A combination of in situ birefringence and depolarized light-scattering experiments was used to study the formation of an ordered cylindrical microstructure in a polystyrene-block-polyisoprene copolymer melt under a shear flow field. We demonstrate that our sample forms an imperfect "single crystal" with a fraction of the cylinders aligned in the flow direction. The aligned regions of the sample coexist with randomly oriented grains. The birefringence experiments enable the characterization of the aligned regions while the depolarized light-scattering experiments enable the characterization of the randomly oriented grains. A model for depolarized light scattering from such samples was developed. It was shown that the usual scattering formulas for grains embedded in an isotropic matrix are applicable provided one recognizes that the scattering vector, q, has transverse (qT) and longitudinal (qL) components even in the small angle scattering limit (qL is the component of q in the propagation direction). This result applies when the analyzer (or polarizer) axis is aligned along the direction of the optic axis of the aligned regions. A simplifying feature of block copolymers is that the product w|qL| ,1, where w is the characteristic grain size, allowing the approximation q ≈ qT. We used our model to study the structure of the block copolymer melt after it had been quenched from the disordered to the ordered state under reciprocating shear flow (strain amplitude ) 133%). Under slow shear flow (shear rate, γ ˘) 0.067 s -1 ), about 60% of the sample consisted of randomly oriented grains and 40% consisted of aligned cylinders. The average grain size and time required to complete the ordering process obtained under slow shear flow were comparable to those obtained under quiescent conditions. Under fast shear flow (γ ˘) 0.67 s -1 ), however, most of the sample (97%) consisted of aligned cylinders, indicating the formation of a wellaligned crystal.

The effect of large-amplitude oscillatory shear flow on a concentrated block copolymer solution with lamellar order was studied by in-situ small-angle neutron scattering. Microstructural changes were studied as a function of temperature, frequency of the oscillatory flow field, and thermal history prior to turning on the shear field. We find that the alignment path depends mainly on thermal history prior to turning on the shear field and is independent of frequency and temperature. At long times, the lamellae were aligned parallel to the shearing plates, regardless of frequency, temperature, and thermal history. We refer to this as the parallel orientation. Monotonic changes from the unaligned to the aligned state were found when the shear field was turned on after the sample was completely ordered. The alignment kinetics, in this case, occurs in two stages. The first stage consists of a rapid rotation of the grains so that the lamellar normals lie in the velocity gradient-vorticity plane. This is followed by a slower process wherein the lamellar normals get increasingly localized in the velocity gradient direction. We also studied ordering kinetics under shear, by turning on the shear field before significant ordering had taken place. In this case, the first stage of ordering resulted in the formation of lamellae aligned perpendicular to the shearing plates in addition to the parallel lamellae, regardless of temperature and frequency. Eventually the perpendicular lamellae were transformed to parallel lamellae via an undulation instability.