Structural elucidation of morphology and performance of the PVDF/PEG membrane

Poly(vinylidene fluoride) (PVDF) membranes were prepared through a phase inversion method, using N, N-dimethylacetamide (DMAc) as solvent, poly ethylene glycol (PEG) as hydrophilic additive and water as the coagulation medium. The ternary phase diagram of the PVDF/DMAc/water system at two different bath temperatures of 25 °C and 60 °C was derived through cloud-point measurements. Besides, the effect of PEG inclusion into PVDF solution was investigated in a ternary phase diagram. The results showed that both parameters of coagulation bath temperature and the PEG content affect the system thermodynamic stability. The morphology investigation of the prepared membranes was carried out by SEM cross-section images. The most regular structure of the membrane was obtained by incorporation of 5 wt.% PEG at 60 °C bath temperature. The AFM analysis also showed the favorable effect of the PEG on membrane surface roughness. The polymorphism study of the membrane crystal forms was carried out by FTIR analysis. The study showed that the higher bath temperature and PEG inclusion disfavored the formation of β crystal, which may be attributed to lower thermodynamic stability of the β crystal. The water flux measurements were carried out using dead-end filtration procedure. The membrane flux and total membrane porosity were increased as the PEG content increased, mainly due to the pore forming effect of PEG additive.