Profiling the molecular interactions between a promising thermoresponsive polymer and ionic liquid: A biophysical outlook

Abstract

Thermoresponsive polymers (TRPs) that undergo significant conformation/aggregation changes in response to changes in temperature have extensive range of potential applications in various fields of science and technology. Among different types of TRPs, poly (N-isopropylacrylamide) (PNIPAM) is a suitable amphiphilic polymer for studying the conformational changes due to its biocompatible behavior with various additives. In order to get deeper insights of the TRPs in the presence of additives, we examined the collapse and aggregation of thermoresponsive polymer (PNIPAM) in aqueous solution containing variable amounts of 1-allyl-3-methylimidazolium bromide ([Amim][Br]). The influence of ionic liquid on the lower critical solution temperature (LCST) of PNIPAM aqueous solution were examined with the aid of comprehensive biophysical techniques such as UV–visible absorption spectroscopy, steady-state fluorescence spectroscopy, thermal fluorescence spectroscopy, viscosity (η) and dynamic light scattering (DLS) measurements. With the addition of 5 mg/mL of [Amim][Br] to the PNIPAM aqueous solution the LCST has been decreased towards lower temperatures. Further with the addition of higher concentration (10 and 15 mg/mL) of [Amim][Br] the decrease in the LCST of PNIPAM has been more pronounced towards lower temperatures. Our experimental results explicitly elucidated that the decrease in the LCST of the PNIPAM with increase in the concentration of [Amim][Br] is due to the substantial variations in the interactions among amide group of polymer, ions of ionic liquid and water molecules. The experimental results of the current investigation can be used for the design of smart polymer-based devices, as its LCST is close to body temperature. The results provide an alternative way to tune the phase transition temperature of the broadly accepted model PNIPAM polymer.