Effect of anions on the phase transition temperature of two structurally isomeric polymers: poly(N-isopropylacrylamide) and poly(2-isopropyl-2-oxazoline)

The effect of Hofmeister anions on the lower critical solution temperature (LCST) of two structural isomers, namely poly(N-isopropylacrylamide) (PNIPAAm) and poly(2-isopropyl-2-oxazoline) (PiPOx), is studied. Following the preparation of these two polymers with the same molecular weights and chain ends, their thermal hysteresis and LCST properties are observed in the aqueous and Hofmeister salt solutions. We investigate the effects of the salt type and salt concentration on the LCST for both polymers. More specifically, the LCST of PiPOx sharply increases as the concentration of SCN− increases, whereas that of PNIPAAm slightly increases prior to decreasing again. It is also confirmed that the boundary between the salting-out and salting-in effects is different for both polymers according to the type of salt present. Furthermore, Raman spectroscopy and two-dimensional correlation spectroscopy are used to examine the conformational changes in both polymers with increasing SCN− (chaotropes) and Cl− (kosmotropes) concentrations. Since the interaction mechanism between the polymer and anion is remarkably different even between isomeric polymers, it is necessary to rationally design the polymer structure to enhance the anion effect of the thermoresponsive polymer.