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 Q15: Structural Relaxation in Dynamic Covalent Polymer NetworksFocus
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Sponsoring Units: DPOLY Chair: Fardin Khabaz, Univ of Akron Room: Room 207 |
Wednesday, March 8, 2023 3:00PM - 3:12PM |
Q15.00001: Glass Transition Behavior of Crosslinked Epoxy/Amine Resins with Prospective Self-Healability Cody T Bezik, Amalie L Frischknecht Self-healing polymers that can dynamically heal damage with no external intervention have enhanced longevity and reduced environmental impact. One strategy to create self-healing polymers is to modify commodity polymers into highly branched networks, taking advantage of strong, glassy cores at the heart of the networks, while retaining mobility at free ends on the periphery of the networks. Adding supramolecular interactions such as hydrogen bonding to those mobile free ends could enable self-healability across a wounded interface. Using large scale coarse-grained molecular dynamics simulations, we calculate the thermomechanical properties of highly branched polymers with hydrogen-bonding end groups. Our model is based on an epoxy system with amine crosslinkers. We vary the ratio of epoxy to amine and the ratio of two different epoxies (one stiff, one flexible), and determine the resulting polymer network architecture and glass transition behavior. We find that experimentally observed anomalies in Tg are reproducible in-silico and explore the molecular basis for these anomalies. |
Wednesday, March 8, 2023 3:12PM - 3:24PM |
Q15.00002: Catalytic Control of Crystallization in Dynamic Polymers Alexa S Kuenstler, Christopher N Bowman Associative interactions in macromolecules introduce significant opportunities to manipulate their static and dynamic behavior. While the influence of ‘sticky’ groups like hydrogen bonding and ionic groups on emergent mechanics, rheology, and morphology have been extensively explored, only recently has the effect of dynamic covalent bonds on such properties begun to be probed. When incorporated into polymer networks, dynamic covalent bonds undergo exchange reactions that introduce plasticity into the network, facilitating rearrangement of network strands. In this work, we explore how bond exchange can be used to control crystallization in polymer networks. We show that the kinetics of thiol-thioester exchange reactions with chemically-identical polymer networks can be controlled through the choice of nucleophilic catalysts. In this way, characteristic timescales of network relaxation can be controlled over three orders of magnitudes (tR ≈ 101 – 103 seconds). Using these materials as model systems, we find that as the rate of bond exchange increases, the rate of crystallization decreases for similar extents of undercooling, and that this can be controlled either through the judicious choice of catalytic species or catalyst loading. Finally, we present a rationale for these observations that provides insight into how manipulation of dynamic bonds can be used to control assembly in polymer systems. |
Wednesday, March 8, 2023 3:24PM - 4:00PM |
Q15.00003: Impacts of Network Heterogeneity on the Macroscopic and Molecular-Scale Force Responses of Polymer Network Invited Speaker: Jennifer E Laaser We investigate the role of crosslink heterogeneity in determining the molecular-scale force responses of polymer networks using two sets of experiments. First, we investigate regularly and randomly-crosslinked networks of poly(n-butyl acrylate) prepared either via coupling of tetrafunctional star polymers (regular networks) or free-radical copolymerization of n-butyl acrylate with a difunctional crosslinker (random networks). We find that the random networks exhibit a significantly earlier onset of strain hardening relative to regular networks with the same modulus, which we attribute to the short strands in the random networks reaching their maximum extension at relatively low strains without being able to relax. We then explicitly probe the relative tension in the short and long strands of a triblock elastomer by incorporating force-responsive mechanophores in the middle of the rubbery midblocks of bidisperse samples. We find that the short strands activate far earlier than the long strands, supporting our hypothesis that the earlier onset of strain hardening is driven by short strands that are pinned to the glassy domains and are unable to relax. Together, these results demonstrate the importance of network topology, dispersity, and pinning of strands to chemical or physical crosslinks in determining both the macroscopic and molecular-scale force responses of polymer networks. |
Wednesday, March 8, 2023 4:00PM - 4:12PM |
Q15.00004: Linear viscoelasticity of polybutadiene vitrimers Ralm Ricarte, Sachin Shanbhag Vitrimers are covalently cross-linked polymer networks that are both insoluble in good solvent but processable at high temperatures. These paradoxical traits are enabled by their cross-links, which engage in dynamic associative exchange reactions. Altering the chemical structure of the vitrimer cross-linker, backbone, or both modifies flow and mechanical properties. Recently, Ricarte and Shanbhag used the inhomogeneous Rouse model (IHR) to elucidate structure-viscoelasticity relationships for vitrimer melts. Their findings suggested that the interplay between chain friction and cross-link exchange controls flow under small-strain conditions. In this work, we experimentally investigate the rheological behavior of polybutadiene (PB) vitrimers which bear cross-links that undergo dioxaborolane metathesis. PB vitrimers were formed by cross-linking commercial PB (Mn = 4.3 kg/mol and Ð = 1.06) with bis-boronic esters via a thiol-ene click reaction. Cross-link density was varied from 1 to 12 mol%. Linear viscoelasticity was evaluated using small-amplitude oscillatory shear (SAOS), stress relaxation, and creep and recovery measurements. For SAOS, the elastic modulus remains constant while the viscous modulus increases as angular frequency decreases. For stress relaxation and creep, terminal relaxation is not observed over a period of 8 hrs, even at temperatures of 160 °C. The rheological data are compared to the IHR model to gauge the impact of XL mobility and temperature on vitrimer relaxation. |
Wednesday, March 8, 2023 4:12PM - 4:24PM |
Q15.00005: Control of Hierarchical Relaxation in Acrylate Vitrimers with Kinetically Distinct Dynamic Bonds Laura E Porath, Nabil Ramlawi, Randy H Ewoldt, Christopher M Evans The rheological relaxation characteristics of a polymer network will determine its macroscopic viscoelastic properties and potential applications. When a polymer network exhibits peaks in the tan delta at a certain frequency, incoming sound or vibration waves at similar frequencies can be dampened by the polymer. Design of the peak placement and width can control which vibrations will be dampened. Some degree of control of the relaxation spectrum has been demonstrated with metal-ion hydrogels. However, a robust understanding of how the polymer chemistry, architecture, and crosslinking lead to relaxation peaks and their associated width of frequency is not yet available. By selecting a crosslinked acrylate vitrimer with a high density of dynamic bonds and through mixing dynamic crosslinkers with bond lifetime differences of orders of magnitude, multiple peaks were observed in the network relaxation spectrum. Oscillatory shear rheology shows a network with one crosslinker exhibits Maxwell behavior typical of a rubbery network, but networks with mixed fast and slow crosslinkers demonstrate a second maximum in the loss modulus which is associated with the faster crosslinker. The multimodal behavior was tunable via the percentage of two dynamic bonds and temperature of the system. |
Wednesday, March 8, 2023 4:24PM - 4:36PM |
Q15.00006: Effect of salt and crosslinking density on viscoelasticity and conductivity of vitrimers Seongon Jang, Brian Jing, Chengtian Shen, Charles M Schroeder, Christopher M Evans Covalent adaptive networks with a topology-conserving bond exchange mechanism (known as vitrimers) are emerging materials replacing conventional polymers due to their reprocessability and self-healing properties. Vitrimers are of key interest in applications such as electrolytes where viscoelasticity and conductivity are controlled by salt addition. In this study, vinylogous urethane (VU)-based vitrimers containing Li salts were prepared with different crosslinking densities using precise linker lengths of ethylene glycol. Stress relaxation experiments show that the ionic dynamic networks exhibited substantially faster relaxation dynamics with smaller characteristic relaxation times compared to neutral networks. Ionic dynamic networks materials exhibited an Arrhenius temperature dependence in relaxation times, which is consistent with expected behavior for vitrimers. In addition, it was found that relaxation times and shear moduli decrease with lower crosslinking densities. Electrochemical impedance spectroscopy was performed to investigate ionic charge transport depending on the network mesh size. Our results show that ionic conductivity increases as the linker length increases. Interestingly, the conductivity data collapse onto a universal curve regardless of crosslinking density when temperature is normalized by the glass transition temperature, which indicates that ionic charge transport is primarily controlled by the segmental chain dynamics of ethylene glycol. 7Li ssNMR suggests that vitrimers with longer linker lengths prefer Li-ethylene oxide coordination rather than Li-VU coordination based on chemical shifts. Overall, this work provides an improved understanding of the effect of salt and crosslinking density on properties of vitrimers, which is essential for applications including solid polymer electrolytes. |
Wednesday, March 8, 2023 4:36PM - 4:48PM |
Q15.00007: Influence of Topological Defects on the Rheology and Dynamics of Vitrimers Jianshe Xia Vitrimers are polymeric networks with dynamic associative covalent bonds, which can be reprocessed like thermoplastics yet still retain the beneficial properties of a crosslinked thermoset material. Here, we put forward a hybrid coarse-grained molecular dynamics and Monte Carlo simulations to investigate the rheology and dynamic properties of vitrimers. We explore the effects of polymer chain structures on the stress relaxation and shear viscosity. We find that vitrimers with uniform and random sticker distributions show larger viscosity and longer relaxation time of stress than gradient and blocky distributions. This is attributed to the fact that more loop defects in gradient and blocky distributions accelerate the relaxation of dynamic cross-linked bonds. Consequently, our simulation results provide some guidelines for understanding and controlling the rheology and dynamic properties of vitrimers with various chain structures. |
Wednesday, March 8, 2023 4:48PM - 5:00PM |
Q15.00008: Effect of network defects on the microscopic dynamics of vitrimers Harsh Pandya, Fardin Khabaz A significant amount of scientific and technological emphasis has been given to the problem of recycling and reprocessing permanently crosslinked polymers known as thermosets. Vitrimers provide a sound solution to this problem owing to their ability to undergo topology rearrangements under a thermal stimulus. The presence of a dynamic covalently bonded network combines the strength of thermosets with the re-processibility of thermoplastics. Nevertheless, there is a gap in the literature on understanding the effects of factors such as bond exchange dynamics and defects in the network structure on the said topological alterations. In this study, we will use a hybrid molecular dynamics-Monte Carlo simulation to investigate the effects of various degrees of defects on the microscopic dynamics of the vitrimer matrix. The correlations between the network defects and the topology freezing temperature will be studied using the volumetric properties of the systems. Then the value of topology freezing temperature will be related to the microscopic dynamics of the network that will be characterized using the mean squared displacement and van Hove autocorrelation function of the crosslinking beads. |
Wednesday, March 8, 2023 5:00PM - 5:12PM |
Q15.00009: Spatiotemporal Vitrimerization of a Thermosetting Polymer using a Photo-Latent Catalyst Chae Bin Kim Globally rising environmental concerns have increased the demand for recyclable polymers and efficient recycling practices. The lack of melt-processability of thermosets significantly limits their recycling because the simplest and most economical way to recycle polymers with a minimum carbon footprint is repairing or re-melting them into new items. To this end, an approach to spatiotemporally converting a thermoset into a processable vitrimer is presented. This was achieved by incorporating a latent, heat-stable photo-active catalyst into an otherwise thermoset, activating associative network exchange reactions only after light exposure and subsequent heating. The thermoset film could be repaired and/or heat-processed by simply exposing the sample to light followed by annealing at elevated temperature and pressure, whereas the repairing/heat-processing failed in the absence of the light exposure step. This approach enables the polymer to initially behave as a thermoset at all temperatures until it requires repair or heat-processing. |
Wednesday, March 8, 2023 5:12PM - 5:24PM |
Q15.00010: Unravelling the Mechanism of Viscoelasticity in Linear and Pendant Polymers with Phase Separated Dynamic Bonds Peyton Carden Incorporation of dynamic bonds within polymer structure enables properties such as self-healing and recyclability. These dynamic bonds, referred to as stickers, can form clusters by phase-segregation from the polymer matrix. These systems exhibit interesting viscoelastic properties with an unusually high and extremely long rubbery plateau. Understanding how viscoelastic properties of these materials are controlled by the hierarchical structure is crucial for engineering of materials for various future applications. Here we studied such systems made from telechelic polydimethylsiloxane chains, as well as pendant functionalized chains by employing a broad range of experimental techniques. We demonstrate that formation of a percolated network of interfacial layers surrounding clusters enhances mechanical modulus in these systems, whereas stickers hopping between the clusters results in terminal flow. Analysis also reveals that the concentration of stickers plays the critical role in viscoelastic properties of these materials, while specific placement of the stickers (chain ends or along the chain) only impacts the behavior on time scale between pulling the sticker out of the cluster and terminal relaxation. From our results, we formulate a general scenario describing viscoelastic properties of polymers with phase-separated dynamic bonds, including the role of architecture. This understanding will foster development of recyclable materials with tunable rheological properties. |
Wednesday, March 8, 2023 5:24PM - 5:36PM |
Q15.00011: Reactive Bead-Spring Models for Associative Polymer Networks Far From Equilibrium Songyue Liu, Thomas O'Connor Associating polymers form dynamic networks of reversible bonds that rearrange their network topology. This makes them promising candidates for adaptive and reprocessable network materials; however, the dynamic bonding produces a hierarchy of chain relaxation modes that give rise to complex flow behavior. Understanding these dynamics requires models that can capture the feedback between dynamic bonding and polymer relaxation in nonequilibrium conditions, but most existing models for dynamic bonding use Monte Carlo algorithms with reaction rates that lose validity under flow. Here we share a new reactive molecular dynamics model that combines a Tersoff bond order potential for associative bond chemistry and a bead-spring model for entangled polymers. The resulting coarse-grained model captures the essential physics of chain dynamics, chain entanglement, and coordinated dynamic bonding and can be tuned to capture a variety of associative bond kinetics. The many-body Tersoff Hamilitonian for dynamic bonding remains valid in nonequilibrium flow conditions. We use this model to simulate canonical Kremer-Grest bead-spring melts with binary associative bonds of varying cohesive strength. We measure the gelation transition with increasing association strength and identify a gel-point at an associative bond strength ~ 1kT. We also assess how chain dynamics and network viscoelasticity change as the degree of gelation increases. |
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