Preparation of Nanofibres from Phenol Liquefied Wood by Electrospinning

1. Introduction

Based on the perspective of abundant raw material sources and sustainable development, liquefied wood (LW) originating from wood biomass has been widely used to prepare various functional materials such as carbon fibres [1], activated carbon fibres [2], adhesive [3], film [4], foam [5] and so on. In this study, using liquefied wood, carbon fibres were successfully prepared by melt spinning, which had a tensile strength of 1.7 GPa [6]. In our laboratory, activated carbon fibres with a specific surface area of more than 2000 m²/g were also prepared by liquefied wood using melt spinning technology [7, 8]. Nanofibres with a high specific surface area in combination with different functionalities in developing filters [9], energy storage [10] and biosensor devices [11] have recently attracted much attention in both academic and commercial areas.

Electrospinning, which obtains fibre diameters in the range of several tens of nanometres to several microns, is a flexible and promising method to produce nanofibres using polymer solutions. Unlike conventional fibre melt spinning techniques, polymers are driven by an electrical force rather than a mechanical force and follow a different thinning mechanism via electrospinning. Typically, a container with a millimetre-sized nozzle is placed with a polymer melt or solution that is subjected to electric fields of several kV up to 40 kV. An anode connected to a syringe that contains polymer solution or melt also connects to a high voltage power supply. The cathode is connected to the collector, which is placed a certain distance from the syringe capillary tip, arranged from 50 mm to 250 mm. In this study, when the polymer solution was fed into the electric field, it became a hemispherical drop at the end of the capillary tip due to the surface tension. Increasing the applied voltage caused a distortion on the droplet due to the increasing charge on it. Afterwards, the polymer fluids were stretched under the electric field forces. Finally, the solvent evaporated and the nanofibres were collected.

Over the past 20 years, more than 100 types of polymers have been used to produce nanofibres such as polyurethane [12], polyethylene oxide and polyvinyl alcohol [13]. Wood fibre materials, such as lignin-based nanofibres, have also been successfully prepared by electrospinning hardwood acetic acid lignin solution [14]. Similar to conventional carbon fibre production, polyacrylonitrile (PAN) is the most commonly used precursor polymer and has also successfully prepared nanofibres in recent publications by electrospinning [15]. The spinnability of polymer solution is known to strongly depend on the properties of the spinning solutions, processing conditions, atmosphere conditions and so on, in which the properties of the spinning solution, including surface tension, electrical conductivity and viscosity, will be important parameters. Typically, polymer solutions that are used for electrospinning have an average molecular weight of more than several hundred thousand. However, as a type of new material, phenol liquefied wood is a kind of sticky fluid with rich phenolic groups. Its main compositions, including cellulosic oligosaccharide, result in a fluid with a relatively low average molecular weight, ranging from a few hundred to several thousand. Theoretically, it is difficult for liquefied wood to be converted into nanofibres by means of electrospinning. However, blending an unspinnable polymer with other easy-to-spin polymers is a commonly used electrospinning approach. For example, chitosan nanofibres have previously been fabricated using the electrospinning process by mixing treated chitosan with poly vinyl alcohol (8 wt% in water) at different ratios [16].

Polyvinyl pyrrolidone (PVP) consists of carbon, oxygen, hydrogen and nitrogen in the repeating unit. It is a well-known polymer that can be used for electrospinning due to its great adhesion, complexation properties and solubility in water and various organic solvents. PVP is typically used in the fabrication of fibres via electrospinning with other materials that are unspinnable, acting as a polymer carrier or a partner. For example, many metal oxide nanoparticles, such as zinc ferrite (ZnFe₂O₄) [17] and Tungsten oxide (WO₃) [18], have been blended with PVP to fabricate various kinds of composite nanofibres by electrospinning. Ethanol, as a type of nontoxic, volatile and economic solvent with a high dielectric constant, is frequently used to dissolve PVP for electrospinning. In the present study, macromolecular PVP was dissolved in ethanol to prepare a solvent for electrospinning. Aiming at the highly effective utilization of wood-based biomass, PVP was used as an additive to increase the average molecular weight of the liquefied wood. In addition, the effects of PVP on the morphologies of electrospun nanofibres and the properties of spinning solution were investigated.

2. Materials and Methods

First, oven-dried fir (Cunninghamia lanceolata) was ground and screened to the size of 60–80 mesh. The wood flour was mixed with phenol and phosphoric acid at a mass ratio of 1:6:0.48. The mixture was loaded in a 500 mL three-neck round-bottom flask and then heated at 160°C in an oil bath for 2.5 hours with continuous stirring. Afterwards, the liquefied wood was obtained. PVP with a molecular weight of 1,300,000 was dissolved in ethanol using a laboratorial magnetic stirrer for 5 hours to prepare the blend solution, which was then used as a solvent. PVP of 6% and 8% by weight was added to the solvent. Next, the electrospinning solution was obtained by mixing the liquefied wood and solvent together. The ratios of liquefied wood to solvent of 1:1 and 7:3 by weight were defined as the electrospinning solution concentrations of 50% and 70%. Therefore, the actual amount of PVP in the four types of LW/PVP/ethanol mixtures were 1.8%, 2.4%, 3% and 4% by weight, respectively. Electrospinning was performed by a self-produced spinning device in our laboratory. For this, the syringe pump and high voltage power were purchased from Beijing Huateng Chemical Equipment Company, Ltd. The anode (positive terminal) that connected the high voltage power supply also connected to the syringe containing the spinning solution. The varied applied voltage caused a distortion on the droplet due to the varied charge on it. By controlling electrostatic, it was possible to form the well-known Taylor cone. Next, the polymer fluids were stretched under electric field forces. The electrospun fibres were then collected on aluminium foil paper. In this case, the tip-to-collector distance was 150 mm during the spinning process, and the spinning voltage varied from 16 to 17 kV, depending on the tip geometry. The spinning solution was fed with a flow rate of 1.50 mL/h by setting up the syringe pump. The morphologies of the resulting nanofibres were characterized by a scanning electron microscopy (SEM, S-3400N) at an accelerating voltage of 5.0 kV.

The effects of the properties of electrospinning solution on the morphologies of the obtained fibres, including surface tension, electrical conductivity and viscosity, were investigated. First, an automatic surface tension metre (QBZY-Automatic Surface Tension Meter) was used to measure the surface tension. Next, the electrical conductivity was measured using a conductivity metre (DDS-11AW Microprocessor Conductivity Meter) at room temperature. Finally, the viscosity of the solution was measured using a rheometer (HAAKE Roto Visco I) at the same shear rate of 100 s⁻¹. All of the tests were carried out three times at around 22°C in order to obtain mean values.

3. Results and Discussion

3.1 Surface morphology

The morphologies of the electrospun nanofibres prepared from PVP/ethanol, as well as the four types of LW/PVP mixed spinning solutions, are shown in Fig.1. It can be observed from Fig. 1a that fine and uniform fibres with a diameter of around 400 nanometres were obtained, which were prepared from 6%PVP/ethanol solution. Similar results can be seen from Fig. 1d, in which the nanofibres were made from 8%PVP/ethanol solution with a diameter of 900 nanometres. With the increasing content of PVP in the spinning solution, the diameters of the electrospun nanofibres increased by roughly two times. It was reported that the diameters of the as-spun fibres from the PVP/ethanol solutions increased from 120 to 250, 400, 750 and 1500 nm, with the PVP concentration increasing from 2 to 4, 6, 8 and 10%, respectively [19]. Obviously, in our case, for the pure PVP/ethanol spinning solution, the results were approximately in-line with previous reports. This proved that the concentration of macro-molecular polymer would play an important role in the morphology of the nanofibres prepared via the electrospinning process. It is observed that the morphology of electrospun nanofibres was still favourable as liquefied wood and the PVP ethanol solution was mixed at a ratio of 1:1 by weight (Figs. 1b and e). There are no beads or crossing points in either of them. However, compared with the fibres in Fig. 1b, the fibres shown in Fig. 1e are more adhesive. In addition, compared with those of pure PVP/ethanol fibres, the diameters of these fibres increased drastically and were recorded as approximately 2.0–4.0 μm. This may have been caused by the structural alteration of the molecular in the fluid. It has been estimated that the type of polymer plays a vital role in dominating the size of the fibre diameters. It has also been reported that wood fibre materials, such as lignin-based nanofibres, were prepared by electrospinning 35/40% hardwood acetic acid lignin (M_w=7851g/mol) solution with diameters about 1.30 and 1.51 μm, respectively [14]. In this case, the value of the diameters was similar to our results but slightly lower. This may have been caused by the choice of different solvents, as in their study, acetic acid (AcOH) rather than ethanol, was used as a solvent. As for the morphologies of nanofibres prepared by the blends of PVP/ethanol solution and liquefied wood at a mass ratio of 3:7, the results showed that there were many net-like structures with a large number of crossing points interspersed among the fibres, as shown in Figs. 1c and f. At the same time, when the proportion of PVP in the ethanol was increased from 6% to 8%, the nanofibres prepared from the PVP/LW/ethanol blend solutions with 50% or 70% liquefied wood showed greater numbers of crossing points. Additionally, the diameters of the fibres in Fig. 1e became uniform and the fibres became more adhesive. The conglutination and formation in the uniformity of fibre morphology may be caused by the rising content of macromolecular PVP, which may have clustered together in the fluid. Furthermore, with the increasing content of liquefied wood, the vaporable ingredients (ethanol) in the blend solution decreased and there were more remnants (micro-molecular liquefied wood) sticking to the fibres. The results showed that the nanofibres derived from liquefied wood via the electrospinning process could be obtained satisfactorily by adding PVP of 6% in the synthesizing electrospinning solvent (ethanol). The best fibre formation belonged to the nanofibres that were electrospun from the solution that contained 50/50% of blend solvent (6%PVP/ethanol)/liquefied wood. In this case, the diameters of the as-spun fibres were recorded in the range of 2.0±0.5 μm. Compared with the fibres that were prepared from liquefied wood by melt spinning with the average diameter of 27∼42 μm [20], the electrospun nanofibres became fine in significant measure.

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Figure 1.

SEM images showing morphologies of electrospun PVP and LW/PVP blend nanofibres: (a) 6% PVP, (b) 6% PVP-50%LW, (c) 6%PVP-70%LW, (d) 8%PVP, (e) 8% PVP-50%LW, (f) 8%PVP-70%LW

3.2 Properties of the molutions

It is well known that the properties of an electrospinning solution, including surface tension, electrical conductivity and viscosity, play vital roles in its spinnability. It is theoretically impossible for liquefied wood, which is composed of many relative low-molecular carbohydrates and phenolic group-rich polymers, to undergo electrospinning without other macromolecular additives. To prepare liquefied wood nanofibres, PVP with an average molecular weight of 1,300,000 g/mol was considered as an additive to synthesize an electrospinning solution. Surface tension, which overcomes the electric force in high voltage electric fields to form a Taylor cone, is closely related to the spinnability of the solution. The surface tensions of electrospinning solutions, increasing from 0.022 N/m to 0.033 N/m with the rising concentration of liquefied wood, are exhibited in Fig. 2. This may have been caused by the decreased proportion of ethanol with a low surface tension of around 0.022 N/m. As for the different proportion of PVP in the same LW concentration, the variations of surface tension were not obvious, as there were minor discrepancies in the actual amounts of PVP in the mixtures. The electrospinning nanofibres derived from the PVP/ethanol blend solutions, which had a surface tension of about 0.022 N/m, were finer and denser than those derived from the liquefied wood electrospinning solution ones (shown in Fig. 1). The higher surface tension led to a stronger resistance to the force of electric field during the electrospinning process. This may reasonably account for the fine and uniform nanofibres from the PVP/ethanol solutions but not the PVP/ethanol/LW blend solutions. On the other hand, the introduction of the PVP/ethanol solution could have lowered the surface tension of the liquefied wood, due to the addition of ethanol. It became much more easy for the electrically charged fluid to overcome the enhanced surface tension when the solvent (PVP/ethanol) was added to the liquefied wood. Therefore, the elongational property of the fluid improved, thus leading to fewer crossing points.

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Figure 2.

Surface tension of synthesizing

The parameters of synthesizing the electrospinning solutions from LW and diameters of the electrospun fibres are shown in Table 1. The conductivities of the spinning solution slightly increased with the addition of liquefied wood. However, there were small differences in the conductivities with the additive amount of liquefied wood. The results showed that the conductivity in the range of 8×10⁻⁴ −12×10⁻⁴ S/m was suitable for the blend solution to prepare the fibres. It was observed that the viscosities of the spinning solution decreased remarkably with the addition of liquefied wood. Additionally, the viscosity of 8%PVP-50%LW spinning solution was about three times that of the 6%PVP-50%LW spinning solution. From this computation, when the liquefied wood was added, the PVP, as the main actor of the viscosity of the blend solution, was diluted to a large extent, leading to a sharp decline in viscosity. Moreover, the reduction of viscosity may also have been caused by the addition of micro-molecular liquefied wood, which resulted in the lower average molecular weight of the blend solutions. This helped to explain the increased fibre diameter (shown in Fig. 1) when the liquefied wood was added to the solution. Fortunately, the electrospun fibres with a diameter of around 1.0–2.0 μm were successfully obtained at a voltage of 16 kV by mixing the liquefied wood with 6%PVP/ethanol solution. The results showed that it was feasible to obtain electrospun fibres by introducing a certain amount of macro-molecular polymer into the liquefied wood, provided that the conductivity and viscosity of the spinning solution were within a certain range.

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Table 1.

Parameters of synthesizing electrospinning solutions from LW

Spinning Solution	Conductivity (10−4 S/m)	Viscosity (10−3 Pa·s)	Diameter (μm)
6%PVP	8	129	0.4±0.1
6%PVP-50%LW	12	4	2.0±0.5
6%PVP-70%LW	11	5	1.5±0.5
8%PVP	8	379	0.9±0.1
8%PVP-50%LW	9	12	4.0±0.5
8%PVP-70%LW	11	9	…
100%LW	4	13	…

4. Conclusions

Electrospun fibres were successfully prepared from liquefied wood by synthesizing an electrospinning solution. This was achieved by adding PVP of 6%. The morphologies of the electrospun fibres with a diameter of 2.0 μm were favourable, as the liquefied wood and 6%PVP/ethanol solution were mixed at a ratio of 1:1 by weight. With the increasing concentration of PVP/ethanol solvent, the surface tension of the blend electrospinning solution decreased. Furthermore, the conductivity slightly increased and the viscosity decreased significantly. This helped the electrical force overcome both the surface tension and viscoelastic forces to stretch the spinning solution into the fibre jet. These characteristics were considered to effectively improve the spinnability of the liquefied wood. Compared with previously reported fibres prepared from liquefied wood by melt spinning, the electrospun nanofibres in this paper became fine in significant measure. In addition, as they are derived from liquefied wood, electrospun fibres have versatile raw material sources. From the perspective of sustainable development, they are a potential renewable and cost effective biomass material.

5. Conflict of Interests

The authors declare that there is no conflict of interest regarding the publication of this article.