Scission of electrospun polymer fibres by ultrasonication

The scope of this research is to study the use of ultrasonication, using a probe sonicator to produce short fibre from electrospun webs of four types of polymer poly(styrene) (PS), poly(methyl methacrylate) (PMMA), poly(acrylonitrile)(PAN) and poly(L-lactic) acid (PLLA). Scissioning of the usually long, continuous non-woven electrospun web would be useful as a means to produce short discrete fibres in significant amounts. Such materials are useful for numerous applications, including composite reinforcement and biomedical applications such as vessels for the containment and release of drugs. Optimisation of the electrospinning parameters were carried out to produce electrospun fibres from the four polymers that were beads-free, and whose nanofiber constituents had submicron diameters. It was found by producing and testing non-woven webs that had roughly similar fibre diameters, as well as similar tensile strength and elastic modulus, that the ductility of the electrospun polymer was the key determinant as to whether the web could be ultrasonically scissioned. Thus, ultrasonication is an effective method to scission brittle/submicron electrospun fibre such as PS and PMMA, whilst post-treatments such as UV-Ozone irradiation and heat treatment of the electrospun fibres were required for polymers such as PAN and PLLA. Without such post-treatment, PAN and PLLA were unable to be scissioned, regardless for how long the samples were sonicated. The success of the post-treatments was due to either reductions in ductility reduction or induced flaws on the electrospun fibres, the latter acting as points for the initiation of failure, facilitating the scissioning of these more ductile materials. The potential mechanisms involved in scissioning were also investigated and discussed, and relate to bubble cavitation and collapse caused by the ultrasonication probe. These mechanism ranged from the effect of the impact of the jet resulting from imploding bubbles leading to erosion and pitting which would create point of weakness for crack initiation, to fibre buckling for long fibres that were oriented parallel to the bubble surface during bubble growth, leading to rotation and buckling of the fibres when the bubble collapsed. The shorter fibres that align normal to the bubble surface experience a difference in velocity for both fibre ends when the bubble implodes. The highest velocity at the end nearest to the bubble wall compared to the farthest end thus leads to tensile failure. The effects of the sonication parameters on the scission process were investigated and it was found that the sonication parameter such as run time, lapsed time, amplitude and solvent types affects the scission efficiency, with the bulk temperature of the solvent and the concentration of dissolved gas in the solvent showing no effect on the short fibre length. The effects of scissioning electrospun PAN prior to and after carbonisation process were also investigated. It was found that the scissioned-carbonised (as-spun web was sonicated prior to carbonisation) showed no significant difference in terms of the surface morphology, carbonaceous quality and electrical conductivity compared with the carbonised-scissioned (the as-spun web was carbonised followed by sonication scissioned). Methods of composite preparation for carbonised PAN short fibre/epoxy were investigated and it was found that the solvent blending method was superior to prepare the composite compared with a high speed mixing technique and no solvent, with some improvement on the flexural strength and modulus observed for the scissioned-carbonised/epoxy compared with the carbonised-scissioned/epoxy composite and the epoxy resin alone.