Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G22: Focus Session: Padden Award Symposium |
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Sponsoring Units: DPOLY Chair: Stephen Cheng, University of Akron Room: 407 |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G22.00001: Exploring conditions for craze initiation and for absence of crazing in polymer glasses Shiwang Cheng, Panpan Lin, Shi-Qing Wang Although crazing in polymer glasses has been extensively studied in the past, it is still difficult to predict and explain when and why crazing would take place. There is even a recent proposal to suggest [1] that craze initiation is ``a frustrated fracture process than rather a yield mechanism'' [2]. In this work, we report the variable parameters that influence crazing. Specifically, we show that a ``young'' glass (prepared by mechanical ``rejuvenation'') can resist crazing and aging promotes crazing. Thus, the degree of vitrification is one variable. Under creep, crazes form faster at a higher tensile stress, showing that crazing is an activation process and depends on the external condition. We also show how the large-scale structure such as the degree of chain networking affects crazing behavior. For example, melt stretching suppresses crazing. Finally, we demonstrate crazing in absence of any ongoing extension when a cold-drawn polymer glass is held fixed when annealing well below T$_{\mathrm{g}}$ during the elastic yielding [3]. These new observations have inspired a molecular picture for large deformation of polymer glasses [4]. \\[4pt] [1] \textit{Polymer }2007\textbf{,} \textit{48}, 1030;\\[0pt] [2] \textit{Polymer }2011\textbf{,} \textit{52}, 2319;\\[0pt] [3] \textit{Phys. Rev. Lett. }2013, \textit{110}, 065506;\\[0pt] [4] submitted to Polymeric Glasses session. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G22.00002: Traversing the crystalline phase space of contorted hexabenzocoronene to maximize charge transport Anna Hiszpanski, Arthur Woll, Nan Yao, Yueh-Lin Loo Alternative crystal structures of molecular semiconductors may exhibit increased intermolecular charge transport, but methods to controllably access non-thermodynamically-favored crystal structures are lacking. Starting with an amorphous film of contorted hexabenozocoronene (HBC) and applying thermal and solvent-vapor annealing to induce crystallization, we have accessed three distinct HBC polymorphs, two of which have previously not been observed. HBC films crystallize as polymorph I upon thermal annealing and as polymorph II upon solvent-vapor annealing with tetrahydrofuran. Subsequent solvent-vapor annealing of polymorph I converts it to polymorph II; thermal annealing polymorph II transforms HBC to yet a different crystal structure, denoted polymorph II'. Though the crystal structure can be tuned through sequential processing, the preferred out-of-plane molecular orientation adopted by HBC is determined primarily by the first processing step. By imposing different processing sequences, we can access films having different polymorphs but the same molecular orientation, and also films having the same polymorph but different molecular orientations, thereby allowing us to decouple the relative contributions of polymorphism and preferential orientation to charge transport. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G22.00003: High Modulus, High Conductivity Nanostructured Polymer Electrolyte Membranes via Polymerization-Induced Phase Separation Lucas McIntosh, Morgan Schulze, Marc Hillmyer, Timothy Lodge Solvent-free, solid-state polymer electrolyte membranes (PEMs) will play a vital role in next-generation electrochemical devices such as Li-metal batteries and high-$T$ fuel cells. The primary challenge is that these applications require PEMs with substantial mechanical robustness, as well as high ionic conductivity. The key to optimizing orthogonal macroscopic properties is to use a heterogeneous composite with well-defined nanoscopic morphology---specifically, long-range co-continuity of high modulus and ion transport domains, which has proven difficult to achieve in commonly-studied diblock copolymer-based electrolytes. We report a simple synthetic strategy to generate PEMs via polymerization-induced phase separation, where the delicate balance between controlled addition of styrene onto a poly(ethylene oxide) macro-chain transfer agent and simultaneous chemical crosslinking by divinylbenzene results in a disordered structure with domain size of order 10 nm. Crucially, both domains exhibit long-range continuity, which results in PEMs that are glassy solids (modulus $\approx$ 1 GPa) owing to the isotropic network of stiff, crosslinked polystyrene, and are highly conductive ($>$ 1 mS/cm at 70 $^{\circ}$C) because ions migrate in channels of low $T_{\rm g}$ poly(ethylene oxide). [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G22.00004: Physical Aging of Thin and Ultrathin Free-Standing Polymer Films: Effect of Stress and Reduced Glass Transitions Justin Pye, Connie Roth While great effort has been made in elucidating the effect of confinement on the glass transition (Tg) in polymers, considerably less work has been done on physical aging. Starting with supported films, we have previously shown that the reduced physical aging rates in ultrathin polystyrene (PS) films can be linked to the reduced Tg near the free surface [Macromolecules 2010, 43, 8296]. We then showed that high molecular weight (MW) free-standing PS films have two reduced Tgs suggesting that two separate mechanisms are acting simultaneously to propagate enhanced mobility at the free surface deeper into the film [PRL 2011, 107, 235701]. To help determine the mechanisms of these two reduced Tgs, we performed physical aging measurements on these high MW free-standing PS films. For thick films (220-1800 nm) in which there are no Tg reductions, we find that the physical aging rate depends strongly on stress caused by thermal expansion mismatch between film and support. This stress, applied to the films as they are quenched into the glassy state, can nearly double the physical aging rate when changing the frame material from polycarbonate to silicon [Macromolecules 2013, DOI:10.1021/ma401872u]. Finally, ultrathin high MW PS films held at a temperature between the two Tgs do exhibit physical aging, indicating that at least some of the film is glassy between these two transitions. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G22.00005: The Origin of Hyperdiffusive Relaxations in Soft Glasses Samanvaya Srivastava, Donald Koch, Lynden Archer Small particles suspended in fluids move randomly over long length- and time-scales. This motion is the expected response of weakly interacting particles to uncoordinated bombardments from the fluid molecules. This feature of suspensions is considered a fundamental characteristic of their equilibrium state and, over long-enough observation times, leads to universal diffusive particle motions. We report on the motions of particles in single component suspensions in which the suspended (particle) and suspending (fluid) phases are chemically linked. We find that even in equilibrated systems these motions are hyperdiffusive. Our observations add to a large number of recent reports, which show that diffusive motion is not the norm in soft matter such as colloidal gels, nanoemulsions and soft nanoparticle glasses. In such systems, particle motions can be highly correlated over long distances and time, belying long-lived, directed forces thought to arise from out-of-equilibrium, metastable states that can drive sudden irreversible structural re-arrangements. We show that hyperdiffusive motion in soft matter does not require such states and can arise naturally from volume fluctuations brought about by thermal forces. We further show that the simplicity of the force dipoles produced by volume fluctuations in our single-component suspensions leads to a physical origin for hyperdiffusion as fundamental as that commonly thought to produce diffusion of particles in dilute suspensions. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G22.00006: On the crossover from Odijk to de Gennes in tube-confined semiflexible polymers Douglas Tree, Kevin Dorfman The problem of a semiflexible polymer confined in a tube was considered solved almost 30 years ago. However, the need to manipulate single molecules of the semiflexible biopolymer DNA for emerging genomic mapping technologies led to measurements of the extension of DNA in nanochannels, which challenged the classic results of Odijk and de Gennes. We have investigated the behavior of semiflexible polymers using an off-lattice implementation of the pruned-enriched Rosenbluth method (PERM), enabling simulation of confined chains that are more than two orders of magnitude longer than chains used in conventional Metropolis methods. Our simulations suggest the presence of additional universal regimes, which arise due to the competing effects of stiffness and excluded volume for polymers in moderate confinement. We have also examined previous interpretations of these regimes in the light of recent results for both the mean polymer extension and extension fluctuations of very long chains. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G22.00007: Polymer Structure and Dynamics under Cylindrical Confinement: Experiments, Simulations and Theory Wei-Shao Tung, Daniel Sussman, Nigel Clarke, Russell Composto, Kenneth Schweizer, Robert Riggleman, Karen Winey Polymer structure and dynamics are perturbed under confinement, especially when the dimension of the confinement is smaller than the polymer's radius of gyration (R$_{\mathrm{g}})$. While most studies focus on thin film confinement, we study cylindrical confinement using experiments, simulations and theory. Our MD simulations study the change in R$_{\mathrm{g}}$, local dynamics, entanglement molecular weight (N$_{\mathrm{e}})$, and diffusion coefficient (D) for polymers in cylindrical confinement with different diameters (d/2R$_{\mathrm{g}}$ $\sim$ 0.4 -- 6). We found increased N$_{\mathrm{e}}$ and D in cylindrical confinement for d/2R$_{\mathrm{g}}$\textless 1.5. Moreover, R$_{\mathrm{g}}$ decreases in the direction of confinement and increases along the cylinder axis. We developed an analytical theory to relate the transformation of chain conformations to the preferential orientation of primitive path steps, and further predicted the increase of N$_{\mathrm{e}}$. Experimentally, we infiltrated 400 kg/mol polystyrene (2R$_{\mathrm{g}}$ $\sim$ 35nm) into anodized aluminum oxide (AAO) membranes (d $\sim$ 18-150nm) to confine polymers into cylindrical nanopores (d/R$_{\mathrm{g}}$ $\sim$ 0.5--4). We use SANS to probe R$_{\mathrm{g}}$, QENS to probe the local dynamics, and elastic recoil detection to measure D of deuterated PS inside PS-filled AAO nanopores. Values obtained from our experiments and literature are quantitatively compared with our simulation results. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G22.00008: Dynamics of entangled rod-coil block copolymers Muzhou Wang, Ksenia Timachova, Alfredo Alexander-Katz, Alexei E. Likhtman, Bradley D. Olsen Polymer science is exploring advanced materials which combine functional domains such as proteins and semiconducting polymers with traditional flexible polymers onto the same molecule. While many studies have focused on equilibrium structure-property relationships, little is known about how the conformational restrictions of rigid domains affect dynamical phenomena such as mechanical properties, processing pathways, and self-assembly kinetics. We have recently introduced a reptation theory for entangled rod-coil block copolymers as a model for this wider class of functional polymeric materials. The theory hypothesizes that the motion of rod-coils is slowed relative to rod and coil homopolymers because of a mismatch between the curvature of the rod and coil entanglement tubes. This effect leads to activated reptation and arm retraction as two relaxation mechanisms that govern the short and long rod regimes, respectively. These results were verified by tracer diffusion measurements using molecular dynamics simulation and forced Rayleigh scattering in both the rod-coil diblock and coil-rod-coil triblock configurations. The tracer diffusion results were then compared to experimental self-diffusion measurements which require a consideration of the motion of the surrounding chains. [Preview Abstract] |
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