Surface Interface Function Regulation Of Polymer Microporous Membranes Based On The Whole Process Of Phase Inversion

The performance of membranes,such as separation,anti-fouling,anti-bacterial and non-clotting performance,is mainly determined by the surface/interface function of membranes.Therefore,it is necessary to modify membrane surface.The traditional phase inversion process only involves the regulation of membrane pore structure,which is a typical physical process.In order to functionalize the membranes in the process of phase inversion,the concept of“chemical phase inversion”was proposed.The purpose of chemical phase inversion is to control the interfacial segregation and cross-linking reaction of functional molecules on membrane and microporous surface around the whole process of phase inversion to modify membrane.According to the different segregation paths of the modifiers,the modification methods can be divided into“Inside-out”segregation and in-situ cross-linking functionalization strategy based on casting solution,“Outside-in”segregation and off-site cross-linking functionalization strategy based on coagulation bath,“Top-down”segregation and interfacial cross-linking functionalization strategy based on microporous membranes.The detail research contents and results are summarized as follows:（1）The modifiers in coagulation bath segregated into membrane by the exchange of solvent and non-solvent,and then fixed in membrane by hydrolysis cross-linking of silane coupling agent.The modified membranes show superior anti-fouling,anti-bacterial and non-clotting properties.Especially for the membrane modified by poly（acrylic acid-sodium styrene sulfonate-vinyltriethoxysilane）P（AA-SSNa-VTES）,the activated partial thrombin time（APTT）could still be extended to 53 s when the area of the membrane was reduced to 1/32 cm².The“Outside-in”segregation mechanism was clarified.Under the driving of surface tension,the diffusion time of coagulation bath on the membrane surface is much less than the gel time,providing a longer time for the segregation of modifiers.Compared with surface grafting and surface coating,the modification time was obviously shortened.（2）Janus membrane with asymmetric wettability was fabricated by controlling modifiers segregation direction in casting solution.Specifically,in phase inversion process,oil-containing non-woven fabric acted as support layer to prevent or slow down the exchange of solvent and non-solvent on bottom surface.Driven by surface tension,hydrophilic modifier poly（vinyl pyrrolidone-vinyl triethoxysilane）P（VP-VTES）in casting solution only segregated from inside to outside and then fixed into membrane by hydrolysis cross-linking of silane coupling agent.According to the segregation path of modifiers,we named this method as“Inside-out”segregation and in-situ cross-linking functionalization strategy based on casting solution.Working collaboratively with micro/nano-structure caused by nonwoven fabric peeling,the water contact angle difference between top and bottom surfaces is¹40°.The prepared Janus membrane exhibited switchable oil/water separation performance due to the asymmetric wettability.It can separate oil-in-water emulsions with its hydrophilic side and can also separate water-in-oil emulsions with its hydrophobic side on simply turning over the membrane.（3）Janus membrane with asymmetric structure and asymmetric wettability was fabricated by salt-induced phase inversion and“Outside-in”segregation strategy based on coagulation bath.The thickness of hydrophilic layer in Janus membrane can be controlled by changing residence time of liquid membrane in coagulation bath.Under the synergistic effect of water and gold nanoparticles embedded in hydrophilic layer,the light absorption performance was enhanced,while the hydrophobic layer played the role of self-floating and heat insulation.The minimized round edge water pathway can also inhibit conduction heat loss.Based on above superior advantages,the integrative Janus PVDF membrane provided a solar steam evaporation rate of 1.02kg/m² h with the solar thermal efficiency of 63.1% under 1 sun illumination,which is superior to the solar-to-steam materials based on golds.（4）In order to enhance evaporation rate,a 3D stereo solar-to-steam generation material based on polytetrafluoroethylene（PTFE）hollow fiber membrane was modified using“Top-down”segregation strategy and polydopamine（PDA）/carbon nanotubes（CNT）,respectively.Different from flat membrane,hollow fiber membrane has self supporting and three-dimensional structure.It can not only absorb solar energy,but also absorb energy coming from PS foam or other materials through heat radiation and heat convection.Under 1 sun illumination,a high evaporation rate of3.73 kg m^-2 h^-1 for single hollow fiber membrane used as photothermal material was record when the material under the hollow fiber membrane was PS foam.The evaporation rate was further increased to 4.88 kg m^-2 h^-1 when the surface of PS foam was covered by carbon cloth with high photothermal performance.This unique evaporation structure can produce freshwater at a rate of at least 10.2 kg m^-2 day^-1which is enough to satisfy 3⁴ people’s daily drinking needs.In summary,the membrane surface can be functionalized by adjusting the segregation path of modifiers.According to the difference of modifiers and the post-treatment process,the membrane can be endowed with excellent anti-fouling,anti-bacterial,non-clotting and photo-thermal conversion properties,respectively.