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

Polyelectrolyte complexes (PECs) are receiving increasing attention because of their stimuli-responsive behaviors with ionic strength, pH, and temperature. Of these, temperature is particularly intriguing in that PECs undergo a glass-melt transition of an unclear nature. Here, we present the thermal behavior of PECs containing weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) as influenced by water content and complexation pH. These are investigated using modulated differential scanning calorimetry (MDSC) and molecular simulation. MDSC reveals a glass-transition-like thermal transition (T_tr) that decreases in value with increasing hydration and with decreasing complexation pH. As complexation pH decreases, the PAA content in the PEC increases and the PAA ionization decreases. Added 1-proponal elevates the transition temperature, as does added urea. Atomistic detail molecular dynamics simulations show an increasing number of water-polymer hydrogen bonds within the hydrated complexes as water content increases. These results suggest that water has a plasticizing effect, and these results support the hypothesis that the transition is caused by a restructuring of the water hydrogen-bonding network within the PEC that then allows for subsequent chain relaxation.