Symmetric diblock copolymers under large amplitude oscillatory shear flow: Dual frequency experiments

Abstract

Conceptually new experiments are described in which the effects of the superposition of two large amplitude oscillatory shear components on the morphology of a lamellar PS-b-PI diblock copolymer of low molecular weight close to T ODT are investigated. First results of such dual frequency experiments are presented for the coupling of two shear components from the lower and intermediate frequency regimes I and II of the shear viscosity leading to parallel and perpendicular orientation behavior, respectively, in single frequency experiments. The employed frequencies were separated by more than two orders of magnitude, rendering possible coupling terms unimportant. Using two-dimensional small-angle x-ray scattering (2D-SAXS) measurements, it is shown that for large rim strain values the perpendicular orientation mechanism completely governs the behavior. In experiments where the applied strain for shear component II leading to perpendicular orientation is significantly reduced with respect to that of component I leading to parallel orientation, a characteristic flip from parallel to perpendicular orientation is observed along the radius of the sample disk. It is further demonstrated that the position of the flipping point along the radius can be altered by varying the amplitude of deformation component II. In these experiments regions of parallel as well as perpendicular orientation exhibiting high order parameters can be achieved. The results are interpreted using concepts for the orientation mechanisms developed in earlier single frequency studies, thereby corroborating the underlying ideas. Furthermore, the results of the dual frequency experiments supply valuable information concerning the competition between different orientation mechanisms not directly obtained from single frequency experiments.