Preparation and characterization of polyvinylidenedifluoride-co-chlorotrifluoroethylene hollow fiber membranes with high alkaline resistance

In the present work, polyvinylidenedifluoride-co-chlorotrifluoroethylene (PVDF-co-CTFE) hollow fiber membranes were prepared via non-induced phase separation (NIPS) using 1-methyl-2-pyrrolidone (NMP) or triethyl phosphate (TEP) as the solvent. The thermodynamic characterization of both systems PVDF-co-CTFE/NMP/water and PVDF-co-CTFE/TEP/water) and the rheological properties of dope solutions were first investigated. Subsequently, the effect of dope concentration and spinning conditions such as air gap on the membrane permeability and mechanical strength was discussed in detail. The rejection of polystyrene with a size of 50 nm was 85, 100 and 100% for PVDF-co-CTFE hollow fiber membranes prepared with PVDF-co-CTFE concentrations of 14,17 and 20wt%, while the permeability was 119, 39 and 2.5 L/m(2) h bar, respectively. The morphologies of hollow fiber membranes were observed using scanning electron microscopy to evaluate the effect of solvent and spinning conditions on the membrane structure. The resultant PVDF and PVDF-co-CTFE hollow fiber membranes were immersed in a strong alkaline solution (NaOH (1 M)+NaClO (4000 ppm)) to evaluate the chemical resistance of PVDF-co-CTFE hollow fiber membranes in comparison with that of PVDF membranes. The mechanical properties of PVDF and PVDF-co-CTFE hollow fiber membranes immersed in the alkaline solution for 180 days were declined 90% and 10% as compared to the original ones, respectively. Moreover, the hollow fiber membranes were characterized using laser Raman spectra, Fourier-transform infrared spectra, differential scanning calorimetry, and X-ray photoelectron spectroscopy, before and after the immersion, to hypothesize the degradation mechanism of PVDF-co-CTFE and PVDF hollow fiber membranes in the alkaline solution. (C) 2018 Elsevier Ltd. All rights reserved.