Influence of Molecular Ordering on Surface Free Energy of Polymer Nanofibres using Scanning Probe Microscopy
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Influence of Molecular Ordering on Surface Free Energy of Polymer Nanofibres using Scanning Probe Microscopy Shuangwu Li, Wei Wang, and Asa H Barber Department of Materials, Queen Mary College, University of London, Mile End Road, London, E1 4NS, United Kingdom ABSTRACT Fibrous materials are used in a variety of applications due to their relatively high surface area to volume as well as anisotropic behavior. Electrospinning is a popular fabrication method which produces polymer nanofibres with a potentially high molecular alignment. In this work we examine the surface free energy of electrospun polyvinyl-alcohol nanofibres and its relation to molecular ordering using scanning probe microscopy adhesion measurements. Comparisons are made with bulk polymer material to show that a high degree of molecular orientation is present at least at the surface of the polymer nanofibre. As a result, the surface free energy of electrospun polymer nanofibres is greater than that of a bulk polymer. This effect indicates that the electrospinning process is effective at polymer alignment over a variety of experimental parameters.
INTRODUCTION Polymer nanofibres have been attractive materials for a wide range of applications because of their large surface area to volume ratio and potentially improved mechanical performance due to high molecular alignment. Their potential applications include tissue engineering scaffolds [1], drug delivery media [2], filtration media [3], protecting clothes, and as reinforcement in nanocomposites [4]. It is well known that the properties and internal molecular structure of polymer solids are greatly affected by their processing conditions. Therefore, an understanding of the processing–structure–property relationship is essential for engineering polymer nanofibres to meet the demands of their applications. A number of processing techniques such as drawing [5], template synthesis [6], phase separation [7], self-assembly [8] and electrospinning [9, 10] have been used to prepare polymer nanofibres. Electrospinning is particularly important for mass production as the method is both simple and easy to scale-up for mass production. Electrospinning uses a high voltage electrostatic field to charge the surface of a polymer solution droplet and thus induce the ejection of a liquid jet through a spinneret [11]. In a typical process, an electrical potential is applied between a droplet of a polymer solution, or melt, held at the end of a capillary tube and a grounded target [12]. When the applied voltage overcomes the surface tension of the droplet, a charged jet of polymer solution is ejected. The route of the charged jet is controlled by the electric field [13]. The jet exhibits bending instabilities caused by repulsive forces between the charges carried with the jet. The jet extends through spiraling loops; as the loops increase in diameter the jet grows longer and thinner until it solidifies and collected on the target [14, 15]. The surface of polymer fibres plays an important role in many applica
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