Polycaprolactone (PCL) is a biocompatible aliphatic polyester with many possible applications in the medical field. PCL nanofibres, produced by electrospinning, could provide new characteristics that are of interest for these applications. However, a key prerequisite is the ability to obtain bead-free fibres with diameters in the nanoscale range. At present the most commonly used solvent for electrospinning PCL is chloroform, but this only leads to fibres in the microscale range. Therefore various solvent systems were examined in this study.The innovative solvent mixture formic acid/acetic acid was found to allow for nanofibres with a diameter ten times smaller than chloroform. Moreover, steady state conditions could be obtained which thus allow electrospinning in a stable and reproducible way. Further it was noticed that the average fibre diameter decreased with decreasing polymer concentration while the diameter distribution decreased with increasing amount of formic acid. Also the humidity, an often overlooked yet important parameter, was noted to affect both diameter 2 characteristics. Generally it can be concluded that the solvent system formic acid/acetic acid could fill the gap in electrospinning PCL since it is readily able to produce uniform fibres in the nanoscale range.

Delamination and brittle matrix fracture has since long been a problem of fibre reinforced composites. This paper investigates if polycaprolactone (PCL) nanofibre nonwovens can increase the interlaminar fracture toughness of resin transfer moulded glass fibre/epoxy laminates, without causing problems during impregnation and without negatively affecting other (mechanical) properties.The mode I fracture toughness was shown to be dependent on both the nanofibre content as well as on how the nanofibres were introduced into the laminates. Almost 100% improvement in fracture toughness could be achieved by electrospinning the PCL nanofibres on both sides of the glass fibre mats prior to impregnation. This led to a mode I fracture toughness of over 1200 J/m 2 . 2Tensile and dynamic mechanical properties of the toughened laminates were not affected by the PCL nanofibres. It could be concluded that even state of the art infusion resins with a high intrinsic fracture toughness can benefit significantly from nanofibre toughening.

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Recently, several types of nanoparticles are frequently incorporated in reinforced epoxy resin composites. A homogeneous dispersion of these nanoparticles is still a problem.Thermoplastic nanofibrous structures can tackle this dispersion issue. Therefore, this paper investigated the effect of electrospun polyamide 6 nanofibrous structures on the mechanical properties of a glass fibre/epoxy composite. The nanofibres were incorporated in the glass fibre/epoxy composite as stand-alone interlayered structures and directly spun on the glass fibre reinforcement. Both ways of nanofibre incorporation have no negative effect on the impregnation of the epoxy. Moreover, the nanofibres remain well dispersed within the matrix. Incorporation of nanofibres increases the stress at failure in the 0°-direction, the best results are obtained when the nanofibres are directly electrospun on the glass fibres. Optical microscopic images also demonstrate that nanofibres prevent delamination when a 90° crack reaches a neighbouring 0° ply. Furthermore, mode I tests showed a small improvement when a thin nanofibrous structure is deposited directly onto the glass fibres. When the composites are loaded under 45°, it is proven that, for an identical stress, the glass fibre composite with deposited nanofibres has less cracks than when interlayered nanofibrous structures are incorporated. Generally, it can be concluded that the addition of polyamide 6 nanofibres improves some mechanical characteristics of a glass fibre/epoxy composite.

In comparison with conventional structures, nanofibrous structures have unique characteristics, such as higher surface-to-volume ratios, smaller pores, and higher porosity. Their hydrophilic nature is a key characteristic for many applications. However, because of their high porosity, it is difficult to measure the hydrophilicity of nanofibrous structures with contact-angle measurements. Therefore, characterization through wicking behavior is more appropriate. The International Organization for Standardization norm on wicking needs some refining to account for the specific nature of highly porous nanofibrous structures. A refined method was used on several structures that differed in the fiber diameter and the polyamide type. The structures with the thickest nanofibers had the highest wicking rates. At equilibrium, the wicking heights of structures of different polyamide types with the same average fiber diameter followed the trend expected from their intrinsic hydrophilicity. In the initial phase, the capillary forces established the wicking behavior. Later in the process, the wicking behavior was determined by the capillary forces and the hydrophilicity. In conclusion, the hydrophilicity of nanofibrous structures can be successfully determined by an optimized wicking procedure, and the fiber diameter is the dominant parameter for the resulting wicking height at equilibrium.

Electrospinning is a process to generate a nanofibrous material. Although the working principle of electrospinning is rather straight forward, it is influenced by many parameters. There is still a serious lack of knowledge concerning the influence of the ambient parameters, for which preliminary knowledge reveals that the relative humidity is of primary importance. This article reports the influence of the relative humidity on electrospun polyamide 6 nanofibres. Mixtures of formic acid and acetic acid are used for steady state electrospinning of polyamide 6 nanofibres, for which a steady state table is determined. When the relative humidity increases, the average fiber diameter decreases and the fraction of the less stable c-phase crystals in the polyamide diminishes. This effect is explained by absorbed water acting as a plasticizer, reducing the Tg of the polyamide. This article shows the importance of working in climatized conditions during electrospinning to obtain reproducible nanofibres.

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