| Thermally induced phase separation(TIPS)technique has been used to prepare commercial membranes in the field of sewage.Many polymeric membranes via the TIPS process such as Polysulfone(PSF)and especially Polyvinylidene fluoride(PVDF)had been reported.This can be ascribed to the fast and relatively simple heat-transfer process which is turn resulted in a membrane with high mechanical strength and narrow pore size distribution.In the previous studies of TIPS,there was no systematic study on the role of non-diluents,and non-diluents played a crucial role in TIPS.Especially,water-soluble non-diluents could double-regulate the membrane structure by the coupling effect of TIPS and NIPS.The main research contents and conclusions are as followsFirstly,considering The unidirectional regulatory role of water-soluble non-diluent on membrane In this work,the effect of double-diffusion between the dope and bore liquid on the membrane microstructure,especially the outer surface and the thickness for the cross-section near the outer skin-layer,was designed and conducted for the first time by using pore-forming agent as bore liquid and adjusting the air gap via solid-liquid(S-L)phase separation of TIPS during the hollow fiber membrane preparation.The reason of employing S-L phase separation were the relatively lower spinning temperature(the obvious diluent evaporation was avoided and the cooling rate slowly was achieved to ensure that dope does not solidify,which are crucial to the diffusion of bore liquid.In addition,the S-L phase usually resulted in a dense and thick skin-layer.The air gap is in the range of 10mm-300mm to regulate the diffusion time of bore liquid within the domain of the cross-section.It can be indicated that the change of the thickness and surface for skin-layer will become obvious with different diffusion of bore liquid and the internal pressure pure water flux has been improved.Secondly,through the study of PVDF/M and water-soluble non-diluents with different molecular weights,the effects of different non-diluents on membrane structure were systematically analyzed.The results showed that non-diluent T,a water-soluble non-diluent with small molecular weight,had no obvious pore-forming effect on the surface.However,after adjusting the air gap to make the poor phase droplets grow up and aggregate,the mass transfer diffusion made the membrane surface have obvious pore structure and the membrane cross-section become penetrated.The pure water flux of the membrane could reach 1394 L(m2 h).The water-soluble PEG400 as a non-diluent is easy to diffuse in coagulation bath,and the effect of increasing the molecular weight of PEG400 on the membrane structure is investigated.The results show that PEG400 also has a strong pore-forming effect in TIPS.When increasing the molecular weight of non-diluent PEG20000 and smaller pore appears on the membrane surface.The rej ection of BSA reached 93.5%,and the external pressure pure water flux reached 207 L/(m2·h).Finally,water-insoluble non-diluent D was used as a non-diluent to compare the changes of membrane structure without NIPS coupling.The results show that non-diluent D has a strong interaction with diluent.Therefore,the structure of membrane cross section can be seen by SEM as closed cellular structure,but when L-L phase separation occurs without NIPS coupling.Non-uniform holes also appeared on the surface of the membrane.the maximum water flux is 352 L/(m2·h). |
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