Modeling the Molecular Mechanisms of Self-healing in Precisely Sequenced Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) are nanostructured copolymers with coexisting rubbery and glassy domains that can display various thermomechanical behaviors. Recent experiments have shown that TPEs can also display rapid intrinsic self-healing when their copolymer sequences are precisely ordered. However, the molecular mechanisms mediating this sequence-sensitive self-healing remain unknown and a topic of active debate. Here, we apply MD simulations to model the self-assembly, dynamics, and welding of self-healing TPEs with controlled monomer sequences. We systematically vary sequences and generate bulk TPEs samples with various self-assembled nanostructures that show consistent trends with observations from small angle neutron scattering. Samples are then cleaved and placed in contact for interfacial welding, following “cut-and-adhere” protocols in experiments. We measure the time dependent recovery of the ultimate strength and toughness and observe mechanical recovery is sensitive to microphase separation and the formation of glassy nanodomains. Sequences that suppress nanodomain formation display rapid welding and weak mechanics, while sequences that promote nanodomain formation cause chains to become pinned in glassy domains which slow interfacial diffusion and strength recovery. Notably, our simulations reveal that strength recovery in nanostructured TPEs is not driven by the interdiffusion and entanglement of individual chains, as in conventional thermoplastics. Instead, we observe that the significantly slower strength recovery in nanostructured TPEs is mediated by the slow, collective dynamics of the glassy nanodomains at the welding interface.