Phase Behavior of Poly(Oxyethylene)–Poly(Oxypropylene)–Poly(Oxyethylene) Block Copolymer in Water and Water–C12EO5 Systems

Abstract

The binary phase diagram of a triblock copolymer poly(oxyethylene) (PEO) poly(oxypropylene) (PPO) poly(oxyethylene) (PEO), (PEO)37(PPO)58(PEO)37 or P105 in water and the ternary system of P105, water, and pentaoxyethylene dodecyl ether (C12EO5) has been studied to understand the miscibility of a small amphiphile, C12EO5 and a copolymer, as well as the mixing effect on the formed liquid crystalline structures. Phase diagrams, small angle x‐ray scattering (SAXS) and differential scanning calorimetry (DSC) were used to characterize these systems. The phase diagram of the binary system is presented together with the characteristic parameters for founded phases, namely, cubic, hexagonal, and lamellar phases. In the ternary system it was found that the small amphiphile and the block copolymer, despite having very different chain lengths are essentially miscible forming single phases. A large amount of C12EO5 can be solubilized in the P105 aggregates whereas P105 is most difficult to dissolve in the C12EO5 aggregates because of the difference in the molecular size. The copolymer is practically insoluble in the lamellar phase of C12EO5 due to the packing constraint. Hence, two lamellar phases coexist in a surfactant‐rich region, at W s = 0.66, where W s is the weight fraction of the total amphiphile in the system. This indicates that the thickness of the lipophilic part of the C12EO5 lamellar phase is too small to allocate the large lipophilic chain of the P105 triblock copolymer.