Preparation of Thermal and pH Dually Sensitive Polyurethane Membranes and Their Properties

With c rystalline polycaprolactone diols (PCL) used as soft segments, and 1,4-butanediol (BDO), dimethylolpropionic acid (DMPA), and N-methyldiethanolamine (MDEA), respectively, used as different pH-sensitive components, three types of thermal and pH-sensitive polyurethanes (PUs) were prepared by using the block copolymerization technique and are referred to as PU(a), PU(b), and PU(c), correspondingly. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and differential scanning calorimetry (DSC) were used to study the distinct functional groups of the PUs and their segmented structures. Their thermal and pH stimuli sensitivity were measured by water permeability (WP) and water swelling (WS) tests. Fourier transform infrared analysis demonstrated that the ‒COOH and ‒N(CH3)‒ groups were successfully introduced into the PU(b) and PU(c), respectively, and these acid–base groups did not change the basic structure of their main chains. X-ray photoelectron spectroscopy analysis showed that the O and N concentration increased on the surface of PU(b) and PU(c), respectively. Differential scanning calorimetry analysis indicated that all three PUs showed phase-separated structures and similar phase-transition temperatures of the soft segments (defined as the switch temperature, Ts), which showed that the introduction of the acid–base functional groups did not much affect the phase-separated structures and crystallization of the soft segments. The WP study indicated that all PUs showed thermal intelligent responses, i.e., the WPs were correspondingly changeable with change of the temperature. In particular, when the temperature varied near the Ts, e.g., from Ts−10°C to Ts + 10°C, the WPs changed markedly. In addition, due to PU(b) and PU(c) containing ‒COOH and ‒N(CH3)‒ groups, respectively, they both showed a pH response in their WP and WS. When the pH changed from 4.0 to 5.5, the WP and WS of PU(b) were obviously changed. The same change also occurred for PU(c) in the pH range from 8.5 to 10.0. We thus conclude that the method utilized is an efficient way to prepare PUs that are dually responsive to both thermal and pH stimuli.