Time-Temperature and Time-Water Superposition Principles Applied to Poly(allylamine)/Poly(acrylic acid) Complexes

The dynamic mechanical and rheological behavior of polyelectrolyte coacervates and complex precipitates is of interest for many applications ranging from health to personal care. Hydration is an important factor, but its effect on the dynamic properties of polyelectrolyte complexes (PECs) is poorly understood. Here, we describe the dynamic behavior of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complex precipitates at varying relative humidity values and temperatures using both dynamic mechanical analysis (DMA) and all-atom molecular dynamics simulations. To mirror the experimental system via simulation, the water content within the PEC is measured and used as the parameter of interest rather than relative humidity. In the experimental DMA, the modulus decreases with both increasing water content and temperature. The data are superimposed into a super master hydrothermal curve using the time-temperature superposition principle and the time-water superposition principle for the first time. The temperature-dependent shift factor (a _T ) follows an Arrhenius relation, and the water-dependent shift factor (a _W ) follows a log-linear relation with the water content in the complex. These results suggest that both temperature and water affect the dynamics of the PEC by similar mechanisms over the range investigated. All-atom molecular dynamics simulations show that an increase in the water content and temperature leads to similar changes in the polyelectrolyte chain mobility with little effect on the number of intrinsic ion pairs, suggesting the validity of time-water and time-temperature superposition principles.