Poly(Vinylidene Fluoride) Surface Modification By UV Irradiation Pretreatment And Functionalization Of Its Membrane

In recent years, fluoropolymers have received much attention because of its excellent physical and chemical properties, such as high-temperature stability, excellent chemical resistance, low water sorption and low dielectric constant, low dielectric loss and so on. Poly(vinylidene fluoride) has been widely used in seperation membrane and biotechnology (vascular sutures and regeneration templates). But just like the other dielectric polymers, the characteristic low surface energy has created numerous technical challenges. For instance, the biomedical equipments could be polluted easily because of the strong adsorption of proteins and low biocompatibility; the low permeability or the reduction of the flux far below the theoretical capacity caused mainly by deposition and accumulation of submicron particles on the membrane surface. In this connection, amount of researches have been devoted to the surface modification of PVDF.Grafting of polymer brushes on the poly(vinylidene fluoride) (PVDF) films was carried out. Peroxide and hydroperoxide initiators were generated via filtered 297 nm UV irradiation pretreatment in nitrogen, followed by air exposure. Homopolymers brushes of methyl methacrylate (MMA) were prepared by the surface-initiated free radical polymerization from the peroxide and hydroperoxide initiators on the PVDF surface. Peroxides and hydroperoxides active species on UV-treated and air-exposed PVDF surfaces were detected by the reaction with 2,2-diphenyl-1-picryl hydrazyl (DPPH) in toluene. The chemical composition and topography of the graft-functionalized PVDF surfaces were characterized by Carbon 1s and Fluorine 1s X-ray photoelectron core-level spectra (XPS), attenuated total reflectance (ATR) FT-IR spectroscopy and atomic force microscopy (AFM), respectively. ATR FT-IR and XPS indicated the formation of polymer brushes on the PVDF surface. ATR FT-IR revealed an increase graft concentration of PMMA with more UV irradiated pretreatment time. Water contact angles on PVDF films were reduced by surface grafting of MMA. The mean roughness of the unmodified film is higher than that of the PVDF-g-PMMA modified film and decreases as the UV irradiation pretreatment time increases.A reverse atom transfer radical polymerization (RATRP) with benzoyl peroxide (BPO)/CuCl/2,2-bipyridine (Bpy) was applied onto grafting of poly(methyl methacrylate) (PMMA) and poly(poly(ethylene glycol) monomethacrylate (PPEGMA) from poly(vinylidene fluoride) (PVDF) microfiltration (MF) membrane surfaces, including the pore surfaces. The introduction of peroxide and hydroperoxide groups onto the PVDF membranes was achieved by ultraviolet (UV) irradiation in nitrogen, followed by air exposure. RATRP from UV pretreated hydrophobic PVDF membranes was then performed for attaching well-defined homopolymer. The chemical composition of the modified PVDF membrane surfaces was characterized by attenuated total reflectance (ATR) FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS). The surface and cross-section morphology of membranes was studied by scanning electron microscopy (SEM). The pore sizes of the pristine PVDF and the PVDF grafted PMMA membranes were measured using micro-image analysis and process software. With increase of graft concentration, the pore size of the modified membranes decreased and became uniform. Kinetic studies of homogeneous system revealed a linear increase in molecular weight with the reaction time and narrow molecular weight distribution, indicating that the chain growth from the membrane surface was a "controlled" or "living" grafting process. The introduction of the well-defined PMMA on the PVDF membrane gave rise to hydrophilicity. Protein adsorption and protein solution permeation experiments revealed that the UV pretreated hydrophobic PVDF membrane subjected to surface-initiated RATRP of methyl methacrylate (MMA) exhibited good antifouling property. To compare antifouling property, the PVDF membrane was also modified by grafting PEGMA polymer according to similar procedures as grafting PMMA.And then the direct preparation of grafting polymerization from commercial PVDF MF membranes using surface-initiated atom transfer radical polymerization was demonstrated. The secondary fluorinated sites and CuCl/4,4-dimethyl-2,2-dipyridine(DMDP) were used for surface-initiated ATRP of hydrophilic monomers. Homopolymers brushes of poly(N-isopropyl acrylamide) (PNIPAAm) were directly prepared by ATRP from PVDF membrane surfaces. The microstructure and chemical composition of grafted PVDF membranes were characterized by ATR-FTIR and XPS. The surface and cross-section morphology of membranes was studied by scanning electron microscopy (SEM). The pore sizes of the pristine PVDF and the PVDF grafted PNIPAAm membranes were measured using micro-image analysis and process software. Solvent effect research revealed an increase in the graft concentration of poly(N-isopropyl acrylamide) (PNIPAAm) with the polarity of solvent. Water contact angles on PVDF membranes were reduced by surface grafting polymerization of PNIPAAm. Temperature- dependent permeability experiments and protein solution permeation experiments revealed temperature sensitivity of polymer brushes-grafted PVDF surfaces.