This work is mainly focused on the preparation, morphology and properties of hierarchical materials based on electrospinning and electrospraying techniques. Here, the hierarchical materials include porous and/or hollow electrospun fibers, porous and/or hollow electrosprayed microspheres and carbon based nanofillers such as carbon nanotubes (CNTs) and graphite nanoplatelets (GNPs) decorated electrospun nanofibers. The main results are summarized as follows:
Biodegradable polycaprolactone (PCL) with controlled porous structures could be obtained by electrospinning PCL/polyoxyethylene (PEO) blend solution, followed by selective removal of the water soluble PEO. The porous structures such as pore size and pore density could be effectively tuned by controlling the weight ratio between the two polymers. On the other hand, more electrospinabe PEO played a vital role in determining the morphologies of the final electrospun products. Long and continuous blend nanofibers and their corresponding porous nanofibers were obtained at a high PEO concentration. Interestingly, the fiber-like materials gradually turned to rice-like nanobeans with decreasing PEO concentrations, which was caused by the decreased macromolecular entanglement.
Nonsolvent assisted electrospinning and electrospraying were proposed to fabricate the porous and/or hollow polymethylmethacrylate (PMMA) fibers and microspheres. Nonsolvent induced phase separation (NIPS) was the main reason for the formation the hierarchical materials. Dichlomethane (DCM) and different alcohols were chosen as the solvent and nonsolvent. DCM possesses a low boiling point and viscosity, and so it was first evaporated during electrospinning or electrospraying, which facilitated phase separation between the nonsolvent and the polymer. On the other hand, the low conductivity of DCM was beneficial to the generation of regular electrosprayed microspheres, because of the low electrostatic forces. Note that the nonsolvent cannot dissolve the polymer but is miscible with DCM. In addition, the nonsovlent used in the experiment always has a higher boiling point than DCM. In terms of the hollow structure, the tendency toward phase separation between the nonsolvent and polymer as well as the surface tension of the nonsolvent, should be taken into consideration. Generally, a large phase separation tendency and a high surface tension for the nonsolvent could lead to the hollow fiber or microspheres. Because the nonsolvent nucleates, grows, and finally emerges more easily if the nonsolvent could strongly phase separated with polymer, and nonsolvent with a large surface tension prefers to stay inside the droplet. For the formation of pores on the surface of fibers or microspheres, apart from NIPS, vapor induced phase separation (VIPS), thermally induced phase separation (TIPS) together with breath figure may also be the contributing factors. Water vapor in the air was condensed on the droplet surface when the very volatile DCM was quickly evaporated and took a large amount of heat from the droplet solution. Pores were finally formed on the vapor-condensed sites when water was subsequently evaporated. Furthermore, the cooling effect could cause TIPS, which was also responsible for the pore formation. When ethanol and butanol served as the nonsolvent, solid electrospun fibers were obtained. However, the hollow fiber with porous shell was present when propanediol was utilized. It was found that a large fiber diameter was beneficial to the creation of surface pores. During electrospraying, hollow microspheres with porous shell were harvested with the addition of ethanol and propanediol into the polymer solution. It was found that propanediol assisted electrosprayed microspheres possessed thinner shell and smaller surface pore compared with microspheres based on ethanol assisted electrospraying, because, propanediol has a higher solubility parameter, corresponding to a stronger phase separation tendency with PMMA. On the other hand, the nucleated propanediol was difficult to grow, due to its much higher viscosity.
Interestingly, more regular microspheres with porous surface were obtained when hexanol was employed as the nonsolvent. Hexanol gathering on the droplet surface could stabilize the interface between the droplet and air, which can prevent the droplet from large deformation, and is therefore beneficial to the formation of regular microspheres. Polymer concentration, flowing rate, voltage, and nonsolvent viscosity could influence the final morphologies of the electrosprayed microspheres.
Ultrasonication was employed to induce CNT and GNP decoration onto the electrospun nanofiber surface. During ultrasonication, microjets and shock waves are generated near the CNT or GNP surfaces after the collapse of the bubble. These jets possessing large energy can push the carbon nanofillers towards the nanofibers at high speeds. When the fast-moving CNTs or GNPs hit the nanofiber surface, interfacial collision between the fillers and nanofibers occurs. As a consequence, the polymer nanofiber may become softened or even partially melted at the impact sites, and thus CNTs or GNPs could be decorated onto the nanofiber surface. It is a simple but quite effective method which possesses many advantages. First, the nanofiller adsorption is carried out under ultrasonication, and thus uniformly dispersed CNTs or GNPs can be decorated on the fiber surface. Additionally, this adsorption can be completed within a short time (generally less than two minutes). Second, it is a surfactant free method. Third, it may be applicable for many different kinds of nanofibers, extending the application of the electrospun polymer nanofiber (EPNF). CNT and GNP size can affect their decoration onto nanofibers. Compared with pristine CNTs, acid treated CNTs became shorter, which facilitated their decoration onto the EPNF surfaces more tightly and densely. On the other hand, GNPs cannot be well anchored onto the nanofiber surface when their sizes were much larger than the fiber diameter. However, GNPs could be uniformly attached or embedded onto the nanofibers, provide that the GNPs size was comparable to the nanofiber diameter. It was found the thermal stability for both CNT and GNP anchored nanofiber composites was enhanced, which originated from the uniform dispersion of CNTs and GNPs on the nanofiber surface and the strong interaction between the nanosized fillers and the polymer. The electrical conductivity was also significantly improved, due to the formation of conductive percolation network as well as the low junction resistance.
| Date of Award | 15 Feb 2013 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Kwok Yiu Robert LI (Supervisor) |
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