Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B4: Glassy Polymers |
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Sponsoring Units: DPOLY Chair: Alexei Sokolov, University of Akron Room: LACC 515A |
Monday, March 21, 2005 11:15AM - 11:51AM |
B4.00001: The Distributions of Tg Values and Physical Aging across Thin and Ultrathin Polymer Films and within Polymer Nanocomposites Invited Speaker: Polymeric glass formers can exhibit amazing changes in glass transition temperature, Tg, relative to bulk when subject to nanoconfinement. At present there is no detailed understanding of this effect nor, until recently, was there a method to determine how far from the surface or interface these effects propagate into the glass former. We have developed a simple, fluorescence / multilayer method that has yielded the first determination of the distribution of Tg values across supported polymer films, revealing that the enhancement of dynamics at a surface affects Tg several tens of nanometers into a polystyrene (PS) film. The extent to which Tg dynamics smoothly transition from enhanced to bulk states depends strongly on nanoconfinement. When films are sufficiently thin that a reduction in thickness leads to an overall Tg reduction, the surface-layer Tg actually increases with a reduction in overall thickness whereas the substrate-layer Tg decreases. These results indicate that the gradient in Tg dynamics is not abrupt and that the size of a cooperatively rearranging region is much smaller than the distance over which interfacial effects propagate. There is no MW dependence of the Tg-nanoconfinement effect in PS; thus, the effect cannot be attributed to radius of gyration, segregation of chains ends to the free surface, or entanglement reduction. However, the effect is strongly dependent on added diluent, repeat unit structure and attractive polymer-substrate interactions: added diluent reduces or even suppresses the effect; poly(4-tert-butylstyrene) exhibits a Tg reduction at a thickness of 300-400 nm, far beyond the thickness at which Tg reductions are observed in PS; and poly(2-vinyl pyridine) (P2VP), which can undergo hydrogen-bonding with hydroxyl units on the surface of glass, exhibits enhancements in Tg at thicknesses below 300 nm. We have also developed an approach for characterizing physical aging in thin and ultrathin films, revealing that the distribution of physical aging is distinct from that of Tg. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:27PM |
B4.00002: Mechanical heterogeneity in bulk, thin-film, and nanocomposite polymeric glasses Invited Speaker: Ultrathin polymer films and nanoscopic polymer structures represent a particularly interesting class of systems for study of the glass transition and cooperative dynamics. Such systems provide a unique test bed to test the limits of validity of traditional continuum mechanics descriptions, and are relevant in technologies being considered for mass-production of next-generation electronic devices. A complete understanding of material properties at nanometer length scales and, in particular, mechanical properties, is crucial for development and optimization of such technologies. Recent experimental data suggest that the thermophysical properties of polymeric materials in small geometries differ from those of the bulk. We use experiments and molecular simulations of polymeric molecules to analyze and explain the origin of such behavior. The overall picture that emerges from our studies is that amorphous, glassy polymeric materials exhibit heterogeneous mechanical properties in nanoscopic structures and in the bulk. Domains measuring several nanometers can have elastic moduli that differ by an order of magnitude. These properties exhibit a Gaussian distribution, whose mean and width vary considerably depending on the geometry of the sample, the presence of surfaces, and the presence (and characteristics) of filler nanoparticles. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 1:03PM |
B4.00003: Kinetics and Thermodynamics of the Glass Transition: Kovacs and Kauzmann Revisited Invited Speaker: The tau-effective paradox was observed by Kovacs in early work on volume recovery; the crux of the paradox is that the effective relaxation times at equilibrium for up-jumps made from different initial temperatures do not agree with one another (i.e., an expansion gap is observed). Our new volume recovery measurements show that the paradox exists to much smaller departures from equilibrium than previously observed and also show that the expansion gap disappears and the paradox resolves itself very near equilibrium. Another paradox related to the glass transition is that of Kauzmann who observed that the entropy of a glass-forming liquid extrapolates to zero at a finite temperature; one proposed resolution is the existence of a thermodynamic Tg below the experimentally observed transition. In an attempt to address whether a thermodynamic Tg exists, the absolute heat capacities of polymethylstyrene and its mixtures with the pentamer were measured. Extrapolation of these measurements to pure polymer gives the equilibrium liquid heat capacity for the polymer at temperatures as much as 180 K below the Tg and 130 K below the Kauzmann temperature. No evidence of a second order transition is observed. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:39PM |
B4.00004: Continuum and Meso-scopic Models for the Nonlinear Relaxation Behavior of Glassy Polymers Invited Speaker: We will describe recent work on developing a physically based constitutive equation for amorphous polymers and present a novel, meso-scale model that begin to bridge the gap between molecular simulations and continuum models. A nonlinear thermoviscoelastic constitutive model has been developed that is thermodynamically consistent, uses generalized strain measures, and where relaxation is controlled by the configurational internal energy. Using this model, we have been able to describe nonlinear stress relaxation, yield, post-yield deformation, physical aging, enthalpy relaxation, and solidification dynamics in the glass transition region. A fluctuation model has been developed that explicitly acknowledges the effect of temporal fluctuations in the thermodynamic variables on the relaxation behavior. The fluctuation model naturally predicts the shape of the relaxation spectra, which is a consequence of the distribution of environments that a small material element experiences. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 2:15PM |
B4.00005: Nanostructure in amorphous poly(n-alkylmethacrylate) melt from dynamic NMR and scattering Invited Speaker: Nanostructures are important aspects of composite polymers, such as block copolymers. Advanced solid state NMR techniques recently revealed, however, that homopolymers can also exhibit pronounced nanostructures. There, they arise from orientational correlations between neighbouring chains or from an incompatibility of different groups within the repeat unit. The presence of such nanostructures has important consequences for the dynamic behaviour and the properties of these materials. As a specific example, the complex dynamics of poly($n$-alkylmethacrylates) is studied by advanced $^{13}$C NMR spectroscopy. Extended backbone conformations involving about five, at most ten repeat units are identified as the molecular units involved in structural relaxation. Syndiotatic and isotactic sequences behave alike. The packing within the locally structured polymer melts is reminiscent of hairy rods with flexible sidegroups, yet much less defined. It also shows up in temperature dependent X-ray scattering. [Preview Abstract] |
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