Hydrophillic Surface Modification On Pvdf Membrane By Poly（L-dopa） Coating And Mpeg-nh₂Grafting

The research of this dissertation focused on the hydrophilic modification of PVDF membranes by poly(L-DOPA) coating and MPEG-NH2grafting. PVDF are widely used as membrane material in application. However, these membranes usually have some limits, such as low permeate flux and easily-fouled due to its hydrophobility property. Surface modification are considered as efficient methods to improve the hydrophilicity of PVDF membranes, like coating and grafting.The L-DOPA coating modification can be conducted with oxidation-crosslinking reaction under low operating temperature for the method is simple and convenient. Based on the characteristics of L-DOPA, we performed surface modification via the self-polymerization-composite of L-DOPA on the PVDF membranes. Besides, the poly(L-DOPA) composite layer with high adhesive property offers a platform for subsequent MPEG-NH2grafting modification, the MPEG-NH2can occur Michael addition and Schiff base reaction with poly(L-DOPA) composite layer. The changes of functional groups were characterized by Fourier Transform Infrared Spectrometer (ATR-FTIR). Morphological changes on the membrane surface were characterized by Scanning electoral Microscopy (SEM) and Atomic Force Microscopy (AFM). To analysis the hydrophilicity and wettability of nascent and modified membranes, the measurements, i. e. contact angle was employed. The antifouling ability and separation efficiency of these membranes was also investigated with oil/water emulsion filtration tests.Firstly, The influences of the coating liquid concentration, time and grafting liquid concentration, time on the membranes performance were assessed. It was found that the best coating conditions are6g/L L-DOPA with the coating time is6h, the best grafting conditions is lg/L MPEG-NH2with the grafting time1h. The surface hydrophilicity and wettability improved significantly after modification, which was indicated by fact that the contact angle of83.9°of nascent membrane, decreased to68.7°of L-DOPA coated membrane, further to42.5°of MPEG-NH2grafted membrane. The reason of the improvement of the modified membrane surface hydrophilicity and wettability was the increasing number of hydrophilic groups such as amino group (-NH2) and hydroxyl group (-OH). After modification, the pure water flux of modification membrane rised from139.9L/(m2-h) of nascent membrane to139.9L/(m2·h) by6g/L L-DOPA coating3h, the water flux of MPEG-NH2grafted membrane reduced to113.4L/(m2·h) though1g/L MPEG-NH2grafted1h.Secondly, we investigated the effect of the modification on the membrane performance. The stability was characterized by the contact angle and the ultraviolet absorption spectrum. The experiments showed that the modified membrane had a preferable resistance to acid, but a poor performance in the alkali condition. The MPEG-NH2grafted membrane had a better performance to the L-DOPA coated membrane, shows that MPEG-NH2layer played a role as a defender to protect the poly(L-DOPA) layer.Finally, the antifouling ability and separation efficiency of these membranes was also investigated with oil/water emulsion filtration tests. The separation experiments showed that the modified membrane had a better anti-fouling ability than the nascent membrane to nonpolar pollutants. By1%surfactant solution at60℃, the water flux recovery rate of coated membrane and grafted membrane were55%and72%, respectively, while the nascent membrane was only46%.,shows that the antifouling ability and stability improved though modification.Compared with the nascent membrane, the hydrophilicity and wettability of the modified membranes improved significantly after L-DOPA coated and MPEG-NH2grafted, and the antifouling ability also investigated with oil/water emulsion filtration tests. The alkali-resistance property of the L-DOPA coated membrane is poor, so how to improve its stability is the key to successful implementation project.