SYMMETRICAL DIBLOCK COPOLYMERS UNDER LARGE-AMPLITUDE OSCILLATORY SHEAR-FLOW - DUAL-FREQUENCY EXPERIMENTS

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 molecula r weight close to T-ODT are investigated. First results of such dual f requency experiments are presented for the coupling of two shear compo nents from the lower and intermediate frequency regimes I and II of th e shear viscosity leading to parallel and perpendicular orientation be havior, respectively, in single frequency experiments. The employed fr equencies were separated by more than two orders of magnitude, renderi ng possible coupling terms unimportant. Using two-dimensional small-an gle x-ray scattering (2D-SAXS) measurements, it is shown that for larg e rim strain values the perpendicular orientation mechanism completely governs the behavior. In experiments where the applied strain for she ar component II leading to perpendicular orientation is significantly reduced with respect to that of component I leading to parallel orient ation, a characteristic flip from parallel to perpendicular orientatio n is observed along the radius of the sample disk. It is further demon strated that the position of the flipping point along the radius can b e altered by varying the amplitude of deformation component II. In the se experiments regions of parallel as well as perpendicular orientatio n exhibiting high order parameters can be achieved. The results are in terpreted using concepts for the orientation mechanisms developed in e arlier single frequency studies, thereby corroborating the underlying ideas. Furthermore, the results of the dual frequency experiments supp ly valuable information concerning the competition between different o rientation mechanisms not directly obtained from single frequency expe riments. (C) 1997 American Institute of Physics.