On the upper critical solution temperature of PNIPAAM in an ionic liquid: Effect of molecular weight, tacticity and water

The upper critical solution temperature (UCST) of 1.0 wt% Poly(N-isopropylacrylamide) (PNIPAAM) in the ionic liquid (IL) 1-ethyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)amide ([EMIM][TFSI]) is strongly affected by the molecular weight and tacticity of the polymer and water content of the IL. UCSTs in IL/water mixtures are tuned in a wide range by modulating the abovementioned parameters: notably UCST spans from about -12 °C for atactic and low MW PNIPAAM in anhydrous IL up to 79 °C for the stereo-regular high MW one in presence of water. In all cases depicted above the transition always remains sharp, a convenient feature to consider in view of possible applications. The significant effect of water on the upward shift of UCSTs, which interestingly is opposite to that observed in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], is related to the water organization in the IL, depending on the concentration therein. The above phenomena are discussed in terms of polymer-IL-water interactions balance.