The effects of cosolvents on the microstructure of the lyotropic liquid crystals formed by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers in water were examined in ternary isothermal systems consisting of a representative PEO-PPO-PEO copolymer (Pluronic P105, (EO)37(PO)58(EO)37), water, and a polar cosolvent (glycerol, propylene glycol, ethanol, or glucose, referred to as "glycols"). The characteristic length scales of the lyotropic liquid crystals were established using small-angle X-ray scattering (SAXS). The SAXS data are analyzed and discussed with respect to (i) influence of the different glycols and the glycol content on the characteristic length scales, (ii) area occupied by one macromolecule at the polar/apolar interface, (iii) location of the different glycols in the microstructure, and (iv) glycol ability to modify the interfacial curvature through swelling the PEO-PPO-PEO macromolecule to different extents. The trends in the glycol effects observed in the ternary phase diagrams ("macroscopic" information) are confirmed and reinforced by the detailed microstructural information obtained here by SAXS. A correlation is found between the glycol effect on the microstructure of the lyotropic liquid crystals (the degree of relative swelling) and the glycol polarity (expressed in terms of octanol/water partition coefficient, dielectric constant or solubility parameter).

The effects of cosolvents such as glycerol, propylene glycol, ethanol, or glucose (referred here as "glycols") on the phase behavior of a representative poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer (Pluronic P105, (EO)37(PO)58(EO)37) are presented. Regions of lamellar, bicontinuous cubic, hexagonal, and micellar cubic lyotropic liquid crystalline structures (established from small-angle X-ray scattering measurements) have been delineated in the four ternary isothermal (25 °C) PEO-PPO-PEO-water-glycol phase diagrams. Pronounced effects on the concentration range of stability of the different phases were found when the glycol was varied from ethanol (the least polar glycol examined here) to glucose (the most polar). For example, glycerol and glucose swell the hexagonal phase to lower copolymer content, while propylene glycol and ethanol do not show this effect. Our experimental observations are discussed in terms of modification of the system "interfacial curvature" affected through the ability of different glycols to swell the PEO-PPO-PEO macromolecule to different extents. A hypothesis is proposed, based on a correlation between the PEO-PPO-PEO-water-glycol phase behavior and the glycol polarity, to account for and predict the effect of glycols on the ternary phase behavior of PEO-PPO-PEO block copolymers.

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