Hydrated polyelectrolyte (PE) complexes
and multilayers undergo
a well-defined thermal transition that bears resemblance to a glass
transition. By combining molecular simulations and differential scanning
calorimetry (DSC) of poly(diallyldimethylammonium) (PDAC) and poly(styrenesulfonate)
(PSS) multilayers, we establish for the first time that dehydration
drives the thermally induced change in plasticization of the complex
and in the diffusion behavior of its components. DSC experiments show
that the thermal transition appears when the assemblies are hydrated
in water but not in the presence of alcohols, which supports that
water is required for this transition. These findings connect PE complexes
more generally to thermoresponsive polymers and liquid crystal phases,
which bear phase transitions driven by the (de)hydration of functional
groups, thus forming a fundamental link toward an integrated understanding
of the thermal response of molecular materials in aqueous environments