EVOLUTION OF MICROSTRUCTURE DURING SHEAR ALIGNMENT IN A POLYSTYRENE-POLYISOPRENE LAMELLAR DIBLOCK COPOLYMER

The process of flow-induced alignment in a lamellar diblock copolymer melt is investigated using simultaneous measurements of shear stress a nd birefringence. Flow birefringence, in situ, during oscillatory shea r clarifies how the orientation distribution evolves toward either ''p arallel'' or ''perpendicular'' alignment, i.e., layers normal to eithe r the velocity gradient or the vorticity axis, respectively. A nearly symmetric, polystyrene-polyisoprene diblock (ODT similar or equal to 1 64 degrees C) is studied at 120 degrees C (T/T-ODT similar or equal to 0.90). The critical frequency (omega(c)') associated with a crossover in the relaxation dynamics from being dominated by the macromolecular response to being dominated by the microstructural response is estima ted to be omega(c)' approximate to 3-7 rad/sat 120 degrees C. At high frequencies (relative to omega(c)'), shearing induces parallel alignme nt, while shearing at lower frequencies leads to perpendicular alignme nt. In all cases, alignment proceeds through a ''fast'' process follow ed by a ''slow'' one. The fast process is dominated by depletion of th e projection of the orientation distribution along either the perpendi cular direction or the ''transverse'' direction (layers normal to the flow). The resulting biaxial distribution is transformed into a well-a ligned uniaxial one during the slow process. Surprisingly, the project ion along the perpendicular direction can disappear faster than the pr ojection along the transverse direction. This occurs during the fast p rocess en route to parallel alignment with sufficiently high frequenci es. As the shearing frequency is lowered, the projection along transve rse orientation decreases faster than that along the perpendicular dir ection.