Saeid Rajabzadeh

Solidification Behavior of Polymer Solution during Membrane Preparation by Thermally Induced Phase Separation

The solidification behavior of poly(vinylidene fluoride) (PVDF) solution during membrane preparation by thermally induced phase separation (TIPS) was investigated. Apparatus newly developed in our laboratory was used to quantitatively measure membrane stiffness during phase separation. In this apparatus, a cooling polymer solution, placed on a stage, is moved upwards and the surface of the polymer solution contacts a sphere attached to the tip of a needle. The displacement of a blade spring attached to the needle is then measured by a laser displacement sensor. Different phase separation modes, such as liquid-liquid (L-L) phase separation and solid-liquid (S-L) phase separation (polymer crystallization) were investigated. In the case of S-L phase separation, the stiffness of the solution surface began to increase significantly just before termination of crystallization. In contrast, L-L phase separation delayed solidification of the solution. This was because mutual contact of the spherulites was obstructed by droplets of polymer-lean phase formed during L-L phase separation. Thus, the solidification rate was slower for the L-L phase separation system than for the S-L phase separation system.

Preparation of a PVDF hollow fiber blend membrane via thermally induced phase separation (TIPS) method using new synthesized zwitterionic copolymer

Antifouling properties of PVDF hollow fiber membrane were improved by the addition of a new synthesized zwitterionic copolymer. This copolymer was synthesized using methyl methacrylate and the zwitterionic monomer 3-(methacryloylamino)propyl-dimethyl(3-sulfopropyl) ammonium hydroxide) (MPDSAH). The copolymer was used as additive in the preparation of a poly(vinylidene fluoride) (PVDF) hollow fiber membrane via thermally induced phase separation. With addition of the copolymer, the crystallization temperature and spherulite growth rate of the polymeric solution decreased sharply. Quartz crystal microbalance with dissipation monitoring results showed that bovine serum albumin (BSA) interactions with the prepared PVDF/copolymer film sharply decreased with addition of the synthesized copolymer. BSA filtration results showed that the membrane had higher antifouling resistance than a membrane without the copolymer additive.

Preparation of robust braid-reinforced poly(vinyl chloride) ultrafiltration hollow fiber membrane with antifouling surface and application to filtration of activated sludge solution

Braid-reinforced hollow fiber membranes with high mechanical properties and considerable antifouling surface were prepared by blending poly(vinyl chloride) (PVC) with poly(vinyl chloride-co-poly(ethylene glycol) methyl ether methacrylate) (poly(VC-co-PEGMA)) copolymer via non-solvent induced phase separation (NIPS). The tensile strength of the braid-reinforced PVC hollow fiber membranes were significantly larger than those of previously reported various types of PVC hollow fiber membranes. The high interfacial bonding strength indicated the good compatibility between the coating materials and the surface of polyethylene terephthalate (PET)-braid. Owing to the surface segregation phenomena, the membrane surface PEGMA coverage increased upon increasing the poly(VC-co-PEGMA)/PVC blending ratio, resulting in higher hydrophilicities and bovine serum albumin (BSA) repulsion. To compare the fouling properties, membranes with similar PWPs were prepared by adjusting the dope solution composition to eliminate the effect of hydrodynamic conditions on the membrane fouling performance. The blend membranes surface exhibited considerable fouling resistance to the molecular adsorption from both BSA solution and activated sludge solution. In both cases, the flux recovered to almost 80% of the initial flux using only water backflush. Considering their great mechanical properties and antifouling resistance to activated sludge solution, these novel membranes show good potential for application in wastewater treatment.

Tailoring the surface pore size of hollow fiber membranes in the TIPS process

A new method was used to tailor the surface pore size of PVDF hollow fiber membranes in the TIPS process by the co-extrusion of different solvents at the outer layer of the extruded polymeric solution. In this method, the membrane properties can be controlled over a wide range of mean pore size (from 24 to 600 nm), water permeability (from 3 to more than 4000 L m−2 h−1 bar−1), and acceptable particle rejection (from 50 to 500 nm), with only a slight change in the membrane mechanical strength. Ternary interaction between the extruded solvent, polymer, and diluent in the polymer dope solution played a major role in controlling the hollow fiber membrane properties. At the interface of the extruded solvent and polymeric solution, segregation of the diluent or PVDF molecules near the outer skin layer of hollow fiber membrane strongly affected the mean pore size and filtration performance over a wide range, from MF membranes to typical UF membranes. Considering the competitive solubility difference between the extruded solvent and the diluent in the polymeric solution and that of PVDF, a road map has been shown for the selection of a satisfactory extruded solvent to obtain membranes with desired properties.

Poly(vinylidene difluoride)/poly(tetrafluoroethylene-co-vinylpyrrolidone) blend membranes with antifouling properties

To inhibit fouling phenomenon in membrane process, a new amphiphilic copolymer, poly(tetrafluoroethylene-co-vinylpyrrolidone) (P(TFE-VP)), was blended with poly(vinylidene difluoride) (PVDF) to fabricate a series of antifouling membranes via non solvent induced phase separation (NIPS) method. The effect of copolymer blend ratios and TFE/VP ratios on membrane properties were evaluated, and the stability of P(TFE-VP) in PVDF membrane was studied. The membrane morphology was controlled by adjusting polymer concentration in dope solution, such that all membranes have similar pore size and density, as well as pure water permeability. In evaluating the effect of TFE/VP ratios, the content of VP in dope solutions was also adjusted to allow a fair comparison. We found that for P(TFE-VP) with a higher VP content, adsorption of BSA on polymer film was negligible. Higher blend ratios of this copolymer resulted in higher surface VP content and better hydrophilicity, but antifouling performance ceased to improve when blend ratio was larger than 1:9 (copolymer:PVDF). Meanwhile, a lower VP content in copolymer resulted in inferior hydrophilicity and severe fouling of the blend membranes. It was also proved that comparing with PVP homopolymer, P(TFE-VP) had satisfying stability inside PVDF membrane.

1.7 PVDF Hollow Fibers Membranes

With the fast growth of population during last century, serious global issues raised that global warming and potable water shortage are the major ones that challenge humankind life. During last decades, membrane technology has developed very quickly and attracted much attention as a great potential for solving humankind problems. Poly(vinylidene fluoride) (PVDF) hollow fiber is one of the technologies that are already well industrialized. In this chapter, preparation, modification, and application of PVDF hollow-fiber membrane are summarized. In preparation part, we especially focus on the thermally induced phase separation and nonsolvent-induced phase separation. These two methods widely have been applied for membrane preparation in industrial scale. For both methods, phase diagram, kinetic effects, and preparation parameters are discussed in detail. In some cases, the results of flat-sheet membranes which expected that could be potentially applied for hollow-fiber membrane preparation in the future are also included. Since virgin PVDF membrane comes with some drawbacks, modification for PVDF membranes (making it more hydrophilic and hydrophobic) is studied. Finally, the important application of PVDF membrane is summarized.

Tailoring both the surface pore size and sub-layer structures of PVDF membranes prepared by the TIPS process with a triple orifice spinneret

In this study, the surface and sub-layer structures of poly(vinylidene fluoride) (PVDF) membranes were effectively tailored by extruding different types of solvents at the outer layer of the polymer solution with a triple-orifice spinneret using the thermally induced phase separation (TIPS) process. The segregation of the diluent or PVDF chains to the interface between the extruded solvent and polymer solution was exploited to tailor the membrane surface structure. In contrast, the diffusion of extruded solvents having good compatibility with PVDF into the polymer solutions changed the phase separation mechanism and resulted in the formation of a novel composite-like structure (spherules connected by the bicontinuous network structure) in the sub-layer of the membrane. This membrane structure enhanced the permeation stability drastically. The membrane properties and structures are summarized based on the change in the competitive ternary interactions between the polymer, diluent, and extruded solvent, and could be used as a guide for selecting appropriate solvents to design and tailor membranes with desired structures and properties.