectometry, and Nuclear Reaction
Analysis. These measurements were performed on blends with
varying compositions, number of arms of the star, and
molecular weight of linear deuterated PB.
Characterizing diffusion behavior of metal nanoparticles in
a polymer matrix is important for understanding
metal/polymer interfacial dynamics. We have applied a
variety of techniques to study such mobility in a
gold/poly(tert-butyl acrylate) system in order to obtain a
more complete picture of the diffusion behavior. X-ray
standing wave along with reflectivity techniques were used
to measure gold particle movement in a time resolved manner.
Diffusion coefficients were also calculated from peak
broadening in Rutherford backscattering spectrometry
experiments. These methods were complemented by TEM, which
shows the two-dimensional distribution of particles.
We present a density functional study of the adsorption of
CO on the Pd(100) surface, to examine how surface strains
affects adsorption isotherms. We find that the coverage, at
a given temperature and presure, is changed substantially by
strain.
Exposure of self-assembled monolayers to x-ray and extreme
ultra violet radiation in the presence of air or oxygen
results in the incorporation of oxygenated functional groups
on the surface of the monolayer. The extent of chemical
modification was determined as a function of dose and oxygen
concentration. Dewetting, symmetric, neutral and asymmetric
wetting of poly(styrene-b-methylmethacrylate) thin films was
observed with increasing concentration of oxygenated
functional groups. The results were analyzed using the
Fowkes-van Oss-Chaudhury-Good approach for determining
solid/liquid interfacial tensions. We will discuss and
demonstrate applications of this technique to tune the
surface properties of substrates for nanopatterning and to
induce macroscopic ordering of thin films of diblock
copolymers.
The morphology of thin films of symmetric diblock copolymers
confined between two hard and flat surfaces was explored by
means of Monte Carlo simulations on a lattice. For such
simulations, the match between the bulk lamellar period and
the simulation box size is crucial to obtain meaningful
results. The simulations were performed in an expanded
grand-canonical ensemble, where the chemical potential and
the temperature of the confined film were specified and its
density was allowed to fluctuate. Our simulations have
revealed various types of morphology, depending on the
surface configurations.
To construct the phase diagram for confined thin films, we
have combined the results of simulations with a
phenomenological theory. The simulations provide valuable
insights into the lowest free-energy morphologies that are
required by the theory; these structures are sometimes
difficult to guess by simple intuition. The theory is useful
for rationalizing the simulation results and for designing
experimental protocols that will permit long-range
patterning of nanoscale features.
We have studied the morphology of Langmuir blodgett films at
the air/water interface of mixed diblock copolymer films.
Solutions of poly(styrene-b-ferrocenyldimethylsilane) and
PS-b-P2VP mixed in a ratio of 20/80 in chloroform were
spread at the air/water interface. The morphology of the
films was studied with AFM as a function of the surface
pressure and the diblock copolymer molecular weight. The
results show that the two diblock copolymers can be induced
to mix at the air/water interface with increasing surface
pressure. A reversible transition from spherical to
cylindrical morphologies is induced in the mixture which can
not be observed in films formed of the two components
separately. The effective surface phase diagram as a
function of block copolymer composition and pressure will be
presented.
The interfacial segregation of block copolymers to the
substrate/polymer interfaces from their mixtures with the
respective homopolymers in thin films is investigated by
neutron reflectivity and lattice-based self-consistent
mean-field calculations; the adsorbed chain configuration
was probed as a function of the ratio of block lengths. The
segment density profiles of either PV2P-PS or PMMA-PS
diblocks adsorbed at the PS/substrate interface are
evaluated. Both experiment and theory reveal evidence for a
broad transition from a "mushroom" to a "wet brush"
configuration of the dangling chains by changing the ratio
of the block lengths.
We cast symmetric poly(styrene-b-2vinylpyridine) (PS-PVP)
diblock copolymer thin films on Si which form lamellae
oriented parallel to the substrate surface, with equilibrium
thicknesses 1.5 L_0, 2.5 L_0, etc, where L_0 is
the equilibrium lamellar period. We investigate films of
initial thickness 2.0 L_0, which form bicontinuous
domain structures (1.5 or 2.5 L_0 thick) after
annealing. If a thick, glassy confining film is placed on
top of the block copolymer film before annealing, island and
hole formation is restricted and the underlying lamellar
morphology is forced to reorganize, whereas a thinner,
elastically deformable confining layer will buckle to
accommodate island and hole growth in the underlying film.
By creating confining films with controlled topologies, we
show that island and hole growth can be oriented, with
domains forming parallel to directions of minimum bending
stiffness of the cover layer topological structure.
The long-range lateral ordering of the lamellar microdomains
of symmetric diblock copolymers of styrene and methyl
methacrylate was studied. Thin films of the block copolymer
were prepared by spin coating solutions of the copolymer
onto a nonpreferential surface which was made by end
grafting poly(styrene-r-methyl methacrylate) random
copolymer having a styrene fraction of 0.58 to silicon
substrates. In the case of very thin films the nonfavorable
interactions with the substrate result in a dewetting. The
contact line is found to pin fluctuations, laterally,
leading to a long-range lateral ordering of the lamellae
that extends microns across the surface. The evolution and
interfacial interactions dependence of the alignment will be
addressed.
The surfaces of polymer thin films preferentially attract
one block of a diblock copolymer and this attraction can
induce layering of spherical microdomains of asymmetric
poly(styrene-b-2vinylpyridine)(PS-PVP) diblock copolymers,
resulting in long range order perpendicular to the surface.
We demonstrate that the lateral structure of such an
asymmetric diblock copolymer film may be controlled by
imposing a topological pattern on the film surface,
resulting in a perfectly packed hexagonal array in 2-D,
i.e., with no grain boundaries and few, if any dislocations.
We observe the structure of these films by sputtering down
to the midplane of a layer of PVP spheres with the oxygen
ion beam of a secondary ion mass spectrometer and then
imaging the exposed surface by scanning force microscopy.
These images show that the topologically confined stripe of
copolymer is ordered in a single crystalline, close packed
structure with hexagonal symmetry for stripe widths as wide
as 4 \mums, with a definite epitaxial orientation
relationship between the edges of the stripe and the close
packed directions in the layer. Such control may prove
useful in lithographic applications of block copolymer
films.
Precise control of the orientation of block copolymer
domains is of interest for the production of self-assembling
nanoporous media. Electric fields are effective in orienting
copolymer domains normal to a surface. In thin films of
poly(styrene-block-methyl methacrylate), a threshold
electric field strength E_t was found, which for film
thicknesses greater than 10 \mum, was independent of film
thickness and could be described by the difference in
interfacial energies of the diblock components with the
substrate. Neutron reflectivity, small angle neutron and
x-ray scattering and off-specular scattering suggest that
interfacial fluctuations are amplified by the electric field
that leads to orientation of the cylinders along the field
lines. In the case of poly(styrene-block-isoprene), a
material with a smaller difference in dielectric constants,
the microdomain orientation occurs at higher field
strengths. Also the reorientation process is more rapid than
in poly(styrene-block-methyl methacrylate).
We have investigated the fracture behavior of A/B/A
assemblies where A is a thick (2mm) beam of a glassy polymer
and B is a thin (1 to 200 microns) layer of another glassy
polymer immiscible with A. The external loading, the
molecular structure at the A/B interface and the thickness
of the B central layer were varied and the fracture
toughness Gc was measured for each assembly. Fracture always
occurred at one of the A/B interfaces and the microscopic
deformation mechanisms depended on the molecular structure
at the interface, on the plastic deformation properties of
the A and B polymers but also on the thermal residual
stresses in the layer and on the degree of mode mixity ahead
of the crack tip, which was imposed by the external loading.
A significant amount of shear stresses at the interface
promoted the formation of oblique crazes in one of the two
polymers and this caused a surprising dependence of Gc on
the layer thickness which will be discussed.
http://umr7615.pcsm.espci.fr/^\simpassade/gb/indexgb.html
This work is concerned with the relationship between the
properties of the pretilt angles and chemical structure of
the alignment layer for a series of novel organo-soluble
side-chain polyimides developed at The University of Akron.
The polyimides were spin cast on ITO glass substrates and
mechanically rubbed with a velvet cloth. Liquid crystal
display cells were constructed with an anti-parallel
geometry using 10\mum glass spacers and filled with the
nematic liquid crystal mixture E7. The pretilt angle, which
is defined as the angle between the liquid crystal director
and the substrate, was measured using the magnetic null
method. Various side-chain polyimide films were prepared and
pretilt angles were determined employing identical
processing conditions. In general, polyimides containing
long flexible aliphatic side-chains (no. carbons >12)
resulted in high pretilt angles (>20^o) however over time
the pretilt shifted to a homeotropic alignment (i.e. 90^o
to the substrate). The stability of the pretilt angles was
improved when polyimide copolymers were used. Further
enhancement of the stability was achieved by crosslinking
the system prior to rubbing. Liquid crystal like side-chains
(cyanobiphenol and biphenol based) resulted in stable
pretilt angles ranging from 20 to 40^o for a spacer length
of six carbons. Several surface techniques were used to
study the effect of rubbing, including; atomic force
microscopy, surfaced enhanced Raman scattering, and contact
angle.
The intoduction of blocky copolymers represents a possible
method of compatibilizing two immiscible polymers in a
blend. However, copolymers do not diffuse quickly to the
interface of a polymer blend system. Therefore, reactive
processing is being investigated as a means to form in-situ
compatibilizers for polymer blends. A model system composed
of poly(bisphenol A-co-epichlorohydrin) blended with
poly(ethylene oxide) that is compatibilized with
difunctional oligomers that are the same structure as the
blend components is currently under investigation. It is
expected that the oligomers can undergo an addition
copolymerization during processing to create the blocky
copolymers at the biphasic interface. Initial tensile
measurements show that the addition of the reactive
oligomers improves the properties of the blend.
Additionally, preliminary results indicate that reactive
oligomers may act as plasticizers and continue to polymerize
at room temperature after the blend is removed from the melt
mixer if insufficiently mixed.
The poor wettability of gelatin solutions on nonpolar
poly(ethylene terephthalate) (PET) and subsequent inadequate
adhesion of cast gelatin films to PET substrates, can be
improved by modifying the surface of the PET. Plasma
treatment of the PET, which creates reactive surface
functional groups, is a suitable surface modification. We
measure the fracture toughness, G_c, of a
semi-crystalline poly(ethylene terephthalate)/gelatin
interface using an asymmetric double cantilever beam (ADCB)
technique as a function of sample thickness, relative
humidity, and plasma parameters. The results indicate that
varying the thicknesses of the two beams, and thus the
mechanical phase angle, is important to control the
direction of crack propagation. Under high relative humidity
G_c increases due to water absorption and plasticization
of the gelatin. The fracture toughness increases as the
product of the plasma power and exposure time of the PET to
the plasma.
In this work we will show the compatibilization effect of
random copolymer AB in homopolymer A and random terpolymer
CDE rubbery system. In particular, we studied the
interfacial property of styrene-butadiene rubber (SBR) in
the interface of polybutadiene(PB) and functionalized
polyisobutylene (Brominated isobutylene-co-methylstyrene,
BIMS). Neutron reflectivity (NR) results indicated that
BIMS/PB interfacial width broadened significantly (from 4nm
to over 30nm) with the addition of SBR in PB. Flory-Huggins
parameters were obtained from interfacial width of BIMS/PB
and BIMS/SBR profiles measured with NR. Flory-Huggins
parameter for SBR/PB was obtained from literature. With
three Flory-Huggins parameters available, the interfacial
property of SBR was studied with SCF (Self-Consistent Field)
modeling. SCF modeling will be compared with NR results.
We investigate a monodisperse polymer in an athermal
solution between two surfaces, through a numerical method
developed by Chhajer and Gujrati which has been successfully
applied to a polydisperse system. The monodisperse polymers
require distinguishing a monomer by its distance from the
end-point, which was not the case with the polydisperse
polymers. Previously, we have applied this method to fixed
length polymer lengths and explored effects next to a
surface and bulk properties. We replace the original cubic
lattice by a modified tree structure of the same
coordination number. This modification allows us to capture
some of the correlations produced by the surface. The model
is then solved by recursive technique. Here we report our
results for a system of fixed length polymers between two
surfaces of finite separation. We report the results of our
computation of various density profiles as a function of the
distance between the two surfaces. We also present results
for the surface entropy and the surface free energy as a
function of surface interaction energy as well as the effect
of chain length.
Structure within thin epoxy films is investigated by neutron
reflectivity (NR) as a function of resin/crosslinker
composition and cure temperature. Variation in the crosslink
density normal to the substrate surface is examined by
swelling the films with the good solvent d-nitrobenzene
(d-NB). The principal observation is a large excess of d-NB
near the air surface. This is not a wetting layer, but
rather indicates a lower crosslink density in the
near-surface region. This effect is due to preferential
segregation of the crosslinker to the air surface, driven by
the lower surface tension of the crosslinker relative to the
epoxide oligamers. The magnitude of the effect is a function
of composition and cure temperature. Exclusion of d-NB from
the region immediately adjacent to the substrate surface is
also observed, possibly indicating a tightly bound layer of
epoxy. Regarding swelling in the bulk of the films, the
behavior is nonsymmetric with departure from the
stoichiometric ratio. The films deficient in curing agent
show greater equilibrium swelling and faster swelling
kinetics than the films with an excess of curing agent.
Kinetics of interdiffusion of a miscible polymer pair, high
density polyethylene (HDPE) and linear low density
polyethylene (LLDPE), was studied experimentally and
theoretically in order to characterize the conditions
required to construct micro- and nanoscopic gradient
morphologies from microlayers. Microlayers were taken into
the melt for a period of time, and the compositional
gradient was fixed by crystallization upon quenching. High
specific interfacial area of microlayers offset the low
diffusion mobility of polymeric chains so that the
microlayer in the melt approached composititional
homogeneity on a laboratory time scale. The progress of
interdiffusion was analyzed by thermal behavior, optical,
electron and atomic force microscopy. The analysis revealed
the role of different fractions and heterogeneity in the
chain microstructure, and allowed us to extract the
diffusion coefficients for elementary chains in the
developing melt blend from the net kinetics of
interdiffusion. It was found that the interlayer boundaries
remained stationary during a characteristic time of
interdiffusion of the component main fractions, and moved at
long times as high molecular weight fractions became
involved in interdiffusion. Interdiffusion of polyethylene
pairs in microlayers was exploited to increase the
concentration of inorganic particles in one of the
components. When microlayers of LLDPE and low density
polyethylene (LDPE) were taken into the melt, significantly
greater mobility of linear LLDPE chains compared to branched
LDPE chains caused a significant shrinkage of LLDPE layers.
Adding a particulate in the LLDPE did not impede the process
of interdiffusion in the melt, and the resultant shrinkage
served to increase the particle concentration.
Epitaxial crystallization of a semicrystalline block
copolymer onto a crystalline organic substrate guides
microphase pattern formation. We used a semicrystalline
triblock copolymer composed of a crystallizable polyethylene
(PE) block, a glassy polystyrene block (PS) and a rubbery
mid-block of alternating ethylene and propylene units (PEP).
The surface interaction between the crystallizable PE block
and the crystalline organic substrate induces a well ordered
vertical lamellar structure of the crystalline block, within
which polymer chains are epitaxially oriented, as evidenced
by bright field imaging as well as selected area electron
diffraction. The oriented pattern of the crystalline PE
induced by the crystallographic epitaxy with the anthracene
substrate simultaneously creates a layering of the PS
component. Such control will be useful for the patterning of
the block copolymers from the Angstrom to the micron scale.
Experiments show that ultra thin free-standing polymeric
films have a lower glass transition temperature (Tg) than
that of the bulk, but a higher Tg when supported on strongly
attractive substrates. Our Tg values obtained by
ellipsometry and local probe calorimetry are fully
consistent. Molecular simulations of such films indicate the
existence of two regions in the films: a low mobility region
near strongly attractive substrates, and a high mobility
region near a free surface. High mobility induces a lower Tg
while low mobility tends to increase it. To understand the
mechanisms behind these changes, self-diffusion coefficients
were determined in bulk, supported and free-standing films.
Tg changes could have serious implications for
nanolithography. Simulations of structure deformation
explored the changes that can be expected in the mechanical
properties of nano scale structures. The results indicate
that the Young's modulus of a polymeric material undergoes
tremendous changes in the sub-100 nanometer range and
feature's aspect ratio, temperature, and substrate's
characteristics influence its elastic behavior.
Chemically heterogeneous surfaces patterned with length
scales from 10s of microns to 10s of nanometers were used as
substrates for thin films of symmetric
poly(styrene-b-methylmethacrylate). Film morphology was
described by alternating regions of symmetric/asymmetric
wetting or symmetric/neutral wetting, and patterns formed in
the films due to differences in thickness between different
wetting regions. Evidence of mass transport of polymer
molecules on the surface of the films was observed. Pattern
features coarsened with annealing time, film surfaces were
smooth over one region even when the film thickness did not
equal the quantized thickness for the type of wetting
present in that region, and perpendicular lamellae were
present at the free surface on patterned films over regions
with neutral wetting. Some of the results were analyzed
using a combination of Monte Carlo simulations and theory.
Recently the issue of physical property deviations in
ultra-thin polymer films has received considerable attention
in the literature. However, the majority of these works
primarily focus on PS and PMMA, two polymers with very high
entanglement molecular weights. In this work, we extend the
thin film studies to polycarbonate, a polymer with one of
the lowest entanglement molecular weights. Polycarbonate
films of variable thickness are spun coat onto Si wafers and
the thermal expansion is observed with specular X-ray
reflectivity. For films supported on a wafer with the native
oxide of Si, the expansion behavior is very bulk-like for
thicknesses greater than 100 ÅFor films thinner than
100 Å\/ (the rms end-to-end length is ~ 160 Åthe
apparent glass transition appears to be depressed. In these
same sub-100 Å\/ films, a negative coefficient of
thermal expansion is observed for temperatures below about
80 ^\circC. Curiously, this negative coefficient of
thermal expansion is no longer observed when the sub-100
Å\/ films are supported on a hydrogen passivated Si
wafer, suggesting that substrate interactions are important.
The origins of these phenomena are to be discussed in
greater detail.
Atomistic molecular dynamics simulations have been used to
delineate the effect of introducing polyhedral oligomeric
silsesquioxane (POSS) moieties substituted by cyclopentyl
(C5POSS) and cyclohexyl (C6POSS) rings as pendant groups on
polynorbornene. Simulations were also performed on
polynorbornene for comparison. Calculated volume-temperature
behavior and X-ray scattering profiles matched well with
experimental results. Most importantly, the effects of
incorporating the POSS moieties into the polymer have been
identified via simulations. These were judged on the basis
of the increase in the glass transition temperature,
retardation of the chain dynamics and improvements in the
calculated elastic tensile, bulk and shear moduli of the
POSS containing polymers compared to the norbornene
homopolymer. The most important conclusion from the study is
that aggregation of the POSS moieties is not required for
the beneficial effects to be realized. Indeed, the
simulations show that there is no tendency for aggregation
to occur among the POSS moieties if they are well dispersed
to begin with. Analysis using various intermolecular
site-site radial distribution functions show the packing of
the polymer chains around the C5POSS is more efficient
compared to that in C6POSS and is attributed to the ungainly
nature of the C6POSS moiety. In addition, the mean squared
displacement of the POSS moieties in the polymer matrix was
found to be very small at all temperatures leading to a
slowing of the segmental dynamics of the backbone polymer
chain, and thereby imparting the macroscopically observed
stiffness. It is reasoned that the chief source of
reinforcement arises from the POSS moieties behaving as
strong anchor points in the polymeric matrix. This has more
to do with the ponderous nature of these moieties versus any
specific intermolecular interactions.
A new synthetic method has been developed to improve the
low-potential polymerization of thiophene. In order to
suppress the side reactions caused by the presence of water
during electropolymerization, 2,6-di-tert-butylpyridine
(DTBP) was added to the BF3-ethyl ether-thiophene(BFEE)
solution. The addition of DTBP leads to an increase of the
conjugation length and improves the packing of the
polythiophene. Highly conductive polythiophene films can be
electrochemically synthesized in the presence of BFEE and
DTBP. The electrical conductivity of the polythiophene is
approximately 1300 S/cm, which is one order of magnitude
larger than that of the polythiophene film without DTBP. The
new Lewis acid/DTBP electrochemical system also has been
demonstrated to work well on other heterocyclic materials.
Successful low-potential electro-polymerization of furan has
been achieved with the oxidation potential lowered by 1100
mV using the DTBP/BEFF system.
The environmental durabilities of several commercial bridge
coatings are being investigated by exposing samples to
accelerated UV irradiation. Primary microscopic techniques
include positron annihilation spectroscopy (PAS), which
detects and characterizes nanometer-scale physical
holes/defects, and electron spin resonance (ESR)
spectroscopy, which detects broken chemical bonds. For the
PAS tests, significant decreases of sub-nanometer defect
parameters are observed as a function of exposure time and
of depth from the surface to the bulk. This is interpreted
as a result of a loss of free-volume and holes fraction and
an increase in cross-link density of the polymers during the
degradation process. For the ESR tests, direct free radical
signals are observed as a function of irradiation time and
chemical environments. A high sensitivity of PAS and ESR
tests to the early stage of degradation is observed.
*Supported by National Science Foundation (CMS-9812717) and
by the Air Force Office of Scientific Research under
Contract Numbers F49620-97-1-0162 and F49620-98-0309
The improvement in oxygen barrier properties of poly
(ethylene terephthalate) through various ways of physical
modification was examined experimentally. Orientation and
heat setting, crystallization from the glassy state at
different temperatures as well as incorporation of platelet
particles by microlayer coextrusion were considered. A model
for correlating the permeability, diffusion and solubility
coefficients with morphological changes was proposed. The
model operated with the fraction of an impermeable phase
that developed as a result of the material modification (an
oriented amorphous phase, crystallites or oriented
platelets) and a permeable amorphous phase of variable
density. It was shown that crystallization frustrated the
molecular packing in the amorphous phase, the densities of
amorphous and crystalline phases being found in good
agreement with previous X-ray results. The oxygen transport
data were amendable to interpretation in terms of
free-volume theory. The transformation from gauche to trans
conformers upon orientation was characterized, and the
density of the oriented impermeable phase was obtained.
Director configurations of nematic liquid crystalline
molecules packed in ellipsoidal domains have been
investigated using mesoscale modeling techniques.
Interactions between the directors were described by the
Lebwohl-Lasher potential. Four different ellipsoidal shapes
(sphere, oblate spheroid, prolate spheroid, and ellipsoid)
were studied under homogeneous and homeotropic surface
anchoring conditions. The model has been characterized by
computing thermodynamic and structural properties as a
function of ellipsoidal shape (prolate and oblate) and size.
The predicted director configuration in ellipsoids resulting
from homeotropic surface anchoring is found to be very
different compared to that in spherical domains. The bipolar
configuration involving homogeneous surface anchoring is
nearly identical in the four cases. The effect of an
external electric field applied at different orientations
with respect to the major axis of the ellipsoid has been
probed as a function of the magnitude of the field and
ellipsoidal size and shape. The orientation of directors is
most easily accomplished parallel and perpendicular to the
major axis for the oblate and prolate spheroids respectively
for homeotropic anchoring and along the bipolar symmetry
axis for homogeneous anchoring. In domains with homeotropic
surface anchoring, the oblate spheroid and elongated
ellipsoid, and for homogeneous anchoring conditions, the
prolate spheroid and elongated ellipsoid are predicted to be
the most efficient geometries for PDLC applications.
Amorphous molecular glasses are used in a variety of
electronic devices. Higher temperature performance requires
the design of higher Tg glasses. QSPR techniques have been
applied to calculate the Tg's of a series of triaryl amines
that may be used in electroluminescent displays. The
molecular descriptors described in Cerius2, (Molecular
Simulations Inc.) were used with a Genetic Function Analysis
to fit the experimental Tg data. For 31 molecules, Tg can be
fitted within +_4C (std. error) and a max. deviation of 11 C
with 5 molecular descriptors. Comparisons of these data with
related QSAR calculations are discussed.
The outgassing behavior of several commercial photoresists
upon exposure to lithographic doses of EUV was studied. The
amount of outgassing was determined by using a quartz
crystal microbalance (QCM) and the chemical composition of
the outgassed species was determined by using a quadrupole
mass spectrometer (MS). The MS results were also used to
obtain outgassing fluxes to compare with the QCM results.
Solvent evolution studies at various PAB times and
temperatures were also performed. Resists evaluated included
four phenolic and one novolac based. Three of the phenolic
resists used a chemically amplified deprotection reaction
mechanism and the other two used a chemically amplified
crosslinking mechanism. Most of the outgassed species that
were identified were deprotection reaction products and
photoacid generator (PAG) decomposition products.
In situ quenching studies
of poly(di-n-octylsilane) by UV-Vis absorption identify the
presence
of at least two competing low temperature Si-backbone conformations with
distinct UV-Vis absorption peaks centered at 354 and 370 nm. The actual
chain
conformation adopted is found to be extremely sensitive to the
preexisting
alkyl side chain packing (which itself depends on the thermal history).
Thermal quenching from the thermotropic mesophase (at 70 ^\circC) to
temperatures well below 0~^\circC suppresses side chain
crystallization and
this stabilizes the more planar form (at 370 nm). More modest thermal
treatments allow for additional side chain packings and new crystal
phases.
Cooling to temperatures near 0~^\circC yields samples dominated by the
354
nm conformational form while cooling to temperatures between 5 and
10~^\circC
gives a UV-Vis absorption with peaks centered at 322, 360 and 374 nm.
X-ray
diffraction and Raman scattering are used to further characterize these
structural phases.
The structures of several poly(silylenemethylene)s have been
studied using X-ray, electron diffraction and molecular
modeling methods. For most of the
poly(di-n-alkylsilylenemethylene)s (alkyl : ethyl to hexyl),
the backbone has an all-gauche conformation giving a 4_1
helical structure and similar packing with a two dimensional
monoclinic ab-projection. Poly(di-n-ethylsilylenemethylene)
(PDESM), however has a pseudo-orthorhombic unit cell
(\gamma*=~92^o) containing two chains (four repeat
units), one at the corner and the second nearly in the
center of ab-projection, indicating that the length of the
alkyl side chains affects the packing without changing the
backbone conformation. The structure of two side chain
liquid crystalline polysilylenemethylenes,
-(SiCH_3R-CH_2)_n- : R =
O(CH_2)_8O-Ph-Ph (PSM-8) and R =
O(CH_2)_11O-Ph-Ph (PSM-11) also have been studied by
X-ray diffraction methods. PSM-8 has both S_B and
S_A liquid crystalline phases with a layer thickness of
26ÅPSM-11 has S_B and S_A phases as well as
two additional crystalline structures (K_0 and K_1)
with a layer thickness of 30 Å. The crystalline structure
has three dimensionally ordered mesogenic groups tilted ~
26^o from the fiber axis.
Two routes to lower permeability of gases in polyether-based
poly(urethane urea) block copolymers are being explored. In
the first, several series of relatively high molecular
weight poly(urethane urea) comb polymers with
polyisobutylene were synthesized. Characterization of the
structure of these amphiphilic copolymers was conducted
using solid state 13C NMR among other methods. Atomic force
microscopy was used to evaluate the microphase-separated
morphology. Permeability of the copolymers to water vapor
and simple gases was measured and the values compared to
predictions from theoretical models. In the second portion
of the study, nanocomposites of poly(urethane urea) and
modified montmorillonite were prepared. A significant change
in the gallery spacing of the composites was noted from
x-ray diffraction experiments and significant reinforcement
of the mechanical properties was obtained at low clay
loadings. Permeability measurements on these materials will
also be discussed.
A series of well-defined poly(urethane urea) multiblock
copolymers was synthesized from MDI, ethylene diamine and
2000 MW poly(tetramethylene oxide), with hard segment
contents between 14 and 47 wt. percent. Atomic force
microscopy and small-angle x-ray scattering were the primary
techniques used to investigate the microphase-separated
structure. Surfaces of solution cast films and
freeze-fractured surfaces were analyzed using a Digital
Instruments MultiMode SPM in the tapping mode. Both
topography and phase images were recorded. The copolymers
have a uniform, single-phase outer layer (soft phase), with
the hard domains primarily located beneath this overlayer.
For all copolymers, AFM images show that the hard domains
are in the form of randomly oriented cylinders. A structural
model for these copolymers is presented.
A series of random poly(lactide) copolymers was synthesized
from L-lactide and D,L lactide using a tin (II) octanoate
catalyst. The copolymers contained from 1.7 to 6.2 percent R
stereochemical defects in otherwise S stereoisomer chains.
Small-angle x-ray scattering experiments were used to
determine the final lamellar microstructure of the
copolymers. Lamellar thicknesses were strongly dependent on
R co-unit concentration and it was concluded that the final
structure contains significant pockets of non-crystalline
material between spherulite fibrils. Equilibrium melting
temperatures were estimated using the Gibbs-Thomson approach
and their variation with comonomer content suggests that
there is significant exclusion of R stereochemical defects
from crystalline regions. Spherulite growth rates of the
D-lactide copolymers were found to be larger than
L-lactide/meso-lactide copolymers at equivalent optical
composition.
We have been examining the microstructure and macroscopic
properties of specific grain boundary and dislocation
defects in optoelectronically active, conjugated crystalline
polymers such as polydiacetylenes and polyalkylbithiazoles.
Here, we discuss molecular models of dislocation structures
in these materials, and compare our results with direct
experimental images of dislocations obtained by low dose
High Resolution Electron Microscopy (HREM). The molecular
models of the defects were constructed by generating a
superlattice containing two neighboring arrays of polymer
chains with different lengths (N, N-1). The simulations were
examined in the limit of N -> infinity, corresponding to an
effective defect density 1/N -> 0. Estimates of the
geometrical distortions associated with the defect and the
contributions of the different molecular mechanisms leading
to the increase in energy were obtained. The models
correspond to edge dislocations with Burger's vector b=[001]
and line directions [100] and [010]. For poly(methyl
bithiazole), it was found that after energy minimization the
dislocation cores tended to dissociate into two partials
separated by an extended stacking fault between neighboring
chains.
A common goal between many scientists is to determine the
structure of Nafion®. Through this structural determination,
a better understanding of the properties (mechanical,
thermal, transport) of ionomers might be achieved, along
with the possibility of controlling the properties. In the
pursuit of this goal, we have utilized neutron scattering
techniques to gain information on the polymer morphology.
All membranes were initialized to a standard state before
using the membranes in experiments or exchanging to a
different counterion. Prompt g activation analysis has been
utilized to evaluate the counterion on the sulfuric acid
group; i.e. evaluate the extent of neutralization, and the
amount of water trapped within the membrane. Samples of N117
were dried and some of these samples were then swollen to
equilibrium in a variety of solvents: water, ethanol, DMF,
DMMP. Other samples of N117 were swollen with water or
ethanol with an increasing volume fraction of solvent. Small
angle neutron scattering experiments were then conducted to
evaluate the structure of the polymer in the dry and solvent
swollen states. Three distinct 'q' features can be noted
from the SANS plots. For the experiments investigating the
effect of increasing volume fraction of water or ethanol
within the membrane, the peak at the highest 'q' value can
be observed to shift to lower 'q' values while the other two
features remain the same. A linear, monotonically increasing
relationship exists for d-spacing of this 'q' value versus
increasing volume fraction solvent plots and is believed to
be indicative of the changing structure of the cluster.
SAXS is a primary tool for assessing the morphology of
semicrystalline polymers. The most common model is a stack
of alternating lamellar crystals and amorphous regions;
spherulitic polymers are represented by an isotropic array
of such stacks. Assuming that lamellae are flat and
infinitely wide, the structure is effectively
one-dimensional, and the (Lorentz corrected) intensity
I_1(s), the correlation function \gamma_1(r), and
the interface distribution function g_1(r) each provide
structural information. I_1(s) gives a measure of the
periodicity within a stack, while \gamma_1(r) and
g_1(r) permit ready estimation of both periodicity and
local crystallinity. Intensity I_1(s) is calculated for
stacks with symmetric and skewed distributions of repeat
periods, various crystallinities, and for different numbers
of lamellae per stack. I_1(s) is used to generate
\gamma_1(r) and g_1(r), as in an experiment. For all
cases the three functions return periodicity in the order
L_I \geq L_\gamma \geq L_g, where the subscript
designates the function. Apparent crystallinity is in the
order \alpha_\gamma \leq \alpha_g. These inequalities
are interpreted in terms of the characteristics of the
disordered lattice. Preliminary calculations are presented
for the case when stacks have a finite width. It is
concluded that the "infinitely wide" assumption is valid for
cylindrical stack diameter D greater than approximately 6
times the average long period L.
Dendritic polymers are macromolecular compounds that contain
a branching point in each structural repeating unit.
Recently liquid crystalline dendritic polymers have proven
to be an interesting new family of mesogenic compounds.
These liquid crystalline dendrimers can exhibit nematic (N),
smectic A (SA) or Smectic C (SC), and hexagonal columnar
phases. In addition, several monodendrons can self-assemble
into cylindrical and spherical supramolecular structures
which produce columnar and cubic mesophases. A series of
side-chain dendritic polynorborenes has been synthesized by
living ring-opening metathesis polymerization. The side
groups are second generation G2(OH) monodendron which are
based on the comformationally flexible AB2 mesogenic
monomer,
13-hydroxy-1-(4-hydroxyphenyl)-2-(4-hydroxy-4”-p-terphenylyl)
tridecane building block. These polymers exhibit multiple
phase transitions during cooling and heating in DSC
experiments. Four different phases are identified in
addition to the isotropic melt using wide angle X-ray
diffraction and small angle X-ray scattering synchrotron
experiments and polarized light microscopy. These phases
include a novel supermolecular SB phase, two SA phases and a
nematic phase. One of the SA phases is metastable and can be
separated by annealing the samples.
Many transition behaviors such as crystallization and glass
transition of polymer thin films are dependent of the
thickness, which has been a very active research topic for
the past several decades. Methods to obtain uniform thin
films and detection of the physical transition processes are
the keys to studying phase transitions of the thin film. We
have chosen a series of poly(ethylene oxide)-b-polystyrene
(PEO-b-PS) to construct a uniform semi-crystalline PEO film
sandwiched by amorphous PS films, whose thickness can be
controlled by the diblock copolymer composition and
crystallization conditions by growing single crystals from
dilute copolymer solution. We employ lamellae single
crystals as an ideal model of a thin film to study the
following problems: 1.) How thin can the amorphous PS film
be and still confine the PEO crystallization and melting,
and 2.) what are the crystallization and melting behaviors
of the PEO film in a nano meter scale confined geometry,
particularly how do the morphology and chain folding change.
As part of a program to study the structure, packing and
miscibility characteristics in polyolefins and their
mixtures, we present new X-ray and neutron diffraction data
sets on linear polyethylene melts. The X-ray structure
factor is compared to new self-consistent PRISM calculations
using a realistic united atom model. Improved agreement with
experiment is obtained by modifying PRISM to yield the
correct compressibility. Self-consistent PRISM calculations
were also carried out on an explicit atom model of
polyethylene to predict the intermolecular pair correlation
function between H atoms. The results were in good agreement
with the H-H intermolecular radial distribution function
obtained from neutron scattering difference experiments on
isotopically substituted polyethylene.
Recent research utilizing solid-state shear pulverization
(SSSP) has revealed its ability to effectively compatibilize
particular immiscible polymer blends and to efficiently mix
polymers with highly unmatched viscosities. New examinations
involving SSSP include thermal analysis and electron
microscopy of a 92/8 (wt. blend; two PE recrystallization peaks are noted, signifying
the occurrence of both heterogeneous and homogeneous
nucleation, the latter associated with blends containing one
component as very finely dispersed droplets. Corroboration
has been provided by SEM in which PE dispersed-phase
diameters as small as 200-300 nm are seen; the dispersed
particles are significantly smaller then those achieved via
melt processing. The ability of SSSP to achieve both fine
dispersion and compatibilization can lead to enhanced
mechanical properties relative to blends produced via melt
processing. Such studies are underway in other systems
including polypropylene/poly(butylene) terephthalate blends.
Single crystals of a series of metallocence catalyzed
ethylene-butene copolymers with similar branch content but
different molecular weight or similar molecular weight but
different branch content were prepared using p-xylene,
octane and dodecane as solvents at different temperatures.
The morphology and structure of crystals were characterized
using transmission electron microscopy , atomic force
microscopy and electron diffraction. The solvent effect,
crystallization temperature, molecular weight and branch
content dependence of the aspect ratio (the length ratio of
the b to a axes in the single crystals) and thickness of the
crystals are studied. The aspect ratio increases with
increasing crystallization temperature for each solvent.
However, the aspect ratio in p-xylene is smaller than those
in octane and dodecane. For the same solvent and temperature
both the aspect ratio and thickness increase with increasing
molecular weight and decreasing branch content.
Sectorization behaviors are studied by polyethylene
decoration and AFM.
Characterization of banded structures obtained from liquid
crystals to confined crystallization has been investigated
using optical and atomic force microscopy. A combined
main-chain/side-chain liquid crystalline (LC) polyester,
PEFBP(n = 11), was focused on to illustrate the formation of
interesting additional bands on crystallization. On
macroscopic scales, both approaches can be applied to
visualize the morphological changes in these banded
textures. In tapping mode AFM, the 3-D topographic banding
images are detected in the height image from Ĕ3.5 \mum LC
bands to Ĕ50 nm confined crystal bands. The sub-surface
fine features of edge-on crystal (KO) lamellae are
differentiated in an extended chain conformation in the
phase image. The formation of complicated banded structures
from liquid crystals to confined crystallization is
recognized as a new element: self-elongated 1-D crystal
growth in the confined LC matrix on crystallization.
We successfully obtain in-situ AFM images during the growth
and melting of sPP single crystals from melt. On a heating
stage, single crystals are melted first, then the samples
are cooled down to a preset crystallization temperature to
study crystal’s growth. From the beginning of the growth of
these single crystals, sectorization can clearly be seen.
There are two kinds of sectors, with different thicknesses
in each crystal. Upon heating, with careful control of the
temperature, the thicker sectors melt first on the edge,
showing notches on the edge of the domain. After kept on the
same temperature, the partially melted thicker sectors
recrystallize and perfect themselves to reserve the shape of
the sector. The further increasing of temperature leads to
the melt of the thinner sectors, then the melt of thicker
sectors follow. This indicates two melting temperatures in
one single crystal. The melting and growth of the single
crystals can be repeated with this thermal cycle. The
surface topography and chain folding of sPP lamellar
crystals are also studied by lateral force microscopy.
A selective solvent and annealing study was done to
investigate the morphology behavior of an I2S miktoarm star
block copolymer and its blends with homopolyisoprene.
Casting from cyclohexane, a selective solvent for
polyisoprene, the neat star shaped I2S block copolymer only
partially microphase separated, and formed a unique layered
morphology inside a homogeneous media. During annealing, the
layered phase and the homogeneous phase both transformed
into a randomly oriented worm morphology. The path of this
transformation is different depending on whether the
starting state is layered or homogenous. The
I2S/homopolyisoprene blend formed a mostly homogeneous phase
after casting from cyclohexane. Annealing produced slow
microphase separation which was observed at various stages
by TEM. Based on these observations, a sporadic nucleation
process of microphase separation is proposed.
We have studied the morpholgy in thin films of various
diblock and triblock copolymers based on isoprene and
(penta-methyl-disilyl-styrene) by Atomic Force Microscopy
and Transmission Electron Microscopy. The structure of the
films obtained by spin coating and by casting onto water of
dilute solutions in toluene is shown to depend strongly on
the wetting properties of the block copolymer on the
considered substrate. The microstructure of the films and
the degree of order achieved (extension of domains, defect
concentration) depend on the evaporation rate of the
solvent. Dewetting of the polymer films leads to the
appearance of holes and disrupts microstructure. The
microstructured copolymer films are accordingly used for the
patterning of substrates such as silicon wafers or
conducting ITO glass. Nanoporous ceramic films with
periodicities of several tens of nanometers are obtained by
combining ozone and UV light treatments of the block
copolymer film. Subsequent Reactive Ion Etching (RIE) can be
used to pattern the substrate.
Moisture causes deterioration on the performance of polymer
composites; however, the molecular mechanisms of moisture
transport and damage remain unclear. We have studied a
poly(bismaleimide) (BMI) carbon fiber composite system. The
weight gain behavior of BMI and its composites deviate
significantly from Fick's law in that no equilibrium uptake
is observed. A modified Fickian diffusion model which can
successfully fit the experimental data is proposed. The
equilibrium uptake factor in Fickian diffusion is modified
to be time dependent. The success of this modification
suggests that the structure of BMI changes during
absorption. Desorption and re-absorption experiments confirm
this speculation. Positronium annihilation results suggest
network relaxation is likely the reason for the structural
change. Moisture uptake of composites is described well by
the same model. Interestingly, the normalized weight gain of
a woven composite is significantly lower than that of neat
BMI resin, suggesting constraining effects by the
fiber-matrix interface. This work is supported by AFOSR
Grant F-49620-98-1-0377.
Simultaneous time-resolved synchrotron small-angle x-ray
scattering (SAXS) and wide-angle x-ray diffraction (WAXD)
were used to follow molecular and lamellae orientation in
the melt of polyethylene near its melting temperature
(T_m as obtained from the DSC peak). A Linkam shearing
stage, designed for use in in-situ x-ray scattering
experiments, was used for application of high shear fields
to the polymer melt. The melt was subjected to a high shear
strain (1400speculate about the development of a shish-kebab structure
having flow induced extended chains of higher molecular
mass, intercalated by chain folded lamellae perpendicularly
oriented to the flow direction. The polymer chains in the
row-like lamellae are also oriented along the flow
direction, the lamellar growth direction being perpendicular
to the flow direction.
This work is supported in part by a NSF grant (DMR 9732653)
and in part by an US-Spain Sci.and Tech.Grant.
The effect of reaction rate on the morphology of reactive
blends has been studied using 75/25 (wt/wt)
mono-carboxylated polystyrene [PS-mCOOH]/poly(methyl
methacrylate) [PMMA] with poly(methyl
methacrylate-ran-glycidyl methacrylate) [PMMA-GMA] as an
in-situ compatibilizer. We changed the amount of PMMA-GMA in
the blend, the molar concentration of GMA, C_GMA,0 in
PMMA-GMA at fixed molecular weight, and the molecular weight
of PMMA-GMA at fixed C_GMA,0. For the blends with
PMMA-GMA having lower C_GMA,0, there exists a critical
amount of PMMA-GMA above which a sharp decrease in the
volume average domain size (D_v) occurs. this amount was
shifted to a smaller value with increasing C_GMA,0 in
PMMA-GMA. We demonstrated that the interfacial graft
reaction between PS-mCOOH and PMMA-GMA at 220 ^oC was
described by the simple second-order reaction kinetics,
i.e., mean field reaction kinetics.
The simulated annealing approach has been adopted to refine
WAXD patterns generated from semicrystalline polymers
fibers. The intensity in reciprocal space is predicted
first, subject to a coordinate transform to map to the film
space. Then a simulated annealing algorithm is applied to
optimize the structure parameters to match the simulated and
experimentally recorded patterns. Other pertinent algorithms
which facilite the 2-D scattering data preparation will be
presented. A sample calculation on data from
poly(trimethylene terephthalate) fibers will be presented.
The macromorphology of isotactic/atactic (iPP/aPP) and
isotactic/syndiotactic (iPP/sPP) polypropylene mixtures is
examined by optical microscopy. At high undercooling the
spherulitic macrostructure of equimolecular weight (Mw=200k)
iPP/aPP blends is volume filling to high aPP concentration
and similar to the iPP homopolymer. The rapid rate of iPP
crystallization obscures the underlying phase behavior of
50/50 (Mw=200k) iPP/aPP blends. At high crystallization
temperature (140-145 C) equimolecular weight (Mw=200k) 50/50
iPP/aPP blends exhibit nodular texture due to melt phase
separation which develops prior to crystallization. The
upper critical solution temperature (UCST) lies below 155 C,
and the blend is miscible at conventional melt processing
temperatures. The UCST behavior is controlled in a
systematic manner by blend molecular weight and aPP
tacticity microstructure. The iPP/sPP pair shows much
stronger tendency for phase separation than the iPP/aPP pair
and is immiscible (Mw=200k) at melt processing temperatures.
The size scale of iPP/sPP phase separated texture is
controlled by the competition between crystallization and
phase separation.
Crystal orientation and phase morphology determined by
self-organization, vitrification, and crystallization have
been investigated in a lamellar forming poly(ethylene
oxide)-b-polystyrene diblock copolymer. The glass transition
temperature of the PS block and melting temperature of the
PEO block are 62°C and 51°C, determined by differential
scanning calorimetry. The order-disorder transition
temperature is observed at 160°C using one-dimensional small
angle X-ray scattering (SAXS) experiments. Therefore, the
PEO blocks basically crystallize in a confined lamellar
space formed by PS glassy layers. The crystal orientation
determined by both two-dimensional SAXS and wide angle X-ray
diffraction shows transitions from perpendicular to the
lamellar surface normal to inclined, and finally parallel to
the LSN, solely depending upon an increase of
crystallization temperature.
A novel organometallic homopolymer,
poly(ferrocenyldimethylsilane) (PFS), when formed into
nanofibers using the electrospinning process, is found to
exhibit a strong diffraction spot that is not observed in
the x-ray powder pattern of unoriented samples. This
structure has been investigated using electron diffraction
(ED) from a single PFS nanofiber. The molecules are oriented
along the fiber axis, as indicated by the optical
birefringence. ED patterns indicate an orthorhombic crystal
structure, different from that in the powder, with the
crystals well ordered along the c axis, but randomly
oriented in the azimuthal direction in the ab plane. PFS has
also been incorporated in a well-defined diblock copolymer
with poly(isoprene) using anionic polymerization. The
crystallization and microphase behavior of the diblock
copolymer has been studied by using transmission electron
microscopy, small-angle and wide-angle x-ray scattering, and
dynamic mechanical measurements.
Wide angle neutron scattering data from a melt of deuterated
poly(ethylene oxide) at 363K and X-ray data from the small
molecule analogs glyme (CH3OCH2CH2OCH3) and diglyme
((CH3OCH2CH2)2O) are presented and compared with results of
molecular dynamics simulations. An explicit atom quantum
chemistry based force field was used in MD simulations of
glyme, diglyme, and 12 and 54 repeat unit PEO melts. We
found good agreement between the experimental structure
factors and those from MD simulations. The structure factor
dependence on PEO molecular weight was obtained from MD
simulations. It exhibited a slight molecular weight
dependence from glyme to diglyme and 12 repeat unit PEO, but
remained essentially unchanged upon further increase of
polymer molecular weight.
Solid-state shear pulverization (SSSP) is a continuous,
mechanical alloying process employing simultaneous effects
of high pressure and shear deformation to pulverize and mix
polymers. Under certain conditions SSSP can result in
limited chain scission and polymeric radical formation. In
immiscible blends, these radicals may be able to recombine
in interfacial regions or regions of high mixing resulting
in block copolymer formation and compatibilization. The
effects of SSSP on amorphous polyamide (PA)/polystyrene (PS)
and PS/low density polyethylene (LDPE) blends have been
studied. As compared to melt-mixed blends, SSSP yields
blends with enhanced blend morphology refinement or
dispersion, and in certain cases enhanced bulk mechanical
properties, particularly elongation at break and impact
strength. Comparisons of dispersed-phase coarsening during
high temperature, liquid-state annealing of the SSSP -
processed and conventionally melt-mixed blends will be
discussed in terms of the potential for achieving effective
compatibilization of particular blends via SSSP.
In this work, a molecular dynamics study of polystyrene
using explicit atom model is presented. Simulations were
performed on atactic polystyrene (16 chains) employing the
force field obtained from our previous simulations on small
chain polystyrene. The x-ray scattering intensities obtained
from radial distribution functions were compared to the
experiment. Simulations reproduce the so-called
"polymerization peak" in the scattering curves. The
intensity of the peak is known to increase with increase in
temperature unlike any other scattering peaks. This
anomalous behavior is also observed from simulations. The
relative intensities of the peak over different temperature
range are in good agreement with the experiment. It is
evident that phenyl-phenyl correlations play an important
role in determining the structure of polystyrene. Extensive
analysis of our simulations reveals that 1-4 interphenyl
interactions contribute to the appearance of "polymerization
peak". The temperature dependence of the peak was
demonstrated by comparing the conformations responsible over
different temperatures.
We have used small angle x-ray scattering (SAXS) and neutron
scattering (SANS) to study the micelle structure of a
polystyrene(PS)-block-poly(hydrogenated
butadiene)(PHB)-block-polystyrene triblock copolymer
in dilute - semidilute solutions in solvents selective for
either the outer PS block (dioxane) or for the middle PHB
block (heptane or decane). Equilibrium structure factors
were measured over the temperature range of 20 - 90 C for
different concentrations of the copolymer, ranging from 4-15solvents. The structure factors were fit to interacting hard
sphere Percus-Yevick model as well as the core-shell model
of micelles. In a time resolved SAXS experiment we were able
to follow the growth of micelles following a temperature
quench in a 4% sample in heptane. The core radius, the hard
sphere interaction radius and the volume fraction of hard
spheres increase with time following an exponential
relaxation. At higher concentrations we were able to observe
the kinetics of ordering. The results agreed with the
Mehl-Johnson-Avrami nucleation theory.
The two-phase morphology of polymerization-induced liquid
crystal/polymer structures are examined using polarized
optical microscopy, high resolution scanning electron
microscopy, and static light scattering techniques.
Specifically, the difference between a highly functional
free-radical monomer and highly functional step-growth
monomer is examined. The free-radical monomer system gels
before the appearance of a liquid crystalline phase and the
polymer morphology consists of small polymer beads which
have aggregated together to form a macroscopic network. The
step growth monomer system, cured at the same composition
and intensity, exhibits liquid crystalline behavior before
gelation while the polymer morphology exhibits a continuous,
dense phase. The size scale of the microstructure probed by
light scattering techniques is an order of magnitude smaller
in the latter case compared to the former case.
Immiscible polymer blends with a cocontinuous microstructure
have a variety of potential applications. These blends can
have unique combinations of mechanical properties allowing
for inexpensive property improvement over homopolymers. They
have also found use in specialty applications such as static
dissipation in electrical housings. Cocontinuity in polymer
blends is difficult to confirm experimentally. It is often
detected using electron microscopy with a variety of image
analysis techniques, solvent extraction, rheology, or a
combination of these methods. In this study, phase
continuity in polystyrene/polyethylene oxide blends was
examined using AC conductivity measurements at 70^oC.
Lithium perchlorate was added to the PEO phase in low
concentrations (<1%) to enhance its conductivity. Solvent
extraction and electron microscopy were used to confirm the
results. A future goal is to correlate the structure with
rheological measurements.
The evolution of the morphology in thin films of
poly(ethylene oxide)-b-poly(butadiene) diblock copolymers
(Mw/Mn=1.04, Mn(PEO)=6000, Mn(PB)=5000) was investigated.
Toluene solutions of the diblock copolymer were spin coated
onto bare silicon wafers. The samples were melted at 80oC
and crystallized at various temperatures and studied by
optical microscopy and AFM. The following was observed. A
direct ordering of lamellar domains parallel to the
substrate was observed just after spin casting. A two
dimensional crystallization within the PEO lamellar layers
was observed retaining the lamellar morphology formed in the
melt state. In addition, a depletion of the film ahead of
the growth front was found. Finally, crystallization also
produced pronounced cracking of the film.
The presence of mechanical anisotropy at multiple length
scales in side group liquid crystalline diblock copolymers
(SG LCBCP's) allows manipulation of the overall morphology
of the material through preferential interaction of external
fields with the structures present in the material. In-situ
SAXS and WAXS are used to follow the evolution of the
morphology of covalently bonded SG LCBCP's under oscillatory
shear, as a function of temperature, shear amplitude and
shear frequency. Suitably chosen SG LCBCP's have shown
promise as bistable electro-optic switches when properly
oriented. Hydrogen bonded analogs of the SG LCBCP's have
been prepared, both from block copolymer and, more
importantly, from simple homopolymer precursors. We report
on the characterization and control of morphology in these
materials through epitaxy and oscillatory shear. The greater
mobility and temperature sensitvity of hydrogen bonded
mesogens affords interesting practical device applications,
such as switchable polarizers.
The mechanical behavior of explosives subject to high acceleration has
been studied in an ultracentrifuge at -10^oC. Melt-cast TNAZ and pressed
TNAZ, LX-14, Composition A3 Type II, PAX-2A, and PAX-3 have been studied.
Failure occurs when the shear or tensile strength of the explosive is
exceeded. The fracture acceleration of pressed TNAZ at -10^oC is about the
same as the fracture acceleration of pressed TNAZ at 25^oC. The fracture
acceleration of the plastic bonded explosives at -10^oC is about two times
greater than the fracture acceleration of the plastic bonded explosives at
25^oC.
The diffusion of organic materials into polymers often causes the polymers
to swell. The diffusion process then becomes non-Fickian, and this
anomalous behavior has been called case-II diffusion. Another aspect of
case-II diffusion is that the polymer may pass from the glassy to the
rubbery state. However, this process is extremely slow and has seemed to be
inaccessible to molecular dynamics techniques. We will show that it
is the partial charges on the organic material that mainly controls the
speed of the diffusion process. By removing the partial charges on the
methanol atoms we are able to study the diffusion of methanol into PMMA
in a reasonable computational time and are able to obtain useful preliminary
information about case-II diffusion.
The prediction of the mechanical properties of polymer
chains has centered so far primarily on the use of empirical
potential energy functions, which are limited by the chosen
classical force-field. A theoretical evaluation of polymer
chain moduli by a molecular orbital approach constitutes a
continuing effort of our laboratories. In this study we
report the prediction of the strain-dependence of the
vibrational frequencies for PPTA as compared to experiment.
A negative slope for strain-induced frequency changes in
PPTA single fibers is calculated, and also observed
experimentally through the use of Raman spectroscopy, of up
to ca. -5 cm-1/yields a value of -13 cm-1/that this slope for PPTA fibers is proportional to Young's
modulus. This may explain in part our overestimation of
these slopes; the fiber is not a single crystal and the
predicted modulus is also overestimated by approximately
1.5.
It is of interest to gain an understanding of the mechanical
behavior of polymer chains at the molecular level in order
to aid in the design of materials with tailored properties.
In this study we examined the geometries and properties of
several series of polymer molecules under strain, e.g.,
polyimide derivatives, using a semi-empirical quantum
mechanical approach. A theoretical value of Young's modulus
was calculated for each polymer, and the mechanical
deformation at the molecular level elucidated. The effects
of substitution on the mechanical performance of a polymer
fiber were evaluated by studying changes in the geometrical
and electronic parameters on the application of strain. This
investigation is a step toward achieving an understanding of
the mechanical behavior of polymer chains and the
development of materials with improved properties.
A method of real-time in situ monitoring of craze
growth in polymers is suggested. The method is based on the
analyses of the light scattering properties of polymer
samples. A possibility of craze detection and
characterization using this method is demonstrated. The
initial assessment of the sensitivity of the method to the
intrinsic absorption and scattering properties of the
material is conducted. Several directions of the future work
are briefly discussed.
We will present applications of a new solid-state NMR
technique, CODEX, for detecting slow (0.1/s to 3000/s)
segmental reorientations with the highest achievable
sensitivity and site resolution, under fast magic-angle
spinning (MAS) conditions. The chemical-shift anisotropy is
recoupled before and after a mixing time by a sequence of
rotor-synchronized 180^o radio-frequency pulses. Segmental
reorientations during the mixing time result in frequency
changes that dephase the magnetization and thus decrease the
mobile-site signal. The correlation function is traced out
by the mixing-time dependence of the intensity, while the
motional amplitude can be estimated from the recoupling-time
dependence. The fraction of mobile groups and the number of
orientational sites is obtained from the long-time
asymptotic intensity, in combination with four-time CODEX. A
detailed CODEX-based characterization of the segmental
dynamics associated with the beta-relaxation in glassy
poly(alkyl methacrylate)s will be presented. We have
quantified the fraction of flipping sidegroups as a function
of the sidegroup size and characterized the slow backbone
motions that are also observed.
We have applied the reptation model in the prediction of the
linear viscoelastic properties (time-dependent shear
modulus) of poly(vinylidene fluoride) (PVDF) melts over wide
range of temperature and molecular weight. All structural
and thermodynamic properties of the PVDF melts needed for
the reptation model predictions, specifically the molecular
weight and temperature dependent density, characteristic
ratio and monomer friction coefficient, were determined from
molecular dynamics simulations of a low-molecular weight
PVDF melts using a quantum chemistry based atomistic force
field.
Accelerated longitudinal shrinkage has been found to occur
in several rigid-rod, polymeric systems at elevated
temperatures. Results show that this shrinkage occurs upon
reaching temperatures at which pendent cleavage, or the
initial stages of carbonization, takes place. Atomistic
simulation and WAXD would suggest that this shrinkage is
initiated by strains occurring at the crystalline lattice
level, resulting from the formation of intermolecular
crosslinks. Various crosslinking reactions have been
discussed. Atomistic simulation results suggest that
shrinkage can be avoided by controlling the type of
crosslink formed in these systems. Thus, attempts to
crosslink dimethyl-pendent poly(p-phenylene
benzobisthiazole) via electron radiation at room
temperature, as well as at elevated temperatures, have been
made. Results of these studies will be presented.
In an attempt to evaluate the effect of intermolecular
hydrogen bonding on compression and transverse properties of
rigid-rod polymeric fibers, methyl pendant poly (p-phenylene
benzobisimidazole) (MePBI) was synthesized and spun into
fibers. The hydrogen bonding in MePBI, as measured by
infrared spectroscopy, is present at temperatures as high as
400 C. The fiber crystal structure has been determined,
using WAXD and molecular simulation, which also shows
significant hydrogen bonding within the system. The
compressive strength of both as-spun and heat-treated
fibers, measured from the recoil test, was found to be 800
MPa. MePBI also shows an increase in torsional modulus and
loop strength, as compared to other methyl-pendant,
rigid-rod systems. The temperature dependence of fiber
tensile, torsional and bending properties will be reported.
A correlation between intermolecular bonding strength, as
measured by ab initio simulations, and compressive strength
will also be presented.
Designing permselective polymers requires better fundamental
understanding and control over the underlying diffusion
processes. To understand how molecular motions influence
diffusion, we have studied a unique series of bisphenol A
polycarbonates modified with cyclohexane linkages in the
backbone. The goal of this research is to determine how
chair-boat-chair conformational transitions of the
cyclohexane units and the coupled motions with neighboring
segments influence permeability and diffusivity. We attempt
to correlate the diffusion behavior to hole fraction and the
magnitude of local segmental motions as well as the type of
penetrant used. We found an increase in permeability when
local molecular motion is enhanced in polymers with the same
hole fraction as determined by positronium annihilation.
This observation apparently contradicts free volume
arguments. Different types of penetrants have also been used
to test the hypothesis that large scale cooperative motions
of polymer segments open up pathways through which small
molecules can diffuse. This work is supported by NSF Grant
DMR 9971569.
Silk is of interest due to its respectable mechanical
properties. These are expected to be influenced by the
crystalline properties such as crystal modulus. X-ray
diffraction with the assumption of uniform stress has been
used for determining the crystal modulus. The values for
silk fibers are reported to be approximately an order of
magnitude lower than those obtained by computational
modeling. This difference is outside the limit of
experimental and computational error and reflects some
conceptual error. We are re-determining the modulus both
experimentally and computationally in an improved manner.
For the X-ray method, this includes making measurements of
fibers with various degrees of crystallinity and
orientation. Extrapolations of the modulus to 100crystallinity and perfect orientation yield 98±10 GPa and
70±14 GPa, respectively. For the computational calculation,
molecular dynamics are being used with periodic boundary
conditions. The frequency of the longitudinal acoustic mode
will be determined and used to calculate the modulus.
Stiffness matrix and stress-strain calculations are also
being made.
Adhesive properties of a model, filled elastomeric adhesive
are measured using an axisymmetic adhesion test with a rigid
glass indenter. Experiments with poly(methyl
methacrylate)-poly(n-butyl acrylate)-poly(methyl
methacrylate) triblock copolymer films are conducted with
two types of complementary experiments to resolve the
surface and bulk contribution to the adhesive behavior. In
the first set of experiments, thermally evaporated gold
particles are deposited onto the triblock copolymer to yield
gold coatings with equivalent thicknesses of 100 nm or less.
The axisymmetric adhesion test is used to measure the force
required to separate the glass indenter from the gold coated
triblock copolymer substrates. The work of adhesion
decreases with increasing thickness of the gold coating.
This result is attributed to a reduction in the true area of
contact between the indenter and the adhesive upon the
addition of the metal particle layer. In the second set of
experiments, effects of rigid particles on the bulk
mechanical properties and adhesive response are probed by
adding spherical aluminum oxide particles to the adhesive.
In both sets of experiments, approaches based on linear
elastic fracture mechanics are used to quantify the adhesive
response.
Amorphous ethylene-styrene interpolymers synthesized by a
metallocene catalyst are viewed as statistical copolymers of
ethylene and ethylene-styrene dyads. These macromolecules
were shown to have a flexibility of polyethylene, which
offers the unusual opportunity to characterize the amorphous
phase of polyethylene without the constraints imposed by
crystallinity. Creep, both linear and non-linear relaxation
as well as recovery were studied in the broad range of
temperatures. The creep and linear relaxation data showed
time-temperature superposition. The creep and relaxation
master curves were analyzed as functions of molecular weight
and comonomer content. Linear relaxation of this
polydisperse system was amendable to description in terms of
both reptation (Doi-Edward's) and double reptation (des
Cloizeaux's) theories if appropriate blending rules were
taken into consideration. However, the double reptation
theory described the data in terms of the relaxation
spectrum better, with fitting parameters consistent with
other experiments. The non-linear relaxation data gave
experimental values of the damping function in excellent
agreement with Doi-Edward's theory. A model of two
co-existing networks for instantaneous recovery after stress
relaxation was developed further in order to incorporate
concepts of reptation theory.
The mechanical and adhesive properties of coalesced latex
blends are of interest due to their applicability in several
areas, including commercial adhesives and latex paints. We
have tested the mechanical and adhesive behaviors of latex
coatings formed by coalescence in water. The coatings are
composed of two latex systems, a softer system with a
particle size of 57nm, and a harder system with a particle
size of 268nm. By varying the proportion of soft particles
(Tg = -30 C) to hard particles (Tg = 20 C) in dispersions,
we are able to show a relationship between the volume
fraction of the hard latices and the effective work of
adhesion. This behavior can be studied through use of an
extension of the JKR method for adhesive testing. In the JKR
method, a rigid hemispherical indenter is brought into
contact with the coating, and we measure the resultant
adhesion . We have also obtained relaxation data for the
latex blends by fitting experimental data to multi-element
Maxwell model. We find that very low volume fractions of the
soft particles significantly influence stress relaxation,
but that larger volume fractions are required to noticeably
affect the adhesive behavior.
A photobleaching method has been used to observe the
segmental dynamics of a polystyrene melt during isothermal
physical aging. Segmental motions were monitored indirectly
through the motion of rigid tetracene molecules dispersed in
the polystyrene matrix. Both rotational and translational
diffusion measurements were performed. Following a
temperature quench to below T_g, either probe rotational
relaxation from an induced anisotropic state, or
translational diffusion across a holographic grating, was
periodically measured as a function of aging time. Rotation
times changed by nearly an order of magnitude during aging,
and down and up temperature jumps showed the expected
asymmetry due to the nonlinearity of the aging process. The
equilibration times and the relative rates of approach
towards equilibrium for the two experiments will be
presented. These equilibration times (10^2-10^5s)
for the temperatures investigated (T_g-3K-T_g-7K)
are the same order of magnitude as equilibration times for
other techniques used to monitor physical aging, such as
volume and enthalpy relaxation.
Neutron scattering from electron beam irradiated and
unirradiated samples of the VDF-TrFE copolymer with a
composition of (50-50) weight % was measured as a function
of temperature in a range encompassing the glass transition
(233 K) and the Curie transition (343 K). The unirradiated
copolymer shows well-defined diffraction peaks for two
lattice spacings (4.60 and 4.73 angstroms at 300 K). These
spacings both increase with temperature up to 330 K where
the peaks go into a single peak with a definite increase in
spacing (4.90 angstroms at 344 K) consistent with a
ferroelectric-to-paraelectric transition. The irradiated
copolymer shows only a single peak from 213 K to 373 K for a
generally larger lattice spacing (4.86 angstroms at 300 K)
with a monotonic increase with temperature matching the
paraelectric dependence. The spacing is increased by
increased temperature or increased dose. Modulated DSC
heating and cooling cycles show broadening and depression of
the ferroelectric transition with irradiation accompanied by
a distribution of lower melting (i.e. smaller size)
crystallites.
We compare star and linear ABC block copolymer
self-assembly, as well as screen for new mesophases in both
systems, by means of self-consistent mean field theory. The
self-consistent field equations were solved in real space
within a simulation box with periodic boundary conditions.
For each type of chain architecture, the parameter space
defined by the volume fractions of blocks A and B was
systematically explored for three different sets of Flory
parameters. In total, over 400 independent quenches were
performed. The study yielded several new microstructures
which are presented.
In certain multi-component, crosslinking polymer systems,
phase separation is induced by polymerization, a process
call polymerization-induced phase separation (PIPS) in which
there exists a competition between reaction rate and phase
separation rate. The final morphology and properties of a
system that experiences PIPS depend on the outcome of this
competition. Thus, by controlling these rates, it would be
possible to control end properties. In fact, this theory has
been applied for the creation of polymer-dispersed liquid
crystals (PDLCs) where cure occurs via condensation
reactions or via free radical polymerization initiated by UV
light. This research examines PIPS in the vinyl ester (VE)
resins, which are popular as matrix materials in polymer
composites. Cure of the VE resins is more complicated
because it uses initiators and therefore requires more time
and offers less control than cure by photopolymerization. To
better understand the PIPS process in the VE resins, this
research separates the two competitive effects and examines
each one in turn. Initial experiments use a model system to
focus on the effect of PIPS in the absence of crosslinking.
Polymer films with thickness commensurate with the size of
the polymer molecule have shown deviation from expected bulk
behavior. A Brillouin light scattering study of
free-standing, ultra-thin polymer films has reported changes
of up to 70 degrees K in the glass transition temperature,
Tg. Using secondary ion mass spectrometry (SIMS), we have
measured diffusion coefficients of ultrathin, free-standing
polystyrene films in order to determine Tg. Both
fluorescence probe and deuterated polymer diffusion were
simultaneously monitored using bilayered films. The top
layer was composed of 0.1 mass ratio of 1,8-dibromopyrene in
1 MMw polystyrene, and the bottom layer consisted of 0.2
mass ratio 20,000 Mw, deuterated polystyrene in 1 MMw
polystyrene. Films were annealed in a range of film
thicknesses and temperatures spanning the expected Tg range
for ultrathin films. Data was fit to Fickian diffusion
profiles to determine diffusion coefficient. It has been
shown that above Tg, diffusion coefficient scales according
to the WLF equation. Below the glass transition, it has been
found that diffusion coefficient exhibits an Arrhenius-type
dependence. Using these characteristics, it was possible to
observe the dependence of diffusion coefficient on finite
film thickness and determine the behavior of Tg as a
function of film thickness.
Considerable flow-induced anisotropy is caused in
thermotropic liquid crystalline polymers (TLCP's) during
injection molding. A new dihydroxy-a-methylstilbene
(DH\alphaMS) copolyester of terephthalatic, isophthalatic,
and 2,6-naphthalenedicarboxylic acids has been evaluated in
injection molded form. The expected anisotropy was observed
with moduli ranging from 1.8 to 4 Msi parallel to and 200 to
600 ksi perpendicular to the flow direction. Tensile
strengths in the range of 21 to 28 ksi were observed
parallel to and as low as 3.6 ksi perpendicular to the flow
direction. Mechanical properties increased in both the flow
and transverse directions with lower melt and mold
temperatures. Fracture initiation (K_Q) was found to be
relatively independent of Mw (by IV), but the rate of change
in energy for fracture, J*, parallel to the flow direction
was found to increase with increasing Mw. Transverse to the
flow direction, energy to break was favored by lower Mw. The
work suggests that processing parameter optimization can
minimize the effects of anisotropy in TLCP's. --
Acknowledgments: S. E. Bales (retired), M. A. Barger, M. T.
Bishop, R. Drumright, M. Lapham (all of Dow Chemical).
Mesophase pitch nanofibers were made with diameters of
several hundred nanometers to a few microns.
Thermogravimetric analysis showed the stabilization reaction
in air began at a temperature below 200^oC, and produced a 6%
weight gain. Changes intensity of aliphatic C-H (2850-3000
and 1400-1450 cm-1) and carbonyl -CO- (1690-1750 cm-1) peaks
in Fourier transform infrared spectra were observed as a
result of stabilization. The intensities of aryl alkyl ether
peaks at 1200-1275 cm-1 increased with stabilization in air.
Solid state NMR results confirmed these changes.
Carbonization in inert gas began when the temperature
reached 300^oC. There was a 25% weight loss during the
carbonization. Element analysis showed that the only carbon
remained. The structures of pitch nanofibers, stabilized
nanofibers and the resulting carbon nanofibers were
investigated with scanning electron microscopy, transmission
electron microscopy and X-ray diffraction.
Orientational coupling due to steric repulsion in bimodal
melts consisting of uniaxially stretched long and short
polymers has been observed recently in Nonequilibrium
Molecular Dynamics Simulations. In this is study, we present
a Molecular Dynamics study of orientational coupling in
dissimilar but homogeneous melts. In particular, we
investigate the effects of steric repulsion in melts which
have two species of polymers having different monomer sizes
and different stiffness. We also investigate chain end
effects in system with triblock polymers composed of stiff
or flexible ends.
The dynamics of fibril formation, skin-core morphology
development, and surface roughening in wet-spun fibers
containing polymers subject to evaporation of solvent are
examined. Recent simulations of morphology development
based on the Kato-Matsuyama free energy for semi-rigid linear
polymers combined with solvent removal and unidirectional
stretching effects have captured all of the above-mentioned
phenomena. Analysis of the results shows that fibrillation takes
place due to unstable phase separation coupled with orientational
ordering. Moreover, fibrillation has been found to proceed in a
manner analogous to the phenomenon of target or spiral wave
break-up, which is commonly observed in nonlinear dynamics.
A continuing investigation of the series
Sr_3CuPt_xIr_1-xO_6, which has been proposed as a
likely example of the Random Quantum Spin Chain Theory
(RQSC), is presented. This series consists of a dilution of
the two isostructural compounds Sr_3CuPtO_6 (AF) and
Sr_3CuIrO_6 (FM) whose mixture yields an approximately
linear chain of randomly distributed ferromagnetic and
antiferromagnetic bonds. Recent results of the AC magnetic
susceptibility for the series Sr_3CuPt_xIr_1-xO_6
show a variety of interesting and unexpected behavior that
departs from the novel low-temperature Curie-like behavior
predicted by the RQSC model. In particular, susceptibility
measurements for x=1 show evidence of long-range
ferromagnetic ordering. An attempt to accurately deduce the
exponents for the critical region(s) using the
susceptibility measurements alone has led to ambiguous
results. Heat capacity data for the series
Sr_3CuPt_xIr_1-xO_6 will presented along with
further analysis and exploration of the critical behavior.
[L36.017] Moments Expansion Study of the Rabi Hamiltonian
Jay D. Mancini, Vassilios Fessatidis (Fordham University), Samuel P. Bowen (Chicago State University), Robert K. Murawski (Stevens Institute of Technology)
[L36.018] Generalized 2nd and 3rd order moments expansion
Robert K. Murawski (Stevens Institute of Technology), Jay D. Mancini, Vassilios Fessatidis (Fordham University), Samuel P. Bowen (Chicago State University)
[L36.019] Schrödinger Green's Function for an Electron in an Anisotropic Harmonic Potential in the Presence of Electric and Magnetic Fields
Vassilios Fessatidis (Fordham University)
[L36.020] Liquid Crystalline Behavior of DNA Fragments
Sean English, Shila Garg (The College of Wooster, Wooster OH 44691)
[L36.021] Fluctuations in a near-critical steady state with temperature gradient
Alexander Patashinski (Northwestern University, Evanston Illinois), Alexander Burin (Northwestern University, Evanston, Illinois)
[L36.022] Molecular Segregation-Desegregation and Ordering: A Hybrid Computer Simulation Approach
Grace Foo (National University of Singapore), Ras Pandey (University of Southern Mississippi)
[L36.023] Modeling Orientational Ordering in Main-Chain LC Polymers
Paul Wessels, Bela Mulder
[L36.024] Dielectric and Photon Correlation Spectroscopy of Filled Nematic Liquid Crystal
Fouad Aliev, Ghanshyam Sinha (Department of Physics, University of Puerto Rico, PR 00931)
[L36.025] Molecular and Collective Relaxation in Liquid Crystals Partially Filling Cylindrical Pores
Zaira Nazario, Timofei Krouglov, Ghanshyam Sinha, Fouad Aliev (Department of Physics, University of Puerto Rico, PR 00931)
[L36.026] Liquid crystals in the SiO_2 opals
Daeseung Kang, Joseph E. Maclennan, Noel A. Clark (Department of Physics and Ferroelectric Liquid Crystal Materials Research Center, University of Colorado, Boulder, CO), Ray H. Baughman (AlliedSignal Corp., Morrstown, NJ)
[L36.027] Structure and Phase Determination of Laterally Attached Side-Chain Liquid Crystalline Polynorbornenes with a One-Carbon Spacer
Gue-Hyun Kim, Coleen Pugh, Stephen Z. D. Cheng* (Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909)
[L36.028] DENDRITIC GROWTH IN NEMATIC LIQUID CRYSTALS
Joshua Martin, Shila Garg (The College of Wooster, Wooster OH 44691)
[L36.029] The influence of ions on the "V-shaped" electrooptic response of ferroelectric liquid crystals with high spontaneous polarization
Martin Copic, Danielle Bundy, Joseph E. Maclennan (Affiliation), Noel A. Clark (Ferroelectric Liquid Crystal Materials Research Center, Department of Physics, University of Colorado at Boulder, CO 80309-0390)
[L36.030] Depletion and Reentrant Phase-transitions with Colloids and Dendrimers
A. G. Yodh, Jian Zhang, Subrata Sanyal (Department of Physics amp; Astronomy, University of Pennsylvania, PA 19104)
[L36.031] Structural Relaxation of Soft Giant Micelles with Liquid Like Order
Reinhard Sigel (FO.R.T.H.-IESL Heraklion, Crete, Greece), Stergios Pispas (Department of Chemistry, University of Athens, Athens, Greece), Dimitris Vlassopoulos (FO.R.T.H.-IESL Heraklion, Crete, Greece), Nikos Hadjichristidis (Department of Chemistry, University of Athens, Athens, Greece), George Fytas (FO.R.T.H.-IESL Heraklion, Crete, Greece)
[L36.032] Heat Transfer and Onset of Convection in a Very Compressible Fluid
Andrei Kogan, Horst Meyer (Duke University)
[L36.033] Twisted and Braided Vortices in Rotating Channel Flow
Jessica Levine, Joseph Niemela (University of Oregon)
[L36.034] Flow Through Layered Porous Media in Gradient Field: Response of Flux Rate and Density Distribution
Ras Pandey (University of Southern Mississippi), Joe Gettrust (Naval Research Laboratory)
[L36.035] Self-consistent Approach to Finite-Temperature Excitations in Dilute Bose Gases
Emil Lundh, P. Ao, Lars M. Jensen (Dept.\ of Theoretical Physics, UmeåUniversity, S-90187 Umeå
[L36.036] Strongly interacting one-dimensional Bose condenstates
B. Tanatar (Bilkent University)
[L36.037] Harmonically Trapped Quantum Gases
M. Grether (Facultad de Ciencias, UNAM, México D.F., México), M. Fortes (Instituto de F\'\isica, UNAM, México D.F., México), M. de Llano (Instituto de Investigaciones en Materiales, UNAM, México D.F., México), J.L. del R\'\io (UAM, Unidad Iztapalapa, México D.F., México), F.J. Sevilla, M.A. Sol\'\is, S. Tapia (Instituto de F\'\isica, UNAM, México D.F., México), A.A. Valladares (Instituto de Investigaciones en Materiales, UNAM, México D.F., México), Universidad Autónoma Metropolitana Collaboration, Universidad Nacional Autónoma de México Collaboration
[L36.038] Stability of Vortices in Two-component Bose Condensates
Hualin Shi, Sui-Tat Chui (Bartol Research Institute, University of Delaware)
[L36.039] Excited States of a Bose Condensate in a Harmonic Trap
Kunal Das, T. Bergeman (SUNY Stony Brook)
[L36.040] Studies of surface spin waves and exchange interactions
Mark van Schilfgaarde (Sandia Nat.Lab.,Livermore,CA), Vladimir Antropov (Ames Lab,Ames,IA)
[L36.041] GW quasiparticle bandstructure of YH_3
Takashi Miyake, Ferdi Aryasetiawan (JRCAT-ATP, 1-1-4 Higashi, Tsukuba, Ibaraki 305-0046, Japan), Hiori Kino (Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106-8666, Japan), Kiyoyuki Terakura (JRCAT-NAIR, 1-1-4 Higashi, Tsukuba, Ibaraki 305-8562, Japan)
[L36.042] A Database of Fermi Surfaces in Virtual Reality Modeling Language
Tat-Sang Choy, Jeffery Naset, Selman Hershfield, Christopher Stanton (Physics Department, University of Florida), Jian Chen (Seagate Technology)
[L36.043] Tight Binding Electronic Structure Calculation of (GaAs)_n/(Ge_2)_n (2 \leq n \leq 4)
Jeffrey Rufinus (University of Wisconsin)
[L36.044] Electronic excitations in II-VI compounds from an ab-initio GW approach
Andrzej Fleszar, Werner Hanke (Inst. for Theoretical Physics, University of Würzburg, 97074 Würzburg, Germany)
[L36.045] The efficiency of the generalized simulated annealing
Y. Xiang, X. G. Gong (Institute of Solid State Physics, Chinese Academy of Sciences, Hefei-230031, China)
[L36.046] Further Consequences of the Canonical sequence Method in Quantum Many-Body Systems
Vassilios Fessatidis, Jay D. Mancini (Fordham University), Robert K. Murawski (Stevens Institute of Technology), Samuel P. Bowen (Chicago State University)
[L36.047] Variational Approach for Approximating the Ground-state Energy of the Rabi Hamiltonian
Samuel P. Bowen (Chicago State University), Vassilios Fessatidis, Jay D. Mancini (Fordham University), Robert K. Murawski (Stevens Institute of Technology)
[L36.048] A Parallel AMR version of the PPM Hydrodynamics Code
Dennis Dinge (The Laboratory for Computational Science and Engineering at The University of Minnesota and Hamline University), Paul Woodward (The Laboratory for Computational Science and Engineering at The University of Minnesota)
[L36.049] Dimensional Crossover of the XC Density Functional
P. Garcia-González (University of York)
[L36.050] First principles study of the Raman spectra of SiS_2 and SiSe_2
Shau Grossman, Koblar Jackson (Physics Dept., Central Michigan University, Mt. Pleasant, MI 48859)
[L36.051] The Fractal Structure of Basic Particles
Shuming Li (University of Science & Techmology Hebei), Lihua Li (University of Minnesota), Shuyun Li (Shijiazhuang Vocational Technology College), Shuwei Li (Hebei BoAi Hospital)
[L36.052] Infrared Photoablation Studies of Arsenic Selenide with the Vanderbilt FEL
Janet Adair, Zsuzsanna Marka, Mike Albert, Shailesh Singh, Norman Tolk (Vanderbilt University, Nashville TN)
[L36.053] Length Scale of Spatially Heterogeneous Dynamics in Supercooled Glycerol
XH Qiu, M.D. Ediger (University of Wisconsin-Madison), S.A. Reinsberg (Max-Planck-Institut fuer Polymerforschung)
[L36.054] Fluid Viscosity Analogs in Granular Materials
Amaria George, Osiel Bonfim (Reed College)
[L36.055] BEAD PILE SYSTEM AS A MODEL FOR SELF-ORGANIZED CRITICALITY
Hanna L. Wagner, Donald T. Jacobs (The College of Wooster, Wooster OH 44691)
[L36.056] Exploring Subsurface Detail of Axial Segregation in Binary Mixtures of Glass Beads in a Horizontal Drum Mixer by Mechanical Means
Kevin A. Parendo (University of Minnesota -- Morris), Joel M. Hanson, James A. Flaten
[L36.057] Droplet Vorticity Alignment in Model Polymer Blends
Kalman Migler (NIST, Polymers Division)
[L36.058] Multilayer Coextrusion Reveals Slip at Polymer-polymer Interfaces
Rui Zhao, Christopher W Macosko (Department of Chemical Engineering and Materials Science,University of Minnesota)
[L36.059] Electrospinning from a Polymer Melt in a Vacuum
Ratthapol Rangkupan, Darrell H. Reneker (Maurice Morton Institute of Polymer Science, The University of Akron, Ohio 44325-3909)
[L36.060] JET-SPLITTING INSTABILITY IN ELECTROSPINNING OF POLY(2-HYDROXYETHYL METHACRYLATE)
Sureeporn Koombhongse, Darrell H. Reneker (Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325)
[L36.061] COLLECTION OF ELECTROSPUN POLYMER NANOFIBERS
Woraphon Kataphinan, Darrell H. Reneker (Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909)
[L36.062] In situ x-ray scattering study of a main-chain thermotropic liquid crystalline polymer under oscillatory shear flow
Nitin Vaish, Wesley R. Burghardt (Northwestern University), Weijun Zhou, Julia A. Kornfield (California Institute of Technology)
[L36.063] Nucleation in Microcellular Thermoplastic Foams
Pieter Spitael, Christopher W. Macosko (Department of Chemical Engineering and Materials Science, University of Minnesota)
[L36.064] Electrospun high performance nanofibers
Wenxia Liu, Zongquan Wu, Darrell H. Reneker (Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909)
[L36.065] Flipping from the Perpendicular to Parallel Orientation in Block Copolymers: An Electron Microscopy Study
Lei Qiao, Karen Winey (Department of Materials Science and Engineering, University of Pennsylvania)
[L36.066] Processing Effects on Block-Copolymer–Based Pressure-Sensitive Adhesives
A. E. O'Connor, C. W. Macosko (University of Minnesota)
[L36.067] Achieving microkelvin control at room temperature
Amy L. Lytle, D.T. Jacobs (The College of Wooster, Wooster OH 44691)
[L36.068] Roughening and De-roughening of the Interface Width in an Electrophoretic Deposition of Polymer Chains
Frank Bentrem, Ras B. Pandey (University of Southern Mississippi)
[L36.069] Pattern Coarsening of Spherical Copolymer Microdomains
Christopher Harrison, Zhengdong Cheng, Paul M. Chaikin, David A. Huse (Department of Physics), Richard A. Register (Department of Chemical Engineering), Douglas H. Adamson (Princeton Materials Institute, Princeton University)
[L36.070] Simulation of a Liquid Crystal at an Amorphous Polymer Surface
T. P. Doerr, P. L. Taylor (Case Western Reserve University)
[L36.071] Structural Properties of a Two-Dimensional Coulomb System
Girija S. Dubey, Godfrey Gumbs (Hunter College/CUNY)
[L36.072] Propagation of Heterogeneous Substrate Induced Ordering in Thick Block Copolymer Films
Lee D. Rockford, Thomas P. Russell (Umass Amherst, Dept. Polymer Science and Engineering), M. Yoon, S. G. J. Mochrie (Massachusetts Institute of Technology, Dept. Physics)
[L36.073] Modeling the Molecular Packing of Copper Phthalocyanine / Poly (1-lysine) Multilayer Thin Films
Angela L. Campbell (Air Force Research Laboratory, Materials & Manufacturing Directorate, AFRL/MLPJ, Wright-Patt AFB, OH 45433), D.R. Wiff (AFRL/MLPO)
[L36.074] Spinodal dewetting in polymer/polymer systems
A. M. Higgins, R. A. L. Jones (Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK), M. Sferrazza (ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK), P. Jukes, J. Sharp, L. Dryden (Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK), J. Webster (ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK)
[L36.075] Novel method of tailoring the surface properties of elastic materials
Kirill Efimenko, Jan Genzer (NC State University, Raleigh, NC)
[L36.076] Mobility of polymer chains at the polymer/air interface
Tobias Kerle, Zhiqun Lin, Ho-Cheol Kim, Thomas P. Russell (University of Massachusetts, Amherst MA01003)
[L36.077] Microstructure of a new Poly[(A-r-B)_x-b-B_y] (y>x) gradient copolymer
Zhiqun Lin, Thomas P. Russell (University of Massachusetts, Amherst, MA., 01003), Elbert E. Huang, Didier Benoit, Craig Hawker (IBM Almaden Research Center, San Jose, CA., 95120)
[L36.078] Rheology of Confined Telechelic Chain under Shear
June Huh, Anna C. Balazs (Chemical and Petroleum Engineering, University of Pittsburgh)
[L36.079] Thickness dependence of the glass transition temperature in thin polymer films
Wang-Cheol Zin (Dept.of Material Science and Engineering, Pohang University of Science and Technology), Jae Hyun Kim, Jyongsik Jang (School of Chemical Engineering, Seoul National University)
[L36.080] Investigation of p-quaterphenyl layers vapor deposited on KCl (001) by Atomic Force Microscopy (AFM)
Jr. Kintzel, D.H. Van Winkle, J.G. Skofronick, S.A. Safron (Florida State University), F. Flaherty (Valdosta State University), D.-M. Smilgies (European Synchrotron Radiation Facility, France), Valdosta State University Collaboration, European Synchrotron Radiation Facility Collaboration
[L36.081] Interfaces of two immicible polymer thin films studied by X-ray Scattering
K. Shin, Y. Seo, M. Rafailovich, J. Sokolov (State University of New York at Stony Brook), H.O. Seeck, S.K. Sinha (Argonne National Laboratory), R. Jones, S. Kumar (Penn State University), R. Kolb (Exxon Research and Engineering Company)
[L36.082] Surface Segregation of Fluorine in Thin Films of Poly (Methyl Methacrylate-co-Tetrahydroperfluorooctylacrylate) (PMMA/TAN) Random Copolymers.
Nora Beck Tan, Wendy Kosik, Joseph Deitzel, Steve McKnight (US Army Research Laboratory, Materials Division AMSRL-WM-MA, APG, MD 21005.), Stephanie Crette, Joseph DeSimone (Department of Chemistry University of North Carolina at Chapel Hill Chapel Hill, NC 27599-3290)
[L36.083] Surface Segregation and Bulk Thermodynamics of Polybutadiene Star/Linear Blends
T. D. Martter, M. D. Foster, G. Lizarraga, S. Xu, R. P. Quirk (U. of Akron, Dept. of Polymer Science, Akron, OH 44325), P. Butler, C. F. Majkrzak (National Institute of Standards and Technology, Gaithersburg, MD 20899), J. D. Demaree (Army Research Lab, AMSRL-WM-ME, APG, MD 21005)
[L36.084] Diffusion Behavior in Metal/Polymer Nanocomposites
Rodney Guico (Northwestern University, Argonne National Laboratory), Kenneth Shull (Northwestern University), Jin Wang, Lynn Rehn, Peter Baldo (Argonne National Laboratory)
[L36.085] Effect of Strain on the adsorption of CO on Pd(100)
H. Metiu, M.W. Wu (Chemistry Department, University of California, Santa Barbara, CA 93106)
[L36.086] Tuning the wetting behavior of block copolymers on self-assembled monolayers exposed to ionizing radiation
Richard Peters, Xiao Yang, Paul Nealey (Department of Chemical Engineering, University of Wisconsin)
[L36.087] Symmetric Diblock Copolymer Thin Films Confined Between Two Hard Surfaces: Simulations and Theory
Qiliang Yan, Qiang Wang, Paul Nealey, Juan de Pablo (Department of Chemical Engineering, University of Wisconsin-Madison)
[L36.088] Phase transition of LB films of mixed diblock copolymer at the air/water interface
Y.S. Seo, K.S Kim, V. Samuilov, M.H. Rafailovich, J. Sokolov, Rob G.H. Lammertink, G.J. Vancso (Affiliation)
[L36.089] Brushes and Mushrooms in Diblock Copolymer/Homopolymer Mixtures
Andreas Terzis, Doros N. Theodorou, Chris Toprakcioglu (Univ. of Patras and FO.R.T.H.-I.C.E./H.T., Greece), Haralambos Retsos, Spiros H. Anastasiadis (FO.R.T.H.-I.E.S.L. and Univ. of Crete, Greece), Greg Smith (LANSCE, Los Alamos), Alain Menelle (C.E.N. Saclay, France), Yves Gallot (Inst. Charles Sadron, France), Georges Hadziioannou (Groningen, Univ., The Netherlands)
[L36.090] Anisotropic Buckling in a Confining Coverlayer Directs Island and Hole Formation in an Underlying Lamellar Block Copolymer Thin Film
M. R. Hammond, G. H. Fredrickson, E. J. Kramer (UCSB)
[L36.091] Long-Range Order of Symmetric Block Copolymer Thin Films
Ho-Cheol Kim, Thomas Russell (Polymer Science and Engineering Department, University of Massachusetts at Amherst)
[L36.092] Controlling the Long Range Ordering of Block Copolymer Micelle Films
R.A. Segalman, H. Yokoyama, E.J. Kramer (UCSB)
[L36.093] Alignment mechanisms of diblock copolymers in electric fields
Jason DeRouchey (UMass-Amherst Polymer Science and Engineering), Thomas Thurn-Albrecht, Tom Russell (UMass-Amherst), Sushil Satija (NIST)
[L36.094] Fracture of interfaces between glassy polymers in a trilayer geometry
Nicolas Passade, Costantino Creton (Laboratoire PCSM-ESPCI, Paris, FRANCE)
[L36.095] The Relationship of Pretilt Angle and Chemical Structure of Rubbed Organo-Soluble Side-Chain Polyimides
Ian K. Mann, F. Bai, Z. Bai, J. Ge, L. Sun, H. Wang, Z. Zhang, Frank W. Harris, Stephen Z.D. Cheng (Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, OH 44325-3909)
[L36.096] Reactive Processing with Difunctional Oligomers to Increase Interfacial Adhesion in Polymer Blends
Charles O'Brien, Kevin Rice+, Mark Dadmun (The University of Tennnessee, Knoxville)
[L36.097] Fracture Toughness of Modified Poly(ethylene terephthalate)/Gelatin Interfaces
B.Y. Asoo, E.J. Kramer (UCSB), C.-A. Dai (Eastman Kodak)
[L36.098] Experimental and theoretical investigation of random copolymer segregation
W Li, B Tang, Y Zhang, D Gersappe, M Rafailovich, J Sokolov (Department of Materials Science and Engineering, SUNY, Stony Brook, NY 11794), D Peiffer, M Lin (Exxon Research and Engineering, Annandale, NJ 08851), J A Dias, K O MacElrath (Exxon Chemical Corp, Bayton, TX 77520)
[L36.099] Numerical Investigation of Monodisperse Polymer in an Athermal Solution Between Two Surfaces
Matthew Yi, Mukesh Chhajer, P. D. Gujrati (University of Akron, Akron OH 44325)
[L36.100] Structure Within Thin Epoxy Films Revealed by Solvent Swelling: A Neutron Reflectivity Study
Hyun Yim, Michael Kent, W. Frere McNamara (Sandia National Laboratories), Robert Ivkov, Sushil Satija (NIST), Jaroslaw Majewski (Los Alamos National Labs)
[L36.101] Interdiffusion of Polyethylenes in Microlayers
Sergei Nazarenko, Eugene Stepanov, Anne Hiltner, Eric Baer (Case Western Reserve University, Department of Macromolecular Science,Center for Applied Polymer Reserach)
[L36.102] Patterning of a Semicrystalline Block Copolymer Thin Film via Epitaxial Crystallization
Cheolmin Park (Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139), Claudio De Rosa (Dipartimento di Chimica, Universita'di Napoli "Federico II" Via Mezzocannone 4, 80134 Napoli, Italy), Lewis J. Fetters (Exxon Research and Engineering Company, Annandale, New Jersey 08801), Bernard Lotz (Centre de Recherches sur les Macromolecules, CNRS Strasbourg, 67083 France), Edwin L. Thomas (Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139)
[L36.103] Size induced glass transition in ultra-thin films and its impact in diffusive and elastic behavior.
J. Andres Torres, David S. Fryer, Paul F. Nealey, Juan J. de Pablo (UW-Madison)
[L36.104] Understanding the morphology and dynamics of ordering of thin films of diblock copolymers on chemically heterogeneous surfaces
Xiao Yang, Richard Peters, Qiang Wang, Juan dePablo, Paul Nealey (Department of Chemical Engineering, University of Wisconsin)
[L36.105] Thermal Expansion Behavior of Thin Polycarbonate Films
Christopher Soles, Wen-li Wu (NIST Polymers Division)
[L36.106] Molecular dynamics simulation study of Norbornene-POSS polymers
R. J. Berry (Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/MLBP), WPAFB OH 45433), R. K. Bharadwaj (Systran Federal Corporation, Dayton, OH 45431), B. L. Farmer (Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/MLBP), WPAFB, OH 45433)
[L36.107] Synthesis and Characterization of High Performance Polyheterocycles
Shuxin Cong, Shi Jin, Stephen Z. D. Cheng (The University of Akron)
[L36.108] Analysis of Sub-nanometer Defects Near Coating Surfaces by Positron Annihilation
Hongmin Chen, Renwu Zhang, Huimin Cao, Peter Mallon, Ying He, Thomas Sandreczki, Y.C. Jean (University of Missouri-Kansas City), R. Suzuki, T. Ohdaira (Electrotechnical Labs, Japan), Bent Nielsen (Brookhaven National Labs)
[L36.109] Oxygen Diffusion In Physically Modified PET
Eugene Stepanov, Sergei Nazarenko, Anne Hiltner, Eric Baer (Case Western Reserve University), Center for Applied Polymer Research Collaboration
[L36.110] Mesoscale modeling study of nematic liquid crystals confined to ellipsoidal domains
B. L. Farmer (Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/MLBP), WPAFB, OH 45433), R. K. Bharadwaj (Systran Federal Corporation, Dayton, OH 45431), T. J. Bunning (Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/MLPJ), WPAFB, OH 45433)
[L36.111] QSPR Calculations of Tg of Electroactive Molecular Glasses
James M. O'Reilly, Santosh Putta, Priyan Patkar (Eastman Kodak Company)
[L36.112] Photoresist Outgassing in EUV
Maharshi M. Chauhan, Paul F. Nealey (The Center for Nanotechnology and the Department of Chemical Engineering, University of Wisconsin - Madison, Madison WI 53706), Harun Solak, Franco Cerrina (The Center for Nanotechnology and the Department of Electrical and Computer Engineering, University of Wisconsin - Madison, Madison WI 53706)
[L36.113] Competing Silicon Backbone Conformations in Poly(di-n-octyl silane)
W. Chunwachirasiri, M.J. Winokur (Dept.~of Physics, Univ.~of Wisconsin, Madison, WI), R. West (Organo-silicon Research Center, Dept.~of Chemistry, Univ.~of Wisconsin, Madison, WI)
[L36.114] Studies of the Structure of Poly(silylenemethylene)s
Soo-Young Park (University of Virginia / Air Force Research Lab.), Tao Zhang, Leonard V. Interrante (Rensselaer Polytechnic Institute), Barry L. Farmer (Air Force Research Lab.)
[L36.115] Structure and Properties of Modified Poly(urethane urea)s: Polyisobutylene Comb Polymers and Nanocomposites
D.M. Weisberg, R. Xu, J.T. Garrett, C.A. Siedlecki, A.J. Snyder, G. Rosenberg, J. Runt (Penn State University)
[L36.116] Microdomain Morphology of Poly(urethane urea) Multiblock Copolymers
J.T. Garrett, C.A. Siedlecki, J. Runt (Penn State University)
[L36.117] Crystallization and Solid State Structure of Poly(lactide) Copolymers
S. Baratian, J. Runt (Penn State University), E. Hall (Cargill-Dow), J.S. Lin (Oak Ridge National Laboratory)
[L36.118] Distortions Near Edge Dislocations in Conjugated Crystalline Polymers
David Martin, Lawrence Drummy, Lebzylisbeth Gonzalez-Ronda, Christian Kübel (Materials Science and Engineering, The University of Michigan)
[L36.119] A Morphological Investigation of Dry and Solvent Swollen Nafion
Sandra Young, Samuel Trevino, Nora Beck Tan, Don Rivin (US Army Research Laboratory), Rick Paul (NIST)
[L36.120] What Does the SAXS Experiment Tell Us?
Buckley Crist (Nortwestern University)
[L36.121] The Structure and Phase Behavior of the Novel Dendritic Liquid Crystalline Polymers
Zhen Liu, Stephen Z. D Cheng (The University of Akron), Peiwei Chu, Virgil Percec (Case Western Reserve University)
[L36.122] Melting and Crystallization of Amorphous-Crystalline Diblock Copolymer
Yan Chen, Lei Zhu, Stephen Z.D. Cheng (Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909)
[L36.123] New X-ray and Neutron Diffraction Data and PRISM Theory for Polyethylene Melts
Anton Habenschuss, Brian K. Annis (Oak Ridge National Laboratory), J. David Londono (E. I. DuPont), John G. Curro, Jeffrey D. Weinhold, Mathias Puetz (Sandia National Laboratories)
[L36.124] Achieving Finely Dispersed Minority Phases in Immiscible Polymer Blends via Pulverization
Andrew Lebovitz, Manuel Anguiano, Klementina Khait, John Torkelson (Northwestern University, Depts. of Chem. Eng. and Mat. Sci. and Eng.)
[L36.125] Studies on Single Lamellar Crystals of Short Chain Branched Polyethylene Copolymers Crystallized From Solution
Qiang Wang, Stephen Z. D. Cheng (Maurice Morton Institute of Polymer Science, The University of Akron), Qiang Fu (Sichuan Union University)
[L36.126] Novel Banded Structures in a Combined Main-Chain/Side-Chain Crystalline Polyester: from Liquid Crystals to Confined Crystals
Jason J. Ge, Wensheng (William) Zhou, John Z. Zhang, Shy-Yeu Wang, Frank W. Harris, Stephen Z.D. Cheng (The University of Akron)
[L36.127] The Growth and Morphology of Syndiotactic Polypropylene Lamellar Crystals Studied by AFM with Heating Stage
Wensheng (William) Zhou, Stephen Z. D. Cheng, Darrell H. Reneker Reneker (The University of Akron), Sergei Magonov (Digital Instruments)
[L36.128] Sporadic Nucleation and Growth in the Microphase Separation Process of an I2S Miktoarm Star Block Copolymer and its Blends with Homopolymer
Lizhang Yang, Darrin J. Pochan, Samuel P. Gido (Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003), Stergios Pispas, Kunlun Hong, Jimmy W. Mays (Department of Chemistry, University of Alabama at Birmingham, Birmingham, AL 35294)
[L36.129] New synthetic route for nanoporous ceramic films based on silicon-containing block-copolymers.
Martin Brinkmann, Vanessa Z.-H. Chan, Edwin L. Thomas (MIT), Apostolos Avgeropoulos, Nikos Hadjichristidis (Dpt Chem., U. Athens), Victor Lee, Robert D. Miller (IBM Almaden)
[L36.130] Moisture Diffusion in Poly(Bismaleimide) and its Composites
L. Bao, A. F. Yee (Macromolecular Sci. & Eng., U. of Michigan, Ann Arbor, MI 48109)
[L36.131] Study of Shear-induced orientation in polyethylene melt near the melting temperature using synchrotron SAXS and WAXD
R.H. Somani (Chemistry Dept., SUNY at Stony Brook.), I. Sics (Instituto de Estructura de la Materia, Madrid, Spain.), B.S. Hsiao, Z.G. Wang (Chemistry Dept., SUNY at Stony Brook.), F. Balta-Calleja (Instituto de Estructura de la Materia, Madrid, Spain.), T. Ezquerra (Instituto Estructura de la Materia, Madrid, Spain), S. Srinivas, A. Tsou (Exxon Chemical Company, Baytown Polymers Center, TX 77522.), SUNY at Stony Brook Team, Instituteo de Estructura de la Materia Madrid Spain Team, Exxon Chemical Company Collaboration
[L36.132] Effect of Reaction Rate on Morphological Change of Reactive Blends
Hyun Kyoung Jeon, Jin Kon Kim (Department of Chemical Engineering and Polymer Research Institute, Pohang University of Science and Technology, Pohang, Kyungbuk, 790-784, Korea)
[L36.133] Full Pattern WAXD Refinement of Oriented Polymer Structures Using Simulated Annealing
Jing Wu, Jerold Schultz (Univ. of Delaware, Chemcial Engineering)
[L36.134] Macromorphology of Polypropylene Homopolymer Tacticity Mixtures
Roger Phillips (Montell USA Inc.)
[L36.135] Confined crystallization in a lamellar forming PEO-b-PS diblock copolymer
Lei Zhu, Stephen Z. D. Cheng*, Bret H. Calhoun, Qing Ge, Roderic P. Quirk (Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, OH 44325-3909), Benjamin Hsiao, Fengji Yeh (Department of Chemistry, The State University of New York at Stony Brook, Stony Brook, New York 11794-3400)
[L36.136] Structural Study of Poly(ferrocenyldimethylsilane) in Electrospun Nanofibers and Poly(ferrocenyldimethylsilane)-b-Poly(isoprene) Diblock Copolymer in Bulk
Zhihao Chen, Mark Foster, Hao Fong, Darrell Reneker, Tae Hee Chang, Roderic Quirk (Dept. Polymer Science, U. of Akron), Rui Resende, Ian Manners (Dept. Chemistry, U. of Toronto)
[L36.137] Comparison of Scattering Data and Molecular Dynamics Simulations for Liquid Poly(ethylene oxide) PEO and Small Molecule Analogs
Brian K. Annis, Anton Habenschuss (Oak Ridge National Laboratory), Christopher J. Benmore (Rutherford Appleton Laboratory), Oleg Borodin, Dmitry Bedrov, Grant D. Smith (University of Utah)
[L36.138] Mixing Efficiency, Coarsening, and Self-Compatibilization in Immiscible Polymer Blends Processed via Solid-State Shear Pulverization
Albert Davydov, Klementina Khait, John Torkelson (Depts. of Chem. Eng. and of Mat. Sci. and Eng., Northwestern University)
[L36.139] Polymerization peak in polystyrene: A molecular dynamics simulation study
Chakravarthy Ayyagari, Dmitry Bedrov, Grant Smith (Department of Materials Science and Engineering, University of Utah)
[L36.140] STRUCTURE AND ORDERING OF MICELLES IN TRIBLOCK COPOLYMER SOLUTIONS
Yong Li, Guangdong Liao, Karl Ludwig, Rama Bansil (Center for Polymer Studies and Department of Physics, Boston University), Cestmir Konak (Institute of Macromolecular Chemistry, Czech Republic), Jyotsana Lal (Argonne National Laboratory, Argonne Illinois)
[L36.141] Morphological Investigation of Two-Phase LC/Polymer Films Formed via Reaction-induced Phase Separation
Greg Hostetter, Greg Beaucage (Dept. Mat. Sci. and Eng., University of Cincinnati), Barry Farmer, Timothy Bunning (Air Force Research Laboratory, AFRL/MLPJ, WPAFB, OH)
[L36.142] Detection of Cocontinuity in Immiscible Polymer Blends
Jeffrey Galloway, Christopher Macosko (Department of Chemical Engineering and Materials Science, University of Minnesota)
[L36.143] Morphology of semicrystalline/amorphous diblock copolymer on thin film
Sheng Hong, William J. MacKnight, Thomas P. Russell, Samuel P. Gido (Department of Polymer sci. and Eng., University of Massachusetts, Amherst, MA 01003)
[L36.144] Morphology of Covalent and Hydrogen Bonded Side Group Liquid Crystalline Block Copolymers: Orientation by Oscillatory Shear and Device Applications
Chinedum O. Osuji (Massachusetts Institute of Technology), Chiyang Chao, Christopher K. Ober (Cornell University), Edwin L. Thomas (Massachusetts Institute of Technology)
[L36.145] Mechanical Behavior of Energetic Materials During High Acceleration
Y. D. Lanzerotti (U. S. Army TACOM-ARDEC), J. Sharma (NSWC, Carderock Div. West Bethesda, MD)
[L36.146] Diffusion of methanol into PMMA studied by molecular dynamics simulations
Mesfin Tsige, M. Mahajan, C. Rosenblatt, P.L. Taylor (Case Western Reserve University)
[L36.147] Molecular Modeling of the Deformation of Polymer Chains
W. W. ADAMS, R. PACHTER, B. L. FARMER (Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, USA), R. J. YOUNG (Manchester Materials Science Centre, UMIST/University of Manchester, M1 7HS, UK)
[L36.148] Electronic Structure and Properties of Strained Polymers
W. N. SIVAK, R. PACHTER, B. L. FARMER, W .W. ADAMS (Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433)
[L36.149] Craze Growth Monitoring Using Light Scattering
Tom Krupenkin (Bell Labs, Lucent Tech., USA)
[L36.150] NMR Characterization of Segmental Dynamics in Glassy Poly(alkyl methacrylate)s Using Centerband-Only Detection of Exchange (CODEX)
Tito J. Bonagamba (UMass Amherst and USP - Instituto de Fisica de Sao Carlos - Brazil), Fabio Becker-Guedes, Eduardo R. deAzevedo (UMass Amherst and USP - IFSC - Brazil), Weiguo Hu, Klaus Schmidt-Rohr (UMass Amherst)
[L36.151] Prediction of the Time-Dependent Shear Modulus of Poly(vinylidene fluoride) from Simulation and Theory
Oleksiy Byutner (University of Utah, Chemical and Fuels Engineering Dpt.), Grant Smith (University of Utah, Material Sciences and Engineering Dpt.)
[L36.152] Effect of Crosslinking and Morphology on Properties of Rigid-rod Polymers
Shawn Jenkins, Karl I. Jacob, Malcolm B. Polk, Satish Kumar (School of Textile and Fiber Engineering, Georgia Institute of Technology), Thuy Dang, F. E. Arnold (Polymer Branch, Air Force Research Laboratory)
[L36.153] Structure and Properties of a Hydrogen Bonded, Rigid-rod Polymer - Methyl Pendant PBI
Shawn Jenkins, Karl I. Jacob, Malcolm B. Polk, Satish Kumar (School of Textile and Fiber Engineering, Georgia Institute of Technology), Thuy Dang, F. E. Arnold (Polymer Branch, Air Force Research Laboratory)
[L36.154] Influence of local segmental motions on diffusion of small molecules through glassy polymers
Vijay Natarajan, Albert F. Yee (Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136)
[L36.155] Crystal Modulus of Silk Fibers: Bombyx mori
Anuchai Sinsawat, R. K. Eby (Dep't of Polymer Science, U. of Akron), Ruth Pachter (Materials and Manufacturing Directorate, AFRL)
[L36.156] Adhesive Properties of Model, Filled Elastomeric Adhesives
Peter Drzal, Elizabeth Cheang, Kenneth Shull (Northwestern University)
[L36.157] Viscoelasticity of Ethylene-Styrene Interpolymers: Molecular Interpretation
Eugene Stepanov, Hong-Yu Chen, Anne Hiltner, Eric Baer (Case Western Reseve University), Center for Applied Polymer Research Collaboration
[L36.158] Adhesion and Stress Relaxation of Coalesced Latex Blends
E. Fabbroni, K.R. Shull (Northwestern University), A.C.I.A. Peters (NeoResins)
[L36.159] Molecular Motions During Physical Aging in Polystyrene
C.L. Thompson, M.D. Ediger (University of Wisconsin-Madison, Department of Chemistry)
[L36.160] Neutron scattering study of electron-irradiated P(VDF-TrFE)
Edward Balizer (NSWC/Carderock Div/West Bethesda, MD), Aime DeReggi (Polymers Div/NIST/Gaithersburg, MD), Dan Neumann (Center Neutron Research/NIST/Gaithersburg, MD), Fred Bateman (Radiation Physics Division/NIST/Gaithersburg, MD)
[L36.161] Star vs Linear ABC Block Copolymer Self-Assembly
Eric Flewelling, Glenn H. Fredrickson, Francois Drolet (University of California, Santa Barbara)
[L36.162] Polymerization-Induced Phase Separation in Vinyl Ester Resins
Manisha Ganglani, John Torkelson, Stephen Carr (Northwestern University)
[L36.163] SIMS studies of Finite Film Thickness Effects on Polymer and Probe Diffusion in Free-Standing Ultra-Thin Polymer Films
Ankit Patel (University of Maryland), Chris White, Wen-li Wu (NIST), Yuxie Pu, M. Rafailovich, J. Sokolov (Stony Brook University)
[L36.164] Anisotropy in a DH\alphaMS Copolyester LC Polymer
Robert Bubeck, James Brewbaker (Michigan Molecular Institute, Midland, MI 48640)
[L36.165] Stabilization and carbonization of mesophase pitch nanofiber
Hao Fong, Darrell H. Reneker (Maurice Morton Institute of Polymer Science, The University of Akron, Akron, OH 44325-3909)
[L36.166] Time-dependent Orientational Coupling in Uniaxially Stretched Bimodal Melts: A Molecular Dynamics Study.
Marc P. Pepin, Gary W. Slater (University of Ottawa)
[L36.167] Dynamics of Fibrillation in Polymer Fibers
Andrew Guenthner, Thein Kyu (The University of Akron)
[L36.168] Heat Capacity of Sr_3CuPt_xIr_1-xO_6
Tom Sangrey, Stephen Irons, Kara Beauchamp (Dept. of Physics, Wesleyan University, Middletown, CT), Mark Smith, Hanno zur Loye (Dept. of Chem. and Biochem., U. of S. Carolina, Columbia, SC)