Polyelectrolyte complexes (PECs) and polyelectrolyte multilayers (PEMs) are growing in interest for medical applications as well as smart materials and surfaces. PECs and PEMs are formed from the ion pairing of oppositely charged macromolecules, and the physical properties of the assembled structure are strongly controlled by the density of ion pairs, which act as crosslinks. These ion pairs, in turn, ultimately influence the glass transition temperature and the dynamics of the PECs and PEMs. Here, we connect how salt or pH dictate the extent of ion-pairing, how water lubricates ion pairs, and how these ultimately influence the glass transition temperature in a quantifiable way. Specifically, 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. Modulated differential scanning calorimetry (MDSC) reveals glass-transition temperatures (Tg) that decreases in value with increasing hydration and with decreasing complexation pH. We show the collapse of all Tg values into a single master curve when plotted against the ratio of water molecules per intrinsic (PAH+–PAA–) ion pair for all pH values explored. Remarkably, this master curve is linear when the natural log of the water to intrinsic ion pair ratio is plotted against the inverse of Tg. This strongly indicates that the thermal transition is heavily influenced by water at the intrinsic ion pair site. Next, PECs consisting of poly(diallyldimethylammonium chloride) and poly(styrene sulfonate) are examined as a function of salt concentration and hydration. We show that this system collapses into a master curve following the same linear relationship, but with salt controlling the extent of ion pairing instead of pH. These results suggest that water plays a dual role in PECs: first by participating in the hydrogen-bonding network within and second by plasticizing the PEC at the ion pair site.

How water, salt, and pH universally influence the glass transition in polyelectrolyte complexes

255th National Meeting (2018)

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