| Membrane distillation (MD) technology, which provided a simple and effectiveapproach for not only utilizing the chemical waste water and heat but also solving the lackof freshwater resources, has gained great attention during the last two decades. The maincause holding back MD development is the relative shortage of membranes with highperformance. So far, polyvinylidene fluoride (PVDF) became a popular membrane materialnot only due to its good hydrophobicity and excellent chemical stability, but also due to thedissolvability in the polar organic solvents resulting in the feasibility of formingmembranes via a phase inversion method. However, the water-soluble polymer additiveswill be left in the membrane during the phase inversion process. And the hydrophobicity ofmembrane reduces due to the residual additives. Based on this issue, two surfacemodification methods, reducing the surface free energy with silane coupling agent andincreasing the surface roughness with nano-particle via surface grafting technology, wereproposed to improve the hydrophobicity of PVDF ultrafiltration membrane in this thesis.Firstly, the PVDF membrane was activated by long-distance and dynamiclow-temperature (LDDLT) Ar plasma flow irradiation using the plasma equipment made byour study group, in order to obtain the active radicals and the peroxides which wouldproduce hydroxyls on the membrane for the further hydrophobic graft reaction. Meanwhile,the proper etching via plasma flow made the membrane surface pore size larger andporosity higher without damage of membrane bulk structure, which increased thepermeation flux of modified membrane. And the effects of the time of plasma irradiation onthe membrane surface morphology and pore size were investigated.Then, the plasma-irradiated membrane surface was grafted with hydrophobicmonomer named octyltriethoxysilane (OcTES). The influences of pH value, concentrationof OcTES and the temperature of heat treatment on the performances of membrane wereinvestigated and optimized. Under the optimal grafting conditions, a certain hydrophobicnano-particle SiO2(R974) was added into OcTES in order to increase the surface roughnesswhich would enhance the hydrophobicity of PVDF membrane. And the influence of theaddition of SiO2on the performance for modified membrane was detected.In addition, the membranes were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), water contact angle (WCA), poresize distribution and degree of grafting (DG) to test the changes of surface morphology,chemical composition, hydrophobicity and pore size. The analysis results revealed that,compared to the original PVDF membrane, the WCA increased from70.1°to121.0°andthe pore size enlarged from25nm to140nm after grafted with OcTES.Furthermore, the separation performances of PVDF membranes were tested via thevacuum membrane distillation (VMD) using5%ethanol aqueous solution. The membranesgrafted with OcTES showed significant increases in both permeation flux (from3.19kg·m-2·h-1to5.39kg·m-2·h-1) and separation factor (from4.94to8.50) in VMD undervacuum degree of86kPa in permeates side. Furthermore, the results of superhydrophobicmodification of PVDF membranes with SiO2showed that the pore size diminished whenthe addition of R974increased, and the WCA of PVDF membrane was improved to153.2°due to the increase of surface roughness. Although the permeation flux deceased a little, theseparation factor soared to9.42, compared to original membrane under the same VMDconditions.In conclusion, the process for hydrophobic modification of PVDF ultrafiltrationmembrane in this thesis had some adventures, such as simple and flexible steps, low-cost,environmental method, and significant improvement in separation performances ofmembrane, which would make the PVDF membrane better for the MD. |
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