Thermoresponsive behavior of an LCST-type polymer based on a pyrrolidone structure in aqueous solution

Abstract

Poly(3-ethyl-N-vinyl-2-pyrrolidone) (C₂PVP) is a new type of thermoresponsive polymer which exhibits phase separation in water above the lower critical solution temperature (LCST) at about 26 °C. The thermoresponsive mechanism of C₂PVP during the heating–cooling cycle has been investigated by means of temperature-dependent FTIR in combination with a two-dimensional correlation (2Dcos) technique and density functional theory (DFT) calculations. Compared with a secondary-amide thermoresponsive polymer such as poly(N-isopropyl acrylamide) (PNIPAM), the unusual phenomenon of an asymmetrical-to-symmetrical transition of the CO peak shape is observed in the conventional IR spectra of tertiary-amide polymer C₂PVP. 2Dcos results confirmed that two species of CO groups (CO⋯2D₂O and CO⋯D₂O) mainly coexist at the initial heating, and they change to the other two CO groups involving the free CO and CO⋯DOD⋯OC after heating, with the latter predominant above the LCST. A slight hysteresis is observed in the cooling process attributed to the existence of a weak “crosslinking” point originating from the structure of CO⋯DOD⋯OC. Based on 2Dcos results, the dehydration process of different groups could be described in the following order: ethyl groups > CO groups > CH₂ groups in ring (“>” means prior to), and the reversible sequence is observed during hydration process. In PNIPAM system, CH₂ groups usually change prior to CO groups, while for C₂PVP the dehydrated rate of CH₂ groups (in ring) lags behind that of CO groups because of the steric hindrance of pyrrolidone structures. It is worth noting that cononsolvency of C₂PVP can be found in water/methanol mixed solvents. The LCST of C₂PVP in water/methanol mixture shifts from 29.5 to 16 °C as the methanol volume fraction reaches 50% and the phase separation totally disappears when the content of methanol is higher than 60%.