COMPLEX PHASE BEHAVIOR IN AQUEOUS SOLUTIONS OF POLY(ETHYLENE OXIDE)-POLY(ETHYLETHYLENE) BLOCK COPOLYMERS

The many similarities in morphological behavior exhibited by diblock copolymer melts and lyotropic surfactant suspensions suggest the existence of common physical principles underlying these phenomena. In an effort to identify such principles, we discuss the phase behavior of aqueous solutions of poly(ethylene oxide)−poly(ethylethylene) (PEO−PEE) block copolymers. The molecular weights of these materials are roughly twice that of common poly(ethylene oxide)−poly(propylene oxide) (PEO−PPO) surfactants, leading to microphase separation and a rich mesophase polymorphism in the absence of solvent. Despite the strongly thermotropic nature of the PEO−water interaction, the phase behavior in solutions of high polymer concentration is primarily lyotropic. Mesophase transitions observed in this regime show characteristics similar to those observed in undiluted block copolymers. A crossover to thermotropic behavior occurs at concentrations on the order of 60 wt % polymer, which corresponds to the formation of a sto...