Anomalous Rheology in a Nanostructured Diblock Copolymer/Hydrocarbon System and Its Kinetic Origin

Nanostructured squalane solutions (5-20 wt %) of a diblock copolymer, poly(styrene-b-hydrogenated isoprene), were prepared by a cosolvent-casting method. The as-cast solutions behaved as viscous liquids with terminal flow behavior at room temperature. Upon heating, the solutions gelled, and they did not return to their starting liquidlike state upon cooling. Small-angle X-ray scattering (SAXS) revealed a random array of spherical micelles in the as-cast solutions, which were hypothesized to be in a nonequilibrium state. This abnormal solidification with increasing temperature was correlated with the formation of body-centered-cubic (BCC) structures. Isothermal SAXS and rheology measurements also indicated that the rate of formation of BCC structures in the as-cast solutions increased with temperature. A diffusion-controlled nucleation-and-growth mechanism was proposed for the ordering process in the as-cast polystyrene-b-hydrogenated polyisoprene/squalane solutions.