Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M17: Structure, Dynamics, and Mechanics of Polymer Networks IFocus Recordings Available
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Sponsoring Units: DPOLY DSOFT DBIO Chair: Gabriel Sanoja, The University of Texas at Austin Room: McCormick Place W-184BC |
Wednesday, March 16, 2022 8:00AM - 8:36AM |
M17.00001: Rate-Dependent Damage and Fracture of Dynamic Networks Invited Speaker: Franck J Vernerey Dynamic networks made of polymer chains connected by weak and transient bonds are found in a majority of biological and synthetic polymers but also in active matter. Owing to their transient bond dynamics, these networks display a rich class of behaviors, from elasticity, rheology, transient fracture, self-healing and growth. This presentation will discuss a theoretical framework, a.k.a., the transient network theory (TNT) that provides a useful tool to track the chain conformation space of a dynamic polymer network. Extending the theory to account for chain rupture at a critical stretch, we scrutinize how damage initiates, grows, and competes with stress-relaxation in these networks. We find that permanent damage is largely rate-dependent and governed by two timescales that respectively describe stress relaxation and self-healing time. Based on this framework, we then explore the fracture of vitrimers that exhibit non-steady crack propagation patterns despite a constant loading rate. This motivates us to extend the TNT and present a time-dependent fracture criterion for crack initiation in dynamic networks. We show that in these networks, fracture is mainly governed by two parameters: the Weissenberg number, that defines the history path of crack-driving force, and an extension parameter that tells how far a crack can grow. These predictions are compared with experimental observation to verify the adequacy of the model. |
Wednesday, March 16, 2022 8:36AM - 8:48AM |
M17.00002: Energy dissipation after chain scission in polymer networks Han Zhang, Ziyu Ye, Robert Riggleman The fracture of end-linked polymer networks strongly affects the performance of these widely-used materials, but a fully quantitative understanding of the fracture behavior is still lacking. In recent years, theories built on the Lake-Thomas theory have emerged to predict the fracture energy by incorporating the effect of defects in the networks. However, experimental studies have suggested that the energy stored in each bond at the time of fracture (U) may be much smaller than the bond dissociation energy, which is a usual assumption in these theories. In this study, we perform molecular dynamics (MD) simulations to model end-linked polymer network systems and analyze the energy dissipated during fracture. Previous studies in our group have shown that a loop-modified Lake-Thomas theory successfully predicts the fracture properties of polymer networks without any adjustable non-physical parameters, but questions around the choice of U remain. Additionally, a dependence of the energy dissipated per bond on the polymer strand length is observed. In this talk, I will describe our simulations attempting to uncover how the energy dissipated depends on the polymer strand length, the network volume fraction, and defect concentration in the network. |
Wednesday, March 16, 2022 8:48AM - 9:00AM |
M17.00003: Fracture mechanics of polymers: plastics and elastomers Shi-Qing Wang, Travis Smith, Chaitanya Ramanand Gupta, Zehao Fan In this presentation we examine a unifying framework for fracture of polymers. The motivation stems from the relatively recent modeling [1,2] of yielding and brittle-ductile transition (BDT) in glassy polymers where chain uncrossability provides chain networking. The inherent mechanical strength of such polymers at or below BDT may be estimated in terms of the network structure and chain pullout in the glassy state. Such knowledge enables us to address the question of whether brittle fracture of polymers such as PS and PMMA and crosslinked rubbers could be understood without invoking the Griffith idea that accounts for the fracture strength in terms of critical energy release rate. Our preliminary results suggest that we can and perhaps should develop a different fracture criterion that also takes the material response into explicit account, which influences the geometric characteristics of intentional precut. |
Wednesday, March 16, 2022 9:00AM - 9:12AM |
M17.00004: Mechanical effects of topological entanglement and knotting of linear chains in a melt Eleni Panagiotou The viscoelastic properties of polymer melts are affected by the entanglement of the constituent chains. However, defining/measuring entanglement remains a challenge. We use tools from topology, the Gauss linking integral and the Jones polynomial, to measure entanglement in polymer melts of linear chains using computer simulations. We use molecular dynamics simulations of coarse grained models of polymer melts of linear chains of varying molecular weight. Our results show that the self and pairwise topological entanglement complexity increases with molecular weight in equilibrium and non-equilibrium conditions. By imposing an oscillatory shear we find that both self and pairwise topological entanglement decrease with increasing loss tangent. All these results suggest that tools from topology can capture entanglement complexity relevant to material properties. |
Wednesday, March 16, 2022 9:12AM - 9:24AM |
M17.00005: Understanding elasticity of polydisperse polymer networks by a stochastic method Weikang Xian, Amitesh Maiti, Andrew Saab, Ying Li Elasticity of polymer network origins from the microscopic connectivity of its constituent polymer chains. Classical theory, like affine and phantom models, is well established and has enjoyed great success, with an assumption that the network comprises only of monodisperse polymer chains. However, variation of chain-length, i.e., polydispersity, is present in a wide range of applications and affects the elasticity of the network. On the other hand, it is possible to tune the elasticity of the network by controlling the polydispersity since sophisticated methods have been developed. Therefore, understanding the structure-property relationship of the polydisperse network is highly valuable. Although efforts have been devoted to clarifying this relationship, most of them rely on phenomenological extensions of the classical theory, usually with extra unphysical assumptions. These extensions might be sufficient to explain certain experimental observations, but do not necessarily elucidate how the polydispersity inherently affects the network elasticity. In order to shed light on the structure-property relationship, we propose a stochastic sampling method that is capable of predicting the elastic moduli of polydisperse polymer networks explicitly. Based on the graph representation of network connectivity, this method effectively estimates junctional fluctuations of chains in the network and predicts the moduli accurately. Large-scale molecular dynamics simulations, with up to over 30 million monomers, are conducted to verify the proposed method. Consistency between theoretical and computational results suggests that the stochastic method is not only efficient but also powerful in evaluation of the elasticity of polymer network with complex microstructures. |
Wednesday, March 16, 2022 9:24AM - 9:36AM |
M17.00006: Fracture, fatigue, and friction of polymers in which entanglements greatly outnumber cross-links Junsoo Kim, Guogao Zhang, Meixuanzi Shi, Zhigang Suo In gels and elastomers, how entanglements affect deformation has long been studied, but how entanglements affect fracture, fatigue, and friction has not. Here we synthesize gels and elastomers in which entanglements greatly outnumber crosslinks. We demonstrate that such polymers resolve a long-standing conflict: crosslinks stiffen polymers but embrittle them. When a polymer of dense entanglements and sparse crosslinks is stretched, before a polymer chain breaks, tension transmits in the chain along its length, and to many other chains through entanglements. This distribution of tension leads to high toughness, strength, and fatigue resistance. Upon swell in water to equilibrium, the hydrogels exhibit low hysteresis, low friction, and high wear resistance. Such an exceptional combination of properties opens doors to broad and immediate applications. |
Wednesday, March 16, 2022 9:36AM - 9:48AM |
M17.00007: Controlling topology and mechanical properties of polymer networks through kinetics of gelation Aaliyah Z Dookhith, Nathaniel A Lynd, Costantino Creton, Gabriel E Sanoja Soft materials are widespread in adhesives, elastomers, and hydrogels, but the molecular design of their mechanical properties remains challenging due to their network-like disordered structure. Recent investigations have quantitatively related the presence of topological defects to the elastic modulus and fracture energy, but a missing link still exists on how those defects are formed during gelation. |
Wednesday, March 16, 2022 9:48AM - 10:00AM |
M17.00008: Effects of Crosslink Homogeneity on the High Strain Behavior of Elastic Polymer Networks Victoria Kong, Thomas Staunton, Jennifer E Laaser The topology of polymer networks affects critical material properties such as elastic modulus and toughness. While the effects of crosslink homogeneity and topological defects are well understood in low strain regimes, understanding their effects in the high strain regime has proven difficult because of premature network fracture. Here, we address this problem using a double network approach, in which the network of interest is swollen in a mixture of monomer and cross linkers that is then polymerized to generate a secondary network that helps dissipate stress, delay fracture, and "lock" the first network in a higher strain state. We examine the effect of crosslink homogeneity by comparing the low and high strain behaviors of randomly and regularly crosslinked butyl acrylate networks. Regularly-crosslinked networks are synthesized via coupling of n-butyl acrylate star polymers, while randomly crosslinked networks with comparable moduli are synthesized by free-radical polymerization. We find that networks with similar crosslinking densities exhibit similar low-strain behavior but that differences emerge in the high-strain regime, reflecting the growing importance of short network strands in transmitting stress in this limit. |
Wednesday, March 16, 2022 10:00AM - 10:12AM |
M17.00009: A Molecular-Level Theory for Predicting Rheological Behavior of Dynamically Associating Polymer Networks Pamela Cai, Andrew J Spakowitz, Sarah Heilshorn, Matthew Webber, Brad Krajina Polymer materials with dynamic associations are ubiquitous in materials applications, from tissue engineering to flexible electronics, due to their tunable physical behavior. Designing new dynamic polymer networks can involve many rounds of synthesis and characterization and would greatly benefit from the ability to predict the mechanical behavior of hypothetical materials. To that end, we present a new theory that incorporates experimentally controllable molecular-level parameters (concentration, chain length, unbinding rate) to fully capture the rheological behavior of physically associating gels. Hyaluronic acid, a biopolymer used in many biomedical applications, serves as the basis for studying biologically relevant polymers and are modified with guest-host molecules to form a dynamic supramolecular network, exhibiting tunable physical properties. Rheological measurements using dynamic light scattering microrheology are compared to theoretical predictions using only molecular level parameters across 6 decades in frequency without the assumption of time-temperature superposition. The resulting fit demonstrates the utility of our theory as a key tool in future design principles of dynamically associating polymer materials. |
Wednesday, March 16, 2022 10:12AM - 10:24AM |
M17.00010: Controlling network relaxation and modulus with dynamic covalent chemistry and salt Christopher M Evans, Brian Jing, Seongon Jang Polymer networks with dynamic bonds are a promising route to sustainable materials, but many questions remain about the relationships between molecular structure and dynamics or mechanics. Dynamic polyethylene oxide (PEO) networks were prepared with two types of exchangeable bonds, one which coordinates to anions and one which coordinates to cations. In the former case, added LiTFSI salt leads to break up of the network and a corresponding drop in both modulus and viscosity. In the latter case, salt increases the modulus while decreasing the viscosity. Li NMR experiments reveal that depending on the length of PEO linkers in the network, the Li can either coordinate with ether oxygens or with the dynamic bond leading to vastly different dynamic mechanical properties even for the same polymer/dynamic bond/salt combination. In all cases, the moduli of the dynamic networks increase with temperature due to entropic elasticity, indicating the network topology is conserved even with salt addition. The environment preferred by the Li cations not only impacts the relaxation of the networks, but also can lead to conductivity when Li is solvated by PEO linkers. Our findings highlight the ability of dynamic chemistry and salt to work in tandem for designing recyclable functional polymers. |
Wednesday, March 16, 2022 10:24AM - 10:36AM |
M17.00011: Dynamic Covalent Polymer Networks under Supersonic Micro-Projectile Impact Zhen Sang, Qing Zhou, Wenpeng Shan, Hongkyu Eoh, Jinho Hyon, Frank Gardea, Svetlana A Sukhishvili, Edwin L Thomas Dynamic bonding brings new features to polymeric materials, including new modes of energy dissipation and self-healing under quasi-static deformation. The behavior of dynamically bonded materials under high strain rates (HSRs) is relatively unexplored and is of interest for a broad range of applications from body armor to spacecraft protection. The laser-induced micro-projectile impact test (LIPIT) was employed to investigate the energy dissipation characteristics during perforation of ultrathin films of Diels-Alder-based polymer (DAP) dynamic networks. The networks are composed of low-molecular-weight furan-attached prepolymers (7000 g/mol) and bismaleimide crosslinkers. The room temperature Young’s modulus can be tuned nearly three orders of magnitude (MPa to GPa) by varying crosslinking density. Micron-sized silica projectiles were launched at incident velocities of 200-800 m/s toward a DAP thin film, yielding extreme strain rates of ~107 s-1. Dissociation of the thermally reversible DA bonds was triggered by compressive shock adiabatic heating, followed by viscoelastic, energy-absorbing flow of DAP material. Post-mortem morphology of DAP films shows a volcano shape and partially recovered-healed, smooth surface perforation. |
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