The role of soft segments on the stress relaxation of thermoplastic elastomers

Thermoplastic elastomers (TPEs) are mostly block copolymers or polymer blends that consist of hard (HS) and soft (SS) segments. Hard segments form crystallites, which act as thermo-reversible crosslinkers. On the other hand, the glass temperature of soft segments is usually lower than room temperature, which enhances processability. Even though TPEs have been used commercially since 1950s, their deformation mechanism is not understood fully at a molecular level. Two players are involved: a crystalline structure and a soft segment. In semi-crystalline homopolymers, tie chain fraction determines the mechanical response. In addition, in TPEs, soft segments can readily dissipate stress due to their higher mobility. Therefore, it is important to understand how each segment contributes to total stress. In this work, we conduct molecular dynamics simulations with a modified Kremer-Grest model. We consider triblock copolymers (HS-SS-HS) with different SS lengths. After cooling, we apply uniaxial and constant engineering strain on these systems. We find that Young's modulus and stress decrease with an increase in SS length. Interestingly, the transition in the deformation mechanism occurs: HS contributes dominantly to stress with short SS while SS contributes dominantly with long SS. To understand this transition, we also calculate nonaffine displacements, and the reorientation and breakage of crystallites during deformation.