Dual Thermoresponsive and pH-Responsive Poly(vinyl alcohol) Derivatives: Synthesis, Phase Transition Study, and Functional Applications

Novel poly(vinyl alcohols) (PVA) functionalized with pendant thermo- and pH-responsive groups were prepared by carbonyldiimidazole (CDI)-mediated couplings of N1,N1-diethylethane-1,2-diamine (DEEDA) with controllable modification degree. Nuclear magnetic response (NMR) and IR have verified the successful modification of PVA. The macro- and microscopic phase transition behavior of the obtained PVA-DEEDA-t (t = 10, 30, 70, and 90 h) was thoroughly characterized using various techniques, including turbidity measurement, NMR, and dynamic light scattering (DLS). PVA-DEEDA-t is demonstrated to possess tunable lower critical solution temperature (LCST) between 58 and 24 °C. LCST is dependent on solution pH and degree of PVA modification (14.1–20.9%). By combining DLS and DOSY characterizations, it can be concluded that both coil-to-globule transition and aggregation occurred to PVA-DEEDA-t during phase transition, while only coil-to-globule transition can be detected for the pristine PVA. 2D NOESY proved that the −NH– segment on PVA-DEEDA-t is in close (<5 Å) proximity to the main chain of PVA, as evidenced by the appearance of NOE signal between −NH– on DEEDA and −CH2–CH– chain of PVA when the temperature increased above LCST. To exploit the functional applications, the PVA-DEEDA-90 h was transformed into gel and film forms. PVA-DEEDA-90 h gel obtained by adding borax enabled controlled drug (e.g., RhB) release due to its temperature- and pH-dependent permeability. The PVA-DEEDA-90 h film was also casted on ITO glass, creating a smart surface with tunable wettability and interfacial ion transportation with high sensitivity toward the pH and temperature.