Fabrication Of Selectively Wettable Nanostructured Films And Electrospun Fibrous Membranes For Self-cleaning And Oil/Water Separation Applications

Selectively wettable surfaces (superhydrophobic/philic) made by nature inspires human beings to develop materials, objects, and processes that function from the macro-scale to the nanoscale for desirable applications. Certain plant leaves, such as Lotus leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical roughness of their leaf surfaces. The self-cleaning surfaces with high contact angle and low contact angle hysteresis (superhydrophobicity) also exhibit low adhesion and drag reduction for fluid flow. Simultaneously, hierarchical membranes fabricated by electrospinning method possessing selective wettability with suitable post-treatment through the use of non-polar material with low surface energy enabled them to exhibit hydrophobicity-oleophilicity and hydrophilicity-oleophobicity properties, which make these materials particularly suited for applications in oil/water separation systems. As in last few years, the increased industrial oily waste water and multiple oil spill disasters have highlighted the challenges of effective oil/water separation, especially the oil/water micro-and nano-emulsions.In this thesis, we first described the theoretical background, parameters and models of the wetting of rough surfaces followed by the fabrication methods used for the preparation of micro-, nano-and hierarchical structured superhydrophobic films. The next logical step is the realization of selectively wettable membrane formation by using electrospinning method. A comprehensive review is presented on historical background, processes, and parameters of electrospinning method to fabricate nano-micro-scale rough membranes. The main contents of the thesis are below:1) A novel fluorinated polybenzoxazine (PBZ) is lucratively designed for the first time to fabricate superhydrophobic films on the glass surface. The2,2-bis(3-flourophenyl-3,4-dihydro-2H-1,3-benzoxazinyl)hexafluoro propane (BAF-fa) is successfully synthesized utilizing conventional one-pot Mannich reaction. The films are developed on the glass surface using simple one-step spray coating method and then cured at220℃for1h. The transformation of hydrophobicity of pure films of P(BAF-fa) to superhydrophobicity has been done with silica modification, thus presented the involvement of binary phenomenon of low surface free energy and surface roughness altogether. The method to fabricate uniform hybrid films showing water contact angle (WCA) of163°, possesses the advantages of being straightforward and inexpensive. The as-prepared hybrid films are superhydrophobic not only for neutral water, but also for water of wide pH range. Additionally, these hybrid superhydrophobic films have shown promising durability in tap water and ethanol at different temperatures. This finding provides an effortless way to fabricate sliding superhydrophobic surfaces with high durability, thus expanding the PBZ role in super water-repellency.2) In order to investigate the influence of hierarchical roughness on the design of low and high adhesion, superhydrophobic films on glass substrate with robust adhesion and dual pinning to the water droplets were fabricated utilizing a novel in situ polymerized F-PBZ having drooping aliphatic chains and incorporated SiO2nanoparticles (SiO2NPs). By employing the F-PBZ/SiO2NPs modification, the as prepared composite films possess the robust adhesion to the glass substrate and superhydrophobic pinned state with WCA of150°and the non-pinned state with WCA approaching to165°. Surface morphological studies have indicated that the wettability of resultant films could be controlled by tuning the surface composition as well as the hierarchical structures. The key role of micro and sub-micro sized structures and the nanometer sized voids is discussed by the investigation of static contact angle, contact angle hysteresis, droplet evaporation and propensity for air pocket formation. The as-prepared films exhibited high adhesion towards the glass substrate with considerable durability in corrosive water and proved their simultaneous use in the transportation of micro droplets, which could be helpful to design large-area and highly scalable superhydrophobic films.3) To cope with the environmental challenges, a facile approach for fabricating superhydrophobic and superoleophilic nanofibrous membranes for gravity driven oil/water separation. The membrane design is then realized by a facile combination of electrospun polyacrylonitrile (PAN) nanofibers coated by in situ polymerized F-PBZ functional layer incorporating SiO2nanoparticles (SiO2NPs). By employing the F-PBZ and F-PBZ/SiO2NP modification, the pristine hydrophilic PAN nanofibrous membranes were endowed with promising superhydrophobicity with a WCAof161°and superoleophilicity with an oil contact angle of0°. This new membrane shows high thermal stability (350℃) and good repellency to hot water (80℃). Furthermore, the as-prepared membranes exhibited fast and efficient separation of oil/water mixtures (3000L m-2h-1) by a solely gravity driven process, which makes them good candidates for industrial oil-polluted water treatments and oil spill clean-up, and also provided new insights into the design and development of functional nanofibrous membranes through F-PBZ modification.4) A novel and scalable strategy was developed for the synthesis of superhydrophilic and pre wetted oleophobic nanofibrous membranes by the facile combination of in situ cross-linked polyethylene glycol diacrylate nanofibers supported on polyacrylonitrile/polyethylene glycol nanofibrous (x-PEGDA@PG NF) membranes. The as-prepared x-PEGDA@PG NF membranes have shown superhydrophilicity with ultralow time of wetting and promising oleophobicity to achieve effective separation for both immiscible oil/water mixtures and oil in water microemulsions solely driven by gravity. These new membranes having good mechanical strength of14MPa and mean pore size between1.5-2.6μm have shown very high flux rate of10,975L m-2h-1with extremely high separation efficiency (residual oil content in in filtrate is26ppm). More importantly, the membrane exhibits high separation capacity, which can separate10L of an oil/water mixture continuously without a decline in flux and excellent antifouling property for long term use. Thus making them an important candidate for treating wastewater produced in industry and daily life, crude oil, especially for high viscosity oil purification.This thesis combine the structure regulation for fabricating novel low surface energy materials in association with intrinsic nanoscale roughness to fabricate self-cleaning films and surface modification of selectively wettable materials (low and high energy materials) to produce nanofibrous membranes for variety of separation of oil/water mixtures with high flux and separation efficiency. Which could be proven to be much more operative than the conventional non-woven and polymeric membranes in efficient oil water separation systems.