Preparation And Properties Of Polyvinyl Alcohol/Functional Mesoporous Silica Composite Membranes Based On Diffusion Dialysis

Diffusion dialysis(DD)separation process is known as its low energy cost,eco-friendly nature and zero secondary pollution,which has attracted more and more concentration of researchers.The core component of the DD process is the membrane and the separation efficiency is decided by the properties of the membrane.Thus,it is turning exactly urgent to search suitable membrane materials for DD process.In this research,polyvinyl alcohol(PVA)and home-made functionalized mesoporous silica particles are selected as organic phase and inorganic phase and then a series of composite membranes are prepared and obtained by solution blending method.The dispersed properties of MS particles with different functionalized groups in the polymer matrix are studied and the structure-function relationships between interfacial structure and separation performance of composite membranes are also detected.The mass transfer model of composite membranes in this research is explored preliminarily and the main content of this study is as follows:In the first section,amino-modified mesoporous silica(AMMS)was synthesized under acidic condition by incorporating tetraethoxysilane(TEOS)as inorganic precursor,poly(ethylene oxide-propylene oxide-ethylene oxide)block copolymer(P123)as structure-directing agent,?-aminopropyl triethoxysilane as surface-modified agent via sol-gel method.The obtained AMMS articles were dispersed in the water and blended with pre-crosslinked polyvinyl alcohol(PVA)solution to prepare casting solutions.The obtained casting solutions were then coating on clean glass plate and a series of composite membranes were obtained after the solvent volatilized absolutely.The composite membranes were heated from room temperature to 100? with a heating rate of 10?/min and kept at 100? for 5 hours.Water uptake(181.7%-289.7%),alkali resistance(with swelling degree in the range of 250%-471%and mass loss in the range of 27.6%-35.3%),mechanical(with TS in the range of 29.9-41.8MPa and Eb in the range of 135.3%-180.4%)and thermal(degradation temperature was higher than 200?)properties of as-prepared membranes were tested.The results indicated that the as-prepared membranes were suitable for application in alkaline diffusion dialysis process with acceptable alkali resistance.Microscopic morphologies of as-prepared membranes confirmed well dispersion of AMMS in PVA matrix and good compatibility between the two phases.NaOH/Na2WO4 system was selected to check separation performance of as-prepared membranes via diffusion dialysis(DD)and the results indicated that UOH and S were in the range of 0.009-0.014 m/h and 23.8-32.5,respectively.The composite membrane exhibited the optimal properties while dosage of AMMS was 2%:UOH was 0.14m/h and S value was 25.The transport mechanism of ions was detected preliminarily and we concluded that existence of-NH2 not only improved the compatibility between PVA and AMMS via the hydrogen bonding,but also changed the electricity property of the organic-inorganic interface regions,which promoted the transport of OH-.In the second section,sulfydryl-functionalized mesoporous silica(MS-SH)particles were synthesized under acidic conditions by incorporating TEOS as inorganic precursor,P123 as structure-directing agent,?-thipropyl triethoxylsilane as surface-modified agent via sol-gel method.The sulfonic acid group-modified MS(MS-SO3H)particles were obtained after the MS-SH was oxidized by H2O2 solution.The MS-SH and MS-SO3H were dispersed in the water and blended with pre-crosslinked PVA solutions,respectively.A series of PVA/MS-SH and PVA/MS-SO3H composite membranes were obtained after coating,solvent volatilization and heating process.The basic properties and microscopic structure of as-prepared composite membranes were characterized and the results indicated that water uptake of samples were in the moderate range of 81.0%-154.0%;swelling degree and mass loss were in the range of 104.0%-225.1%and 2.9%-17.7%after being immersed into 2M NaOH solution for 60 hours under 25?;involvement of MS-SH and MS-SO3H enhanced thermal stability of-OH obviously,which could be attributed to the adsorption ability of mesopores to the PVA chains and the hydrogen bonding between-OH and-SO3H.The results of SEM and TEM showed that MS-SH and MS-SO3H dispersed uniformly in the PVA matrix and the mesoporous structure kept well in the composite membranes,which demonstrated the well compatibility between the organic and inorganic phases.NaOH/Na2WO4 mixture was used to detect separation performance of as-prepared membranes via diffusion dialysis and the results indicated that UOH and S of composite membranes were in the range of 0.010-0.020 m/h and 48-85,respectively.The composite membranes exhibited higher ion flux and selectivity than those of pure PVA membrane.The UOH and S of PVA/MS-SO3H membranes enhanced synchronously and this was contrary to the"tradeoff effects",which demonstrated that-SO3H in the interfacial region possessed multiple functions:constructing the transport channels for Na+ as well as prevent the leakage of WO42-.The results also confirmed that separation performance of composite membranes can be regulated by adjusting the charge properties in the interfacial regions and thus membranes with anti-tradeoff effects could be obtained.In the third section,MS was synthesized firstly under acidic conditions by incorporating TEOS as inorganic precursor,P123 as structure-directing agent via sol-gel method.Then the MS was modified by adsorbing phosphotungstic acid(HPW)under vacuum condition and the obtained sample was signed as MS-HPW.The MS and MS-HPW were blended with pre-crosslinked polyvinyl alcohol(PVA)solution and a series of composite membranes were obtained after casting,solvent-volatilization and thermal treatment.The basic properties and microscopic structure of as-prepared membranes were tested and the results indicated that ion exchange capacities(IECs)and water uptake(WR)were in the range of 0.015-0.031 mmol/g and 98.3%-196.6%,respectively;TG and DTG curves demonstrated that involvement of MS-HPW promoted the stability of-OH and carbon backbones,thus the thermal stability of composite membranes enhanced obviously;Results of mechanical testing showed that TS of composite membranes(62.49-82.72MPa)was higher than those of pure PVA membrane(58.55MPa),while Eb declined slightly as increasing dosage of MS and MS-HPW.Alkali resistance of the hybrid membranes was tested in 25? 2M NaOH solution and the results were acceptable:swelling degree was in the range of 134.3%-160.2%and mass loss was in the range of 3.2%-12.2%,which demonstrated that the obtained membranes were suitable for the application of alkaline DD process under room temperature.The separation performances of as-prepared membranes were tested by using alkaline diffusion dialysis(DD)process of NaOH/Na2WO4 system and the results were encouraging:UOH and S of PVA/MS membranes decreased obviously with the increasing dosage of MS,while PVA/MS-HPW hybrid membranes exhibited higher ion flux and selectivity than that of pure PVA membrane.The composite membranes possessed optimal separation performance while the dosage of MS-HPW was 6%:UOH and S were 0.0175m/h and 88.8,respectively.The transport mechanism of ions was detected and we concluded that adsorbed HPW in the mesopores constructed the electronegative channels for Na+,and the-OH from PVA promoted the transport of OH-via hydrogen bonding.The separation performance of composite membrane promoted obviously under the conjunct action of the both sides.