Nanoscale strain-stress mapping for a thermoplastic elastomer revealed using a combination of in situ atomic force microscopy nanomechanics and Delaunay triangulation

Styrene block copolymer-type thermoplastic elastomer (TPE) has a nanoscale phase-separated structure and undergoes topological changes in response to external stimuli. A typical example is the macroscopic stress relaxation in TPEs while maintaining the elongated state. To understand the influence of structural changes on the mechanical properties of TPEs, in this work, we evaluate a styrene-ethylene-butylene-styrene-type TPE using an in situ atomic force microscopy (AFM)-based nanomechanical approach while maintaining 50% strain. According to AFM deformation and modulus maps of the same area at different relaxation times, it is found that the hard polystyrene domain splits in the early stage of relaxation, resulting in a constant rise in the number. At the same time, the modulus of the soft poly(ethylene-butylene) matrix continues to decrease. Both the domain number and matrix modulus stabilize in the late stage of the relaxation. Further finite element analysis explains the above phenomenon. The distribution of stresses and strains under the material microscopically evolves toward homogenization with relaxation time. The internal stress network achieves an elastic behavior after sufficient relaxation at the expense of hard-domain splitting.