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 temperatur e, pH, and molecular weight. Pluronic-PAA undergoes temperature- and pH-res ponsive aggregation in aqueous solutions, as shown by solubilization of flu orescent dyes and spin-labeled probes. Stable gel structures cross-linked b y aggregates are formed above the critical micellization temperature (cmt). The aggregates provide a nonpolar microenvironment to pyrene that is insen sitive to the ionization. The microenvironment of pyrene below the cmt at l ow pH is nonpolar, suggesting the presence of hydrophobic microdomains unre lated to the temperature-induced micellization. Full ionization of PAA lead s 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 microvisc osity of the environment of the spin probe.