Polymer Surface Modification via Noncovalent Binding of Functional (Macro)molecules : Investigation of Entrapment Strategy and Mechanism
Entrapment technique, served as one kind of polymer surface functionalization strategies, has been developed in recent decades. All previous studies focused on polar polymer surface modification. In this thesis, aiming to polymer surface hydrophilic and antifouling modification, a variety of (macro)molecules have been applied to entrap into polar polyestersulfone (PES) and nonpolar polypropylene (PP) surface respectively. It was found that the modification conditions for PES are not suitable for PP membrane surface; and the conditions for PP membrane are not exactly effective for PP film. Furthermore, the entrapment mechanism has been studied and discussed.
In the beginning of this thesis, polar PES microfiltration (MF) membrane has been used as base polymer. Two different routes, abbreviated as E1 and E2 have been tested for PES surface modification. In E1, the modifiers were anticipated to diffuse into swelling region in modifier solution, and then they were fixed into PES surface by deswelling in water (solvent extraction); in E2, the base polymer was swollen in solvent, and the modifiers were anticipated to entrap into PES surface quickly in water solution. Present studies revealed that PES surface can be hydrophilic modified via entrapment of poly(ethylene oxide) (PEO)-containing homo-/copolymers; and E1 showed much better efficiency than E2. Therefore, E1 was selected as the entrapment approach for the following studies of PP surface modification.
Then nonpolar PP surface was endowed with hydrophilicity, thermo-responsibility, as well as cationic charge after entrapment with corresponding modifiers respectively. For instance, it was validated that entrapment of small amphiphilic molecule octaethyleneglycol monooctadecylether (C18EO8) into Membrana PP MF membrane surface improved outer surface and inner surface hydrophilicity, as well as the corresponding antifouling properties. Wettability and water flux of poly(butyl acrylate)-b-poly(N-isopropylacrylamide) (PBA-b-PNIPAAm) entrapment modified PP membrane surface were time-dependent, which had abrupt change at the lower critical solution temperature (LCST) of PBA-b-PNIPAAm. This thermo-responsive property could be further used for protein desorption. In addition, non-porous Membrana PP plate surface was also functionalized with the same procedure, and modified PP plates showed similar modification efficiency and surface properties as for porous PP membranes. Moreover, zeta potential measurement validated that the Celgard PP film surface showed cationic after entrapment with methyl and octyl groups quarternized poly(n-butyl acrylate)-block-poly(2-dimethylaminoethyl methacrylate) (PBA-b-PqDMAEMA). The mechanism of entrapment behavior was further investigated in the final section of this thesis. In case of Membrana PP MF surface modification in nonpolar solution, entrapment of a variety of ethyleneoxide-containing substances into PP surface was studied. All results revealed that PEGs were ineffective, while many nonionic amphiphilic substances, especially some tri-block copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) were very effective for PP surface hydrophilic modification. The relationship between modifier structure and architecture and entrapment behavior was investigated by studying the reverse micellization of amphiphilic modifiers in nonpolar solutions via pyrene-probe fluorescence and 1H NMR spectroscopy. The balanced structure of nonionic tri-block modifiers, the lowest reverse critical micelle concentration (RCMC) had been observed. It was concluded that a balance block copolymer structure and architecture promoting the self-association in the nonpolar solvent is the basis for a high entrapment efficiency. In case of modification in polar solution, the swelling degree for diffusion of modifier is one important factor. Moreover, the deswelling step in a second solvent is another important factor to entrap the modifier into base polymer surface.
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