Study On Modification Of PVDF Membranes By PEI Cross-linking And Interfacial Polycondensation

Poly(vinylidene fluoride)(PVDF)membranes have been extensively applied to scientific research and industrial process due to its outstanding properties such as high thermal stability,good chemical resistance and membrane forming properties.However,the applications of PVDF membranes are limited by two major problems: fouling in water treatment and wetting in membrane contactors.The hydrophilic modification is an important aspect of the PVDF membrane research.Firstly,the PVDF film with sponge-like pore was fabricated.Secondly,the PVDF membrane was treated with PEI-3000.Finally,the polycondensation reaction of PEGDE and PEI was used to form a new interception layer to improve the anti-pollution performance.Firstly,we studied the mechanism of PVDF sponge-like pore membranes formation and the post-treatment conditions of the membranes.The PVDF membrane was prepared by changing the composition of coagulation bath,and we explored the best post-treatment conditions of PVDF membranes by sodium hypochlorite.The factors that affect the treatment including p H,temperature,time and the concentration of sodium hypochlorite were investigated.The physicochemical properties,morphology and microstructure of the membranes were characterized.Our results showed that the coagulation bath is an important factor affecting the membranes structure.The membranes with sponge-like pore were fabracted by using the Ca Cl2(25wt%)as coagulation bath andthe average pore size of the membranes is 0.1108 ± 0.005?m.We found that treatment of sodium hypochlorite solution can increase the water flux of membranes by decreasing the PVP content.The optimum conditions of the treatment were not affected the structure of the membranes.When the time was 36 h,the temperature was 25?,the concentration of sodium hypochlorite was 5000 ppm and p H = 9,the membrane had the highest water flux of 1325.43 L·m-2·h.Secondly,PEI was used to treat PVDF membranes.The effects of the composition of treatment liquid,the molecular weight of polyethyleneimine and the content of polyethyleneimine on the properties of the membranes were investigated.The results show that PVDF membranes with carbon-carbon double bonds can be obtained by the reaction of dehydrofluorination with sodium hydroxide.Simultaneous treatment with PEI and sodium hydroxide can lead to the dehydrofluorination reaction and the Michael addition reaction,and N is introduced into the PVDF membranes,which provides the conditions for the next chapter.The results of SEM show that the structures of membranes did not obviously change and N was successfully introduced into the PVDF membrane after modification.The results of XPS show that the membraneswere partially oxidized and the O content was 7.94% after the sodium hydroxide modification.After the simultaneous modification with sodium hydroxide and polyethyleneimine,the O content of the membranes increased to 8.19% and the N content increased from 0 to 5.45%.In the last part,thecrosslinking reaction between amine group and polyethylene glycol diglycidyl ether performed.The hydrophilicity and anti-fouling properties of the membranes were improved because the hydrophilic group was formed in the membranes.XPS results show that the crosslinking reaction has occurred and the oxygen content of the membrane increased to 13.45%.Meanwhile,the EDS surface mapping scan also finds the oxygen elements.The results of static adsorption of catalase,dynamic adsorption of catalase and membrane resistance show that the hydrophilicity and anti-pollution properties of the modified membranes are improved significantly.The contact angle of the film decreased from 77.5° to 54.5°,and the contact angle was only 16.5° after 10 min.When the PEI content ? 4g,the membranes flux loss rate is less than 40% while the membranes flux recovery rate is more than 90%.The membranes resistance calculation shows that the proportion of self-resistance increases and the resistance of the pollution layer decreases during the operation.The anti-pollution performance of the membranes is remarkably improved.