Molecular Mechanisms Governing the Mechanics of Polymeric and Protein-Based Materials

Wang, Ao

doi:https://doi.org/10.21985/n2-8rsf-hn43

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Molecular Mechanisms Governing the Mechanics of Polymeric and Protein-Based Materials

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Since their discovery, polymeric and protein-based materials are key to technological innovation and are indispensable in our lives. However, these natural and man-made soft materials usually have complex and hierarchical structures, with basic functional units in submicron scale. Therefore, a major challenge in understanding and designing these materials lies in linking molecular scale design parameters to macroscale mechanical properties. Addressing this challenge requires methods than can transcend multiple length and time scales while addressing chemical and microstructural complexity of these materials. For this purpose, in this thesis, we attempt to investigate the molecular mechanisms governing the mechanics of polymeric and protein-based materials using molecular dynamics (MD) simulations. The first part of this thesis focuses on developing a computational procedure to measure and design the viscoelastic properties of elastomers directly from molecular-scale features. By carrying out high-frequency oscillatory shear MD simulations, we investigated the effect of chemical composition, crosslink density, and microstructure on the linear and non-linear viscoelasticity of ethylene propylene diene monomer rubber (EPDM). For small deformations in the glassy regime, we found that the simplest measure of local molecular stiffness, namely the Debye-Waller factor, is predictive of linear viscoelasticity. Large-amplitude oscillatory shear simulations revealed that dissipation becomes strongly influenced by polymer entanglements. Furthermore, utilizing time-temperature superposition in MD, we were able to capture rheological properties over 7 orders of magnitude in frequency, and the resulting master curves were used to predict key constants that determine the viscoelasticity of EPDM. The second part of the thesis covers studies on the tensile properties of titin protein-based fibers manufactured using synthetic biology approaches. Inspired from recent experiments that suggested that titin fibers can match the strength of biological superfibers such as silks, we examined molecular mechanisms that could explain these observations. MD simulations indicate that the non-bonded interactions, including Van der Waals and electrostatic interactions, can strongly hinder slippage between immunoglobin-like (Ig) domains in fibrils; while the staggered alignment of single fibers is pivotal to transferring shear stress. During stretching, the tension load is mainly born by the Ig domain. The strong cross-beta structure of Ig domains and close contact between single titin fibrils contribute to the high modulus and strength of the titin fiber. Meanwhile, the linkers between adjacent Ig domains contribute to the high extensibility of the titin fiber. These results establish the relation between the micro-scale hierarchical structures of titin protein-based fibers and their outstanding macroscopic mechanical properties, which could potentially guide the design of other high-performance protein-based fibers. The third part of this thesis focuses on bio-adhesives consisting of curli nanofibers, where we studied the detachment mechanisms of amyloid fibrils from surfaces to get deeper insight into adhesion mechanisms of natural and engineered biofilms. Amyloid nanofibers, such as curli nanofibers, have proven capable of adhering strongly to abiotic surfaces. We carried out coarse-grained MD simulations to determine the detachment mechanisms of single amyloid fibers from surfaces. We discovered that the amyloid nanofibers can undergo three different peeling processes when pulled at a constant rate normal to the surface. Computational phase diagrams built from parametric studies indicate that strong nanofibers with high cohesive energy detach by peeling smoothly away from the substrate while weak fibers break prematurely. At intermediate ratios, hinge formation occurs and the work of peeling the nanofiber is twice the adhesive energy due to the additional energy required to bend the nanofiber during desorption. Varying the geometry of amyloid subunits revealed that the work of peeling decreases for thicker nanofibers, suggesting that the tape-like monomeric structure of amyloids may facilitate better adhesive performance.

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