UNDULATIONS AND DISORDER IN BLOCK-COPOLYMER LAMELLAE UNDER SHEAR-FLOW

The effect of shear flow on a concentrated block copolymer solution wi th lamellar microstructure was studied by in-situ small angle neutron scattering. Microstructural changes were determined from scattering me asurements in all three principal directions (flow, velocity gradient, and neutral directions). The lamellae were found to align along the v elocity direction but exhibited two orientations: one parallel to the shearing surfaces and the other perpendicular to the shearing surfaces . The perfection of the perpendicular alignment was always significant ly greater than that of the parallel alignment. In fact, our sample in the parallel alignment was, to a large extent, disordered (i.e., liqu idlike). In some cases, we found transient, well-aligned, perpendicula r lamellae that eventually degenerated to poorly ordered, parallel lam ellae. In one case, a systematic increase in lamellar undulations was evident prior to disordering. These results are not in agreement with current theories, which only predict enhanced order in block copolymer s under shear flow.