Liquid-liquid phase separation in dilute solutions of poly(styrene sulfonate) with multivalent cations: Phase diagrams, chain morphology, and impact of temperature

The dilute solution behavior of sodium poly(styrene sulfonate) is studied in the presence of trivalent Al³⁺ and bivalent Ba²⁺ cations at various levels of excess NaCl. The study evaluates the phase behavior and the morphology of the polyelectrolyte chains with increasing extent of decoration with the Al³⁺ and Ba²⁺ cations and analyses the effect of temperature on these decorated chains. The phase behavior is presented in the form of the cation concentration versus the respective poly(styrene sulfonate) concentration, recorded at the onset of precipitation. Whereas poly(styrene sulfonate) with Al³⁺ exhibits a linear phase boundary, denoted as the “threshold line,” which increases with increasing poly(styrene sulfonate) concentration, Ba²⁺ cations show a threshold line which is independent of the poly(styrene sulfonate) concentration. An additional re-entrant phase, at considerably higher cation content than those of the threshold lines, is observed with Al³⁺ cations but not with Ba²⁺ cations. The threshold line and the re-entrant phase boundary form parts of the liquid-liquid phase boundary observed at the limit of low polymer concentration. The dimensions of the polyelectrolyte chains shrink considerably while approaching the respective threshold lines on increase of the Al³⁺ and Ba²⁺ cation content. However, subtle differences occur between the morphological transformation induced by Al³⁺ and Ba²⁺. Most strikingly, coils decorated with Al³⁺ respond very differently to temperature variations than coils decorated with Ba²⁺ do. As the temperature increases, the poly(styrene sulfonate) chains decrease their size in the presence of Al³⁺ cations but increase in size in the presence of Ba²⁺ cations.