Poly(N-isopropylacrylamide) grafted poly(vinylidene fluoride) copolymers for temperature-sensitive membranes

Amphiphilic graft copolymers having poly(vinylidene fluoride) (PVDF) backbones and poly(N-isopropylacrylamide) (PNIPAAm) side chains were synthesized via γ-ray pre-irradiation induced graft polymerization in water phase suspension. No organic solvent was used in the synthesis process, which was harmless to environment. The resulting copolymers, PVDF-g-PNIPAAm, were characterized by nuclear magnetic resonance (NMR) and thermogravimetric analysis (TGA). It was showed that the PNIPAAm chains were successfully grafted onto PVDF main chains, and the weight percentage of PNIPAAm chains in the graft copolymers increased almost linearly with the increase of the monomer concentration in reaction solution. The synthesized PVDF-g-PNIPAAm was used as an additive in the preparation of PVDF porous membranes by immersion precipitation. The surface chemical compositions of the prepared blend membranes were analyzed using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found the weight fraction of PNIPAAm chains on membrane surface was much higher than that in PVDF-g-PNIPAAm copolymer due to the surface segregation of hydrophilic chains. The membrane hydrophilicity, surface morphology and water permeability were evaluated by water contact angle measurement, scanning electron microscopy (SEM) observation and water permeation experiment, respectively. It was indicated that both water contact angle and water permeability exhibited an abrupt change when the temperature was elevated to above low critical solution temperature (LCST, 32 °C) due to the shrinkage of PNIPAAm chains.