Evolution of entanglements during the response to a uniaxial deformation of lamellar triblock copolymers and polymer glasses

TL;DR

This study uses molecular dynamics simulations to investigate how entanglements in lamellar triblock copolymers evolve under uniaxial deformation, revealing the role of initial entanglements and loop chains in stress response.

Contribution

It introduces a boundary-driven tensile protocol to analyze entanglement evolution in triblock copolymers, highlighting the influence of loop chains and initial entanglements on mechanical behavior.

Findings

Initial entanglements influence the preyield stress response.

Entanglement networks debond during strain-hardening.

Loop chains reinforce memory effects in entanglement dynamics.

Abstract

Using coarse-grained molecular-dynamics simulations, a generic styrene-(block)-butadiene-(block)-styrene (SBS) triblock copolymer under lamellar conformation is used in order to investigate the mutual entanglement evolution when a structure of alternating glassy (S)/rubbery (B) layers is submitted to an imposed deformation. By varying the amount of loop chains between each phase, i.e. noncrossing chains, it is possible to generate different types of S/B interface definitions. A specific boundary driven tensile strain protocol has been developed in order to mimic "real" experiments and measure the stress-strain curve. The same protocol is also applied to a reference state consisting in a directed glassy homopolymers, as well as to an isotropic glassy polymer. The evolution of initial mutual entanglements from the undeformed samples during the whole deformation process is monitored. It is shown for all considered systems that initial entanglements mostly participate to the preyield regime of the stress-strain curve and that this network is debonded during the strain-hardening regime. For triblocks with a non-null amount of crossing chains, the lower the amount is, the longer the memory effect of the initial entanglement network in the postyield regime is. On the fly distributions of entanglements, which depart from the postyield regime, depict memory effects and long time correlations during the strain-hardening regime. For triblocks, loop chains reinforce these effects.