Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B14: Invited Session: Trends and Perspectives on Fundamental Polymer Physics |
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Sponsoring Units: DPOLY DFD Chair: James Forrest, Univesrity of Waterloo Room: 301-303 |
Monday, March 3, 2014 11:15AM - 11:51AM |
B14.00001: Role of enhanced segmental mobility in the deformation of polymer glasses Invited Speaker: Mark Ediger The mechanical properties of polymer glasses, including plastic flow, are important for many applications. In contrast to the flow of polymer melts, plastic flow is poorly understood at a fundamental level. One reason for this is that the deformation of polymer glasses typically occurs in a highly nonlinear regime, e.g., doubling the strain rate has little impact on the flow stress. Eyring proposed that stress increases the rate of molecular rearrangements in solids and this is the source of nonlinearity in many models. In this talk, experiments measuring molecular mobility during constant strain rate deformation of a polymer glass will be described for the first time. In these experiments on PMMA, the mobility initially increases in the pre-yield regime, by a factor of up to 160, as compared to the undeformed glass. After yield, the mobility remains constant even as the stress is decreasing; this non-Eyring effect is consistent with the view that the sample is being pulled higher on the potential energy landscape. For the range of strain rates investigated, mobility and strain rate are linearly correlated, consistent with the view that enhanced segmental mobility enables the flow of polymer glasses. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:27PM |
B14.00002: Monte Carlo Field-Theoretic Simulations for Melts of Symmetric Diblock Copolymer Invited Speaker: Mark Matsen Monte Carlo field-theoretic simulations (MC-FTS) are performed on melts of symmetric diblock copolymer for invariant polymerization indexes extending down to experimentally relevant values of $\bar{N} \sim10^4$. The simulations are performed with a fluctuating composition field, $W_-({\bf r})$, and a pressure field, $W_+({\bf r})$, that follows the saddle-point approximation. Our study focuses on the disordered-state structure function, $S(k)$, and the order-disorder transition (ODT). Although short-wavelength fluctuations cause an ultraviolet (UV) divergence in three dimensions, this is readily compensated for with the use of an effective Flory-Huggins interaction parameter, $\chi_e$. The resulting $S(k)$ matches the predictions of renormalized one-loop (ROL) calculations over the full range of $\chi_e N$ and $\bar{N}$ examined in our study, and agrees well with Fredrickson-Helfand (F-H) theory near the ODT. Consistent with the F-H theory, the ODT is discontinuous for finite $\bar{N}$ and the shift in $(\chi_e N)_{\rm ODT}$ follows the predicted $\bar{N}^{-1/3}$ scaling over our range of $\bar{N}$. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B14.00003: A direct quantitative measure of surface mobility in a glassy polymer Invited Speaker: Elie Raphael Thin polymer films are widely used in applications and have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature $T_{\textrm{g}}$. Above $T_{\textrm{g}}$ the entire film flows, while below $T_{\textrm{g}}$ only the near surface region responds to the excess interfacial energy. An analytical thin film model for flow limited to the free surface region is developed and shows excellent agreement with sub-$T_{\textrm{g}}$ data. The system transitions from whole film flow to surface localized flow over a narrow temperature region near the bulk $T_{\textrm{g}}$. The experiments and model provide a measure of surface mobility in a sample geometry where confinement and substrate effects are negligible. \\[4pt] This work has been done in collaboration with Y. Chai, Department of Physics and Astronomy and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1; T. Salez, Laboratoire de Physico-Chimie Theorique, UMR CNRS Gulliver 7083, ESPCI, Paris, France; J.D. McGraw, Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada, L8S 4M1; M. Benzaquen, Laboratoire de Physico-Chimie Theorique, URM CNRS; K. Dalnoki-Veress, Department of Physics and Astronomy, McMaster University and Laboratoire de Physico-Chimie Theorique, UMR CNRS; and J.A. Forrest, Department of Physics and Astronomy and Guelph-Waterloo Physics Institute,University of Waterloo. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:39PM |
B14.00004: On the Anomalous Diffusion of a Polymer Chain in an Unentangled Melt Invited Speaker: Jorg Baschnagel The dynamics of polymer chains in unentangled melts is commonly described by the Rouse model. However, various experimental and simulation studies show that certain dynamical phenomena in unentangled melts cannot be explained by the Rouse theory. One of the puzzling observations is the anomalous diffusion of the center-of-mass (CM) of a polymer chain for times $t < t_N$, where $t_N\propto N^2$ is the Rouse time of a polymer consisting of $N$ monomers. We explore two attempts to explain this observation: (i) an approach based on the effective interactions between the CMs in the melt and (ii) an approach based on the hydrodynamic flow and viscoelasticity of the melt. For approach (i) we find a partial success [1]: The theory accounts for the anomalous motion by yielding a negative power-law tail for the CM velocity autocorrelation function (CM VAF), $C_{\mathrm{cm}}(t) \propto - N^{-1}t^{-5/4}$. This prediction is in good agreement with molecular-dynamics (MD) simulations utilizing Langevin dynamics with a strong damping constant. On the other hand, for simulations with momentum conserving dynamics (i.e., the experimentally relevant situation) the prediction of approach (i) is qualitatively incorrect. In the latter case, the CM VAF rather scales as $C_{\mathrm{cm}}(t) \propto - N^{-1/2}t^{-3/2}$. This behavior can be rationalized by approach (ii). The predictions of approach (ii) are found to be good quantitative agreement with the MD simulations [2]. \\[4pt] [1] J. Farago, A. N. Semenov, H. Meyer, J. P. Wittmer, A. Johner and J. Baschnagel, Phys.\ Rev.\ E \textbf{85}, 051806 (2012).\\[0pt] [2] J. Farago, H. Meyer, J. Baschnagel and A. N. Semenov, Phys.\ Rev.\ E \textbf{85}, 051807 (2012). [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 2:15PM |
B14.00005: New questions in classical polymer physics Invited Speaker: Steve Granick This talk will show some of the most famous problems of polymer physics (the nature of entanglement, microrheology in the noncontinuum limit, pulled chains) can be enriched using deep imaging based on modern fluorescence imaging methods. These methods generate huge statistics regarding polymers whose internal conformational rearrangements can be large enough to image directly. Averages are obtained and also the fluctuations around them. From such investigations some aspects of classical polymer understanding are confirmed, but in other aspects we find surprises. Combining this with recent findings using super-resolution microscopy, a consistent new picture emerges. -- Work performed in collaboration with Juan Guan, Kejia Chen, Lingxiang Jiang, John King, Subhalakshmi Kumar, Changqian Yu, and Chi Hang Boyce Tsang. [Preview Abstract] |
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