Temperature-Switchable Polymer: Uniting Deep Eutectic Solvents with Poly(N-isopropylacrylamide) and Poly(N-vinyl caprolactam)

As analogs of ionic liquids (ILs), deep eutectic solvents (DESs) have attracted considerable attention as benign liquid formulations in the fabrication of polymeric materials because of the numerous advantages and functionalities of these liquid formulations, along with their ability to satisfy the principle of sustainability. Herein, the effectiveness of choline chloride (ChCl)-based DESs as a cosolvent was studied in the development of a versatile platform for regulating the thermal behavior of poly(N-isopropylacrylamide) (PNIPAM) solutions. The hydrogen bond donor (HBD) groups in DESs selectively influence the dehydration mechanism of PNIPAM. UV–visible spectroscopy, steady-state fluorescence, Fourier transform infrared spectroscopy (FTIR), and dynamic light scattering (DLS) analyses demonstrated the predisposition of HBD groups toward complex hydration network around PNIPAM. Furthermore, the lower critical solution temperature (LCST) was investigated using temperature-dependent fluorescence spectroscopy, demonstrating a decrease in the LCST of PNIPAM in the presence of DESs. The LCST declined most steeply in the presence of choline chloride/lactic acid (ChCl/LA), whereas only negligible variations were observed for choline chloride/urea (ChCl/urea) and choline chloride/ethylene glycol (ChCl/EG). PNIPAM and poly(N-vinyl caprolactam) (PVCL) were compared to understand the unique molecular interactions of DESs, which clarified the involvement of eutectic solvents in altering hydrogen bonding between the polymers and surrounding water molecules. Disruption of the hydrogen bond interactions resulted in early hydrophobic collapse of the polymers. Moreover, this change depended on the nature of the HBD groups in the DESs. This study highlights the fundamental spectroscopic insights of thermoresponsive polymers in sustainable eutectic solvents for potential use as pulsatile drug carriers.