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

Abstract

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 experimental DMA, 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 (aT) follows an Arrhenius relation, and the water-dependent shift factor (aw) follows a log-linear relation with 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. Allatom molecular dynamics simulations show that an increase in water content and temperature lead to similar changes in polyelectrolyte chain mobility with little effect on the number of intrinsic ion pairs, suggesting the validity of time-water and time-temperature supposition principles.

Recent progress in the science of complex coacervation

“…37,57 The horizontal shift factor τ c (γ →γτ c ) accounts for changes in the stress-relaxation behaviour of the material, which is attributed to the weakening of electrostatic interactions via charge screening (Figure 11c). 37,57 This is analogous to the traditional time-temperature superposition, which has also been reported for coacervates 53,59 and polyelectrolyte complexes, 200 except that salt concentration weakens interactions (and expedites dynamics) rather than temperature.…”

“…Surface properties such as the interfacial tension between the surfactant and coacervate phase have been measured using atomic force microscopy 38,54 and surface forces apparatus. 55,56 The structure and dynamics of bulk coacervates have been characterized using rheology 37,47,53,[57][58][59][60][61][62][63][64][65] along with X-ray and neutron scattering. 62,66,67 Finally, the thermodynamics of bulk coacervation has been characterized using isothermal titration calorimetry (ITC).…”

“…try (DSC), thermogravimetric analysis (TGA), electrochemical impedance spectroscopy (EIS), and dynamic mechanical testing (DMA) were compared by molecular dynamics (MD) simulations by Sammelkorpi and coworkers. 63,200,[203][204][205][206][207][208][209][210] These reports ultimately determined that the temperature of the thermal transition between samples prepared at different water, salt, pH, solvent, and other additive contents could be collapsed onto a universal curve by normalizing based on the water concentration per intrinsic ion pair (Figure 12d). 203,206 Furthermore, it was possible to linearize this universal curve by plotting the natural log of the ratio of (water/intrinsic ion pair) as a function of (1/T tr ) (Figure 12e).…”

“…These results were further extended to show that it was possible to perform both a time-temperature and a time-water superposition at constant salt because the variation in the intrinsic ion pair concentration was minimal. 200 Ultimately, these three areas of study into coacervate dynamics consider a wide range of time/length scales; the work on glass transition-like thermal transitions considers local segmental and water dynamics, 194,203,[205][206][207][208][209] while the time-salt superposition work borrows from the 'sticky Rouse' theories that consider the dynamics of the entire chain conformation. 37,57,201 These areas of recent progress thus remain only brief glimpses at a rich dynamic behavior of coacervate dynamics and rheology, and suggest the importance of hydration and other atomistic-level effects in the practical application of these materials.…”

A Salt Controlled Scalable Approach for Formation of Polyelectrolyte Complex Fiber†

“…However, the hydration effect of the salt ions also has an impact on mechanical properties. As aforementioned, water is an effective plasticizer to PEC materials due to the lubrication effect and increase of free volume, which leads to a small breaking strength and a large elongation . But the salt ions would trap water molecules via hydration, thus weakening the lubrication effect .…”