Study on the Construction and Hydrophilic Modification of Phenolphthalein Poly(Ether Sulfone) Nanofibrous Composite Membranes

Author:Song Guo Cheng

Supervisor:hu zu ming


Degree Year:2019





Today’s world,water pollution problem has become an increasingly serious obstacle which will obstruct the sustainable development of society.Water pollution caused by natural or man-made factors has menaced the health of human beings and the global ecological environment,it has become a global issue which need to solve urgently.Among the wastewater treatment technologies,the membrane separation technology has developed rapidly due to its advantages of high separation efficiency,low energy consumption,environmental friendliness,easy operation and so on.Among the categories of separation membranes,the nanofibrous membranes with good permeability,high rejection ability and wide application prospects have been widely used in water treatment field due to their superior properties of large porosity,uniform and dense structure and light weight,etc.In recent years,the solution-blowing as a new technology to prepare nanofibers has attracted broad interests once discovered owing to its easy and safe operation,widely raw material,high efficiency and low energy consumption,et al.Phenolphthalein Poly(ether sulfone)(PES-C)as a high-performance engineering plastics is an ideal material for membrane separation.The excellent solubility and processability provide the theoretical basis for it to prepare nanofibers via solution-blowing.However,the membrane fouling as a major issue to hinder the development of the membrane separation technology would not only reduce the separation efficiency and add the operation cost,but also affect the lifetime of membranes,so enhancing the membrane antifouling ability has become the main tendency of the modified research.The antifouling ability can be effectively improved via increasing the hydrophilicity of membranes,so after the PES-C nanofibrous membranes were prepared via solution-blowing,and the optimum spinning process was determined,it was applied in filtration and the research of hydrophilic modification was studied in two ways:firstly,the hydrophilic substances were directly mixed with PES-C spinning solution or depositing on the surface of nanofibers to obtain the nanofibrous composite membranes;secondly,the ultrathin integrated hydrophilic barrier was formed on the surface of PES-C nanofibrous supporting substrates to prepare the nanofibrous composite membranes.The research contents were listed as follows:(1)The PES-C nanofibrous membranes were prepared via solution-blowing and the spinning parameters were discussed,the influences of different concentrations,gas pressure,receiving distance and solution injection rate on the morphology and structure of nanofibers were studied in detail and the optimal conditions were confirmed.The response surface methodology was used to discuss the impact of spinning parameters on the mean diameter of nanofibers,and the response surface model between the spinning process parameters and mean fiber diameter was established and verified the acutal results.Then the phenolphthalein with different proportions were added into polysulfone(PSF)to prepare modified PSF,and the corresponding nanofibers were prepared via solution-blowing under similar conditions.In order to improve the hydrophilicity of the membranes,different contents of carboxybetaine poly(arylene ether sulfone)(PES-CB)with zwitterionic groups were added into PES-C spinning solution to prepare PES-C/PES-CB nanofibrous composite membrane.The structure and properties of nanofibrous membranes were characterized and analyzed in detail.The results showed that the optimal spinning parameters were as follows:the concentration of spinning solution was 25 wt%,gas pressure was 0.1 MPa,solution injection rate was 4 ml/h,protruding distance of inner nozzle was 10 mm and receiving distance was 25 cm,the mean diameter of PES-C nanofibers was about 490 nm.The PES-C nanofibers prepared under the ideal conditions had smooth and homogeneous appearance,high productivity,and stable spinning process.The response surface model with the mean diameter of PES-C nanofibers as response values was relatively accurate and had certain guiding significance for the practical spinning process.The modified PSF nanofibrous membranes with different phenolphthalein contents were prepared via solution-blowing,and the results showed that the thermal resistance of the nanofibers was enhanced with the increase content of phenolphthalein.The 5 wt%weight loss temperature and Tg of PSF nanofibers were 415℃and 195℃respectively while they were 495℃and 270℃respectively for PES-C nanofibers.The addition of 30 wt%PES-CB was comparatively most suitable ratio according to the morphology and wettability of the PES-C/PES-CB nanofibrous composite membrane.The filtration tests of starch suspension revealed that the antifouling abilities of PES-C and modified PSF nanofibrous membranes were relatively poor(the flux recovery ratio(FRR)was about 55%),while the antifouling ability of nanofibrous composite membrane with PES-CB addition was improved obviously.When the addition of PES-CB was 30 wt%,the FRR of the composite membrane increased to about 80%.The FRR of PES-C and modified PSF nanofibrous membranes were only about 25%in oil-water separation tests,while the FRR of the PES-C/PES-CB nanofibrous composite membranes increased to about 60%.(2)PES-C/SiO2 nanofibrous composite membranes were prepared via adding nano-silica powder and colloidal silica into PES-C spinning solution respectively,the results showed that the thermal properties of the composite membranes had been slightly improved with the increase content of silica.It was observed that under the same conditions,the aggregation of nanoparticles on the surface of composite nanofibers with nano-silica powder addition was serious,while the silica dispersed relatively homogeneous and the fibers surface was comparatively smooth with colloidal silica addition.In the following filtration tests of starch suspension,the antifouling ability of membranes exhibited better under slightly pressure,the nanofibrous composite membrane with colloidal silica addition owned better antifouling performance(FRR>70%)comparing with the membrane adding nano-silica power(FRR≈60%).The FRR of nanofibrous composite membrane with colloidal silica addition was about 40%while it was about 30%with nano-silica power addition in oil-water separation tests.Overall,the filtering effect of nanofibrous composite membrane with5 wt%colloidal silica addition was relatively best.(3)The PES-C nanofibrous membrane was immersed in certain concentration of PVP solution,the polyvinylpolypyrrolidone(PVPP)was deposited on the surface of nanofibers to obtain PES-C/PVPP nanofibrous composite membrane.Meanwhile the PES-C/PVP nanofibrous composite membranes were prepared via adding different contents of polyvinylpyrrolidone(PVP)into PES-C spinning solution.The results showed that the hydrophilicity of membranes was improved obviously with the modified of PVPP and PVP,when the depositing degree of PVPP was about 10.5 wt%,or the addition content of PVP was 5 wt%,the membranes were superhydrophilic.In the subsequent filtration tests of starch suspension,the FRR of PES-C/PVP nanofibrous composite membrane with5 wt%PVP content reached about 90%at 5 kPa,the FRR of PES-C/PVPP nanofibrous composite membrane was above 88%(depositing degree≥10.5wt%)while the total flux reduction ratio(TFR)was about 60%.The result of circulating filtration tests showed that the PES-C/PVPP nanofibrous composite membrane had good reusability,and the stable water flux of each cycle was about 1500L/m2·h.The FRR of PES-C/PVP nanofibrous composite membrane(5 wt%PVP)was about 90%while it was about 85%for PES-C/PVPP nanofibrous composite membrane(depositing degree≈10.5wt%)during the process of oil-water separation tests.The results showed that the membranes with PVPP coating had excellent antifouling ability and reliability,it was easy to operate and desirable for industrial application.(4)The PVPP was sprayed on the surface of PES-C nanofibrous membrane via solution-blowing apparatus,then the composite membrane based on PES-C nanofibrous substrate was prepared by hot-pressing treatment.The result showed that an integrated and smooth barrier on the surface of nanofibrous membrane was formed with 1.5 h spraying time and 4 ml/h injection rate.Then the polydopamine(PDA)nanoparticles were single side deposited on the PVPP barrier layer and the optimal depositing time was confirmed,finally the N-[Tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid sodium salt(TAPS-NA)was immobilized on the surface of PDA coating through covalently interactions.In the subsequent filtration tests of protein solutions,the membrane with PVPP layer was proved to be applied in ultrafiltration which the rejection and FRR for Bovine serum albumin(BSA)solution were above 75%.After PDA coating and TAPS-NA immobilization,the BSA rejection and FRR of the membranes both reached above 80%while the TFR decreased to about 45%due to the superhydrophilicity and electrostatic repulsion effect.In the separation test of oil/water emulsions,the nanofibrous composite membranes with PDA coating could maintain a high permeate flux(Jp≈240 L/m2·h,0.05 MPa),the FRR and oil rejection ratio were all above 80%while the TFR was only about 37%.In the circulating filtration test of BSA solution and oil/water emulsions,the flux attenuation of each cycle was similar which was favorable for the long-term use of nanofibrous composite membranes.The results showed that the membranes with PVPP ultrathin hydrophilic layer on the surface of nanofibrous substrate which were prepared via solution-blowing apparatus had good antifouling properties and could quickly and effectively separate the filtrate.(5)The composite membrane with PES-C nanofibrous membrane as substrate and sodium alginate(SA)as hydrophilic barrier layer was fabricated via a vacuum filtration technique,then the ultrathin SA layer was subsequently dealt with the crosslinking reaction and the influence of SA with different contents on the morphology of barrier layer were discussed.The results showed that when the theoretical depositing degree(TDD)was 2.5 mg/cm2,it was able to achieve an integrated and ultrathin SA barrier layer.With the increase content of SA depositing,the hydrophilicity and mechanical strength of the membranes were enhanced.In the subsequent filtration test of BSA solution,due to the electrostatic repulsion effect,the BSA rejection of nanofibrous composite membrane(TDD=5 mg/cm2)could reach more than 88.7%,and the FRR was above 85%while the TFR decreased to about 30%.During the filtration test of lysozyme(Lys)solution,because of the electrostatic adsorption effect between the membranes and Lys,the reduction of water flux was serious and the TFR was more than 50%which meaned the antifouling ability of Lys was relatively weak.In the separation test of oil/water emulsions,when TDD was more than 4 mg/cm2,the FRR was above 80%and the oil rejection ratio was above 85%.In conclusion,the superhydrophilic PES-C/SA nanofibrous composite membranes with excellent antifouling ability and high separation efficiency could be simple prepared via vacuum filtration,and was able to apply in ultrafiltration.The results showed that under the suitable conditions,PES-C could be fabricated via solution-blowing,and the nanofibrous composite membranes obtained by different ways of hydrophilic modification possessed characteristics of low-pressure high-flux performance,good antifouling ability and recyclability,and could effectively and quickly separate filtration medium.The modified method in the paper was easy to operate and control under mild conditions,and was very desirable for the continuous process in industrial application.