• improving the fracture toughness of the ply interfaces via epoxy/elastomer blends and

• reducing the mismatch of elastic properties (and stress concentrations) at the interfaces between the laminated plies.

1. reduced nanoscopic confinement of matrix polymer chains;

2. variation in properties of nanoscale inorganic constituents; many studies have reported that the mechanical, conductivity, optical, magnetic, biological and electronic properties of several inorganic nanoparticles significantly change as their size is reduced from the macroscale to the microlevel and nanolevel; and

3. nanoparticle arrangement and creation of a large polymer/particle interfacial area.

15.2.1 Laminated Carbon nanotubes (CNT)/epoxy FRP nanocomposites

Nanoscale fillers such as carbon nanotubes (CNTs) and carbon nanofibres (CNFs) offer new possibilities for low-weight composites with extraordinary mechanical, electrical and thermal properties. Taking into consideration their high axial Young’s modulus, high aspect ratio, large surface area, low density and excellent thermal and electrical properties, these fillers can be used as modifiers for the polymer matrices of fibre-reinforced polymer composites leading to advanced mechanical behaviour. However, with nanotube-reinforced polymer composites there has only been a moderate strength enhancement, which is significantly below the theoretically predicted potential. To achieve the full potential of nanotubes, there are two critical issues that have to be solved:

Nevertheless, based on a scaling argument correlating the radius (r), fibre strength (σ) and interface strength (τ) with the energy absorbed per unit cross-sectional area by fibre pull-out (i.e., G_pull-out ~ rσ²/τ), it was shown very recently that improvements in toughness in polymer/CNT nanocomposites cannot be attributed to the nanotube pull-out mechanism, as the pull-out energy significantly decreases when the fibre radius is scaled down to the nanoscale. In line with this argument, many studies have reported reductions in toughness with the incorporation of CNTs, even at low loadings. Further evidence from work with other nanoscale fillers suggests that conventional toughening mechanisms may not transfer to polymer nanocomposites directly.

In general, weakly interacting nanotube bundles and aggregations of nanotubes result in a poor dispersion state that significantly reduces the aspect ratio of the reinforcement. The reason for the weak interfacial bonding behaviour lies in the atomically smooth, non-reactive surface of the nanotubes, which does not efficiently transfer the load from the polymer matrix to the nanotube lattice. To solve this problem, a number of methods have been developed to maximize the benefits of nanotubes in polymer composites, i.e. surfactant-assisted dispersion,¹ sonication at high power,² in situ polymerization,³ electric field or magnetic-induced alignment of nanotubes,^{4, 5} plasma polymerization⁶ and surface modifications such as inorganic coating,⁷ polymer wrapping⁸ and protein functionalization.⁹

One key area where nanocomposites can make a significant impact is in addressing interlaminar toughness in fibre-reinforced composites. The improvement of the interlaminar toughness of fibre-reinforced composites has been the focus of research for a considerable time, since it is directly related to the dynamic as well as the damage-tolerance performance of the composite. The problem has been addressed in various ways: stitching, Z-pinning or interleaving, with a notable increase in toughness while also providing improvements in mechanical properties, such as fatigue life. Other approaches focus on tailoring the matrix or interface properties in order to provide the necessary interlaminar fracture toughness. Matrix toughening may be performed through chemical modification or, more recently, with the incorporation of fillers in the matrix material. Interface modification can also be performed by grafting, which tailors the chemical compatibility between the fibres and the matrix.

Gojny et al.¹⁰ investigated the interlaminar shear strength of nano-reinforced FRPs and described an efficient technique (mini-calendering) for dispersing carbon-based nanoparticles in epoxy resins. The application of a mini-calender to disperse carbon nanotubes (and carbon black) proved to be an efficient approach for reaching a good state of dispersion and enabled the manufacture of high volumes of nanocomposites. The resulting nanotube/epoxy composites exhibit a significant increase in fracture toughness as well as an enhancement in stiffness even with low nanotube content. Gojny et al.¹¹ also investigated the influence of CNTs on the interlaminar shear strength of a glass-fibre-reinforced polymer (GFRP) composite. They reported an increase of + 19% in interlaminar shear strength with a weight fraction as low as 0.3 wt% of amino-functionalized doublewall CNTs (DWCNT-NH₂) in the epoxy matrix, Fig. 15.1.

It has been claimed that the nanometre size of the particles means they can be used as modifiers in fibre-reinforced polymers. Composites have been produced via the resin-transfer-moulding (RTM) process and the particles were not filtered by the glass-fibre bundles. A follow-up review by the same research team reported that the interlaminar shear strength of the nanoparticle-modified GFRP was significantly improved (+ 16%) whilst adding only 0.3 wt% of CNTs.¹² The interlaminar toughness (G_Ic and G_IIc) was not affected in a comparable manner. The laminates containing CNTs exhibited a relatively high electrical conductivity at very low filler content.

Zhao et al.¹³ fabricated CNTs and continuous carbon-fibre (T300) reinforced unidirectional epoxy-resin matrix composites. They prepared CNTs by catalytic decomposition of benzene using the floating transition method at 1100–1200 °C. Benzene was used as a carbon source and ferrocene as a catalyst with thiophene. The CNTs used were straight with a diameter of 20–50 nm, internal diameter of 10–30 nm and length of 50–1000 μm. The volume fraction of continuous carbon fibre (first filler) in the composites without second filler (CNT) was 60%. The flexural strength of the composites reached a maximum value of 1780 MPa when the weight per cent of CNT in the epoxy-resin matrix was only 3%. The study concluded that the flexural strength and modulus of the composites increased at first and then decreased with an increase of the CNT content in the epoxy-resin matrix.

Hsiao et al.¹⁴ and Meguid and Sun¹⁵ investigated the tensile and shear strength of nanotube-reinforced composite interfaces by single lap shear testing. They observed a significant increase in the interfacial shear strength for epoxies with contents between 1 and 5 wt% of multi-walled nanotubes (MWNTs) when compared with the neat epoxy matrix. In particular, instead of processing and characterizing CNT/ polymer composites, Hsiao et al.¹⁴ explored the potential of using CNTs to reinforce the adhesives joining two composite structures. In the study, different weight fractions of MWNT were dispersed in epoxy to produce toughened adhesives. The reinforced adhesives were used to bond the graphite fibre/epoxy composite adherends. This experimental study showed that adding 5 wt% MWNT to an epoxy adhesive effectively transferred the shear load from the adhesive to the graphite fibre system in the composite laminates and improved the average shear strength of the adhesion by 46% (± 6%). A significant enhancement of the bonding performance was observed as the weight fraction of CNTs was increased. As shown in Fig. 15.2 (left), the 5 wt% MWNT effectively transferred the load to the graphite fibres in the adherends and the resulting failure was in the graphite fibre system. On the other hand, for epoxy adhesives containing no MWNTs (see Fig. 15.2 (right)), failure occurred at the epoxy along the bonding interface and no significant fractures of the graphite fibre were observed. Despite the promising results, the researchers concurred that further experiments involving increasing MWNT weight fractions and more detailed scanning electron microscopy (SEM) observations are required in order to understand and model the role of MWNTs in enhancing adhesion.

Various studies can be found on the incorporation of CNFs in polymeric matrices giving the final mechanical and electrical properties of these materials. As in all cases where nano-sized fillers are involved, the development of highperformance CNF/polymer composites requires a homogeneous dispersion of CNFs in the polymeric matrix because it is crucial to the composite’s performance. Early studies by Hussain¹⁶ reported that matrix reinforcement with nanowhiskers can damage the fibres in composite materials. As such, he incorporated microscale and nanoscale Al₂O₃ particles in filament-wound carbon-fibre/epoxy composites. He observed an increase in modulus, flexural strength, interlaminar shear strength and fracture toughness when the matrix was filled at 10 vol% with alumina particles (25 nm diameter). This effect stemmed largely from the large surface area of the filler and the ability of the particles to mechanically interlock with the fibres. Hybrid reinforced composites consisting of two or more different types of reinforcing fibres have also been studied in polymer matrix composite systems. It has also been reported that hybridization by incorporating whiskers into the matrix causes fibre damage resulting in a decrease in ultimate strength. However, the work claimed that the incorporation of a rigid spherical filler, especially a fine or nano-sized filler, did not cause serious damage to the fibre surfaces.

Mahfuz et al.¹⁷ studied the tensile response of carbon-nanoparticle/whisker- reinforced composites and observed a 15–17% improvement in tensile strength and modulus. Iwahori and Ishikawa¹⁸ reported compressive strength improvements in carbon-fibre-reinforced polymer (CFRP) composite laminates by using cup-stacked carbon nanofibres (CSCNFs) dispersed in epoxy as three-phase composites. Iwahori et al.¹⁹ went a step further and employed two types of CSCNF with different aspect ratios, i.e. with a fibre length of 500 nm to 1 μm (AR10) and a fibre length of 2.5 to 10.0 μm (AR50), respectively. These two types of CSCNF were dispersed into the epoxy resin. At the first trial stage, a manual fabrication process of the composite plates by impregnation of the diluted compound with the same epoxy into a dry carbon-fibre fabric was employed, followed by hot-press curing. Compression strength improvements of around 15% were attained in the three-phase composites, in comparison with the control case with no CSCNFs. Encouraged by the promising mechanical properties, they also manufactured cup- stacked carbon nanotubes (CSCNTs) dispersed in CFRP fabric to obtain more stable mechanical properties than manual fabrication processes. Figure 15.3 shows typical CSCNTs manufactured. They evaluated the mechanical properties of the CSCNT-dispersed CFRP and found an improvement in stiffness and strength (e.g. compressive strength) in two-phase and three-phase nanocomposite materials. The researchers accepted that another key issue in pre-impregnated composite fibre (prepreg) development is the optimization of the aspect ratio of CSCNFs.

Although the details of this process were not disclosed, it is noteworthy that it has one advantage for good dispersion because of the multiple number of edges of graphene sheets on the CSCNF surface. Such edges may help to increase interaction between the CSCNF and polymer. A good dispersion of the CSCNF was suggested in the micrographs for the three-phase composites made through the prepreg route. There was a large improvement of the compression strength for these three-phase composites made using the prepreg method compared with manually impregnated samples. For a T-700 CF UD prepreg sample, the compression strength in the fibre direction improved by 25% in comparison with the control sample (no CSNF). However, the elastic modulus during compression of this composite was not affected as naturally expected. More recently, Yokozeki et al.²⁰ investigated the damage accumulation that occurred in carbon-fibre- reinforced nanocomposite laminates under tensile loading. The nanocomposite laminates used in the study were manufactured from prepregs consisting of traditional carbon fibres and epoxy resin filled with CSCNTs. The thermomechanical properties of the unidirectional carbon-fibre-reinforced nanocomposite laminates were evaluated, and cross-ply laminates were subjected to tension tests to observe the damage accumulation of matrix cracks. As shown in Fig. 15.4, the number of matrix cracks in CSCNT-dispersed CFRP is much less than in conventional CFRP. A clear retardation of matrix-crack accumulation in CSCNT-dispersed CFRP laminates (both 5 wt% and 12 wt%) compared with laminates without CSCNT can be observed. Fracture toughness associated with matrix cracking was evaluated based on an analytical model using the experimental results. It was suggested that the dispersion of CSCNTs resulted in fracture-toughness improvement and residual thermal strain decrease, which was considered to cause the retardation of matrix-crack formation.

In the work of Wichmann et al.,¹² different nanoparticles, such as fumed silica and carbon black, were used to optimize the epoxy matrix system of a glass-fibre- reinforced composite. Their nanometre size enabled their application as particle reinforcement in FRPs produced by a modified RTM, without being filtered by the glass-fibre bundles. Figure 15.5 is a schematic of a modified-RTM device. An electrical field was applied during curing in order to enhance the orientation of the nanofillers in the z-direction. The interlaminar shear strengths of the nanoparticle- modified composites were significantly improved (+ 16%) and an increase in fracture toughness of 42% was observed by adding only 0.3 wt% of CNTs. The interlaminar toughness was not affected in a comparable manner. Only the fumedsilica nanocomposites exhibited a negligible decrease in Young’s modulus. However, with 0.5 vol% of epoxy-functionalized fumed-silica nanoparticles, K_Ic increased by 55%. The laminates containing CNTs exhibited relatively high electrical conductivity at very low filler content, which suggests functional properties such as stress–strain monitoring and damage detection.

It should be acknowledged that traditional fibre-reinforced composite materials with excellent in-plane properties perform poorly when out-of-plane through-thickness properties are important. Composite architectures with fibres designed orthogonal to the two-dimensional (2D) layout in traditional composites could alleviate this weakness in the transverse direction, but efforts so far have only had limited success. Nevertheless, the combination of a nanotube-modified matrix together with conventional fibre reinforcements (e.g. carbon, glass or aramid fibres) could lead to a new generation of multifunctional materials. Besides electrical conductivity, which can be induced by the carbon nanoparticles, an additional z-reinforcement can be expected. The fibre orientation in structural components is usually in plane (x- and y-directions), leading to fibre-dominated material properties in these directions, whereas the z-direction remains matrix dominated. With regard to the nanometric size, carbon nanoparticles allow an infiltration between the microscale fibres. The use of CNTs at the reinforcing phase should improve the matrix properties, especially in the z-direction, which is equivalent to improving the interlaminar properties.

One of the applications of CNT-reinforced polymers for filament-wound CFRP was demonstrated by Spindler-Ranta and Bakis.²¹ An amount of 1 wt% singlewalled nanotubes (SWNTs) was added to an epoxy polymer matrix. However, this study concluded that SWNTs did not produce any noticeable effect on the CNT-reinforced composites and filament-wound CFRP rings. In contrast, Veedu et al.²² reported significant improvements in the interlaminar fracture toughness, hardness, delamination resistance, in-plane mechanical properties, damping, thermoelastic behaviour and thermal and electrical conductivities. They presented an approach to three-dimensional(3D) through-the-thickness reinforcement, without altering the 2D stack design, using interlaminar CNT forests, which provide enhanced multifunctional properties along the thickness direction. The CNT forests allowed the fastening of adjacent plies in the 3D composite. They grew MWNTs on the surface of micro-fi bre fabric cloth layouts, normal to the fibre lengths, resulting in a 3D effect between plies under loading. These nanotube- coated fabric cloths served as building blocks for multilayered 3D composites, with the nanotube forests providing interlaminar strength and toughness under various loading conditions.

15.2.2 Laminated layered silicates/epoxy FRP nanocomposites

In the early 1990s, the Toyota research group synthesized polyamide-6-based clay nanocomposites that demonstrated the first use of nanoclays as a reinforcement for polymer systems.²³ They concluded that nanoclays not only influenced the crystallization process, but that they were also responsible for morphological changes. Recognizing these benefits, many researchers, using a variety of clays and polymeric matrices, have produced nanocomposites with improved properties.²⁴

Haque et al.,²⁵ using a similar manufacturing process (i.e. vacuum-assisted resin infusion moulding or VARIM), showed a large improvement of the mechanical properties of their S2-glass-fibre laminates with a very low layered- silicate content. They showed that, by dispersing 1 wt% nanosilicates, S2-glass/ epoxy-clay nanocomposites exhibited an improvement of 44%, 24% and 23% in interlaminar shear strength, flexural strength and fracture toughness respectively. Similarly, the nanocomposites exhibit approximately 26 °C higher decomposition temperatures than conventional composites. The increased properties at low loading were attributed to several factors:

The clays were also presumed to decrease the mismatch in the coefficient of thermal expansion, significantly reducing residual stresses and leading to higher quality laminates. Increased interfacial bonding, matrix agglomeration and coarse morphology were observed on the fractured surface of the low loading nanocomposites. The degradation of these properties at higher clay loadings was believed to be caused by phase-separated structures and also by defects in the cross-linked structures. However, the authors acknowledged that further work was necessary in order to produce clay-epoxy nanocomposites with a fully exfoliated structure.

Similarly, Chowdhury et al.²⁶ employed the VARIM process to manufacture woven carbon-fibre-reinforced polymer matrix composites. They investigated the effects of nanoclay particles on the flexural and thermal properties. Different weight percentages of a surface-modified montmorillonite mineral were dispersed in SC-15 epoxy using sonication. The nanophased epoxy was then used to manufacture 6000 fibre tow-plain weave carbon/epoxy nanocomposites using the VARIM technique. Flexural test results of thermally post-cured samples indicated a maximum improvement in strength and modulus of about 14% and 9%, respectively. Dynamic mechanical analyses (DMA) of the thermally post-cured samples showed a maximum improvement in the storage modulus of about 52% and an increase in the glass transition temperature of about 13 °C. In terms of mechanical and thermal properties, 2 wt% nanoclay seems to be an optimum loading for carbon/SC-15 epoxy composites. Microstructural studies revealed that nanoclay promotes good adhesion of the fibre and matrix, thereby increasing the mechanical properties.

Lin et al.²¹ successfully prepared layered silicate/glass-fibre/epoxy hybrid composites using a vacuum-assisted resin transfer moulding (VARTM) process. Figure 15.6 shows a schematic of the experimental set-up for the closed-mould VARTM process. They selected clay and short-length glass fibres to reinforce an epoxy resin. To study the effects of fibre direction on clay distribution in the hybrid composites, unidirectional glass fibres were placed in two directions: parallel and perpendicular to the resin flow direction. The intercalation behaviour of the clay and the morphology of the composites were investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The complementary use of XRD and TEM revealed an intercalated clay structure in the composites. Dispersion of the clay in the composites was also observed using SEM. The clays were dispersed both between the bundles of the glass fibres and within the interstices of the fibre filaments. The mechanical properties of the ternary composites were also evaluated. The results indicated that introducing a small amount of organoclay to glass-fibre/epoxy composites enhanced their mechanical and thermal properties, confirming the synergistic effects of glass fibres and clays in the composites.

Aktas et al.²⁸ developed a novel approach for the characterization of nanoclay dispersion in polymeric composites using electron microprobe analysis (EMPA). Dispersion analysis was performed on three sets of centre-gated discs fabricated by RTM. The first set was neat epoxy polymer without reinforcement, whereas the second set comprised 17 vol% randomly oriented chopped glass-fibre preforms. The final set, in addition to the glass-fibre reinforcement, contained 1.7 wt% Cloisite 25A nanoclay. After curing, a sample along the radius of a nanoclay- reinforced disc was analysed with an electron microprobe analyser. The scanning electron micrographs indicated that the nanoclay exists in clusters of various sizes ranging from over 10 μm down to submicrometre scale. Nanoclay clusters larger than 1.5 μm were analysed by digital image processing of scanning electron micrographs taken along the sample radius. The dispersion of nanoclay clusters smaller than 1.5 μm was quantified by compositional analysis via wavelength dispersive spectrometry (WDS). The distribution of nanoclay clusters larger than 1.5 μm was found to be approximately constant along the radius with an average value of 1.4% by volume. Similarly, nanoclay clusters smaller than 1.5 μm were found to be distributed evenly with an average value of 0.41 wt%. In addition, the glass transition temperature improved by 11% with the addition of nanoclay.

Gilbert et al.^{29, 30} and Timmerman et al.³¹ demonstrated that fracture toughness and mechanical properties are increased by the incorporation of metal and inorganic particles. They developed La PolynanoGrESS (layered polynanomeric graphite epoxy scaled system), which utilizes the nanoparticle effect in an epoxy matrix and scales to a continuous carbon-fibre-reinforced composite system. Typically, Timmerman et al.³¹ modified the matrices of carbon-fibre/epoxy composites with layered inorganic clays and a traditional filler to determine the effects of particle reinforcement, both microscale and nanoscale, based on the response of these materials to cryogenic cycling. The mechanical properties of the laminates studied were not significantly altered through nanoclay modification of the matrix. The incorporation of a nanoclay reinforcement at a suitable concentration resulted in laminates with microcrack densities lower in response to cryogenic cycling than those seen in unmodified or macro-reinforced materials. Lower nanoclay concentrations resulted in a relatively insignificant reduction in microcracking and higher concentrations displayed a traditional filler effect.

Brunner et al.³² extended the work of Timmerman et al., using epoxy with a relatively small amount of nano-sized filler as a matrix in fibre-reinforced laminates. They focused on investigating whether a nano-modified epoxy matrix yields improved delamination resistance in a fibre-reinforced laminate compared with a laminate with neat epoxy as the matrix material. To start with, neat and nano-modified epoxy specimens without fibre reinforcement were prepared for a comparison of the fracture toughness of the matrix material itself. Additional properties of the neat and nano-modified epoxy were also determined (partly taken from Timmerman et al.³¹) and compared. The study reported an improvement in fracture toughness up to about 50% and energy release rates increased by about 20% with the addition of 10 wt% of organosilicate clay.

Several other studies have described property enrichment due to the addition of nanoclay to composite matrices. For instance, Schmidt,³³ Mark³⁴ and Hussain et al.¹⁶ demonstrated the technology of dispersing Al₂O₃ particles in a matrix and investigated their effect on the mechanical properties of CFRP. The incorporation of filler particles resulted in higher fracture toughness by significantly improving the toughness of the matrix and crack deviation. Studies on carbon/SiC-epoxy nanocomposites reported a 20-30% improvement in mechanical properties.³⁵ Mohan et al³⁶ evaluated the tensile performance of S2-glass/epoxy composites dispersed with alumina nanoparticles up to 1.5% weight fraction and found an increase of 12% in tensile modulus and 8% in tensile strength. Kornmann et al.³⁷ successfully synthesized epoxy-layered silicate nanocomposites based on diglycidyl ether of bisphenol A and an anhydride-curing agent. A manufacturing process using hand lay-up, vacuum bagging and hot pressing was also developed to produce glass-fibre-reinforced laminates with this nanocomposite matrix. Transmission electron microscopy indicated that silicate layers dispersed in the epoxy matrix have long-range order with an interlamellar spacing of about 9 nm. X-ray diffraction analysis confirmed this nanostructure both in the nanocomposites and in a fibre-reinforced composite based on the same matrix. Scanning electron micrographs of the laminate with a nanocomposite matrix showed that nanolayers stacked at the surface of the glass fibre, thus possibly improving the interfacial properties of the fibres. Flexural testing of the laminates showed that the nanolayers improved the modulus and the strength, by 6% and 27% respectively. Dynamic mechanical analyses ofthe epoxy and nanocomposite plates and their corresponding laminates showed a systematic glass transition temperature decrease of the nanocomposite-based materials. This, the researchers suggested, explained the larger water uptake observed at 50 °C in the plate and the laminate based on a nanocomposite matrix compared with those based on the pristine epoxy.

Karaki et al.³⁸ incorporated layered clay, alumina and titanium dioxide into an epoxy matrix and fabricated continuous carbon-fibre-reinforced polynanomeric matrices to study tension-tension fatigue behaviour. They found that the number of microcracks in each layer depended on the type of particles and their concentration. Wang et al.³⁹ demonstrated that the exfoliated clay with only 2.5 wt% in epoxy showed a significant improvement in fracture toughness and concluded that an increase in the number of microcracks and the fractured surface due to crack deflection resulted in the toughness increase. Siddiqui et al.⁴⁰ investigated the mechanical properties of nanoclay-dispersed CFRP, and showed that the interlaminar fracture toughness is higher than that of conventional CFRP. Ragosta et al.⁴¹ showed that critical stress intensity factors of epoxy/silica nanocomposites increased with an increase of silica content. Work by Seferis and co-workers⁴² incorporated nano-sized alumina structures in the matrix and interlayer regions of prepreg-based carbon-fibre/epoxy composites. Subramaniyan et al.⁴³ observed that the addition of 5 wt% of nanoclay increased the elastic modulus of epoxy resin under compression by 20% and that the compressive strength of glass-fibre composites with nanoclay when made by the wet lay-up technique increased by 20-25%. Subramaniyan and Sun⁴⁴ showed that polymers can be toughened significantly using a relatively small amount of nanoclay particles based on three-point bending tests of edge-notched specimens with sharp crack tips. A parallel study by the same authors,⁴⁵ reported that the compressive strength of unidirectional GFRP with nanoclays increased compared with conventional GFRP.

Hackman and Hollaway⁴⁶ studied potential applications of clay nanocomposite materials in civil engineering structures. They concluded that there is the possibility to increase the service life of materials subjected to aggressive environments because of the increased durability of glass-fibre and carbon-fibre composites. Liu et al.⁴⁷ demonstrated the improvement of fracture toughness and the reduction of water diffusivity of epoxy/nanoclay composites. Ogasawara et al.⁴⁸ investigated the helium gas permeability of silicate-clay (montmorillonite) particle/epoxy nanocomposites. They reported that the incorporation of increasing amounts of montmorillonite particles reduced helium gas permeability. With an increase of montmorillonite loading, gas diffusivity decreased while gas solubility increased. Helium diffusion was found to be in agreement with numerical results based on the Hatta-Taya-Eshelby theory.⁴⁹’⁵⁰ They revealed that the dispersion of nanoscale platelets in a polymer is effective in improving the gas barrier property. The study appreciated the fact that surface-modified clays are amenable for making organic/clay nanocomposites because of the weak bonding force between the layers of montmorillonite.,⁴⁸ In the study, a typical less viscous epoxy of Epikote 807 base resin was used for better dispersion. It was shown that a loading of the nanoclay of about 4 vol% (about 6 wt%) reduced the diffusion coefficient to 1/10, and that theoretical predictions based on an aspect ratio of 0.001 agreed well with the experimental results.

15.2.3 Polyamide FRP nanocomposites

Electrospun nanocomposite fibres have great potential in applications where both a high surface-to-volume ratio and strong mechanical properties are required, such as high-performance filters and fibre-reinforcement materials. Since the mechanical properties of fibres generally improve substantially by decreasing fibre diameter, there is considerable interest in the development of continuous electrospun polymer nanofibres. In this respect, Lincoln et al.⁵¹ reported that the degree of crystallinity of polyamide-6 (PA-6) annealed at 205 °C increased substantially with the addition of montmorillonite (MMT). This implied that the silicate layers could act as nucleating agents or growth accelerators. In contrast, a study by Fong et al.⁵² showed a very similar overall degree of crystallinity for electrospun PA-6 and PA-6/Cloisite-30B nanocomposite fibres containing 7.5 wt% of organically modified MMT (OMMT) layers.

Fornes and Paul⁵³ found that OMMT layers could serve as nucleating agents at 3% concentration in PA-6/OMMT nanocomposites but retarded the crystallization of PA-6 at a higher concentration of around 7%. In addition, the differences in the molecular weight (MW) of PA-6 and the solvent used for electrospinning were also expected to have different effects on the mobility of PA-6 molecular chains and the interactions between the PA-6 chains and OMMT layers, which may also affect the crystallization behaviour of PA-6 molecules during electrospinning. Li et al.⁵⁴ manufactured PA-6 fibres and nanocomposite fibres with an average diameter of around 100 nm by electrospinning using 88% aqueous formic acid as the solvent. The addition of OMMT layers in a PA-6 solution increased the solution viscosity significantly and changed the resulting fibre morphology and sizes. TEM images of the nanocomposite fibres and ultra-thin fibre sections and wide-angle X-ray diffraction results showed that the OMMT layers were well exfoliated inside the nanocomposite fibres and oriented along the fibre axial direction. The degree of crystallinity and crystallite size both increased for the nanocomposite fibres and, more significantly, for the fibres electrospun from the 15% nanocomposite solution, which exhibited the finest average fibre size. As a result, the tensile properties of electrospun nanocomposites were greatly improved. Young’s modulus and the ultimate strength of electrospun nanocomposite fibrous mats improved to 70% and 30%, respectively, compared with PA-6 electrospun mats. However, the ultimate strength of nanocomposite fibrous mats electrospun from 20% nanocomposite solution decreased by about 20% due to their larger fibres. Young’s modulus of PA-6 electrospun single fibres with a diameter around 80 nm was almost double the highest value reported for conventional PA-6 fibres and could be improved by about 100% for electrospun nanocomposite single fibres of similar diameters.

Liang et al.⁵⁵ described a fibre that consisted of a nano-Fe₂O₃-particle/PA-6 nanocomposite. The thermal stability of the composite material was enhanced by about 16 °C (from 440 °C to 456 °C) by the addition of Fe₂O₃ nanoparticles with 15.0% content (part per hundred parts of resin). The Fe₂O₃-reinforced materials processed by melt spinning displayed an improved tensile modulus compared with similarly processed pure PA-6; the improvements in tensile strength and modulus were about 21% and 112%, respectively. Moreover, this fibre absorbed ultraviolet and visible light.

In another interesting study,⁵⁶ a range of polymer matrices, including polyvinyl alcohol, poly (9-vinyl carbazole) and polyamide, were examined. To compare production methods, polymer composite films and fibres were produced. It was found that, by adding various mass fractions of nanofillers, both Young’s modulus and hardness increased significantly for both films and fibres. In addition, thermal behaviour was seen to be strongly dependent on the nanofillers added to the polymer matrices. Wu et al.⁵⁷ prepared carbon-fibre-reinforced and glass-fibre- reinforced PA-6 and PA-6/clay nanocomposites. The fabrication method involved first mechanically mixing PA-6 and PA-6/clay with E-glass short fibres (6-mm long) and carbon fibres (6-mm long), separately. A twin-screw extruder at a rotational speed of 20 rpm extruded the fibres. The temperature profiles of the barrel were 190–210–230–220 °C from the hopper to the die. The extrudate was pelletized, dried and injection moulded into standard test samples for mechanical property tests. The injection-moulding temperature and pressure were 230 °C and 13.5 MPa, respectively. The research found that the tensile strength of PA-6/clay containing 30 wt% glass fibres was 11% higher than that of PA-6 containing 30 wt% glass fibre, while the tensile modulus of the nanocomposite increased by 42%. The flexural strength and flexural modulus of neat PA-6/clay were found to be similar to PA-6 reinforced with 20 wt% glass fibres. It was concluded that the effect of nanoscale clay on toughness was more significant than that of the fibre. The heat distortion temperatures of the PA-6/clay and PA-6 were 112 °C and 62 °C, respectively. Consequently, the heat distortion temperature of the fibre- reinforced PA-6/clay system was almost 20 °C higher than for the fibre-reinforced PA-6 system. The notched Izod impact strength of the composites decreased with the addition of the fibre. Scanning electron microphotographs showed that the wet-out of glass fibre was better than carbon fibre. The study concluded that the mechanical and thermal properties of the PA-6/clay nanocomposites were superior to those of the PA-6 composite in terms of heat distortion temperature, tensile and flexural strength and modulus without sacrificing their impact strength. This was attributed to nanoscale effects and the strong interaction force that existed between the PA-6 matrix and the clay interface.

Regarding short fibres, Akkapeddi⁵⁸ prepared PA-6 nanocomposites using chopped glass fibres. A typical experiment used a commercial grade PA-6 with a molecular weight of 30 kg/mol and specially designed functional organo- quaternary ammonium-clay complexes (organoclays) based on MMT or hectorite- type clays. Freshly dried PA-6 (moisture < 0.05%) was blended with 3–5 wt% of a selected organoclay powder and extruded at 260 °C in a single step under high shear mixing conditions. Alternatively, the organoclay was master-batched first into PA-6 (at 25 wt% loading and then re-extruded in a second step with more PA-6 to dilute the clay content to < 5 wt%). Conventional chopped glass fibre, 10 μm in diameter and about 3 mm in length, was then added as an optional reinforcement through a downstream feed port at zone 6 of a twin-screw extruder. The glass fibre was compounded with the molten, premixed PA-6 nanocomposite either as a one-step extrusion process or in a second extrusion step. The extrudate was quenched in a water bath and pelletized. The pellets were dried under vacuum at 85 °C and injection moulded into standard ASTM test specimens. As shown in Fig. 15.7, significant improvements in modulus were achievable in both the dry and the moisture-conditioned states for PA-6 nanocomposites compared with standard PA-6, at any given level of glass-fibre reinforcement. In particular, a small amount (3–4 wt%) of nanoscale dispersed layered silicate was capable of replacing up to 40 wt% of a standard mineral filler or 10–15 wt% of glass fibre to give equivalent stiffness at a lower density. In addition, improved moisture resistance, permeation barrier and fast crystallization/mould cycle time contribute to the usefulness of such composites.

Vlasveld et al.⁵⁹ developed a three-phase thermoplastic composite, consisting of a main reinforcing phase of woven glass or carbon fibres and a PA-6 nanocomposite matrix. The nanocomposites used in this research had moduli that were much higher than unfilled PA-6, including above T_g and moisture-conditioned samples. Flexural tests on commercial PA-6 fibre composites showed a decrease of flexural strength on increasing temperature. The researchers claimed that the strength of the glass-fibre composites was increased by more than 40% at elevated temperatures and the temperature range for a given minimum strength was increased by 40–50 °C. The carbon-fibre composites also showed significant improvements at elevated temperatures, although not at room temperature. Based on flexural tests on PA-6-based glass and carbon-fibre composites over a large temperature range up to near the melting point, it became clear that for these fibre composites it is important to have a reasonably high matrix modulus. Both glass and carbon composites were very sensitive to a decrease of the matrix modulus below values around 1 GPa. At higher moduli, the carbon-fibre composites were more sensitive to the matrix modulus than glass-fibre composites. The modulus of unfilled PA-6 decreased below the (arbitrary) 1 GPa level just above T_g. It is noteworthy that the nanocomposites used in this research had moduli that were much higher and stayed above the 1 GPa level up to 160 °C, which was more than 80 °C higher than for unfilled PA-6. The nanocomposites also showed much higher moduli in moisture-conditioned samples. Even in moisture-conditioned samples tested at 80 °C, the modulus was much higher than for the dry unfilled PA-6, which again was well above 1 GPa. DMA measurements indicated that the nanocomposites did not show a change of T_g and that the reduction of the modulus on absorption of moisture was due to the T_g decrease.

Vlasveld et al.⁶⁰ investigated fibre-matrix adhesion in glass-fibre-reinforced PA-6 silicate nanocomposites. The main reinforcing phase consisted of continuous E-glass fibres, whereas the PA-6-based matrix was a nanocomposite reinforced with platelets of exfoliated layered silicate. Two different types of nanocomposite were used with different degrees of exfoliation of the silicate layers: one with unmodified silicate and one with an organically modified silicate. They developed nanocomposite laminates by the sol–gel and modified-diaphragm methods. The preparation of the PA-6 nanocomposites consisted of melt-compounding Akulon® K122D with Somasif® MEE and Somasif® ME-100 by means of a co-rotating twin-screw extruder at 240 °C. For the Somasif® MEE nanocomposite materials, an 11 wt% MEE master batch was initially compounded. To obtain the various concentrations of the MEE nanocomposite, the master batch was extruded for a second time without dilution for the 11 wt% nanocomposite, or diluted with Akulon® K122D to concentrations of 6.1 and 2.7 wt%. The 2.5 wt% Somasif® ME-100 nanocomposite material was produced by diluting a 10% ME-100 master batch with Akulon® K122D in the extruder. (All percentages are weight percentages of silicate measured with a thermogravimetric analyser (TGA) after heating for 40 min at 800 °C in air.) Two demands in the preparation of the single- fibre fragmentation specimens had to be met: the fibre had to lie straight in the centre of the specimen and the matrix material of the specimen had to be thin enough to be transparent, since the fibre fragments were examined and measured using an optical microscope. A Fontijne hot-plate press heated to 240 °C was used to produce the films for the single-fibre fragmentation test specimen. Single fibres were carefully extracted from a fibre bundle and placed at a distance of approximately 2 cm apart parallel to each other between the PA or nanocomposite films. The hot-plate press was used to melt the polymer films and a pressure of 0.8 N/mm² was applied for 30 s to provide the necessary bonding with the fibre.

After cooling between cold metal plates, tensile test specimens were prepared. It was observed that the ultimate strength and stiffness increased by adding 1% SiO₂ nanoparticles, while little improvement in fatigue behaviour was found. It was concluded that the failure mechanism was by interfacial de-bonding and that both the addition of nanoparticles and moisture conditioning had a negative effect on the bonding between the matrix and the glass fibres. In addition, the researchers noted that, in the composites formed, adhesion between the nanocomposites and the carbon fibres (Fig. 15.8) was probably worse than between the unfilled PA-6 and the matrix, reducing the potentially positive influence of the increased matrix modulus.

An assessment of reactively processed anionic polyamide-6 (APA-6) for use as a matrix material in fibre composites was conducted by van Rijswijk et al.⁶¹ They also compared it with melt-processed PA-6 and PA-6 nanocomposites. A specially designed lab-scale mixing unit was used to prepare two liquid material formulations at 110 °C under a nitrogen atmosphere: a monomer/activator mixture in tank A and a monomer/initiator mixture in tank B, as shown in Fig. 15.9. After individually degassing both tanks (15 min at 100 mbar), the two material feeds were mixed using a heated (110 °C) static mixer and dispensed (1:1 ratio) into a heated (110 °C) buffer vessel with nitrogen protective environment. A stainless-steel infusion mould was used together with a 3-mm-thick stainless-steel cover plate (not shown) to manufacture neat APA-6 panels (250×250×2 mm). Homogeneous heating of the mould was obtained by placing it in a vertically positioned hot flat platen press. Silicon tubes connected the resin inlet of the mould to the buffer vessel and the resin outlet to a vacuum pump. Infusion from bottom to top was necessary to prevent entrapment of air. A pressure-control system was used to precisely set the infusion and curing pressures (absolute pressure in the mould cavity). Loss of control over the pressure in the mould cavity due to solidification of resin in the unheated outlet tube had to be prevented. To avoid this, a buffer cavity was machined in the mould near the outlet to slow down the infusion, hence giving ample time to stop the resin flow before it was able to exit the mould. For each infusion pressure, the infusion time to reach the buffer cavity was determined visually because the steel cover plate had been replaced by a glass one. Additionally, a resin trap and a cold trap were placed directly after the mould to protect the vacuum pump.

The mechanical properties of APA-6 and HPA-6 (Akulon® K222D, low-MW injection-moulding grade hydrolytically polymerized PA-6) nanocomposites were compared with injection-moulded neat HPA-6. As expected, the HPA-6 nanocomposite had the highest modulus over the entire range of temperatures (20–160 °C) and moisture content (0–10 wt%) tested. However, APA-6 came close and had the highest maximum strength due to its characteristic crystal morphology, which was directly linked to the reactive type of processing used. This same morphology, it was claimed, also made APA-6 slightly less ductile compared with melt-processed HPA-6. Compared with the melt-processed HPA-6, APA-6 polymerized at 150 °C and the HPA-6 nanocomposite had a higher modulus at a similar temperature, or a similar modulus at a higher temperature (40–80 °C increase). It is noteworthy that such an increase in maximum-use temperature, related to the heat distortion temperature, can seriously expand the application possibilities for PA-6 and PA-6 composites. For all PAs, temperature and moisture absorption reduced the modulus and the strength and increased the maximum strain, which was directly related to the glass transition temperature. It was observed that moisture absorption reduced T_g to below the testing temperature. However, the effect of both was in essence the same. Retention of the mechanical properties of APA-6 after conditioning at 70 °C for 500 h and subsequent drying was demonstrated. Conditioning by submersing in water at the same temperature, however, resulted in a brittle material with surface cracks, as is common with most polyamides, which was caused by continued crystallization and the removal of unreacted monomer. Given the fact that submersion at elevated temperatures is usually not an environment in which PA-6 and its composites are applied, this property reduction was therefore not detrimental for the application of these materials. The overall conclusion of this comparative study into the application of polyamides as matrix materials in fibre composites was that both APA-6 and the HPA-6 nanocomposites outperformed melt-processed HPA-6 in terms of modulus and maximum strength. Therefore, the researchers concluded that both ‘improved’ PAs may be expected to enhance matrix-dominated composite properties like compressive and flexural strength, provided that a strong fibre-to-matrix interphase is obtained.

Another comparative study was conducted by Sandler et al.⁶² on melt-spun PA-12 fibres reinforced with carbon nanotubes and nanofibres. A range of MWNTs and carbon nanofibres were mixed with a PA-12 matrix using a twin-screw microextruder and the resulting blends spun to produce a series of reinforced polymer fibres. The work aimed to compare the dispersion and resulting mechanical properties for nanotubes produced by the electric arc and a variety of chemical vapour deposition techniques. A high quality of dispersion was achieved for all the catalytically grown materials and the greatest improvements in stiffness were observed using aligned, substrate-grown, carbon nanotubes. The use of entangled MWNTs led to the most pronounced increase in yield stress, most likely as a result of increased constraint of the polymer matrix due to the relatively high surface area. The degree of polymer and nanofiller alignment and the morphology of the polymer matrix were assessed using X-ray diffraction and differential scanning calorimetry (DSC). The carbon nanotubes were found to act as nucleation sites under slow-cooling conditions, the effect scaling with effective surface area. Nevertheless, no significant variations in polymer morphology as a function of nanoscale-filler type and loading fraction were observed under the melt-spinning conditions applied. A simple rule-of-mixture evaluation of the nanocomposite stiffness revealed a higher effective modulus for the MWNTs compared with the carbon nanofibres, a result of improved graphitic crystallinity. In addition, this approach allowed a general comparison of the effective nanotube modulus with those of nanoclays as well as common short glass and carbon-fibre fillers in meltblended PA composites. The experimental results further highlighted the fact that the intrinsic crystalline qualities, as well as the straightness of the embedded nanotubes, were significant factors influencing reinforcement capability.

15.2.4 Poly (ether ether ketone) (PEEK) FRP nanocomposites

Jen et al.⁶³ manufactured AS-4/PEEK APC-2 nanocomposite laminates and also studied their mechanical responses. The experimental procedures were as follows: firstly, the nanoparticles were diluted in alcohol (50 ml alcohol: 2 g SiO₂) and stirred uniformly. Then, 16 plies of [0/90]_4s cross-ply and [0/± 45/90]_2s quasi- isotropic prepregs were cut, SiO₂ solution was spread on the prepregs in a temperature-controlled box, and after evaporation of alcohol the nanoparticles were found to weigh in the range 111–148 mg/ply. The next step was to repeat the spreading for 5, 8, 10 and 15 plies, followed by consolidation of the stacked plies in a hot press to form a laminate 2 mm thick. The consolidation process is shown in Fig. 15.10.

Next, the laminates were cut into specimens and tested according to ASTM D3039M. The tensile tests were repeated at 50, 75, 100, 125 and 150 °C. These tests measured stress–strain, strength and stiffness and the obtained data were compared with those for the original APC-2 laminate (no SiO₂ nanoparticles) and showed that the optimal content of SiO₂ nanoparticles was 1% by total weight. The ultimate strength increased by about 12.48% and elastic modulus by 19.93% in quasi-isotropic nanolaminates, whilst the improvement for cross-ply nanocomposite laminates was less. At elevated temperatures the ultimate strength decreased slightly below 75 °C and the elastic modulus reduced slightly below 125 °C; however, both properties degraded highly at 150 °C (≈ T_g) for the two layups. Finally, after constant stress amplitude tension-tension (T–T) cyclic testing, it was found that both stress-cycle (S–N) curves were very close, being below 10⁴ cycles for cross-ply laminates with or without nanoparticles, and the S–N curve of the nanolaminates was slightly reduced after 10⁵ cycles.

Sandler et al.⁶⁴ produced poly (ether ether ketone) nanocomposites containing vapour-grown carbon nanofibres using standard polymer processing techniques. Macroscopic PEEK nanocomposite master batches containing up to 15 wt% vapour-grown CNF were prepared using a Berstorff co-rotating twin-screw extruder with a length-to-diameter ratio of 33. The processing temperatures were set to about 380 °C. The strand leaving the extruder was quenched in a water bath, air dried and then regranulated followed by drying at 150 °C for 4 h. Tensile bars according to the ISO 179A standard were manufactured on an Arburg Allrounder 420 injection-moulding machine at processing temperatures of 390 °C, with the mould temperature set to 150 °C. Prior to mechanical testing, all samples were heat treated at 200 °C for 30 min followed by 4 h at 220 °C in an attempt to ensure a similar degree of crystallinity of the polymer matrix. Macroscopic tensile tests were performed at room temperature with a Zwick universal testing machine. The cross-head speed was set to 0.5 mm/min in the 0–0.25% strain range and was then increased to 10 mm/min until specimen fracture occurred. An evaluation of the mechanical composite properties revealed a linear increase in tensile stiffness and strength with nanofibre loading fractions up to 15 wt%, while matrix ductility was maintained up to 10 wt%. Electron microscopy confirmed the homogeneous dispersion and alignment of the nanofibres. An interpretation of the composite performance by short-fibre theory resulted in rather low intrinsic stiffness properties of the vapour-grown carbon nanofibre. Differential scanning calorimetry showed that an interaction between the matrix and the nanoscale filler could occur during processing. However, such changes in polymer morphology due to the presence of nanoscale filler need to be considered when evaluating the mechanical properties of such nanocomposites.

Schmidt’s investigation⁶⁵ involved multifunctional inorganic–organic composite sol–gel coatings on glass surfaces. The sol–gel process allowed the fabrication of ceramic colloidal particles in the presence of organo-alkoxysilanes carrying various perfluoroalkyl groups and the synthesis of multifunctional transparent inorganic–organic composites. The report claimed that, in addition, these composites can be used as controlled-release systems or designed as graded systems. Using this approach, a coating with a very low surface free energy (with antisoiling properties) and temperature stability up to 350 °C, a controlled-release system for permanent wettability (anti-fogging) and systems containing metal colloids for optical effects were developed. Lin⁶⁶ and Wang et al.⁶⁷ studied the effects of wear and friction by adding SiC nanoparticles to PEEK. The latter studied the effect of the synergism between nanometre SiC and PTFE on the wear of PEEK. Fine powders of PEEK (ICI grade 450P, η = 0.62) having a diameter of approximately 100 μm were prepared. Nanometre SiC, smaller than 80 nm, was used as a filler. The PTFE powders (25 μm in diameter), nanometre SiC and PEEK were completely mixed ultrasonically and dispersed in alcohol for ~ 15 min. Then the mixture was dried at 110 °C for 6 h to remove the alcohol and moisture. Finally, the mixture was moulded into block specimens by compression moulding, in which the mixture was heated at a rate of 10 °C min^− 1 to 340 °C, held there for 8 min and then cooled in the mould to 100 °C. After release from the mould, the resultant block specimens were prepared for friction and wear tests. A tribological study found that the incorporation of PTFE into PEEK filled with 3.3 vol% nanometer SiC had a detrimental effect on the tribological properties of the SiC-PTFE-PEEK composite. The morphologies of the worn surfaces and the properties of the transfer films deteriorated, while the load-carrying capacity of the SiC-PTFE-PEEK composite was also adversely affected. The researchers claimed the reason for this was due to SiF_x, which formed on the original surface and the worn surface during the compression moulding and sliding friction processes, as a result of a chemical reaction between the nanometre SiC and PTFE. The chemical reaction and the formation of SiF_x dominated the tribological behaviour of the SiC–PTFE–PEEK composites filled with various contents of PTFE and 3.3 vol% nanometre SiC. When the PTFE volume percentage was low then the SiF_x caused friction and the wear of the SiC–PTFE–PEEK composite to rise. However, at high volume percentages the low-friction PTFE dominated the friction and wear behaviour and friction decreased as the percentage of PTFE increased. Chemical reactions and the formation of SiF_x led to changes in the worn-surface morphologies and a detrimental effect on the characteristics of the transfer films.

15.2.5 Polyimide polyarylacetylene (PAA) and poly (p-phenylene benzobisoxazole) (PBO) FRP nanocomposites

Ogasawara et al.⁶⁸ directed their investigations to improving the heat resistance of a relatively new phenylethynyl-terminated imide oligomer (Tri-A PI) by loading of MWNTs. They fabricated MWNT/Tri-A PI composites containing 0, 3.3, 7.7 and 14.3 wt% MWNT using a mechanical blender without any solution (dry condition) for several minutes. The volume fractions of MWNT were calculated to be 2.3, 5.4 and 10.3 vol% from the density of the MWNT (1.9 g/cm³) and the cured polyimide (1.3 g/cm³). Scanning electron micrographs showed the particle size of the imide oligomers to be in the range 0.1-10 μm, and the MWNTs were not dispersed uniformly in the mixture. The loss of aspect ratio during mechanical blending was not significant; therefore MWNTs can be used in various ways; for example, they are suitable for mechanical blending with imide oligomers. The preparation of the nanocomposite involved melt mixing of the MWNT/imide oligomer at 320 °C for 10 min on a steel plate in a hot press and then curing at 370 °C for 1 h under 0.2 MPa of pressure with a PTFE spacer (thickness 1 mm). The resulting composites containing 3.3, 7.7 and 14.3 wt% MWNT exhibited relatively good dispersion at a macroscopic scale. Tensile tests on the composites showed an increase in the elastic modulus and the yield strength, and a decrease in the failure strain. Figure 15.11 shows the effect of MWNT concentration on the Young’s modulus of the composites.

Dynamic mechanical analysis showed an increase in the glass transition temperature with incorporation of the carbon nanotubes. The experimental results suggested that the carbon nanotubes were acting as macroscopic cross-links and were further immobilizing the polyimide chains at elevated temperatures. As to the reason why dispersed MWNTs increased the heat distortion temperature, the researchers explained that the dispersed MWNTs impede the molecular motion in the polyimide network at elevated temperatures. The other property improvements in this material are that MWNT showed some potential for controlling electric conductivity and electromagnetic wave absorbability. Although static properties were obtained, discussions were not given and it is evident that more research is required to prove that the suggested phenomenon is a true cause of the higher glass transition temperatures.

There is increasing development in the use of polyarylacetylene (PAA) in advanced heat-resistant composites due to its outstanding heat resistance and excellent ablative properties. Fu et al.⁶⁹ reviewed the advantages of PAA resin over the state-of-the-art heat-resistant resin. The main potential applications ofPAA resin are in conventional resin matrix composites with ultra-low moisture outgassing characteristics and improved dimensional stability, which are suitable for spacecraft structures, as an ablative insulator for solid rocket motors and as a precursor for carbon–carbon composites. Carbon-fibre-reinforced PAA composites (carbon fibre/PAA) undoubtedly play a very important role in all these fields. Unfortunately, the mechanical properties of the carbon-fibre/PAA material are not yet sufficiently satisfactory to replace widely used heat-resistant composites such as carbon- or graphite-reinforced phenolic resin. The mechanical properties of carbon-fibre-reinforced resin matrix composites depend on the properties of the carbon fibre and the matrix, especially on the effectiveness of the interfacial adhesion between the carbon fibre and the matrix.

PAA has a high content of benzene rings and hence a highly cross-linked network structure, which renders the material brittle. Moreover, the chemically inert characteristics of the carbon-fibre surface lead to weak interfacial adhesion between the fibres and the non-polar PAA resin. To ensure that the material can be used safely in complicated environmental conditions and to exploit the excellent heat-resistant and ablative properties more effectively, it is necessary to improve the mechanical properties of the carbon-fibre/PAA composites. This can be achieved in two ways. One method is to improve the properties of the PAA resin by structural modification or by intermixing with other resins, such as phenolic resin. The other is treatment of the carbon-fibre surface. Treatment of the carbon-fibre surface has been studied for a long time and several methods, such as heat treatment, wet chemical or electrochemical oxidation, plasma treatment, gasphase oxidation and the high-energy radiation technique have been demonstrated to be effective in the modification of the mechanical interfacial properties of composites based on polar resins such as epoxy. For instance, Zhang et al.⁷⁰ treated carbon fibres by oxidation–reduction followed by coating with vinyltrimethoxysilane-silsesquioxane (VMS-SSO) to improve the interfacial mechanical properties of the carbon-fibre/PAA composites. The carbon-fibre surface-treatment process is shown in Fig. 15.12.

Polar functional groups, including carboxyl and hydroxyl, were imported onto the carbon-fibre surface after the oxygen plasma oxidation treatment. The quantity of carboxyl on the carbon-fibre surface decreased and that of hydroxyl increased after LiAlH₄ reduction. The LiAlH₄ reduction time was decided according to the experimental parameter in Lin.⁷¹ The VMS-SSO coating was grafted onto the carbon-fibre surface by a reaction between the hydroxyl in VMS-SSO and that on the carbon-fibre surface. The VMS-SSO coating concentrations and treatment time were decided according to Zhang et al.,⁷² who had optimized VMS-SSO coating treatment parameters. The investigation found that the interlaminar shear strength of the carbon-fibre/PAA composites increased by 59.3% after treatment.⁷⁰ The conclusion was drawn that carbon-fibre surface oxidation-reduction followed by coating with silsesquioxane is an effective method to improve the interfacial mechanical properties of carbon-fibre/PAA composites. This kind of method could be widely used for different resin matrix composites by changing the functional groups on silsesquioxanes according to that on the resin.

Poly (p-phenylene benzobisoxazole) (PBO), a rigid-rod polymer, is characterized by high tensile strength, high stiffness and high thermal stability. Kumar et al⁷³ found that PBO/CNT-reinforced fibres exhibited twice the energy- absorbing capability of plain PBO fibres. The nanocomposites were prepared as follows: ~ 4.3 g (0.02 mol) of 1,4-diaminoresorcinol dihydrochloride, ~ 4 g (0.02 mol) of terephthaloyl chloride and ~ 12 g of phosphoric acid (85%) were placed into a 250 ml glass flask, equipped with a mechanical stirrer and a nitrogen inlet/outlet. The resulting mixture was dehydrochlorinated under a nitrogen atmosphere at 65 °C for 16 h and subsequently at 80 °C for 4 h. At this stage, 0.234 g of purified and vacuum-dried HiPco nanotubes was added to the reaction flask. The mixture was heated to 100 °C for 16 h while stirring and then cooled to room temperature. P₂O₅ (8.04 g) was added to the mixture to generate poly (phosphoric acid) (77% P₂O₅). The mixture was stirred for 2 h at 80 °C and then cooled to room temperature. Further P₂O₅ (7.15 g) was then added to the mixture to bring the P₂O₅ concentration to 83% and the polymer concentration to 14 wt%. The mixture was heated at 160 °C for 16 h with constant stirring. Stir opalescence was observed during this step. The mixture was finally heated to 190 °C for an additional 4 h while stirring. An aliquot of the polymer solution was precipitated, washed in water and dried under vacuum at 100 °C for 24 h. An intrinsic viscosity of 14 dl/g was determined in methanesulfonic acid at 30 °C. A control polymerization of pure PBO was also carried out under the same conditions without adding SWNT. For PBO/SWNT (90/10) composition, 0.47 g of purified HiPco tubes (SWNT) was added to the mixture. The sequence of steps and polymerization conditions were the same for a PBO/SWNT (95/5) sample. The intrinsic viscosity values of PBO and PBO/SWNT (90/10) were 12 and 14 dl/g, respectively. Single-walled nanotubes were well dispersed during PBO synthesis in polyphosphoric acid (PPA). PBO/SWNT composite fibres were successfully spun from the liquid crystalline solutions using dry-jet wet spinning. The addition of 10 wt% SWNT increased PBO fibre tensile strength by about 50% and reduced shrinkage and high-temperature creep. The existence of SWNT in the spun PBO/ SWNT fibres was evidenced by a 1590 cm¹ Raman peak.

15.4.1 Thermal stability and fire retardancy

The commercial importance of polymers has been driving the use of composites in various fields, such as aerospace, automotive, marine, infrastructure and military applications.⁷⁵ Performance during use is a key feature of any composite material. Whatever the application, there is a natural concern about the durability of polymeric materials. The deterioration of these materials depends on the duration and the extent of interaction with the environment. Degradation of polymers includes all changes in chemical structure and physical properties due to external chemical or physical stresses caused by chemical reactions, involving bond scissions in the backbone of the macromolecules that lead to materials with characteristics different (usually worse) from those of the starting material (Fig. 15.13). As a consequence of degradation, the resulting smaller fragments do not contribute effectively to the mechanical properties, the article becomes brittle and the life of the material becomes limited. Thus, any polymer or its (nano)composite that is to be used in outdoor applications must be highly resistant to all environmental conditions.

Research indicates that the modified epoxy nanocomposites possess better flame retardant properties than conventional composites. With the Kissinger method, the activation energies for the thermo-oxidative degradation of epoxy nanocomposites are less than those of the pure epoxy in the first stage of thermo-oxidative degradation. However, in the second stage of thermo-oxidative degradation the activation energies for epoxy nanocomposites are generally higher than those of the pure epoxy. For example, the main mechanism for layered silicate is barrier formation, which influences flame spread in developing fires. Several minor mechanisms are significant, but important fire properties such as flammability or fire load are hardly influenced. Hence combinations with aluminium hydroxide and organo-phosphorus flame retardants need to be evaluated. It has been shown that carbon nanotubes can surpass nanoclays as effective flame-retardant additives if the carbon-based nanoparticles (single- and multi-walled nanotubes as well as carbon nanofibres) form a jammed network structure in the polymer matrix, so that the material as a whole behaves rheologically like a gel.⁷⁶

The thermal degradation of nanocomposites depends on clay loading, structure and the nature of the ambient gas. Recently Leszczyńska et al.⁷⁷ reviewed the thermal stability of various polymer matrices improved by montmorillonite clay and their influencing factors in detail. For the majority of polymers, due to their hydrophobic character, the clay must be modified with a surfactant in order to make the gallery space sufficiently organophilic to permit it to interact with the polymer. In fact, several factors were found to govern the thermal stability of nanocomposite materials, such as the intrinsic thermal resistance of the polymer matrix, nanofiller content, the chemical constitution of the organic modifier and the chemical character of polar compatibilizers as well as the access of oxygen to the composite material during heating. For the surface modification of clay, the surfactant is usually described as an ‘onium’ salt, but in fact ammonium salts are most commonly used. The quaternary ammonium ion is nominally chosen to compatibilize the layered silicate with a given polymer resin. However, the molecular structure (length and number of alkyl chains and unsaturation) is also the determining factor of thermal stability of polymer/MMT nanocomposites.

A possible mechanism for the degradation of modifiers in silicates has been given by, among others, Hwu et al.⁷⁸ and Leszczyńska et al.,⁷⁷ and research has shown that surfactants degrade between 200 and 500 °C. The amount of surfactant lost during thermogravimetric analysis of various organoclays indicates that surfactants with multiple alkyl tails have greater thermal stability than those with a single alkyl tail. It has been proposed that organic modifiers start decomposing at a temperature of around 200 °C, and the small molecular weight organics are released first while the high-molecular weight organic species are still trapped by the organic layered silicate matrix. With increasing temperature, high-molecular organic polymer chains may exist between the interlayers until the temperature is high enough to lead to their further decomposition. The incorporation of silicate layers with a high-aspect ratio of decomposed/charred material on the clay surface acts as a carbonaceous insulator. Silicate has an excellent barrier property, which prevents permeation of various degraded gaseous products.

The addition of clay enhances performance by acting as a superior insulator and mass transport barrier to the volatile products generated during decomposition. The clay acts as a heat barrier, which could enhance the overall thermal stability of the system as well as assisting in the formation of char during thermal decomposition. In a nanocomposite, the temperature at which volatilization occurs is higher than for a microcomposite. Moreover, thermal oxidation of the polymer is strongly slowed in a nanocomposite with high-char yield, both by a physical barrier effect, enhanced by ablative reassembling of the silicate, and by a chemical catalytic action due to the silicate and the strongly acid sites created by thermal decomposition of the protonated amine silicate modifier.

Polymers that show good fire retardancy on nanocomposite formation exhibit significant intermolecular reactions, such as inter-chain aminolysis or acidolysis, radical recombination and hydrogen abstraction. For polymers that degrade through a radical pathway, the relative stability of the radical is the most important factor in predicting the effect that nanocomposite formation has on the reduction in the peak heat release rate. The more stable the radical produced by the polymer, the better the fire retardancy, as measured by the reduction in the peak heat release rate of the polymer/clay nanocomposite.

CNTs are increasingly finding applications as thermal management materials and are also being considered in potential interface and attachment techniques.⁷⁹ Interfaces between materials have a significant effect on the thermal impedance of electronic systems and in practice they can be the dominant factor in achieving effective thermal transfer. The interface materials and processes in question are the methods used to join an electronic device to a thermal transfer medium (e.g. substrate, heat pipe, heat sink), including coatings and bonding techniques. In this respect, they may need to perform the tasks of attachment, stress/strain relief and thermal transfer. The simplest of all interfaces is a dry joint (two surfaces pushed together). In this case, interface thermal resistance can be significant and will depend on the surface materials: their hardness, co-planarity, roughness and the applied pressure to hold the surfaces together. To enhance heat transfer across the interface, thermally conductive materials can be introduced to improve surface coupling and conductivity. Commercialization is being pursued by several companies, e.g. MER Corporation is developing a CNT interface material with Mitsubishi. The material is a film on a 2-μm-thick porous (20% CNT, 80% pores) material, which is filled and laminated with polyethylene or epoxy resin to improve its mechanical properties. This system has similar thermal properties to conventional phase change materials. A hydrocarbon condensed onto the nanotubes improves wetting but may limit thermal conductivity. Nanotube sheets, in particular, have been used to thermally fuse together two polymer sheets in a transparent and seamless fashion.

This concept of a CNT dry adhesive is very attractive in terms of the potential for a high thermal conductivity interface. Zhao et al.⁸⁰ and Johnson⁸¹ recently reported the development of ‘dry adhesive/Velcro’, a novel CNT technology for thermal interface materials. MWNT ‘adhesives’ are electrically and thermally conductive (comparable to commercially available thermal paste). When grown on a surface in an array, CNTs have an extremely large surface-to-volume ratio and bind to each other and to surfaces through van der Waals interactions. When acting collectively, van der Waals forces can provide significant adhesive strength (~ 12 N/cm²) regardless of the hydrophobicity of the surfaces. Similar work was reported by Xu et al.,⁸² who developed CNT interfaces for improved thermal and electrical management, which achieved a thermal interface resistance of 20–31 mm². K/W at a pressure of 0.445 MPa. Additionally, the combination of a CNT array and a phase change material (load 0.35 MPa) produced a minimum resistance of 5.2 mm².K/W. Hu et al.⁸³ described an exploratory thermal interface structure, made of vertically oriented CNTs directly grown on a silicon substrate, which has been thermally characterized using a 3-omega method. The effective thermal conductivities of the CNT samples, including the effects of voids, were found to be 14 W/m.K to 83 W/m.K in the temperature range of ~ 22-50 °C, one order higher than the best thermal greases or phase change materials. These results suggest that vertically oriented CNTs can potentially be a promising next- generation thermal interface solution. However, fairly large thermal resistances were observed at the interfaces between the CNT samples and the experimental contact. Minimizing these contact resistances is critical for the application of these materials. Additionally, the potential for a low insertion force, ‘flexible’ (stress relieving) and reworkable die and substrate attachment system would be of significant benefit for assembly and maintenance. This research has demonstrated the potential of this new technology; however, there are still significant issues in terms of strength, performance and reliability to address prior to these materials being acceptable as an attachment system in a high-performance application. Butt joints give poor thermal resistance and research into devising a low thermal resistance joint is ongoing.

A limiting factor in the use of CNTs is the transfer of heat flux from one nanotube to another in an efficient manner, e.g. in a butt joint. CNTs can be woven into mats to produce a low-density, high-thermal-conductivity material. This can be put into a metal composite by pressure or squeeze casting, or epoxy can be added as a filler to give rigid mats; however, these approaches are still at an early stage. Although expectations of CNTs are very high for their use in composites, there has been some speculation on the results they produce when mixed with polymers. For instance, CNTs are good conductors by themselves but they may not exhibit the same level of conductivity when integrated into other materials. Experiments have shown that thermal conductivity has increased by two or threefold, when it should have been close to 50-fold. The problem is that CNTs vibrate at much higher frequencies than the atoms in the surrounding material, which causes the resistance to be so high that thermal conductivity is limited. Inducing stronger bonds between the nanotube and the other materials might help in solving the problem.

The addition of CNTs to a polymer matrix could increase the glass transition, melting and thermal decomposition temperatures due to their constraint effect on the polymer segments and chains. It is important to improve the thermal endurance of polymer composites.⁸⁰ Thermal management is a growing need in aerospace and defence applications alike. As device engineers continue to follow Moore’s Law and devices get smaller and more powerful, the management of heat has become a serious issue. However, thermal transport in nanostructures requires an understanding of heat transport beyond that gained at the continuum level and necessitates advances in measurement methods. This arena is one that offers many opportunities for exploration. An increased understanding of interfaces, dispersion and percolation networks and morphology characteristics is expected to provide pervasive and significant advances in the field of nanocomposites. Improved thermal management is essential for meeting market-driven performance, lifetime and cost requirements. This is particularly crucial for the next generation of designs with their need for enhanced functionality, higher volumetric power densities and increased reliability. In many cases, such as all-electric aircraft, military applications and many sensing systems, thermal management solutions also have to perform in harsh environments. The main issues with the current technique are ensuring that the silicon is kept below 350 °C during CNT processing (this is not an issue with SiC devices) and the need to apply a clamping pressure (1–4 atm) to affect a joint. Potential applications include microprocessors and power electronics, such as those used in military appliances.

15.4.2 Electronic properties

Despite the excellent dispersion of CNTs and CNFs, the percolation thresholds in various systems are drastically different. Ultra-low percolation thresholds in the range 0.0021–0.0039 wt% and, in contrast, high percolation thresholds in the range 3–5 wt% have been reported in the literature. Major uncertainties are associated with the type and quality of nanotubes, that is, a wide variety of synthesis methods have been employed to obtain nanotubes of different sizes, aspect ratios, crystalline orientation, purity, entanglement and straightness. It has been reported that, when the aspect ratio of CNTs was reduced from 411 to 83–8.3 in epoxy/CNT nanocomposites, the corresponding percolation threshold increased from 0.5–1.5 and > 4 wt%, respectively, indicating that the aspect ratio is a predominant factor.⁸⁴

In contrast, for an aspect ratio of 300, Kim et al.⁸⁵ reported a percolation threshold of 0.011–0.011 vol% in epoxy/CNT nanocomposites. Even with an aspect ratio of 1000, Allaoui et al.⁸⁶ obtained a percolation threshold of 0.6 vol%. Moreover, it is also rather interesting to note that, even with the same kind of nanotubes, the percolation threshold varied (from 0.0225 to 10 wt%) depending on the matrix materials. Although the differences can be qualitatively explained based on the type and nature of the matrix/resin and the cross-linking density, concrete knowledge is still lacking. In fact, it is very difficult to control the local cross-linking density as it in turn depends on the nature of fillers, their disentanglement and orientation. With polyvinyl alcohol as the matrix and the same Hyperion nanotubes as fillers, Shaffer and Windle⁸⁷ reported a percolation threshold between 5 and 10 wt% for nanotubes. Sandler et al.⁸⁸ also reported a percolation threshold between 0.0225 and 0.04 wt% in epoxy nanocomposites based on these nanotubes. Although differences in melt viscosity, cross-linking density (and percentage crystallinity for thermoplastics) may qualitatively explain the observed variations with different matrices, proper experimental evidence is still lacking.

In comparison to carbon nanotubes where the graphitic layers are parallel to the nanotube axis, carbon nanofibres, in general, can show a wide range of graphitic layer orientations with respect to the fibre axis. This dramatically affects the percolation threshold of the materials. Nevertheless, it is important to note that the ultra-low percolation thresholds reported in some systems were in fact in samples prepared at a very small scale.

Recently, super-capacitors have attracted much attention because of their high capacitance and potential applications in electronic devices. It has been reported that the performance of super-capacitors with MWNTs deposited with conducting polymers as active materials is greatly enhanced compared with electric double-layer super-capacitors with CNTs, due to the Faraday effect of the conducting polymer. Additionally, polymer/CNT nanocomposites could have many potential applications in electrochemical actuation, electromagnetic interference shielding (EMI), wave absorption, electronic packaging, self-regulating heaters and PTC resistors. Furthermore, synthetic DNA has been proposed for quantum dot and nanotube- based computing systems.⁸⁹ The use of DNA as a structural material has opened up many new possibilities for the engineered nanoscale fabrication of computing systems. Beginning with efforts to crystallize DNA to determine its natural structure, the development of synthetic DNA as a structural material for nanoscale fabrication has produced a nanoscale barcode and various 2D and 3D structures.

Nanotube transistors have also been produced using integrated nanotubes, which may lead to large-scale integration. The patterned growth of CNTs on silicon wafers may prove necessary for the integration of nanotubes in aerospace and defence electronics. Also, coiled nanotubes could have even more interesting applications in various areas than their straight counterparts. For example, the conduction of electricity through a coiled nanotube generates an inductive magnetic field, an indication that coiled nanotubes, unlike straight nanotubes, could be of use as electromagnetic nanotransformers or nanoswitches. Recent research and developments have led to the possibility that nanotubes will be useful for downsizing circuit dimensions. Presently, current-induced electromigration causes conventional metal wire interconnects to fail when their diameter becomes too small. The covalently bonded structure of CNT militates against the similar breakdown of nanotube wires and, because of ballistic transport, the intrinsic resistance of the nanotube should essentially vanish. Experimental results have shown that metallic SWNTs can carry up to 10⁹A/cm²compared with current densities for normal metals of only 10⁵A/cm².

Due to scalability and low power consumption, field emitters are attractive candidates for a wide variety of space applications, particularly where budget is a major consideration. Field emission (FE) using carbon nanotubes has already been demonstrated as ideal for high-voltage low-current electrical power applications such as field-emission electric propulsion (FEEP), colloids, micro-ion thrusters and perhaps even small electrodynamic (ED) tethers.⁹⁰ A typical cathode delivers up to 10 mA with gate voltages in the range of 200 to 400 volts and weighs about 20 grams. CNT FE fits well with requirements for micro-satellites, which are considered to be a viable alternative for a variety of applications, and microtechnology, by contributing to a substantial reduction of mass, volume and power requirements for small satellites and satellite sub-systems.

Carbon nanotube technology could have a dramatic breakthrough for magnetic devices, especially magnets, for space and aircraft applications. The basic electronic properties of semiconducting CNTs change when placed in a magnetic field. Nanotube band gaps are comparable with silicon and gallium arsenide, which are currently the mainstays of the computer industry because their narrow band gaps correspond with how much electricity it takes to flip a transistor from ‘on to off’. Superconductivity has been reported by both doping a SWNT with caesium, potassium or rubidium and packing small buckyballs inside it.⁹¹ With the possibility of the band gap of carbon nanotubes disappearing altogether in the presence of stronger magnetic fields, CNTs could take over the role of silicon and gallium arsenide, potentially revolutionizing the computer industry. If either the promise of superconductivity with current densities of 106 A/cm². or higher or the greatly improved strength of a composite material based on carbon nanotube fibres is achieved, there could be significant weight reductions in magnets. Therefore, the most urgent need is to determine the superconducting properties of possible CNTs. That is, we need to know the feasible current density as a function of temperature, magnetic field and strain.

15.4.3 Field emission and optical properties

Carbon nanotubes possess the right combination of properties (nanometre-sized diameter, structural integrity, high electrical conductivity and chemical stability) to make good electron emitters. Research on electronic devices has focused primarily on the use of SWNTs and MWNTs as field-emission electron sources for flat panel displays, lamps, gas discharge tubes providing surge protection and X-ray and microwave generators. A potential applied between a nanotube-coated surface and an anode creates a high electric field due to the small radius of the nanofibre tips and the lengths of the nanofibres. The local fields cause electrons to tunnel from a nanotube tip to the tunnel. This process of nanotube-tip electron emission differs from that of bulk metals because it arises from discrete energy states instead of continuous electronic bands and its behaviour depends on the nanotube-tip structure, whether SWNT or MWNT.

The importance of electromagnetic interference (EMI) shielding has also increased in the electronics and communication industries, especially in space and military uses, due to the widespread use of packed highly sensitive electronic devices. Kim et al.⁹² designed radar-absorbing structures (RASs) with a load- bearing ability in the X-band. Glass/epoxy plain-weave composites with excellent specific stiffness and strength, containing MWNTs to induce dielectric loss, were fabricated. Fabrication involved impregnation of glass/epoxy plain-weave composites by mixing a matrix and MWNTs. The MWNT-filled fabric composites were dried for 5–7 min at 100 °C. Drying times increased with MWNT content. As the viscosity of the premixture increased rapidly above 3.0 wt%, the researchers reported that they found it difficult to maintain the uniformity of MWNTs in the matrix. Specimens were cured at a stabilized pressure of 3 atm and vacuum- bagged in an autoclave initially for 30 min at 80 °C and then for 2 h at 130 °C. Observation of the microstructure of the composites revealed that the uneven distribution of MWNTs could induce high dielectric loss, which was confirmed through a measurement of permittivity.

The optimal design of a two-layered RAS, consisting of MWNT-added glass/ epoxy fabric composites, was performed by linking a genetic algorithm with a method for the reflection and transmission of electromagnetic waves in a multilayered RAS. As a result, a two-layered RAS was designed having 90% absorption of electromagnetic (EM) energy over the entire X-band. An RAS fabrication process was proposed based on the non-linearity of the thickness per ply with MWNT content and the number of plies. A comparison between the theoretical and experimental reflection losses confirmed that the process can be used in the fabrication of multilayered RASs. However, the authors commented that further studies directed to broadening the absorbing bandwidth of a RAS, comprising a multilayered RAS and a frequency selective surface, are required. Furthermore, it was reported that GE Plastics has been using CNTs in a poly(phenylene oxide)/ polyamide blend for automotive mirror housings for Ford⁹³ to replace conventional micron-size conducting fillers, which would require loadings as high as 15 wt% to have satisfactory anti-static properties; however, this level of CNT loading would result in poor mechanical properties and a high density of the final composite.

While mechanical properties in this range have been observed for individual SWNTs, those observed for assemblies of these nanotubes in nanotube yarns and sheets are much lower, which restricts the performance of actuators based on them. At high levels of charge injection into a CNT, the predominant cause of actuation is electrostatic, as with dielectric elastomer actuators (DEAs).⁹⁴ However, the electrostatic forces are repulsive interactions between like charges injected into the nanotubes, rather than between two electrodes. Charges are injected by applying a voltage between an actuating nanotube electrode and a counter electrode, through an ion-containing solution, as depicted in the upper image of Fig. 15.14 (where the counter electrode is another CNT), leading to charging (Fig. 15.14).

Electrostatic repulsive forces between like charges on the CNT work against the stiff carbon-carbon bonds in the nanotubes to elongate and expand the nanotubes, though quantum mechanical effects can predominate over electrostatic forces at low levels of charge injection. Unlike dielectric elastomers, strains are low (< 2%) since CNTs are extremely stiff. Actuation is generally achieved in films or yarns (Fig. 15.14 bottom) composed of many nanotubes. The porous nature of the films and fibres enables fast ion transport with response times of < 10 ms, with effective strain rates of 19%/s and effective power-to-mass ratios of 210 W/kg (half that of a high revving electric motor). The achievable response rate decreases with increasing nanotube yarn or sheet thickness, increasing interelectrode separation, and decreasing electrolyte ionic conductivity. A high work density combined with good temperature stability (> 450 °C in air, > 1000 °C in an inert environment) make CNTs prime candidates for situations where weight and temperature are important, such as aerospace and defence applications. Strains are relatively small compared with other polymer actuators (but an order of magnitude larger than found in typical piezoceramics). Strain could be increased by employing electrolytes such as highly purified ionic liquids that can withstand very large potentials without reacting electrochemically. CNT actuators have recently been shown to actuate when used as electrodes in a fuel cell. This possibility is exciting because the energy density of fuel cells is much higher than that of batteries, helping to enable autonomous applications of CNTs and possibly artificial muscle technology.

Research into field effect transistors (FETs) aims to replace the source/drain channel structure with a nanotube. Transistors assembled with carbon nanotubes may or may not work, however, depending on whether the chosen nanotube is semiconducting or metallic, over which the operator has no control. It might be possible to peel back layers from a MWNT to achieve the desired properties, but advances in microlithography are still needed to perfect this reduction method. Recent developments have focused media attention on nanotube nanoelectronic applications. Crossed SWNTs have been used to produce three- and four-terminal electronic devices along with non-volatile memory that functions like an electromechanical relay.

Irradiated by a camera flash, a random network of polyaniline (PANI) nanofibres was turned into a smooth and shiny film due to the highly efficient photothermal conversion and low thermal conductivity of PANI, which demonstrated a versatile new technique for making polymers into potentially useful structures.⁹⁵ One of the great advantages of the flash-welding technique is that an area can be selectively welded by using a photomask, differing from other welding techniques like microwave welding. It is used to fabricate polymer films to pre-designed patterns. In addition, the technique can rapidly create asymmetric films, which are widely used in many applications such as separation membranes, chemical sensors and actuators.

Flat panel displays are one of the more lucrative applications of carbon nanotubes but are also one of the most technically complex. Nanotubes have an advantage over liquid crystal displays since they have low power consumption, high brightness, a wide viewing angle, a fast response rate and a wide operating range. In a flat panel display, electric fields direct field-emitted electrons toward an anode where phosphorus produces light. One demonstration used nanotube/ epoxy stripes on the cathode glass plate and phosphor-coated indium-tin-oxide (ITO) stripes on the anode plate. The pixels were at the intersections of the cathode and anode stripes. Pulses of ± 150 V were switched between the anode and cathode stripes to produce an image. Various types of carbon nanotube are being considered for future field-emission displays (FEDs). Samsung is a leading player in CNT-based FE displays.⁹⁶ They have produced several prototype CNT-FEDs, including a 9-in. red-blue-green (RGB) colour FED that can operate at video frame speed. One beauty of the Samsung display is that the CNT-based cathode materials are printable inks.

In addition to flat panel displays, another potentially important application of CNT is in polymer-based light-emitting devices. The advantages of organic light- emitting diodes (OLEDs) based on conjugated polymers are low cost, low operating voltage, excellent processability and flexibility.⁹⁷ However, their low quantum efficiency and stability have limited their application and development. Carbon nanotubes are also being considered as electron emitters for high- brightness lighting elements (Fig. 15.15).

Nanotube-based lamps are similar to displays, comprising a nanotube-coated surface opposing a phosphor-coated substrate, but they are less technically challenging and require less investment. With lifetimes expected in excess of 8000 h, they could replace environmentally problematic mercury-based fluorescent lamps used in stadium-style displays. More recently, lighting elements for the three primary colours have been produced and a brightness > 10 000 cd/m² has been demonstrated.⁹⁸ The frequency characteristics of the cold cathode element were studied over a range of frequencies. The cut-off frequency in these devices was found to be determined by the vacuum gap capacitance between the cathode and the gate electrode. The devices were repeatedly operated at 50 kHz without any degradation. It is quite feasible to use cold cathode lighting elements to assemble a full-colour display with video frame speed for large pixel screens. Nanotube-based gas discharge tubes might also find commercial use in protecting telecommunications networks from power surges. Another application arises if a metal target replaces the phosphorescent screen at the anode. This causes the accelerating voltage to increase, producing X-rays instead of light. The compact geometry of this nanotube-based X-ray generator could lead to use as an X-ray endoscope and for medical exploration.

Non-linear optical organic materials, such as porphyrins, dyes and phthalocyanines, have optical limiting properties, which can be used to control light frequency and intensity in a predictable manner in photonic devices. However, these are narrow-band optical materials. The combination of the unique properties of CNTs with conducting organic polymers (e.g., polyaniline, polypyrrole, polythiophene, poly(3,4-ethylenedioxy thiophene), poly(p-phenylene vinylene) and poly(m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene)) makes these materials interesting multifunctional systems with great potential in many applications such as super-capacitors, sensors, advanced transistors, high-resolution printable conductors, electromagnetic absorbers, photovoltaic cells, photodiodes and optical limiting devices. As a result of the optical limiting performance and the good photoconductivity, special attention has been given to CNTs functionalized with polymers such as poly (N-vinyl carbazole).

15.4.4 Age and durability performance

The study of the degradation and stabilization of polymers is an extremely important area from a scientific and industrial point of view and a better understanding of polymer degradation will help to increase the life of a product.⁹⁹ Polymer degradation in broader terms includes biodegradation, pyrolysis, oxidation, mechanical, photo- and catalytic degradation. Because of their chemical structure, polymers are vulnerable to harmful effects in the environment. In the following sections, epoxy and polyamides are considered. It is important to note that little attention has been given to the durability of polymer nanocomposites compared with their preparation techniques and the evaluation of mechanical properties.

15.4.5 Impact resistance and energy absorption

Damage due to low- and high-velocity impact events weakens the structure of composite materials due to a continuous service load. Furthermore, an impact may generate different types of flaws before full perforation, i.e. sub-surface delamination, matrix cracks, fibre debonding or fracture, indentation and barely visible impact damage (BVID). Over time, these effects can induce variations in the mechanical properties of composite structures (the primary effect of a delamination is to change the local value of the bending stiffness and the transverse-shear stiffness), leading to possible catastrophic failure. It has been reported that the energy-absorption capability and related properties of polymer matrices can be engineered by adding nanoscale fillers. For example, rigid nano-sized particles such as SiO₂, TiO₂, CaSiO₃, Al₂O₃ powder, CNTs and clay nanoplatelets have been used, and some important findings are summarized in this section.

Typical fillers for the reinforcement of polymer matrices are particles (e.g. silica or aluminium oxide particles), tubes (e.g. nanofibres or nanotubes) and plates (e.g. nanoclay platelets). Significant enhancement of the impact strength of polymeric nanocomposites has been achieved by adding amino-functionalized MWCNTs or small amounts of single-walled carbon nanotube (SWCNTs). The sensitivity of FRPs to intrinsic damage (delamination, matrix cracking and fatigue) and their minimal multifunctionality requires a significant effort to improve their performance to meet spacecraft application standards. Currently, however, there are many questions surrounding the incorporation of nanoparticles, like CNTs and CNFs, in FRPs with regard to manufacturing methodology and structural integrity. The toughening of particle-modified semi-crystalline polymers is related to the inter-particle distance, s, for any type of added particle. The distance s depends on both the concentration, u, and the average size, d, of the particles.

A number of experiments have shown that fracture toughness improved with the addition of clay nanoplatelets to epoxy when the clay nanoplatelets were not fully exfoliated and intercalated clay nanoplatelets were present. Such very high improvements are not typically observed for composites reinforced with conventional micro-particles. Subramaniyan and Sun⁴⁴ reported that core/shell rubber (CSR) nanoparticles, having a soft rubber core and a glassy shell, improved the fracture toughness of an epoxy vinyl ester resin significantly more than MMT nanoclay particles with the same weight fraction. However, hybrid blends of CSR and nanoclay were found to give the best balance of toughness, modulus and strength. The same investigators highlighted that, when the nanoclay particles were used to enhance the polymer matrix in a conventional glass-fibre-reinforced composite, the interlaminar fracture toughness of the composite was less than that of the unreinforced composite. It was suggested that a possible reason for this result was the arrangement of the nanoclay particles along the fibre axis.

Two factors affect the capacity of rigid particles for energy dissipation at high loading rates:

Therefore, the optimal minimal rigid-particle size for polymer toughening should fulfil two main requirements: to be smaller than the critical size for polymer fracture and also to have a debonding stress that is small compared with the polymer matrix yield stress.

Most of the studies that noted an increase in toughness by incorporating CNTs in polymers (both thermosets and thermoplastics) have attributed it mainly to the nanotube pull-out mechanism and the bridging of cracks in the matrix. The nanotube pull-out mechanism was inspired by conventional polymer/fibre composites where fibre/matrix debonding and fibre pull-out (including work done against sliding friction in pulling out the fibre) govern the extent of energy absorption. Hence, the very high interfacial areas in polymer/nanotube composites are expected to result in drastic improvements in the work necessary for fracture due to nanotube pull-out. To explain and confirm the pull-out mechanism, Barber and co-workers¹¹⁹ studied the pull-out of individual nanotubes by attaching them to the end of a scanning probe microscope (SPM) tip and pushing them into a liquid epoxy polymer (or liquid melt of polyethylene-butene). After the polymer had solidified, the nanotube was pulled out and the forces were recorded from the deflection of the SPM tip cantilever. Although this provided an estimate of the interfacial strength of individual nanotubes, it is not directly relevant to pull-out toughness measurement, which depends on many factors like the alignment/ orientation and flexible/entanglement nature of the nanotubes. Even by increasing the embedded length of the nanotube in the resin, the nanotube breaks instead of being pulled out from the polymer.

Nanotube-reinforced structures have up to eight times higher tensile strength and advanced energy dissipation mechanisms, so more damping can be achieved with smaller and lighter structural designs. The number of CNT walls and their size affects stress concentration in the composite, and thus short and even round particles are strongest (like diamonds, etc.). However, longer fibres are flexible and may be better for damping. A CNT may act as a nanoscale spring and a crack-trapping material in composites. These damping phenomena could be multiplied when CNTs are dispersed. Orientation and geometry (waviness) of CNT particles may affect the mechanisms of energy dissipation and fracture mechanics. Maximum stiffness is achievable when the longitudinal orientation of CNTs is 90°. Notably, open-end CNTs do not collapse, fail or buckle due to higher stress concentrations, while many researchers have used closed-end CNT-reinforced composites. Thus isolated SWNTs may be desirable for damping applications due to their significant load-bearing ability because of CNT/matrix interactions. Defects of the carbon particles likely limit performance. In future, CNTs may serve as storage containers for sealant or multi-purpose particles of another material.

A Kireitseu et al.¹²⁰ study on the rotating fan blades of turbine engines represents another feasible aerospace or defence application. The authors considered a large rotating civil engine blade, illustrated in Fig. 15.19, which is typically hollow and usually has stiff rib-like metallic structures in order to increase rigidity and maintain the cross-sectional profile of the blade. They suggested a foam-filled fan where the metal structure or traditional fillers are replaced with a CNT-reinforced syntactic foam and, also, with a CNT-reinforced composite layer on the top. Results of the damping behaviour and impact toughness of the composite sandwiches showed that CNT-reinforced samples have better impact strength and vibration-damping properties over a wide temperature range. Experiments conducted using a vibrating clamped beam with the composite layers indicated up to a 200% increase in the inherent damping level and a 30% increase in the stiffness with some decrease (20–30%) in the density of the composite. The crosslinks between the nanotubes and composite layers also served to improve the load transfer within the network, resulting in improved stiffness properties. The critical issues to be considered include: the choice of nanotubes and related matrix materials for vibration damping; tailoring the nanotube/matrix interface with respect to the matrix; and the orientation, dispersion and bonding of the nanotubes in the matrix. It is anticipated that significant weight, thickness and manufacturing cost reductions could be achieved in this way.

The critical parts in aerospace vehicles depend on both the strength and toughness of the materials they are made of, while the associated technology places strict limitations on the weight of the different components. Notably, matrix toughening may be performed through chemical modification or, more recently, with the incorporation of fillers in the matrix material. An interface can be modified by grafting in order to tailor the chemical compatibility between the fibres and the matrix. In this way, despite their size, nanofillers can have a significant presence in a small zone. In contrast, only a few micro-particles participate in plastic zone deformation. It then follows that nanofillers can lead to better fracture properties of a brittle matrix, resulting in enhancement of the matrix fracture toughness, which can lead to an overall advanced fracture behaviour. Additionally, by involving more nanofibres during delamination the nanofibres can act as reinforcing fibres, thus increasing fracture toughness. These approaches may offer alternatives to the conventional interlaminar toughness improvement of fibre- reinforced composites (especially high-performance, high-temperature composites), which has been addressed in various ways, such as by tailoring the matrix or interface properties, stitching, Z-pinning or interleaving. These approaches also improve mechanical properties, such as fatigue life. Coiled CNTs (Fig. 15.20) are another option for advanced composite reinforcement technology.

Traditional straight carbon nanotubes are recognized as strong reinforcements, which substantially enhance the strength of composites. However, the drawbacks, such as the decrease of fracture toughness and subsequent increase of the brittleness of the composites, may restrict the utilization of these reinforcements for some composite structures. Coiled nanotubes are ideal reinforcements for composite and polymer-based materials. These reinforcements can provide moderate strength improvement as well as enhancing the fracture toughness of the composites without substantially increasing the weight and damaging the nanotube structures.

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