Poly(Vinyl Alcohol)/Silica Organic-Inorganic Hybrid Membranes For Pervaporation Dehydration

Hydrophilic polymeric membranes are easily swollen in an aqueous solution,resulting ina decrease of selectivity.They were normally modified by chemical crosslinking to reducetheir swelling and increase selectivity,while accompanied with a sacrifice in waterpermeability.Therefore,the objective of this work is to decrease the swelling of polymericmembranes in an aqueous solution and enhance their water permselectivity.For this purpose,poly(vinyl alcohol) (PVA) as a hydrophilic membrane material and silica precursors asinorganic components were used to prepare PVA/silica organic-inorganic hybrid membranesfor pervaporation dehydration of ethanol.The swelling properties of the PVA membraneswere studied by experiments and molecular simulation.With the assistance of molecularsimulation and inverse gas chromatography (IGC),γ-aminopropyltriethoxysilane (APTEOS)and 1,2-bis(triethxysilyl)ethane (BTEE) were finally used to prepare PVA/silica hybridmembranes,whose physicochemical properties,surface topography,swelling properties andpervaporation performances were investigated in detail.For the swollen PVA membranes,PVA chains stretched easily and their mobilityincreased,big holes were thus formed in the PVA matrix,resulting in a decrease in themembrane selectivity.Water located mostly around the hydrophilic region of the hydroxylgroups of PVA chains;and hydrogen bonding was formed between water and PVA.Waterclusters were formed in the swollen PVA,whereas ethanol molecules dispersed almostindividually in the PVA matrix.Hybridization of PVA membrane by C_nH_2n+1Si(OH)₃ decreased the mobility of PVAchains,increased amorphous region in PVA matrix.Based on the results of C_nH_2n+1Si(OH)₃hybridized PVA membranes studied by molecular simulations,APTEOS and BTEE werechosen to modify the PVA membranes.APTEOS and BTEE hybridization adjusted themicrostructure of the PVA membranes and remarkably decreased the mobility of PVA chainsFurthermore,solubility and diffusion behavior of water and alcohol in the PVA/APTEOShybrid membranes were studied by IGC.It is noted that the hybridization of PVA by APTEOS enhanced water permeability of the PVA membranes and their sorption selectivity anddiffusion selectivity for water/alcohol,which agreed with the above simulation results.PVA/APTEOS,PVA/BTEE,and PVA/PSS (polysilisesquioxane) hybrid membraneswere prepared.These three kinds of hybrid membranes have lower swelling degree than thepristine PVA membrane,and the hybrid membranes containing low silica content have higherwater permeability and permselectivity than the pristine PVA membrane.However,hybridization of PVA by silica was meanwhile accompanied with a decrease in thehydrophilicity of PVA membrane at high silica content.Q-PVA membranes were thussynthesized with the aim to increase their hydrophilicity,and then modified by glutaraldehydeand APTEOS to reduce the swelling of the q-PVA membranes.As expected,the q-PVA and itsmodified membranes have higher water permeability and selectivity than the pristine PVAmembrane.Interaction parameters of the modified PVA membranes with water and ethanol weremeasured,which were influenced by hydrophilicity and structure of the membranes.Furthermore,there is a direct relation between both water/ethanol sorption selectivity andpermselectivity with the interaction parameters for the modified PVA membranes.Thisconclusion may be taken as a guideline to choose and design membrane materials.