Effect of Water on the Thermal Transition Observed in Polyelectrolyte Complexes

Polyelectrolyte complexes (PECs), formed by the association of oppositely charged polyelectrolytes in solution, undergo a glass transition-like event of an unclear nature. This transition event has been detected using calorimetry or mechanical techniques. The observed thermal transition temperature (T$_{\mathrm{tr}})$ is influenced by both the complexation condition and subsequent addition of water. Recent simulation work suggests that water with the polyelectrolyte plays dual roles: weakening polyelectrolyte ion pairs and also undergoing a subtle dehydration process with increasing temperature. Here, we present the influence of water on two pairs of PECs, poly(diallyldimethylammonium chloride) $-$poly(sodium 4-styrenesulfonate) (PDAC$-$PSS), and poly(allylamine hydrochloride)$-$poly(acrylic acid) (PAH$-$PAA), in which the PEC structure and composition are affected by complexation conditions, NaCl concentration or pH values. Modulated differential scanning calorimetry (MDSC) reveals a T$_{\mathrm{tr}}$ that decreases in value with increasing hydration and decreasing polyelectrolyte-polyelectrolyte ion pairing. We show the collapse of all T$_{\mathrm{tr}}$ values into a single master curve when plotted against the ratio of water molecules per polyelectrolyte-polyelectrolyte ion pair.