Strain-Adaptive Self-Assembled Networks of Linear-Bottlebrush-Linear Copolymers

HY Liang and ZL Wang and AV Dobrynin, MACROMOLECULES, 52, 8617-8624 (2019).

DOI: 10.1021/acs.macromol.9b01859

We study the strain-adaptive behavior of the self-assembled networks of linear-bottlebrush-linear (LBL) triblock copolymers using a combination of analytical calculations and molecular dynamics simulations. Interactions between immiscible blocks result in microphase separation and formation of soft and strain-adaptive composite networks. Such unique network properties are manifestations of the architectural asymmetry of the two blocks: (i) flexible linear chains that aggregate into domains and (ii) bottlebrush strands that form a soft matrix. The mechanical response of the networks is a two-stage process, which starts with the extension of the bottlebrush network strands (elastic deformation regime) followed by the pulling out of the linear chains from L-domains (yielding regime). In the elastic stage of deformation, the stress strain curves are described by a nonlinear network deformation model, which considers bottlebrush strands as semiflexible chains with an effective Kuhn length. During the yielding stage, forces generated in bottlebrush strands become sufficient to pull linear chains from the aggregates. This pull out process creates a new interface between linear and bottlebrush blocks and occurs at a constant force. This is manifested in a linear dependence of the true stress on the network elongation ratio, sigma(true) proportional to lambda, for uniaxial network deformations. The two-stage network deformation process is incorporated into a unifying model of strain-adaptive network deformation. The model predictions are confirmed by molecular dynamics simulations of uniaxial deformation of self-assembled LBL copolymer networks and by experimental results for copolymers consisting of poly(dimethylsiloxane) bottlebrush block and two poly(methyl methacrylate) linear chain blocks with different compositions and block lengths.

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