Surface properties of electrospun polymer nanofibres

Abstract

Fibrous materials are used in a variety of applications due to their relatively high surface area to volume ratio as well as anisotropic behaviour. Electrospinning is a popular fabrication technique which produces polymer nanofibres with a potentially high molecular orientation. The surface of polymer fibres plays a significant role in many applications thus measurement of their surface properties is essential but challenging due to their relatively small size. In this thesis, ultrafine nanofibres have been produced by electrospinning with their nanofibre morphology controlled by varying different processing parameters. Atomic force microscopy (AFM) adhesion contact mechanics and individual nanofibre wetting measurements have been conducted to explore surface properties of the produced electrospun polymer fibres. Results using traditional Owens-Wendt plots applied to our nanomaterials show electrospun nanofibres have a higher dispersive surface free energy compared to bulk polymer film but a lower polar contribution, giving a total surface free energy in excess of bulk equivalents. A novel proposed model indicates that this nanofibre dispersive surface free energy is intimately linked to density of the polymer and ultimately the molecular spacing or orientation for the polymer chains. Comparisons are made with bulk polymer films to show that a high degree of molecular orientation is present at least at the surface of the polymer nanofibre. Structure investigations on electrospun fibres of polyvinyl alcohol using FTIR and XPS surface techniques explore how an increase in hydrogen bonds formed within nanofibres rather than on the fibre surface enhance this dispersive contribution but lowers the polar contribution. The wetting behaviour of electrospun fibre is extended to assemblies at length scales above individual fibres to highlight how superhydrophobic surfaces can be produced from nanofibre networks with defined spacings and geometries. This superhydrophobicity was adequately described by a Cassie-Baxter model modified to account for the fibrous geometry.