The Impact of Conformational Entropy on Non-linear Mechanical Behavior in Thermoplastic Elastomers

Thermoplastic elastomers (TPEs) are multiblock copolymers composed of hard (crystallizable) and soft blocks that form thermo-reversible network structures. They are widely used in various products due to their excellent mechanical properties and processability. However, their complex network morphologies challenge our understanding of their mechanical response beyond the linear regime. Using non-equilibrium molecular dynamics simulations, we reveal that not only crystallinity but also the conformational entropy of amorphous segments is major factor determining the mechanical response in the non-linear regime. We consider ABA-type (hard-soft-hard) TPEs, maintaining the length of the hard segment while varying the length of the soft segment (N_SS). Our results indicate that important mechanical characteristics in the non-linear regime (the slope M of the stress-strain curve, the remaining energy density W_R, and the residual strain γ_R) show a non-monotonic dependency on N_SS. By disintegrating the stress into energetic and entropic contributions, we find that the interplay between those two contributions should lead to the observed non-monotonic trends. Furthermore, we quantify the conformation entropy of the amorphous network strands (mainly the soft segments) by using the Wall equation, and reveal that it is the main contributor to the entropy changes in the non-linear regime.