Research Of Poly(ethylene-co-vinyl Alcohol)(EVOH) Microporous Membrane Preparation Via Thermally Induced Phase Separation

Membrane separation technology is simple, energy-saving, high-effective, no secondary pollution, operational in room temperature, and has been applied in various fields. In water treatment aspect, membrane separation has been used in salt-water desalination, waster water treatment, and purified water production. In present, the membrane materials used in water treatment are polypropylene (PP), polysulfone (PSF), poly(vinylidene fluoride) (PVF), et.al. Most of these membranes are hydrophobic and thus the separation flux tends to decline during the operations due to solute adsorption and pore blocking. Some hydrophilic materials, such as polyvinyl alcohol (PVA), cellulose acetate (CA), have bad mechanical intensity and solvent resistance.Poly (ethylene-co-vinyl alcohol) (EVOH) is a semi-crystalline random copolymer, can crystallize over whole composition range of ethylene. EVOH has very low gas permeability, excellent chemical resistance and mechanical properties and been widely used as packing material, such as food, cosmetic, chemical product. Because EVOH has hydrophilic vinyl alcohol segment, it has superior properties over other hydrophobic polymers for the prevention of membrane fouling. Recently, EVOH membranes have been the subjects of significant research interest for water treatment applications. In this thesis, EVOH microporous membranes have been prepared via thermally induced phase separation (TIPS), and some research have been done about physical chemical topic and membrane morphology control in preparation process.In TIPS, the phase diagrams provided the critical information about phase behavior of the system and the base to control membrane morphology. Predication of phase diagram is a kind of interesting work. The binary interaction model was applied to estimate the phase diagrams of copolymer and diluent systems. The S-L and L-L phase separation curves of EVOH/ 1,4-butanediol, EVOH/1,3-propanediol, and EVOH/glycerol have been calculated. The fair agreement between the experimental data and the calculated values indicated the feasibility to predict the phase separationbehavior of copolymer-diluent systems on the basis of binary interaction model. In order to confirm the importance of incorporating intra-molecular interaction, calculations with and without the consideration of intra-molecular interaction were performed and compared. And it was found that better results were obtained if intra-molecular interaction was introduced.Phase diagram is important in TIPS, if phase behavior of the system can be controlled, which would be significative. The mixtures of 1,4-butanediol and poly (ethylene glycol)(PEG400) were used as diluents. The phase diagrams of this system were studied. The morphology of microporous membrane was observed by scanning electron microscope (SEM). It was found that the composition of the binary solvents could influence the temperature of L-L phase separation and the morphology of the membrane.A hollow fiber module is more efficient than a flat sheet module because the hollow fiber module has more surface area for separation per unit volume of module than the flat sheet module. For this reason, hollow fibers are more commonly used in industry than flat sheet. EVOH38 hollow fibers were prepared via TIPS process. Water, methanol and acetone were used to extract the diluent in the fibers, respectively. Bigger shrinkage of fibers during extractant evaporation was observed when water or methanol was used. The results of calculation for interactive parameters indicated that the molecules of water and methanol had stronger interaction with EVOH38. Therefore, the affinity of extractant with polymer would be the crucial factor for the shrinkage during extractant evaporation. Besides, the analysis of X-ray diffraction in extraction process indicated that water and methanol could reach the amorphous region of the polymer and destroy the structure of crystal, so that the polymer would be easier to be deformed and shrunk. The smaller pore size and porosity at inner layer of fibers were observed on the SEM images of cross-section of hollow fibers treated with water and methanol. The denser morphology must result from the larger volume shrinkage and the collapse of porous structure caused by the extractants due to the better affinity of the two extractants with the polymer. The measurement results of water and gas permeability are coincident with the SEM images and shrinkage data.In the last chapter of this thesis, Poly (ethylene-co-vinyl alcohol) with ethylene content of 44mol% (EVOH44)/PVP (K30) membranes were prepared via thermally induced phase separation (TIPS). Poly (ethylene glycol) with average number weight of 300 (PEG300) was used as diluent. Differential scanning calorimetry (DSC) andattenuated total reflectance infrared (ATR-FT-IR) spectra were used to investigate the compatibility of EVOH44 and PVP (K30). From disappearance of the melting peak of EVOH, T_g of PVP in DSC, the shift of PVP carbonyl and C-O bonds of EVOH in ATR, it was indicated that they are compatible. The effects of PVP content on phase diagram, PVP distribution and membrane morphology were studied. It was found that the binodal point shifted to higher temperature and proportion of PVP between polymer matrixes to pore surface decreased with PVP content increasing. It was also found that the pore size of the membranes increased as PVP content increased. The hydrophilicity and protein adsorption properties of EVOH/PVP membranes were also measured; the results indicated that they had better hydrophilicity and protein antifouling property compared with pure EVOH membrane.