Thermoresponsive behavior of poly[trialkyl-(4-vinylbenzyl)ammonium] based polyelectrolytes in aqueous salt solutions

This contribution reports 16 new thermoresponsive systems based on poly[trialkyl-(4-vinylbenzyl)ammonium] chlorides. When salts are introduced into solution of the water-soluble polymers, thermoresponsive behavior is observed. The type of transition is dependent on the length of the alkyl chain: the polymer with an ethyl chain has an upper critical solution temperature (UCST) whereas the polymers with butyl or pentyl chains have a lower critical solution temperature (LCST). The magnitude of the effect of a given salt on the solution behavior is dependent on the type of the used electrolyte. With all polymers, the strength of the effect increases in the following order: NaH$_{2}$PO$_{4}$ < Na$_{2}$CO$_{3}$ < Na$_{2}$SO$_{4}$ < NaCl < sodium pentanesulfonate < NaNO$_{3}$ < NaSCN < lithium trifluoromethanesulfonate < lithium bis(trifluoromethane) sulfonimide. For monovalent salts, the order follows a reversed Hofmeister series. The results can be used to design new thermosensitive systems by matching the alkyl content of a polycation with an effect of the small molecular salt. Nearly an unlimited number of thermoresponsive polymers can be designed following the approach introduced in this article.