Polyvinyl alcohol (PVA); Dimethylformamide (DMF); * Unit for modulus values is MPa unless otherwise stated.
Electrospun CNT/polymer composite nanofibres and their mechanical strength
Open access
Published: 01 February 2010
DOI: 10.5772/8160
With the rapid development in nanoscience and nanotechnology, there is an ever increasing demand for polymer fibres of diameters down to a nanometre scale having multiple functionalities. Electrospinning, as a simple and efficient nanofibre-making technology, has been used to produce polymer nanofibres for diverse applications. Electrospun nanofibres based on polymer/carbon nanotube (CNT) composites are very attractive multifunctional nanomaterials because they combine the remarkable mechanical and electronic properties of CNTs and the confinement-enhanced CNTs alignment within the nanofibre structure, which could greatly improve the fibre mechanical, electrical and thermal properties. In this chapter, we summarise recent research progress on electrospun CNTs/polymer nanofibres, with an emphasis on fibre mechanical properties and structure-property attributes. Outlook towards the challenge and future directions in this field is also presented.
Carbon nanotubes (CNTs) which were first reported by Oberlin
Structure of carbon nanotubes
Carbon nanotube is a new form of carbon which has a seamless hollow cylindrical structure, consisting of carbon hexagons with both ends capped with fullerene molecule. There are two general types of carbon nanotubes, namely single walled nanotubes (SWNTs) and multi walled nanotubes (MWNTs). SWNTs consist of one single layer of hexagonal carbon atom rolled into tubular form the diameter of which ranges from 0.4 to over 3 nm, while MWNTs have several concentric cylinders with diameter from 1.4 to over 100 nm [3]. SWNTs are varied in chiral angle i.e. the angle at which the atoms of CNTs are twisted about the main axis of the CNTs. According to their chiral angle and diameters, SWNTs are classified to three forms of structures including armchair, zigzag and intermediate [4] as shown in Figure 1. The chiral angle governs the electrical conductivity. While the armchair structure exhibits very high conductivity, the zigzag and the intermediate forms show semi-conductivity.
Several techniques have been developed to synthesise SWNTs and MWNTs. The most used methods are carbon arc discharge [2], laser ablation of carbon [5] and chemical vapour deposition (on catalytic particles) [6]. Either vacuum or process gas is used in the production. The defining issue in selecting an appropriate method is the ability to produce nanotubes on a large scale. Advances in catalysis and continuous growth processes are making CNTs more commercially feasible.
Mechanical properties
CNTs combine outstanding mechanical, electronic, thermal properties, and low density. However, their extraordinary mechanical properties due to carbon-carbon sp2 bonds and cylinder structure set them apart from many other different materials and other forms of carbons. Soon after Ijima’s discovery and before the large scale production of CNTs, computer simulation was used to calculate the rigidity of SWNTs [7]. The calculated Young’s modulus was 1500 GPa. However, later studies predicted that Young’s modulus of CNTs is approximately 1 TPa [8]. It was not until 1997 that the first direct mechanical measurement was carried out on arc-MWNTs using atomic force microscopy and an average Young’s modulus value of 1.28 TPa was obtained [9]. The highest Young’s modulus and tensile strength measured for MWNTs produced by chemical vapour deposition method is 0.45 TPa and 4 GPa, respectively. The highest measured tensile strength for arc-MWNTs is 63 GPa [10]. Mechanical measurements on SWNTs did not commence until the late 1990’s due to difficulties in handling them [11]. The highest measured values for Young’s modulus and tensile strength of SWNTs are 1.47 TPa and 52 GPa, respectively [12]. The tensile strength of SWNTs could be more than 5 times higher than that of a steel fibre with the same diameter, yet only one-sixth of its density [13][14].
Electronic properties
The remarkable electronic properties of CNTs make them particularly attractive for the creation of miniaturized electronic component. While the resistivity values for high quality graphite, the measured resistivity copper is approximately 0.40 and 0.017 µΩm, respectively, and the measured resistivity of CNTs falls into the range of 0.05 µΩm ~ 10 mΩm [15]. Due to structural defects catalytically produced CNTs are expected to have a higher resistivity, similar to those of disordered carbon (i.e of the order of 10~100 µΩm). Earlier studies suggested that electronic properties of CNTs could vary widely from tube to tube and they could be metallic or semiconducting depending on their structure (
Carbon nanotubes/ polymer composites
Because of the exceptional properties and large aspect ratio, incorporation of nanotubes into polymer matrix has been proven to be a promising approach leading to structural materials and composites with excellent mechanical and physical properties. Various researches have been conducted and comprehensive reviews on mechanical properties of carbon nanotubes/polymer composites have been given by Coleman
The significant challenges in processing of CNTs/polymer composites lie in the uniform dispersion and orientation of nanotubes within the polymer matrix. CNTs tend to aggregate to form tight bundles due to strong van der Waals interactions and small size [24][25]. Dense and entangled CNTs network in the bundles prevent the CNTs from dispersing uniformly within the polymeric resins. As a result of uneven dispersion, the physical and mechanical properties of the composite material are considerably lower than the expected.
The difficulties in obtaining uniform dispersion were highlighted in the literature [26][27][28][29][30][31][32][33] and several techniques have been developed to improve the CNT dispersion. Ultrasonication has been used to de-aggregate CNT bundles and force the nanotubes to disperse uniformly throughout the material [27]. Other investigators have utilised solution-evaporation methods with high-energy sonication [26], surfactant-assisted processing through the formation of a colloidal intermediate [34][35] and melt spinning [36], as well as mechanical stretching of nanotube/polymer composites [37][38]. Others considered deposition of carbon nanotubes suspension under a magnetic field [39] and onto chemically modified substrate [40]. The most commonly used techniques to fabricate CNT/polymer composites are solution casting [41][42], melt processing [43][44], electrospinning [45] and
One of the interesting features for CNTs is the nucleation crystallisation of surrounding polymer molecules when the CNTs are dispersed into some polymer matrices. It has been observed that the introduction of CNTs to some polymer matrices alters the crystallisation dynamics of polymers [42][50]. In other words, the presence of CNTs induces crystallisation of host polymer. As a result of the crystalline polymer layer formed around the embedded nanotubes, the interaction between the CNT and polymer matrix is enhanced leading to improved mechanical properties.
Since 1998, several publications have reported the nucleation crystallinity of polymer in CNT/polymer composites [51][52][53][54][55][56][57]. The transmission electron microscopy (TEM) studies by McCarthy
It was also reported that the presence of CNTs in polyvinyl alcohol (PVA) improved the composite mechanical properties more than in other polymer systems [42][64][65][66][67] [68][69]. The nucleation crystallisation of PVA was confirmed by microscopic investigations of fractured PVA composite film, which showed that a thick PVA coating covered the nanotubes [69]. It has been established that the presence of ordered polymer coating is the main reason leading to the enhanced mechanical strength [66].
Electrospinning is a relatively low cost, fast and versatile method to produce continuous nanofibres mainly from polymer solutions. This technique has not been well studied until last decade even though it was invented in 1934 [70].
Basic electrospinning principle
A basic electrospinning setup, as shown in Figure 2a, consists of a container for polymer solution, a high-voltage power supply, spinneret (needle) and an electrode collector. During electrospinning, a high electric voltage is applied to the polymer solution and the electrode collector leading to the formation of a cone-shaped solution droplet at the tip of the spinneret, so called “Taylor cone” [71]. A solution jet is created when the voltage reaches a critical value, typically 5-20 kV, at which the electrical forces overcome the surface tension of the polymer solution. Under the action of the high electric field, the polymer jet starts bending or whipping around stretching it thinner. Solvent evaporation from the jet results in dry/semidry fibres which randomly deposit onto the collector forming a nonwoven nanofibre web in the most cases (Figure 2b).
Extensive research has been carried out on various aspects of electrospinning including operating parameters (e.g. applied voltage, feeding rate, distance between the nozzle and collector), material properties (e.g. viscosity, surface tension, conductivity), spinningability of many different polymers [72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87] [88], process modelling [88] [89] [90] [91] [92] [93] [94] [95], nanofibre characterisations and morphology [87].
Electrospun nanofibres
Fibres obtained from electrospinning vary from uniform fibres to fibres with an irregular beads-on-string structure. The morphology of the electrospun fibres are dependent upon a number of factors including the polymer solution parameters such as molecular weight, molecular weight distribution, electrical conductivity, surface tension, viscosity and solvent, and the operating parameters such as electrical field, the distance from the nozzle tip and the collector and the flow rate of the polymer, as well as ambient conditions [84][96][97]. The diameter of electrospun fibres can be in the range between several microns to tens of nanometres. The small fibre diameter and large aspect ratio lead to extremely high surface-to-volume (weight) ratio, which makes the electrospun nanofibre desirable for many applications [98]. Recently, several review articles have been published on electrospinning [99][100][101], which demonstrate the great potential of electrospun nanofibres in diverse application fields.
Mechanical properties of electrospun polymer nanofibres
Although several experimental investigations have been carried out on the mechanical properties of nanofibre mats, only a few studies have been reported on the stress-strain behaviour of single electrospun nanofibres [102][103][104]. These studies have demonstrated that single electrospun nanofibres have promising mechanical properties. Gu
CNT/polymer composites have been fabricated by a number of processing methods including melt processing, solution processing and
It has been established that electrospinning a polymer solution containing well-dispersed carbon nanotubes leads to nanocomposite fibres with the embedded carbon nanotubes orienting parallel to the nanofibre axis due to large shear forces in a fast fibre-drawing process [106]. It has been demonstrated that electrospinning is a potential method for aligning and debundling CNTs [107]. The improved CNT alignment within the fibres plus low cost and fairly simple spinning process has made this technique promising for producing CNT/polymer composite nanofibres.
The reported electrospun CNT/polymer composites including corresponding solvent and CNT concentration are listed in table 1. While CNTs have potential to be embedded into various polymer matrices, some polymers cannot be easily electrospun into nanofibres.
Carbon nanotube dispersion
A common method for preparing CNT/polymer solution for electrospinning involves making nanotube dispersion and polymer solution separately and then mixing them together. In general, interest has been focused on achieving homogenous nanotube dispersion in a polymer solution, which will affect the orientation and distribution of CNTs in the resultant nanofibres.
Polymers | Solvents | CNTs | CNTs (wt%) |
Diameter (nm) |
Tensile strength* (MPa) (Modulus, MPa) |
Ref |
PAN | -------- | SWNTs/ MWNTs |
1 | 180 | -------- | [108] |
PAN | DMF | MWNTs | 1-20 | 50~300 | 285~312 (6.4~14.5GPa) |
[109] |
PAN | DMF | MWNTs | 1.5,7 | 20~140 | ------ | [110] |
PAN | DMF | SWNTs | 1-10 | 50~400 | 20~30 | [111] |
PAN | DMF | MWNTs | 1 | 200~2000 | -------- | [112] |
PAN | DMF | SWNTs | 1~4 | 50~200 | (140 GPa) | [45] |
PAN | DMF | MWNTs | 2-20 | 100-300 | 37-80 (2-4.4GPa) |
[113] |
PVA | Water | MWNTs | 4.5 | 295~429 | 4.2~12.9 | [114] |
PVA | Water | SWNTs | 10 | 315~447 | 5.9~6.0 | [115] |
PEO | Ethanol/Water | SWNTs | 3 | ---- | (0.7~1.7GPa) | [116] |
Polymethyl methacrylate | Chloroform | MWNTs | 0.5-2 | 200~6000 | ------ | [117] |
Polymethyl methacrylate | DMF | MWNTs | 1-5 | 100~800 | ------ | [118] |
Polyurethane | DMF | SWNTs | 1 | 50~100 | 10~15 | [119] |
Polycaprolactone | Chloroform/ Methanol | MWNTs | 7-15 | 100~550 | -------- | [120] |
Polylactic acid | DMF | SWNTs | 1-5 | 1000 | ------- | [121] |
Regenerated silk fibroin | Formic acid | SWNTs | 0.5-5 | 147 | 2.8-7.4 (180~705) |
[122] |
Regenerated silk | Formic acid | SWNTs | 1 | 147~153 | 13.9~58.0 (633.8~6549.3) |
[123] |
Polybutylene terephthalate | 1,1,1,3,3,3 Hexafluoro-2-propanol | MWNTs | 5 | 250~3500 | (1.79 GPa) | [124] |
Polycarbonate | Chloroform | MWNTs | 4 | 350 | ------ | [125] |
Nylon 6,6 | Formic acid | MWNTs | 2-20 | 150~200 | ------- | [126] |
Polystyrene | DMF/ tetrahydrofuran | MWNTs | 0.8,1.6 | 300,4500 | ---- | [127] |
The stable dispersion of CNTs can be achieved by using surfactants (e.g. sodium dodecyl sulphate), large amphiphilic polymers (e.g. polyvinyl pyrrolidone) and natural macromolecules (e.g. polysaccharide, Gum Arabic ) which can be adsorbed onto the hydrophobic nanotubes [116][128]. Nevertheless, the most common method for dispersing individual nanotubes is ultrasonication treatment. While ultrasonication is a valuable technique to overcome the entanglement of nanotubes and break up the agglomerates, it could also introduce defects and irregularity into the CNTs [128].
Purity is also an important factor affecting the composite quality. CNTs are typically purified by chemical methods to remove amorphous carbon and metal catalyst. The purification treatment also facilitates the dispersion of the CNTs in solvents as well as improves the nanotube-matrix interaction [15]. It was demonstrated that the caps of SWNTs could be removed by treatment with hot nitric acid due to the formation of carboxylic acid and hydroxyl groups at the nanotube ends [129][130]. Apart from this, the purified CNTs can be functionalised to get better dispersability and additional functions [131]. There are several approaches to chemical functionalisation of nanotubes [105][132]. Although functionalised CNTs showed improved dispersion, the electronic and photonic properties of nanotubes would be altered as well [133].
Structure and morphology
Alignment of individual nanotubes within the host polymer is a crucial step, in particular with the applications for reinforcement. Electrospinning is expected to make CNTs align with the fibre axis during the fibre formation process because of the high stretching ratio. Strategies to make the best use of this capability have been employed to improve the dispersion of CNTs in polymer solution using surface functionalised CNTs and ultrasonication treatment.
Salalha
In other similar studies [116][134], well dispersed SWNTs were incorporated into PEO nanofibres by electrospinning. It was shown that nanotube alignment within the nanofibres depended strongly on the quality of the initial dispersion. Ko
There have been several studies on controlling the spatial orientation of electrospun CNT/polymer composite nanofibres [109] [137] [138] [139] [140]. Modification of electrospinning setup has been a common approach to produce aligned electrospun composite nanofibres. This includes employing rotating collectors such as a disk or mandrel to collect nanofibres aligned in the rotating direction.
Ge
nanofibres had significantly smaller diameter, ranging from 250 nm to 750 nm, than the neat nylon-6 fibres and the fibre diameter reduced with the increase in the concentration of CNTs. This is consistent with what Ra
Huang
Zhang
Yee
Mechanical properties
Most studies on CNT/polymer composites have been driven by improving the mechanical strength. This is of particular importance for electrospun nanofibres, because the relatively low bulk mechanical properties hinder their applications in some areas. Due to the small size, measuring the tensile properties for individual electrospun nanofibres is difficult. A few experimental investigations on mechanical properties of electrospun CNT/polymer nanofibres have been reported [45][124][145][146]. In these studies, atomic force microscopy (AFM) has been used as a tool to study the mechanical behaviour of single electrospun composite nanofibres.
Ko
A novel method to study the mechanical deformation of electrospun composite nanofibres has been described by Kim
Ye
Elastic deformation of MWNTs in electrospun MWNTs/PEO and MWNTs/PVA nanofibres were studied by Zhou and co-workers [149]. The degree of elastic deformation was found to increase as the modulus of the polymer matrix increased. A simplified model was also proposed to estimate the elastic modulus ratio of MWNTs and polymers. To confirm the validity of the model, the results were compared with that from AFM measurement.
As for CNT reinforced electrospun nanofibre mats, several research groups studied the mechanical behaviour of electrospun nanofibre membranes. These studies have demonstrated that CNTs improve the mechanical properties of electrospun polymer nanofibres as long as they are well dispersed into the polymer matrix. For example stress-strain analysis showed that the tensile strength of SWNT reinforced polyurethane (PU) nanofibre membrane was enhanced by 46% compared to pure PU nanofibre mat [119]. However, this value was further increased by 104% for PU membranes containing ester-functionalized SWNTs. This improvement in the mechanical strength was attributed to improved dispersion of the SWNTs as well as enhanced interfacial interaction of nanotubes with the PU matrix because of modified nanotube surface [119]. Recently, Yoon
The upper limit of CNT concentration in electrospun nanofibres is also confined by the extent of CNT dispersion. Hou
The importance of fibre alignment on the mechanical properties has been well established. In a study by Jeong et al. [154], aligned electrospun MWNT/PVA membranes have been reported. The tensile strength of these membranes increased from 5.8 MPa to 12.9 MPa by adding 1wt% of MWNTs. In a recent study, however, Blond et al. [155] achieved a higher level of reinforcement. They produced aligned SWNT/PVA nanofibre membrane with the strength of up to 40 MPa using a rotating drum collector followed by mechanical stretching.
It has been demonstrated that CNTs nucleate crystallisation in CNT/polymer composite films [50][57][66][67]. The presences of crystalline polymer coating around the nanotubes significantly enhance the stress transfer and therefore the mechanical properties of composites [42]. It is normally believed that crystallisation of polymers is a slow process involving orientation of polymer molecules and solidification. Therefore, nucleate crystallisation of polymer should occur mainly in composite films that normally take a long time for evaporation of solvent during the film casting process, and a fast drying and solidification process, such as in electrospinning, could hinder the nucleation crystallisation because the polymer molecules have not sufficient time to orient around nanotubes. In a recent study, Naebe
Post-electrospinning treatment, using methanol for instance, was found to be an effective way to increase the mechanical properties of electrospun PVA nanofibres [156]. Naebe
Influence of polymer types
Different types of polymers, including semi-crystalline, amorphous and elastomeric polymers, have been used to fabricate CNT-containing composite nanofibres [119] [125] [127] [128] [145]. It was revealed that flow-induced crystallisation might have occurred during electrospinning of semi-crystalline polymers, and the polymer crystals were oriented along the fibre axis [128] [134]. On the other hand, it was shown that nanotubes aligned well during electrospinning of CNT/polymer nanofibres. Since the presence of oriented polymer crystals has a significant influence on mechanical properties, it is complicated to evaluate the real contribution of CNTs regarding the improvement in the mechanical performance of electrospun composite nanofibres.
With the amorphous polymers, only a few studies on CNT/polymer nanofibres have been reported [125] [127] [145]. Although enhanced mechanical properties were reported for the nanofibres, the role played by polymer morphologies (i.e. crystalline, amorphous, and rigid) was not fully understood.
Influence of carbon nanotube types
SWNTs and MWNTs differ from one another in their size and dispersability in solution and polymer matrix as well as in mechanical and electrical properties [3]. However, few papers have reported on the influence of CNT types on the structure-property relationship of electrospun nanofibres.
Dror et al. [128] and Salalha et al. [134] studied the effect of SWNTs and MWNTs on the formation of electrospun PEO nanofibres. On the basis of X-ray diffraction, it was demonstrated that while the PEO crystal orientation in electrospun nanofibres was not affected by the inclusion of SWNTs, the incorporation of MWNTs into PEO matrix had a detrimental effect on the degree of the crystal orientation. Nevertheless, no data on mechanical properties of CNT/PEO nanofibres was reported. Electrospun MWNT/PVA and SWNT/PVA nanofibres have been reported [114] [115]. It was observed that the SWNTs and MWNTs induced different crystal phases in the PVA. With the same CNT concentration, the tensile strength of MWNT/PVA nanofibres showed no significant difference to that of SWNT/PVA ones.
Electric and thermal properties
The formation of electrospun CNT/polymer nanofibres has been explored for possible improvement in the electrical and thermal properties of polymer. As for electrical conductivity, most polymers possess a very low conductivity and the presence of CNTs provides a platform for inherently conducting polymer nanofibres suitable for many applications. Incorporation of CNTs into polymer nanofibres was found to increase the electrical conductivity of composite nanofibres [109]. The electrical properties of electrospun MWNT/PAN composite fibres were investigated by two independent groups [109] [141]. Ge
Electrospun MWNT/nylon composite nanofibres were also prepared and the electrical properties were examined as a function of the filler concentration [126]. The MWNT/nylon nanofibres were electrospun on the ITO coated glass and a metal coated glass electrode was placed on the composite fibre sheet. The filler concentration was varied from 0 to 20 wt% and the I~V characteristics were examined. As shown in Figure 5, the I~V curve indicates a non-ohmic behaviour, which changed with the filler concentration. Similar electrical behaviour was also reported for SWNT/PVDF [157] and MWNT/PEO [158] composite nanofibres.
In an attempt to define the parameters that determine the conductivity of the nanofibre mats, McCullen
In a rather different approach to studying the electrical conductivity of polymer nanofibres, Kang
Sundaray and co-workers [117] described the electrical conductivity of single electrospun MWNT/PMMA composite nanofibres. Alignment of MWNTs in the direction of the fibre axis was confirmed by bright field TEM images. The room temperature DC electrical conductivity of an electrospun MWNT/PMMA fibre showed a ten-orders increase compared to pure PMMA fibre. Percolation threshold of the composite nanofibre was well below the 0.05% w/w of CNTs loading and the conductivity increased with increase in MWNT concentration.
Not many papers reported on the thermal properties of electrospun CNT/polymer composite nanofibres. Thermal analysis has been carried out on the electrospun composite nanofibres to understand the relationship between the presence of carbon nanotubes and thermal properties. It was indicated that the presence of CNTs enhanced the thermal stability of polymer nanofibres.
The effect of heat treatment on SWNT/PAN composite fibres was investigated using TEM by Ko
Applications
Electrospun nanofibres have a broad range of applications due to the combination of simplicity of fabrication process and their unique features. While several reviews on polymer nanofibre applications have been published [99][100][101][160], the works on CNT/polymer nanofibres have been mainly focused on developing a fundamental understanding of the fibre structure property relationships. Conducting electrospun CNT/polymer nanofibres have been demonstrated to be attractive for a large variety of potential applications, such as in optoelectronic and sensor devices [161]. For example, electrochemical biosensors were fabricated using electrospun MWNT/polymer composite nanofibres [162] [163]. In a recent study, the electrospun MWNT/poly(acrylonitrile-co-acrylic acid) nanofibres were found to enhance the maximum current of glucose oxide electrode and the enzyme electrode could be used several times without significant decrease in current [162]. Electrospun PVA nanofibres containing chitosan grafted MWNTs also exhibited sensory ability to hydrogen peroxide and potassium ferricyanide [163]. This nanofibre-based sensor demonstrated more sensitive response and intense current as well as faster electric charge transport than those of film-based sensors. Other potential applications of electrospun CNTs/polymer nanofibres include tissue engineering scaffolds, composite reinforcement, drug carriers for controlled release and energy storage. Given the advantages of CNT/polymer nanofibres in mentioned fields above, the number of investigations on these topics is very small.
The use of the electrospinning technique to incorporate carbon nanotubes (CNTs) into polymer nanofibres has been shown to induce alignment of the nanotubes within the polymer matrix, leading to significant improvements in fibre strength, modulus and electrical conductivity. To realise their commercial applications, considerable work is still required. This includes a thorough understanding of the structure–property relationship for various electrospun polymer nanofibres, the effective incorporation of carbon nanotubes into polymer fibres with a high loading content, and large scale production of composite nanofibres of consistent and high quality but at a low cost [164] [165] [166]. Core-shell CNT/polymer nanofibres are also a subject that warrant further research [167].
Published: 01 February 2010
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