Effect of Water on the Thermal Transition Observed in Poly(allylamine hydrochloride)-Poly(acrylic acid) Complexes

Here, we present the thermal behavior of polyelectrolyte complexes (PECs) containing weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) as influenced by water content and complexation pH. Modulated differential scanning calorimetry (MDSC) reveals a glass-transition-like thermal transition (T-tr) that decreases in value with increasing hydration and with decreasing complexation pH. We show the collapse of all T-tr values into a single master curve when plotted against the ratio of water molecules per intrinsic (PAH(+)-PAA(-)) ion pair for all pH values explored. Remarkably, this master curve is linear when the natural log of the water to intrinsic ion pair ratio is plotted against the inverse of T-tr. This strongly indicates that the thermal transition is heavily influenced by water at the intrinsic ion pair site. Other water-solvent mixtures are also explored, for which T-tr appears to depend only on water content, regardless of the added solvent. These results suggest that water plays a dual role in PAH-PAA complex: first by participating in the hydrogen-bonding network within and second by plasticizing the PEC. A hypothesis for the thermal transition is proposed in which hydrated PECs undergo a two-step thermal transition caused by an initial restructuring of the water-polyelectrolyte hydrogen-bonding network, followed by chain relaxation.