Complex fluids under shear: Block copolymers

Abstract

A survey is given on the effects of large amplitude oscillatory shear (LAOS) flow on the orientation behavior of lamellar poly (styrene-b- isoprene) (PS-b-PI) diblock copolymers with emphasis on the elucidation of common behavior with other layered complex fluids like lyotropic lamellar phases. It is described that for LAOS at temperatures close to the order-disorder-transition temperature, T ODT, three orientation regimes related to the dynamic shear viscosity of the block copolymers are observed: parallel orientation at high, perpendicular orientation at intermediate, and parallel orientation again at very low shear frequencies. It is demonstrated that one of the key issues to understand these non- equilibrium type experiments is the behavior of instabilities leading to flips of orientation from parallel to perpendicular alignment and vice versa. An orientation diagram showing the position of these instabilities in the amplitude-frequency space is derived experimentally suggesting a consistent phenomenological picture of orientation phenomena of layered systems under shear.