Preparation Of Hierarchically Nanostructured Electrospun Fibers And Its Application In Oil/Water Separation

This work focused on the problem of oil spillage often occurred during the economic and social developments which have posed great threats both on ecological environment and human health. In view of relationship between the structures of sorbent and sorbate, we will develop an oil sorbent with a high oil sorption capacity, a high uptake capacity, an excellent recyclablity and reusability based upon the hierarchical structures of materials to overcome the low oil sorption capacity, poor oil/water selectivity and recyclablity existing in current oil sorbents. Efficient oil cleanup from water surface can not only solve the problems caused by oil spillage to protect ecological environment and avoid damages to relevant industry resources and human life but also save energy source reducing the economical losses immensely, which shows a great practical and realistic significance.Many hierarchical structures with micro-and nanoscales have been created by nature during its evolution throughout the millennia, enabling the living body to exhibit unique and novel functions such as superhydrophobicity, structural colors, thermal insulation, and high adhesion, which provides new ideas and learning ways for human beings to develop new materials to meet the application demands. Recently, electrospinning as an efficient method for generation of polymeric fibers with diameters both on micro-and nanoscales has gained increasing attention due to its unique advantages, such as a widely materials can be electrospun into fibers, fine structures both of fibers and their assembles are regulable, multivariate techniques can be combined with electrospinning, process for fiber preparation are extendable. Researchers are trying to prepare fibers with controllably hierarchical structures to endow themselves with novel properties, thus meet the requirements of function and performance in application areas.Hierarchical structures with micro-and nanoscales widely existed in nature and the new opportunities offered by inspiration arising from these structures for creation of new materials were reviewed firstly. Subsequently, we showed a brief review of the present research on fabrication of hierarchical structures via electrospinning. In this work, the fibers with hierarchical structures, such as rough surfaces with numerous papillae, wrinkled surfaces, porous surfaces, porous cores, etc., were fabricated via electrospinning directly. The fiber assembles exhibits a three dimensional porous structure, which have a high specific surface area (SSA) and pore volume due to the introduction of hierarchical structures. These typical structures can not be obtained via conventional spinning techniques, which were characterized by a field emission scanning electron microscopy, nitrogen adsorption measurement analyzer, and synchrotron radiation small-angle X-ray scattering. The formation of hierarchical structures as well as their regulation rules was presented. These results laid a fundamental support to fabrication of electrospun fibers with a high SSA and a high pore volume. Base on these studies, we investigated the effects of intrinsic property and hierarchical structures of the fibers on their wettability. Then we used these fibers to soak up oil on water surface and made analysis on the oil sorption mechanism and process. Subsequently, the structures both of fibrous mats and single fiber were regulated and optimized via multi-nozzles electrospinning and core-shell electrospinning to improve the oil sorption performance. The main results and conclusions of this thesis are presented as following.Polystyrene (PS) fibers with hierarchical structures and a high SSA were prepared via electrospinning directly. The SSA of as-prepared fibers was about67times magnitude larger than that of fibers without hierarchical structures. The results demonstrated that the as-prepared fibers were porous materials with mesopores and macropores. These porous structures and fiber surface morphologies can be controlled via tuning the weight ratio of a solvent with a high volatility to a solvent with a low volatility. The sole solvent with a high volatility enables the resultant fibers to show a flat-shape with porous surface or collapsed beads with porous surface. As increasing of low volatility solvent in co-solvent, the collapsed beads change into elliptical or circular shape and the fibers show wrinkled surfaces or smooth surfaces. The hierarchical structures of electrospun fibers are governed by the competition between phase separation (thermally induced phase separation and non-solvent induced phase separation) and solidification of the fluid jet in electrospinning, which is affected by the variation of solvent composition and relative humidity. Additionally, the processing parameters such as solution feed rate, applied voltage, and work distance can also affect the fiber morphology to some extent, which can be varied for large scale preparation in a specific range. Relative humidity plays a key role in the fabrication of PS fibers with hierarchical structures and a high SSA, usually it can be maintained around40%. Hybrid PS fibers with hierarchical structures were obtained via electrospinning PS solution with an addition of inorganic nanoparticles. It has been demonstrated that the distribution of nanoparticles within PS fibers can be regulated via tuning the solvent composition. For a given amount of nanoparticles, the nanoparticles was imbedded inside the fibers and only a small amount was present on fiber surfaces when a high volatility solvent used, as the low volatility solvent increased in mixed solvent, the nanoparticles began to come out from the inside of fibers to their surfaces. For a given solvent system, the nanoparticles on fibers surfaces were increased as the increasing of nanoparticles. For the different solvent systems, the hierarchical structures of hybrid PS fibers were almost not affected by the addition of nanoparticles when a high volatility solvent used. As increasing of low volatility solvent increased in mixed solvent, the SSA and total pore volume of as-spun fibers were increased immensely. The nanoparticles can increase total pore volume of as-spun fibers instead of change the pore size as low volatility solvent used, but as the mixed solvent used, both pore size and pore volume were increased. A small amount of nanoparticles does not change the initial thermal decomposition temperature, but a higher amount does.The wettability and oil sorption capacity of as-prepared PS fibers with hierarchical structures were investigated. The influence factors of wettability as well as corresponding regulation rules were also elucidated. The fibrous mat with a conversion of hydrophobicity and hydrophilicity was obtained and we also provided an elucidation on oil sorption mechanism and process as well as their influence factors. It shows that the wettability of as-prepared fibrous mats was governed by the intrinsic properties of group in polymer molecular chain and surface morphology of fibrous mats. The hierarchical structures can boost the hydrophobicity or hydrophilicity of the fibrous mats. The electrospun PS fibers with hierarchical structures exhibit an oleophilictiy-hydrophobictiy due to their nonpolarity and low surface energy, which shows an oil sorption capacity of113.87g/g for motor oil. The oil sorption process is a dynamic process. The oil wetted fiber surface firstly and absorbed by fiber surface, filling in the voids among fibers, and then diffused into the pores within fibers by capillary action. The oil sorption capacity of fibrous mats was overwhelmingly influenced by interfibre voids instead of intrafibre porosity, which mainly depended on fiber diameters, porous structures, SSA, porosity, etc.Reusable oil sorbent with good mechanical properties can be obtained by adding a certain amount of polyurethane (PU) fibers with good mechanical properties into PS fibrous mats via multi-nozzles electrospinning. Moreover, it also revealed the effects of structures of composite fibrous mats on oil sorption, which laid a solid foundation for preparation of reusable oil sorbent with a high oil sorption capacity. The results show that the bulkiness of composite fibrous mats becomes poorer with the addition of PU fibers, showing a mats-like instead of cotton-shape. With the increasing of PU fibers in composite fibrous mats, the SSA, pore size, water contact angle, oil sorption capacity of as-prepared fibrous mats decreased, however, the mechanical properties improved a lot and oil sorption capacity decreased with the increase of reusable times. The drop of oil sorption capacity for the first reusable time is larger while that for the second time is quite small.PU reinforced composite PS/PU fibers with hierarchical structures can be obtained via core-shell electrospinning, which overcomes the inefficiency of pure PS fibers in reusable property and low oil sorption capacity of composite fibrous mats. The effects of solution properties and processing parameters in electrospinning on fiber morphology and that on oil sorption capacity were elucidated. Results show that the fiber gradually changes from flat-shape with a high concentration of core solution into almost circle, meanwhile, the SSA and total pore volume increased thus the fiber shows core-shell structure, but lower bulkiness of fibrous mats and much larger density of fiber assemblies result in a lower oil sorption capacity. For a given high concentration of core solution, a high applied voltage results in the formation of nanopores on the composite fiber surface showing a larger SSA and pore volume compared with a low applied voltage in electrospinning, exhibiting a higher oil sorption capacity. For a given core solution, solvent composition in shell has a certain influence on fiber structures, however, it shows a little influence on oil sorption capacity. Compared with pure PS fibrous mats, the mechanical properties of composite fibrous mats have a substantial improvement, the break elongation and break stress has raised about10～15times reaching up to more than50%and3～5times, respectively. After5times reuse, the fibrous mats still show a high oil sorption capacity comparable with traditional polypropylene nonwoven, approximately3～5times that of wool-based nonwoven.In this thesis, we have prepared electrospun fibers with hierarchical structures and high SSA controllably via one-step, and we also obtained regulation rules of the typical structures. For the first time, we investigated the effects of hierarchical structures of as-prepared single fiber on its oil sorption capacity via element trace method with the help of soft X-ray spectromicroscopy beamline. According to the relationship between the demands from materials in application and their structural properties, we improved the mechanical properties of oil sorption fibers with a relative high sorption capacity via multi-nozzles electrospinning and core-shell electrospinning. Consequently, we obtained the composite fibrous mats composed of PS fibers with hierarchical structures and PU fibers with a high elacticity as well as the high elastic PU reinforced composite PS/PU fibers. These fibers exhibit a wide application prospect and practical value both in oil cleanup from water surfaces and oil/water separation.