The use of polymer–carbon nanotube composites in fibres
A general review of carbon nanotube (CNT)–polymer fibres is undertaken in this chapter with main efforts focusing on the mechanical, electrical and sensing properties of CNT–polymer fibres. The review introduces issues related to the preparation of CNT–polymer fibres and orientation of CNTs, necessary to fully exploit the mechanical and electrical properties of CNT–polymer fibres. It is demonstrated that CNTs have great potential for a wide range of applications. Their large aspect ratio, excellent electrical conductivity and ultra-high mechanical properties make them outstanding candidates to be multi-functional nanofillers for polymer fibres.
The discovery of ways to produce highly orientated polymers has given tremendous stimulus to both basic polymer science and industrial developments since the 1970s.1,2 The first successful methodology to produce ultra-stiff and strong fibre with the molecules aligned in the direction of the fibre axis was by Kwolek et al.3 Here, a solution-based method was used to spin fibres from rigid rod p-aramid molecules with outstanding stiffness and strength, which was eventually developed into a commercial product known as Kevlar®.
It has been known since the 1930s1 that a fully extended flexible polymer chain such as polyethylene (PE) would also be extremely stiff in the direction of the chain axis. Fully aligned chains have been achieved by two different methods: solution (gel) spinning or solid state deformation. The first successes were achieved by Capaccio and Ward,4 where an oriented polymer network was achieved by solid state drawing between Tg (glass transition temperature) and Tm (melting temperature). A number of subsequent studies on solid state drawing were then carried out on PE, but the real commercial breakthrough study was carried out by Smith and Lemstra5 using gel spinning. Here, ultra-high molecular weigh polyethylene (UHMWPE) was spun from a disentangled semi-diluted solution, which, after drawing, resulted in fibres of 90 GPa stiffness and 3 GPa strength. These fibres were developed into commercial products by DSM and its alliance Honeywell, under the trade name of Dyneema® and Spectra®, respectively. Major applications are in ropes, cables, protective clothes and helmets.6
Under the pressure of environmental and recycling issues, recent development in orientated polymers has concentrated on single polymer composites, where the same polymer is used as both the matrix and fibre in the composites. Two remarkable achievements from Ward et al.7 and Peijs and co-workers,8–11 based on all-PP composites, were developed into commercialized products under the trade names of Curv® and PURE®, respectively.
Due to the one-dimensional structure of carbon nanotubes (CNTs), oriented polymer–CNT composites fibres or tapes attracted much attention, as such an oriented system could result in a high mechanical reinforcing efficiency.12,13 Furthermore, the introduction of such nanofibres into oriented polymer systems can mimic the structure of natural materials such as bone and tooth, creating ‘designed’ composite materials with additional levels of hierarchy (see Fig. 21.1). Therefore, CNT–polymer fibres as a research topic have recently been widely investigated. Researchers are mainly focusing on the mechanical,13–17 electrical18–22 and sensing properties.23,24
There are two main categories of polymer–CNT fibres: one consists of polymer, where the CNT content is typically below 10 wt%; another type mainly contains CNTs. For the first category, to obtain a certain degree of orientation for the polymer matrix and CNTs, as-prepared CNT–polymer composites are often spun into fibres by different means, including gel spinning,25 electro-spinning,26–28 melt spinning,29–32 and solid state drawing.20–22,33,34 As expected, fibre spinning conditions play an important role on the final properties especially the speed, temperature and post-treatment.
For the second category, methods including wet spinning,35 dry spinning from CNT forests,36,37 and direct spinning from chemical vapour deposition (CVD) synthesis,38,39 have been used to produce neat CNT fibres (ribbons or yarns) in micrometre or millimetre size. These methods have the potential to be used to produce neat continuous CNT fibres or yarns on an industrial scale. Especially the method reported by Li et al.,39 where CNT fibres are directly collected from CVD reaction are of interest for future high-strength fibre development. The fairly simple procedure and high mechanical property expectation of CNTs open up the possibility of ultimately producing neat CNT fibres with comparable or even higher mechanical properties than widely used carbon fibres. Nevertheless, so far the mechanical properties of most neat CNT fibres are still well below those of carbon fibres and further study is still needed to optimize such a process.
Due to the one-dimensional structure of CNTs, oriented polymer–CNT composite fibres or tapes generate intense interest as such oriented systems could result in high mechanical reinforcing efficiency. Ajayan et al.40 were the first to consider drawing as a method to align CNTs in a polymer matrix. Since then, oriented polymer fibres or tapes containing CNTs have been extensively investigated. It is observed that CNTs could be aligned by process-induced shear, which could be caused by spinning or drawing.20,29 The orientation of CNTs in polymer fibres has been studied extensively by different methods, including Raman (see Fig. 21.2), SEM (see Fig. 21.3), and TEM,29 etc.
21.2 (a) Typical Raman spectra of an oriented CNT–polymer composite; each curve is the spectrum for the indicated tape orientation with respect to the incident polarization; (b) relative Raman intensity as a function of the angle ψ (0 – π/2) between the polarization direction and the sample axis for PVA-SWNT nanocomposite tapes at draw ratio 5, indicating a high degree of alignment of the nanotubes. Reprinted with permission from reference 13.
21.3 SEM of MWNT–co-PP composite surface for: (a) isotropic film; (b) solid-state drawn tape; (c) solid-state drawn plus annealed tape. Please note the relaxed anisotropic nanotube bundle structure in the latter. Reprinted with permission from reference 22.
Figure 21.2 presents a Raman study of the alignment of SWNTs. It has been observed that the Raman spectra intensity of the tangential mode G band (1500–1700 cm–1) monotonically decreases with increasing the angle between the drawing direction and the polarization direction of the polarizer for SWNTs, and this phenomenon can be used to demonstrate the orientation effect of CNTs.13
As shown in Fig. 21.3, MWNT networks can be observed by SEM at high accelerating voltage as previously reported by Loos et al.41 It is shown that MWNTs can be highly aligned after solid state drawing. This could be caused by the high shear rate during solid state drawing. Furthermore, interesting ‘relaxed hairy’ MWNT bundles have been observed after thermal annealing. The process is explained as following: after the solid-state drawing process, MWNT bundles are highly oriented and constrained in the surrounding polymer matrix. MWNTs in these bundles are entangled and under tension due to the deformation induced by the solid-state drawing process. Upon annealing above or close to the melting temperature of the polymer, these MWNT bundles, together with the polymer chains, have the ability to relax to a more isotropic state. Similar to the entropy-driven relaxation process in oriented polymers, the increase in polymer mobility allows the relaxation of the nanotube bundles, bringing more disorder into the system. As a result, local lateral contacts between MWNT bundles are created.21
Kumar et al.16 reported an interesting approach based on a rigid rod polymer poly-p-phenylenebenzobisoxazole (PBO). Here PBO was synthesized in the presence of SWNTs to produce lyotropic liquid crystalline solutions which were spun into composite fibres using dry jet wet spinning. The tensile strength of these PBO–SWNT fibres containing 10 wt% SWNTs was around 50% higher than that of control PBO fibres containing no SWNTs. However, it should be noted that the tensile strength of commercial PBO fibre (Zylon HM) is 5.8 GPa compared to 2.5 GPa as found in their control study.
Another interesting method used to align CNTs in polymer composite fibres is electrospinning. This technique has been used to produce man-made fibres since the 1930s and involves electrostatically driving a jet of polymer solution out of a nozzle onto a metallic counter electrode. In 2005, Zhou et al.42 described electrospinning as a method to fabricate PEO and poly(vinyl alcohol) (PVA) nanofibres containing MWNTs. Recently, Wang et al.26 and Kannan et al.43 showed good reinforcement efficiency of CNTs in such electrospun PVA fibres.
Due to their one-dimensional and outstanding mechanical properties, CNTs have become very interesting for reinforcing polymer fibres.12–14,16,18,19,25,27,29,33,34,44–53 However, it is difficult to compare the mechanical reinforcement results from the literature as there are many differences in these systems. It is shown that some basic models, such as the simple rule of mixture can be used to calculate and compare the effective mechanical properties of CNT in polymer matrix.13,15,26,48 Table 21.1 lists the calculated effective CNT modulus and strength from data reported in the literature. Again there are two categories of highly oriented CNTs fibres: one are fibres that mainly or solely consist of CNTs (such as work from Dalton et al.,49 Zhang et al.54 and Koziol et al.38); another type are polymer fibres containing small amounts of CNTs (typically < 10 wt%).
The highest mechanical properties of neat CNT spun fibre that have been reported are by Koziol et al.,38 where a modulus of 350 GPa and strength of 9 GPa are obtained. Its modulus is approaching the value of carbon fibre, while its best reported tensile strength value is indeed higher than any other man-made fibre. However, it needs to be noted that there is a large amount of scatter in their experimental data (see Table 21.1), while a 1 mm gage length is used in their tensile text experiments to minimize the influence of defects by CNT ends in the yarns. Nevertheless, the properties have yet to approach the theoretical values of CNTs of 1000 GPa for modulus and 50–150 GPa for tensile strength. The discontinuous nature of these CNT yarns is believed to be responsible for this as the strength is not determined by the intrinsic strength of the CNTs but by the nanotube-nanotube interactions and overlap length. Considering the variety of applications and importance of carbon fibre in a wide range of fields, such a spinning method reported by Koziol et al. to produce neat CNT fibre, the possibility of replacing carbon fibre is indeed exciting. However, more attention needs to be paid to the realistic potential and advantages of such CNT yarns compared to carbon fibres, rather than a one-to-one replacement.
The second category of polymer fibres contains relatively small amounts of CNTs. The first report of such a fibre is by Andrews et al.,55 where the modulus increased 150% and strength increased 90% after adding 5 wt% SWNT in a petroleum pitch matrix. Their fibre results in a back calculated modulus for the CNTs of 1296 GPa and strength of 13 GPa. The mechanical properties of polymer–CNT fibres based on PBO,16 PVA,1,13,77,81 PP47,56,57 and PAN,58,59 are investigated in the literature and are listed in Table 21.1. The great advantage of CNTs as mechanical reinforcing fibre is its extremely high tensile strength of nearly 150 GPa compared with 7 GPa for conventional carbon fibre. Modulus, on the other hand, is less of interest as a property as a Young’s modulus of 1000 GPa for CNTs is less impressive when compared with that of ultimate high modulus carbon fibres of 600–800 GPa. As listed in Table 21.1, only a few studies have achieved an effective tensile strength above 10 GPa.13,15,29,59–63 The highest effective reinforcement in strength so far was achieved by a group led by Kumar,63 where an effective strength of 116 GPa was achieved by gel spinning PVA–SWNT fibres. A slightly lower effective strength of 88 GPa was reported by Wang et al.13 earlier, where a threefold increase in strength was obtained by adding 1 wt% SWNT in an oriented PVA matrix (see Fig. 21.4). It was concluded that a high level of dispersion, interfacial interaction and alignment of nanofillers were essential to achieve this true mechanical reinforcement in CNT-based composites.13
21.4 Stress–strain curves for oriented PVA–SWNT tapes with a draw ratio of 5, showing a strong increase in tensile strength with the addition of small amounts of SWNTs to the polymer. Reprinted with permission from reference 13.
In terms of melt processed polymer–CNT fibres, it is demonstrated that better reinforcement is obtained at low filler content and low draw ratios.15,52 This is caused by poorer dispersion at higher filler content. As the fibres are transformed into a more organized structure at higher draw ratios, they are more sensitive to the cracks initiated by CNT bundles due to over-drawing of the system.
Another important factor to take into account is that for ultimate reinforcement the use of single-walled nanotubes (SWNTs) is essential. In terms of cost and ease of dispersion, multi-walled nanotubes (MWNTs) are the most attractive candidates to reinforce polymer composites. However, when nanotubes are used as reinforcing elements in composites, the total volume they occupy, including their hollow part, needs to be considered for their reinforcing efficiency.64 Especially in the case of MWNTs, where the stress needs to be transferred from the outer layer to the inner layers, there are implications for their effective properties. As the interaction between individual graphene layers is known to be low, this can easily result in telescopic interlayer sliding in MWNTs, leading to a significant reduction in the effective properties of MWNTs since only the outer layers carry the load.65
From a theoretical perspective, at small Vf(CNT) the composite properties are largely dominated by the polymer matrix. However, at a critical Vf(CNT) the properties are dominated by the CNTs. Let us envisage a stress–strain curve for a highly aligned and therefore linear elastic PE fibre and for a single-walled CNT (PE, E = 100 GPa, σ = 3.5 GPa, ε* = 3.5%1 and CNT, E = 971 GPa, σ = 126 GPa, ε* = 13%66,67) for example (Fig. 21.5).
At the failure strain of the matrix, a tensile stress of 34 GPa exists in the CNT. As the matrix fails, the load is transferred to the CNTs. However, if the Vf is low, the macroscopic load cannot be sustained and the composite fails. When Vf reaches a critical value, the CNTs can sustain the load (assuming load transfer remains). Therefore, a critical concentration exists for the transition from properties which are matrix dominated to properties which are CNT dominated.
Figure 21.6 describes the failure of a polymer matrix reinforced with SWNTs. The plot describes a specific system, where the strength of the matrix and SWNT concentration is set. Then, a universal plot of Ef/Em vs. Vf vs. yields a graph describing all reinforcing scenarios. Since we are describing only CNTs, the fibre/matrix property ratios are purely defined by the matrix choice.
21.6 Variation of composite tensile strength as a function of Vf. Below a critical volume fraction , the composite fails when the matrix fails, while above the composite fails when the fracture strength of SWNTs is reached.
Figure 21.7 demonstrates that for SWNT composites, low strength and high strain composites benefit the most from being reinforced by CNTs. Conversely, stiff matrices with low strains – typically the characteristics of a high-performance fibre – need a large volume fraction of CNTs to fully exploit the intrinsic strength of the CNT. Take, for example, PBO (Zylon HM, Toyobo), which has a tensile modulus of 270 GPa and failure strain of 2.5%. Here, a volume fraction of ~ 20% would be needed to fully exploit the potential strength of CNTs. Matrices such as i-PP (E = 15 GPa, σ = 500 MPa, ε = 4%9,10,68), however, require a far lower critical volume fraction at ~ 0.8%.
Wang et al.13 showed one of the highest contributions to σc from σCNT The matrix here is a low draw ratio (λ = 6) PVA. It is known, however, that PVA can be drawn to more than 20 times and show an E modulus of 70 GPa and σ of 2.3 GPa.1,2 Therefore, for PVA at a draw ratio of 6, is ~ 0.16%, but this increases to ~ 3.4% for a more commercially interesting PVA fibre drawn to λ = 21.
Therefore, to achieve a noticeable amount of increase in fibre strength for high performance polymer fibre, relatively large amounts of CNTs are often needed. However, it is well known that dispersing large amounts of CNTs in a polymer matrix is very difficult. Many studies have investigated the dispersion qualities of carbon nanotube in different polymer matrices12,14,15,23,26,46,49,69 with the goal of developing composites with individually isolated carbon nanotubes. A model system for studying this dispersion is PAV as at low volumes (< 1%) of CNTs, good dispersions can be achieved and it can be drawn into stiff, strong fibres.12,13,49 PVA has also been shown to have good interfacial interaction with CNTs.26 Dispersion quality is a critical composite issue since aggregate bundles possess poor interfacial properties between adjacent tubes. Aggregation also causes a reduction in effective aspect ratio and larger aggregates are typically an entangled network of tubes which can only show poor alignment upon fibre drawing.46 Poorly dispersed nanotube bundles may act as stress concentration points if they exceed the critical flaw size for a particular material. Current techniques can normally well disperse relatively small amounts of CNT in a polymer matrix (e.g. 1 wt% in PVA matrix),13 but it is still a major challenge to perfectly disperse the high CNT loadings required for highly oriented polymer fibres (typically 3–10% for high performance PVA fibres).
Thanks to their outstanding conductivity and exceptionally large aspect ratio, CNTs are considered one of the best conductive fillers for conductive polymer composites (CPCs). However, the conductivity of oriented polymer–CNT fibres or tapes is reported to decrease (see Fig. 21.8 (a)) upon drawing for various polymer matrixes.16,19,70 This is explained by the breakdown of local contacts between CNTs during solid state drawing.20 The aspect ratio of the conductive filler is found to play an important role in the conductivity of the oriented composites.19,70 The drawing process applied to the composites is shown to align the CNT network (see Fig. 21.3). Recently, the effect of CNT orientation on the conductivity of polymer fibres was extensively studied by Du et al.71 They made a series of PMMA–SWNT fibres with different degrees of nanotube alignment by controlling the melt spinning conditions. The degree of alignment was quantified using the full-width at half-maximum (FWHM) of the S WNT obtained from X-ray study, where the higher FWHM corresponds to less alignment. The conductivity decreases with increasing alignment and they form optimal orientation percolation between 20° to 40°. It also shows that intermediate levels of orientation give higher conductivity than isotropic samples (see Fig. 21.9).
21.8 (a) The effect of solid state drawing on the resistivity of PP tapes containing MWNT or carbon black (CB); (b) the effect of solid state drawing (dashed arrows) and annealing (solid arrows) on the percolation threshold of CPCs based on co-PP and MWNTs. Reprinted with permission from references 20 and 21.
21.9 Electrical conductivity of a 2 wt.% SWNT–PMMA composite along the alignment direction with increasing nanotube isotropy. Nanotube alignment is assessed using X-ray scattering where FWHM = 0 is perfectly aligned and FWHM=180 is isotropic. Inset: a log-log plot of electrical conductivity vs. reduced FWHM determines the critical alignment, FWHMc. Reprinted with permission from reference 71.
The percolation threshold of highly oriented CPCs based on CNTs is relatively high (~ 5 wt%19) compared to the values obtained for isotropic systems.20,72,73 However, the conductivity of oriented fibres or tapes is found to increase during thermal treatments or annealing.18,21,22,44,49,74–77 This increase is explained by an improvement of local contacts between conductive regions caused by thermal energy. Therefore, it is possible to reduce the percolation threshold in oriented systems by thermal treatment. However, thermal treatment will destroy the mechanical properties of mono-component fibres or tapes if the annealing temperatures are near or above the melting temperature of the matrix.18,44,77 Recently, a new concept was described by Deng et al.21,22 for the creation of multifunctional polymer nanocomposite tapes (or fibres) that combine high stiffness and strength with good electrical properties and a low percolation threshold of CNTs. The concept is based on a bicomponent tape (or fibre) construction consisting of a highly oriented polymer core and a CPC skin based on a polymer with a lower melting temperature than the core, enabling thermal annealing of these skins to improve conductivity through a dynamic percolation process while retaining the properties of the core and hence those of the tape (or fibre). The percolation threshold in the CPC skins of the highly drawn conductive bicomponent tapes could be decreased from 5.3 to 1.1 wt% after annealing (see Fig. 21.8 (b)). Such a method has provided the industry with a simple way to produce conductive fibre with high strength and conductivity. It also suggests another route to control the morphology and conductivity of the conductive network formed by nanofillers.
Polymer–CNT composites have been studied to sense external stimuli such as bio-molecules,79 chemicals,79–82 gases,83,84 vapour,80–83 liquid,85 mechanical stress or strain23,86 and temperature.87 The exposure of the CPCs to the external stimuli can result in changes of electrical properties, which can be considered as a signal. As shown in Fig. 21.10,23 a mechanical strain applied to an elastomer–CNT composite can result in a clear electric signal which can be used for sensing. Applications for sensors based on polymer–CNT composites are expected in a wide range of fields, such as: building applications, smart textiles, medical applications and protective clothing.
Damage sensing in structural composites is another important possible application for polymer–CNT composites.24 It has been demonstrated that conducting carbon nanotube networks formed in a thermoset polymer matrix can be used as highly sensitive sensors for detecting the onset, nature, and evolution of damage in advanced polymer-based composites. The internal damage accumulation can be monitored in situ using electrical measurements. After the onset of damage and subsequent reloading of the damaged structure, there is a remarkable shift in the sensing curve, indicating irreversible damage (as shown in Fig. 21.11). These results demonstrate promise for evaluation of automatic self-healing approaches for polymer composites and development of enhanced life prediction methodologies.24
21.11 (a) Load–displacement and resistance curves for the 0° specimen with centre ply cut to initiate delamination; (b) load–displacement and resistance curves for the 0/90 specimen; (c) resistance curves for initial loading (undamaged) and reloading (damaged) laminates. Reprinted with permission from reference 24.
A general review on polymer–CNT fibres has been given and main efforts have been focused on the orientation of CNT in polymer fibres, and the mechanical and electrical properties of polymer–CNT fibres. It is demonstrated that CNTs have the potential for a wide range of applications in the polymer fibre field. Their one-dimensional character, large aspect ratio, excellent conductivity and ultra high mechanical properties make them outstanding candidates as multi-functional nanofillers.
Multi-functional polymer fibres are becoming an interesting topic in the field of polymer–CNT composites. Thanks to their intrinsic multi-functionality, CNTs have been demonstrated as able to provide polymer fibres with electrical properties, mechanical properties and sensing ability. A combination of these properties could be obtained in specific composites, whereas the properties can be tailored by engineering these materials from the nano to mm level in order to fulfil desired applications. However, due to their nano-size and large aspect ratio, there are still difficulties in dispersing them into polymer matrix, especially at higher loadings.
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