PATTERN AND SEGMENT RELAXATION IN A BLOCK-COPOLYMER MELT FOLLOWING STEP SHEAR-FLOW

The relaxation dynamics of a homogeneous poly(styrene)-poly(butadiene) star diblock copolymer melt following step shear flow is studied as i ts temperature is lowered through the microphase separation temperatur e (MST). Transient Raman scattering, birefringence, and mechanical rhe ometry measurements are used to investigate segment and composition pa ttern relaxation dynamics. The birefringence was determined to be prim arily form birefringence, and its relaxation rate was found to decreas e dramatically in the vicinity of the MST. At temperatures close to th e MST, the shear stress, first normal stress difference, and segment o rientation anisotropy (from Raman scattering measurements) relaxation rates also decreased. Slowing down of the first normal stress differen ce and segment orientation anisotropy relaxations was found to be less dramatic than that of the birefringence relaxation but more pronounce d than that of the shear stress relaxation. A decrease in the relaxati on rate of segment orientation anisotropy close to the MST is proposed to result from the coupling of segmental and composition pattern rela xations due to tethering of chain segments to the domain interface.