Preparation And Modification Of Hydrophilic Microporous Membranes Via Thermally Induced Phase Separation

Membrane separation technology is simple, energy-saving, high-effective, and has been applied in various fields. At present, membrane materials used in water treatment are polypropylene (PP), polyethylene (PE) and poly (vinylidene fluoride) (PVF), et al. Membranes made of such polymers have good thermal stability and chemical resistance. However, the relatively poor hydrophilicity of this type of membrane limits its applications, for the loss of permeation flux caused by adsorptive fouling of biological molecules. The conventional hydrophilic polymers such as poly (vinyl alcohol) and cellulose acetate are usually poor in thermal stability and chemical resistance. An alternative way is to make the surfaces of hydrophobic membranes hydrophilic by chemical modifications. However, thermal instability and technical complexity of these membranes made them undesirable in practical application.Compared with the immersion precipitation method, thermally induced phase separation (TIPS) process is applicable to crystalline polymers that cannot be dissolved at room temperature. It is easy to control the pore size and prepare membranes with good mechanical strength and high porosity. In this paper, the crystalline poly (ethylene-co-vinyl alcohol) (EVOH) and poly (ethylene-co-acrylic acid) (EAA) were selected to prepare hydrophilic microporous membranes with high porosity and mechanical strength.First of all, graft reaction of poly (ethylene glycol monomethyl ether) (MPEG) onto EAA through the esterification reaction was performed. Water contact measurements and protein adsorption experiments indicated that the hydrophilicity and antifouling properties of films were improved after EAA was grafted with MPEG. The micro -porous membranes of EAA-g-MPEG and EAA were prepared via TIPS process. Phase diagrams for those systems were compared. It was found that the binodal curve shifted to the lower temperatures after EAA was grafted with MPEG, while the change of the dynamic crystallization temperatures was rather small. The membrane morphology was investigated, which revealed that the pore size of EAA-g-MPEG membranes was smaller than that of EAA membranes.In order to economize energy resources in industrial applications, we hope that graft reaction of EAA and MPEG goes on for predetermined time, and then the solution is cooled to prepare the membrane. However, not all the MPEG in the solution can react with EAA. Therefore, the effect of MPEG on the phase diagram of EAA-g-MPEG/MPEG/di-n-octyl phthalate (DOP) system was investigated by varying the composition of MPEG and DOP. The binodal temperature originally shifted to lower temperature as the proportion of MPEG increased, then increased remarkably when the ratio of MPEG to DOP was more than 1/6. The domain growth kinetics of EAA/DOP, EAA-g-MPEG/DOP and EAA-g-MPEG/MPEG/DOP systems was studied by light scattering method. It was found that the domain growth rate of EAA-g-MPEG/MPEG/DOP system was lower than EAA/DOP and EAA-g-MPEG/ DOP systems at a certain quench temperature. Moreover, the membrane morphologies were compared, indicating that the pore size became smaller when MPEG was added as a component of diluent.Most of the commercialized hollow fiber membranes are hydrophobic and need to be pre-treated by ethanol before use. In this paper, EVOH containing 38mol% ethylene (EVOH38) hollow fiber membranes with good water permeabilities were prepared via TIPS process using binary solvents of 1,4-butanediol and PEG400. In order to further improve the tensile strength of hollow fibers, the supported EVOH hollow fiber membranes were prepared with glass fiber braid as supporting material. The obtained hollow fiber membranes presented good tensile strength and has potential in membrane bioreactor (MBR) applications.In order to investigate the effect of cooling nonsolvent on the membrane preparation via TIPS, EVOH/PEG300 system was chosen and two kinds of cooling nonsolvent were used. Results showed that when water was used as cooling nonsolvent, the membrane presented an asymmetric structure consisting of a porous skin, macrovoids near the top and lacy structures near the bottom. In contrast, when methanol was used as the precipitation medium, it showed particulate morphology on the top surface and cellular pores near the bottom. The mutual diffusion coeffcients between diluent and nonsolvents were calculated and discussed relating to membrane formation.EVOH microporous membrane were surface modified by introducing phos-phorylcholine (PC) groups. The morphologies and mechanical properties of hollow fiber membranes were examined, which indicated that the modification had exerted little effect on membrane structures. Water contact angle and protein adsorption measurements indicated that the hydrophilicity was effectively improved and the bovine serum albumin adsorption was significantly suppressed after modification. The permeation experiments revealed that PC-modified EVOH membranes had higher pure water fluxes with enhanced flux recovery and reduced flux loss from protein adsorptive fouling.SiO₂ particles with two kinds of diameter (2μm and 30nm) were blended with EVOH38/PEG400 systems and made into membranes via TIPS. Then the diluent and SiO₂ were extracted in succession to obtain EVOH/SiO₂ membranes and EVOH membranes. The effect of SiO₂ on the phase diagram was investigated. Then the effects of cooling rates and particle sizes on the membrane morphology, porosity and water permeability were studied. Results indicated that the phase separation temperatures were not influenced much by adding SiO₂. It was also found that slower cooling rate and SiO₂ size with 2μm resulted in higher porosity. Moreover, when the SiO₂ size was larger than the nearest distance between two pores formed by TIPS, the improvement of pore connectivity was more effective.