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 Rayleigh scattering has been used to determine the diffusivities parallel and perpendicular to the lamellar planes, for poly(ethylene-propylene)-poly(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 examine the conformational dynamics in block copolymer solutions. For PEP-PEE in squalane, the stress-optic relation is approximately valid, but the stress-optic coefficient increases in the ordered state. The location of the ODT is consistent with the dilution approximation. For polystyrene-polyisoprene (PS-PI) diblock and triblock copolymers, the stress-optic relation fails completely. There is evidence that the fluctuation regime (i.e., in the disordered state but near the ODT) may be considerably broader in block copolymer solutions than in melts. Furthermore, the onset of structure in the solution is accompanied by substantial form birefringence. In general, the optical signals are more complicated than the Theological ones, but also much more sensitive to small changes in temperature or concentration.