Structure, Interactions, and Phase Transitions in Block Copolymer Solutions

-Alice Gast, Stanford University

Polystyrene/polyisoprene (PS/PI) diblocks suspended in decane serve as a model system for our small angle x-ray and neutron scattering investigation of diblock copolymer solutions. Melts of PS/PI without solvent exhibit ordered morphologies including lamellae and close-packed cylinders depending on the block asymmetry. These same diblocks self-assemble in decane 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 both face-centered and body-centered cubic arrays depending on the coronal and core dimensions. These ordered morphologies in the melt and micellar crystals provide limiting reference states for the poorly understood high concentration regime in between. As we increase the polymer concentration we observe a curious melting of the micellar crystals before the onset of anisotropy. We apply models of the micelle structure to qualitatively describe the disordering process; 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 occur as melt conditions are approached. We monitor the degree of anisotropy to estimate the concentration for the onset of these shape transitions. Implications for flow behavior and rheological properties of these solutions will be discussed.

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