Aggregation Phenomena in Aqueous Solutions of Hydrophobically Modified Polyelectrolytes. A Probe Solubilization Study

Aggregation in the solutions of a hydrophobically modified polyelectrolyte, poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(acrylic acid) (Pluronic-PAA), has been studied as a function of temperature, pH, and molecular weight. Pluronic-PAA undergoes temperature- and pH-responsive aggregation in aqueous solutions, as shown by solubilization of fluorescent dyes and spin-labeled probes. Stable gel structures cross-linked by aggregates are formed above the critical micellization temperature (cmt). The aggregates provide a nonpolar microenvironment to pyrene that is insensitive to the ionization. The microenvironment of pyrene below the cmt at low pH is nonpolar, suggesting the presence of hydrophobic microdomains unrelated to the temperature-induced micellization. Full ionization of PAA leads to weakening of hydrophobic microdomains below the cmt. Above the cmt, 12-doxylstearic acid and spin-labeled steroid hormone can be solubilized into Pluronic-PAA aggregates. Solubilization results in a rise in the microviscosity of the environment of the spin probe.