Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F55: Advancing Polymer and Biopolymer Physics though Simulation and Theory II: Functional Polymers and GelsFocus
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Sponsoring Units: DPOLY DCOMP DBIO GSNP Chair: Yaroslava Yingling, North Carolina State University Room: BCEC 254B |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F55.00001: Effects of Ion Group Distribution on the Structure and Dynamics of Amorphous Ionomers Gary Grest, Dipak Aryal, Dvora Perahia Ionic clusters control the structure, dynamics, and transport in ionic polymers. These ionic groups associate into random clusters in melts, where the distribution and morphology of the clusters impact the transport in these materials. Using fully atomistic molecular dynamics simulations we study melts of polystyrene sulfonate with 10% sulfonation with random, precise and blocky distributions of of the SiO3- sulfonate groups along the backbone. We find that for random distribution, the clusters in average are elongated, whereas the clusters are more globular when the sulfonate groups are equally spaced along the backbone. Elongated and globular coexists for blocks of sulfonate groups. The average ionic cluster size is comparable for the random and precise distributions and larger for the blocky distribution. We show that the low wavevector peak in the structure factor S(q) is much more well-defined for the precise and block cases compared to the random one. Interestingly, over the time scales of 1000 ns studied, the mobility of the block chains is higher than the random and precise, even though the ionic clusters are larger. The Na+ counterions are condensed and move at the same rate as the sulfonate groups. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F55.00002: Modeling Explicit-Ion Effects in Weak Polyelectrolyte Complexes Vikramjit Rathee, Hythem Sidky, Ben J Sikora, Jonathan Whitmer Polyelectrolytes may be classified into two primary categories (strong and weak) depending on how their charge state responds to the local environment. Both of these find use in many applications, including drug delivery, gene therapy, layer-by-layer films, and fabrication of ion filtration membranes. The mechanism of polyelectrolyte complexation is, however, still not completely understood. Here we detail computer simulations aimed at providing a comprehensive picture of the titration and complexation behaviour of these weak polylectrolytes and perform thermodynamic investigation through coarse-grained molecular simulations permitting us to calculate the free energy of complex formation and its dissection into energetic and entropic contributions. Our investigations indicate that entropic contributions indeed dominate the free energy of complex formation for strong polyelectrolytes, which is in agreement with experiments, but are less important than energetic contributions when weak electrostatic coupling or weak polyelectrolytes are present. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F55.00003: Polyelectrolytes under spatial and dielectric confinement Trung Nguyen, Monica Olvera de la Cruz Understanding the conformational behavior of polyelectrolytes inside viral capsids and lipid vesicles is crucial for gene delivery and drug loading applications. In this talk, we focus on the collective effects of dielectric mismatch and spatial confinement on the conformational behavior of a model polyelectrolyte chain and counterions confined in a spherical cavity. We show that surface polarization due to dielectric mismatch between the media inside and outside the cavity influences the chain conformational behavior. Specifically, it is found that surface polarization increases the work required to change the equilibrium (unconfined) chain conformation and favors the collapsed conformations with smaller size. The roles of surface charge density, electrostatic strength between the monomers and counterions and multivalent counterions are also discussed. The findings therein offer insights into the effects of dielectric mismatch, which were often overlooked in previous studies on confined polyelectrolytes. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F55.00004: Electronic Structure of Polymer Dielectrics: The Role of Chemical and Morphological Complexity Lihua Chen, Rohit Batra, Raghavan Ranganathan, Gregory Sotzing, Yang Cao, Ramamurthy Ramprasad The electronic structure of polymers contains signatures that correlate with their short-term and long-term integrity when subjected to large electric stresses. A detailed picture of the electronic structure of realistic models of polymers has been difficult to obtain, mainly due to the chemical and morphological complexity encountered in polymers. In this work, we have undertaken a comprehensive analysis of the electronic structure of six model polymers displaying chemical and morphological diversity, namely, polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA), polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), using first-principles density functional theory computations and classical molecular dynamics simulations. In particular, we have studied the role of monomer chemistry, tacticity and large-scale morphological disorders in shaping the electronic structure of these polymers. Appropriate connections and comparisons between the computed results and the available experimental data have also been provided. Critical insights on physico-chemical and electronic structures relationships are revealed, providing a pathway for understanding the factors that control electrical conduction and degradation of polymers. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F55.00005: Microscopic origin of the morphotropic-like phase boundary in P(VDF-TrFE) Bing Zhang, Wenchang Lu, Yang Liu, Haibibu Aziguli, Wenhan Xu, Qing Wang, Jerry Bernholc Ferroelectric materials with compositions near a morphotropic phase boundary (MPB) exhibit dramatically enhanced electromechanical properties, which could be utilized in advanced actuators, transducers, sensors and energy-harvesting applications. We have recently observed [1] a morphotropic-like phase boundary for the first time in an organic material, poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)], which makes it possible to develop high-performance electroactive polymers. This talk describes the results of our extensive first-principles calculations for this class of polymers. We find that chirality introduced by the TrFE monomers plays the central part in the structural competition that leads to a phase boundary between the trans-planar and the 3/1-helical chain conformations, explaining and supporting the experimental results. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F55.00006: Molecular Modeling of Polyetherimides Chengyuan Wen, Shengfeng Cheng Molecular modeling plays an increasingly important role in quantifying properties of polymeric materials and clarifying molecular mechanisms underlying material behavior. We use both atomistic and coarse-grained molecular dynamics (MD) and Monte Carlo (MC) simulations to investigate polyetherimides (PEIs), which are widely used for automotive industry, aircrafts, medical instruments, and chemical devices. We have developed a transferrable coarse-grained MD model of PEIs that is able to capture their thermal expansion properties and mechanical moduli, and demonstrated the importance of including entropic correction terms in the calculation of potentials of mean force when the coarse-grained force field is trained with atomistic MD simulations. To understand the dispersity of branched PEIs, we have developed a MC simulation tool based on the Gillespie algorithm for their polymerization reaction including branching. The comparison between MC simulations, the Stockmayer theory, and experiments on the molecular weight distributions of branched PEIs will be discussed. We use simulations to clarify the application range of the Stockmayer theory and to investigate the branching behavior of PEIs when the theory is not applicable. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F55.00007: A unifying perspective on rigidity in under-constrained materials Matthias Merkel, Karsten Baumgarten, Brian P Tighe, M. Lisa Manning We present a novel approach to understand rigidity in under-constrained materials, including sub-isostatic spring networks as well as 2D and 3D vertex models for dense biological tissues. We show that the onset of rigidity is determined by a purely geometric criterion. This allows us to analytically predict the elastic material properties close to the transition, which depend only on few geometric coefficients. We obtain exact expressions for the magnitudes of bulk modulus and shear modulus discontinuities at the rigidity transition, several scaling relations of the shear modulus, and the magnitude of the anomalous Poynting effect. Moreover, we show that the ratio of the excess shear modulus to the shear stress is inversely proportional to the critical shear strain with a prefactor of three, which we expect to be a general hallmark of rigidity in under-constrained materials induced by geometric incompatibility. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F55.00008: Quantum Chemistry Simulations for Dynamic Network Polymers Michael Buche, Zachary Sparrow, Yuval Vidavsky, Robert Distasio, Meredith Silberstein Conventional elastomers are crosslinked by covalent bonds. These covalent crosslinks create a permanent network, substantially increasing stiffness but decreasing stretchability. Alternatively, if polymers can be made with crosslinks that are strong, but able to break and reform, they will have similar stiffness benefits as the covalently crosslinked material without sacrificing stretchability, resulting in a high toughness material. Additionally, properties such as self-healing and solid-liquid transitions are also possible through this mechanism. In this talk, I will discuss a dynamic network polymer synthesized using organometallic coordination compounds as crosslinks, focusing on the behavior of the crosslinks as investigated using density functional theory. The potential energy surface of these crosslinking structures is reshaped as force is applied. The reaction paths of the structures are discovered using the growing string method, and are studied under increasing force. These studies yield force-dependent kinetics information that can be used to create a physics-based constitutive model of the material to compare with experimental mechanical tests. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F55.00009: Molecular Simulation Study of Orientational Dynamics in a Cross-linked Epoxy Network Ketan S Khare, Frederick Phelan Jr. Subtle changes in the chemistry of cross-linked epoxy networks have a drastic impact on the thermomechanical properties of cross-linked epoxy, which indicates the strong connection between chemical topology and polymer physics in these materials. Here, we study the orientational dynamics of cross-linked epoxy using atomistic molecular dynamics simulations. Specifically, we characterize the reorientation of vectors defined at atomic and molecular length-scales using first and second order Legendre polynomials (C1 and C2, respectively) at temperatures across the glass transition. For atomic length scales, we use the bond vectors of selected groups of atoms. For molecular length scales, we use end-to-end vectors of the epoxy monomer and the cross-linker that comprise the network. We then use the time—temperature superposition (TTS) principle to create master curves of C1 and C2. We find that while TTS can be successfully applied at molecular length scales, the topological location of the atoms can cause a distinct signature in the orientational dynamics of the bond vectors, which can be teased out using TTS. We are interested in using atomistic simulations to connect molecular insight with macroscopic properties. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F55.00010: Soft Nanoparticles as Adhesives for Gel-like Materials Ryan Sayko, Zhen Cao, Heyi Liang, Andrey Dobrynin It is known that nanoparticles can act as effective adhesives for soft polymeric materials. Using a combination of molecular dynamics simulations and theoretical calculations, we study the ability of soft nanoparticles to glue together gel-like surfaces. In particular, we have shown that at the interface between two gel-like surfaces nanoparticles can be in a bridging state, Pickering state, or engulfed state. The depth of indentations produced by nanoparticles into soft substrates is controlled by nanoparticle size, nanoparticle and substrate Young’s modulus, and their surface properties. Using molecular dynamics simulations, we have elucidated relationships between deformations of the nanoparticles and substrates and their elastic and surface properties. By varying moduli of nanoparticles and gel-like substrates, we find that it is possible to observe a coexistence of both bridging state and Pickering state. Furthermore, we use the Weighted Histogram Analysis Method to calculate the work required for separation of two gels which interface is reinforced by nanoparticles. To explain our simulation results, we have developed a model relating the work of substrate separation with the physical parameters describing contact phenomena in nanoparticle/substrate systems. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F55.00011: Modeling mechanics of large colloidal microgel suspensions Svetoslav V Nikolov, Alberto Fernandez-Nieves, Alexander Alexeev Large colloidal suspensions, comprised of soft and deformable microgel particles, exhibit a rich mechanical and phase behavior. As the particle density increases above close packing, individual particles start to deform and interpenetrate. Experimentally, these systems exhibit a high energy storage capacity and fast response times which makes them particularly attractive as self-healing and reconfigurable materials. The soft nature of the particles and their responsiveness to stimuli, such as temperature and pH, allows for external control of the emergence of liquid, liquid-crystal, and glassy phases. In our work, we utilize dissipative particle dynamics to construct large colloidal suspensions and study how the mechanical behavior of these suspensions changes with packing fraction and solvency. The colloidal suspensions are represented by an array of soft and deformable polymeric gels which are comprised of interconnected of polymer chains. We seek to determine how the behavior of individual chain parameters affects the underlying mechanical behavior of the suspensions in this multiscale system. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F55.00012: Tuning microscopic interactions in dry (co-)polymer systems for improved thermal conductivity Debashish Mukherji, Joerg Rottler Polymers are an important class of soft matter whose properties are dictated by large fluctuations. |
Tuesday, March 5, 2019 1:39PM - 2:15PM |
F55.00013: Advancing Polymer and Biopolymer Physics through Simulations and Theory: a few examples Invited Speaker: Alexander Grosberg In the talk, a few examples of successfull theoretical developments in biopolymer physics will be given. |
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