Thermoresponsive Poly(Ionic Liquid)s in Aqueous Salt Solutions: Salting‐Out Effect on Their Phase Behavior and Water Absorption/Desorption Properties

Here, a thermoresponsive phase behavior of polymerized ionic liquids (PILs) composed of poly([tri‐ n ‐alkyl(vinylbenzyl)phosphonium]chloride) (poly([P nnn VB ]Cl) is reported, where n (the number of carbon atoms of an alkyl chain) = 4, 5, or 6 after mixing with aqueous sodium chloride solutions. Both monomeric [P 555VB ]Cl and the resulting poly([P 555VB ]Cl) linear homopolymer show a lower critical solution temperature (LCST)‐type phase behavior in aq. NaCl solutions. The phase transition temperature of the PIL shifts to lower value by increasing concentration of NaCl. Also the swelling degree of cross‐linked poly([P 555VB ]Cl) gel decreases by increasing NaCl concentration, clearly suggesting the “salting‐out” effect of NaCl results in a significant dehydration of the poly([P 555VB ]Cl) gel. The absorbed water in the PIL gel is desorbed by moderate heating via the LCST behavior, and the absolute absorption/desorption amount is improved by copolymerization of [P 555VB ]Cl with more hydrophilic [P 444VB ]Cl monomer.