Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H33: Where Simulation, Theory, and Experiment Meet Across Time ScalesFocus Industry

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Sponsoring Units: DPOLY FIAP DCOMP Chair: K. Michael Salerno, Sandia National Laboratory Room: 336 
Tuesday, March 15, 2016 2:30PM  2:42PM 
H33.00001: Simulation of chain diffusion in diblock copolymer microstructures using dynamical selfconsistent meanfield theory Douglas Grzetic, Robert Wickham We simulate chain diffusion in ordered phases of a diblock copolymer melt, using our recentlydeveloped dynamical selfconsistent meanfield theory [\emph{J. Chem. Phys.} \textbf{140}, 244907 (2014)]. This theory enables us to study large length and time scales in these dense systems, while remaining connected, in a selfconsistent manner, to the microscopic physics of Brownian chains whose beads interact via a speciesdependent modified LennardJones potential. In the LAM and HEX phases, chain diffusion perpendicular to the microdomain interface is exponentially suppressed with increasing segregation, while parallel diffusion is unaffected. In the BCC phase, diffusion is isotropic and is gradually suppressed with increasing segregation. Chain diffusion is also isotropic in the gyroid phase, but does not vanish with increasing segregation. Instead, the diffusion constant asymptotes to a value consistent with chain diffusion being restricted to the interface of the threedimensional gyroid network of struts, characterized by a network tortuosity value of $1.72$. Finally, we measure the outofequilibrium evolution of the anisotropy in the chain diffusion as metastable LAM transforms to stable HEX over long times. [Preview Abstract] 
Tuesday, March 15, 2016 2:42PM  2:54PM 
H33.00002: Effects of Structured Ionomer Interfaces on Water Diffusion: Molecular Dynamics Simulation Insight Dipak Aryal, Dvora Perahia, Gary Grest The dynamics of solvent molecules across structured ionomers interfaces is crucial to innovative technologies with selective controlled transport. These polymers consist of ionizable blocks facilitating transport tethered to mechanical stability enhancing ones, where their incompatibility drives compounded interfaces. Here water penetration through the interface of an ABCBA copolymer is probed by atomistic molecular dynamics simulations where C is a randomly sulfonated polystyrene with sulfonation fractions $f \quad =$ 0 to 0.55, B is poly (ethylenerpropylene) and A is poly (tbutyl styrene). For $f $\textgreater 0, a twostep process with slow diffusion at the early stages is observed where water molecules transverse the hydrophobic rich surface before reaching the hydrophilic regime. Water molecules then diffuse along the percolating network of the ionic center block. Increasing the temperature and sulfonation fraction enhances both the rate of diffusion and the overall water uptake. [Preview Abstract] 
Tuesday, March 15, 2016 2:54PM  3:06PM 
H33.00003: Towards a Modeling Framework for Thermodynamics and Transport Coefficients in Polyelectrolyte Assemblies Ronald Larson, Ali Salehi A continuum description of polyelectrolyte (PE) equilibrium gelation, and the kinetics of assembly is developed, accounting for PE chain diffusion, complexation, network relaxation is reported here. Using a combination of FloryHuggins and FloryRehner free energy model, an upperconvected Maxwell model to describe polyelectrolyte gel stress and relaxation, and a Poisson equation for the electrostatic potential profiles, we develop a model that can account for both equilibrium properties of PE gels and for transport of PE's and ions during layerbylayer assembly. As PE chains diffuse, counterions readjust themselves to minimize the net local charge, but fail to do so completely as they would have to pay a significant entropic penalty. Diffusion of PE chains predominantly driven by the electrostatic field induced by the entropy of counterions is characterized by pulselike PE composition profiles. Even without considering chain complexation, we demonstrate that it is possible to at least qualitatively explain the nonmonotonic variation of PEM growth kinetics versus salt concentration, observed experimentally. [Preview Abstract] 
Tuesday, March 15, 2016 3:06PM  3:18PM 
H33.00004: Topological Constraints in Directed Polymer Melts Adam Nahum, Pablo Serna, Guy Bunin Polymers in a melt may be subject to topological constraints, as in the example of unlinked polymer rings. How to do statistical mechanics in the presence of such constraints remains a fundamental open problem. We study the effect of topological constraints on a melt of directed polymers, using simulations of a simple quasi2D model. We find that fixing the global topology of the melt to be trivial drastically changes both the static and dynamic properties. Polymers wander in the transverse direction by a distance which is only logarithmic in their length, and monomers subdiffuse logarithmically slowly. This is in sharp contrast to expectations from existing theories. To cast light on the suppression of the strands' wandering, we also analyse the topological complexity of subregions of the melt, finding it is also logarithmically small. We comment on insights the results give for 3D melts, directed and nondirected. [Preview Abstract] 
Tuesday, March 15, 2016 3:18PM  3:30PM 
H33.00005: Scaling of viscosity with rate, pressure, and temperature: Linking simulations to experiments Vikram Jadhao, Mark Robbins Elastohydrodynamic lubrication (EHL) is important in many practical devices and produces extreme pressures ($>1$ GPa) and shear rates ($10^510^7$ s$^{1}$). This makes EHL fluids ideal candidates for bridging the gap between experimental and simulation studies of viscosity. There is an ongoing debate about whether the highrate response of simple molecules like squalane follows a powerlaw Carreau model or a thermal activation based Eyring model. We use molecular dynamics simulations to investigate the rheological response of squalane for a wide range of rates ($10^510^{10}$ s$^{1}$), pressures (0.1 MPa to 3 GPa), and temperatures ($100313$ K). We find that experimental and theoretical results can be collapsed onto a master curve consistent with Eyring theory over more than 20 orders of magnitude in rate. Extrapolating Eyring fits to simulations at $10^7$ s$^{1}$ and above yields Newtonian viscosities $\eta_0$ that are consistent with available lowrate experiments, and allows predictions to much higher pressures and lower temperatures. There is no indication of a diverging viscosity at finite stress, since log $\eta_0$ rises sublinearly with pressure up to 6 GPa and $\eta_0 > 10^{12}$ Pas. Correlations between chain conformations and Eyring parameters are also presented. [Preview Abstract] 
Tuesday, March 15, 2016 3:30PM  3:42PM 
H33.00006: Unified forcelevel theory of multiscale transient localization and emergent elasticity in polymer solutions and melts Zachary E. Dell, Kenneth S. Schweizer A unified, microscopic, theoretical understanding of polymer dynamics in concentrated liquids from segmental to macromolecular scales remains an open problem. We have formulated a statistical mechanical theory for this problem that explicitly accounts for intra and intermolecular forces at the Kuhn segment level. The theory is selfconsistently closed at the level of a matrix of dynamical second moments of a tagged chain. Two distinct regimes of isotropic transient localization are predicted. In semidilute solutions, weak localization is predicted on a mesoscopic length scale between segment and chain scales which is a power law function of the invariant packing length. This is consistent with the breakdown of Rouse dynamics and the emergence of entanglements. The chain structural correlations in the dynamically arrested state are also computed. In dense melts, strong localization is predicted on a scale much smaller than the segment size which is weakly dependent on chain connectivity and signals the onset of glassy dynamics. Predictions of the dynamic plateau shear modulus are consistent with the known features of emergent rubbery and glassy elasticity. Generalizations to treat the effects of chemical crosslinking and physical bond formation in polymer gels are possible. [Preview Abstract] 
Tuesday, March 15, 2016 3:42PM  3:54PM 
H33.00007: Failure of Tube Models to Predict the Linear Rheology of Star/Linear Blends Ryan Hall, Priyanka Desai, Beomgoo Kang, Maria Katzarova, Qifan Huang, Sanghoon Lee, Taihyun Chang, David Venerus, Jimmy Mays, Jay Schieber, Ronald Larson We compare predictions of two of the most advanced versions of the tube model, namely the Hierarchical model by Wang et al. (J. Rheol. 54:223, 2010) and the BOB (branchonbranch) model by Das et al. (J. Rheol. 50:207234, 2006), against linear viscoelastic data on blends of monodisperse star and monodisperse linear polybutadiene polymers. The star was carefully synthesized/characterized by temperature gradient interaction chromatography, and rheological data in the high frequency region were obtained through timetemperature superposition. We found massive failures of both the Hierarchical and BOB models to predict the terminal relaxation behavior of the star/linear blends, despite their success in predicting the rheology of the pure star and pure linear. This failure occurred regardless of the choices made concerning constraint release, such as assuming arm retraction in fat or skinny tubes, or allowing for disentanglement relaxation to cut off the constraint release Rouse process at long times. The failures call into question whether constraint release can be described as a combination of constraint release Rouse processes and dynamic tube dilation within a canonical tube model of entanglement interactions. [Preview Abstract] 
Tuesday, March 15, 2016 3:54PM  4:06PM 
H33.00008: Challenging SlipLink Models: Predicting the Linear Rheology of 1,4Polybutadiene Blends of WellCharacterized Star and Linear 1,4Polybutadienes Maria Katzarova, Priyanka Desai, Beomgoo Kang, Ryan Hall, Qifan Huang, Sanghoon Lee, Taihyun Chang, David Venerus, Jimmy Mays, Jay Schieber, Ronald Larson The discrete sliplink model (DSM) is a singlechain meanfield model for entanglementdominated polymer dynamics. The model has been used successfully to make predictions about the linear and nonlinear rheology of monodisperse homopolymer melts, polydisperse melts, or blends. By using recent advances in coarsegraining, hierarchical modeling, and graphics processors, the model is amenable to predictions of wellentangled branched polymers. Here, the parameters of the most coarsegrained member of the hierarchy are fit to the dynamic modulus of monodisperse linear chains and applied to symmetric 4arm polybutadiene (PBd) starlinear blends with roughly 20 entanglements per star arm, but with no parameter adjustment. Agreement with data is quantitative. This detailed model is further used to examine assumptions and approximations typically made in tube models for blending, including factorization in the time domain. Failure of these assumptions point towards possible fixes to tube models. [Preview Abstract] 
Tuesday, March 15, 2016 4:06PM  4:18PM 
H33.00009: Multiscale simulations of polymer melt flow in an abrupt contraction and expansion channel Takashi Taniguchi, Kohei Harada We investigated a flow of a polymer melt with a molecular weight distribution in a channel with 4:1:4 contraction and expansion geometry by using a multiscale simulation (MSS) method here a macroscopic model and microscopic molecular model are directly connected through the velocity gradient field and stress field. In the MSS method, we introduced Lagrangian particles which contain many chains to precisely maintain the microscopic states. As the microscopic polymer model, a sliplink model is used. As a result, we clarified the relation between the macroscopic flow behavior and molecular level information such as a local polymer configuration and spatial distribution of number of entanglements per chain. [Preview Abstract] 
Tuesday, March 15, 2016 4:18PM  4:30PM 
H33.00010: Modeling Structure Property Relations and Failure Mechanisms of PPTA Fibers using Reactive Molecular Dynamics Dundar Yilmaz Failure mechanisms of poly(pphenylene terephthalamide (PPTA) under extreme tensile deformation has been studied using reactive potentials with molecular dynamics simulations. Amorphous PPTA systems with different molecular weights generated using an inhouse developed amorphous builder. Tensile modulus of amorphous PPTA has been calculated as up to 6.7 GPa. Nitrogen and carbon vacancy defects were introduced to both crystalline and amorphous systems. The tensile modulus of defectsfree crystalline PPTA calculated as 350 GPa. Introduction of 5{\%} nitrogen vacancy defects reduced the tensile modulus to 197 GPa. PPTA fibers generated with skin core structure where skin region composed of PPTA chains in crystalline order and core region was composed of unordered PPTA chains vice versa. Relation between ratios of skin and core regions and mechanical properties of the fiber studied. Tensile load was mostly accommodated through stretching of bonds between amide group and phenyl groups. Under extreme tensile deformation PPTA chains failed at these CN bonds. [Preview Abstract] 
Tuesday, March 15, 2016 4:30PM  4:42PM 
H33.00011: Chain Ends and the Ultimate Tensile Strength of Polyethylene Fibers Thomas C. O'Connor, Mark O. Robbins Determining the tensile yield mechanisms of oriented polymer fibers remains a challenging problem in polymer mechanics. By maximizing the alignment and crystallinity of polyethylene (PE) fibers, tensile strengths $\sigma\sim67$GPa have been achieved. While impressive, firstprincipal calculations predict carbon backbone bonds would allow strengths four times higher ($\sigma\sim20$GPa) before breaking. The reduction in strength is caused by crystal defects like chain ends, which allow fibers to yield by chain slip in addition to bond breaking. \\ We use large scale molecular dynamics (MD) simulations to determine the tensile yield mechanism of orthorhombic PE crystals with finite chains spanning $10^210^4$ carbons in length. The yield stress $\sigma_y$ saturates for long chains at $\sim6.3$ GPa, agreeing well with experiments. Chains do not break but always yield by slip, after nucleation of 1D dislocations at chain ends. Dislocations are accurately described by a FrenkelKontorova model, parametrized by the mechanical properties of an ideal crystal. We compute a dislocation core size $\xi=25.24${\AA} and determine the high and low strain rate limits of $\sigma_y$. Our results suggest characterizing such 1D dislocations is an efficient method for predicting fiber strength. [Preview Abstract] 
Tuesday, March 15, 2016 4:42PM  4:54PM 
H33.00012: Microscopic deformation mechanisms in model thermoplastic elastomers by molecular dynamics simulation Amanda Parker, J\"org Rottler Thermoplastic elastomers (TPEs) can be formed by exploiting the nanostructured morphology of triblock copolymers. Glassy endblocks phase separate to form spherical regions which act as physical crosslinks for the soft rubbery phase. Molecular dynamics simulations of TPEs allow us to relate the microscopic mechanisms active during plastic deformation to the macroscopic stress response. A coarsegrained beadspring model of linear ABA triblock copolymers which forms the desired spherical morphology is used for pure stress and pure strain uniaxial deformations. The systems are first equilibrated using a soft pair potential. We observe increased strain hardening in triblocks when compared to homopolymers of the same chain length in accordance with experiments. We connect variations in the stress response for systems of different chain lengths to the nonaffine deformation of chains and to the scale of phase separated regions. The stress response is also compared to rubbery elasticity models, taking into account the evolution of chain entanglements during deformation. We observe void formation at the interfaces of glassy regions or where these regions have broken up at large strain. [Preview Abstract] 
Tuesday, March 15, 2016 4:54PM  5:06PM 
H33.00013: ComputerAided Design of Photocured Polymer Networks Swarnavo Sarkar, Sheng LinGibson, Martin Chiang Lightinitiated free radical polymerization is widely used for manufacturing biomaterials, scaffolds for micomolding, and is being developed as a method for fast 3D fabrication. This process has a large set of control parameters in the composition of the photocurable matrix and the photocuring conditions. But a quantitative map between the choice of parameters and the properties of the resultant polymer is currently unavailable. We present a computational approach to simulate the growth of a polymer network using the stochastic differential equations of reactions and diffusion for a photocuring system. This method allows us to sample trajectories of a growing polymer network in silico. Thus, we provide a computational alternative to synthesize and probe a polymer network for properties like the degree of conversion, structure factor, density of states, and viscosity. We present simulation results that agree with the universal features observed in photopolymerization. Our proposed method enables a thorough and systematic search over the entire parameter space to discover interesting combinations for synthesis. [Preview Abstract] 
Tuesday, March 15, 2016 5:06PM  5:18PM 
H33.00014: Molecular Description of Yield in Densely Crosslinked Epoxy Thermosets Sandipan Chattaraj, Prita Pant, Dnyanesh Pawaskar, Hemant Nanavati In densely crosslinked networks, macroscopic yield is a transition from deformations of bond lengths and angles, to cooperative deformation of multiple effective network chains via bond torsions. In this work, we examine this yield in terms of the "activation number", $\nu$, of microscopic effective chains between crosslinks. $\nu$ is the number of effective network chains, in one Eyring activation volume, V*. It is thus a measure of the number of network chains ‘activated’ at yield, for cooperative deformation. Microcompression experiments have been performed on SU8 micropillars, to determine its V* value. SU8 is an important epoxy thermoset, which is used extensively in the microelectronics industry, in microfluidics and microelectromechanical systems (MEMS). The effective chain length based on Arruda and Boyce's 8chain model, compares well with the rms length, obtained by chain conformer analyses. We find that $\nu$ $\sim$ 24, at room temperature, for DGEBAbased epoxies including SU8 and DGEBAamine networks, over a range of network junction functionalities and V*. That $\nu$ corresponds very well with the reduced temperature, T/Tg, also demonstrates its viability as a molecular descriptor of yield in densely crosslinked thermosets. [Preview Abstract] 
Tuesday, March 15, 2016 5:18PM  5:30PM 
H33.00015: Toward a predictive model for elastomer seals. Nicola Molinari, Musab Khawaja, Adrian Sutton, Arash Mostofi Nitrile butadiene rubber (NBR) and hydrogenatedNBR (HNBR) are widely used elastomers, especially as seals in oil and gas applications. During exposure to wellhole conditions, ingress of gases causes degradation of performance, including mechanical failure. We use computer simulations to investigate this problem at two different length and timescales. First, we study the solubility of gases in the elastomer using a chemicallyinspired description of HNBR based on the OPLS allatom forcefield. Starting with a model of NBR, C=C double bonds are saturated with either hydrogen or intramolecular crosslinks, mimicking the hydrogenation of NBR to form HNBR. We validate against trends for the mass density and glass transition temperature for HNBR as a function of crosslink density, and for NBR as a function of the fraction of acrylonitrile in the copolymer. Second, we study mechanical behaviour using a coarsegrained model that overcomes some of the length and timescale limitations of an allatom approach. Nanoparticle fillers added to the elastomer matrix to enhance mechanical response are also included. Our initial focus is on understanding the mechanical properties at the elevated temperatures and pressures experienced in wellhole conditions. [Preview Abstract] 
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