Upper Critical Solution Temperature (UCST) Behavior of Polyguanidinium in Aqueous Media

Abstract

Liquid−liquid phase separation (LLPS), driven by specific associative interactions, plays a pivotal role in the formation of molecular condensates. Previously, our research [Oh, S.-H. et al. Macromolecules 2023, 56, 3989−3999] unveiled LLPS in aqueous solutions of guanidinium-containing polymers induced by monovalent salt addition, displaying upper critical solution temperature (UCST) phase behavior. This behavior is mainly attributed to the face-to-face π−π stacking of guanidinium groups when the Coulombic repulsion is attenuated. In this work, we characterize the UCST behavior using transmittance, static light scattering, and small-angle neutron scattering, elucidating the pertinent thermodynamic parameters. The cloud point and spinodal temperatures gradually increase as the concentration of the counteranion increases across a wide temperature range between 0 and 100 °C. Also, theta temperature (T θ ) and the effective interaction parameter (χ eff ) for solutions change in a quantitative manner with the anion concentration. The entropic and enthalpic contributions to χ eff are found to decrease and increase with the anion concentration, respectively. This increase in the enthalpic penalty between the monomer and the solvent is attributed to the attractive monomer−monomer interactions via π−π interaction of guanidinium pairs at higher anion concentrations, inducing phase separation. Our findings provide thermodynamic insights into the significant role of associative interaction for molecular condensations in aqueous media and the molecular mechanisms driving UCST phase behavior.