Birefringence and Depolarized Light Scattering of an Ordered Block Copolymer Melt under Shear Flow

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). Thi...