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

The many similarities in morphological behavior exhibited by diblock c opolymer melts and lyotropic surfactant suspensions suggest the existe nce of common physical principles underlying these phenomena. In an ef fort to identify such principles, we discuss the phase behavior of aqu eous solutions of poly(ethylene oxide)poly(ethylethylene) (PEO-PEE) bl ock copolymers. The molecular weights of these materials are roughly t wice that of common poly(ethylene oxide)-poly(propylene oxide) (PEO-PP O) surfactants, leading to microphase separation and a rich mesophase polymorphism in the absence of solvent. Despite the strongly thermotro pic nature of the PEG-water interaction, the phase behavior in solutio ns of high polymer concentration is primarily lyotropic. Mesophase tra nsitions observed in this regime show characteristics similar to those observed in undiluted block copolymers. A crossover to thermotropic b ehavior occurs at concentrations on the order of 60 wt % polymer, whic h corresponds to the formation of a stoichiometric complex of three wa ter molecules per ethylene oxide repeat. This crossover is accompanied by changes in the structural dimensions, long-range order, and mechan ical characteristics of the mesophases. Transitions in these dilute so lutions resemble those previously observed in surfactant systems. Thes e findings suggest that the complex structure of hydrated PEG plays a central role in determining the phase behavior of the system.