Properties of aqueous solutions and gels of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(acrylic acid)

Gelation in aqueous solutions of poly(acrylic acid) (PAA) grafted onto poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (Pluronic F127) has been studied. The gel point and onset of micellization are observed in Pluronic-PAA solutions at the same temperatures, confirming that the gelation occurs via formation of micelles that act as thermoreversible cross-links. The salt- and pH-dependent expansion or folding of the polyelectrolyte segments changes the gel elasticity. The increased polarity due to enhanced polymer ionization at higher pH or with salt addition causes the poly(propylene oxide) (PPO) segments to aggregate at lower temperatures. The weak dependence of the polymer concentration at the maximum reduced viscosity on the salt concentration is attributed to hydrophobic interactions among PPO segments.