Phase Behavior and Microstructure in Binary Block Copolymer/Selective Solvent Systems:  Experiments and Theory

Abstract

A combined experimental and theoretical study of phase behavior and structure in binary systems of block copolymers and a selective solvent is presented. The block copolymers used here consist of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), and the solvent is water, selective for the PEO block. The concentration−temperature phase diagrams of (EO)8(PO)47(EO)8 (Pluronic L92) and (EO)11(PO)70(EO)11 (Pluronic L122) in water have been determined experimentally. These two block copolymers are unique in that they form both normal (H1, “oil-in-water”) and reverse (H2, “water-in-oil”) hexagonal lyotropic liquid crystalline structures in binary systems with water over the same temperature range. This is the first time the H2 structure is reported in a binary PEO−PPO−PEO/water system. Thereafter, the predicted phase diagram of (EO)20(PO)69(EO)20 in water is given, employing a self-consistent mean-field lattice theory with internal degrees of freedom. The free energy for a number of possible microstructures was calculated as a function of the polymer concentration, and the extensions of the one-phase and two-phase regions were established. The model predicted the composition and temperature stability ranges of disordered solution, micellar solution, normal hexagonal, lamellar, reverse hexagonal, and reverse cubic ordered phases which appeared at increasing (EO)20(PO)69(EO)20 concentration, in good agreement with the experimental Pluronic L122 phase behavior. Moreover, the model provides the volume fraction profiles for the PEO, PPO, and water components in the self-assembled microstructures, information which is not readily accessible from experiments. Finally, an increased segregation among the species and an increased domain spacing at increasing polymer length were found. The data are consistent with the scaling behavior predicted for the domain size in weakly segregating block copolymer systems.

Effect of Pharmaceutically Acceptable Glycols on the Stability of the Liquid Crystalline Gels Formed by Poloxamer 407 in Water

“…Hence, differences in the ternary phase diagrams observed on the basis of the phase boundaries can be considered as indications for the degree of swelling of the copolymer blocks and conclusions for the location of the glycol in the self-assembled structure can be drawn. Reverse (water-in-oil) liquid crystalline phases were not obtained in any of the systems studied since the initial preferred curvature of Poloxamer 407 does not favor the formation of reverse structures in the absence of an apolar solvent (42,55).…”

Vesicle formation in mixture of a PEO-PPO-PEO block copolymer (Pluronic P123) and a nonionic surfactant (Span 65) in water

“…An opaque dispersion obtained after hydration and swelling of Pluronic P123 film at 60 °C changed into a transparent solution by cooling to room temperature (~25 °C) (see Figure 1a). This is due to the transformation of the large assemblies and/or aggregates such as liquid crystal gels formed at 60 °C into the spherical micelles at room temperature (~25 °C) [44]. Namely, vesicles are not formed from Pluronic P123 alone in aqueous solutions at room temperature (~25 °C) [22][23][24].…”

Simulation study of intra- and intermicellar ordering in triblock-copolymer systems

“…[8][9][10][11][12] In this approach it is assumed that the polymer self-assembles into an aggregate with a fixed geometry ͑e.g., spherical or cylindrical͒. The free energy of the aggregate and its structural properties can then be calculated within the mean-field approximation.…”