Relaxation Times of Solid-like Polyelectrolyte Complexes of Varying pH and Water Content

The effect of complexation pH and water on the relaxation time and dynamics of polyelectrolyte (PE) complexes (PECs) and coacervates remains poorly understood. Here, we describe the dynamic mechanical behavior of solid-like PECs containing weak PEs poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) at varying complexation pH, relative humidity, and temperature with support from molecular dynamics simulations. Time−temperature, time−water, and time−intrinsic ion pair superposition principles are applied to obtain the relaxation times. It is shown that the natural log of relaxation time in hydrated PAH/PAA PECs is inversely proportional to the volume fraction of water (ln τ ∼ ϕ_w ⁻¹ ) for a given complexation pH. For all complexation pH values examined, the natural log of relaxation time collapsed into a single line when plotted against the ratio of the number of intrinsic ion pairs to water molecules (ln (τ) ∼ number of intrinsic ion pairs/ number of water molecules). Taken together, this suggests that the relaxation of solid-like, hydrated PAH/PAA PECs is mediated by bound water at the intrinsic ion pair.