Multi-scale coarse-grained molecular dynamics simulation to investigate the thermo-mechanical behavior of shape-memory polyurethane copolymers

Abstract

Shape-memory polyurethanes (SMPUs) are smart semi-crystalline polymers with heat-induced shape memory. The segmented SMPU copolymer simulated in this study consisted of an isocyanate (hard segment) that acts as a netpoint to stabilize the network, and a polyol (soft segment) that, owing to polymer crystallization, acts as a molecular switch. The ratio of hard to soft segments is a determining factor in the thermo-mechanical behavior and shape-memory performance of SMPU copolymers. However, the limitations of the scale of the conventional all-atom molecular dynamics simulation makes it difficult to observe mesoscale phenomena such as the phase-segregated morphology and polymer crystallization. To address this problem, we have developed a coarse-grained (CG) molecular dynamics (MD) model with fewer degrees of freedom by treating multiple atoms as a single bead. We derived the CG intra- and inter-bead potentials of the CG MD model such that the structural and thermodynamic properties would be identical to those of the all-atom reference model. Consequently, polymer crystallization was verified to occur at room temperature with the developed CG potentials. We subsequently investigated the effects of hard-segment content (HSC) on the thermal transition at SMPU melting temperature. We also investigated the influence of HSC on the thermo-elastic behavior and shape-memory properties of SMPU copolymers by simulating a 4-step thermo-mechanical cycle. The results revealed that the microstructure significantly affects macroscopic shape-memory behavior. We expect that this study will be used to improve the prediction and design of the thermo-mechanical behavior of segmented polyurethane or other semi-crystalline polymers.