Bulletin of the American Physical Society
2023 APS March Meeting
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session G12: Structural Relaxations in Permanent and Dynamic Covalent Polymer Networks and GelsInvited
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Sponsoring Units: DPOLY Chair: Ralm Ricarte, FAMU-FSU College of Engineering Room: Room 235 |
Tuesday, March 7, 2023 11:30AM - 12:06PM |
G12.00001: Forensics of Polymer Networks Invited Speaker: Andrey V Dobrynin Polymer networks intertwine many aspects of our lives from consumer goods to biological tissues. It is now well understood that their mechanical response is a result of convolution of distinct structural parameters: chemical composition, strand conformation, and network topology. Unfortunately, since the discovery of rubber vulcanization by Charles Goodyear in 1839, the internal organization of networks has remained a sealed “black box”, due to inability of modern techniques to characterize crosslinked systems in details once they are formed. This long-standing problem of polymer science is addressed by developing a forensic-style method based on the analysis of a network non-linear response to deformation. This methodology provides information about strand degree of polymerization between crosslinks, contribution of loops and entanglements in network elasticity as well as fraction of stress-supporting strands. Application of the developed framework to networks with trapped entanglements highlights a transition from crosslink- to entanglement-controlled network elasticity with increasing degree of polymerization of network strands between crosslinks and illustrates how specific feature of this transition manifested in changes of entanglement and structural shear moduli characterizing different modes of network deformation. For networks with comb and bottlebrush strands, it allows to establish dependence of their Kuhn length on molecular architecture. Furthermore, the decoded structural information enables classification of different network types according to the effectiveness of stress distribution between strands and quality control to evaluate human error in network synthesis. |
Tuesday, March 7, 2023 12:06PM - 12:42PM |
G12.00002: Cavitating Gels: Macroscale Failure to Molecular Structure Invited Speaker: Alfred J Crosby Failure, or fracture, remains one of the most important, yet most challenging, topics in materials science. This challenge is especially true for soft materials, such as hydrogels and biological tissues. Understanding failure in these systems is critical for developing important technologies, from robust prosthetic materials to protective gear for mitigating debilitating injuries. However, the ultra-soft nature of these materials makes quantifying failure processes difficult. Here, I present recent advances in understanding how cavitation-based deformations lead to either reversible or irreversible damage in model polymer gel materials. These advances were developed as a result of a multi-university, multi-disciplinary study that combined the use of precisely-defined polymer networks, molecular dynamic simulations, and new characterization methods to develop and validate theoretical relationships between molecular architecture and macroscale cavitation and fracture. Collectively, these results enhance fundamental understanding of ultra-soft gel materials and the mechanisms that control their failure across a range of size scales. |
Tuesday, March 7, 2023 12:42PM - 1:18PM |
G12.00003: Mechanics of Non-Concatenated Ring Polymers – Effects of Topology Revealed by Molecular Simulations Invited Speaker: Ting Ge While many aspects of the statics and dynamics of non-concatenated ring polymers have been elucidated in the past several decades, the transformation of ring polymers into practically useful materials has remained less explored. Using molecular simulations with perfect control of polymer topology, we make elastomers and thermoplastics out of non-concatenated ring polymers and investigate their mechanical properties during large deformation and failure, which serve as an essential foundation of the proper function of polymeric materials. Advancing the knowledge of structure-property relationships in ring polymer mechanics is urgent given recent developments in the precise synthesis and characterization of ring polymers. The simulations reveal that the elastomers made of cross-linked ring polymers are significantly more stretchable than cross-linked linear polymers. Compared to linear polymers, the entanglements between ring polymers do not act as effective cross-links. As a result, the stretchability of cross-linked ring polymers is determined by the maximum extension of polymer strands between cross-links, rather than between trapped entanglements as in cross-linked linear polymers. The simulations also reveal that the thermoplastics made of ring polymers below the glass transition temperature fail through a mechanism similar to the crazing in their linear counterparts under tensile loading. The stable craze formation indicates the existence of an entanglement network in glassy ring polymers. Nevertheless, the entanglement network consists of only a fraction of the topological constraints that force ring polymers to be in self-similar loopy globular conformations. The structural features of the ring polymer craze and the drawing stress during the craze formation are related to the underlying entanglement network. Apart from the simulations, theories have been developed to delineate the mechanical behavior of both ring elastomers and ring thermoplastics. The simulations and accompanying theoretical studies demonstrate tuning polymer topology as a transformative pathway to design polymer mechanics, propelling the establishment of a new paradigm of topological polymer chemistry entering materials science. |
Tuesday, March 7, 2023 1:18PM - 1:54PM |
G12.00004: Correlation between dynamics and macroscopic rheology of vitrimers Invited Speaker: Fardin Khabaz Vitrimers are a class of polymers that bring together desirable mechanical properties of thermosets with the reprocessing of thermoplastics. This ability arises from the rearrangement of the vitrimer network via a bond shuffling mechanism while its cross-link density remains preserved. As the properties of the material are directly affected by the dynamics of the polymer chains, it is critical to understand the link between the macroscopic behavior and the microscopic dynamic of the molecules. Molecular dynamics simulations can provide detailed molecular mechanisms of the system under macroscopic stress-induced deformations. We present a simulation methodology that utilizes coarse-grained molecular dynamics in conjunction with a Monte Carlo method to capture the bond exchange in vitrimers. Constant stress is applied to the system below its glass transition temperature while monitoring the strain (e.g., creep). Vitrimer shows accelerated dynamics under applied shear stress compared to that of thermoset. This different behavior between the two networks is reflected in the mean squared displacement of the crosslinker. In addition, the motion of the crosslinker shows non-affine displacement, resulting in regions of enhanced mobility (i.e., dynamics heterogeneities). |
Tuesday, March 7, 2023 1:54PM - 2:30PM |
G12.00005: Tuning the rheological properties of dynamic covalent hydrogels through crosslinking bond exchange kinetics for biomedical applications Invited Speaker: Adrianne M Rosales The dynamic properties of the natural extracellular matrix (ECM) allows for a continuous exchange of information with resident cells that changes over time. In addition, the dynamic properties of ECM polymers provide mechanical stress relaxation, thereby protecting and enhancing cell function. Despite the importance of matrix dynamics, nearly all commercially available cell culture platforms are static in nature or have limited control over viscoelasticity. Within this context, we developed a poly(ethylene glycol) (PEG) hydrogel platform using a fast reversible thia-conjugate addition crosslinking chemistry to impart viscoelastic properties at physiologic conditions. By controlling the aromatic substituents on a benzalcyanoacetamide, we demonstrate that we can preferentially control the forward reaction kinetics, which contributes to the overall modulus of the hydrogel. Importantly, we can perform these manipulations with minimal effect on the reverse reaction rate kinetics, thereby holding the stress relaxation properties of the hydrogel relatively constant. Furthermore, we showed the kinetics of bond exchange are tuned over several orders of magnitude with pH and that the developed hydrogels exhibit a regime of shear thickening under continuous shear. Finally, our data indicate good cytocompatibility with encapsulated fibroblasts and human mesenchymal stem cells, which promise suitability for cellular applications such as injectable cell delivery vehicles to localize delivery of therapeutics to a specific site. Overall, these data suggest a route to decoupling forward and reverse reaction rate kinetics in dynamic covalent PEG hydrogels, thereby expanding the toolbox for viscoelastic ECM mimics. |
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