Do additives shift the LCST of poly (N-isopropylacrylamide) by solvent quality changes or by direct interactions?

The phase transition of thermoresponsive poly(N-isopropylacrylamide) is studied under the influence of additives considered as model substances for drugs. A series of aromatic compounds with similar structures, mainly benzaldehydes, is chosen. The lower critical solution temperature (LCST) is determined by differential scanning calorimetry and ¹H-NMR. All additives cause a down shift of the LCST, which depends on additive molecular structure and concentration. Since the LCST shifts are not correlated to hydrophobicity or solubility of the additive, the detailed substitution pattern is discussed as the controlling factor. The question whether LCST shifts can be explained by either the additives affecting the solvent quality or by specific interactions of additives with the polymer is addressed by LCST determination in dependence on polymer concentration. Though both factors are relevant, specific additive-polymer interactions are shown to play a major role in controlling the LCST.