Melting of ordered arrays and shape transitions in highly concentrated diblock copolymer solutions

Polystyrene/polyisoprene (PS/PI) diblocks suspended in decane serve as a model system for the investigation of highly concentrated diblock copolymer solutions. Bulk melts of PS/PI with no solvent exhibit ordered morphologies including lamellae, close-packed cylinders, etc. that depend on the block asymmetry. These same diblocks self-assemble in decane at low concentrations to form monodisperse, spherical micelles with a dense polystyrene core and a diffuse polyisoprene corona. Strongly interacting polymeric micelles, observed at modest polymer concentrations, order into cubic arrays that include both face-centered and body-centered cubic crystals depending on the length scale of the repulsion relative to the core dimension. These ordered morphologies of the melt and micellar crystals provide limiting reference states for the poorly understood high-concentration regime studied in this work. As we increase the polymer concentration, we observe a curious melting of the micellar crystals before the onset of anisotropy. Since the melting of the micellar crystal is not predicated upon shape transitions, we return to tethered-chain models of our spherical polymeric micelles to qualitatively describe the disordering process in terms of a loss of the osmotic pressure gradient between micelles. One system exhibits a reentrant disorder-order-disorder-order phase transition. Finally, the development of anisotropy in the scattering pattern is linked to shape transitions that develop as melt conditions are approached. We monitor the degree of anisotropy to estimate the concentration for the onset of these shape transitions.