Tacticity and Counterion Modulated Temperature Response of Weak Polyelectrolytes: The Case of Poly(methacrylic acid) Stereoisomers in Aqueous Solutions

The temperature-induced formation of nanoparticles of two poly(methacrylic acid) (PMA) isomers, atactic (aPMA) and isotactic (iPMA), displays an opposite thermoresponsiveness. The formation of nanoparticles is investigated in dilute aqueous solutions in the presence of cations of different charge (Na+, Mg2+, and La3+) first by UV spectroscopy and visual observations. In the aPMA case, aggregation and phase separation take place with increasing temperature, whereas with iPMA intermolecular associates gradually disintegrate upon heating the solutions; however, no macroscopic phase separation is observed, and the process is completely reversible. Stable associates exist in iPMA solutions at all temperatures if higher valency ions (Mg2+ and La3+) are present, whereas with aPMA, only La3+ ions induce irreversible (stable) aggregation. Detailed structural characteristics of iPMA and aPMA aggregates are determined by dynamic and static light scattering and additionally characterized by pyrene fluorescence measurements, which reveal microgel-like nature of the aggregates. The observed opposite thermoresponsiveness is supported by calorimetric measurements. Behavior in PMA solutions of both isomers with added Mg2+ ions is governed by strong hydration of Mg2+ ions and by their monodentate binding mode to carboxylic groups on PMA.