Temperature Dependent Behavior of Isotactic and Atactic Poly(Methacrylic Acid) in the Presence of MgCl2 and CaCl2

Abstract

Temperature (T) induced nanoparticle formation of isotactic (iPMA) and atactic (aPMA) poly(methacrylic acid) chains is investigated in aqueous solutions in the presence of divalent cations (Mg²⁺ and Ca²⁺) by UV spectroscopy, visual observations and pyrene fluorimetry. In aPMA solutions, aggregation and phase separation take place with increasing T. The onset of aPMA aggregation (heating) and re-dissolution of the aggregates (cooling) is shifted to lower Ts with increasing ionic strength of the solutions. iPMA associates partly decompose upon heating, but stable nano-sized particles are present at all Ts. Structural characterization of the aggregates was carried out by dynamic and static light scattering measurements at 25 °C. Results revealed that aggregates of both PMAs with Mg²⁺ ions involve a lot of water and have a rather even mass distribution. This is attributed to strong hydration of Mg²⁺ ions and their preference towards monodentate binding to carboxylate groups. Differently, Ca²⁺ ions bind more strongly and in a bidentate manner, they lose the hydration water and form aggregates with several structural elements, depending on chain tacticity.