The complex influence of the oscillatory shear on the melt of linear diblock copolymers.

The phase morphologies of symmetric linear diblock copolymers subjected to the oscillatory shear are investigated with the aid of dissipative particle dynamics simulations. The frequency dependent reorientations of the lamellar phase (LAM) have been identified. We find that the parallel orientation of LAM (i.e., the lamellar normal is parallel to the velocity gradient) appears at high shear frequency, whereas the perpendicular orientation of LAM (the lamellar normal being perpendicular to the velocity gradient) takes place at low shear frequency. In both of the cases, the reorientations undergo similar processes: the original LAM phase prepared in equilibrium breaks down rapidly, and it takes a very long time for the perfectly oriented LAM being reformed. Moreover, the shear-induced isotropic to lamellar phase transitions are observed when the oscillatory shear amplitude is large enough. It indicates that the shear amplitude plays a dominant role in the order-disorder transition. The viscosity and the modulus of the melt are found to be dependent on the shear amplitude and the shear frequency in a complex way.