Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A56: Symposium Honoring William W. Graessley IFocus

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Sponsoring Units: DPOLY Chair: Ralph Colby, Pennsylvania State Univ Room: LACC 515B 
Monday, March 5, 2018 8:00AM  8:36AM 
A56.00001: Molecular theory for Polymer Rheology: When the Mist Clears – and When it Swirls Back Again Invited Speaker: Thomas McLeish A day's unforgettable conversation with Bill Graessley was spent preparing for our jointlywritten chapter for Stealing the Gold, a book dedicated to the innovative science of Sir Sam Edwards. The idea was that Bill would write about the phenomenology of polymer melt rheology before the advent of tube theory, and I would cover subsequent developments. I was struck by the years of experience and vast grasp of the data on melt flow and relaxation phenomena that Bill had garnered over the years. He described his encounted with the DoiEdwards theory as 'the mist blowing away' or 'the fog clearing'  one of those moments where all the objects one has blundered into blindly suddenly take on their mutual relationship within a landscape clear to view. 
Monday, March 5, 2018 8:36AM  8:48AM 
A56.00002: Following the footsteps of Graessley to open the Pondera’s box in nonlinear polymer rheology Jianning Liu, Yi Feng, Shiqing Wang Modern research on nonlinear rheological responses of entangled polymers was initiated by the pioneering studies of Bill Graessley and others. One of Graessley's results [1] sent the earliest warning about lack of complete understanding after the creation of the tube model [2]: Polybutadiene melts showed ultra “strain softening” when examined with large stepwise shear. Along with Osaki, Graessley emphasized the importance to understand the anomaly associated with the stress relaxation. This presentation reviews the history of nonlinear polymer melt rheology, a subject we have devoted a decade of intense efforts [3] and indicates how the stress relaxation behavior has intrigued us all and revealed considerable insight into the essence of nonlinear responses. 
Monday, March 5, 2018 8:48AM  9:00AM 
A56.00003: Measurements of shear viscosity and normal stresses in entangled polymers Salvatore Costanzo, Giovanni Ianniruberto, Giuseppe Marrucci, Dimitris Vlassopoulos We present a simple method to measure the viscosity as well as both the first and second normal stress differences of polymers in nonlinear shear flow. It is based on the use of conepartitioned plate (CPP) geometry in order to measure over a wide range of shear rates without edge fracture problems. In particular, we employ a modular CPP setup with two different diameters of the inner transducer, mounted on a rotational straincontrolled rheometer. This setup overcomes limitations of previous approaches based on CPP, such as moderate temperatures, the need for multiple measurements with different volume of samples, and yields data over a wide temperature range by performing a twostep measurement on two different samples. The method has been successfully tested with two entangled polystyrene solutions at elevated temperatures. Results compare favorably with the limited literature data, especially on the second normal stress difference, as well as with predictions obtained with a recent tubebased model of entangled polymers accounting for shearflowinduced molecular tumbling. Limitations and possible improvements of the proposed simple experimental protocol are also discussed. 
Monday, March 5, 2018 9:00AM  9:12AM 
A56.00004: Dynamics of Entangled, Polydispersed Polymer Melts Gary Grest, Brandon Peters, K. Michael Salerno, Ting Ge, Dvora Perahia While essentially all theoretical and computational studies of entangled polymer melts have focused on monodispersed samples, the best polymer synthesis routes result in some dispersity, albeit narrow, in the distribution of molecular weights (M_{w}/M_{n} ~ 1.011.04). The effects of dispersity on chain mobility and the stress relaxation have been studied using molecular dynamics simulations of entangled, disperse coarsegrained polyethylene melts. Polymer melts with M_{w}/M_{n} = 1.0 to 1.16 for times of ~ 500 μs, which is on the order of the terminal stress relaxation time at T=500 K, were studied. The chain lengths were set to follow a SchultzZimm distribution for the same average M_{w} = 36 kg/mol. We find that the entanglement time and tube diameter are unaffected. The average diffusion constant however, increases with dispersity, as the contribution of the shorter chains moving faster outweighs the longer chains moving slower. The stress autocorrelation function was fit to the theoretical expression proposed by Likhtman and McLeish. The resulting plateau modulus, terminal time and viscosity all decrease with increasing dispersity, corresponding to an apparent increase in the entanglement molecular weight. 
Monday, March 5, 2018 9:12AM  9:24AM 
A56.00005: ForceBased Theory of the Shear Rheology of Entangled Polymer Melts Shijie Xie, Kenneth Schweizer We qualitatively extend the reptationtube theory for the stressstrain curve and chain stretch dynamics under continuous startup shear deformation. New theoretical aspects include a dynamic tension blob scaling model for predicting the interchain grip force that causes stretching, a delayed onset of chain retraction, a distribution of entanglement spacings and grip loss strains, and shearratedependent accelerated stretch relaxation for fast deformations (Rouse Wi_{R}>1). The convective constraint release idea is modified to be consistent with delayed retraction guided by the physical idea the overshoot is an elasticviscous crossover. Calculations are performed for the stressstrain response, chain stretch dynamics, and orientational stress and relaxation time. For Wi_{R}>1, qualitatively different results are predicted compared to existing tube models for the shear rate dependence of the overshoot stress and strain, degree of chain stretch at the overshoot and in the steady state, and the long time stress. Connections between nonlinear dynamics at the overshoot and in the steady state suggest common underlying physics. Very good agreement is found between theory and experiments and simulations. 
Monday, March 5, 2018 9:24AM  9:36AM 
A56.00006: LinearNonlinear Dichotomy of Rheological Responses in ParticleFilled Polymer Melts Wentao Xiong, Xiaorong Wang In the present study, systematic rheometric measurements are carried out to explore quasisinusoidal responses in the nonlinear regime of carbon blackfilled monodispersed polyisoprene melts with molecular weights ranging from 3 to 390 kg/mol. Experimental results show that these quasisinusoidal responses emerge from a sharp transition as the molecular weight of the matrix, M_{n}, approaches and passes through a characteristic molecular weight M_{c*}. Below M_{c*}, the system typically shows the classic nonlinearity, where storage modulus G’ decreases as strain amplitude increases and the resulting stress waveforms are distorted from sinusoidal waves. Above M_{c*}, the system displays anomalous nonlinearity, where the stress responses at any given strain remain surprisingly sinusoidal regardless the drop of modulus G’. In this manner, the system actually exhibits characteristic linearnonlinear dichotomy in the rheological responses in the nonlinear regime. The critical point M_{c*} is found to be a few times of the entanglement molecular weight M_{e}. 
Monday, March 5, 2018 9:36AM  9:48AM 
A56.00007: Effects of Branching on Rheology of Polyethylene Combs: A Molecular Dynamics Simulation study Sidath Wijesinghe, Dvora Perahia, Gary Grest Linear, branched, and star polymers exhibit distinctive rheological behavior, critical to their processing, depending on their topology. Early studies by Graessley et al. have shown that the effects of branches differ below and above the entanglement length. Lohse and coworkers demonstrated that branches on the length scale of the entanglement length of the backbone results in increased viscosity. Using coarse grained (CG) molecular dynamics simulations we study the effects of degree of branching, including their length and density on the rheology of entangled polyethylene (PE) melts, with branch lengths above and below the entanglement length while branching density is varied. Our CG models are able to capture the flow properties observed by experimental studies and provide fundamental new correlations between branching lengthdensities and rheology for entangled branched PE. As expected branched PE chains diffuse slower than their linear analogs. For polymer melts with same M_{W}, diffusion is predominantly governed by the branch length and only slightly affected by the branching density. Beyond the new model for CG of branched polymers, this study has provided new insight into longstanding challenges in polymer rheology. 
Monday, March 5, 2018 9:48AM  10:24AM 
A56.00008: Chi parameters from simulations Invited Speaker: Scott Milner The Flory–Huggins χ parameter describes the excess free energy of mixing and governs phase behavior for polymer blends and block copolymers. For chemically distinct nonpolar polymers, χ is dominated by mismatch in cohesive energy density. For chemically similar polymers, the entropic part of χ arising from nonideal packing can be significant. To investigate this, we perform molecular dynamic (MD) simulations for beadspring chains differing only in stiffness. Using thermodynamic integration, we extract χ as low as 10^{4} per monomer, in goo agreement with fieldtheory based predicitons of Fredrickson et al. We also obtain χ for the archetypical coarsegrained model of enthalpic polymer blends: flexible beadspring chains with different LJ interactions between A and B monomers. Using this χ and selfconsistent field theory (SCFT), we predict the interfacial profile for phaseseparated binary blends, in good agreement with MD simulations for immiscible blends. Applied to atomistic simulations, our method should be able to predict χ for new polymers from chemical structures. 
Monday, March 5, 2018 10:24AM  10:36AM 
A56.00009: Revisiting the Chain Behaviors of a Polymer at the Theta Point Pengfei Zhang, Nayef Alsaifi, ZhenGang Wang As one of the key concepts in polymer solutions, the theta point is defined as the vanishing of the osmotic second virial coefficient, and it is highly relevant to many important topics such as the coilglobule transition and phase separation in polymer solutions. In spite of many theoretical and simulation studies, there is currently no consensus on the shift of the theta point and the chain size change (i.e., whether chains are expanded or contracted). Here, based on a discrete Gaussian chain model, we investigate the theta point and the chain behaviors using a fourparameter model, which includes the bare second and third virial coefficients as well as the squaregradient term accounting for the finite interaction range. Our model is free of the divergence problem commonly encountered in previous studies based on the continuous Gaussian chain model, and thus can give unambiguous predictions and clarify the discrepancies in the literature. We find that the sign of the chain size change relative to the ideal counterpart depends on the relative strength of the bare third coefficient and the coefficient of the squaregradient term. 
Monday, March 5, 2018 10:36AM  10:48AM 
A56.00010: Electrostatic and hydrophobic interactions in NaCMC aqueous solutions: effect of degree of substitution and solvent medium on structure and rheology. Carlos Gonzalez Lopez, Ralph Colby, Joao Cabral Sodium carboxymethyl cellulose (NaCMC) is an anionic, weak, semiflexible polyelectrolyte, extensively used in industry. We employ SANS, light scattering, and rheology to probe the conformation and dynamics of NaCMC solutions across a wide range of molecular weight (Mw), degree of substitution, salt and polymer concentrations. In saltfree solution, the overlap concentration and correlation length show the expected c* ∝ N^{–2} and x ~ c^{1/2} dependences. The entanglement crossover scales as c_{e} ∝ N^{–0.6±0.3}, in strong disagreement with scaling theory for which c_{e} ∝ c* is expected. A second crossover, to a steep concentration dependence for specific viscosity (η_{sp} ∝ c^{3.5±0.2}), commonly assigned to the concentrated regime, follows c** ∝ N^{–0.6±0.2} which thus suggests instead a dynamic crossover related to entanglement. The scaling of c* and c_{e} in high salt solution show neutral polymer in good solvent behaviour, characteristic of highly screened polyelectrolyte solutions. The crossover between the saltfree and high salt solution is not well described by current scaling theories for flexible or semiflexible polyelectrolytes. Backbone hydrophobicity becomes important for poorly substituted samples at high concentrations. 
Monday, March 5, 2018 10:48AM  11:00AM 
A56.00011: Using the Semidilute Unentangled Regime to Measure NumberAverage Molecular Weight for Ionic Polymers Joshua Bostwick, Aijie Han, Xiuli Li, Louis Madsen, Ralph Colby We use scaling models to derive four methods to calculate the number density of chains and hence numberaverage molecular weight M_{n}, from polyelectrolyte chain dynamics in the semidilute unentangled concentration regime. Each method uses combinations of measurements that yield the number density of chains in ways that are insensitive to solvent quality and the presence of salt. We test these methods using pulsedfieldgradient NMR to measure diffusion coefficient, the shear rate dependence of steady shear viscosity to measure viscosity and relaxation time and the correlation length from smallangle Xray scattering. We will test these methods by investigating the sodium salt of sulfonated polystyrene (where M_{n} is already known) and a select group of other ionic polymers for which M_{n} has been elusive. This group of techniques shows promise for breaking the bottleneck of molecular weight determination for general ionic polymers, thus enabling accelerated development and application of these systems. 
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