Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F52: Padden Award SymposiumFocus
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Sponsoring Units: DPOLY Chair: Amalie Frischknecht, Sandia National Laboratories Room: BCEC 253B |
Tuesday, March 5, 2019 11:15AM - 11:39AM |
F52.00001: 24 MINUTE DELAYED START TIME
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Tuesday, March 5, 2019 11:39AM - 11:51AM |
F52.00002: Dynamic Heterogeneity in Entangled Linear and Ring Polymers: Single Molecule Studies Reveal Surprises due to Molecular Architecture Yuecheng Zhou, Charles Schroeder We directly observe the non-equilibrium dynamics of single ring polymers suspended in semi-dilute solutions of linear chains. Our results show that ring polymers fluctuate drastically in chain extension even at steady-state, yet ring polymers exhibit markedly less molecular individualism during transient stretching compared to linear chains. We hypothesize that ring polymer extension fluctuations arise due to threading of linear polymers through open ring polymer chains in flow. The fluctuation frequency as a function of strain rate and concentration is quantified, and trends are consistent with increasing concentration. We further study the relaxation of linear polymers in entangled solutions of purely linear chains. Our results show dynamic heterogeneity in relaxation such that single polymer relaxation trajectories exhibit either a single-mode or double-mode exponential decay, which starkly contrasts relaxation behaviors from dilute and semi-dilute unentangled solutions. We interpret the power law scalings of these relaxation times as a function of concentration, and our single molecule results are discussed in the context of the classic tube model and reptation theory. Our results show that molecular behavior is markedly heterogeneous in non-dilute polymer solutions. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F52.00003: Leveraging conductivity-enhancing pathways in homopolymer-blended block polymer electrolytes Melody Morris, Ryan Nieuwendaal, Joseph Dura, Thomas H Epps Block polymer (BP) electrolytes are appealing alternatives to liquid, gel, or homopolymer systems because the mechanical, thermal, and conductive properties are decoupled such that enhanced stability and performance are achievable. As a method to increase conductivity, polystyrene-b-poly(oligo-oxyethylene methacrylate) [PS-b-POEM] BPs were blended with a more mobile, ion-conducting POEM homopolymer; the blends then were doped with lithium salts. By adding homopolymers of different molecular weights, wet brush or dry brush regimes were achieved, as confirmed via neutron reflectometry. The wet brush polymer blends increased the mobility of both the polymer and lithium as suggested by the reduction of Tg,POEM and via line narrowing in variable temperature Li-7 solid-state nuclear magnetic resonance measurements, respectively. However, the dry brush blend demonstrated an overall higher ionic conductivity, likely due to the presence of homopolymer-rich ion channels. The results of this study provide key design parameters to promote enhanced conductivity via homopolymer-rich pathways in BP-based electrolytes. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F52.00004: Designing Coacervate-forming Systems Using Charge Sequence Tyler Lytle, Charles Sing Oppositely-charged polyelectrolytes can associatively phase separate in a salt solution via a process known as ‘complex coacervation’. Coacervation is driven in part by a large entropic gain due to counterion condensation and release. This drives coacervation by replacing condensed counterions with the oppositely-charged polyelectrolyte, leading to a significant increase in the counterion translational entropy. The magnitude of this entropy change can be tuned by altering the sequence of charged and neutral monomers, which leads to significant changes in phase behavior. We have developed a theoretical model to understand this connection between sequence and phase behavior by mapping the coacervate molecular structure to a 1D adsorption model that can be evaluated using the transfer matrix method. This transfer matrix theory uses inputs from Monte Carlo simulations to determine the phase separation and is able to determine the phase separation of sequence-defined polyelectrolytes. Theoretical results exhibit qualitative agreement with experimental and simulation results. These sequenced systems provide insights into the phase separation of intrinsically-disordered proteins and provides a method to use sequence specificity to tune the phase behavior of coacervate-based materials. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F52.00005: Interaction parameters governing self-assembly of ion-containing block copolymers Whitney Loo, Nitash Balsara Block copolymers have been studied for use in lithium metal solid-state batteries due to their ability to decouple ion transport and mechanical properties. It is not surprising that several theoretical models have been developed to describe the effect of salt on self-assembly including the counter-ion entropy model of Rabin, the Born solvation approach of Wang, the ionic self-consistent field theory (SCFT) of Sing and de la Cruz, the polarizable field theory of Fredrickson, and the free ion SCFT of Qin. In order to discriminate between theories, we have determined the morphology of a series of block copolymer/salt mixtures as a function of composition, chain length, salt concentration and temperature. The effect of salt on chain dimensions is determined independently from neutron scattering experiments on homopolymer/salt mixtures. The effective interactions between the blocks are determined by two methods: from characterization of disordered concentration fluctuations and from the locations of phase boundaries. Comparisons between experiment and theory will be presented. The experiments show the presence of coexisting phases at some phase boundaries (e.g. two BCC phases with different lattice constants) that have not been seen in salt-free systems nor captured by any theory. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F52.00006: Spontaneous degrafting of polyelectrolyte brushes from solid substrates Yeongun Ko, Jan Genzer Polymer brushes grafted covalently to solid substrates may degraft in some instances. We study the stability of strongly and weakly charged polymeric grafts derived from poly(2-dimethyl aminoethyl methacrylate) (PDMAEMA). We employ surface-initiated atom transfer radical polymerization to prepare polymer assemblies featuring gradients of grafting density and molecular weight. The degree of permanent charge in strong electrolytes is adjusted by reacting PDMAEMA with methyl iodide to a given extent. We interrogate the stability of those surface-grafted polymers under various pH at ionic strength values in solution. Swelling of the brush due to electrostatic charging in aqueous media (adjusted by varying pH and degree of quaternization) generates tension along the grafted macromolecular backbone. Such tension focuses at the bottom-most section of the polymer brush close to the substrate. This lowers the activation energy for breaking labile chemical bonds either in the initiator itself or the head-group chemistry of the initiator that links the initiator to the underlying substrate. Weak polyelectrolyte brushes are more stable than strong polyelectrolyte grafts. The stability of brushes decreases with increasing pH and decreasing ionic strength of surrounding solution. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F52.00007: Crystallization Modes of Poly(3-dodecylthiophene)-based Block Copolymers Depend on Regioregularity Jin-Seong Kim, Jonathan P Coote, Junghun Han, Gila E Stein, Bumjoon Kim Conjugated block copolymers (BCPs) can self-assemble into highly ordered nanostructures in a melt state. However, when cooled below the melting temperature, crystal growth can disrupt the self-assembled structure and produce a poorly-ordered fibrillary texture. Here, we demonstrate that crystallization modes of poly(3-dodecylthiophene) (P3DDT)-based BCPs can be tuned through regioregularity (RR) and morphology. When cylindrical BCP structure is formed, crystal growth tends to break through the morphology even with low RR, showing both “break-out” and “template” crystallization modes. In contrast, when lamellar structure is formed, crystal growth remains confined by the second block domain with high RR. This morphology-dependent behavior is attributed to geometric compatibility of P3DDT crystal growth and the self-assembled symmetry: In a lamellar phase, the P3DDT chain orientations at the block interface are compatible with crystal growth, and both the alkyl-stacking and π-π growth directions are unrestricted within a lamellar sheet. In a cylindrical phase, the radial orientation of P3DDT chains at the block interface is not compatible with crystal growth, and the hexagonal close-packed symmetry only allows for one direction of unrestricted crystal growth. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F52.00008: Quantitative assessment of tie chains for charge transport in conjugated polymers Kaichen Gu, Chad Ray Snyder, Jonathan Onorato, Christine Luscombe, Lynn Loo Intercrystallite molecular connections are widely recognized to greatly impact the macroscopic properties of semicrystalline polymers. Because it is challenging to directly probe such connections, theoretical frameworks have been developed to quantify their concentrations and predict the mechanical properties accordingly. Tie-chain connectivity similarly impacts the electrical properties in semicrystalline conjugated polymers. Yet, its quantitative impact has eluded the community. Here, we applied the Huang-Brown model, a framework commonly used to describe the structural origins of mechanical properties in polyolefins, to quantitatively elucidate the effect of tie chains on the electrical properties of a model conjugated polymer. We found that a critical tie-chain fraction of 10E-3, below which intercrystallite connectivity limits macroscopic charge transport, and above which intracrystallite disorder is the bottleneck. It follows that tensile straining polymer films with tie chains above percolation threshold reorients the interconnected crystallites, leading to charge-transport anisotropy in the films. Our study shows the importance of connected crystalline domains by tie chains for efficient charge transport and implicates the importance of long and rigid polymer chains. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F52.00009: Solvent quality and polymer concentration effects in linear and cyclic polymer solutions Thomas Gartner, Michael Hore, Arthi Jayaraman Manipulating chain architecture has long been explored as a strategy to tailor the properties of polymer melts and solutions; as such, there is a rich body of literature probing the physics of nonlinear polymer architectures. Herein, we use coarse grained (CG) molecular dynamics (MD) simulations, Polymer Reference Interaction Site Model (PRISM) theory, and small-angle neutron scattering (SANS) experiments to study linear and cyclic polymer solutions. Specifically, we examine the chain scaling exponent (ν) and effective polymer-solvent and polymer-polymer interactions as a function of architecture, solvent quality, and polymer concentration. We find striking quantitative agreement between our computational and experimental results, and we show that the linear and cyclic ν and effective interactions are similar in good solvents but significantly depart from one another as solvent quality decreases. These surprising trends are most pronounced at low polymer concentrations due to the balance between available intra- vs. inter-chain contacts in the linear and cyclic architectures. We also discuss the benefits and drawbacks of varying CG model resolution, providing guidance for others to make the correct choice of model when studying the polymer physics of similar systems. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F52.00010: Neural-Network Assisted Self-Consistent Field Theory for Block Copolymer Simulations Karim Gadelrab, Alfredo Alexander-Katz Self-consistent field theory (SCFT) provides impressive predictive capabilities for the equilibrium phases of a variety of block copolymer systems. When solved to high accuracy, qualitative as well as quantitative results can be compared to experiments. In this work, we exploit neural networks (NN) unique capability as a universal function approximator to evolve the fields in SCFT for several iterations during free energy minimization. To fully evolve the system, we use a hybrid algorithm mixing a proper PDE solver with the trained NN. The hybrid approach is verified on a diblock copolymer system. Convergence is achieved in all cases, independent of computational cell size, and molecular characteristics (volume fraction, and degree of block incompatibility). The lessons drawn from the NN-SCFT implementation can be extended to other energy minimization problems such as density functional theory (DFT). |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F52.00011: Interfacial and Wetting Considerations in High Throughput Nanoscale 3-D Transfer Molding Michael Deagen, Edwin Chan, Linda Schadler, Chaitanya Ullal Assembly of soft matter into three-dimensional structures at the nanoscale holds promise in fields including microfluidics and photonics, and transfer molding offers a low-cost, large-area approach to patterning such structures. In transfer molding, an ink is coated into a nano-patterned stamp, cured, and transferred to a substrate in a layer-by-layer fashion. We studied wetting, surface treatment, and adhesion in the context of continuous processing techniques with a focus on selective filling of stamps, a size scale dependence on adhesion following plasma exposure, and comparison of batch and continuous surface treatments. Residual-layer-free transfer molding was achieved through identification of wetting regimes related to the coating process combined with plasma doses orders of magnitude lower than the optimal dose suggested in prior literature on plasma bonding. Comparison of plasma treatment and corona discharge treatment for multi-layer adhesion further underscored the importance of brief surface treatments for both process throughput and stamp lifetime. This work brings transfer molding to new size scales, opening new opportunities for research in low-cost, multi-layer nano-patterning. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F52.00012: Close-packed Structures of Block Copolymer Micelles Induced by the Size of Crystallites Liwen Chen, Sangwoo Lee, Han Seung Lee Block copolymer micelles are versatile model spherical particles in the investigation of self-assembling structures of spherical particles and associated phase transformation phenomena. We investigated the close-packed structures of spherical poly(butadiene-b-ethylene oxide) (PB-PEO) diblock copolymer micelles dispersed in water using synchrotron X-ray scattering measurements. Remarkably, rapid thermal quenching of disordered PB-PEO micelle solutions to different temperatures induced three representative close-packed structures: face-centered cubic (fcc), random stacking of 2-dimensional hexagonal close-packing (rhcp), and hexagonal close-packing (hcp). Careful examination of the 2-dimensional scattering patterns revealed that the clear correlation between the type of close-packed structures and the size of crystallites controlled by the depth of thermal quenching: the smallest crystallites stabilize hcp, and as the size of crystallites increases, the hcp transforms to rhcp and eventually settled to fcc. This observation shows the interfacial tension effect is crucial for the selection of the metastable crystal structures confined in small crystallites. |
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