Modification Of Porous PVDF Membranes By Hyperbranched Polymers

Due to their unique structures, highly branched, three-dimensional polymers such as dendrimers or hyperbranched polymers attract increasing attention. Dendrimers are perfectly branched macromolecules, with a degree of branching (DB) of 1.0, which are only accessible by time-consuming multi-step syntheses. An economically interesting alternative are the randomly branched hyperbranched polymers, which can easily be produced on large scale by a one-pot polymerization of appropriate AB₂ monomers. During the past decades numerous interdisciplinary research projects on such polymers have been started and a wide variety of applications has been proposed. To date, few efforts have been made to explore the feasibility of using hyperbranched polymers to impart traditional membrane materials with special functionalities. In this research, several hyperbranched polymers were used as modifiers for PVDF porous membranes. The effects of the hyperbranched polymers on the structure and properties of PVDF porous membranes were systematically investigated. The experiment methods and results are summarized as follows.Hyperbranched aliphatic polyethers with narrow molecular weight distribution have been prepared via anionic polymerization of glycidol with rapid cation-exchange equilibrium. The synthesized hyperbranched polyglycerol (HPG) was characterized by FTIR, NMR, GPC, etc. The degree of branching (DB) of HPG calculated from the relative integration of ¹³C-NMR spectrum is 0.54. The number average molecularweight (((M_n)|-)) obtained from GPC is 3265, with a dispersity of 1.48. The HPG was blended with PVDF to fabricate porous membranes via immersion precipitation process. It was found that the remaining degree of HPG in the final membranes was 50, 59 and 52 mol% for M5, M10 and M20, respectively. Water contact angle measurements indicated the improvement in the hydrophilicity of the membrane surfaces with the addition of HPG to the casting solutions. For example, the water contact angle decreased from the pure PVDF membrane's 88°to 64°when the content of HPG reached 2 wt% in the cast solution. Also, the addition of HPG increased the pore size of the membrane surfaces and the porosity of the membrane bulks, leading to an improvement in the water flux of the PVDF membranes. Specially, by comparing the effects of HPG and PEG on the properties of PVDF membranes, it was found that HPG was more effective than PEG in improving the hydrophilicity of PVDF membranes when the molecular weights of HPG and PEG are comparable. However, the HPG still showed a tendency to diffuse out of the membrane in practical applications, and appropriate methods should be employed to improve the stability of HPG in the final membranes.To inhibit the loss of HPG during the immersion precipitation process and to enhance the stability of HPG in the final membrane, HPG was partially crosslinked in the cast solutions using 4,4'-oxydiphthalic anhydride (ODPA) as the crosslinking agent. The results indicated that, when the crosslinking degree increased from 0 to 30%, the remaining degree of HPG in the final membranes after the immersion precipitation process also increased from 48 mol% to as high as 85 mol%, suggesting crosslinking is an effective approach to suppress the loss of HPG. Furthermore, XPS analysis demonstrated that the content of HPG in the membrane surface increased when the membrane containing crosslinked HPG was treated by shaking in water at a relatively high temperature (60°), leading to a decrease in water contact angle and the improvement in fouling-resistance. However, a higher crosslinking degree resulted in a decrease in the porosity of both the surface and the bulk, and therefore, the reduction in water flux. In order to optimize the membrane performance, a small amount of poly(vinylpyrrolidone) (PVP) was used as an additive, and membranes with both good hydrophilicity and high water flux were fabricated.To endow hydrophobic poly(vinylidene fluoride) (PVDF) membranes with reliable hydrophilicity and protein-resistance, an amphiphilic hyperbranched-star polymer (HPE-g-MPEG) with about 12 hydrophilic arms in each molecule was synthesized by grafting methoxy polyethylene glycol) (MPEG) to the hyperbranched polyester (HPE) molecule using terephthaloyl chloride (TPC) as the coupling agent, and blended with PVDF to fabricate porous membranes via phase inversion process. XPS analysis results indicated that the MPEG segments of HPE-g-MPEG enriched at the membrane surface substantially. Based on the XPS results, a model depicting the deformation of the HPE-g-MPEG molecules during the immersion precipitation process and the sketch of molecular conformation in the final membrane surfaces was proposed. It was also found that the water contact angle decreased to as low as 49°for the membrane with a HPE-g-MPEG/PVDF ratio of 3/10. More importantly, the water contact angle of the blend membrane changed little after being leached continuously in water at 60℃for 30 days, indicating a quite stable presence of HPE-g-MPEG in the blend membranes. Furthermore, the blend membranes showed lower static protein adsorption, higher water and protein solution fluxes, and better water flux recovery after cleaning than the pure PVDF membrane.Three MPEGs with different molar mass (((M_n)|-) = 350, 750, and 2000, respectively) were grafted to the hyperbranched polyester (HPE) molecule using terephthaloyl chloride (TPC) as the coupling agent, and blended with PVDF to fabricate porous membranes via phase inversion process. With the increase in MPEG arm length, the MPEG arms in hyperbranched-star polymer were more inclined to enrich at the membrane surfaces and endowed the membranes with improved hydrophilic property. The substantial coverage of MPEG segments prohibited the protein absorption at the membrane surfaces effectively, and consequently, resulting in enhanced anti-fouling properties during the foulant solution filtration processes.Three kinds of amphiphilic polymers, including the tri-block copolymer of (polyethylene oxide)-(polypropylene oxide)-(polyethylene oxide) (I, EPTBP), the comb-like copolymer of polysiloxane with polyethylene oxide and polypropylene oxide side chains (II, ACPS) and the hyperbranched star copolymer of polyester-g-methoxyl polyethylene glycol (III, HPE-g-MPEG), were blended with PVDF to fabricate porous membranes via a phase inversion process, respectively, and the effects of the different structures of the amphiphilic polymers on the properties of the blend membranes were compared. XPS and elemental analysis results indicated that the three amphiphilic polymers showed good stability during the immersion precipitation process. The retention degree of the three amphiphilic polymers in the final membranes is 54%, 73% and 85% for EPTBP, HPE-g-MPEG and ACPS, respectively. It was also found that all the three amphiphilic polymers, especially ACPS, enriched obviously in the surface layer of the blend membranes. The calculated enrichment degree ((O/F)_surface /(O/F)_bulk) is 1.4, 1.9 and 2.9 for the blend membranes containing EPTBP, HPE-g-MPEG and ACPS, respectively, which is in the same sequence as the retention degree in the membrane bulks. This enrichment resulted in a membrane surface with much better hydrophilicity and protein resistance than that of the pure PVDF membrane.PVDF based porous membranes were prepared by blending PVDF with an amphiphilic hyperbranched-star (HPE-g-MPEG-Ac) polymer via a typical phase inversion process. And then the prepared porous membranes were filled and swollen by a liquid electrolyte solution to form polymer electrolytes. It was found that the introduction of HPE-g-MPEG-Ac resulted in a remarkable increase in porosity and a considerable reduction in crystallinity of the PVDF porous membranes, which favored the liquid electrolyte uptake and, consequently, led to a substantial increase in ion conductivity at ambient temperature. The blend membrane with a HPE-g-MPEG-Ac/PVDF ratio of 3/10 showed that the ion conductivity was as high as 1.76×10^-3 S/cm at 20℃, which was more than 4 times that of the pure PVDF membrane. Also, the prepared polymer electrolytes showed a wide electrochemical stability up to 4.5 V versus Li⁺/Li even for the blend membrane with the highest HPE-g-MPEG-Ac content in this study.