Annealing Effects on Orientation in Dynamically Sheared Diblock Copolymers

The frequency dependence of the lamellar orientation is investigated for a nearly symmetric polystyrene−polyisoprene diblock copolymer of low molecular weight under large-amplitude oscillatory shear near the order−disorder transition temperature ( T ODT ). It is demonstrated that in the case of low frequencies not only the linear viscoelastic response but also the thermal history of the diblock copolymer must be known in order to predict the orientation behavior. For samples annealed at temperatures far below T ODT but above the glass transition temperatures of both blocks, parallel alignment of the unit normal with respect to the velocity gradient direction is observed. For samples quenched from above T ODT, preferential perpendicular alignment can be achieved. It is shown, however, that only for annealed samples uniform orientation with optimum order parameters is obtained.