Crystal size effect on large deformation mechanisms of thermoplastic polyurethane

Abstract

Thermoplastic polyurethane, a phase-separated polymer containing amorphous soft domains and crystal hard domains, is a widely used high-performance polymer. However, how the crystal size affects the mechanical properties of thermoplastic polyurethane remains unclear. In this work, molecular dynamics simulations are carried out to reveal the atomistic deformation mechanisms coupled to crystal sizes. The atomistic models contain finite crystal hard domains in a representative volume element with periodicity in three dimensions. With comprehensive analysis in tension, compression, and shear, we find that the crystal size affects the timing and difficulty of deconstruction and rotation in crystal hard domain, cavitation in amorphous soft domain, and therefore determine the structural strength at different deformation stages. For example, smaller crystal size renders higher yield strength yet lower ultimate tensile strength. The discovered crystal size effect allows us to envision a gradient nano-crystal thermoplastic polyurethane with its customizable yet exceptional mechanical properties. By engineering the spatial distribution of crystals with different sizes, the gradient nano-crystal thermoplastic polyurethane can be strong in tension, yet soft in compression. Our current work deepens the understanding of the deformation mechanisms of thermoplastic polyurethanes and provides insights into the rational design of block copolymer materials with desirable mechanical properties.