Membrane formation process and mechanism of PVDF-g-PNIPAAm thermo-sensitive membrane Original Research Article

The copolymer of poly (vinylidene fluoride) grafting poly (N-isopropylacrylamide) (PVDF-g-PNIPAAm) was synthesized and its flat sheet membranes were prepared via phase inversion method with N, N-dimethylformamide (DMF) as solvent and ethanol aqueous solutions as coagulation bath. The membrane formation process and mechanism were studied by the phase diagram (thermodynamics) and the ultrasonic time-domain reflectometry (kinetics). The membrane morphology was characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and Atom force microscope (AFM). The permeability at different temperature was studied by filtration experiment. The results showed that the coagulation bath composition influenced the membrane formation process and mechanism greatly. When water was used as coagulation bath, instantaneous liquid–liquid demixing dominated the demixing process and resulted in faster membrane-forming speed and typical finger-like macrovoids. With the increase of ethanol content in coagulation bath, the demixing process changed from instantaneous liquid–liquid demixing to delayed liquid–liquid demixing and resulted in slower membrane-forming speed and uniform spongy pores. The special solubility of PNIPAAm in ethanol aqueous solution greatly affected the membrane formation process and the final membrane structure. The permeability and the temperature-sensitivity of PVDF-g-PNIPAAm membrane could be regulated by varying the coagulation bath composition simply.