Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F18: Padden Award SymposiumPrize/Award
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Hide Abstracts |
Sponsoring Units: DPOLY Chair: Darrin Pochan, University of Delaware Room: 277 |
Tuesday, March 14, 2017 11:15AM - 11:27AM |
F18.00001: Elastocapillarity in Soft Matter: From Wetting and Adhesion to Interface Reinforcement Zhen Cao, Andrey Dobrynin Elastocapillarity, the fine interplay between capillary and elastic forces, determines contact phenomena in soft matter at micro- and nano-scales. Using a combination of the molecular dynamics (MD) simulations and theoretical calculations, we developed a unifying model able to describe a wetting-like and adhesion-like contact between a particle and substrate. In the framework of this model a deformation of a particle or substrate is a universal function of the elastocapilliary number -- $\gamma^{\ast }$/$G^{\ast }a$, where $\gamma^{\ast } $and $G^{\ast } $are effective surface tension and effective shear modulus of the particle/substrate system, and $a $is a radius of contact. In the adhesion regime the elastocapillary number is smaller than unity, while in the wetting regime this parameter is larger than unity. This approach was extended to elucidate conditions of interface reinforcement between soft materials by nanoparticles. The prediction of the model were confirmed by MD simulations showing that the work required for separation of two gels glued together by nanoparticles could be up to 10 times larger than the work of adhesion between two neat gel surfaces. [Preview Abstract] |
Tuesday, March 14, 2017 11:27AM - 11:39AM |
F18.00002: Light-responsive viscoelastic timescales in bio-inspired metal-coordinate supramolecular hydrogel mechanics Scott Grindy, Niels Holten-Andersen Stimuli-responsive hydrogels are currently an active subject of research for biological structural applications such as adhesives. Conventional, covalently crosslinked hydrogels are typically too weak to function mechanically, so researchers have used dynamic and reversible crosslinks in attempts to improve mechanical performance. Recently, we showed that, in a PEG-based hydrogel crosslinked by bio-inspired Histidine:M2$+$ coordinate bonds, the characteristic mechanical relaxation timescale can be controlled by selecting the transition metal ion acting as the crosslink center and that by using mixed transition metals, we are able to design hydrogels with multiple hierarchical relaxation timescales while controlling the magnitude of the timescales by varying the relative metal concentrations. This platform of PEG-His hydrogels represents a straightforward method for creating hydrogel materials with precisely-engineered viscoelastic energy dissipation properties. Here, we expand on this platform by exploiting the chemistry of metal-coordinate complexes to create hydrogels with UV-responsive viscoelastic properties, where the oxidation state of the transition metal can be altered using UV light. Careful selection of the metal ion crosslinks and UV exposure allows a diverse set of (pre-UV properties) - (post-UV properties) pairs. Such a precise level of control over hierarchal energy dissipation modes enables optimization of hydrogel mechanics for a wide array of loading contexts. [Preview Abstract] |
Tuesday, March 14, 2017 11:39AM - 11:51AM |
F18.00003: Morphology Control and Interfacial Activity Study of Semi-Crystalline Graft Copolymers Hyeong Jun Kim, Jin-Seong Kim, Youngkwon Kim, Ryan Hayward, Bumjoon Kim Self-assembly of copolymers containing conjugated segments has been considered a promising approach to achieve desirable nanostructures for efficient organic electronics. Most conjugated polymers, however, often exhibit a high degree of crystallinity, which significantly increases the complexity of their phase behaviors. Here, we explored graft-copolymer architectures with conjugated polymer backbones to regulate self-assembly. A series of poly(3-hexylthiophene) (P3HT) based graft copolymers were prepared, and the effectiveness of the graft architectures was demonstrated by comparing with the corresponding block copolymers. We found that: 1) crystallization of P3HT was suppressed in a tunable fashion due to steric hindrance from grafted chains, leading to production of thermally annealed well-ordered nanostructures; 2) graft architectures accumulate at the of P3HT/fullerene interface in bulk heterojunction solar cells, resulting in higher thermal and mechanical; and 3) grafting of hydrophilic side chains with high areal chain densities on P3HT crystals allowed for stable dispersion in aqueous solvent, enabling fabrication of green-solvent processable conjugated polymer devices. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F18.00004: Study of the Air-Water Interfacial Behavior of Block Polymer Micelles: Toward Rational Design of Polymer Lung Surfactants Hyun Chang Kim, You-Yeon Won Lung surfactants play a critical role in the lung's ability to process air by lowering alveolar surface tension. For potential lung surfactant applications, we have been testing a wide range of block copolymer micelles to identify candidate materials that are capable of replicating the surface-tension-lowering properties of natural lung surfactants. This talk will discuss the results obtained from two representative systems: aqueous micelles formed by PLGA-PEG and PS-PEG block polymers. Water-spread PLGA-PEG micelles form stable monolayers at the air-water interface. However, PLGA-PEG micelles are not strongly bound to the air-water interface, and thus unable to produce low surface tension (\textless about 10 mN/m) at high compression. Experiments suggest that the tendency of PLGA-PEG micelles to submerge into the water subphase is controlled by such parameters as the molecular weight and grafting density of the PEG brush chains, and the curvature of the PEG grafting surface. This behavior can be precisely modeled by the Daoud-Cotton theory. In contrast, we found that water-spread PS-PEG micelles are typically completely pinned to the air-water interface, and thus are able to produce an extremely low surface tension (close to 0 mN/m) at high compression. The exact origin of this behavior is not understood yet. We suspect that the PEG brush chains in PS-PEG micelles are less hydrated than in ordinary PEG brush situations, prohibiting the micelles from submerging into the aqueous phase. Transverse proton NMR relaxation measurements support this explanation. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F18.00005: Large strain deformation behavior of polymeric gels in shear- and cavitation rheology Seyed Meysam Hashemnejad, Santanu Kundu Polymeric gels are used in many applications including in biomedical and in food industries. Investigation of mechanical responses of swollen polymer gels and linking that to the polymer chain dynamics are of significant interest. Here, large strain deformation behavior of two different gel systems and with different network architecture will be presented. We consider biologically relevant polysaccharide hydrogels, formed through ionic and covalent crosslinking, and physically associating triblock copolymer gels in a midblock selective solvent. Gels with similar low-strain shear modulus display distinctly different non-linear rheological behavior in large strain shear deformation. Both these gels display strain-stiffening behavior in shear-deformation prior to macroscopic fracture of the network, however, only the alginate gels display negative normal stress. The cavitation rheology data show that the critical pressure for cavitation is higher for alginate gels than that observed for triblock gels. These distinctly different large-strain deformation behavior has been related to the gel network structure, as alginate chains are much stiffer than the triblock polymer chains. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F18.00006: Negative Transference Numbers in Polymer Electrolytes Danielle Pesko, Ksenia Timachova, Nitash Balsara Energy density and safety of conventional lithium-ion batteries is limited by the use of flammable organic liquids as a solvent for lithium salts. Polymer electrolytes have the potential to address both limitations. The poor performance of batteries with polymer electrolytes is generally attributed to low ionic conductivity. The purpose of our work is to show that another transport property, the cation transference number, t$+$, of polymer electrolytes is fundamentally different from that of conventional electrolytes. Our experimental approach, based on concentrated solution theory, indicates that t$+$ of mixtures of poly(ethylene oxide) and LiTFSI salt are negative over most of the accessible concentration window. In contrast, approaches based on dilute solution theory suggest that t$+$ in the same system is positive. In addition to presenting a new approach for determining t$+$, we also present data obtained from the steady-state current method, pulsed-field-gradient NMR, and the current-interrupt method. Discrepancies between different approaches are resolved. Our work implies that in the absence of concentration gradients, the net fluxes of both cations and anions are directed toward the positive electrode. Conventional liquid electrolytes do not suffer from this constraint. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F18.00007: Investigating Kinetic Pathways During Solvent Vapor Annealing with Soft Shear via In Situ Small-Angle Neutron Scattering Cameron Shelton, Ronald Jones, Thomas Epps Solvent vapor annealing with soft shear (SVA-SS) is a block polymer (BP) thin film annealing technique that directionally aligns nanostructures by exploiting solvent swelling/deswelling differences between the film and a polydimethylsiloxane (PDMS) pad adhered to the free surface. Although studies have demonstrated the potential of SVA-SS to generate well-aligned domains, the restructuring mechanism and effect of key parameters requires investigation to improve control over self-assembly. In this work, we conducted in situ small-angle neutron scattering experiments to explore the kinetic pathways of nanostructure alignment of poly(d-styrene-b-isoprene-b-d-styrene) thin films during SVA-SS. We compared results to SVA (without shear) and determined that alignment occurred through domain breakup and reformation initiated by PDMS swelling and deswelling, respectively. Additionally, changes in parameters such as PDMS elasticity and deswell rate resulted in nonlinear trends in domain directionality and ordering that were not apparent by small-area atomic force microscopy analysis. By relating the key thermodynamic effects to measured kinetic pathways for alignment, we have generated a more optimized approach to direct BP thin film self-assembly using SVA-SS. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F18.00008: Chain Fodling Patterns of Semicrystalline Polymer Fromed via Different Crystallization Pathways: Roles of Chain Network Shichen Yuan, Toshikazu Miyoshi Crystallization of polymer chains has been a debatable matter due to a lack of experimental techniques to access chain-level structure during and after crystallization. Our group developed a novel strategy to trace chain trajectory of isotope labeled polymer chains by~$^{13}$C-$^{13}$C Double Quantum (DQ) NMR.~$Isotactic$-polypropylene ($i$PP) shows polymorph depending on crystallization kinetics, and metastable mesomorphic and~$\beta$~forms experience phase transitions into stable~$\alpha$~form via melting and re-crystallization by increasing temperature. In this study, we investigated chain trajectory of~$i$PP in metastable crystalline forms obtained by rapid quenching, nuclear agent, stable~$\alpha$~forms obtained via phase transition from metastable forms and quiescent crystallization from the melt. Comparing experimental DQ buildup curves with spin-dynamics simulation, it was revealed that~$\beta$~and mesomorphic forms adopt adjacent re-entry cluster with average folding number of 4-5, which are very close to that in the stable~$\alpha$~form after phase transition and in the melt-grown crystals. The results indicate that available kinetics does not influence average folding number during crystallization, and that invariance of chain network dominate chain-folding process. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F18.00009: Distinct Thermophysical and Interfacial Properties Associated with Low Molecular Weight Cyclic Polystyrene in Bulk and Confined States: Tg and Fragility Lanhe Zhang, Ravinder Elupula, Scott Grayson, John Torkelson Cyclic or ring polymers represent an exciting class of topologically distinctive polymers. The influence of ``end-to-end'' tethering and the unusual conformational properties associated with cyclic topologies have led to polymer dynamics significantly different from the linear counterpart. Bulk cyclic polystyrene ($c$-PS) exhibits very weak $T_{\mathrm{g}}$- and fragility-molecular weight (MW) dependences compared to linear PS. In stark contrast to the substantial $T_{\mathrm{g}}$-confinement effects in linear PS, a nearly completely suppressed confinement effect is discovered in low MW $c$-PS. The cyclic topology strongly restricts polymer-substrate interactions. Therefore, the near elimination of the $T_{\mathrm{g}}$-confinement effect in $c$-PS originates mainly from a very weak perturbation to $T_{\mathrm{g}}$ near the free surface. Upon nanoscale confinement, linear PS films have been shown to have significantly reduced fragility compared to bulk. Despite having similar bulk fragility as high MW linear PS, low MW $c$-PS films show major suppression in fragility reduction with decreasing thickness. Due to a lack of chain ends, properties associated with the ring structure are not prone to be perturbed by either MW reduction or confinement. This result indicates a strong correlation between the susceptibility of fragility perturbation and the susceptibility of $T_{\mathrm{g}}$ perturbation, caused by chain topology and/or by confinement. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F18.00010: Effects of chain stiffness on the performance of conjugated polymers Wenlin Zhang, Enrique Gomez, Scott Milner Conjugated polymers, though still outperformed by their inorganic counterparts, are promising materials for flexible electronics, including field-effect transistors and solar cells. In order to design new materials and optimize existing polymers, we want to establish concrete links between chain properties, structural order, and electronic properties. Here we emphasize that a fundamental chain parameter, the persistence length, which characterizes the bending stiffness of chain backbones, is critical to the performance of conjugated polymers. The backbone stiffness affects not only chain conformations, but also configurational order for semiflexible chains. Using molecular dynamics simulations and analytical theories, we demonstrate that chain stiffness, together with the nematic interactions between backbone moieties, governs nematic phase behaviors and molecular packing at interfaces for conjugated polymers. The structural order, as a function of chain stiffness, in turn enhances charge transport. Because we can efficiently predict persistence lengths, liquid crystallinity, and interfacial ordering based on chemical structures, our overall work can help screen novel semiconducting polymers for high performance electronic devices. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F18.00011: Invariant fast surface diffusion on ultra-stable and aged molecular glasses Yue Zhang, Zahra Fakhraai Recent studies have reported surface diffusion on molecular glasses to be orders of magnitude faster than bulk diffusion with a lower activation barrier. The enhanced surface mobility is hypothesized to be responsible for the ultra-stable glass formation by physical vapor deposition. Here in this study, we directly measure the surface diffusion on ultra-stable glasses using tobacco mosaic virus as probe particle to test this hypothesis. Surface diffusion is also measured on physically aged and ordinary liquid-quenched glasses as a comparison. At two measuring temperatures below bulk glass transition temperature T$_{\mathrm{g}}$, the surface diffusion coefficients remain invariant on these glasses regardless of orders of magnitude of variations in their bulk relaxation dynamics. Our results suggest that the fast surface diffusion is decoupled from the bulk relaxation dynamics when measured below T$_{\mathrm{g}}$. [Preview Abstract] |
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