CAREER: Mechanics of Recyclable Thermoset Polymers

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This Faculty Early Career Development (CAREER) grant will aim to fundamentally understand the microstructure-property linkages of novel recyclable thermoset polymers (vitrimers). The ubiquity of plastics in our daily lives presents challenges due their potential threats to the environment and natural habitats arising from their low recyclability. Conventional thermoset polymers, whose attractive physical and mechanical properties make them important structural materials, are particularly insidious. The irreversible crosslinking at the molecular level seriously impedes their reprocessing and recycling ability. Novel vitrimer chemistries offer exciting avenues to develop mechanically robust, recyclable thermosets. However, predicting the emergence of defects (microstructure) during reprocessing and their impact on the mechanical characteristics (property) is difficult. The challenge is rooted in unraveling the damage processes associated with the statistics of microscale defect structures resulting from vitrimer chemo-mechanics at various length-scales and time-scales. The research project addresses these principal issues via novel multiscale computational mechanics. Its technological relevance is rooted in the need for a predictive modeling and simulation framework to enable damage-tolerant vitrimers for structural applications. The research program is integrated into a broader educational goal of creating an immersive learning experience at two levels: (i) underrepresented student groups including students with disabilities, (ii) an integration of granular art in education (K-12), and (iii) development of a graduate course on the microstructure-sensitive failure mechanics of materials. Transesterification based covalent adaptive network polymers (vitrimers) derive their reprocessing ability via bond exchange reaction chemistry. An attractive approach to recycling such vitrimers involves pulverizing them into granular powders and reprocessing them via thermomechanical compaction. During this process, individual particles undergo non-linear, dissipative deformation, and welding at inter-particle contact regions through novel chemo-mechanical healing processes. The resulting new solid embodies fingerprints of recycling in the form of micromechanical defect populations. These defect microstructures redefine their viscoelastic stiffening, strengthening, and damage tolerance, which depends on the mechanics at multiple length-scales: (i) granular thermo-chemo-mechanics of healing; (ii) defect micromechanics of damage evolution, and (iii) continuum micromechanics of recycled components. This project will address fundamental questions associated with the mechanics of recycled thermosets via a multiscale computational mechanics platform: (a) at the mesoscale, by discrete element modeling and simulation of granular ensembles endowed with vitrimers chemo-mechanics to quantify emergent statistics of the defect-damage linkages, and (b) at the macroscale, by formulating and computationally implementing a micromechanically informed statistical viscoelastic damage mechanics. The computational frameworks will be calibrated, validated, and assessed against a suite of existing experimental datasets while offering mechanistic predictions for the design of damage-tolerant recyclable thermosets.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.