Rehydration of thermoresponsive poly(monomethoxydiethylene glycol acrylate) films probed in Situ by real-time neutron reflectivity

The rehydration of thermoresponsive poly(monomethoxy diethylene glycol acrylate) (PMDEGA)films exhibiting a lower critical solution temperature (LCST) type demixing phase transition in aqueous environments, induced by a decrease in temperature, is investigated in situ with real-time neutron reflectivity. Two different starting conditions (collapsed versus partially swollen chain conformation) are compared. In one experiment, the temperature is reduced from above the demixing temperature to well below the demixing temperature. In a second experiment, the starting temperature is below the demixing temperature, but within the transition regime, and reduced to the same final temperature. In both cases, the observed rehydration process can be divided into three stages:first condensation of water from the surrounding atmosphere, then absorption of water by the PMDEGA film and evaporation of excess water, and finally, rearrangement of the PMDEGA chains. The final rehydratedfilm is thicker and contains more absorbed water as compared with the initially swollen film at the same temperature well below the demixing temperature.