Modeling viscoelastic properties of triblock copolymers: A DPD simulation study

Abstract

Gel systems based on self-assembled, amphiphilic ABA triblock copolymers in midblock-selective solvent form stable, spatially extended networks with controllable morphology and tunable viscoelastic behavior. In this work, we systematically evaluate the mechanical properties of these gels using morphology calculations, and a nonequilibrium oscillatory shear technique based on the dissipative particle dynamics (DPD) method. Our simulations demonstrate that low molecular weight triblock copolymers with incompatible blocks self-assemble into micelles connected with bridges and loop-like chains comprised of the solvent-selective polymer midblocks. The fraction of bridges, Φ b , generally increases with increasing relative volume of the midblock, x, defined as the ratio of midblock and endblock volumes (x = V B/ V A ). For our model, Φ b reaches a plateau at approximately x > 9 for a strongly selective solvent. At this limit, the value of Φ b increases from 0.40 to about 0.66 as the copolymer concentration, c, increases from 0.2 to 0.5; however, this increase is less significant at higher concentrations. The elastic response of the gel studied here is comparable with the Rouse modulus. The elastic modulus increases with polymer concentration, and it exhibits a broad peak within 6 < x < 12. Finally, we present an approximate method to predict the elastic modulus of unentangled ABA triblock copolymers based solely on the morphology of the micellar gel, which can be gleaned from equilibrium DPD simulations. We demonstrate that our simulation results are in good qualitative agreement with other theoretical predictions and experimental data.