Preparation Of Poly(Vinyl Chloride) Porous Membranes Via A Combined Process Of TIPS And NIPS

Membrane separation has wide attention as an efficient separation technology, whilemembrane is the core of membrane science. As membrane material, poly (vinyl chloride)(PVC) owns excellent properties and low price. However, preparation method reported forPVC membrane was non-solvent induced phase separation (NIPS) and the membranesstructure obtained was finger-like that made mechanical properties of membraneinsufficient for practical applications. One way to solve this issue is to developmultichannel membranes that are at the cost of a flux loss. The membranes prepared viathermally induced phase separation (TIPS) can own excellent mechanical properties, butthe temperature needed in this process using sole diluent is too high for PVC thermalstability. Therefore, it is hard to obtain PVC membrane via classical TIPS method for itslow decomposition temperature.In this work, PVC membrane was prepared via a combined process of TIPS and NIPSchoosing biphenyl ketone (DPK), a mixture of N,N-dimethyl acetamide(DMAc)/N,N-dimethyl formamide (DMF) at a certain ratio as a combined diluent, underguidance of solubility parameter theory,(polyethylene glycol600as additive, ethyl alcoholas extraction agent). This work focuses on mechanism of membrane formation, preparationof membrane and regulation of membrane structure. Scanning electron microscope (SEM)and a capillary flow aperture were used to characterize membrane structure. The effects ofconditions for membrane preparation on membrane structure and properties wereinvestigated detailedly. The separation properties of membrane were studied via pure waterflux and bovine serum albumin (BSA) retention tests. The mechanism of membraneformation was explored initially and the effects of DPK content and PVC concentration onmembrane structure and properties were investigated, as well.The results showed that the morphology of casting solution converted to gel state quicklyand then changed from numerous crystal to white nascent membrane during the coolingprocess. Bi-continuous structure was observed via SEM after extracted by ethyl alcohol, butno pure water flux was tested for its dense surface. In this process, TIPS was the only driving force and the decreasing of temperature weakened the interaction of molecules.Liquid-liquid phase separation occurred first and then solid-liquid phase separation andcrystallization until membrane shaped up.Mass transfer and heat exchange existed at the same time while choosing ice water ascoagulation bath. In this process, TIPS and NIPS cooperated together or competed eachother effecting membrane structure. During cooling process, mass transfer and decrease oftemperature increased the viscosity of system and the delayed phase separation dominatedthe NIPS process, meanwhile liquid-liquid phase separation occurred during the coolingprocess and then the decrease of DPK content caused by mass transfer enhanced thecrystallization of system companied by solid-liquid phase separation. The increase of DPKcontent made the membrane structure change from finger-like pores to uniform spongy.When the content of DPK is lower than41.3wt%, there were finger-like pores existing inmembrane. While at41.3wt%and over, the membrane structure was spongy without largerpores. As the polymer concentration increases, membrane structure changed fromfinger-like to uniform spongy. When it was lower than14wt%, finger-like pores can be seenand pores size decreased with the increasing of polymer concentration. The optimalconditions for membrane formation were PVC concentration14wt%and DPK content41.3wt%. The membrane had a mean pore size of35.4nm, a tensile strength about8.5MPa,a porosity of65%, pure water flux about23.6L/(m2·h) under0.1MPa and25oC, BSArejection about68%.