Water and the Glass Transition Temperature in a Polyelectrolyte Complex

Hydrated polyelectrolyte complexes, H-PECs, have recently started attracting renewed interest as a class of highly solvated/plasticized blends. H-PECs are observed to undergo a transition in mechanical properties close to room temperature. Whether this is a true glass transition has been questioned recently: the material has an unusually low modulus in the "glassy" state and molecular dynamics simulations have suggested temperature-induced dehydration and water structure changes are responsible for the transition. Using in situ infrared spectroscopic methods on thin films of a widely studied H-PEC we find no definitive evidence for changes in the hydration state of functional groups, the water content, or water structure on passing through T_g for stoichiometric and nonstoichiometric H-PECs. These complexes represent a promising platform for fundamental studies of the glass transition, since the coupling between chains can be modified by "doping" the material with salt, which breaks ion pairing cross-links. The Fox equation was used to estimate T_gs for paired and unpaired oppositely charged repeat units.