Micellar ordering in concentrated solutions of di- and triblock copolymers in a slightly selective solvent

Micellar ordering in semidilute solutions of polystyrene-polyisoprene diblock and triblock copolymers in the slightly selective solvent di-n-butyl phthalate has been studied using rheology and small-angle X-ray scattering (SAXS). Ordering as a function of temperature has been investigated for a range of polymer concentrations 0.1 less than or equal to phi less than or equal to 0.4. For phi < 0.2, the rheological response is liquidlike and SAXS shows that there is no intermicellar order in the liquid; however, the solution viscosity shows a strong maximum near 50 degrees C. Above a crossover concentration phi approximate to 0.2, ordering of micelles is indicated by the presence of a sharp structure factor peak. The ordered micellar structure, identified as hexagonal for 0.2 less than or equal to phi greater than or equal to 0.3 and lamellar for phi less than or equal to 0.3, persists up to an order-disorder transition at T approximate to 40 degrees C for the diblock and T approximate to 50 degrees C for the triblock solutions studied. The rheological characteristics of the ordered solutions are reminiscent of those found in ordered block copolymer melts. At higher temperatures, for example approximately 20-30 degrees C above the ODT for the phi = 0.2 solutions, indications of chain aggregation disappear from the rheological properties; however, some evidence of chain association persists to still higher temperatures in the SAXS profiles. The domain spacing, d, in the ordered solutions obtained from the principal structure factor peak position, shows a crossover at phi approximate to 0.2, in agreement with theology. At high concentrations, d scales as d similar to phi(-1/3), suggesting a three-dimensional contraction of the microstructure, and thus micelles of finite length. This concentration dependence is opposite to that previously observed for ordered block copolymer solutions in neutral solvents, due to the solvent selectivity. The results are gathered in a rather rich phase diagram for this system.