Session J32 - Poster Session I.
POSTER session, Wednesday morning, March 19
Exhibit Hall, Conv. Center

[J32.01] A Neutron Reflectivity Study of Co\-po\-ly\-mer/Ho\-mo\-po\-ly\-mer Interfaces

The behavior of the interface between polystyrene (PS) and polymethylmethacrylate (PMMA) was studied using neutron reflectometry. A random copolymer, denoted P(S_f-r-MMA_1-f) where f is the styrene fraction, located at the interface, was manipulated to observe the effects that copolymer composition and layer thickness have on the interface. Neutron reflectivity measurements show that for random copolymer layers with deuterated styrene fractions at four different compositions (f=0.3,0.5,0.8,0.9), material from the random copolymer layer was taken up uniformly in the more soluble of the two adjacent homopolymer layers. They also show that for varying thicknesses of f=0.8 random copolymer, a ``pure'' copolymer layer is achieved only for thicknesses greater than 127\,ÅIn addition, a strong composition- and interface-dependence of the effective \chi_S-MMA for the copolymer/homopolymer system was found. Specifically, interfacial widths between the random copolymer and the PS and PMMA homopolymers are asymmetric and much broader than predicted if \chi_S-MMA were assumed to be composition-independent and equal to the value determined from studies of PS/PMMA diblock copolymers.

[J32.02] Physics of Friction in Disposable Plastic Syringes

Nosocomial applications of disposable plastic syringes demand excellent frictional behavior with no stick-slip over a broad velocity range and, simultaneously, a tight seal between stopper and barrel. However, when used in syringe pumps at slow injection speeds, stick-slip motion is frequently observed and high "break-out" forces are often necessary to initiate plunger movement after extended storage times. We have traced this frictional behavior to a velocity-dependent interaction between the elastomeric stopper and the plastic syringe barrel mediated by the syringe lubricant, almost universally a polydimethyl siloxane fluid. Lubricant properties were altered by crosslinking the surface of the silicone oil in an oxygen plasma. Changes in surface chemistry and morphology of the crosslinked oil were correlated with changes in frictional performance.

[J32.03] Escape and Imprisonment Transitions for Grafted Polymer Chains Compressed by Finite Obstacles

The compression of singly-grafted chains by very large obstacles is a classical problem that was tackled many years ago. It is now possible, using an atomic force microscope tip, or the surface forces apparatus, to compress individual grafted chains by obstacles not much larger than the unperturbed chain size. We study this here for chains in theta and poor solvents and also for diblock copolymers. In all cases we show that the chain can undergo an escape transition from an imprisoned state at weak compressions to an escaped state at strong compressions. In the escaped state the chain forms a tether from the grafting site to the edge of the obstacle and the remainder of the chain escapes from under the obstacle. This system undergoes something analogous to a first-order phase transition, and we discuss some related effects such as hysteresis and super-compression which occur in this system.

[J32.04] A Surface Force Study of Polymer-Surfactant Mixtures

We study the interactions between a cationic modified guar and Sodium Dodecyl Sulfate (SDS). The bulk properties have been investigated by rheology and spectrofluorimetry. A Surface Force Apparatus (SFA) was used to determine the structure of the adsorbed polymer-surfactant layer. The bulk behavior is typical of a polymer-surfactant solution where the polymer and the surfactant bear opposite charges. For low enough SDS concentrations, the surfactant and the polymer mix in a single phase. A two phase region is reached at charge equivalence and redissolution of a very low viscosity solution by charge inversion is observed for higher SDS concentrations. At the mica surface, the cationic polymer adsorbs in a configuration with thickness of the order of 250 nm. When SDS is added, the change in both the layer thickness and the forces profile indicate a strong structural modification upon increasing surfactant concentration of the adsorbed layer.

[J32.05] Adsorption of Random Copolymers on a Solid Surface

We consider adsorption of random AB copolymers on a solid surface. The monomers A and B differ by their affinities to the surface, with adsorption energies (in unit of k_BT) \epsilon_A,B=\epsilon\pm\eta respectively. Using a scaling argument which employs typical compositional fluctuations of monomers in contact with the surface, we find that at \theta conditions, the adsorption-desorption transition is first order, with the thickness of the adsorption layer scales as 1/\eta^2 at the transition. The scaling predictions are confirmed by Monte-carlo simulations.

[J32.06] Interaction Between Surfaces Coated with Charged Polymers

This abstract was not submitted electronically.

[J32.07] The Effect of Block Copolymer Adsorption on Thin Film Dewetting Kinetics

Using optical microscopy, the dewetting kinetics of thin polystyrene films from a solid substrate is studied as a function of block copolymer adsorption at the polymer/solid interface. The block copolymer is poly(styrene-block-methylmethacrylate) (PS-b-PMMA) which contains short MMA and long S blocks. The MMA block adsorbs to the silicon oxide surface, whereas the S block extends into the matrix polymer. The matrix is formed by a polystyrene (PS) homopolymer. The dewetting kinetics of PS films, with and without PS-b-PMMA, is investigated by measuring the hole diameter as a function of time, at constant temperature for various molecular weights and concentrations of PS-b-PMMA in the film. We find that the presence of adsorbing block copolymer at the polymer/solid interface can significantly retards the dewetting dynamics of thin polymer films by increasing the adhesion between the polymer film and the substrate. These macroscopic results are correlated with the molecular aspects of block copolymer adsorption, such as the volume fraction profile and the coverage of the adsorbed layer.

[J32.08] Surface Treatment of Polyethylene Film Using Oxygen Plasma

The surface modification of polyethylene by oxygen plasma were performed with respect to plasma power, oxygen pressure and treatment time. Changes in the chemical structure of the film surface were probed by FTIR and ESCA. The plasma ensures a quick functionalization on the polyethylene surface with O-containing chemical groups within a few seconds. There is no further formation of oxygen containing moieties after two minutes. The surface wettability and morphology of treated polymer were also examined using contact angle measurement, atomic force microscopy (AFM) and scanning electron microscopy (SEM). It is found that the surface roughness was accentuated by lower plasma power, and surface lateral aggregation of polyethylene fibrils was observed to proceed within a few seconds.

[J32.09] Computer Simulations of the Pullout of Chains with Internal Vibrational Modes

Along with chain scission(T. P. Doerr, P. L. Taylor, J. Chem. Phys. 101), 10107 (1994); F. A. Oliveira, P. L. Taylor, J. Chem. Phys. 101, 10118 (1994), chain pullout(T. N. Krupenkin, P. L. Taylor, Phys. Rev. B 52), 6400 (1995); T. N. Krupenkin, P. L. Taylor, Macromolecules 28, 5819 (1995) is one of the primary mechanisms involved in the fracture of a polymer solid. In order to develop a more detailed understanding of the mechanical behavior of realistic polymer systems undergoing fracture, the commercial molecular modeling software package Cerius was used to perform molecular dynamics simulations on amorphous polyethylene systems out of which single chains are pulled at approximately constant speed. The internal vibrational modes of the chain are found to be excited by the pullout process. The energy and the force as a function of time necessary to pull a chain out depend on the speed of pullout. The relevance to the pullout process of the internal vibrational modes of the chain will be discussed.

[J32.10] Polymer-Solid Interfaces: Effects of Functional Groups on the Structure and Strength

At polymer-solid interfaces, the chain structure and dynamics are important for adhesion. The effect of functional groups randomly attached to polymer chains on the structure and strength of bulk polymer-solid interfaces was explored. Computer simulation, based on a Self-Consistent Field Lattice Model (SCFM) developed by N.D.Theodorou, was used to examine the influence of the concentration of functional groups (\phi) and the interaction strength (\chi) between functional groups and the solid surface. It was found that (A) with increasing \chi (\phi constant), the interfacial polymer chains become less aniosotropic and can approach isotropy with high \chi; (B) with increasing \phi (\chi constant), interfacial chains become more isotropic at first and then turn to be more anisotropic, suggesting an optimal \phi for bonding. Peeling tests of Aluminum-carboxylated polybutadiene (Al-cPBD) interfaces showed a peak fracture energy at 5-6% [-COOH]. Strength development of Al-cPBD interfaces shows very long time dependence.

[J32.11] NMR-Determined Partitioning of Water Between Voids and the Polymer Matrix in an Electronics Packaging Material

Electronics packaging materials (EPM) are usually composites of organically coated glass particles, carbon black and a thermosetting polymer matrix. An unwanted characteristic of EPM's is absorption of water. Sometimes, during the soldering of these packages onto circuit boards, the associated heat would drive water vapor to the larger interfaces, thereby causing either delamination or cracking of the package. We used proton NMR along with magic angle sample spinning to examine the state of water in wafers of this packaging material. We were able to distinguish between water in voids and water within the polymer matrix. It was established that a) the thermodynamically preferred environment was the polymer matrix, b) the fractional void volume is 0.002 and c) rapid heating caused no significant irreversible damage of the kind that would accelerate water transport to the surface.

[J32.12] Adhesive Properties of Thermoreversible Gels

Fracture mechanics analysis based on JKR theory provides a useful method to investigate the adhesive behavior of low modulus materials, such as acrylic elastomers diluted with solvent. We have developed a thermoreversible system based on a triblock copolymer with poly(methyl methacrylate) (PMMA) endblocks and a poly(n-butyl acrylate) midblock, in which we are able to explore its interfacial properties. These low modulus gels are formed when the triblock copolymer is dissolved in warm 2-ethyl hexanol, a selective solvent for the midblock, and cooled to room temperature. As shown through dynamic rheological studies, this thermoreversible gel exhibits a linearly elastic response, thus, making it an excellent model system for JKR experiments. In these trials, hemispherical lenses formed from 5 to 10 volume percent polymer, are loaded and unloaded against a variety of substrates including flat, rigid surfaces coated with polystyrene and PMMA. Results obtained from measuring the contact area and lens displacement throughout loading cycles, indicate that these gels have no adhesion hysteresis and are completely elastic. Furthermore, by applying this fracture mechanics approach to this system, we are able to obtain estimates of the gel's modulus as a function of composition, which are consistent with rheological experiments for these highly diluted copolymers.

[J32.13] Modifying Surface Interactions With Random Copolymer Brushes

Random copolymers of polystyrene and polymethylmethacrylate, P(S-r-MMA), were end anchored onto a Si substrate. Solution casting films of hydroxy terminated P(S-r-MMA) onto a Si substrate followed by annealing at 140C and thorough rinsing with toluene resulted in the anchoring of the copolymer to the substrate with removal of residual, nonanchored copolymer. The kinetics of the anchoring process was measured by using ellipsometry and reflectivity to determine the time dependence of the film thickness. This can be related to the diffusion and attachment of the hydroxy endgroup to the surface. X-ray photoelectron spectroscopy and contact angle measurements were used to assess the variation in brush concentration as a function of distance from the free surface and as a function of P(S-r-MMA) composition. The effect of grafting density and molecular weight of the brush on the ability of the P(S-r-MMA) to modify the surface interactions will be discussed.

[J32.14] Block Copolymer Films on Neutral Substrates: Controlling Domain Orientation

Neutron reflectivity, SANS, and optical microscopy have been used to study the morphology of ordered P(dS-b-MMA) block copolymer films (Mw=50 kg/mol), on substrates coated with end-grafted P(S-r-MMA) random copolymer brushes. Three distinct morphologies were observed as f, the styrene content of the brush, was varied from 0 to 1. For f\geq0.65 or f\leq0.50, the lamellae orient parallel to both film surfaces, and an incomplete top layer forms for incommensurate thicknesses (island formation). The layer adjacent to the brush is composed of dPS or PMMA, respectively, for high f or low f brushes. When the interfacial energies of the brush with the dPS and PMMA blocks are nearly equal, 0.5\leqf\leq0.65, a novel morphology is seen. Adjacent to the brush, the lamellae orient perpendicular to the substrate, while adjacent to the vacuum interface they orient parallel to the free surface. The top surface of the film is flat in this case for all thicknesses (no island formation), since the thickness of the vertical lamellar layer can take on any value.

[J32.15] Development of Block Copolymers for Minimization of Adhesion in Aqueous Environments

The aim of our work is to develop polymers which, when applied to surfaces, can reduce the thermodynamic work of adhesion in aqueous environment to zero. We have designed a series of acrylic block copolymers which have an uncharged, hydrophilic block and one or two hydrophobic poly(methyl methacrylate) (PMMA) blocks. The hydrophilic block can have a lower surface energy than the anchoring PMMA block, and is expected to segregate to the top surface of the film, thereby avoiding the problems associated with reconstruction of the polymer surface when exposed to air. Block copolymers of PMMA and poly(t-butyl acrylate) (PTBA) are synthesized anionically, and ester interchange reactions are utilized to convert the PTBA block to a hydrophilic polymer. Triblock copolymers can form thermally reversible hydrogels, so that films with thicknesses beyond a monolayer will retain their structural integrity. The contact angles of water on these surfaces indicate that a hydrophilic layer is located at the free surface. The JKR technique based on the contact between the surfaces and low modulus gels is applied to measure the adhesive interactions in water.

[J32.16] Thermal Fatigue Testing of Polymer Interfaces

Fracture of epoxy underfill/polyimide protective coating (chip passivation) interfaces has been a major cause of failure in the direct chip attach (DCA) microelectronic packages. Cracks initiate at and propagate along the interface primarily due to the stresses induced by thermal cycling (during operation). We have developed a simple method to investigate the thermal fatigue crack growth rate da/dN and have used it to study the crack growth at a generic underfill/polyimide interface. The sample is prepared as a multilayered cantilever beam by capillary flow of the underfill over a polyimide coated aluminum sheet. Since most underfills are opaque, it is difficult to measure the crack growth visually. However crack growth along the interface from the free end changes the displacement of this end of the beam and we measure this displacement using a non-contact method at the lowest temperature in each thermal cycle. The change in beam displacement is readily converted into crack growth knowing that the bonded length of the beam is bent into a circle by the thermal stress while the unbonded length remains straight. The crack growth rate is correlated with the crack extension force amplitude due to the thermal stresses.

[J32.17] Mechanisms of Probe Tack Adhesion of Model Acrylic Elastomers

The adhesion mechanisms of model acrylate homopolymers and copolymers are studied with an instrumented probe tack test. A video camera positioned under the transparent glass substrate records the bonding and debonding process while the force displacement curve is acquired. This setup allows to couple the observation of the cavitation and fibrillation mechanisms, occurring during the debonding of the film from the stainless steel probe, with the mechanical measurement of stress and strain. The transitions between different debonding mechanisms are critically dicussed in terms of the bulk and surface properties of the adhesive and its molecular structure.

[J32.18] Neutron Reflectivity and Surface Circular Dichroism Study of Poly (dimethyl siloxane) Monolayers

We study poly (dimethylsiloxane) (PDMS) monolayers spread at the air/water interface using neutron reflectivity and surface circular dichroism techniques. Previous studies of surface pressure-area isotherms of PDMS were used to speculate that a series of conformational changes occur as the surface concentration increases. The surface density profiles of PDMS obtained from neutron reflectivity measurements reveal more detailed structural information of PDMS at the interface. As the surface concentration increases, the density of the monolayer continuously approaches that of the bulk polymer, while the thickness evolves via discrete steps. Surface circular dichroism is used to probe the possibility that these discrete steps correspond to conformational ordering of the chains into interpreted helices or stacked geometry.

[J32.19] Electrostatic self-assembly of polyions on charged substrates

The kinetics of formation of self-assembled monolayers is studied for polystyrene sulfonate(PSS) adsorbed on oppositely charged surfaces of amine terminated self-assembled monolayers(SAM) and polyallylamine(PAA). During the early stages of deposition in both cases, an inhomogeneous deposition is noted as measured by atomic force and friction force microscopy. Island formation of unperturbed PSS coils on defect sites is observed during the initial stage of deposition. Longer deposition times result in an equilibration of the polymer layers into highly flattened macromolecular chains. AFM and FFM measurements are combined with ellipsometer and X-ray reflectivity results to quantitate the layer thicknesses and roughness with time.

[J32.20] Reinforcement of Polymer Phase Boundaries by Mixtures of Diblock and Triblock Copolymers

The fracture toughness G_c of polystyrene(PS)/poly(2vinylpyridine) (PVP) interfaces reinforced with either the pure diblock dPS_510-PVP_540 or pure triblock PVP_290-dPS_470-PVP_290 copolymers is rather low at moderate areal chain density \Sigma = 0.1 chains/nm^2 even though a chain scission to crazing transition is observed at lower \Sigma. At large \Sigma where lamellar overgrowths have formed G_c for the triblock-reinforced interfaces is much larger than that for the diblock reinforced interfaces. Here we look at the reinforcement produced by 1:1 and 1:3 mixtures of the triblock and diblock copolymers at PS/PVP interfaces. The 1:3 mixture produces some increase in G_c over that of the pure diblock at \Sigma= 0.1 but little, if any, at large \Sigma. On the other hand the G_c for the 1:1 mixture is much larger than that for the pure diblock copolymer at equivalent \Sigma. Cross-sectional transmission electron microscopy of the interface shows that the lamellae that form at large \Sigma of the 1:1 mixture have many more defects than the lamellae formed from pure diblock copolymer and this difference is probably responsible for the strengthening observed. The defects may result from a slowing of the kinetics of lamellar organization caused by adding the triblock copolymer to the mixture.

[J32.21] Wetting Behavior in a Three-Component Polymer Blend

Previously, we presented interfacial segregation experiments on a three-component, two-phase A/BC polymer system, where B and C are miscible with each other but not with A. In this study, we aim to understand the transition from two to three phases in BC/A system. By judicious choice of blend thermodynamics, we are able to present the complete wetting of the C rich phase at both the polymer/polymer and polymer/air interfaces. A blend of SAN (B) and d-PMMA (C) thin film (3000Åwas deposited on a thick PC (A) bottom layer. Upon annealing at 200\degC where AB phase separates, the depth profile was examined using forward recoil spectrometry (FRES). It was found that the d-PMMA rich phase wets both the PC and air interfaces. Thus a three-layer structure was constructed from the original SAN and d-PMMA blend. From the morphology point of view, the d-PMMA rich phase encapsulates the SAN rich phase in the PC matrix. The kinetics of the growth of the wetting layers will! a! lso be presented as a function of temperature and SAN composition. The wetting behavior we report here thus provides a novel approach for tailoring the morphology in three-component, multiphase polymer blends, as well as for polymer thin film technologies.

[J32.22] Surface Induced Metastability in Polymer Solutions

Thermodynamic properties of materials are significantly different near a surface as compared to those in the bulk because of additional constraints at the surface. This difference is further enhanced for polymers due to their connectivity. We study the phase behavior of polymers in solution near various types of surfaces, using a simple lattice model which is solved on a tree structure. We find various transitions which can be either confined to the surface or it can be a simultaneous surface and bulk transition. We also find that it is possible to produce a metastable state in the bulk by controlling the surface properties. We are able to predict surface segment density, surface free energy and surface entropy for polydisperse linear polymers ans randomly branched polymers in interacting solutions.

[J32.23] Penetration Of Free Chains Into Tethered Chain Layers In Good Solvent Conditions: An Experimental Study Involving Langmuir Diblock Copolymer Monolayers

Neutron reflectivity is used to study the penetration of free polymer chains into a layer of tethered chains under good solvent conditions. The tethered chain system is a Langmuir monolayer of highly asymmetric PDMS-PS diblock copolymers on the surface of ethyl benzoate. The small PDMS blocks anchor the copolymers to the surface while the larger PS blocks dangle into the subphase liquid. By varying the deuteration scheme, the segmental concentration profiles of both the dangling PS blocks and the free PS chains in solution are determined. For 170K tethered PS blocks and 40K free PS chains, and a free chain concentration of 0.06 g/ml, the free chains are largely squeezed out of the brush as the surface density of tethered chains is increased from the dilute regime to the strongly interacting regime.

[J32.24] Characterization of Adhesion in Pressure Sensitive Adhesives with a Spherical Indenter

A combination of interfacial effects and bulk viscoelastic behavior controls the performance of pressure sensitive adhesives. We have studied these issues using commercially available adhesive transfer tapes. Using a spherical indenter, these experiments measure the displacement of the indenter into the adhesive as a function of the applied normal contact force. With the assumption of linear elastic behavior under small displacements, accepted theories of this contact problem are used to approximate the contact area and calculate a plateau modulus from the loading data. Fracture mechanics approaches based on linear elasticity also allow us to calculate the energy release rate (i.e. driving force for adhesive failure) from the load/displacement relationship measured during unloading. To alleviate uncertainty in the calculation of the plateau modulus and energy release rate for thicknesses not within the ranges of accepted theories, a finite element model simulates the behavior of the polymer layer upon loading and pull-off of the indenter. Future work will include modeling the adhesive with a viscoelastic constitutive model and characterizing the effects of geometry, substrate material, and loading rate on the adhesive properties.

[J32.25] Interfacial Widths and Interfacial Segregation in Poly(styrene-co-methyl methacrylate) Random Copolymer Systems

The interfacial widths between poly(styrene-co-methyl methacrylate) (P[S-ran-MMA]) and deuterated polystyrene (dPS) or deuterated poly(methyl methacrylate) (dPMMA) were investigated using neutron reflectivity. Random copolymers of 0.49, 0.73, and 0.85 mol.% styrene were used to examine the effect of copolymer composition. After a 4 hour anneal at 150 ^oC, the interfacial width between the compositionally symmetric random copolymer (0.49) and dPS was smaller than with dPMMA. This result is consistent with our previous blend miscibility results which exhibit asymmetric miscibility for the compositionally symmetric (P[S-ran-MMA]). Interfacial segregation was studied in bilayer samples containing of a dPS film and a blend of dPMMA and 5 or 10% P[S-ran-MMA]. Copolymer segregation to the interface between dPS and dPMMA was found in all samples as prepared and after annealing. -----------------------------------------------------------

[J32.26] Using Heterogeneous Substrates to Control Diblock Copolymer Morphology

Patterned substrates comprised of sub-micron metallic lines on silicon wafers have been prepared with non-lithographic techniques, in order to study the behavior of blends and diblock copolymers at heterogeneous surfaces. An effort has been made to capitalize on the surface segregation effects afforded by the preferential interactions between specific components of a diblock copolymer or blend, and a distinct surface. Whereas thin diblock films form lamellar domains parallel to a uniform silicon substrate, patterned substrates should drive the lamellar formation normal to the plane of the substrate. The strength of the driving force and consequently the effectiveness of lamellar reconstruction is strongly dependent on the degree of commensurability between the diblock or blend molecular weight and the metal/oxide linewidth, which varies between 60 and 100 nm. Results from films of several molecular weight blends and block copolymers will be discussed.

[J32.27] Relaxation Dynamics and Small Molecule Diffusion in Thin and Ultrathin Polymer Films

Deviations from bulk polymer behavior have been observed in ultrathin polymer films (less than 200 nm thick). Using second harmonic generation (SHG) characterization of probe reorientation dynamics (from \mus to days) shown previously to be fully coupled to the cooperative segmental dynamics of the polymer \alpha-relaxation, a broadening of the relaxation time distribution is seen in ultrathin polymer films as compared to bulk films. This broadening could reflect increasing heterogeneity in local-scale polymer chain environment as film thickness approaches chain dimensions and cause an enhancement of small molecule translational diffusion. A novel experimental technique involving fluorescence nonradiative energy transfer (NRET) is being employed to study diffusion of small probe molecules in ultrathin polymer films. Probe translational diffusion will be compared to average rotational relaxation times measured using SHG characterization above and below T_g. Studies on thin films have shown that probe translation is less temperature dependent than rotation above T_g while they have similar temperature dependencies below T_g. Effects of film thickness and polymer-substrate interactions on average relaxation times and on the temperature dependence of diffusion and segmental relaxation will be discussed.

[J32.28] Surface Modification and Etching of Polystyrene Films in Oxygen and Argon Plasmas

Surface modifications of polystyrene films in oxygen and argon plasmas have been performed. The etching rate of polystyrene films in argon plasma is much lower than that in oxygen plasma, but demonstrates similar dependence on gas pressure. In both oxygen and argon plasmas, the etching rate increases sharply with the decreasing pressure in the low pressure range as a result of dominant ion sputtering. The sample loading effects were investigated in both oxygen and argon plasmas. It showed that the influence of sample area on etching rate was significant in argon plasma in both low and high pressure ranges while in oxygen plasma such trend was observed only in high pressure range. The surface wettability was studied using contact angle measurement. The contact angles were found close to 0 degree shortly after plasma treatment, and wettability degradation was detected after long time aging. The surface morphology was examined with atomic force microscopy.

[J32.29] Complex formation in rare-earth metal ion doped thin films of chitosan

A systematic study has been made on the influence of doped rare-earth metal ions (Er+++ and ND+++) on the molecular interactions present in thin polymer films fabricated from chitosan-dilute acetic acid solutions (chitosan/HAc). FTIR spectroscopy (including NIR, MIR and FIR) coupled with XPS indicate a weak complexation between the metal ions and amine groups of chitosan. Specifically, the FIR spectra show broad bands near 250, 480 and 550 cm-1 for the metal-doped films suggestive of metal ion-ligand vibrations. XPS results indicate that multiple chemical states of N with an increased percentage of a higher binding energy state nitrogen were caused by a weak interaction with the doped metal ions. Slight differences in microroughness between the doped and undoped films as observed by X-ray reflectometry may also be associated with the doping. The NIR and MIR spectra do not show any significant changes for all the doped and undoped films, implying that the basic molecular conformation of chitosan polymers is not changed by the weak complexation.

[J32.30] Annealing Effects in Charged Pinned Micelles.

Using scaling arguments and 2d numerical self-consistent field model, we investigate the effect of charge annealing on the equilibrium structure of charged pinned micelles. Annealing of charge is known to provide an "abnormal" type of behavior in laterally homogeneous polyelectrolyte films. We focus on the case of laterally inhomogeneous layers. We consider weak polyacid molecules sparsely tethered onto an inert planar substrate and immersed in a salt-free solution. Similar to the case of quenched polyelectrolytes, one finds pinned micelles under poor solvent conditions. However, the scaling dependences for the micelle parameters are different from those for a quenched system. We analyze also how the annealing of charge affects the transition into a homogeneous state of the layer.

[J32.31] Simulations of the Interaction of Small Molecules with Styrene-Butadiene and Styrene-Butadiene-Acrylonitrile Polymer Thin Films

We have simulated atomistically the structure and energetics of Styrene-Butadiene(SB) and Styrene-Butadiene-Acrylonitrile(SBA) random copolymer thin films. The films attain bulk density in their interior region. The surface layer is about 5 ÅThe orientation of the backbone bonds is random in the interior region of the films, while orienting preferentially in the direction of the plane of the film at the surface. The surface energy for the SBA films is significantly higher than for the SB films and the reason was found to be the preferential location of Acrylonitrile groups on the surface. The comparison of density and surface energy with experimental data is very good. We studied the interaction of toluene, water and hexadecane with SB and SBA films. At small concentrations, toluene is found to wet the films and spread uniformly with random structural orientation. Hexadecane also wets the surface, its chains showing an extended conformation. The water molecules are seen to form drops and do not prefer the SB and SBA surfaces. We will present results of wetting and non-wetting interaction energies, surface coverage, and conformational characteristics of these molecules and low molecular weight surfactants at different concentrations of these molecules on the films.

[J32.32] Viscoelastic Responses in Partial Crosslinked Epoxy During Physical Aging

Crosslink polymers are of particular interest for many structural applications with improve thermal properties. In this study, a mixture of 1,6 hexanediamine and heptylamine was crosslinked with a di-functional epoxy resin. Depending on the molar ratio of diamine and monoamine used, the glass transition can be controlled. The network shows a sharp glass transition. Step-strain stress relaxation experiments were performed on two networks with different degree of crosslink. Effect of physical aging on viscoelastic responses at several temperatures were also examined. Time-Aging Time-Temperature superposition principle was investigated. The effect on rate of aging as affected by temperature was also studied and results will be presented.

[J32.33] Conformational study and atomistic modelling of polybenzoxazine

A Rotational Isomeric State(RIS) model for polybenzoxazine(PBO) of which the repeating unit has a form [-Ph(OH)(CH_3)-CH_2-N(CH_3)-CH_2-], with substituents on the aromatic ring arranged with OH para to CH_3, and OH ortho to CH_2, has been obtained. The torsional state of the C(sp)-O bond is treated as a variable in designing the RIS model. Atomistic models of amorphous PBO were obtained from the RIS-Monte Carlo PBO single chain by carrying out molecular mechanics and molecular dynamics under cubic periodic boundary conditions at bulk density of 1.1 g/cm^3. The cohesive energy density and solubility parameter are calculated. The amorphous structure was analyzed using the radial distribution function and orientational correlation of the phenyl ring and free volume fraction with respect to the probe size. Atomistic PBO thin films of thickness of ~28 Åwere constructed in vacuum by employing two-dimensional periodic boundary conditions on the above equilibrated bulk parent chain. The density profile and composition distribution profile with respect to the distance from the center of mass were obtained.

[J32.34] Infrared Study of Curing in Bismaleimide Resins

Bismaleimide resins based on Bismaleimidodiphenylmethane and Diallyl Bisphenol A are used currently in fiber-reinforced composites for many aerospace applications. In general these composites are cured at temperature between 200 C to 250 C. However, "full" cure can not be attained due to the diffusion control mechanism at late stage of network formation. In this study, the curing of BMI resins with different molar ratio were examined using fourier transform infrared spectroscopy. In addition, mono-functional hardener was also used to probe the detail curing mechanisms. The advancement in glass transition temperature is also characterized based on isothermal frrequency sweep dynamical mechanical technique. The rate of glass transition advancement is to be with infrared results.

[J32.35] Correlations Between Positron Annihilation Lifetime Spectroscopy and Small Angle X-Ray Scattering in Glassy Polymers

Positron Annihilation Lifetime Spectroscopy (PALS) is a structural technique capable of quantifying some aspects of the glassy state in polymeric materials. Specifically, PALS enables one to sample the so-called free volume, or the nanometer-sized voids, within the glass. As with any analytical technique, results that can be corroborated by various other experimental methods are very reassuring and helpful from a calibration point of view. To this end, we believe that we have established the first experimental evidence linking small angle x-ray scattering (SAXS) and PALS. The SAXS technique is well established as a method capable of measuring inhomogeneous regions of electron density. By performing both the PALS and the SAXS measurements as a function of temperature on PMMA, we observe both the radius of the SAXS heterogeneities and the PALS average hole radius to change in a similar manner. This suggests that there is a direct correlation between what PALS and SAXS are capable of measuring.

[J32.36] Chain Conformation of Amorphous Substituted Poly(paraphenylenes)

High molecular weight, substituted poly(paraphenylenes) (SPPP) are emerging as a family of heat-resistant, high modulus resins which have many potential military and civilian applications. In addition to a high thermal-oxidative stability (500 C) and a large tensile and flexural moduli (7-10 GPa), the molecular architecture, para-linked phenyl units with various benzyol substituents randomly distributed along the backbone, leads to amorphous polymers exhibiting traditional thermoplastic behavior. Given the rigid nature of the paraphenylene backbone, these structural characteristics are surprising. Thermomechanical experiments and scattering experiments (small-angle neutron scattering and wide-angle x-ray diffraction) have been used to determine the effect of repeat-unit orientation and substituent size on the conformation of the polymer chain. Understanding the influence of molecular architecture on chain conformation in these systems will lead to new structure-property relationships that may be applicable to the design of other high-performance rigid-rod systems.

[J32.37] Response of Polymer Blends to Oscillating Pressure Fields

The response of a polyolefin blend to oscillating pressure fields were studied by in-situ small angle neutron scattering. This blend exhibited a lower critical solution pressure. The pressure was cycled so that the sample oscillated from the one-phase to the two-phase region. At small quench depth and low frequency, the scattering intensity at all scattering vectors was in-phase with the applied field. Deviations from this simple behavior was observed with increasing quench depth and/or decreasing frequency.

[J32.38] Domain Shape Transitions in Emulsified Polyolefin Blends

The shape of dispersed domains in emulsified polyolefin blends was studied by transmission electron microscopy. Model polyolefins - polyethylene (PE), head-to-head polypropylene (PP), and PE-PP diblock copolymers - were synthesized via anionic polymerization. We find that increasing block copolymer concentration leads to an abrupt transition from spherical domains to cell-like domains with faceted walls. The cell-like domains were aggregated and formed large macrophases. The characteristic size of the domain-rich phase was larger than the size of the PE droplets formed in the absence of the block copolymer. This indicates strong adhesive interactions between the domains.

[J32.39] Molecular Dynamics Study of Uniaxial Deformation of Amorphous Polyethylene

We employ a recently developed extended ensemble molecular dynamics simulation method to study the constant strain rate, uniaxial deformation of an amorphous polymer. The resultant stress-strain curves show the same general features as observed in experiment: linear behavior at low strains followed by macroscopic yield. Increasing strain rate raises the yield stress. The density of the material decreases (increases) during tensile (compressive) deformation. The microscopic structural changes associate with abrupt drops in the stress-strain curve are identified. These changes are generally associated with torsional bond angle relaxation events. Localized structural changes also occur in the "linear" regions of the stress-strain diagram. Macroscopic yield occurs when the deformation in these localized regions become sufficiently cooperative and correlated.

[J32.40] Shear Deformation Behavior of Modified Poly(Phenylene ether)

This abstract was not submitted electronically.

[J32.41] The Distribution of Relaxation Times of the \alpha-relaxation of PVDF

The dielectric constant of the \alpha-relaxation of semi-crystalline poly(vinilidene fluoride) (PVDF) in the \alpha-phase has been measured as a function of frequency and temperature and master curves at different reference temperatures have been obtained. This results has been analyzed in three ways: 1) assuming a distribution of relaxation times using the Havriliak-Negami (HN) model; 2) obtaining by integration of the imaginary part of the dielectric constant, \epsilon ^\prime\prime(ømega), the decay correlation function, \phi (t), and fitting it to the Kohlrausch-Williams-Watts (KWW) expression and 3) by applying the Simulated Annealing Direct Signal Analysis (SADSA) to obtain from \epsilon ^\prime\prime(ømega) the distribution of relaxation times G(\ln(\tau)) without the need of assuming a functional dependence for the distribution function. It was found that both, the HN and the KWW models could be used to describe the experimental results for the \alpha-relaxation of PVDF and the parameters of both distributions as a function of temperature do not vary with the temperature suggesting that the distribution of relaxation times does not change in the temperature range studied in this work, which justifies the use of the time-temperature superposition principle. Also comparisons are made between the ``real'' distribution function obtained by SADSA and the HN and KWW distribution functions.

[J32.42] Effect of Confinement on the Glass Transition of Semi-Crystalline BPAPC

The glass transition temperature, T_g, has been measured by Thermally Stimulated Depolarization Currents (TSDC) and Dynamic Scanning Calorimetry (DSC) for a series of Bisphenol-A Polycarbonate specimens spanning the crystallinity range from amorphous to 21.8%. Both characterization techniques show a decrease in the glass transition temperature as the sample's crystallinity increases. This result has been interpreted as a consequence of the confinement of the glassy amorphous material inside the crystalline regions. This striking size effect is due to the reduction in the space among the crystallites to sizes less than the characteristic length of the cooperative rearrangements, \xi, at T_g. The chain segments diffusion at T_g over distances lower than \xi results in the observed T_g decrease as the crystallinity increases. The confinement of the amorphous phase is also observed by the broadening of the dynamic mechanical spectra of the \alpha relaxation in the crystallized BPAPC samples. The interference with the size of the cooperativity at T_g by the crystallites, allows to explain the changes observed in this semi-crystalline polymer without the need to invoke the immobilization of amorphous chain segments by the connections to the crystals.

[J32.43] Structure and Morphology of Mesophases Formed by Supramolecular Assembly of Dendrimers

Electron microscopy has been used to investigate structure and morphology of the columnar hexagonal mesophase formed by dendrimers based on molecular engineering of a second generation monodendron, denoted G2-ABG. The column diameter (~ 44-52
) was measured as a function of core group architecture and thermal history. The mesophase also could be aligned by appropriate surface conditions, facilitating identification and characterization of the mesophase. Homeotropic ordering of columns was obtained on a carbon substrate, whereas in-plane orientation was found on a glycerin (or water) surface. The planar texture was typified by long continuous curved columns, which suggests flexible molecular stacking within the columns. This morphology was compared to that of similar fluorinated compounds. Various defects that can occur in the system have been observed.

[J32.44] Reversible Gelation of Arborol Dendrimers

Reversible gels of arborol didenron polymers were studied by light scattering, thermal analysis, freeze-fracture electron microscopy, and small angle X-ray scattering. The results are consistent with self-assembly of the molecules into very long, stiff linear arrays. The spacing between the molecules in the stacks was measured.

note

[J32.45]

This abstract was not submitted electronically.

[J32.46] Solvent-dependent swelling of PAMAM starburst dendrimers

The molecular dimensions of PAMAM (poly(amido amine)) dendrimers in solution are found to depend on solvent quality. The hydrodynamic properties of the dendrimers are observed by holographic relaxation spectroscopy and dynamic light scattering. For low generation dendrimers the hydrodynamic radii in water, methanol and butanol are similar, indicating a compact structure of the molecules which are only little penetrated by solvent molecules. For higher generations a significant swelling under good solvent conditions is observed. This finding supports theoretical models that predict a high density core followed by a region which is accessible to solvent molecules, and where the monomer density of the dendrimer is dependent on the solvent quality.

[J32.47] Solubility of Dendritic Polymers

The recent introduction of dendritic polymers into the commercial market has lead to increased interest in the physical properties and potential applications of these nanomolecules. The expense of these materials has encouraged exploration into their use as additives in hybrid polymer and polymer/solvent systems. Successful incorporation of dendrimers and hyperbranched polymers into hybrid systems depends on their ability to mix with other system components; however, little is known about the solubility of the new commercial materials or their compatibility with commodity plastics. This issue is addressed in the current study, where the solubility of commercially produced dendritic and hyperbranched polymers in common solvents is explored through prediction and experiment. The influence of core chemistry, end-group chemistry, and molecular weight are considered.

[J32.48] Novel Dendrimer Crystals

This abstract was not submitted electronically.

[J32.49] A SAXS Study of Correlations within Dendritic Polymer Systems

The intramolecular/intermolecular correlations within various dendrimer systems have been investigated with SAXS. Polyamidoamine, Polyol, and polyethyloxazoline molecules were examined while in solution, as part of blends, and as part of interpenetrating polymer networks (IPNs). The spherelike shape of the dendrimer is contrasted with the more starlike shape of the dendrigaft polymers, while hyperbranched polymers exhibit considerable amount of polydispersity. Additionally, IPNs where identified that exhibited correlated spacings between dendrimers in the polymeric matrix.

[J32.50] Percolation Transition and Condensation in Branched Polymers

We consider a lattice model of branched polymers containing tri-functional branches. As shown earlier(P.D. Gujrati, J. Chem. Phys. 98, 1613 (1993)), there is a line of first- order condensation polymerization transition, terminating in a critical point. A line of percolation transition emerges out of the polymerization line. We have numerically obtained these two lines. We present our results and various interesting features.

[J32.51] Interplay of crystal structure, morphology and energetics in syndiotactic polypropylene

Since the recognition that the stable structure of syndiotactic polypropylene (sPP) is based on full antichiral packing of helices(Lovinger et al, Macromolecules, 26, 3494 (1993) ), a number of studies have confirmed the packing scheme, refined the original model, and proposed various types of structural disorders. The most recent models are motivated by our discovery of c-axis related defects and involve structures resulting from energy minimization through potential energy calculations or from refinement of X-ray data from bulk specimens. These models introduce shifts and/or rotations in the molecular helices along and transverse to the chain axes. We examine and discuss these various models based upon detailed, high resolution evidence from hk0 and hkl single-crystal electron-diffraction data. Further implications of antichiral molecular packing and packing defects are illustrated with self- and heteroepitaxy, and growth sector-dependent properties. 1

Part J of program listing