Preparation of thermoresponsive hydrogels via polymerizable deep eutectic monomer solvents

Abstract

We report the preparation of thermoresponsive poly(N-isopropylacrylamide) (polyNIPAM) hydrogels via the free radical polymerization of deep eutectic monomer solvents (DEMs) for the first time, where NIPAM serves as a polymerizable hydrogen bond donor. DEMs were prepared by a simple heating and stirring protocol using NIPAM and either choline chloride (ChCl) or acetylcholine chloride (AcChCl) (as hydrogen bond acceptor) in various ratios to yield low-melting point liquids (as low as 15 °C). 1D and 2D NMR spectroscopy supported the association of the NIPAM and choline salts present within the DEM structure, in addition to the low self-diffusion coefficients of the species present compared to when dissolved in water. Thermogravimetric analysis demonstrated an enhanced thermal stability of the DEMs compared to NIPAM. Hydrogels prepared by free-radical polymerization of the prepared DEMs in the presence of N,N′-methylenebisacrylamide (BIS) as crosslinker showed a significant increase in reaction rate compared to the equivalent reaction in water, which was attributed in part to the high viscosity of the DEMs. These gels exhibited thermoresponsive swelling behaviour when immersed in water. Specifically, gels prepared via DEMs featured reduced swelling capacity and increased mechanical strength relative to those prepared by aqueous polymerization, attributed to a significant increase in cross-linking density.