Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P25: Focus Session: Dynamics and Structure in Polymer Melts and Glasses |
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Sponsoring Units: DPOLY Chair: Ramanan Krishnamoorti, University of Houston Room: Colorado Convention Center 203 |
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P25.00001: Evolution of stress and entanglements during deformation of glassy polymers Invited Speaker: Simulations of the mechanical properties of model polymer glasses have been performed over a wide range of entanglement densities, temperatures, strain rates and chain lengths. Primitive Path Analysis (PPA) is used to examine the corresponding changes in entanglement structure during deformation. Results for the initial yield stress, strain hardening and crazing will be presented. The initial yield stress is a function of strain rate and the thermal history of the sample. Strain hardening can be fit to entropic network models for the stress-strain curve. The stress shows a neo- Hookean response at low entanglement density $\rho_e$ and Langevin strain-hardening at high $\rho_e$. As expected from network models, entangled polymers deform affinely at scales larger than the entanglement length. However simulations and experiments show strain hardening decreases with increasing temperature while entropic models predict a linear increase. Our results show that strain hardening scales with the flow stress rather than temperature and that substantial strain hardening occurs for unentangled chains. Studies of craze formation show that it does not lead to entanglement loss in our systems. Instead, small scale motions concentrate entanglements at the nodes between fibrils. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P25.00002: Role of Fluctuations in Predicting the Glass Formation Line Grigori Medvedev, James Caruthers Problems with application of the Ehrenfest relations to predicting pressure dependence of the glass transition temperature are well documented in the literature. The resolutions of this problem proposed by various authors range from claiming general inapplicability of the standard phase transition theory to glasses to postulating additional thermodynamic variables to describe glassy state. In this paper we follow a different approach based on explicit acknowledgement of the dynamic heterogeneity of materials observed at a nanometer length scale in and near the glassy state. As a result, the macroscopic relaxation response of a material (defining the glass transition) emerges as an average over an ensemble of local responses which vary from one location to another due to thermodynamic fluctuations. Because of the strong non-linearity of the relaxation time as function of its variables, regions with different values of thermodynamic parameters unevenly contribute to the average effectively shifting the macroscopic relaxation time and, thus, the glass transition point. Moreover, since the magnitude of fluctuations depends on temperature, the glass formation line rotates in the volume-temperature plane. We show that the fluctuation model provides improvement in predicting the glass formation line in PMMA and a number of other polymeric glass formers for which literature data are available. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P25.00003: Entanglement Theories: Packing vs. Percolation Richard Wool There are two emergent theories of polymer entanglements, the Packing Model (Fetters, Lohse, Graessley, Milner, Whitten, $\sim $'98) and the Percolation Model (Wool $\sim $'93). The Packing model suggests that the entanglement molecular weight M$_{e}$ is determined by M$_{e}$ = K p$^{3}$, where the packing length parameter p = V/R$^{2}$ in which V is the volume of the chain (V=M/$\rho $Na), R is the end-to end vector of the chain, and K$\approx $357 $\rho $Na, is an empirical constant. The Percolation model states that an entanglement network develops when the number of chains per unit area $\Sigma $, intersecting any load bearing plane, is equal to 3 times the number of chain segments (1/a cross-section), such that when 3a$\Sigma $ =1 at the percolation threshold, M$_{e}\approx $31 M$_{j}$C$_{\infty }$, in which M$_{j}$ is the step molecular weight and C$_{\infty }$ is the characteristic ratio. There are no fitting parameters in the Percolation model. The Packing model predicts that M$_{e}$ decreases rapidly with chain stiffness, as M$_{e}\sim $1/C$_{\infty }^{3}$, while the Percolation model predicts that M$_{e}$ increases with C$_{\infty }$, as M$_{e}\sim $C$_{\infty }$. The Percolation model was found to be the correct model based on computer simulations (M. Bulacu et al) and a re-analysis of the Packing model experimental data. The Packing model can be derived from the Percolation model, but not visa versa, and reveals a surprising accidental relation between C$_{\infty }$ and M$_{j}$ in the front factor K. This result significantly impacts the interpretation of the dynamics of rheology and fracture of entangled polymers. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P25.00004: Interdiffusion at Ring-Shaped Polystyrene / Its Deuterated Counterpart Bilayer Interfaces Daisuke Kawaguhi, Atsushi Takano, Keiji Tanaka, Toshihiko Nagamura, Naoya Torikai, Robert Dalgliesh, Yushu Matsushita Time evolution of interfacial thicknesses between a cyclic polystyrene (c-hPS) / its deuterated counterpart (c-dPS) and a linear polystyrene (l-hPS) / its deuterated counterpart (l-dPS) bilayer films was investigated by neutron reflectivity as a function of molecular weight. The interfacial thickness of (c-hPS/c-dPS) film with molecular weight of 15k was almost equivalent to that of (l-hPS/l-dPS) one at any given annealing time. In contrast, the interfacial thickness of (c-hPS/c-dPS) film with molecular weight of 115k was significantly larger than that of (l-hPS/l-dPS) film at any given annealing time, indicating that the diffusion constant of c-PS ($D_{c})$ is qualitatively larger than that of l-PS ($D_{l})$. It might be explained in terms of less topological constraint of the entanglement for the cyclic PS at high molecular weight region. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P25.00005: Monte Carlo Simulation of the Glass Transition in Polyethylene Rajesh Khare, Orestis Alexiadis, Vlasis Mavrantzas, Job Beckers, Arlette Baljon The end-bridging (EB) Monte Carlo (MC) move has been used to simulate united atom models of bulk and end-grafted polyethylene both above and below the glass transition temperature (Tg). In previous work, EB move has been shown to lead to significant improvement in the relaxation of melts consisting of long polymer chains. We find that although a reasonable fraction of EB moves get accepted below glass transition in our MC simulation, the autocorrelation function of the chain end-to-end vector does not relax completely at these low temperatures. The temperature dependence of enthalpy and density is used to calculate the Tg for both bulk and thin film systems. For the film consisting of chains grafted on a hard surface, the Tg is slightly lower than that for the bulk, as expected in a system with unfavorable polymer-surface interactions. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P25.00006: Glassy Dynamics and Pressure Effects in Polymer Melts Erica J. Saltzman, Kenneth S. Schweizer A statistical mechanical theory of collective dynamic barriers, slow segmental relaxation and the glass transition in polymer melts has been developed by combining and extending methods of mode coupling, density functional and activated hopping transport theories. Previously, atmospheric pressure results were obtained for the crossover and glass transition temperatures, collective barrier and segmental relaxation time. A cooperativity parameter is introduced based on dynamic consequences of local chain stiffness, and its effects on dynamic fragility are discussed. The theory is extended to elevated pressures, which are found to broaden the deeply supercooled regime and reduce the dynamic fragility while retaining a universal Rossler-Sokolov scaling law for the temperature dependence of the alpha relaxation time. The ratio of the dynamic crossover temperature (ideal mode coupling critical temperature) and kinetic glass transition temperature plays an essential role in the dynamics at all pressures. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P25.00007: Anomalous Surface Dynamics Near Tg in Supported Polystyrene Films by XPCS Zhang Jiang, Mrinmay Mukhopadhyay, Sunil Sinha, Sanghoon Song, Hyunjung Kim, Laurence Lurio The reduction of the glass transition temperature (Tg) in supported thin polymer films is of great interest. One proposed explanation is that close to the surface region there exists a thin layer with very low viscosity. Here we report a recent XPCS (X-ray photon correlation spectroscopy) measurement from silicon supported polystyrene films. At just above Tg, we have found a relaxation mode of the surface fluctuations at least 100 times faster than the capillary wave theory predicts. Surprisingly, this mode does not show significant molecular weight dependence. At higher temperatures (T-Tg$>$=50C), the surface relaxation becomes normal, as predicted by the capillary wave theory [1]. \newline \newline [1] H. Kim, et al., Phys. Rev. Lett. 90, 68302 (2003) [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P25.00008: Theory of Segmental Relaxation and Physical Aging in Polymer Glasses Kenneth Schweizer, Kang Chen A predictive statistical mechanical theory of collective dynamic barriers and segmental relaxation of deeply supercooled polymer melts has been recently developed and widely applied [1]. The theory is based on a dynamic density functional perspective and the concept of a confining nonequilibrium free energy due to interchain forces. Dynamical constraints are primarily quantified by the temperature, pressure and material dependent dimensionless amplitude of long wavelength thermal collective density fluctuations, S0. This theory has now been generalized to the nonequilibrium glass based on the idea of a freezing in of the structural component of density fluctuations. Below Tg an apparent crossover of the segmental relaxation time to an Arrenhius form is predicted. Physical aging is addressed based on a simple first order kinetic equation for the time evolution of S0. At intermediate time scales after a quench the relaxation time generally grows with aging time as a power law with a temperature dependent exponent. The theoretical approach can be generalized to treat nonlinear mechanical properties including stress-strain response, yielding, modulus softening, strain hardening, and stress acceleration of relaxation and aging. [1] K.S.Schweizer and E.J.Saltzman, J.Chem.Phys. 121, 1984 (2004). [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P25.00009: Power Law Behavior of Dynamics in Simple Glass Formers John McCoy, Julieanne Heffernan, Joanne Budzien, Douglas Adolf Simulation results for the diffusive behavior of polymer chain/ penetrant systems are analyzed. Both freely jointed and freely rotating chains are studied. In all cases, the characteristic times, $\tau $, extracted from the diffusion constants are found to be single valued functions of the packing fraction, $\eta $. The functions $\tau (\eta )$ are found to be power-laws with exponents that are sensitive to both chain stiffness and particle type. For a specific system type, all measures of motion extrapolate to zero (or infinity) at a single $\eta _{0}$. In addition, ($\eta _{0}-\eta )$ can be interpreted as a ``scalar metric'' of the ``distance'' to the glass ``transition.'' [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P25.00010: Cure of Bisphenol M Dicyanate Ester/Polycyanurate under Nanoscale Constraint Qingxiu Li, Sindee Simon It is well known that properties are affected by constraint at the nanoscale. Although thermosetting resins have been cured in the presence of nanoparticles and nanotubes, cure of thermosetting resins under the well defined nanoscale constraints imposed by controlled pore glass or similar matrices has not been previously documented. In this work, we investigate the isothermal curing of bisphenol M dicyanate ester/polycyanurate under various nanoscale constraints, including within an aluminum oxide nanofilter, in unsilanized controlled pore glass, and in silanized controlled pore glass. Differential scanning calorimeter and Fourier transform infrared spectroscopy are used to monitor the evolution of the glass transition temperature and the conversion, respectively, as a function of pore size and pore surface chemistry. For the glass transition temperatures of the polycyanurate networks cured in the silanized controlled pore glasses, only nanoconfinement effects are observed; whereas for the material cured in the unsilanized controlled pore glasses, both a nanoconfinement and a surface effect are observed. Furthermore, curing under nanoscale constraint accelerates the cure of bisphenol M dicyanate ester. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P25.00011: Probing Chain Entanglement in Polymer Glasses in Sub-nano Level Gi Xue, Xiaoliang Wang, Dongshan Zhou, Pinchuan Sun A new approach to characterize the chain distance of polymer glasses in isotopic blends using a dipolar filter NMR experiment was developed using ultra fast magic angle spinning (MAS) to selectively enhance sensitivity and resolution of the signals corresponding to the entanglement region. When the deuterated chain entangles with the hydrogenous one within 0.8 nm, the strong 1H dipolar interaction will be diluted. Ultra fast MAS (25-kHz) is used to further average the residual dipolar interaction in the entanglement region. And a dipolar filter experiment is used to suppress the signals from strong 1H-1H dipolar interactions that exist in hydrogenated polymer chains. Based on the above three aspects, an unique 1H signal enhancement effect was found selectively in the overlapping region where the distance between two chains is shorter than 0.8 nm, which can be used as a sensitive probe to characterizing the entanglements in sub-nano level. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P25.00012: Synchrotron X-ray scattering study of structure and dynamics of thin block copolymer films Hyunjung Kim, Heeju Lee, Young Joo Lee, Sanghoon Song, Youngsuk Byun, Zhang Jiang, Sunil K. Sinha, Adrian R{\" u}hm, Suresh Narayanan We have studied the structure and the dynamics of block copolymer films of poly(styrene)-b-poly(dimethylsiloxane) in the melt using X-ray Photon Correlation Spectroscopy. Block-copolymers exhibit internal interactions and therefore an internal structure (in our case spherical micelles). This ought to have a strong influence on the physical properties of the thin films. It can be expected that the dynamics is strongly altered once the film thickness reaches the characteristic length scale in the polymer, which is in our case given by the micelle diameter. The surface tension obtained from static grazing incidence scattering data shows that a PDMS layer segregates to the free surface of the film. The dynamics results are compared with the theory of overdamped thermal capillary waves on thin films. Both the surface dynamics and the micelle dynamics, which were selectively measured by changing the incident angle, will be discussed. It was supported by Korea Science {\&} Eng. Foundation / Seoul Research {\&} Business Development Program. [Preview Abstract] |
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