Effect of Glycols on the Self-Assembly of Amphiphilic Block Copolymers in Water. 1. Phase Diagrams and Structure Identification

Abstract

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.

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

“…6). This grouping of glycols is also consistent with the results of our study on the Pluronic P105 self-assembly (48,50).…”

“…Propylene glycol likely belongs to this group. The correlations shown here are basically the same as those found for Pluronic P105 (48), and hence support the relationship between the effect of glycols on the PEO-PPO-PEO block copolymer self-assembly and the glycol polarity.…”

“…For more details on the SAXS measurements see Ref. (48). The positions of the Bragg diffraction peaks registered in the SAXS spectra were evaluated from the desmeared spectra deeming this way methodologically more precise.…”

“…In order to further probe the mechanism of the glycol effects on the Poloxamer block copolymer phase behavior, we have correlated parameters that can describe the main features of the ternary phase diagrams studied here to the glycol relative polarity (48). As a measure of the glycol polarity we have used complex parameters such as the octanol/water partition coefficient and the solubility parameter, which have the advantage of capturing a number of features related to the solvent polarity (data for the glycols studied are given in Table 1).…”

“…Using these general dependencies we have proposed a hypothesis for the role of polar organic solvents based on their relative polarity (48). Depending on their octanol/water partition coefficient the solvents can be classified as PEO-resembling or PPOresembling (respectively, with log P lower than that of ethylene glycol and higher than that of propylene glycol; see Table 1).…”