Electronic structure basis of strength and toughness in fluoropolymers

Fluoropolymers (FPs), such as teflon, FEP, PVDF, and ETFE, are a family of fluorocarbon-based plastic resins with different monomeric units of C-F bonds. They have excellent thermoplastic characteristics suitable for industrial applications. While they are engineered by adding or subtracting fluorines with other chemical agents like chlorine and ethylene, a common theoretical platform that can explain the distinctive thermo-mechanical behaviors of different FPs is yet to emerge. From electronic structure calculations using density function theory (DFT) simulations and electron population analyses, we find that electron redistribution is the electronic structure basis for governing mechanical behavior under different deformation conditions. Considering six polymers as example FP variants, we explore their mechanical behaviors by applying uniaxial loading along the C-C chains. Taking atomic variability and arrangement as indicators of chemical heterogeneity, we find that it plays a key role in controlling strength and toughness. Lower chemical heterogeneity and the symmetry of C-H rather than C-F bonds increase strength and toughness of a given polymer. These findings are expected to provide crucial guidance in designing new FPs with unprecedented extreme mechanical properties.