Mechanical Evaluation of Thermal Transitions in Polymer Nanofibres Using SPM
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1025-B10-02
Mechanical Evaluation of Thermal Transitions in Polymer Nanofibres Using SPM Wei Wang, Shuangwu Li, and Asa H. Barber Department of Materials, Queen Mary, University of London, Mile End Road, London, E1 4NS, United Kingdom ABSTRACT Polymer nanofibres produced by electrospinning techniques have unique mechanical properties due to their large surface area to volume ratio and potentially high molecular orientation. The effects of temperature on mechanical properties is challenging to measure due to the small fibre diameters produced. In this paper, scanning probe microscopy (SPM) is successfully used to elucidate the mechanical performance of individual electrospun polyvinyl alcohol (PVA) nanofibres over a range of temperatures. As observed in the results, thermal transitions have a dramatic effect on the mechanical behaviour of the nanofibres and are highlighted using Atomic Force Microscopy (AFM) techniques analogous to dynamic mechanical thermal analysis but at the nanoscale. Interestingly, nanofibre thermal transitions are shown to be mediated by fibre diameter and the driving force of reducing the surface area of the nanofibre.
INTRODUCTION Polymer one-dimensional (1-D) nanostructures have been gained much interest recently due to their unique properties and potential applications in many fields [1-3]. Electrospinning is a particularly straightforward synthetic method which has been successfully used to produce continuous ultrafine fibres with diameters typically ranging from 5µm to below 5 nm [4-6]. Unlike other processing, the fibre formation via electrospinning is achieved by applying a high voltage to charge a viscous polymer solution (or melt) contained in a syringe or spinneret. A critical voltage applied to the solution causes ejection of a polymer liquid-jet from the spinneret due to charge repulsion within the solution itself. The ejected polymer solution is elongated and drawn in the electrostatic field. The solvent evaporates during this period and the relatively dry fibres are finally deposited on a grounded substrate. The very large surface area to volume ratio (S/V) and extremely long length (high aspect ratio) of electrospun polymer nanofibres gives rise to interesting applications in composite materials where they may act as reinforcing fibres [7]. The effectiveness of electrospun polymer nanofibres as reinforcement is dependent on molecular chain orientation along the fibre direction induced by the large shear force and rapid solidification in electrospinning [8-10]. The fibrous geometry and structure of electrospun polymer nanofibres are also related to their thermal behaviour. Thermal properties including glass transition temperature (Tg), peak crystallinization temperature (Tc), melting temperature (Tm) and thermal degradation of various electrospun polymer fibre mats are typically characterized using Differential Scanning Calorimetry (DSC), Dynamic Mechanical Thermal Analysis (DMTA) and Thermal Gravimetric Analysis (TGA) techniques [11-16]. However, the results are averaged over
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