Preparation And Properties Of Polymeric Cation Exchange Membranes With Different IPN Structures

The ionic selectivity,stability and ion transportation rate are usually utilized to characterize the performance of ion exchange membranes(IEMs).It is therefore very important to select membrane materials with corrosion resistance,aging resistance,high ion flux and selectivity.In this research,different interpenetrating polymer network(IPN)are constructed to modify polytetrafluoroethylene(PTFE)and special chlorosulfonated polyethylene rubber(CSM)with excellent chemical resistance to obtain IEMs with different separation performance.The microscopic morphologies,mechanical properties,thermal stability and alkali resistance of the resulting IEMs were tested.The separation performance(including ion flux and selectivity)of the membranes was simulated by the alkaline diffusion dialysis(DD)experiment of NaOH/Na2WO4 system.The research content of the thesis includes the following three parts:In the first part,a latex type interpenetrating polymer network technology(LIPN)was used to prepare a series of carboxylic acid type PTFE-based cation exchange membranes.The latex type interpenetrating polymer network structure could effectively improve the thermoplasticity after the interpenetrating polymerization process.Since commercial PTFE concentrates contain fluorinated emulsifiers which are sensitive to pH,butyl acrylate(BA)and methyl methacrylate(MMA)were firstly polymerized to form a protective layer out of PTFE,aiming to avoid the demulsification of PTFE emulsions.Then,butyl acrylate(BA)and functional monomer methacrylic acid(MAA)were added to initiate polymerization.A PTFE/P(BA-MMA)/P(BA-MAA)hydrophilic emulsion with three-layer core-shell structure can thus be produced.By controlling the addition amount of BA and MAA,a series of casting solutions were prepared and dried under ambient conditions on a polytetrafluoroethylene template to form corresponding membranes,which were denoted as A1-A4.The as-prepared membranes with ion exchange capacities(IECs)of 1.77 to 2.62 mmol/g showed water uptake of 36.1-57.9%,the swelling ratio of 289.8%to 515.0%,the mass loss rate of 2,53%-4.32%as well as good mechanical(the tensile strength(TS)of 9.58-21.7 MPa,the elongation at break(Eb)of 26.3%to 96.7%)and thermal stability,with initial degradation temperature up to 300 ℃.Since the carboxylic acid groups of as-prepared membranes were located at the surface of PTFE particles and was susceptible to attack by OH-,the swelling raio of membranes was expected to be high.However,considering that the shell layer of the latex particle was a crosslinked interpenetrating structure,the slight weight loss of membranes was observed.The diffusion dialysis testing results indicate that the hydroxide permeability coefficients(UOH)of as-prepared membranes are in the range of 0.0063-0.0082 m/h,while the separation factors(S)are in the range of 11.1-15.4.The results above implies the potential applications of the as-prepared membranes in alkaline waste water treatment system.The second part is implemented based on the first part.In order to improve the performance of the membrane,especially the separation performance.The LIPN PTFE/P(BA-MMA)/P(BA-MAA)emulsion prepared in the first part was blended with a pre-crosslinked polyvinyl alcohol(0.5%GA cross-linking)solution.A semi-interpenetrating cation exchange membrane with semi-IPN structure was obtained by regulating the ratio of the emulsion to the PVA,which was labeled as M series membrane.The results show that the dialysis coefficient(UOH)of M series membrane ranges from 0.01098 to 0.02862 m/h,and the separation factors(S)are of 18.6-31.3.The ion flux and ion selectivity of M series membranes are significantly higher than those of A series membranes.The water uptake of the M series membrane ranges from 44.4 to 136.3%,indicating that the addition of the PVA dispersion medium can increase the hydrophilicity of the membrane,and the hydrophilic group of the latex particles can be contact with the solution;the swelling ratio of the M series membrane is 144.1-273.8%,and the mass loss rate is 5.76-10.84%.Since the M series membrane is encapsulated by PVA dispersion medium,the swelling ratio becomes smaller,but the weight loss is nevertheless increased;the tensile strength(TS)is 31.9-67.2 MPa.The results indicate that the overall performance of the cation exchange membrane with PVA dispersion medium is significantly higher than that of the cation exchange membrane directly dried by latex particles.In the third part,a carboxylic acid type CSM-based cation exchange membrane was prepared using sequential interpenetrating polymer network technique(SIPN).In the toluene solution of chlorosulfonated polyethylene,the crosslinking of methacrylic acid(MAA)and triallyl isocyanurate(TAIC)forms the casting solution with IPN structure,followed by dried,hot pressed and vulcanization,giveing a sequential IPN structure.Changing the amount of cross-linking agent TAIC can regulate the microstructure of the cation exchange membrane.As a crosslinking agent,TAIC can be polymerized with MAA to form a network structure.The casting solution of SIPN structure is then dried and vulcanized with TAIC as a vulcanizer to obtain CSM serials membrane.In this paper,the addition amount of TAIC is 0.1-0.2%relative to the total mass of MAA and CSM rubber,which can regulate the microstructures of resulting cation exchange membrane,indicating the significance of TAIC.The properties including the microstructure,water content,alkali resistance,thermal stability,mechanical properties and diffusion dialysis experiments are evaluated.The ion exchange capacity(IEC)of the membranes is at the range of 2.1 to 3.9 mmol/g.The water content of the membrane ranges from 45.9%to 51.4%.For the separation property,the dialysis coefficient(Uoh)of the membrane is ranged from 0.0128 to 0.0209 m/h,while the dialysis coefficient(Uw)is at the range of 0.000278 to 0.000508 m/h,the membrane separation factors(S)ranges from 36.8 to 45.7.With respect to the mechanical performance,the tensile strength ranges from 18.5 to 27.2 MPa,and the elongation at break ranges from 96.7 to 273.3%.Additionally,excellent thermal ability with an initial decomposition temperature of 200 ℃ is detected.When the 0.007 g of TAIC,1 g of MAA and 2 g of CSM is added,the overall performance of the membrane reaches the best.Specifically,the TEM image shows the most uniform IPN structure,and the tensile strength(TS)is 27 MPa with the elongation at break(Eb)of 273.3%,As well as the swelling ratio of the membrane is only 60.7%.Additionally,the dialysis coefficient(UOH)is 0.0185 m/h along with tungstate permeability coefficient(Uw)of 0.000443 m/h.Separation coefficient(S)reaches up to 41.83.It is worth noting that when the amount of TAIC or MAA monomer is too high,the degree of crosslinking between MAA and TAIC becomes larger,due to the smaller flow area of CSM solution.In this case,PMAA becomes the continuous phase,and CSM becomes the dispersed phase,leading to the package being reversed by PMAA network structure.Thus,separation performance of membranes declines accordingly.