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 c opolymer solutions. Bulk melts of PS/PI with no solvent exhibit ordere d morphologies including lamellae, close-packed cylinders, etc. that d epend on the block asymmetry. These same diblocks self-assemble in dec ane at low concentrations to form monodisperse, spherical micelles wit h a dense polystyrene core and a diffuse polyisoprene corona. Strongly interacting polymeric micelles, observed at modest polymer concentrat ions, order into cubic arrays that include both face-centered and body -centered cubic crystals depending on the length scale of the repulsio n relative to the core dimension. These ordered morphologies of the me lt and micellar crystals provide limiting reference states for the poo rly understood high-concentration regime studied in this work. As we i ncrease the polymer concentration, we observe a curious melting of the micellar crystals before the onset of anisotropy. Since the melting o f the micellar crystal is not predicated upon shape transitions, we re turn to tethered-chain models of our spherical polymeric micelles to q ualitatively describe the disordering process in terms of a loss of th e osmotic pressure gradient between micelles. One system exhibits a re entrant disorder-order-disorder-order phase transition. Finally, the d evelopment 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.