Strain Adaptive Stiffening in Self-Assembled Bottlebrush Networks

We study the strain adaptive behavior of the self-assembled linear-brush-linear (ABA) triblock copolymer networks using analytical calculations, molecular dynamics simulations and experiments. During microphase separation of the architecturally distinct blocks, stiff bottlebrush strands are physically crosslinked by aggregates of linear chains and form a network with extreme softness and intense strain-stiffening. The mechanical response of such networks under uniaxial deformations is a two-stage process, which starts with extension of the bottlebrush network strands (elastic regime) followed by pulling out of the linear chains from A-domains (yielding regime). In the elastic regime, the stress-strain curves can be described by a network model, which considers bottlebrush strands as worm-like chains. In the yielding regime, force generated in bottlebrush strands is sufficient to pull linear chains from the aggregates. This pull-out occurs at a constant force which results in a linear dependence of the true stress on the network elongation ratio, σ_true∝λ. Such two-stage process is confirmed by molecular dynamics simulations of the self-assembled ABA copolymer networks and experimental results on PMMA-bbPDMS-PMMA plastomers.