Morphological Control Of Electrospun Jets And The Fiber Structures&Properties

Electrospinning is one of the easiest methods for prepraring micro or even nano scale fibers. It uses a high voltage field to draw melt or solution into very fine fibers. After attracting extensive attention at the end of20th century, electrospinning has rapid progress, especially for the application of materials in scaffold materials, filtration materials, catalysts carriers, reinforcement materials, inorganic nanofibers, anti-fake materials and etc. However, little breakthrough has been made in the theory of electrospinning in the21st century. The main reason is that the existing measurement methods are not competent for characterizing the process of the electrospinning process. The extremely fine jet in electrospinning process flies very fast. Besides, the driving force is electric force, which is difficult to determine. Mass and heat transfer and jet solidification also complicate the electrospinning process. Besides, the electrospun products have various morphologies, which are not easy to control. So in this research, new methods are used for studying jet morphology, interaction and movement parameters in order to prepare materials with different scales and morphologies. The properties of these obtained materials are studied and modulated.The forming mechanisms and morphology evolutions of microspheres, beaded structures and bead-free fibers in electrospinning of poly (hydroxybutyrate-co-hydroxyvalerate)(PHBV)/CHCl3solution are studied by dry-jet-wet-electrospinning (DJWE) method. The solution viscosity raises, with increasing the polymer concentration. And the obtained products are micropaticles at very low concentration, then beaded fibers and at last even fibers with increasing concentration. But if ethanol is added into solution, the conductivity increases, and beaded structures decrease with ethanol content. The morphology of microspheres, which is obtained from electrospinning of2wt%PHBV solution, varies with collection distance. When collected at very low distance, the microparticles show irregular but smooth spheres. When the distance increases to5cm, particles like blood cells can be observed. Porous and regular spheres can be obtained in larger collection distance. With assistance of fluorescent materials, the morphology variation is found to relate to particle solidification process. When collected at low distance, the spheres do not have solidified surface. So the double-diffusion is very violent, which leads to the irregular and surface collapse. While collected in large distance, the surface has solidified already before falling into the bath. So the double-diffusion is moderate. And the obtained structure is regular. In the subsequent shrinkage process, the porous structures appear. The beads with beanpod-like structures and twice the volume of normal beads can be obtained at the boundary of stability and instability section in electrospinning of6wt%PHBV solution. It can be inferred that the jet is firstly stretched at the defects and formed the primary beads and fibers between adjacent beads. Then, if the fibers solidify and are stronger than the beads, the primary beads are further stretched and secondary beads which were common in electrospun fiber appeared. Otherwise, the fiber might be break down by the electric field force. For the bead-free fibers, when collected at low distance, the jet contains much solvent. After falling into bath, the jet has severe double-diffusion at the interface. The inner part of jet shrank heavily and caves on the surface occurred. But if collected at longer distance, the fibers show smooth surface. After contact angle test, electrospun products prepared by DJWE are more hydrophobic than that prepared by normal electrospinning, which indicates the DJWE products have fewer adhesions and more loose structure.Electrospun solutions of high conductivity are used as research system in electrospinning. When the concentration of PAN/DMF solution reaches16wt%, some nanofibers with diameter of tens nanometers can be observed at the tie points of two micrometer fibers. These fibers do not come from electrospinning process, but from electrospinning of the unsolidified parts of fiber which have already fallen on collector. When electrospun fibers fall on collector, the positive charges on the fiber surface is repulsed by the electrical field and negative charges migrate onto the fiber. Then following positively charged fibers fall onto the negatively charged fibers, and new electrical field forms between the two fibers. The tie point of the two fibers has the largest electric field strength. So the unsolidified parts on the fiber will be secondary-electrospun and the nanowebs mainly forms on that spot. The nanowebs can also be observed in PHBV W/O emulsion electrospinning. Some high conductivity solutions using DMF as solvent, if electrospun at relative humidity lower than40%, can get ringlike deposition. Otherwise, if the conductivity is lower or other solvents are used, no ringlike deposition can be obtained, no matter how to adjust the electrospinning parameters. This deposition pattern is found the result of the balance of jet mutual repulsion and jet flying randomness. Higher concentration solution jet has stronger repulsion and less randomness, so the jet will be pushed out and deposit into a ring. Electrospinning parameters, such as collection distance, voltage, feeding rate, affect the morphology of the deposition pattern. Besides, if the spinning time is longer than10cm, a larger and glossier ring outside the previous one will appear. The two rings are separated and have no intersection. The out-ring structure is the result of the repulsion of jet by the first ring. Because there are some residue charges on the first ring, after certain amount, the jet will be push out to larger area. The second ring has better fiber alignment as a result of larger repulsion.High speed rotating method is applied for preparing electrospun aligned mats from solutions of different conductivities. Via developing a mathematical modeling, the orientations of the fibers in the mats prepared by various rotating speeds are calculated. The flying speeds of the PAN jet in vertical and perpendicular directions can also be estimated by speed decomposition. The result shows that the speeds in both vertical and perpendicular directions decrease when the concentration of PAN solution increases from10wt%to14wt%. But the vertical speed decreases sharper. The result infers that the jet from higher concentration solution flies slower and has longer flying time. So the jet has longer time to interact to reduce the randomness. The vertical speeds of12and14wt%are quite low, so the fibers deposits into a ringlike pattern. The mechanical properties of the electrospun mats increases with higher rotating speed. But even if the rotating speed higher than600m/min, the mat orientation and mechanical properties are not good enough. Though the jet from PHBV solution of low conductivity is even slower than the PAN jets, the surface charge density is too low to repulse mutually. So the ringlike deposition pattern cannot be obtained. The tensile strength increased and the elongation at break decreased with the increasing orientation when the tensile direction was parallel to the rotating direction. But in the perpendicular direction, the work of fracture and the elongation at break decreased, and the strength varied little. The fractures tended to parallel the rotating direction with increasing orientation factors in both tensile directions.The ringlike deposition, combined with the DJWE method, is used for preparing well-aligned PAN electrospun continuous yarns. A modified DJWE apparatus is developed to collect yarns. This structure can be wound at much higher speed in the electrospinning process. And fibers in the obtained yarn have very good alignment. Higher concentration the solution is, the fast winding speed can be applied. When the concentration is14wt%and relate humidity is20%, the winding speed reaches the maximum, that is65m/min. The fiber alignment increases with the increasing concentration and winding speed. According to theoretical calculation, when the deposition pattern is ringlike, the amount of defects in the yarn is very low. The yarn strength also increases with alignment. The as-spun yarn can also be post-treated. The draw ratio in boiling water can reach up to5times. With higher draw ratio, the diameter of the fibers in the yarn decrease, and the strength increases obviously. The molecules in the fiber can also be observed in infrared dichroism and X-ray diffraction. When the draw ratio reaches up to5times, the tensile strength of the drawn yarn is580MPa and elongation at break is12%. The yarn is a probable candidate for precusor of high performance carbon nanofibers.