Order-disorder transition and critical micelle temperature in concentrated block copolymer solutions

The phase behavior of symmetric styrene-isoprene (SI) diblock copolymers in selective solvents in the vicinity of the order - disorder transition (ODT) was investigated by small-angle neutron scattering (SANS). Particular emphasis is placed on the region just above the ODT, where a disordered phase of micelles is observed. To understand morphological changes in more detail, one of the blocks is deuterated, i.e., PS-dPI and dPS-PI, and the scattering length density (SLD) of the solvents used were identical to the SLD of the corona chains. Two approaches for the analysis of SANS data were taken: the generalized indirect Fourier transformation (GIFT) and direct model fitting with relevant form and structure factors. With increasing temperature, the micellar aggregation number decreases, and the core radius is roughly maintained by the increased solvent swelling of the core. The critical micelle temperature (cmt) is experimentally described in the present study by an abrupt decrease in the size, aggregation number, and volume fraction of micelles. The micelles were found to dissociate into free chains approximately 20-30 °C higher than the T _ODT, and the cmt is found to be quite close to the mean-field spinodal temperature T _s. A mean-field temperature, T _ME, is located by the crossover temperature from the linear behavior of I _max ^-1 vs T ^-1 plot, and the structure factor above the experimentally determined T _MF is also found to be in good agreement with the Leibler-Landau-type mean-field theory. We examined two different ODTs: body-centered cubic lattices to disorder and hexagonal cylinders to disorder, with similar results in both cases.