- Open Access
Study of electrospun polycarbosilane (PCS) nanofibrous web by needle-less technique
© Sinha; licensee Springer 2014
- Received: 3 February 2014
- Accepted: 17 March 2014
- Published: 1 July 2014
This study reports on the various functional characteristics of silicon carbide (SiC) nanofibrous web. The SiC nanofibrous web was spun by the electrospinning technique using Nano Spider (needle-less) machine. The as-spun nanofibrous web was cured to 180°C and subsequently, pyrolized at 1000°C under inert nitrogen (N2) atmosphere to convert into silicon carbide nanofibrous web. The various properties of SiC web is characterized by using FESEM, Thermal Analysis, X-ray Diffraction, Energy Dispersive Spectroscopy (EDX), Atomic Force Microscopy and Surface Profilomertry. FESEM microphotographs indicated the interconnected fibres leading to pores of prepared SiC Nanofibrous web. Deep rooted fibre surface porosity was revealed by AFM. The thermal behavior of as-spun, cured and pyrolized PCS webs are influenced by the heat treatment at different temperatures. The surface roughness changes with the heat treatment of PCS nanofibrous webs. The pyrolized web carries higher surface roughness as compared to as-spun and cured webs. The EDX plots indicated the presence of C and Si elements in pyrolized PCS nanofibrous web.
- Nanofibrous web
A brief introduction to the process of electrospinning in the context of technical grade composite nanofibres is presented in this section. Furthermore, this section elaborates a brief review of current and past research activities that focus on the development of ceramic electrospinning and their applications in various areas.
Electrospinning is an attractive and simple process capable of spinning fibres from melt or polymer solution using an electric field (Salem 2007; Chase et al. 2011). Electrospinning process provides a straightforward electrohydro-dynamical mechanism to produce fibres with diameters even less than 100 nm (Frenot and Chronakis 2003). Under the influence of an electric field, a pendant droplet of the polymer solution at the spinneret is deformed into a conical shape. Such deformation is dependent on solvent physical properties like good miscibility, low boiling point, low surface tension, high conductivity and low dielectric constant (Salem 2007).
The electrospinning of technical grade composite nanofibres is still challenging and attractive for technical applications like biomedical, filtration, conductive and antimicrobial etc. Various synthetic & natural polymers such as; polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), chitosan, polyvinyl acetate (PVAc), polyacrylonitrile (PAN) based carbon fibre, polyvinylidene fluoride, polymethyl methacrylate (PMMA), polyamic acid are used for preparation of composite nanofibres. These polymer either blended or filled with nanoparticles or carbon nanotubes (CNTs) and have been successfully electrospun into composite nanofibres (Bhardwaj and Kundu 2010; Peresin et al. 2010; Choi et al. 2004, 2008; Ding et al. 2009; Geng et al. 2005; Luhrs et al. 2009; Sundaray et al. 2008; Jirsak et al. 2010; Wang et al. 2009; Ko 2006; Sundaray 2006). Out of these, polyvinyl alcohol (PVA) is one of the most popular polymer, which is employed as a matrix due to its high solubility in water and its good compatibility with many salts, including zinc acetate and copper nitrate (Chuangchote and Supaphol 2006; He et al. 2010).
Now a day, there is much demand and usage of high performance fibre carrying multi-functional characteristics. In this direction, electrospinning has been further explored for the generation of ceramic nanofibre. Various ceramic nanofibres mostly oxide based are fabricated by the combination of two conventional techniques: electrospinning and sol-gel (Yang et al. 2010; Lu et al. 2009). In sol-gel technique, mainly four steps involves; (1) particle to form desired colloidal solution, (2) Colloidal solution either sprayed, coated or electrospun on substrate, (3) sol particles to form gel in a state of a continuous network (4) Finally, pyrolyze the remaining organic or inorganic components to form an amorphous or crystalline ceramic compounds. Various oxide nanofibres and composites can be synthesized by this approach are MgO, ZnO, CuO, NiO, TiO2, SiO2, Cr2O3, Al2O3, SnO2, Fe2O3, NiFe2O4, LiFePO4 and Pt and in addition to these, hybrid fibres, strontium ferrite composite nanofibres and Mg & Zn mixed nanofibers can also be prepared by using this method (Lu et al. 2009; Wanga et al. 2010; Park et al. 2010; Kim et al. 2009; Hota et al. 2009; Sundarrajan and Ramakrishna 2007; Shukla and Kumar 2008; Yang 2007; Chen et al. 2010; Nakanea et al. 2013.; Dai et al. 2013; Ayku et al. 2013). Preparation and magnetic properties of lanthanum- and cobalt-codoped M-type strontium ferrite nanofibres was recently fabricated by electrospinning of solution, which exhibits significant improvements in the magnetic properties than the nanoparticles obtained by the sol-gel process (Moallemian et al. 2013). Therefore, not much importance has been paid to sol-gel process due to its certain limitation e.g. weak bonding and low degree of functional properties, low wear-resistance and high permeability.
Hence, now attention has been focused to electrospinning the ceramic nanofibre from base ceramic polymers. Generally, ceramic nanofibres are made by the electrospinning of ceramic precursors followed by calcination at higher temperatures (Teo and Ramakrishna 2009; Goldstein 2004; Huanga et al. 2003). In fact, Silicon carbide (SiC) is one of the well-known non-oxide ceramics used as a high-temperature structural material and it’s also demonstrate numerous potential applications in the field of high-tech application areas like aerospace, composite materials, semiconducting devices. These high technical applications are mainly due to their excellent properties like high thermal conductivity, high thermal stability, high strength & hardness and good resistance to oxidation and corrosion (Wallenberger et al. 1999; Shin et al. 2008). In this segment, Polycarbosilane (PCS) is the promising material for fabricating Silicon carbide (SiC) nanofibrous ceramic webs. SiC fibres are derived from PCS with the process, which is more over similar to that of carbon fibre preparation process. The first invention of SiC fibres are oxygen containing fibre having poor thermal stability and second generation SiC fibres are defined by oxygen free carbon with Silicon in the form of SiC microstructure is stable at higher temperature (Wallenberger et al. 1999). Additionally, due to high surface area to mass ratio, an enhanced efficiency of the ceramic nanoweb could be exploited. Various previous researchers reported about pure SiC web formation and its composites. Moreover, fabrication of pure SiC nanowebs was done by syringe and needle type electrospinning set up and also by solution blowing method (Shin et al. 2008). But the conventional nozzle based electrospinning process has significantly lower production rate (0.1–0.2 g/hour) as compared to the needle-less electrospinning process (1–2 g/min).
Looking at the governance of SiC fibres in various applications, an attempt was made herewith for development of pure SiC nanofibrous web through needle-less technique. This research work is carried out on a latest electrospinning technique having very high throughput rate. The present work has additional aim to fabricate SiC nanofibrous web having well connected fibres leading to pores network, which may be exploited for filtration purpose. This developed nanofibrous SiC web was subjected for evaluation of various characteristics using FESEM, Thermal Analysis, X-ray Diffraction, Energy Dispersive Spectroscopy (EDX), Atomic Force Microscopy (AFM) and Surface Profilomertry. A better understanding of the process and techniques, opens the future path for further researching on nanofibrous coating of various densities on high performance fabric of different morphological structures. In summary, an optimized electrospinning process was established to achieve desired properties of SiC nanofibrous (high performance fibre) web at a very high throughput rate (g/m2). Such high performance SiC web is obtained by precise control of various process parameters and that could be noted as a high point of this study.
Spinnable and amorphous grade of Polycarbosilane (PCS) with average molecular weight of 800 was used as basic polymeric material. The solvent mixture of DMF and toluene in ratio of 20% (v/v) was prepared by gentle stirring for 12 hours at ambient temperature. Subsequently, PCS was added to the mixed solvent at concentration of 1.2 g/ml and stirred for 24 hour at room temperature to obtain uniform solution, which is described elsewhere (Shin et al. 2008).
Electrospinning, curing and pyrolization
Surface morphology and diameter characterization
The morphology of electrospun fibres were examined by using the Field Emission Scanning Electron Microscopy (Zeiss EV 050). The diameters of nanofibres were estimated by Image J software using the captured FESEM images. The average fibre diameter was determined from 30 measurements of the random fibres taken from different areas of the scanned images. The 3D structure of nanofibrous web is investigated by Atomic Force Microscope (Agilent 5500 SPM AFM). The FESEM integrated with a Phoenix Energy Dispersive X-ray (EDX) detector was used to analyze the surface structure of pyrolized web.
Thermogravimetric analysis (Universal V47A TA) is employed to monitor the changes, which occurred during heating of web under nitrogen (N2) atmosphere with a heating rate of 10°C/min with a temperature range between 30–1000°C. The obtained TGA curves are interpreted for the thermal behavior of the samples by matching the endothermic peaks. Moreover, TGA experiments were performed under identical conditions of pyrolization.
Differential Scanning Calorimetry (DSC Q 200 V24.8) was used to measure the physical properties of samples under N2 atmosphere with a heating rate of 10°C/min in a temperature range between 30–300°C.
To understand the structural properties, the Wide Angle X-ray (WAXD) diffraction traces of webs are recorded with a Phillips diffractometer (Model like Expert Pro) using copper K alpha radiation in reflectance mode (40 Kv/20 mA) at a scanning speed of 0.02 degrees/second. The samples are scanned for 2θ angle from 10 to 80 degrees.
The topography (roughness) of the nanofibrous samples were measured by μScan (Nano Focus Inc.) using a confocal sensor for as-spun to pyrolized webs.
Morphology of as-spun, cured and pyrolized nanofibrous webs
Thermal behaviour of webs
Structural identification of PCS and SiC
The EDX plot of SiC nanofibrous web is s presented in Figure 7. The spectrum revealed the existence of Si and C in the nanofibres. The EDX mappings of C/Si ratio of 0.97 (approximately 1), which clearly reveals the formation of microcrystalline SiC on pyrolysis of PCS (Wallenberger et al. 1999; Lu et al. 2010).
Surface profilemetry analysis
In the present study, we successfully fabricated and evaluated the functional properties of PCS nanofibre web by using latest Nano Spider machine, having very high throughput rate. A precise degree of process control was carried out to fabricate such SiC web. The fabricated SiC web is having average fibre diameters of approximately 1.5 μm. Heat treatment has significant influence on the morphology of PCS nanofibrous web. The as-spun PCS fibre is straight and becomes thinner & more irregular after pyrolization. SiC nanofibrous web is having interconnected fibre and leading to pores network and it is also confirmed that surface porous structure is deeply rooted, as revealed by 3D AFM image. The formation of pores network by the interconnected fibres could be exploited for filter applications. There are chances of new crystalline morphology formation during electrospinning of as-spun PCS web as indicated by the XRD and DSC. The formation and growth of new crystalline morphological structure is due to stress on the PCS fluid during the high speed electrospinning process. Surface profilemetry revealed increase in surface roughness with the heat treatment of PCS webs. The SiC web is having highest roughness followed by cured and as-spun PCS webs, as it is confirmed by surface profilemetry in quantitative term and values. A thermally stable SiC web is fabricated, as confirmed by TGA analysis. DSC and TGA results correlate to each other. The ratio of C/Si is approximately 1 in SiC nanofibrous web, confirming high yield of SiC fibres without any impurities or other compounds. In summary, an optimized process was established, which is having scope for bulk production and also explore light weight nano coating on various high performance fabrics.
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