Phase Behavior of a Mixture of Poly(oxyethylene)−Poly(dimethylsiloxane) Copolymer and Nonionic Surfactant in Water

The A−B-type silicone copolymer, Me 3 SiO−(Me 2 SiO) 23 −Me 2 SiCH 2 CH 2 CH 2 −O−(CH 2 CH 2 O) 51.6 H (Si 25 C 3 EO 51.6 ), forms only lamellar liquid crystal (L α ) in water over a whole range of concentrations. Rich phase behavior was observed upon addition of conventional nonionic surfactant, C 12 EO 5, whose molecular weight is ∼1/10 of Si 25 C 3 EO 51.6 . For both amphiphiles, f, the volume fraction of hydrophilic part in amphiphile, is the same, and f = 0.5. C 12 EO 5 alone forms aqueous micellar solution (W m ), normal hexagonal (H 1 ), bicontinuous cubic (V 1 ), lamellar (L α ), and reverse micellar solution (O m ) phases in water with increasing surfactant concentration. On the other hand, W m, discontinuous micellar cubic (I 1 ), L α, reverse bicontinuous cubic (V 2 ), and reverse hexagonal (H 2 ) phases are successively formed in water at a 70/30 weight ratio of Si 25 C 3 EO 51.6 /C 12 EO 5 . A reduction in the effective cross-sectional area per amphiphile at hydrophobic surface ( a S ) takes place upon addition of C 12 EO 5 in the mixed system. A small amphiphile, C 12 EO 5, is dissolved in the copolymer L α phase, and a relatively small amount of surfactant occupies a rather large area at an interface of aggregates, although the a S for Si 25 C 3 EO 51.6 in the L α phase is more than double that for C 12 EO 5 . Hence, the surfactant mainly dictates the morphology or amphiphile-layer curvature in the mixed system. The copolymer is practically insoluble in the L α phase of C 12 EO 5 due to the packing constraint. Hence, two L α phases coexist in a surfactant-rich region at W S = 0.75, where W S is the weight fraction of total amphiphile in the system. On the other hand, the copolymer L α phase changes to the I 1 phase with increasing the surfactant mixing fraction at W S = 0.55. The spherical micelle in the I 1 phase has a double-layer structure in which the surface is covered by surfactant and the core is pure poly(dimethylsiloxane) chain.