The production and evaluation of highly-aligned nano-fibre preforms using electro-spinning

Shao, Siheng (2023). The production and evaluation of highly-aligned nano-fibre preforms using electro-spinning. University of Birmingham. Ph.D.

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Abstract

Electro-spinning is an established manufacturing technique that is used to produce fibres in the nano-metre diameter. The electro-spun fibres are generally deposited on a grounded static electrode where the orientation of the deposited fibres is random. In the context of using these fibres as reinforcements in the production of composites, ideally, unidirectional and continuous fibres are desirable. Whilst a number of elegant and innovative techniques have been reported for producing aligned fibres, at the time of writing, there were no publications on techniques to extract the aligned fibres in an efficient manner to enable spooling and post-processing. Such a technique was developed in this study.

The starting point for the current project was COMSOL modelling where various electrode configuration and materials were considered. It transpired that by shielding the unwanted electric field lines between the charged spinneret (needle) containing a polymer solution, and modifying the geometry of the grounded static electrode, it was possible to control the trajectory of the electro-spun polymer jet, from the charged needle, to oscillate between the ends of the electrode. The ideal material for the shield was polytetrafluoroethylene (PTFE) and this new electro-spinning configuration is referred to as the ‘Vee-shield’ method because of the profile of shield. The fixture consists of a V-shaped PTFE shield at 60° with a 0.6 cm wide integral rectangular base that is mounted on a copper disk with a 10 cm diameter. The continuous extraction of the aligned nano-fibres was demonstrated using a motorised spooling unit. The polymer that was selected for this study was polyacrylonitrile (PAN) and the solvent was dimethyl sulfoxide (DMSO). In the static set-up, approximately 91% of the fibres were deposited to within 3°. When the spooling rig was used, a tape of cellulose paper was used as a mobile substrate. It was hauled off at 0.07 mm/minute, where 78% of the fibres were aligned to within 3°. A patent was recently secured for this method for producing highly aligned nano-fibres (patent number: WO2020120985A1).

After the fibre alignment technique was developed, attention was given to improving the tensile properties of the electro-spun fibres. This required the development of an end-tabbing method that was repeatable, where the alignment of the fibres was assured, and where the gauge length of the tensile specimen was not compromised by the end-tab resin. The bonding adhesive selected was a photo-curable UV resin that fulfilled the above-mentioned requirements.

The feasibility of improving the tensile mechanical properties of the electro-spun fibres was investigated by cold-drawing. The Young’s modulus at 0.015 strain, and the ultimate tensile strength of the as-spun and cold-drawn fibres were 2.9 ± 0.28 GPa and 76.7 ± 3.3 MPa and 8.5 ± 0.3 GPa and 88.9 ± 4.2 MPa, respectively.

Finally, the effect of oxidation of the electro-spun PAN fibres and the carbonisation temperature were studied. Since PAN shrinks during oxidation and carbonisation, a custom-designed rig was used to apply a constant tension, within a tube furnace, during the heat-treatment operation. The oxidation of PAN was carried out at 300 °C in air and the carbonisation at 600, 900 and 1,200 °C in a nitrogen atmosphere. The Young’s modulus and the ultimate failure stress increased as a function of the heat-treatment temperature. For example, Young’s modulus increased from 7.4 ± 0.7 GPa (as-spun fibre) to 37.2 ± 1.1 GPa (carbonised at 900 °C). In the case of the ultimate failure stress, it increased from 195.0 ± 10.2 MPa to 378.1 ± 33.1 MPa for the as-spun and carbonised samples (at 900 °C), respectively.

XRD and Raman spectroscopy were used to study the graphitic characteristics of the oxidised and carbonised fibre where it was established that the sp2 characteristics increased with heat-treatment temperature. With reference to the Raman spectra, the ID/IG ratio for the carbonised fibre was 0.87 ± 0.04 and it represents one of the highest values reported in literature for PAN samples that were treated at the same temperature.

A TEM-based technique was developed to estimate the fibre volume fraction for the carbonised fibres. This aspect was carried out in collaboration with the Electron Microscopy Centre in the School of Metallurgy and Materials at the University of Birmingham.

The work has shown that the tensile strength and the Young's modulus of electro-spun PAN fibres can be increased by cold-drawing.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):