CAREER: Mechano-Adaptive Polymers with Reversible Strain Stiffening and Softening via Active Control of Metal-Ligand Interactions

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NON-TECHNICAL SUMMARYThe mechanical and elastic properties of synthetic polymers are one reason why they have become ubiquitous in modern life. However, comparatively, biomaterials exhibit unmatchable dynamics and complex mechanical behaviors. This has motivated materials scientists and engineers for years to push the limit of synthetic polymers to approach the functionalities of biomaterials. This project is inspired by the mechanics of biological cells. Actin, which plays an important role in supporting cell mechanical integrity, shows stiffening followed by softening under mechanical stress. The stiffening protects the cell from initial impact while the softening prevents catastrophic fracture. These mechano-responses are fully reversible. Imparting this mechanical reversibility of “stiffening-softening” to synthetic polymers will enable unprecedented mechano-adaptation and allow the materials to actively adjust their shape and stiffness in response to the mechanical environment, becoming resistant to both low and high impacts. These properties have not been realized in synthetic polymers. As such, this project aims to develop polymeric materials with tailored structures and molecular interactions to achieve similar mechanics. The results will advance the limited understanding of controlling the nonlinear mechanics of complex molecular networks. It will set the stage toward materials that emulate mechanobiology to have force-regulated sensing, signaling, and morphing functionalities. The project will be integrated with strategic education and outreach plans centered on virtual reality (VR) demonstrations to engage a diverse group of students and promote polymer science education in Northern Nevada. TECHNICAL SUMMARYBiomaterials exhibit unmatchable dynamics and complex mechanical behaviors compared to synthetic polymers. Supramolecular polymers with tunable noncovalent bonds provide an opportunity to bridge this gap. Intensive research has been conducted to elucidate the interdependencies of material properties, polymer architectures, and the characteristics of the associative groups. However, active control of the dynamics of the associative bonds by mechanical force is still understudied. Thus, this project will elucidate the use of this dynamic parameter by focusing on the change of sticker kinetics during mechanical perturbation in a nonequilibrium state, in order to access the complex nonlinear viscoelasticity for responsive networks. The nonlinear mechanics of interest is the “stiffening-softening paradox” of living cells. They stiffen under low strains to resist the initial impact, soften at high strains to prevent catastrophic fracture, and recover their original forms upon load removal, representing dynamic adjustments of their mechanical property according to the surrounding environment. The project plans to develop a solvent-free polymer that contains tailored noncovalent bonds, whose associative interactions can be actively adjusted with force in a multi-component system assembled from linear-bottlebrush-linear triblock copolymers. Each component would provide complementary properties to realize complex nonlinear mechanics. This study will address the fundamental question as to how different structural components independently dominate the macroscopic viscoelasticity in certain force ranges, and when their coupled effects become dominant. A combination of synthetic approaches and mechanical & spectroscopic characterization methods will be used to probe their interactions. Understanding the interplay between multi-component systems will serve as a springboard to emulate living materials, which rely on multi-component hierarchical structures to optimize living functions. .This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.