Birefringence and depolarized light scattering of an ordered block copolymer melt under shear flow

A combination of in situ birefringence and depolarized light-scattering exp eriments was used to study the formation of an ordered cylindrical microstr ucture in a polystyrene-block-polyisoprene copolymer melt under a shear flo w field. We demonstrate that our sample forms an imperfect "single crystal" with a fraction of the cylinders aligned in the flow direction. The aligne d regions of the sample coexist with randomly oriented grains. The birefrin gence 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 fro m such samples was developed. It was shown that the usual scattering formul as for grains embedded in anisotropic matrix are applicable provided one re cognizes that the scattering vector, q, has transverse (q(T)) and longitudi nal (q(L)) components even in the small angle scattering limit (q(L) 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 a xis of the aligned regions. A simplifying feature of block copolymers is th at the product w\q(L)\ much less than 1, where w is the characteristic grai n size, allowing the approximation q approximate to q(T). We used our model to study the structure of the block copolymer melt after it had been quenc hed from the disordered to the ordered state under reciprocating shear flow (strain amplitude = 133%). Under slow shear flow (shear rate, (gamma) over dot = 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 fl ow were comparable to those obtained under quiescent conditions. Under fast shear flow ((gamma) over dot = 0.67 s(-1)), however, most of the sample (9 7%) consisted of aligned cylinders, indicating the formation of a well-alig ned crystal.