Fabrication Of Micro/nanofibres By Blown Bubble Spinning And Its Mechanism

Nanotechnology has become one of the leading force in promoting scientific and technological progress in the 21 st century, and how to use low-energy and efficient way to obtain a variety of nanomaterials with special structures is an important issue in nanotechnology research. As a one-dimensional nanomaterial, nanofibres have greatly expanded the applications of traditional textile industry due to their remarkable properties such as small diameter, huge specific surface area, high porosity and others, and the rapid development of technologies in the preparation of structured nanofibres will further promote the exploitation of new nanofibrous products, accelerate the process of textile technology and industry transformation.In this thesis, a brief overview of the history and the current research focusing on the recent electrospinning, meltblown and solution blowing methods in fabricating nano-(ultra-) fibres was given, summarizing the main features and potential benefits of the three spinning methods and also pointing out the existing problems they encounter. On the basis of in-depth analysis of unique advantages in the bubble electrospinning as well as significant roles which the airflow plays in the textile processing and textile applications, a novel spinning method called blown bubble spinning was first proposed.According to the mechanism of the blown bubble spinning——the surface tention of bubble films produced by polymer solutions is overcome by using the airflow with a certain temperature and velocity, leading to the stretch and breakup of bubbles; and the jets from ruptured bubble films were further pulled by the blowing air and solidified to nanofibres with the evaporation of the solvent, an experimental apparatus was designed and built, and four kinds of polymers(polyvinyl alcohol, polyvinyl butyral, silk fibroin and polyamide) were carried out by the blown bubble spinning for fabrication of nanofibres, comparing with the electrospinning method. The results indicated that all products by blown bubble spinning were fiber bundles composed of numerous ultra-/nanofibers, similar to nanofibrous yarns, instead of mats formed by nanofibres by electrospinnning. In addition, polyamide nanofibres and its fibrous bundles showed smallest diameter and best morphology among the four polymers.In order to study the mechanism of the blown bubble spinning, the relationship between ambient temperature, humidity, bubble radius and bubble surface tension was firstly analysed and a modified bubble Young-Laplace equation was developed, theoretically suggesting that much finer nanofibres could be obtained by means of lowering the surface tension of bubbles with decreasing the bubble radius, reducing the relative humidity when the temperature outside bubble is greater than the temperature inside bubble, and improving the temperature outside bubble; then, a high-speed camera was utilized to observe the behavior of bubble dynamics in the absence and presence of gas flow field, finding that the thickness of the bubble wall was reduced to one sixth when bubbles burst, compared to the initial thickness, and the jets would oscillate under the effect of air flow around the spinning line; at the same time, multiple jets on account of the bubble breakup brought about the separation of initial fibrous jets, and achieved the drawing and coalescence by the flow, formed into fibrous bundles consisting of nanofibres; moreover, the equations of the jet motion was established based on the stress analysis of bubble jets in the gas flow filed.To investigate the effect of the spinning process parameters on the diameter and morphology of nanofibres and fibrous bundles fabricated by the blown bubble spinning, the initial airflow temperature, the initial airflow rate, the foaming tube diameter, the spinning distance and the angle between hot-air nozzle and foaming tube were selected as experimental parameters and related single-factor experiments were carried out. The results showed that the diameter of nanofibres and fibrous bundles decreased with the increase of the initial airflow temperature, the initial airflow rate and the spinning distance, but increased with the foaming tube diameter and the angle; Through variance analysis, all five parameters were significant factors affecting the diameter and morphology. Considering spinnability, fiber quality, energy consumption, ease of processing equipment and operation convenience, optimized parameters were the initial airflow temperature of 160 ℃, the initial airflow rate of 40m/s, the foaming tube diameter of 10 mm, the spinning distance of 35 cm and the angle of 40°, based on the judgement that the diameter of nanofibers was about 400 nm as well as that of fibrous bundles 40μm or so.With the purpose of investigating the influence of properties of spinning solutions on the structures and properties of products obtained by the blown bubble spinning, three different concentrations of polyamide 6/66 spinning solution were first prepared and four different types of surfactants including silicone quaternary ammonium salt, betaine, Tween60 and sodium dodecyl benzene sulfonatewere chosen as additives on the basis of optimized processing parameters; changes in the surface tension and the rheological behavior of solutions with or without adding surfactants in formic acid solutin were analysed by adjusting the concentration and the ratio of polyamide 6/66 and surfactants. The results indicated that in the absence of surfactants, the surface tension of the spinning solutions and the zero shear viscosity got enhanced as the polyamide 6/66 concentration increased; after adding different concentrations of surfactants with different types, the surface tension of the spinning solutions and the zero shear viscosity had different change trends. The surface tension and zero shear viscosity minimized when using the non-ionic surfactant Tween60, from 41.08 m N/m to 33.38 m N/m and from 1.73 Pa.s to 1.51 Pa.s, respectively. Next, the blown bubble spinning experiments were performed using different spinning solutions with or without adding surfactants and the corresponding products were subjected to structural analysis by scanning electron microscopy, infrared spectroscopy, X-ray diffraction and property analysis by thermal stability and mechanical properties. The results showed that in the absence of surfactants, the diameter of nanofibres and fibrous bundles, thermal stability and breaking stress increased with rise of the solution concentration, but its aggregation structure had no obvious change; after using different concentrations of surfactants with different types, the structures and properties of nanofibres and fibrous bundles exhibited different change trends but no clearly uniform law. The optimized parameters were 12% PA6/66, 1.5% Tween60 through comprehensive analysis; at that condition, both the morphology and the properties of the obtained nanofibrous bundles owned the best performance, showing that the diameter of inner nanofibres was 317.8±54.8nm, the diameter of fibrous bundles was 18.6±1.5μm, thermal decomposition temperature was 439.7 ℃, breaking stress and initial modulus were 6.51 ± 0.47 MPa and 17.49±1.34 MPa, respectively, and elongation at break was 24.67 ± 2.18%.In order to further enhance and improve the performance of polyamide 6/66 nanofibrous bundles, different mass fraction of multi-walled carbon nanotubes(MWCNTs) with different treatments were added to the PA6/66 solution, and changes in the properties of the mix spinning solutions were discussed.It was found that the surface tension of the solution after adding MWCNTs by Tween60/ultrasonic treatment was lower than those of solutions after adding MWCNTs by ultrasonic treatment or untreatment as well as the zero shear viscosity of the solution was minimum and MWCNTs dispersion was best with longest stability. Then, the impact of different mass fraction of MWCNTs with different treatments on the structures and properties of products obtained by the blown bubble spinning was investigated; the diameters of the composite nanmofibres and nanofibrous bundles were larger than that of pure PA6/66 nanmofibres and nanofibrous bundles, and all diameters become enlarged with increasing the MWCNTs content; however, under the same MWCNTs concentrations with different treatments, the diameter of the composite nanmofibres and nanofibrous bundles were minimum, respectively 284.4±53.4nm and 40.4±5.3μm when adding 0.5% MWCNTs by Tween60/ultrasonic treatment; but the aggregation structure of the composite nanofibrous bundles indicated no significant variation even though MWCNTs was introduced with different treatments. Whereas the thermal stability, mechanical and electrical properties of the composite nanofibrous bundles after adding MWCNTs had been greatly improved compared to those of pure PA6/66 nanofibrous bundles; among them, the composite nanofibrous bundles demonstrated optimum properties when the concentration of MWCNTs treated by Tween60/sonication was 1.5%, with improving the thermal decomposition temperature by 38.5℃, the breaking stress and the initial modulus by 196.88 % and 80.91%, while the elongation at break decreased by 17.27%, but the conductivity increased by 7 orders of magnitude from 10-13S/m to 10-6S/m.In conclusion, the blown bubble spinning is a short aprocess for fabricating nanofibres and fibrous bundles. According to the research on the blown bubble spinning technology and its mechanism, it contributes to reveal the dynamics behavior of the polymer bubbles in the gasflow field and to understand the mechanism of the nanofibers’ formation. Moreover, it is of great theoretical value and practical significance to further design and improve the blown bubble spinning methods and related apparatuses for continuously achieving high performance nanofibrous yarns in one-step procduction.