Structure Control Of Hydrophilic Ethylene-Acrylic Acid Co-Polymer Microporous Membranes Via Thermally Induced Phase Separation

Thermally induced phase separation(TIPS) is one of the most versatile and widely utilized methods to produce a variety of microporous membranes of controlled pore size. The TIPS process is based on the phenomenon that the solvent quality decreases when the temperature is decreased. Thus, upon removal of thermal energy by cooling or quenching a polymer-diluent solution, phase separation occurs. After the phase separation, the diluent is removed, typically by solvent extraction, and the extratractant is evaporated to yield a microporous structure. The TIPS process, which is applicable to a wider range of polymers including crystalline polymers, requires few parameters in determing the microporous structure and forms more microporous structures compared with other process. Recently, the TIPS process has had many new developments, which focued on membrane structures control as well as preparation of anistropic membranes and hydrophilic membranes.In this paper, hydrophilic microporous membranes were prepared via the TIPS process using hydrophilic ethylene-acrylic acid co-polymer(EAA). Choice of diluents and extractants for EAA3003 systems and Effects of diluents and extractants on the microporous structures of membranes in the TIPS process were studied. It was found that choice of diluents and extractants was one of most important methods to control membrane structures. After DOP was selected as diluent for EAA3003 system, the thermodynamic phase diagram of EAA300/DOP system was determined, and effects of thermodynamic and kintic factors on the membrane structures were also investigated, hydrophobic HDPE membranes were produced in comparing with the hydrophilic EAA membranes and methods of structure control of HDPE membranes were discussed. Furthermore, anistropic or asymmetric membranes were produced by inducing a polymer concentration gradient in the membrane solution prior to cooling.It was found that under high temperature (180) and with low shearing strength, soybean oil, sesame oil, peanut oil, DOP and DPE were effective as diluents for EAA system to prepare microporous membranes via TIPS, while the mixture of castor oil, dibutyl-o-phthalate with EAA3003 was obviously layered. Compatibility of EAA with epoxy soybean oil was poor, and EAA remained grainy after mixing for 4 hours. In the case of Oleic acid, the mixture became reddish brown, which may be resulted from chemical reaction between the two components. Although liquid paraffin, DOTP, butyl stearate and the mixtures of soybean oil/oleic acid(1/1), liquid paraffm/DOP(1/1)could blend uniformly with EAA3003, the intensity of these mixtures were very poor. The viscosity of the solutions of soybean oil/DOP (1/1), soybean oil/ liquid paraffin(1/1), liquid paraffin/soybean oil(2/8) with EAA3003 were so high that they were inappropriate to use as diluent for EAA3003. It was suggested that the choice of diluents had great effects on the membrane structure. Pore size increased in the order of EAA3003-DPE, EAA3003 -soybean oil, EAA3003-DOP, EAA3003-sesame oil, EAA3003-peanut oil at 30wt%. Pore sizes were about 4.9781um, 1.8020um, 1.7244um, 1.5742um, and 0.5861um for the systems of EAA3003/DPE, EAA3003/soybean oil, EAA3003/DOP, EAA3003/sesame oil and EAA3003/peanut oil respectively. EAA3003/Soybean oil system had a leafy structure, while other systems had a cellular structure.Seven different extractants were used to remove DOP from EAA films to produce microporous membranes. SEM micrographs suggested that all the membrane structures were resembled with obvious spherulites, especially for the membrane prepared with acetone. Pore size of the membrane prepared with methanol was very large, which was accorded with it's high porosity. The membrane prepared with isobutanol was mostly opaque, and only on the top surface some pores was found. Porosity were shown to increase with increasing boiling point and decreasing solubility parameter difference between EAA and the extractants. As drying time increased, porosity of the membranes increased initially, then dec...