Phase Behavior and Rheological Analysis of Reverse Liquid Crystals and W/I2 and W/H2 Gel Emulsions Using an Amphiphilic Block Copolymer

This article reports the phase behavior determination of a system forming reverse liquid crystals and the formation of novel disperse systems in the two-phase region. The studied system is formed by water, cyclohexane, and Pluronic L-121, an amphiphilic block copolymer considered of special interest due to its aggregation and structural properties. This system forms reverse cubic (I-2) and reverse hexagonal (H-2) phases at high polymer concentrations. These reverse phases are of particular interest since in the two-phase region, stable high internal phase reverse emulsions can be formed. The characterization of the I-2 and H-2 phases and of the derived gel emulsions was performed with small-angle X-ray scattering (SAXS) and rheometry, and the influence of temperature and water content was studied. The H-2 phase experimented a thermal transition to an I-2 phase when temperature was increased, which presented an Fd3m structure. All samples showed a strong shear thinning behavior from low shear rates. The elastic modulus (G') in the I-2 phase was around 1 order of magnitude higher than in the H-2 phase. G' was predominantly higher than the viscous modulus (G ''). In the gel emulsions, G' was nearly frequency-independent, indicating their gel type nature. Contrarily to water-in-oil (W/O) normal emulsions, in W/I-2 and W/H-2 gel emulsions, G', the complex viscosity (vertical bar eta*vertical bar), and the yield stress (tau(0)) decreased with increasing water content, since the highly viscous microstructure of the continuous phase was responsible for the high viscosity and elastic behavior of the emulsions, instead of the volume fraction of dispersed phase and droplet size. A rheological analysis, in which the cooperative flow theory, the soft glass rheology model, and the slip plane model were analyzed and compared, was performed to obtain one single model that could describe the non-Maxwellian behavior of both reverse phases and highly concentrated emulsions and to characterize their microstructure with the rheological properties.