Internal surface coating and photochemical modification of polypropylene microfiltration membrane (1997)

Membrane surface modification is of interest to most membrane scientists because the surface properties of most commercial membranes are often nonoptimal for particular applications. This thesis focuses on the further development of a thin film coating on the internal surface of polypropylene microfiltration membrane. A modified interfacial polymerizatIon of 1,6-naphthalene disulfonylchloride containing a photoactive diazoketone (DK) group on the side chain and 1,8-octanediamine is studied. Various fabricatIon conditions including concentration of monomers, organic solvent selection, crosslinking agent, selection of an acid acceptor and reaction time were investigated. The choice of organic solvent was found to be critical for obtaining an even, smooth coating layer without significantly changing the membrane morphology. Mass of the polysulfonamide coating layer was typically 10-15 wt% of the base membrane. The coating layer at the membrane internal surface demonstrated a high stability to Soxhlet extracting using CHCl₃ for 48 hours. A mechanism of this new type of interfacial polymerization is qualitatively proposed, and investigated experimentally. The first reported study of end-group analysis and molecular weight measurement of coating polysulfonamide provided a solid ground to evaluate the hypothesis. Most results from experiments either strongly support the proposed mechanism or can be reasonably interpreted by the proposed mechanism. The mechanistic study defines the most important principles of this new type of interfacial polymerization which can be varied to control the coating process, and gives a better understanding of this new coating technology. General applicability of this coating technology was tested by applying this technology to polyester and polyamide. The results indicate that this new technology is generally applicable and can be used to coat condensation polymers to the internal membrane surface as an even thin layer, without significantly changing the morphology of the polyolefin microfiltration membranes. The chemical functionality of polysulfonamide coated membrane surface could be further modified via photochemical transformation of the diazoketone group contained in the coating polysulfonamide, in the presence of an appropriate medium. Thus, membranes containing indene acid, glycolic acid, bromoethyl ester, and polyethylene glycol ester functionalities were obtained. The photochemical reaction conversion was found to be approximately 80%. The highest charge density (0.097 meq/g) could be achieved on the glycolic acid ester membrane. The hydrophilicity of the membrane surface is increased by polysulfonamide coating and further increased by photochemical functionalization. The microfiltration flux and separation performance of these membranes with polystyrene latex (PSL) and carboxylate modified latex (CML) spheres, was examined. The extent of fouling of the membranes was studied by SEM. A lower separation of PSL and a higher pure buffer flux were found for the functionalized membrane compared to the polysulfonamide coated control (DK) membrane. Pure buffer flux was increased after photochemical modification. These results suggest that the increase of hydrophilicity of the photochemically modified membranes was the dominant factor affecting the membrane separation performance. The degree of fouling of the functionalized membranes was significantly decreased which would presumably lead to an improvement of membrane effective life-time. A remarkable change of polypropylene microfiltration membrane performance was found after the addition of Triton X-100 to the PSL feed solution. This phenomenon was explained in terms of the change of membrane surface properties. Polyethylene glycol ester membranes were introduced to mimic the membrane physically adsorbed Triton X- 100. The results suggested that the Triton X-100 in the testing solution not only temporarily modified the membrane surface properties to be more hydrophillic, but also prevented the PSL particles from aggregating, particularly in the membrane pores.