Micellar Ordering in Concentrated Solutions of Di- and Triblock Copolymers in a Slightly Selective Solvent

Abstract

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 ≤ φ ≤ 0.4. For φ < 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 °C. Above a crossover concentration φ ≈ 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 ≤ φ ≤ 0.3 and lamellar for φ ≥ 0.3, persists up to an order−disorder transition at T ≈ 40 °C for the diblock and T ≈ 50 °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 temperature...