Complex Phase Behavior in Aqueous Solutions of Poly(ethylene oxide)−Poly(ethylethylene) Block Copolymers

Abstract

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 stoichiometric complex of three water molecules per ethylene oxide repeat. This crossover is accompanied by changes in the structural dimensions, long-range order, and mechanical characteristics of the mesophases. Transitions in these dilute solutions resemble those previously observed in surfactant systems. These findings suggest that the complex structure of hydrated PEO plays a central role in determining the phase behavior of the system.

Phase Behavior of a Block Copolymer in Solvents of Varying Selectivity

“…The occurrence of coexisting phases is quite common in aqueous surfactant systems and in some block copolymers, 28 particularly the poly(ethylene oxide)-poly(butylene oxide) block copolymers in water. [30][31][32][33] The observation of coexistence is not surprising in itself, given that for a two-component system one expects coexistence regions along each phase boundary.…”

“…Aqueous block copolymer solutions systems have received the most attention, particularly in the case of the ethylene oxide-propylene oxide-ethylene oxide (Pluronic) copolymers. 21,[28][29][30][31][32][33][34][35] In this case, water is selective for ethylene oxide, and the polymer solubility decreases with increasing temperature. Phase diagrams have been constructed via theory and experiment and in general show a rich polymorphism with structures that are similar to those observed in neat block copolymers.…”

Synthesis and Applications of Hollow Particles

“…Amphiphilic block copolymers in water can selfassemble into various ordered mesospheres [66][67][68][69][70] .…”

Amphiphilic block copolymer nanocontainers as bioreactors

“…Despite their larger size and higher flexibility similar considerations must hold for the amphiphilic block copolymers. In fact, decreasing the lengths of the hydrophilic blocks at constant hydrophobic block lengths causes a transition from spherical to worm-like micelles and finally to vesicular structures [12,[14][15][16]. This tendency towards the formation of less-curved aggregates with decreasing hydrophilic block length corresponds to the observation with low-molecular-weight non-ionic oligo (ethylene oxide monoalkyl ethers) when the number of ethylene oxide units is reduced (i.e., their packing parameter increases) [17].…”

“…A similar influence is also observed in the lyotropic phase behavior of the block copolymers at higher concentrations. Here the existence region of the lamellar phase grows at the cost of spherical and hexagonal phases of higher local curvature with a decreasing length of the hydrophilic blocks [8,14,[18][19][20][21].…”