Surface-selective and controllable photo-grafting for synthesis of tailored macroporous membrane adsorbers
Photo-grafting is a straightforward and promising technique for surface modification of polymeric membranes. This work emphasized on the development and investigation of surface-selective photo-grafting method from polar organic solution; on the other hand, on the preparation of membrane adsorbers via the proposed grafting methods and evaluation of the resulting membrane adsorbers. Two novel surface-selective photo-grafting methods have been developed: synergist immobilization and iniferter immobilization methods. Hydrophilized polypropylene (PP) microfiltration (MF) membrane, whose surface polymer layer contains polyacrylate, was used as base membrane for both methods; track-etched polyethylene terephthalate (PET) MF membranes (PET200 and PET400) were used for extension of synergist immobilization method and further investigation; methanol or acetonitrile solution of acrylamide (AAm) with/without cross-linker (EDMA) was applied for investigation of grafting mechanism. For synergist immobilization method, the synergist (tertiary amino groups) for photo-initiator benzophenone (BP) was introduced onto the membrane surface via an aminolysis reaction with diethyl ethylenediamine (DEEDA). The reaction conditions have been optimized. The proposed grafting mechanism was verified by the significant difference in degree of grafting (DG) between original and aminolysed membranes. In order to better understand and improve this novel method, detailed investigation of functionalization parameters and affecting factors has been carried out. The grafted membranes were characterized by ATR-IR, contact angle, SEM, permporometry, liquid permeability and zeta potential. The obtained results demonstrated that the highest surface-selectivity of photo-grafting could be achieved only under the optimum grafting conditions, i.e., inert solvent to excited BP should be used to reduce/avoid homopolymerization in bulk solution; appropriately low UV intensity should be applied to exclude the uncontrolled side grafting reaction (another functionalization mechanism was discovered at high UV intensity based on direct generation of starter radical); appropriately low BP concentration was used to reduce the non-selective photo-grafting. Thus, the grafted layer could be well controlled by immobilized synergist concentration, UV irradiation time, monomer concentration and initiator concentration. In addition, this method has been successfully applied to track-etched PET membrane, and it is also expected to functionalize other polymeric membranes with similar chemical structure. For iniferter immobilization method, the reaction conditions for immobilization of photo-iniferter (dithiocarbamate group) have been optimized. The grafting mechanism has been verified by the relationship between DG and photo-iniferter concentration. Detailed investigation with respect to grafting efficiency, uniformity on the whole membrane surface and controlled grafted layer structure has been carried out. This grafting method exhibited high grafting efficiency, uniform modification and high controllability. However, the re-initiation efficiency was low for the selected grafting system based on the significant difference in DG value obtained by continuous and intermittent UV irradiation. Via developed synergist immobilization method and conventional photo-initiator adsorption method, three types of anion-exchange membranes (low and high grafting density and slightly cross-linked grafted layer) have been prepared in aqueous solution of (2-(methacryloyloxy)ethyl) trimethylammonium chloride (MAETMAC) with/without EDMA, using hydrophilized PP MF membrane as support. The effect of grafted layer architecture on protein binding capacity and liquid permeability has been investigated. Buffer/elution solution permeability, static and dynamic protein binding behaviors have been determined for selected resulting anion-exchange membranes. Analyses demonstrated that cross-linking of grafted layer and high grafting density can improve the liquid permeability of membrane adsorbers. But the protein binding capacity was relatively low for high grafting density membrane. In comparison, the membranes with slightly cross-linked grafted layer exhibited improved overall performance. In addition, compared to conventional adsorption method, synergist immobilization method is a more efficient and suitable grafting technique for the preparation of anion-exchange membranes with three-dimensional grafted layer based on the higher grafting efficiency and better dynamic performance for membrane adsorbers prepared via this method. For the preparation of affinity membrane, track-etched PET400 membrane was grafted with a special functional copolymer with bisphosphonate ester groups via synergist immobilization method from acetonitrile solution. The resulting affinity membrane showed high binding capacity for selected proteins. Especially, it was found that markedly higher binding capacity and affinity have been achieved for lysozyme than for cytochrome C, both proteins with similar pI value and protein size. With this affinity membrane, the protein separation has been realized in the 1:1 mixture solution of lysozyme and cytochrome C with a very high selectivity. Using hydrophilized PP MF membrane, MIP thin-layer composite membranes have been prepared via synergist immobilization. However, the imprinting effect was not observed probably due to the influence of synergist on the stability of formed complex between functional monomer and template and template concentration in bulk solution. The optimization of composition for MIP has been performed. Iniferter immobilization method would be a promising alternative. MIP thin-layer composite membranes have been synthesized via this method, but the evaluation and further investigation is still in progress.
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