Study On Preparation Of High-performance Polyvinylidene Fluoride Membrane By Low Temperature Thermally-induced Phase Separation Method

In this paper,the new role of non-solvent polyethylene glycol-400(PEG400)in the low temperature thermally-induced phase separation(L-TIPS)was analyzed,PEG400 were used to regulate the phase separation mechanism of the dope solution,and then the membrane structure and performance were improved.The membrane structure was regulated by gradually increasing the PEG400 content,the role evolution of water-soluble non-solvent PEG400 on the phase transformation mechanism and the effect on tensile strength of the polyvinylidene fluoride(PVDF)membrane were studied.The results showed that the increase of PEG400 content could increase the cloud point temperature of dope solution,accelerate the thermally induced phase separation(TIPS)effect and inhibit the negative effect of the non-solvent induced phase separation(NIPS)effect on membrane structure.Meanwhile,the increase of PEG400 content could shift the TIPS effect of the dope solution system from solid-liquid to liquid-liquid phase separation and inhibit the growth of PVDF spherulites during the TIPS process.The dual role mechanism of PEG400 together eliminated the NIPS finger-like voids and the TIPS sphere-packed aggregation structure,which had the negative impact on tensile strength of the PVDF membrane.The PVDF flat membrane showed excellent mechanical properties;the tensile strength of the membrane reached 3.12 MPa(more than 3-6 times higher than the membranes reported),pure water flux reached 391 L/(m2·h),and the rejection rate of carbon ink was 100%.The L-TIPS method was adopted to prepare a high-performance PVDF hydrophobic membrane with solvent triethyl phosphate and non-solvent PEG400 as a fully water-soluble mixed diluent.In the process of L-TIPS membrane formation,the competing mechanisms of TIPS and NIPS as well as the effect of PEG400 were regulated by changing the coagulation bath temperature.Meanwhile,the unidirectional solidification process of dope solution along the outer to the inner parts of the cross-section radius of the membrane was analyzed.The results showed that the tensile strength of the membrane increased with the increase of the external coagulation bath water temperature,which was the opposite of the general rule(i.e.,the tensile strength declines with the increase of the coagulation bath temperature).In this experiment,the water-soluble additive PEG400 was employed to regulate the membrane structure,and more specifically to increase the cellular structure(a positive impact on tensile strength)and reduce the sphere-packed aggregation structure(a negative impact on tensile strength).Thus,the tensile strength of the membrane increased.When the coagulation bath temperature was 60?,the tensile strength of the membrane reached 6.21 MPa(more than 2-4 times higher than the hydrophobic membranes reported).Meanwhile,owing to the enhancement of the pore-forming effect of PEG400,the PVDF hollow-fiber hydrophobic membrane also had a high flux.The gas permeability coefficient reached 0.23 ml/(m2·s·Pa).The vacuum membrane distillation flux reached 30.6 kg/(m2·h),which was 14.6 times higher than that of the coagulation bath temperature at 15?.