Abstract
Nanofibres are fibres having diameters in the nano scale, and they exhibit high properties due to their high surface area to volume ratios compared to fibres in the micro scale.
Boron nitride is a compound of boron and can have hexagonal (h-BN), cubic (c-BN), wurtzite (w-BN) and rhombohedral (r-BN) crystal structures. Hexagonal boron nitride has a similar structure to graphene. Hexagonal boron nitride has unique physical and chemical properties such as high temperature resistance and good resistance to oxidation.
In this study, nano sized hexagonal boron nitride (at wt %0.15, %0.30 and %0.45) added polyvinyl alcohol (PVA) based nanofibres and 100% PVA nanofibres are produced via electrospinning. The morphologies of the produced nanofibres are analysed by scanning electron microscopy (SEM), and the tensile strengths of the nanofibres are evaluated.
Key words: electrospinning, nanofibres, hexagonal boron nitride, polyvinylalcohol
Introduction
Nanofibres are fibres having diameters in the nano scale, and they exhibit high properties due to their high surface area to volume ratios compared to fibres in the micro scale [Nijuguna, 2008].
Electrospinning is a widely used method for production of nanofibres where a high voltage is applied on a polymeric solution. Other methods for producing nanofibres are drawing, template synthesis, phase separation and self-assembly [Ramakrishna, 2005].
Boron nitride is a compound of boron and can have four different crystal structures: hexagonal (h-BN), cubic (c-BN), wurtzite (w-BN) and rhombohedral (r-BN) [Mirkarimi, 1997]. Hexagonal boron nitride has unique physical and chemical properties such as high temperature resistance and good resistance to oxidation as well as being electrically insulative [Han, 2008; Sutter, 2011; Liu, 2013].
In the literature there are few studies on nanofibres having hexagonal boron nitride reinforcement and those studies are mainly investigating the morphological and thermal properties of the nanofibres rather than their mechanical properties due to their fragile structures [Hwang, 2010; Uslu, 2012; Koysuren, 2012].
Experiment
Partially hydrolised PVA (polyvinylalcohol) polymer (Merck brand, having a MW of 70000) was dissolved in pure water at 16 wt% to produce nanofibres. Nano-sized hexagonal boron nitride (h-BN) at 70-80 nm particle size (Grafen Chemicals, Turkiye) was dissolved in absolute ethanol (Sigma Aldrich) to have 1 wt% h-BN solution.
The PVA and h-BN solutions were mixed at appropriate ratios to obtain hBN reinforced PVA nanofibres having 0.15 wt%, 0.30 wt% and 0.45 wt% of h-BN. Pure PVA nanofibres were produced using the PVA solution. The sample codes and percentages of the nanofibres are given in Table 1.
Nanofibres were produced in Istanbul ITA Tekstil Research Laboratories using a KatoTech brand electrospinning machine which works with a movable single syring
e and has a cylinder to collect nanofibre webs. The working parameters were 26,6 kV applied voltage, 0,003 ml/min solution feed rate, 10,05 m/min cylinder speed and 31,60 m/min syringe travelling speed. The distance between the syringe and the cylinder was 12 cm during the production.
The tensile strength of the produced nanofibres were evaluated. Nanofibres have very tiny and sensitive structures that might be easily broken when handling for mechanical testing, for this reason the tensile testing process requires care when handling the nanowebs. Nanofibre webs were cut at 4x6 cm rectangles and tested at 10 mm/min extension and 30 mm gauge length on a universal testing machine (Tinius Olsen, 10 kN) using a 100N load cell.
The morphological structures of the nanofibres were examined at Erciyes University with a Leo 440 scanning electron microscope at 20 kV. And the AFM (atomic force microscopy) analyses of the nanofibres were conducted at Nanomagnetics Instruments (Ankara, Turkey) using one of their AFM products named ezAFM on the dynamic mode using a PPP-NCLR cantilever.
Results and discussion
Tensile strength results of the nanofibres are given in Table 2. The tensile strength and elongation percent of the nanofibres showed increase with h-BN reinforcement.
The morphological structures of the nanofibres were examined at Erciyes University with a Leo 440 scanning electron microscope at 20 kV. The images of the samples are given in Figures 1-4. The nanofibres have diameters in the nano scale and they don’t have beads in their structures.
The AFM (atomic force microscopy) analysis of the pure PVA nanofibre sample is shown in Figure 5.