Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session D12: Out-of-Equilibrium: Structure and Dynamics of PolymersInvited
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Sponsoring Units: DPOLY Chair: Yangyang Wang, Oak Ridge National Laboratory; Shiwang Cheng, Michigan State University Room: Room 235 |
Monday, March 6, 2023 3:00PM - 3:36PM |
D12.00001: New Computational Tools to Address a Hard Non-equilibrium Problem: Crystallization of Polyethylene Invited Speaker: Ronald G Larson New simulation methods are described that can contribute to solving an old problem: understanding how molecular weight and branching distribution, rheology and processing conditions, and the resulting semi-crystallinity, control the end properties, such as modulus and toughness, of polyethylene film. In this talk, a practical method is presented that accounts for the effects of short- and long-chain branching on rheological properties of commercial polyethylene through use of an optimal ensemble of chains, that can be used to predict chain orientation in a processing flow such as film blowing. Molecular dynamics simulations are used to determine rates of primary and secondary nucleation of polyethylene crystals including the influence of branching. We show that both primary and secondary nucleation proceed through a nematic-like intermediate. We show that short-chain branches are partially expelled from the growing crystal to an extent dependent on branch length and by measuring the degree of expulsion, we can infer the defect energy induced by trapping of branches of various lengths in the crystal. Future work will apply these insights to the development of a film blowing model that includes new insights into both rheology and crystallization. |
Monday, March 6, 2023 3:36PM - 4:12PM |
D12.00002: Single Polymer Dynamics of Ring-Linear Blends and Entangled Solutions Invited Speaker: Charles M Schroeder Single polymer dynamics provides a powerful window to directly view the nonequilibrium behavior of polymers in flow. In recent years, single molecule techniques have been used to uncover fascinating and unexpected phenomena in polymer physics, including the importance of molecular individualism, dynamic heterogeneity, and molecular subpopulations that underlie the dynamic behavior of materials. In this talk, I will discuss our group’s recent work in extending the field of single polymer dynamics to architecturally complex polymers such as rings, in addition to entangled solutions of linear chains in flow. Ring polymers are a unique class of macromolecules that lack free ends and show qualitatively different dynamics compared to linear polymers. In dilute solution, ring polymers undergo a coil-stretch transition in extensional flow that is different than linear polymers due to a coupling between chain architecture and hydrodynamic interactions (HI), resulting in an ‘open loop’ conformation in flow. We further study the dynamics of rings in shear flow using a custom flow-gradient shear device for fluorescence microscopy. In shear flow, single rings undergo end-over-end tumbling events and tank-treading-like motion that is markedly different than linear chains. Upon increasing polymer concentration, we also study the dynamics of rings in semi-dilute solutions of pure linear chains or blends of ring-linear chains. Surprisingly, the relaxation dynamics of rings in semi-dilute unentangled solutions reveals the emergence of multiple molecular sub-populations, which is analogous to the relaxation of linear chains in the entangled regime. Finally, we study the nonequilibrium stretching dynamics of rings in semi-dilute blends of ring-linear polymers in extensional flow, which reveals unexpected fluctuations in chain extension, even at ‘steady-state’. Brownian dynamics simulations are used to complement single molecule experiments, which show that ring extension fluctuations arise due to a combination of intermolecular HI effects and threading of linear polymers through rings. Overall, our work shows that molecular behavior is markedly heterogeneous in non-dilute polymer solutions, which showcases the power of single molecule techniques to understand the nonequilibrium dynamics of soft materials. |
Monday, March 6, 2023 4:12PM - 4:48PM |
D12.00003: Developing a qualitative chain-level description of polymer mechanical properties Invited Speaker: Shi-Qing Wang In many polymer products, better mechanical properties are achieved through nonequilibrium structures acquired from specific processing procedures. Today sustainable (biodegradable, recycled) polymers cannot replace PE, PP and PET because of their inadequate mechanical characteristics. Sustainability of polymers remains an empty promise unless we develop a basic chain-level understanding mechanical behavior of polymeric materials in terms of what structures are required for desirable mechanical properties. By reviewing the available phenomenology concerning processing-structure-property relationship in the literature we emphasize that (a) preservation and enhancement of chain networking are essential, (b) non-equilibrium structures such as geometric condensation (GC) of the chain network can be generated during processing to promote toughness and ductility. We will describe the concept of crystalline chain network (CCN), suggest that predrawing of semicrystalline polymers can result in GC envisioned previously for glassy polymers, and show why molecular strategies such as strain-induced nano-confined crystallization preserves CCN to render certain sustainable polymers such as poly(lactic acid) superbly ductile and strong. |
Monday, March 6, 2023 4:48PM - 5:24PM |
D12.00004: Initial Solvent Induced Non-Equilibrium Effect of Polymer Nanocomposites Invited Speaker: So Youn Kim Since the degree of particle dispersion can determine the physical properties of polymer nanocomposites (PNCs), plenty of studies have focused on the intrinsic parameters of PNCs such as the concentration/size/chemistry of nanoparticles/polymers relevant to the particle microstructure. While the consideration of these parameters is based on PNCs being in their equilibrium states, PNCs can be kinetically trapped in a nonequilibrium state during the multiple steps of processing. In other words, processing conditions can contribute more significantly to particle dispersion and the properties of PNCs beyond the effects of the intrinsic parameters. |
Monday, March 6, 2023 5:24PM - 6:00PM |
D12.00005: Atomistic simulation of microphase separation and flow-induced crystallization above the melting point of entangled polymers under elongational flow Invited Speaker: Brian J Edwards This presentation will focus on recent work in the MRAIL Group at the University of Tennessee consisting of united-atom simulations of entangled polyethylene solutions and melts of linear C1000H2002 undergoing both planar and uniaxial elongational flow. Flow-induced phenomena in entangled solutions of linear C1000H2002 polyethylene dissolved in n-hexadecane and benzene solvents were simulated via nonequilibrium molecular dynamics at concentrations of 14.5C* and 13.5C*, respectively, of the coil overlap concentration, C*. The simulations revealed that both solutions undergo a chemical phase separation when subject to planar extensional flow at extension rates faster than the inverse Rouse time of the solution. The onset of phase separation initiated after roughly two Hencky strain units of deformation for both solutions and attained a stationary state at about ten Hencky strain units. Furthermore, the simulations revealed that at very high extension rates the polymer phase forms semicrystalline domains regardless of the solvent; however, the critical extension rate for flow-induced crystallization appeared to be affected by a number of variables, including solution temperature and the chemical nature of the solvent. Similar qualitative behavior was observed in atomistic simulations of the C1000H2002 melt under both planar and uniaxial elongational flow. Based on the assumption that the global crystallization process followed a first-order reversible kinetic rate expression with a lag time, kinetic rate constants were calculated as functions of the Deborah number that allowed quantification of the flow-induced crystallization phenomenon exhibited by the simulated system under planar elongational flow at a temperature high above its quiescent melting point. Similar results for both PE melts and solutions were also found for uniaxial elongational flows, with very subtle differences arising in the thermodynamic phase diagram. |
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