Unravelling the interactions between biomedical thermoresponsive polymer and biocompatible ionic liquids

Studies on the phase behavior of thermoresponsive polymers (TRPs) in the presence of ionic liquids (ILs) are an emerging area for the preparation and design of new polymeric materials. The search to understand the influence of ILs on polymers has come into the limelight as a great challenge. Hitherto, limited work on the phase transition behavior of TRPs in the presence of ILs is available. In this work, we studied the phase behavior of poly-N-isopropylacrylamide (PNIPAM) in presence of cholinium chloride ([Ch]Cl), cholinium acetate ([Ch][Ac]), cholinium bitartrate ([Ch][Bit]) and cholinium dihydrogen citrate ([Ch][DHCit]) using various techniques such as UV–Visible absorption spectroscopy, steady-state fluorescence spectroscopy, thermal fluorescence spectroscopy, viscosity (ƞ) and dynamic light scattering (DLS). All cholinium-based ILs studied show qualitatively and quantitatively a similar phase behavior, suggesting it to be quite resilient with respect to changes in the anion of the ILs. However, concentration and orientation of ILs have a varied effect on the phase transition temperature and on the aggregation behavior of PNIPAM. Our temperature dependent experimental results explicitly signifies that lower critical solution temperature (LCST) values decrease with increasing the temperature and concentration of studied ILs, which indicates that hydrophobic interactions are dominating. Anions of IL with their charge densities, hydration capacities and hydration energies leads to the hydrophobicity of PNIPAM + IL aqueous solution. High polarity, owing to the charge of the carboxylate groups in [Ch][DHCit] and [Ch][Bit], and hydrogen bond acceptor capability of the Cl⁻ anion causes their affinity for water inducing ability. This is the first report on the influence of cholinium-based ILs on the phase behavior of the PNIPAM. The current research work provides significant information on the phase transition and aggregation behavior of TRPs in ILs, which paves the way for potential applications in various fields.