We have used Brillouin light scattering and ellipsometry to compare the glass transition temperature T_g of freely-standing films (FSFs) of polystyrene (PS) with T_g values obtained for PS films with one and two surfaces confined by SiO layers. These measurements have been performed as a function of film thickness h for two different molecular weights. For FSFs the reductions in the T_g value compared with the bulk value depend on both the film thickness(J.A. Forrest, K. Dalnoki-Veress, J.R. Stevens and J.R. Dutcher, Phys. Rev. Lett. 77), 2002 (1996). and the molecular weight of the polymer, and are as large as 65 K for h = 29 nm. This is the first study in which reductions in the glass transition temperature of polymer thin films due to chain confinement have been observed directly. Also, we find that the T_g reductions for FSFs are much larger than those observed for films with either one or two confining surfaces which have T_g reductions < 10 K for h > 30 nm. Surprisingly, there is little difference between the results for films with one and two confining surfaces: the confinement of only one film surface dramatically suppresses the large T_g reductions that are observed for FSFs.
[F29.02] Polymer Interdiffusion near an Attractive Surface
Eric Lin, Wen-li Wu (NIST)
The interdiffusion between deuterated and hydrogenated poly(methyl methacrylate) (PMMA) bilayers on a silicon substrates are studied using neutron reflectometry. Lower layer thicknesses are prepared ranging from 0.4 to 3.6 radii of gyration (R_g) of the polymer chain. The concentration profiles of the deuterated layer were measured at different times after annealing above the polymer glass transition temperature. The rate of interdiffusion decreases significantly as the lower layer size decreases. The thinnest layers have effective diffusion constants almost 2 orders of magnitude smaller than that of the 3.6R_g layer. The results here suggest that the attractive interaction energy between the polymer and the substrate affects the chain dynamics at least 2R_g from the substrate surface. The change in the polymer dynamics is strongly affected by the initial chain conformation in the confined geometry of the initial film sizes.
[F29.03] Confinement effects on diffusion in polymer multilayers
R.A.L. Jones, E. Sivaniah, M. Sferrazza (Cavendish Laboratory, University of Cambridge, UK), J. Penfold (Rutherford Appleton Laboratory)
We have measured the collective diffusion coefficient of polystyrene and deuterated polystyrene on length scales approaching and less than the radius of gyration of the polymer chains. This has been achieved by preparing multilayers of alternating thin films of polystyrene and deuterated polystyrene; neutron reflectivity is used to follow the decay, on annealling, of the Bragg peaks corresponding to the fundamental and harmonic spatial frequencies of this artificially created composition modulation. The decay of the composition modulation is not well described by a single exponential relaxation, and the collective diffusion coefficient extracted from the long time limit of the decay is smaller than the bulk diffusion coefficient, and decreases with increasing wave-vector.
[F29.04] Relaxation of Polymeric Interfaces below the Glass Transition Temperature
Charles Laub (U. of Connecticut), Alamgir Karim, Sushil Satija (NIST), Gian P. Felcher (Argonne National Laboratory)
We observe by neutron reflectivity a rapid relaxation at the interface of bilayers of polystyrene when heated at a temperature just below the glass transition temperature (about100 C) of the polymer. The effect is truly a relaxation to be distinguished from the interdiffusion that takes place above the glass transition temperature. Bilayers were prepared by spin coating the bottom layer and floating on the top layer. Two sets of samples were prepared with a floated layer 100 nm thick: one with a relatively thick (45 nm) bottom layer, the second with a bottom layer 15 nm thick. The first set comprised a batch of four different matched molecular weights (from 50k to 1000k). A first relaxation was seen at an annealing temperature of 60 C, but no further relaxation took place at temperatures below 90 C. At the last temperature the interfaces of all matched molecular weights relaxed over a weight-independent thickness of 4 nm. The second set of samples (with thin bottom layer) relaxed at lower temperatures: at 87.5 C the interface was as thick as the layer itself . Further experiments are required to pin down the details of the relaxation phenomena and the transition from relaxation to interdiffusion in thin polymer bilayer films.
[F29.05] Surface Relaxation of Polystyrene Films
Y. Liu, T. P. Russell (Department of Polymer Science and Engineering, University of Massachussetts, Amherst, MA), J. Diaz, A. Cossy-Favre, M. G. Samant, J. Stohr (IBM Almaden Research Center, San Jose, CA), H. R. Hugh (University of Wollongong, Australia)
Near edge X-ray adsorption fine structure , NEXAFS, spectroscopy was used to investigate the relaxation of a supported polystyrene film near a free surface after the imposition of a small deformation. The NEXAFS dichroic ratio was obtained for both Auger and total electron yield processes as a function of temperature to determine the orientation of the polymer in the first 1 and 10 nm from the free surface. Complete relaxation of the polymer was not seen for temperatures less than the bulk Tg. No evidence of enhanced mobility at the free surface was found . A planar relaxation of the polymer was found in the first nanometer from the free surface, whereas in the first 10 nm, the dominant relaxation was normal to the surface.
[F29.06] Friction Measurements of mica versus High Polymers at the Glass Transition Temperature using the Surface Forces Apparatus
Manfred Heuberger, Gustavo Luengo, Jacob Israelachvili (Dept. Chem. Eng., UCSB, Santa Barbara, CA 93106)
We are using a bimorph-driven translation stage and a lateral spring in the surface forces apparatus to measure the friction of high polymers like PMMA and PBMA. Plastic deformations of the polymer around the contact zone can be observed during sliding using a newly developed extension of the optical FECO technique. We report on a dynamic transition from smooth sliding to stick-slip motion that occurs in the initial stages of sliding and suggest the presence of dissipative mechanisms on the molecular level. We link our observations to the well known Schallamach waves which were previously observed and described in macroscopic tribo-systems involving rubber sliding on a hard wall.
[F29.07] Viscoelastic Properties and Slip Behavior of Confined Polymer Melts
Rajesh Khare, Juan de Pablo, Arun Yethiraj (Departments of Chemistry and Chemical Engineering, University of Wisconsin, Madison WI 53706.)
We investigate the viscoelastic properties and wall slip behavior of confined polymers using a combination of continuum mechanics and molecular simulations. Oscillatory shear flow molecular simulations are used to obtain the frequency dependence of the loss and storage moduli for simple fluids and polymer melts. Simple fluids display a frequency at which the fluid behavior changes from being predominantly viscous to predominantly elastic. In contrast, polymer melts show predominantly elastic behavior at all frequencies investigated. The slip behavior of bead-spring chains sheared between FCC lattice walls is compared to that of a realistic (united atom) model of polyethylene sheared between graphite walls. Results are similar except that the polyethylene fluid sometimes displays more than one slip plane which is not observed in the bead-spring chains. The effects of chain topology, relative magnitudes of polymer-polymer and polymer-wall interactions, and confinement on viscoelastic properties and slip behavior are also investigated.
[F29.08] Dynamics of polymers in nanoscopic confinements.
Evangelos Manias (Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853), Emmanuel P. Giannelis (Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853)
Layered-silicate based polymer nanocomposites in the form of intercalated hybrids represent new model systems to study the dynamics of polymer chains in extreme --nanoscopic-- confinements or at the solid-polymer interface. The structure of the nanocomposite materials is a multilayer with alternating polymer and mica-type silicate layers with a periodicity of a few nanometers. The effect of the wall-polymer interactions on the intercalation kinetics of the polymer chains between the layers is explored by X-ray Diffraction and Rutherford Backscattering analysis. Furthermore, for these highly adsorbed coils in nanometer geometries, the chain and segmental dynamics of the polymers are further investigated by Differential Scanning Calorimetry and Nuclear Magnetic Resonance.
[F29.09] Polymer Dynamics at the Polymer-Polymer Interface
K. A. Welp, R. P. Wool (University of Delaware, Dept. of Chemical Engineering, Newark, DE, 19716)
We explore the interdiffusion of centrally labeled (50%) chains (HDH) with end labeled (each 25%) chains (DHD) of poly(styrene) with neutron reflection. The D-depth profile observed exhibits a characteristic 'Ripple' behavior(G. Agrawal, R. P. Wool, W. D. Dozier G. P. Felcher, J. Zhou, S. Pispas, J. W. Mays, and T. P. Russell, J. of Polym. Sci. Part B, 1996, 34.). New experimental data on high molecular weight (400k) material is presented. The depth profiles measured are compared to predictions based on reptation, Rouse and Mode Coupling(C. J. Grayce, G. Szamel, and K. S. Schweizer, J. Chem. Pys. 1992 102 2222.) dynamics. Phenomenological predictions based on recently proposed models are also discussed. Comparisons of magnitude, time dependence of the observed and predicted profiles strongly supports the reptation model for polymer dynamics in the melt.
[F29.10] Flow-Induced Surface Zippering of an Adsorbed Polymer Chain
M.M. Kohonen, David R.M. Williams (Institute of Advanced Studies, The Australian National University, Canberra ACT 0200, Australia.)
In many practical situations adsorbed polymers are exposed to shear flows. We study the dynamics of a grafted polymer chain in good and theta solvents in the presence of weak surface adsorption and an applied shear flow. For zero flow the chain undergoes a well-known adsorption transition at a critical adsorption energy per monomer. In the presence of an applied shear flow the chain stretches along the surface. Due to non-linear terms in the stretching response this results in a confinement of the chain in the direction perpendicular to the surface. This confinement increases the adsorption. These effects result in a "zippering" transition, whereby the chain absorbs in a line along the surface. We investigate the statics and dynamics of this transition both theoretically and by computer simulation.
[F29.11] Molecular Dependence of Reorientation in Unidirectionally Rubbed Surface Alignment Layers
Geoffrey Hietpas, James M. Sands, David Allara (The Pennsylvania State University)
Films of poly(biphenyl dianhydride-p-phenylenediamine) (BPDA-PDA), ranging in thickness from 5-300 nm and supported on native-oxide-covered Si substrates, were unidirectionally rubbed at a loading of 2g/cm2 for 300 cm, conditions similar to those used in the preparation of flat panel liquid crystal displays. High precision polarized infrared difference spectroscopy measurements, supplemented by spectral simulations, show evidence for a reversible chain reorientation in an ~1-3 nm thick surface layer in which well-ordered chains in initial, randomly oriented domains align parallel to the rubbing direction. The data further show that the responsive domains consist predominantly of chains with planar, unperturbed structures while domains containing perturbed, non-planar imide rings are largely unresponsive to buffing. Chain re-alignment is observed to be a molecular-scale roughening process in which extensive perturbations of the imide ring structures arise, including out-of-plane rotation of the rings and bond distortions. A mechanism is suggested in which steric interactions arise between adjacent chains in the planar stacking layers during chain reorientation.
[F29.12] Relaxation of Liquid Crystal Alignment Layers
David C. Rich (), Enid K. Sichel, Peggy Cebe (Tufts University)
A new method for investigating thermophysical transitions in liquid crystal alignment layers is discussed. The technique involves curing a set of alignment films at an array of temperatures after the films have been brushed with a cloth, but before liquid crystal cells are constructed from the films. When a thermal transition in the polymer is initiated by a post-brush cure, the aligning ability of the brushed films is destroyed. The technique is demonstrated using polyamide- imide, PMDA-APB polyimide, poly(phenylene ether sulfide) and PVA poly(vinyl alcohol) alignment films. The technique is advantageous for examining brush-aligned surfaces which, due to surface roughness, can not be examined using conventional ellipsometry .
[F29.13] Investigation of Thermomechanical and Viscoelastic Phenomena in the Curing of Rigid-Rod Polyimide Thin Films.
C.K. Chiang, Aime S. De Reggi (Polymer Division, NIST, Gaithersburg, MD.)
Two types of polyimide thin films were prepared by spin-coating precursor solutions on silicon wafers and curing in a temperature cycle extending to 350\degC. One type of polyimide was unoriented and served as control, while the other was oriented relative to the silicon surface due to its rigid rod molecular conformation. The orientation resulted in anisotropic thermomechanical properties after cooling from 350\degC to room temperature, which is indicated by wafer-curvature measurements throughout the temperature cycles. A curvature peak was observed in the solvent-loss temperature region beginning around 80\degC with time dependent effects.