Entropic Contribution to the Nonlinear Mechanical Properties of Thermoplastic Elastomers

Thermoplastic elastomers (TPEs), multiblock copolymers containing both hard (crystallizable) and soft segments (SSs), are employed in various applications due to their excellent mechanical properties. However, their mechanical response during deformation, especially at a nonlinear regime beyond the initial elastic deformation, remains elusive. Understanding the structure-property relationship for the nonlinear regime is, therefore, crucial for developing novel TPEs. In this work, we show by employing nonequilibrium molecular dynamics simulations that not only the crystallinity but also the conformational entropy of segments should account for the complicated mechanical response in the nonlinear regime. We consider triblock copolymers with two hard segments (HSs) at the ends and one SS in the middle. We fix the size of the HS but vary the size (N SS) of the SS. We find from our simulations that important mechanical characteristics in the nonlinear regime (the slope M of the stress-strain curve in the nonlinear regime, the remaining energy density W R, and the residual strain gamma R) exhibit nonmonotonic trends with N SS. We decompose the stress into energetic and entropic contributions and find that the interplay between those two contributions should lead to such nonmonotonic trends of mechanical properties in the nonlinear regime. We also show that the conformational entropy of the amorphous chains (mainly the SSs) bridging crystalline domains plays a critical role in the entropy changes in the nonlinear regime.