Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F55: Padden Award Symposium |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Jan Genzer, North Carolina State Univ Room: LACC 515A |
Tuesday, March 6, 2018 11:15AM - 11:27AM |
F55.00001: Effect of Charge on Interfacial Activity and Micelle Formation of Ion-containing Block Copolymers at the Oil-Water Interface Ha-Kyung Kwon, Kenneth Shull, Monica Olvera De La Cruz Understanding the role of charge interactions in the self-assembly of ion-containing block copolymers continues to be a challenge, especially in systems such as partially quaternized polystyrene-block-poly(2-vinylpyridine) (PS-Q2VP) where multiple factors, such as ionic correlations, ion solvation, and Flory-Huggins interactions, are at play. Here, we present the results of a combined theory and experiments-based approach to investigate the effect of charge interactions on the micelle formation and interfacial activity of PS-Q2VP block copolymers at the oil-water interface. We use the SCFT-LS method to investigate the self-assembled morphologies of PS-Q2VP in solvents, to delineate the role of solvent quality and charge interactions on micelle morphology. We characterize the interfacial activity and micelle formation of PS-Q2VP via a combination of pendant drop measurements and x-ray scattering techniques and demonstrate that charge interactions can be a powerful tool in controlling the interfacial adsorption and self-assembly of PS-Q2VP block copolymers. These results represent a promising model system for understanding and utilizing the role of charge interactions on the phase behavior of ion-containing block copolymers using theory and experiments. |
Tuesday, March 6, 2018 11:27AM - 11:39AM |
F55.00002: Anisotropic Ion Diffusion and Electrochemically Driven Transport in Nanostructured Block Copolymer Electrolytes Ksenia Timachova, Irune Villaluenga, Steven Greenbaum, Nitash Balsara Nanostructured block copolymer electrolytes have the potential to enable solid-state batteries with lithium metal anodes. Improving the performance of these electrolytes requires understanding the molecular mechanisms that govern ion motion. We present complete characterization of ion transport in lamellar block copolymers with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI). Individual self-diffusion coefficients of the Li+ and TFSI- ions are measured by pulsed-field gradient NMR. The data indicate that salt diffusion in nanostructured block copolymers is locally anisotropic, and this enables determination of a diffusion coefficient parallel to the lamellae, D||, and a diffusion coefficient through defects in the lamellae, D⊥. This is the first direct measurement of anisotropic ion diffusion in polymer electrolytes. We find agreement between the diffusion coefficient D⊥ and electrochemically determined diffusion coefficients, indicating that the performance of nanostructured block copolymer electrolytes in batteries is limited by ion transport through defects. This finding motivated the design of block copolymers where the diffusion pathways for the Li+ and TFSI- are fundamentally different: defects only limit the transport of TFSI- while Li+ diffuses isotropically. |
Tuesday, March 6, 2018 11:39AM - 11:51AM |
F55.00003: Frank-Kasper Phases in Block Polymers: Stable or Metastable? Akash Arora, Kyungtae Kim, Ronald Lewis, Meijiao Liu, Wei-hua Li, Anchang Shi, Frank Bates, Kevin Dorfman Block polymers are a class of soft materials that self-assemble at mesoscopic length scales to form a variety of ordered structures. In recent years, several experimental studies witnessed the formation of complex low-symmetry structures in sphere-forming block polymers. These structures, commonly referred to as Frank-Kasper (FK) phases, contain particles of disparate sizes packed in a tetrahedral closed-packed arrangement with multiple local coordination environments. In this work, we use self-consistent field theory (SCFT) to examine the stability of different FK phases in AB diblock copolymers. We performed extensive SCFT calculations and observed that several FK phases exist on the free energy surface of diblock copolymers. More importantly, the associated free energies differ only marginally (10-3 kBT). Recently, our group experimentally discovered two new FK phases in poly(isoprene-b-lactide) diblock copolymers. Using SCFT, we will discuss the thermodynamic stability of these phases and demonstrate the importance of various molecular parameters that govern the relative stability of these complex phases in AB diblocks. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F55.00004: In situ SAXS/SANS Characterization of Supramolecular Hydrogels under Deformation Chao Wang, Clinton Wiener, Bryan Vogt, Robert Weiss Double network hydrogels were the first synthetic tough hydrogels with their toughness attributed to energy dissipation through a sacrificial covalent network to prevent catastrophic failure, but this leads to significant hysteresis. Supramolecular hydrogels can overcome this limitation as energy is dissipated by rearrangement of physical associations (hydrophobic, ionic, H-bond). These associations are reversible, but the mechanisms associated with the network rearrangement are not well understood. Here, we examine a family of hydrogels based on amphiphilic statistic copolymer that are crosslinked by hydrophobic association during uniaxial elongation where the nanostructure evolution during deformation is elucidated by time-resolved SAXS. Nanostructures of hydrogels are dependent on both the strain rate and the crosslink density (as defined by copolymer composition) with the relative change in the nanostructure evolving from rubber-like (ν=0.5) to glass-like (ν=0.33) as rate increases. These differences are related to time scales for stress relaxation processes relative to the strain rate. Combined SANS and SAXS measurements demonstrate plastic deformation of hydrophobic aggregates and segmental pull-out of hydrophobes contribute to the energy dissipation of these hydrogels. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F55.00005: Phase behavior and magnetic field response of liquid crystalline block copolymers with labile mesogens Youngwoo Choo, Manesh Gopinadhan, Masafumi Fukuto, Lalit Mahajan, Dennis Ndaya, Rajeswari Kasi, Chinedum Osuji Self-assembly of liquid crystalline block copolymers(LCBCPs) in magnetic fields can yield highly aligned states. The presence of labile mesogens in such systems can modify the field response and phase behavior. We explore a cylinder-forming polylactide(PLA)-based LCBCP blended with labile cyanobiphenyl mesogens. The critical field strength to achieve well-aligned microdomains decreases to sub-1 T values on addition of small amounts of mesogens. At higher mesogen content, the system undergoes a swelling-induced transition to spherical PLA microdomains embedded in a nematic LC matrix. Strikingly, the spherical microdomains order on a rotationally degenerate FCC lattice in which the [001] lattice vector is parallel to the applied field. We systematically explore the system morphology between aligned HEX cylinders and FCC spheres as a function of composition and investigate the effects of field strength and cooling rate on orientational order using in situ X-ray scattering and transmission electron microscopy. Our results demonstrate that the addition of labile mesogens significantly enhances the field response of these materials and provides a pathway to the formation of textured cubic systems that are otherwise not amenable to alignment in a uniaxial magnetic field. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F55.00006: Identifying the Factors Governing Flow-Enhanced Nucleation in n-Alkanes with Molecular Simulation David Nicholson, Gregory Rutledge The application of a flow field is known to drastically accelerate the rate of crystal nucleation in semicrystalline polymer materials. Using non-equilibrium molecular dynamics (NEMD), nucleation studies are performed under shear and uniaxial extension for monodisperse melts of short (C20) and long (C150) alkanes, as well as for bimodal mixtures composed of both short and long chains. These studies reveal how the acceleration in the nucleation rate correlates with macroscopic measureable quantities and conformational statistics of the flowing melt. The observed correlations are used to evaluate of the capacity of various models for flow-induced nucleation to describe the NEMD data. It is observed that the monodisperse nucleation rate for C150 shows excellent agreement with a literature model based on the stretching of the end-to-end vector across the range of strain rates and flow fields studied. A similar correlation is found for the bimodal mixtures, however it is the amount of local stretching on the Kuhn segment length scale that correlates with the nucleation rate across the range of flow conditions and melt compositions. Based on this result, a mechanism in which crystalline clusters are formed from locally stretched chain segments is proposed. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F55.00007: Slow relaxations of grafted polymer due to confinement by neighboring chains Ryan Poling-Skutvik, Jacinta Conrad, Ramanan Krishnamoorti The behavior of nanoparticles dispersed in solutions plays a crucial role in environmental remediation, nanocomposite processing, and targeted drug delivery methods. These applications require well-dispersed particles, commonly achieved by grafting polymers to the particle surface. This end-tethered topology leads to strong deviations in the properties of grafted chains compared to those of free chains. Using neutron scattering techniques, we directly measure the structure and dynamics of grafted polymer chains in semidilute solutions. The grafted polymer relaxations initially follow the Zimm model but are slower than expected at longer time scales. These slow dynamics are caused by the confinement of neighboring chains, underscoring the importance of grafting on the dynamic properties of grafted polymers. Further, we disperse the grafted nanoparticles into solutions of linear polymers that act as crowding agents to compress the grafted corona. As the corona densifies due to an increase in osmotic pressure, the grafted polymers become more confined and exhibit slower relaxations. We identify grafting density as a crucial variable controlling the relaxations of grafted polymers and demonstrate that transverse and lateral relaxations decouple even at moderate grafting densities. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F55.00008: Impact of Aspect Ratio Control of Nanorod Surfactants on Shaping Block Copolymer Particles Kang Hee Ku, Youngkwon Kim, Jae Man Shin, Young Jun Lee, Bumjoon Kim Self-assembly of block copolymers (BCPs) within interface-engineered emulsion droplets is a simple and powerful tool in producing shape-controlled polymeric particles. In this talk, we describe the influence of nanorod (NR) length on manipulating interfacial properties of BCP particles. A series of cylinder- and lamella-forming polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and aspect ratio-controlled NRs were used, where the ratio of the NR length to the size of the NR-hosting domain (l/L) was tunable in a range from 0.07 to 2.40. Selective arrangement of NRs on the P4VP domain of the particle surface enabled the production of lens-shaped particles and striped ellipsoidal particles. In particular, the value of l/L was the key parameter in determining the location of the NRs in the BCP particles. Lens-shaped particles were produced only for 0.36 ≤ l/L ≤ 0.96, whereas ellipsoidal particles were formed for much wider range of l/L ≥ 0.83 without upper limit. This difference is attributed to larger entropic penalty for the NRs confined within the cylinders than that within the lamellae. The dynamics of NRs at the interface boundary of BCP particles was also investigated by performing a dissipative particle dynamics simulation of the NRs within a confined BCP emulsion system. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F55.00009: Extensional Relaxation Times of Dilute and Semi-Dilute Polymer Solutions Jelena Dinic, Leidy Jimenez, Madeleine Biagioli, Vivek Sharma Liquid transfer and drop formation in jetting, printing, coating, spraying and atomization applications involve complex free-surface flows, including the formation of columnar necks that undergo spontaneous capillary-driven thinning and pinch-off. The progressive self-thinning of neck is often characterized by self-similar profiles and scaling laws that depend on the relative magnitude of capillary, inertial and viscous stresses for simple, Newtonian fluids. Stream-wise velocity gradients that arise within the thinning columnar neck create an extensional flow field that can orient and stretch macromolecules, contributing extra elastic stresses and extensional viscosity that change the pinch-off dynamics for polymeric complex fluids. Here, we employ the dripping-onto-substrate (DoS) rheometry protocols for visualizing and analyzing the pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop. We show that the magnitude and concentration-dependence displayed by extensional viscosity and extensional relaxation time are quite distinct from the values obtained in response to shear flow, as the progressive screening of excluded volume and hydrodynamic interactions depends on polymer concentration as well as on degree of stretching and conformational anisotropy. |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F55.00010: Molecular Insights into the Extensional Flow of Polymers Thomas O'Connor, Mark Robbins Nonlinear extension flows are common in polymer processing but remain a challenging theoretical problem. These flows dramatically stretch chains and deform the entanglement network far from equilibrium. Analytic models make conflicting assumptions about chain dynamics and entanglements in nonlinear flow, and testing these models requires knowledge of molecular conformations that is not available from experiments. Here, we present molecular dynamics simulations of extensional flows in entangled polymer liquids, for Rouse-Weissenberg numbers 0.06 to 52, and Hencky strains up to 12. We measure the transient viscosity, resolving the linear viscoelastic limit and the rate dependence of the nonlinear viscosity. Our generic, bead-spring simulations reproduce experimental trends for polystyrene melts and solutions, implying a universal nonlinear rheology in extension. Characterizing the microscopic structure of flowing liquids reveals a direct connection between stress and chain entropy with a rate-independent entanglement density, and explains the scaling of nonlinear viscosity with chain and entanglement length at different rates. These results answer critical questions about the fundamental nature of entanglement and chain confinement in aligned and deforming polymers. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F55.00011: Proton Transport through a Crystalline Polymer: Lamellar Water Channels in Chain-Folded Precisely Sulfonated Polyethylene Edward Trigg, Taylor Gaines, Kenneth Wagener, Mark Stevens, Karen Winey Recent advances in polymer synthesis have allowed precise control over polymer architectures and microstructures, opening the door for the design of highly controlled nanoscale morphologies with previously inaccessible properties. Our previous work has shown that precisely periodic placement (every 21st carbon) of carboxylic acid groups along linear polyethylene gives rise to regular chain folding at the location of each acid group, and concurrent crystallization of the alkyl segments. Here we demonstrate that sulfonic acid groups along linear polyethylene give rise to similar chain folding, and that the resulting acid layers become hydrated upon exposure to humidity, providing high proton conductivity. This is the first time, to our knowledge, that controlled polymer folding has been utilized for proton or ion transport, and that high proton conductivity has been reported within a crystalline polymer. To supplement the experimental methods, atomistic molecular dynamics simulations were performed. These simulations confirm our characterization of the structure, and show that the ordered hydrated layers improve proton diffusion relative to tortuous channels of the same chemical composition. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F55.00012: Low-Temperature Ionic Conductivity Enhanced by Disrupted Ice Formation in Polyampholyte Hydrogels Xinda Li, Hemant Charaya, Guy Bernard, Vladimir Michaelis, Janet Elliott, Byeongdu Lee, Hyun-Joong Chung The phase behavior of water in hydrogels has a broad impact on various applications. Our previous study showed the polyampholyte hydrogel has the potential to be used as an aqueous gel electrolyte in the electrochemical storage devices at –30 °C, due to the enhanced low-temperature conductivity. In this study, the impact of polymer structures on the enhanced low-temperature ionic conductivity of polyampholyte hydrogel was explained with a model charge-balanced polyampholyte, poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino) propyl] trimethylammonium chloride), whose polymer and water structures were probed by variable-temperature SAXS and WAXS, respectively. Here, an interconnected globular network structure of polymer-rich phase at low temperature preserved ion-conducting channel of non-frozen water molecules at low temperatures. This hypothesis was further supported by MAS-NMR. This result provides an insight to design gel electrolytes for enhanced low-temperature performances. |
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