Rheology of symmetric diblock copolymers

Abstract

Using large-scale simulations with the graphics processing unit (GPU) accelerated Highly Optimized Object-oriented Many-particle Dynamics (HOOMD)-blue software we investigate the anisotropic, dynamic-mechanical properties of lamellae-forming diblock copolymer melts. Both, the shear-stress autocorrelation function in equilibrium as well as stress-controlled simulations of shear flow are considered. The latter are implemented in (HOOMD)-blue via reverse nonequilibrium molecular dynamics simulation (RNEMDS).We find that copolymer lamellae that are perpendicularly oriented to the shear flow have similar rheological properties than unstructured homopolymer melts. Copolymer lamellae with the metastable parallel and the unstable transverse orientation exhibit similar dynamic-mechanical behavior in the linear regime. Most notably, they are characterized by a high dissipation.(RNEMDS) enables the study of oscillatory shear flow as well as the investigation of nonlinear rheology. Our simulation results for a soft, coarse-grained model at very high frequencies indicate that the finite speed of stress propagation has to be accounted for in the quantitative analysis, and we devise an analytic prediction for the nonlinear velocity and spatially varying stress profiles as a function of the loss and storage moduli.