Glassy dynamics of model complex coacervate films with variable interaction strength quantified by the critical salt concentration

Abstract

Complex coacervates are widely studied in manufacturing, food processing, personal care, and therapeutics. While much research pertinent to coacervates has recently focused on their phase behavior, their glassy dynamics remains largely unexplored. We anticipated, based on a combination of the generalized entropy theory (GET) of the dynamics of glass-forming liquids and recent molecular dynamics simulation studies, that variations in these material's polymer charge density should significantly alter the glass transition temperature (T_g) and fragility (m) because of the known effect of charge on the cohesive energy density of polymer materials. To test this hypothesis, we performed dielectric spectroscopy measurements on model complex coacervate films formed by blending poly-diallyldimethyl ammonium chloride (PDDA) and adenosine triphosphate (ATP). The critical salt concentration (c_salt) was taken to be a quantitative measure of molecular interaction strength, ε. The films were vacuum annealed before dielectric measurements were conducted over a wide temperature range extending down to temperatures close to T_g. As anticipated, we find that increasing ε increases T_g, but progressively decreases m. We also find that our coacervate films exhibit high apparent dielectric permittivity (>10³) at room temperature for a moderate frequency of 1 kHz, which naturally explains the observed high responsiveness of such materials to even relatively weak electric fields (≈1 V cm⁻¹). Finally, we show that moisture tends to plasticize the glassy dynamics of these materials, i.e., reduce T_g. These trends are expected to hold rather generally for complex coacervate materials arising in diverse manufacturing and biophysical contexts.