Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K55: Advancing Polymer and Biopolymer Physics though Simulation and Theory IIIFocus
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Sponsoring Units: DPOLY DCOMP DBIO GSNP Chair: Frederick Phelan Jr., National Institute of Standards and Technology Room: BCEC 254B |
Wednesday, March 6, 2019 8:00AM - 8:12AM |
K55.00001: Monte Carlo Modelling of Phase Separation in Polymer Blends That Contain Branched Molecules Emma Wood, Nigel Clarke Thermoplastic-toughening of thermosetting epoxy resins is becoming increasingly prevalent, particularly for aerospace composites. As the mechanical properties of these materials are highly dependent on morphology, it is extremely important to understand phase separation within blends of branched and linear polymers. Unfortunately, the Flory-Huggins model has significant limitations for systems containing branched polymers. Entropies of branched polymers are often much lower than for linear polymers of the same molecular weight, but the mean field approach of the Flory-Huggins model does not account for this. The impact of polymer conformation on energetic interactions is also ignored. Here, we have developed a simulation approach capable of overcoming these difficulties and calculating free energy for polymers with branched architectures. We have also used Monte Carlo methods to directly simulate concentration fluctuations within industrially relevant blends, and have therefore gained insight into their phase separation characteristics. |
Wednesday, March 6, 2019 8:12AM - 8:24AM |
K55.00002: Interface repulsion and arrest of coarsening in thin films of homopolymer blends due to thermal oscillations Marcus Mueller, Louis Pigard In equilibrium the interface potential that describes the interaction between two AB interfaces in a binary blend of A and B homopolymers is attractive, and this interface attraction gives rise to a coarsening of the blend morphology even in the absence of interface curvature. Using continuum models we demonstrate that the time-periodic variation of the Flory-Huggins parameter (or temperature or solvent concentration) qualitatively alters this behavior, i.e., for suitable parameter we find that AB interfaces repel each other and adopt a well-defined distance. We explore for which oscillation periods and amplitudes this interface repulsion occurs and how the preferred interface distance depends on these parameters. Using particle-based simulations we explicitly demonstrate that this dissipative self-assembly of a homopolymer blend results in a lamellar structure with multiple planar interfaces in a thin film geometry. |
Wednesday, March 6, 2019 8:24AM - 8:36AM |
K55.00003: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 8:36AM - 8:48AM |
K55.00004: Coarse-grained Modeling for Polymer Solutions via the Mori-Zwanzig formalism Shu Wang, Wenxiao Pan In this talk, we present a new method to establish implicit-solvent coarse-grained (CG) modeling for polymer solutions to conserve the dynamical properties of polymers. In the CG modeling, tens to hundreds of atoms were grouped as one CG particle; and the CG dynamic equations were rigorously derived from the atomistic data. The solvent-mediated dynamics of polymers was accurately captured via the generalized Langevin equation (GLE) with a non-Markovian memory kernel based on the Mori-Zwanzig formalism. The computational cost for direct evaluation of the non-Markovian memory kernel and generation of colored noise was significantly reduced by exploiting the equivalence between the non-Markovian dynamics and Markovian dynamics in an extended space. A higher-order time-integration scheme was developed to further accelerate the CG simulations. To assess, validate, and demonstrate the established CG modeling, we have applied it to simulate four different types of polymer solution systems. We find that the proposed CG modeling effectively conserves the velocity autocorrelation function and diffusivity of polymers and enables two orders of speedup in computer time, compared with the reference molecular dynamics simulations. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K55.00005: Simulating Diblock Copolymer Micelles in Binary Explicit Solvents Dong Meng, Jing Zong Amphiphilic block copolymers form nanoscale assemblies when dissolved in a selective solvent. Such self-assembled structures have wide-ranging applications as drug delivery vehicles and nanoreactors, etc. A powerful method to manipulate the assemblies is to vary the composition of solvent mixtures. Unlike single solvent solution, computational studies of amphiphilic block copolymers in solvent mixtures are rarely reported due to high computational cost associated with the necessity of treating solvents explicitly. Here, the Field-Accelerated Monte Carlo [1] simulation is employed in the expanded grand canonical ensemble to study the micelle formation of diblock copolymers in binary solvents: one selective solvent and one good solvent for both blocks. We investigate effects of molecular weight and solvent composition on micelle morphology, critical micelle concentration, and micelle size and aggregation number. It is found that distribution of the good solvent is highly inhomogeneous, concentrating at micelle interface and partitioning unevenly outside/inside micelle cores. Solvent intake by micelle cores increases with polymer molecular weight, affecting the way micelle size and aggregation number change with solvent composition. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K55.00006: Assessment of an Anisotropic Coarse-Grained Model for Cis-Polybutadiene Obtained by a Bottom-up Approach Ioannis Tanis, Claire Lemarchand, Rousseau Bernard, Laurent Soulard As it has been shown in previous coarse-grained simulations of polymer systems, the spherical representation utilized for the coarse-grained beads cannot capture anisotropic effects that play a crucial role on the structural features as well as on the dynamic response of such systems [1-2]. |
Wednesday, March 6, 2019 9:12AM - 9:48AM |
K55.00007: New Insights into the Glass Transition from Computational Prediction and Evolutionary Design Invited Speaker: David Simmons In most polymers, the glass transition is one of the most important phenomena determining performance properties including mechanical response, processability, and transport behavior. For this reason, understanding and controlling the glass transition is a longstanding goal of polymer science and soft condensed matter physics. However, the vast range of timescales associated with glass formation, coupled with a lack of an agreed-upon theoretical description of the problem, have posed major challenges to achieving this goal. Here I describe a new approach to this problem, combining efficient molecular dynamics simulations, physics-based heuristics, machine learning, and evolutionary algorithms to predict, understand, and design the glass transition. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K55.00008: Thermodynamics and kinetics of diblock copolymer micelles: chain architecture effect Prhashanna Ammu, Elena Dormidontova Diblock copolymers are actively used in various application, thus understanding the factors affecting thermodynamics and kinetics of self-assembly is of obvious importance. Computer modelling is a useful tool to analyze and compare behavior of various complex systems including diblock copolymers. Using dissipative particle dynamics simulations we investigate the effect of chain architecture on diblock copolymer self-assembly and the chain exchange kinetics using the example of diblock copolymers with a ring-shaped core or corona block in comparison with their linear diblock copolymer counterpart. We found a striking difference in both the equilibrium micelle size and kinetics of chain exchange in these systems. Furthermore, mixed micelles containing block copolymers of both types exhibit an unexpected synergism of chain exchange, which is not present in mixed micelles composed solely of linear chains. The origin of this effect and its implication for micelle self-assembly and practical applications of self-assembled nanostructures will be discussed. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K55.00009: Simulation of Free Surface of Block Copolymers Daniil Bochkov, Gaddiel Ouaknin, Frederic Gibou Due to their self-assembling properties, block copolymers are used in a variety of engineering applications, from patterning of microchips to targeted drug delivery. The study of these materials has been significantly accelerated by the powerful self-consistent field theory (SCFT), which has been particularly effective at studying polymer self-assembly in bulk or in confinements with a-priori known geometry. However, in many situations the surface of polymer material is free to deform (e.g, polymer/air interface) and its shape must be determined simultaneously while solving the SCFT equations. In this talk, we present a computational framework for simulating free surfaces block copolymers based on an analytical shape sensitivity analysis. Specifically, we consider an incompressible polymer melt described by the SCFT equations and derive an analogue of the Young-Laplace equation for block copolymers. Selective interactions of surrounding materials with distinct polymer chain blocks are taken into account by a new approach for imposing boundary conditions, consistent with the incompressibility property. To demonstrate the capabilities of this methodology, we present examples of suspended and substrate-supported diblock copolymer droplets. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K55.00010: Systematic and Many-Chain-Simulation-Free Coarse Graining of Polymer Melts: Structure-based Coarse Graining of the Kremer-Grest Model Yan Wang, Qiang Wang Our group recently proposed the systematic and simulation-free strategy for coarse graining of polymeric systems , where the well-developed polymer reference interaction site model (PRISM) theory, instead of the many-chain molecular simulation (MCMS), is used to obtain the structural and thermodynamic properties of both the original and coarse-grained (CG) systems. Our strategy is much faster than those using MCMS, thus effectively solving the transferability problem of coarse graining. It also avoids the problems caused by the finite-size effects and statistical uncertainties of MCMS, particularly for the original system, which are the reasons why its coarse graining is needed. Here we apply our strategy to the structure-based coarse graining of the well-known Kremer-Grest model for homopolymer melts, where the self-consistent PRISM theory is used to avoid the assumption of ideal-chain conformations in the original system. This paves the way to quantitatively applying our strategy to more realistic polymers and their multiscale modeling. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K55.00011: Extensional Rheology of Neat and Contaminated Ring Polymer Melts Thomas O'Connor, Ting Ge, Michael Rubinstein, Gary Grest Molecular simulations are applied to study unconcatenated ring polymer melts in uniaxial extensional flow. Melts of neat rings and neat linear chains with the same length, and rings contaminated with a small fraction of linear chains are elongated to steady-state for a wide range of Rouse Weissenberg numbers. The rate dependence of the steady-state stress and viscosity are compared for the three systems and correlated with changes in microscopic chain conformations. Extensional flows stretch and orient both ring and linear chain conformations along the extension axis. However, linear chains can stretch to twice the length of a ring with the same molecular weight. The significantly stronger response of linear chains to extensional flow suggests a small fraction of linear chain contamination might dominate the extensional viscosity of a nominally neat ring melt. Simulations of ring melts contaminated with varying fractions of linear chains are used to predict the experimental signature of linear contamination. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K55.00012: Stress Relaxation in Highly Oriented Melts of Entangled Polymers Austin Hopkins, Thomas O'Connor, Mark Owen Robbins Molecular dynamics simulations are used to study stress and conformational relaxation in entangled polymer melts deformed far from equilibrium by uniaxial extensional flow. Melts are elongated to a Hencky strain of 6 at Rouse Weissenberg numbers from 0.16-25, producing states with highly aligned chains. Then flow is ceased and the system is allowed to relax until twice the equilibrium disentanglement time. The relaxation of the stress is correlated with changes in the conformation of chains and the geometry of the tube confining them. The primitive path length of chains relaxes towards its equilibrium value on the equilibrium Rouse time and the orientation of the primitive path then relaxes on the equilibrium disentanglement time. Both results are counter to predictions of several recent models that suggest a large reduction in the entanglement density that persists for the disentanglement time, raising fundamental questions about the nature of entanglement in aligned molten polymers. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K55.00013: Plasticizing polymers with small-molecule additives: a not so simple picture revealed by a simple molecular model Kushal Panchal, Oluseye Adeyemi, Roozbeh Mafi, Li Xi Processing and manufacturing of many polymer products require the addition of plasticizers for tuning the flow properties of the melt as well as the thermo-mechanical properties of the product (lower Tg and improved material flexibility and ductility). These effects are often collectively described as plasticizers reducing the inter-chain friction and "softening" the material. However, a detailed molecular mechanism has not been fully revealed. We use molecular simulation to show that these effects are often not monotonically correlated. Most notably, additives that are better at reducing Tg may not be as effective at reducing the material stiffness. Indeed, our simulation shows that by simply changing the size of plasticizers, the Tg and Young's modulus vary in opposite directions. In-depth analysis of the free volume distribution and molecular mobility indicates that plasticizers have different effects on the dynamics at different length and time scales. Ongoing research focuses on the plasticizer effects on polymer viscoelasticity. Findings of this study reveal the rich complexity of the plasticization phenomena. In particular, the notion of "plasticization" is indeed a collection of a wide range of physical phenomena that are only loosely correlated at best. |
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