Functionalized CNTs/PVDF Nanocomposite Membrane Preparation、Characterization And Properties Study

Poly(vinylidene fluoride)(PVDF) has become one of the most popular polymericmembrane materials in microfiltration (MF) and ultrafiltration (UF) due to its excellentchemical resistance, good thermal stability and mechanical properties. However, due to itsinherent hydrophobic characteristic, PVDF membranes are often susceptible to fouling, andsubsequently suffer serious flux losses.In this work, to endow hydrophobic PVDF membranes with reliable hydrophilicity andbiofouling resistance，blend modification for improving the fouling resistance of PVDFmembranes was investigated. The acid functionalized multi-walled carbon nanotube(MWNTCOOH) and hyperbranched poly (amine-ester)(HPAE) functionalized carbon nanotube(MWNTHPAEand SWNTHPAE) were successively synthesized, and then used as additive toprepare PVDF nanocomposite membranes by phase inversion method. Various techniquessuch as transmission electron microscope (TEM), scanning electron microscopy (SEM), x-rayphotoelectron spectroscopy (XPS) and contact angle goniometry, static protein adsorption aswell as permeability experiments and anti bacteria experiments were applied to characterizethe effect of modified CNTs on the morphology, permeability, anti-fouling performance, aswell as the anti-fouling mechanism of the nanocomposite membranes. We also investigatedthe effect of PEG molecular weight on the morphology, permeability and anti-foulingperformance of the PVDF/MWNTHPAE-1.5nanocomposite membranes. The main experimentmethods and results were summarized as follows:The MWNTHPAEand SWNTHPAEwere characterized by XPS、FTIR、TGA and SEM. Allthese characterizations confirmed that HPAE was attached to the MWNTHPAE/SWNTHPAEsurface. MWNTs and SWNTs displayed poor dispersion in DMF and casting solution， andprecipitated out in several hours after ultrasonicated mix due to agglomeration. MWNTCOOHdisplayed stable dispersion for several hours, but precipitated out later. The MWNTHPAEandSWNTHPAEdisplayed homogeneous dispersion in DMF and casting solution， and remainedstable for several weeks of storage without macroscopic precipitation. The obviousagglomerations of MWNTs and SWNTs can be seen in TEM image of PVDF nanocompositemembrane. The MWNTHPAEand SWNTHPAEwere randomly dispersed at the individualnanotube level in the scanned area and no obvious agglomerations can be observed. Theparticle size of MWNTHPAEwas smaller than that of MWNTs. The dispersion of MWNTHPAEin DMF was improved with an increase of PEG molecular weight.SEM was applied to characterize the surface and cross-section morphologies of membranes with different blend compositions. The formation of macrovoids, pore size andporosity were firstly promoted with increasing the modified CNTs/PVDF ratio and thensuppressed. The pure water flux showed an upward trend with increase in the concentration ofmodified CNTs and then decreased accordingly. The BSA rejection showed the oppositetrend. The pure water flux was at maximum when the content of MWNTCOOH, MWNTHPAEand SWNTHPAEwas1.5%,1%and1%, respectively. The pore size and porosity ofPVDF/PEG/MWNTHPAE-1.5increased with an increase of PEG molecular weight, and thepure water flux increased and the BSA rejection decreased accordingly.XPS spectrum showed that the oxygen and carben molar ratio on the surface of thePVDF/MWNTHPAE-1.5and PVDF/SWNTHPAE-1.5was higher than the PVDF membrane,which indicated that MWNTHPAEand SWNTHPAEwere incorporated to the surface of PVDF.Furthermore, the appearance of N peak in the XPS spectrum of the PVDF/MWNTHPAE-1.5and PVDF/SWNTHPAE-1.5at binding energies of400eV, clearly confirms that PVDF wasdoped with MWNTHPAEand SWNTHPAE. Moreover, MWNTHPAE, as an individual addition，could not be flushed away from the membrane surface. The contact angle measurementindicated that the hydrophilicity of the nanocomposite membranes was significantly improved.Zeta potential data and AFM measurements showed the nanocomposite membranes exhibitedrelatively low negative charge and great roughness compared to the PVDF membrane. Thehydrophilicity of PVDF/PEG/MWNTHPAE-1.5improved with an increase of PEG molecularweight.Static protein adhesion experiments revealed that nanocomposite membranes reduced theamount of protein adsorbed to the surface. The flux recovery ratio (FRR) values of thenanocomposite membranes were increased with the content of modified CNTs.Simultaneously, the irreversible fouling ratio (Rir) values were reduced for the nanocompositemembranes. It could be concluded that the antifouling capability of nanocomposite membranewas considerably improved with an increase of modified CNTs. For example, the PVDFcontrol membrane possessed the lowest FRR value of78.4%, while FRR values of thePVDF/MWNTHPAEnanocomposite membranes were increased from85.3%to95.7%with theratio of MWNTHPAE/PVDF increasing from0.5%to2%. Simultaneously, the Rirvalue ofPVDF control membrane was the highest (21.6%) among the membranes, and the Rirvalueswere reduced from14.7%to4.3%for the PVDF/MWNTHPAEnanocomposite membranes.We examined the resistance to bacterial adhesion by SEM and flow cytometers. After theconstant exposure to filtered water, we observed less bacterial adhesion to the surface of thePVDF/MWNTHPAEand PVDF/SWNTHPAEnanocomposite membrane. The higher themodified CNTs/PVDF ratio is, the less bacterial adhesion is. In contrast, significant adhesionof bacterial cells was observed on the surface of PVDF membrane. Under static E. coliadhesion tests, there is no difference between the PVDF/MWNTHPAEnanocomposite membrane and PVDF. Fewer bacteria can be found on the PVDF/SWNTHPAEnanocompositemembrane surface, with decreased cell adhesion at higher SWNTHPAEconcentration. Resultsproved that hydrophilicity/hydrophobicity showed stronger relationship than surfaceroughness and surface charge for the bacterial adhesion.