Temperature and pH-Dependent Response of Poly(Acrylic Acid) and Poly(Acrylic Acid-co-Methyl Acrylate) in Highly Concentrated Potassium Chloride Aqueous Solutions

In this study, the phase transition phenomena of linear poly(acrylic acid) (PAA) and linear or star-shaped poly(acrylic acid-co-methyl acrylate) (P(AA-co-MA)) in highly concentrated KCl solutions were investigated. The effects of polymer molecular weight, topology, and composition on their phase transition behavior in solution were investigated. The cloud point temperature (T(CP)) of polymers drastically increased as the KCl concentration (C(KCl)) and solution pH increased. C(KCl) strongly influenced the temperature range at which the phase transition of PAA occurred: C(KCl) of 1.0–2.2 M allowed the phase transition to occur between 30 and 75 °C. Unfortunately, at C(KCl) above 2.6 M, the T(CP) of PAA was too high to theoretically trigger the crystallization of KCl. The addition of hydrophobic methyl acrylate moieties decreased the T(CP) into a temperature region where KCl crystallization could occur. Additionally, the hydrodynamic diameters (D(h)) and zeta potentials of commercial PAA samples were examined at room temperature and at their T(CP) using dynamic light scattering. The salt concentration (from 1 to 3 M) did not impact the hydrodynamic diameter of the molecules. D(h) values were 1500 and 15 nm at room temperature and at T(CP), respectively.