DIFFUSION, RHEOLOGY, AND FLOW BIREFRINGENCE IN BLOCK-COPOLYMER LIQUIDS NEAR THE ORDERING TRANSITION

Measurements of the diffusion and relaxation of block copolymer chains near the order-disorder transition (ODT) are reported. Forced Rayleig h scattering has been used to determine the diffusivities parallel and perpendicular to the lamellar planes, for poly(ethylene-propylene)-po ly(ethylethylene) (PEP-PEE) melts. The anisotropy is relatively weak ( i.e., less than a factor of 4), but increases steadily as temperature is decreased. Rheology and flow birefringence have been employed to ex amine the conformational dynamics in block copolymer solutions. For PE P-PEE in squalane, the stress-optic relation is approximately valid, b ut the stress-optic coefficient increases in the ordered state- The lo cation of the ODT is consistent with the dilution approximation. For p olystyrene-polyisoprene (PS-PI) diblock and triblock copolymers, the s tress-optic relation fails completely. There is evidence that the fluc tuation regime (i.e., in the disordered state but near the ODT) may be considerably broader in block copolymer solutions than in melts. Furt hermore, the onset of structure in the solution is accompanied by subs tantial form birefringence. In general, the optical signals are more c omplicated than the rheological ones, but also much more sensitive to small changes in temperature or concentration.