Interactions between a biomedical thermoresponsive polymer and imidazolium-based ionic liquids: A comprehensive biophysical investigation

Understanding the phase transition behavior of thermoresponsive polymers (TRPs) and ionic liquids (ILs) aids in the development of novel polymeric ionic liquid (PIL)-based materials. Further, knowledge of the molecular interactions between TRPs and ILs leads to novel biophysical prospects, such as producing TRPs with phase transition behaviors nearer to physiological body temperature that can be employed in drug delivery systems. Predicted interactions between TRPs and ILs were used to probe the changes in the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM) upon adding 1-ethyl-3-methylimidazolium nitrate ([Emim][NO₃]) or 1-butyl-3-methylimidazolium nitrate ([Bmim][NO₃]) ILs to aqueous PNIPAM solutions. Absorption and steady-state fluorescence spectroscopy were used to elucidate molecular interactions and variations in the absorbances of aqueous TRP and IL mixtures, and temperature-dependent dynamic light scattering, and fluorescence spectroscopy measurements were used to determine LCSTs. Aggregation behavior, changes in coil to globule transition behavior, absorbances, and fluorescence intensities were identified in aqueous PNIPAM, PNIPAM-[Emim][NO₃] and PNIPAM-[Bmim][NO₃] solutions. IL additions increased the LCST of PNIPAM, and this effect was more pronounced in ILs, perhaps due to the shorter alkyl chain on the imidazolium cation. The current study aids the design of PIL-based materials and could stimulate novel developmental concepts.