However, these membranes suffer from fouling tendency as the incorporation of hydrophobic CNTs in polyamide membranes leads to facile irreversible adsorption of organic/inorganic/ biological impurities. It is shown that the fouling problems can be circumvented by altering the surface morphology of the membrane. Since CNTs can be easily transformed from hydrophobic to hydrophilic by acid treatment; therefore, incorporation of CNTs functionalized with hydrophilic/amphiphilic groups is considered very advantageous for long-term uninter‐ rupted performance of theses membranes [184–188]. In this direction, Choi et. al. [187] have fabricated a polysulfone/MWCNT hybrid membrane by phase inversion process. The surfacemodified MWCNTs provide hydrophilicity and conductivity to the membrane. The authors reported that the pore size of the membrane increased with increase in CNT loading up to 1.5 wt% but further loading of CNTs increased viscosity of blend solution which led to decrease in pore size. The membrane with 4 wt% of MWCNT has pore size just smaller than pure polysulfone membranes and exhibited higher flux and good salt rejection ability. Nevertheless, the CNTs reinforced polymeric membranes are still a new concept and more efforts in the direction are necessary to further improve their performance and address the negative

The major drawbacks such as high operating temperature, elevated costs and complex fabrication protocols associated with the conventional metal oxide-based sensors have prompted researchers to look for new avenues to come up with new materials not plagued by these shortcomings. This insatiable quest has unfolded the arena of CNTs-based polymer nanocomposites. The synergistic combination of the remarkable electrical transport and mechanical properties of CNTs and easily tailorable electroactive nature of conducting polymers has the potential to give one of the best-sensing platforms for the efficient gas/ chemical vapour detection [99, 189]. Therefore, dedicated efforts have been made to combine CNTs with various conducting polymers such as polyaniline [190–192], poly(3,4-ethylene‐ dioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)[193], polypyrrole (PPy) [194], hexa‐ fluoroisopropanol substituted polythiophene (HFIP-PT), poly(3-hexylthiophene) (P3HT) [195], poly 3-methyl thiophene [98], to fabricate portable, more stable, highly sensitive, costeffective and energy efficient chemiresistors especially for the detection of extremely minute

 **h) Salt rejection (%)**

In conclusion, it is notable that CNTs-based polymer nanocomposites present an array of possibilities for their use in various technology developments. The electrical properties and dependent applications of CNTs-based nanocomposites (e.g. EMI shielding, antistatic) are

**Figure 21.** Selective response of the drop coated and AC-DEP assembled PEDOT/PSS-SWCNTs nanocomposite films to various vapours of 10 ppm. Reproduced from [193], with permission from Elsevier.

already satisfactory, though the scope for improvement cannot be ruled out especially in terms of cost reduction. In the context of thermal properties, CNTs-polymer nanocomposites with good thermal stability and appreciable thermal conductivity have already been exploited for heat sink and thermal-interfacing applications. However, for mechanical properties in particular, some key challenges need to be addressed and resolved so that the full potential of CNTs can be realized. Out of these challenges, the ability to acquire homogeneous dispersion of CNTs and their alignment in the polymer matrix remain major bottlenecks. The limitations faced due to this adversely affect the available filler surface area and thus the load transfer between the filler and the matrix leading to a compromise in terms of mechanical properties. With the increased understanding of functionalization chemistry in the recent years, the issue of dispersion has been partly circumvented. However, to compete with the existing carbon fibre-based nanocomposites, further efforts are needed in this direction. Novel functionaliza‐ tion routes are needed which enable maximum possible homogeneity in CNTs dispersion with least sacrifice on the part of mechanical properties of nanocomposites. Due to the large costs involved in the synthesis and processing of these nanocomposites, their commercial viability also needs mention in terms of key challenges. Furthermore, the high thermal conductivity of CNTs can be capitalized only when the high interfacial thermal resistance of nanotube networks can be minimized. In a nutshell, the future of CNTs-based polymer nanocomposites decisively hinges on the success achieved in the address of these key challenges.

### **Author details**

Waseem Khan, Rahul Sharma and Parveen Saini\*

\*Address all correspondence to<EMAIL_ADDRESS><EMAIL_ADDRESS>
Polymeric and Soft Materials Section, Materials Physics and Engineering Division, CSIR-National Physical Laboratory, New Delhi, India

### **References**

already satisfactory, though the scope for improvement cannot be ruled out especially in terms of cost reduction. In the context of thermal properties, CNTs-polymer nanocomposites with good thermal stability and appreciable thermal conductivity have already been exploited for heat sink and thermal-interfacing applications. However, for mechanical properties in particular, some key challenges need to be addressed and resolved so that the full potential of CNTs can be realized. Out of these challenges, the ability to acquire homogeneous dispersion of CNTs and their alignment in the polymer matrix remain major bottlenecks. The limitations faced due to this adversely affect the available filler surface area and thus the load transfer between the filler and the matrix leading to a compromise in terms of mechanical properties. With the increased understanding of functionalization chemistry in the recent years, the issue of dispersion has been partly circumvented. However, to compete with the existing carbon fibre-based nanocomposites, further efforts are needed in this direction. Novel functionaliza‐ tion routes are needed which enable maximum possible homogeneity in CNTs dispersion with least sacrifice on the part of mechanical properties of nanocomposites. Due to the large costs involved in the synthesis and processing of these nanocomposites, their commercial viability also needs mention in terms of key challenges. Furthermore, the high thermal conductivity of CNTs can be capitalized only when the high interfacial thermal resistance of nanotube networks can be minimized. In a nutshell, the future of CNTs-based polymer nanocomposites

**Figure 21.** Selective response of the drop coated and AC-DEP assembled PEDOT/PSS-SWCNTs nanocomposite films to

various vapours of 10 ppm. Reproduced from [193], with permission from Elsevier.

32 Carbon Nanotubes - Current Progress of their Polymer Composites

decisively hinges on the success achieved in the address of these key challenges.

\*Address all correspondence to<EMAIL_ADDRESS><EMAIL_ADDRESS>
Polymeric and Soft Materials Section, Materials Physics and Engineering Division, CSIR-

**Author details**

Waseem Khan, Rahul Sharma and Parveen Saini\*

National Physical Laboratory, New Delhi, India

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## **Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling**

Hai M. Duong, Feng Gong, Peng Liu and Thang Q. Tran

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62510

#### **Abstract**

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*Mater.* 16, 1005–1009 (2004).

Aligned carbon nanotube (CNT) composites have attracted a lot of interest due to their superb mechanical and physical properties. This article presents a brief overview of the synthesis approaches of aligned CNT composites. The three major methods for fabricating aligned CNT fibers are first reviewed, including wet-spinning, dry-spinning and floating catalysts. The obtained CNT fibers, however, have limited mechanical and physical properties due to their porous structure and poor CNT alignment within the fibers. Appropriate treatments are required to densify the fibers to enhance their properties. The main approaches for the densification of CNT fibers are then discussed. To further enhance load transfer within CNT fibers, polymer infiltration is always used. Typical studies on polymer infiltration of CNT fibers are reviewed, and the properties of the obtained composites indicate the superiority of this composite fabrication method over the conventional dispersion method. Since aligned CNT composites are usually obtained in structures of long fiber or thin film, it is difficult to measure the thermal conductivity of these composites. An off-lattice Monte Carlo model is developed to accurately predict the thermal conductivity of aligned CNT composites.

**Keywords:** Aligned CNT composite, CNT fiber, fiber densification, polymer infiltra‐ tion, Monte Carlo model

### **1. Introduction**

Both single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs), which possess remarkable multifunctional properties such as high Young's modulus of 1 TPa [1], ultrahigh thermal conductivity of 2000–3500 W/mK [2,3], and outstanding electrical conductivity of 3×104

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S/cm [4], have attracted much attention over the past decades [5]. Carbon nanotubes (CNTs) have been considered promising effective additives for developing high-performance compo‐ sites. Currently,inthe widely-usedapproaches forfabricationof CNT-basedcomposites,CNTs arerandomlydispersedintothematrix.Thisapproach,however,typicallyresults inlowvolume fraction, poor dispersion and random orientation of CNTs in matrices, inducing very limited enhancementsandmuchlowerpropertiesthanexpected.Toovercometheselimitations,various CNT Structures such as buckypapers [6], CNT arrays [7–9], and CNT yarns [10] have been developed to pre-arrange the CNTs prior to fabricating composites.

Among those CNT post-treatments, assembling CNTs into CNT fibers has attracted tremen‐ dous attention. In general, there are three major methods for production of CNT fibers: (1) wetspinning from CNT/acid or polymer solutions [11–13]; (2) dry-spinning from vertically aligned CNT arrays [10,14–17]; and (3) direct-assembling from CNT aerogels formed in chemical vapor deposition (CVD) [5,18–24]. The first method is also known as the wet-spinning method, while the others are known as dry-spinning methods.

The obtained CNT fibers commonly possess satisfactory mechanical and electrical properties [5], and even higher strength and better flexibility than commercial carbon fibers and polymer fibers [25]. In order to further improve their properties (mechanical strength in particular), polymer infiltration is usually performed on the CNT fibers to obtain CNT fiber/polymer composites. The polymer can greatly enhance the inter-tube load transfer, inducing high mechanical strength of the composites.

### **2. Methods for assembling CNTs into CNT fibers**

### **2.1. Spinning from CNT Solution**

In 2000, Vigolo et al. [11] first fabricated CNT ribbons and fibers via the coagulation spinning approach that was widely used to synthesize polymer fibers. **Figure 1**(**a**) shows the schematic of the experimental setup used to make nanotube ribbons. In this method, SWNTs are homogeneously dispersed in a solution of sodium dodecyl sulfate (SDS), which helps prevent CNTs from agglomeration. The CNT dispersion is then injected into the co-flowing stream of a polymer solution that contains 5.0 wt.% of polyvinylalcohol (PVA) to form CNT ribbons, as shown in **Figure 1**(**b**). **Figure 1**(**e**) shows a typical SEM image of the as-obtained CNT ribbon, revealing a preferential orientation of the nanotubes along the ribbon's main axis. After the ribbons are washed and dried, most of the surfactants and polymers are removed. The ribbons are collapsed into fibers due to capillary force, as shown in **Figure 1**(**c**). These fibers are more flexible than traditional carbon fibers, as shown in **Figure 1**(**d**). The diameter of CNT fibers varies from 10 to 100 μm depending on fabrication conditions. The tensile strength, modulus and electrical conductivity of the obtained fibers are 300 MPa, 40 GPa and 10 S/cm, respectively.

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510 49

S/cm [4], have attracted much attention over the past decades [5]. Carbon nanotubes (CNTs) have been considered promising effective additives for developing high-performance compo‐ sites. Currently,inthe widely-usedapproaches forfabricationof CNT-basedcomposites,CNTs arerandomlydispersedintothematrix.Thisapproach,however,typicallyresults inlowvolume fraction, poor dispersion and random orientation of CNTs in matrices, inducing very limited enhancementsandmuchlowerpropertiesthanexpected.Toovercometheselimitations,various CNT Structures such as buckypapers [6], CNT arrays [7–9], and CNT yarns [10] have been

Among those CNT post-treatments, assembling CNTs into CNT fibers has attracted tremen‐ dous attention. In general, there are three major methods for production of CNT fibers: (1) wetspinning from CNT/acid or polymer solutions [11–13]; (2) dry-spinning from vertically aligned CNT arrays [10,14–17]; and (3) direct-assembling from CNT aerogels formed in chemical vapor deposition (CVD) [5,18–24]. The first method is also known as the wet-spinning method, while

The obtained CNT fibers commonly possess satisfactory mechanical and electrical properties [5], and even higher strength and better flexibility than commercial carbon fibers and polymer fibers [25]. In order to further improve their properties (mechanical strength in particular), polymer infiltration is usually performed on the CNT fibers to obtain CNT fiber/polymer composites. The polymer can greatly enhance the inter-tube load transfer, inducing high

In 2000, Vigolo et al. [11] first fabricated CNT ribbons and fibers via the coagulation spinning approach that was widely used to synthesize polymer fibers. **Figure 1**(**a**) shows the schematic of the experimental setup used to make nanotube ribbons. In this method, SWNTs are homogeneously dispersed in a solution of sodium dodecyl sulfate (SDS), which helps prevent CNTs from agglomeration. The CNT dispersion is then injected into the co-flowing stream of a polymer solution that contains 5.0 wt.% of polyvinylalcohol (PVA) to form CNT ribbons, as shown in **Figure 1**(**b**). **Figure 1**(**e**) shows a typical SEM image of the as-obtained CNT ribbon, revealing a preferential orientation of the nanotubes along the ribbon's main axis. After the ribbons are washed and dried, most of the surfactants and polymers are removed. The ribbons are collapsed into fibers due to capillary force, as shown in **Figure 1**(**c**). These fibers are more flexible than traditional carbon fibers, as shown in **Figure 1**(**d**). The diameter of CNT fibers varies from 10 to 100 μm depending on fabrication conditions. The tensile strength, modulus and electrical conductivity of the obtained fibers are 300 MPa, 40 GPa and 10 S/cm, respectively.

developed to pre-arrange the CNTs prior to fabricating composites.

**2. Methods for assembling CNTs into CNT fibers**

the others are known as dry-spinning methods.

48 Carbon Nanotubes - Current Progress of their Polymer Composites

mechanical strength of the composites.

**2.1. Spinning from CNT Solution**

**Figure 1.** (a) Schematic of the experimental setup used to make nanotube ribbons. (b–d) Optical micrographs of nano‐ tube ribbons and fibers. (b) A single folded ribbon between horizontal and vertical crossed polarizers (scale bar = 1.5 mm); (c) A freestanding nanotube fiber between two glass substrates (scale bar = 1 mm); and (d) Tying knots reveals the high flexibility and resistance to torsion of the nanotube microfibers. (e) Scanning electron micrograph shows SWNT bundles are preferentially oriented along the main axis of the ribbon (scale bar = 667 nm) [11].

Vigolo's technique is remarkable for further studies on fabricating continuous CNT fibers on a large scale, although this technique has some disadvantages. For example, the drawing of these as-spun gel fibers is slow (∼1 cm/min), and the solid fibers are short (∼10 cm) [26]. In addition, the fibers' mechanical properties are low compared with those of component individual nanotubes. Moreover, mechanical performance is improved, mostly because PVA chains in a CNT fiber enhance load transfer efficiency between CNTs. However, the existence of the non-conductive PVA leads to lower electrical and thermal conductivity of the obtained CNT fibers than pure CNT sheets [27]. Thus, fibers composed solely of CNTs are desirable. In 2004, Ericson et al. [12] developed a method to fabricate well-aligned macroscopic fibers composed solely of SWNTs. In this method, purified SWNTs are dispersed in 102% sulfuric acid, which charges SWNTs and promotes their ordering into an aligned phase of individual mobile SWNTs surrounded by acid anions. This ordered dispersion is extruded into continu‐ ous lengths of macroscopic neat SWNT fibers. The obtained fibers possess a Young's modulus of 120 ±10 GPa and a tensile strength of 116 ±10 MPa. Because these pure CNT fibers do not contain polymers, they demonstrate better electrical and thermal properties than fibers containing polymers, with an electrical conductivity of 500 S/cm and thermal conductivity of ~21 W/m K. In another method proposed by Behabtu et al. [13], high-quality CNTs are dissolved in chlorosulfonic acid and extruded into a coagulant (acetone or water) to remove the acid. The forming filament is further stretched and tensioned to ensure high CNT alignment in the structure. The resulting fibers possess a Young's modulus of 120 ± 50 GPa and strength of 1.0 ± 0.2 GPa. The tensile strength shows a tenfold improvement over wet-spun fibers fabricated using the method developed by Ericson et al. [12]. At the same time, they display outstanding electrical conductivity (~29000 ± 3000 S/cm) and thermal conductivity (~380 ± 15 W/m K).

#### **2.2. Spinning from vertically aligned CNT arrays**

Just like drawing a thread from a silk cocoon, CNT fibers can be synthesized from a vertically aligned CNT array. In 2002, Jiang et al. [10] spun a 30-cm-long CNT fiber from a CNT array (~100 μm in height). In 2004, Zhang et al. [14] modified this technique by introducing twist during spinning. In this method, the nanotube arrays (~30 μm in height) are grown on an iron catalyst–coated substrate by CVD. Afterward, yarns are drawn from the array and twisted with a variable-speed motor. **Figure 2**(**a**) and (**b**) clearly show the SEM images of the structures formed during the spinning process. The obtained fibers have a tensile strength of ~460 MPa, modulus of ~30 GPa and electrical conductivity of 300 S/cm.

Since then, many efforts have been made to optimize spinning processes and to improve the performance of CNT fibers. Spinning fibers from higher CNT arrays can effectively improve fiber performance. For example, Zhang et al. [15] reported the spinning of CNT fibers from relatively long CNT arrays (0.65 mm), which resulted in the strength and Young's modulus of the CNT fibers reaching 1.91 GPa and 330 GPa, respectively. Furthermore, Li et al. [16] spun CNT fibers from 1 mm CNT arrays. Their tensile strength reached up to 3.3 GPa, which is much higher than that of CNT fibers from the 0.65 mm array. In aiming to achieve the goal of providing a continuous process for the solid-state fabrication of CNT yarns from CNT forests, Lepro et al. [17] spun fibers from CNT forests grown on both sides of highly flexible stainless steel sheets, instead of the conventionally used silicon wafers, as shown in **Figure 2**(**c**), (**d**) and (**e**). They reported that the catalyst layer is shown to be re-usable, decreasing the need for catalyst renewal during a proposed continuous process.

**Figure 2.** (a) and (b) SEM images of a carbon nanotube yarn in the process of being simultaneously drawn and twisted during spinning from a nanotube forest outside the SEM [14]; (c–e) Spinnable CNT forest grown on flexible stainless steel substrate [28].

Studies have found that not all CNT arrays can be spun into fibers, and the degree of spinna‐ bility of CNTs is closely related to the morphology of CNT arrays [29,30]. Several research groups have made efforts to study the mechanism of spinning fibers from CNT arrays. Kuznetsov et al. [28] developed a structural model for the drawing of sheets and fibers from CNT arrays. Huynh et al. [30] studied the roles of catalyst, substrate, temperature, gas flow rates, reaction time with acetylene, etc. to identify and understand the key parameters and develop a robust, scalable process. More recently, Zhu et al. [31] pointed out that the entangled structures at the ends of CNT bundles are critical for the continuous drawing process. Further fundamental studies of this mechanism are critical for fabricating spinnable CNT arrays and improving the properties of CNT fibers.

### **2.3. Spinning from CNT aerogels**

(~100 μm in height). In 2004, Zhang et al. [14] modified this technique by introducing twist during spinning. In this method, the nanotube arrays (~30 μm in height) are grown on an iron catalyst–coated substrate by CVD. Afterward, yarns are drawn from the array and twisted with a variable-speed motor. **Figure 2**(**a**) and (**b**) clearly show the SEM images of the structures formed during the spinning process. The obtained fibers have a tensile strength of ~460 MPa,

Since then, many efforts have been made to optimize spinning processes and to improve the performance of CNT fibers. Spinning fibers from higher CNT arrays can effectively improve fiber performance. For example, Zhang et al. [15] reported the spinning of CNT fibers from relatively long CNT arrays (0.65 mm), which resulted in the strength and Young's modulus of the CNT fibers reaching 1.91 GPa and 330 GPa, respectively. Furthermore, Li et al. [16] spun CNT fibers from 1 mm CNT arrays. Their tensile strength reached up to 3.3 GPa, which is much higher than that of CNT fibers from the 0.65 mm array. In aiming to achieve the goal of providing a continuous process for the solid-state fabrication of CNT yarns from CNT forests, Lepro et al. [17] spun fibers from CNT forests grown on both sides of highly flexible stainless steel sheets, instead of the conventionally used silicon wafers, as shown in **Figure 2**(**c**), (**d**) and (**e**). They reported that the catalyst layer is shown to be re-usable, decreasing the need for

**Figure 2.** (a) and (b) SEM images of a carbon nanotube yarn in the process of being simultaneously drawn and twisted during spinning from a nanotube forest outside the SEM [14]; (c–e) Spinnable CNT forest grown on flexible stainless

Studies have found that not all CNT arrays can be spun into fibers, and the degree of spinna‐ bility of CNTs is closely related to the morphology of CNT arrays [29,30]. Several research groups have made efforts to study the mechanism of spinning fibers from CNT arrays. Kuznetsov et al. [28] developed a structural model for the drawing of sheets and fibers from CNT arrays. Huynh et al. [30] studied the roles of catalyst, substrate, temperature, gas flow rates, reaction time with acetylene, etc. to identify and understand the key parameters and

modulus of ~30 GPa and electrical conductivity of 300 S/cm.

50 Carbon Nanotubes - Current Progress of their Polymer Composites

catalyst renewal during a proposed continuous process.

steel substrate [28].

In both of the above-mentioned methods, individual CNTs are first produced in the form of CNT powders or arrays. In this method, fibers are achieved through the post-process of spinning. Unlike the in-direct methods, CNT fibers can be assembled directly in a CVD process in which individual CNTs are synthesized. In 2000, Zhu et al. [18] first reported the direct synthesis of 20cm long ordered SWNTs with a diameter of approximately 0.3mm using a floating catalyst CVD method in a vertical furnace. In 2004, Li et al. [19] reported a method for the direct spinning of long CNT fibers from aerogels formed during CVD. **Figure 3**(**a**) is a schematic of this direct spinning process. In this method, reaction precursors are mixed and introduced into a tube furnace operated at 1200°C. In a reducing hydrogen atmosphere, the

**Figure 3.** (a) Schematic diagram of the direct spinning process for CNT fibers [21]; (b) and (c)SEM micrographs of a fiber that consists of well-aligned MWNTs [19]; (d) Schematic diagram of the CNT fiber-spinning process using a hori‐ zontal furnace [24].

nanotubes form an aerogel in the hot zone of the furnace and are stretched into cylindrical hollow socks which are then pulled and collected continuously out of the furnace as fibers. **Figure 3**(**b**) and (**c**) show SEM images of the aligned CNT fibers after condensation by acetone vapor. CNT fibers, spun directly and continuously from aerogels, demonstrate both high strength (up to 357 GPa) and high stiffness (8.8 GPa), which are comparable to those of commercial fibers [20].

### **3. Methods for enhancing CNT alignment**

The as-spun CNT fibers fabricated using the above methods usually have a porous structure, and the CNTs within the fibers have poor alignment [14,19,32]. Hence, the CNT fibers should be further densified to obtain a more closely-packed structure and better alignment of the CNTs. Since the van der Waals interaction strongly depends on the contact area between CNTs, much space and many pores between CNT bundles could lower the degree of this interaction. By applying the densification process, the densified CNT fibers can have reduced interspace between CNTs and an improved contact area, leading to an increased van der Waals interac‐ tion. As a result, these highly dense structures have a stronger van der Waals interaction between CNT bundles, hence improving fiber performance [32–34].

### **3.1. Enhancing CNT alignment for fibers spun from the wet-spinning technique**

While classical composite fibers consist of CNTs embedded in a polymeric matrix, fibers fabricated by the wet-spinning technique consist of an interconnected network of polymers and CNTs. The spinning conditions, such as the flow velocity of the polymer solution and the injection rate of the CNT solution, has no measurable effect on the CNT orientation in the resulting fibers. However, when the fibers are immersed in an appropriate solvent or heated, the network of polymers and CNTs can be loosened and stretched, resulting in a significant improvement in CNT alignment. For example, Vigolo et al. [35] enhanced the CNT alignment of their SWNT/PVA fibers by re-wetting, swelling and re-drying the fibers vertically under a tensile load with a weight attached to the end of the fiber. The solvent used in the study was comprised of water, acetone and acetonitrile. Once re-wetted and swollen by the solvent, the fibers could be stretched up to 160% with significantly improved alignment, as shown in **Figure 4**. This indicates that the networks of CNTs and absorbed polymers form cross linked assemblies that can be elastically deformed. As a result, their strength and stiffness increased from 10 GPa and 125 MPa, to 40 GPa and 230 MPa, respectively, after the stretching.

Separately, Miaudet et al. [36] reported an increase in tensile strength of CNT/PVA fibers from 1.4 GPa to 1.8 GPa after the hot-stretched treatment. The reduction of PVA chain alignment from ±27° to as low as 4.3°, and the nanotube alignment to as low as 9°, suggested that better alignment of the CNT and PVA chains was the main reason for the improved mechanical performance of the hot-stretched fibers.

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510 53

**Figure 4.** SEM images of an as-spun fiber (a), and (b) a stretched CNT fiber [35].

### **3.2. Enhancing CNT alignment for fibers spun from dry-spinning and floating catalyst techniques**

CNT fibers spun from dry-spinning and floating catalyst techniques can be densified by applying a mechanical force in their lateral direction. The densification methods can be classified into two categories: indirect approaches (such as liquid densification, twisting [32], drawing through dies [37], or an aligning and tension system [38]); and direct approaches (such as rubbing/false twisting [39] and mechanical compression [40]).

#### *3.2.1. Indirect approaches*

nanotubes form an aerogel in the hot zone of the furnace and are stretched into cylindrical hollow socks which are then pulled and collected continuously out of the furnace as fibers. **Figure 3**(**b**) and (**c**) show SEM images of the aligned CNT fibers after condensation by acetone vapor. CNT fibers, spun directly and continuously from aerogels, demonstrate both high strength (up to 357 GPa) and high stiffness (8.8 GPa), which are comparable to those of

The as-spun CNT fibers fabricated using the above methods usually have a porous structure, and the CNTs within the fibers have poor alignment [14,19,32]. Hence, the CNT fibers should be further densified to obtain a more closely-packed structure and better alignment of the CNTs. Since the van der Waals interaction strongly depends on the contact area between CNTs, much space and many pores between CNT bundles could lower the degree of this interaction. By applying the densification process, the densified CNT fibers can have reduced interspace between CNTs and an improved contact area, leading to an increased van der Waals interac‐ tion. As a result, these highly dense structures have a stronger van der Waals interaction

commercial fibers [20].

**3. Methods for enhancing CNT alignment**

52 Carbon Nanotubes - Current Progress of their Polymer Composites

between CNT bundles, hence improving fiber performance [32–34].

**3.1. Enhancing CNT alignment for fibers spun from the wet-spinning technique**

from 10 GPa and 125 MPa, to 40 GPa and 230 MPa, respectively, after the stretching.

performance of the hot-stretched fibers.

Separately, Miaudet et al. [36] reported an increase in tensile strength of CNT/PVA fibers from 1.4 GPa to 1.8 GPa after the hot-stretched treatment. The reduction of PVA chain alignment from ±27° to as low as 4.3°, and the nanotube alignment to as low as 9°, suggested that better alignment of the CNT and PVA chains was the main reason for the improved mechanical

While classical composite fibers consist of CNTs embedded in a polymeric matrix, fibers fabricated by the wet-spinning technique consist of an interconnected network of polymers and CNTs. The spinning conditions, such as the flow velocity of the polymer solution and the injection rate of the CNT solution, has no measurable effect on the CNT orientation in the resulting fibers. However, when the fibers are immersed in an appropriate solvent or heated, the network of polymers and CNTs can be loosened and stretched, resulting in a significant improvement in CNT alignment. For example, Vigolo et al. [35] enhanced the CNT alignment of their SWNT/PVA fibers by re-wetting, swelling and re-drying the fibers vertically under a tensile load with a weight attached to the end of the fiber. The solvent used in the study was comprised of water, acetone and acetonitrile. Once re-wetted and swollen by the solvent, the fibers could be stretched up to 160% with significantly improved alignment, as shown in **Figure 4**. This indicates that the networks of CNTs and absorbed polymers form cross linked assemblies that can be elastically deformed. As a result, their strength and stiffness increased

### *<IP_ADDRESS>. Liquid densification*

The alignment and mechanical performance of as-spun CNT fibers can be improved through liquid densification. In this method, a liquid such as acetone or ethanol is absorbed into the fibers and subsequently evaporated, resulting in a dense CNT structure. The fibers are densified due to the surface tension of the solvent and the fiber diameter is reduced accord‐ ingly. The densification process slightly improves nanotube alignment (**Figure 5**) and enhances load transfer between nanotubes, ensuring that most of them are fully load-bearing. Liu et al. [41] studied the mechanical properties of twisted fibers with and without acetone densification. The diameter of the yarn changed from 11.5 to 9.7 μm after shrinking. Although the maximum strain of the yarn remained unchanged (~2.3%), the Young's modulus (~56 GPa) of the shrunk fiber was slightly greater than before shrinking (~48 GPa). Liu et al. [41] also reported a change in diameter and maximum load of twisted fibers before and after shrinking. After acetone shrinking, the diameter reduction ranged from 15% to 24%, and the load increase ranged from 15 to 40%, indicating that tensile strength was enhanced. Among common solvents (water, ethanol and acetone) used to shrink CNT yarns, acetone showed the best shrinking effect.

**Figure 5.** (a) SEM images of a twisted fiber before, and after shrinking (b) [41].

### *<IP_ADDRESS>. Twisting*

The as-spun fiber is relatively loose with noticeable spaces between CNT bundles. Increasing the twist angle is an effective method for densifying CNT fibers. Since it brings CNTs into closer contact with each other, twisting improves the friction coefficient μ between CNTs, therefore contributing positively to fiber strength [15]. Zhang et al. [15] compared the tensile behaviors of twisted and untwisted CNT fibers spun from the same 650 mm height array. After twisting, the diameter of the fiber decreased from 4 to 3 μm (**Figure 6**), while tensile strength increased from 0.85 GPa to 1.9 GPa [15].

**Figure 6.** SEM images of as-spun CNT fiber (a), and the same fiber after post-spin twisting (b) [15].

### *<IP_ADDRESS>. Drawing through dies*

The as-spun CNT fibers can be densified by being drawn through dies of different diameters. The average measured fiber diameter was determined by die size. Sugano et al. [37] densified CNT fibers spun from CNT arrays by drawing them through densifying dies with different diameters (d = 30, 35, 55, 75 μm) (**Figure 7**(**a**)). The fibers were deformed elastically after drawing CNTs through the die, and their density increased with decreasing die diameter. As CNT fibers are held together by van der Waals forces between MWNTs, these forces increased due to higher apparent density with decreasing distance between MWNTs, as shown in **Figure 7**(**b**). As a result, their strength was significantly enhanced after treatment.

**Figure 7.** (a) Schematic view of untwisted CNT yarn in the process of being drawn from the aligned MWNT array and past the sheet through a die; (b) SEM image of surface morphologies of the resulting CNT fiber.

#### *<IP_ADDRESS>. Aligning and tension system*

**Figure 5.** (a) SEM images of a twisted fiber before, and after shrinking (b) [41].

54 Carbon Nanotubes - Current Progress of their Polymer Composites

increased from 0.85 GPa to 1.9 GPa [15].

*<IP_ADDRESS>. Drawing through dies*

The as-spun fiber is relatively loose with noticeable spaces between CNT bundles. Increasing the twist angle is an effective method for densifying CNT fibers. Since it brings CNTs into closer contact with each other, twisting improves the friction coefficient μ between CNTs, therefore contributing positively to fiber strength [15]. Zhang et al. [15] compared the tensile behaviors of twisted and untwisted CNT fibers spun from the same 650 mm height array. After twisting, the diameter of the fiber decreased from 4 to 3 μm (**Figure 6**), while tensile strength

**Figure 6.** SEM images of as-spun CNT fiber (a), and the same fiber after post-spin twisting (b) [15].

The as-spun CNT fibers can be densified by being drawn through dies of different diameters. The average measured fiber diameter was determined by die size. Sugano et al. [37] densified CNT fibers spun from CNT arrays by drawing them through densifying dies with different diameters (d = 30, 35, 55, 75 μm) (**Figure 7**(**a**)). The fibers were deformed elastically after

*<IP_ADDRESS>. Twisting*

The aligning and tension system is one of the most effective methods of enhancing CNT alignment and performance of CNT fibers. Tran et al. [38] first modified the traditional dryspinning process to improve CNT alignment of their CNT fibers (**Figure 8**(**a**)). In this modified system, a capstan effect rod system (CERS) is added to a dry-spinning system to regulate tension and torque to the fibers. As the fibers pass through a CERS, the increased tension extends and aligns the bundles (**Figure 8**(**b**) and **8**(**c**)). This process has two effects: (i) aligning CNTs in the fibers during the initial tensioning; and (ii) condensing the CNT bundles. The first effect increases contact length between bundles, and the second effect reduces the distance between CNTs. The significant increase in fiber strength from 0.45 to 1.2 GPa after the treatment is due to better alignment of the fiber bundles and higher fiber compaction.

**Figure 8.** (a) The schematic of modified CNT yarn spinning; (b) SEM images of surface morphologies of CNT fiber spun from traditional process; and modified process (c) [38].

Generally, the drawback of the indirect approaches is their low densifying forces. The liquid densification method, for example, employs the surface tension of volatile solvents such as acetone or ethanol to densify the CNT fibers. Its densifying force is therefore limited by the low surface tension of the solvents used [32]. Similarly, the compressive force produced by drawing CNT fibers through a die results from the drawing forces and the die size used. These drawing forces are limited by fiber strength, while a significantly smaller die could damage the fiber structure, resulting in poor strength [37]. Therefore, CNT fibers cannot be adequately densified with these methods, and their performance remains unsatisfactory.

### *3.2.2. Direct approaches*

Direct approaches are considered the best solution to overcome the above limitations. As the densifying forces are applied directly to CNT fibers, the forces can condense the fibers into a much denser structure [40].

#### *<IP_ADDRESS>. Rubbing/false twisting*

CNT fibers can be densified using several traditional textile twistless methods such as rubbing. Miao et al. [39] used a rubbing roller system (**Figure 9**(**a**)) to densify CNT web drawn from a vertically aligned CNT forest into a compact twistless yarn. As the system used a constant rate false twisting process, there is only a temporary twist on the incoming side of the yarn, and the yarn on the outgoing side (and thus the final yarn) will be twistless. As can be seen in **Figure 9**(**b**), the resulting yarn consists of a high packing density sheath with CNTs lying straight and parallel to the yarn axis, and a low density core with many microscopic voids. With an increased contact length between CNT bundles in the high packing density sheath, the mechanical performance of the core-sheath structured, twistless carbon nanotube yarns are significantly higher than that of their corresponding twist-densified yarns.

Advanced Fabrication and Properties of Aligned Carbon Nanotube Composites: Experiments and Modeling http://dx.doi.org/10.5772/62510 57

**Figure 9.** (a) The schematic of the core-sheath, twistless CNT yarn fabricated by a rubbing roller system; and (b) SEM image of the resulting yarn.

#### *<IP_ADDRESS>. Mechanical compression*

**Figure 8.** (a) The schematic of modified CNT yarn spinning; (b) SEM images of surface morphologies of CNT fiber

Generally, the drawback of the indirect approaches is their low densifying forces. The liquid densification method, for example, employs the surface tension of volatile solvents such as acetone or ethanol to densify the CNT fibers. Its densifying force is therefore limited by the low surface tension of the solvents used [32]. Similarly, the compressive force produced by drawing CNT fibers through a die results from the drawing forces and the die size used. These drawing forces are limited by fiber strength, while a significantly smaller die could damage the fiber structure, resulting in poor strength [37]. Therefore, CNT fibers cannot be adequately

Direct approaches are considered the best solution to overcome the above limitations. As the densifying forces are applied directly to CNT fibers, the forces can condense the fibers into a

CNT fibers can be densified using several traditional textile twistless methods such as rubbing. Miao et al. [39] used a rubbing roller system (**Figure 9**(**a**)) to densify CNT web drawn from a vertically aligned CNT forest into a compact twistless yarn. As the system used a constant rate false twisting process, there is only a temporary twist on the incoming side of the yarn, and the yarn on the outgoing side (and thus the final yarn) will be twistless. As can be seen in **Figure 9**(**b**), the resulting yarn consists of a high packing density sheath with CNTs lying straight and parallel to the yarn axis, and a low density core with many microscopic voids. With an increased contact length between CNT bundles in the high packing density sheath, the mechanical performance of the core-sheath structured, twistless carbon nanotube yarns

densified with these methods, and their performance remains unsatisfactory.

are significantly higher than that of their corresponding twist-densified yarns.

spun from traditional process; and modified process (c) [38].

56 Carbon Nanotubes - Current Progress of their Polymer Composites

*3.2.2. Direct approaches*

much denser structure [40].

*<IP_ADDRESS>. Rubbing/false twisting*

Wang et al. [40] reported that CNT fibers densified by the pressurized rolling system showed highly packed structures with a densification factor of up to 10 (**Figure 10**(**a**)). Moreover, the densified fibers can reach an impressive average strength of 4.34 GPa, which is the highest extrinsic CNT fiber strength reported to date [40]. In addition, Tran et al. [24] presented a modified densification method to produce a highly packed CNT structure. As shown in **Figure 10**(**b**), CNT fibers were sandwiched between two sheets of A4 paper and pressed by a stainless-steel spatula with an applied force of approximately 100 *N*, at 45° to the fiber axis. The spatula was subsequently slid across the A4 paper, along the fiber axis, to compress and mechanically densify the fibers into a ribbon shape while the compressive force was main‐ tained. The CNT ribbon in this study also showed a densification factor of up to 10, while the strength and electrical conductivity of the densified fibers approached impressive values of 2.81 GPa and 12,000 S/cm, respectively. The study showed that the mechanical densification treatment may have increased the CNT bundle size and inter-CNT contact, and induced better alignment (**Figure 10**(**c**) and (**d**)), resulting in improved properties of the densified CNT fiber.

**Figure 10.** The schematic of mechanical densification methods using (a) pressurized rolling system, and (b) spatula; SEM images of the surface morphology of the (c) as-spun CNT fiber, and (d) densified CNT ribbons [24].

### **4. Mechanical and physical properties of aligned CNT polymer composites**

The outstanding physical and mechanical properties of aligned CNT assemblies make them promising for the research and development of high-performance composites. The final properties of the composites are affected by many factors, such as, morphology of individual nanotubes and impregnating method.

#### **4.1. Impregnating method**

Pure CNT assemblies including fibers and films have an ineffective load transfer between CNTs as the CNTs interact with each other via weak van der Waals forces. Among all methods used to enhance load transfer between CNTs, polymer impregnation is one of the most effective treatments in enhancing the mechanical properties of CNT assemblies. This rein‐ forcement mainly stems from the enhanced inter-tube load transfer and the crystallinity of the impregnated polymer. Several impregnating methods are used to fabricate aligned CNT polymer composites.

#### *4.1.1. Dip coating/soaking*

Dip coating or soaking is widely used to impregnate polymer into CNT assemblies. In this method, CNT assembles are immerged into polymer solution for sufficient infiltration and then taken out for curing. Liu et al. [32] reports the mechanical properties of PVA impregnated fibers spun from CNT arrays. **Figure 11** shows the schematic of the CNT/polymer fiber manufacturing process and compares the tensile properties of a CNT/PVA fiber with two types of pure CNT fibers. As can be seen, the CNT/PVA composite fiber with 19 wt.% PVA possesses a tensile strength of 1.95 GPa. This result is 255% higher than that of simply twisting the CNT fiber and 103% higher than that of a CNT fiber subjected to twisting and shrinking by acetone. The greater strength of the CNT/PVA fiber stems from the decrease in fiber diameter due to the high wettability between dimethyl sulfoxide (DMSO) and CNTs, and the increase in tensile load due to improved load transfer efficiency between CNTs after PVA impregnation (**Figure 11**(**b**)).

**Figure 11.** (a) Schematic of the CNT/polymer fiber manufacturing process; and (b) stress-strain curves of a typical SWNT/PVA yarn and two types of pure SWNT yarns. [32]

Similarly, Tran et al. [24] reported an outstanding enhancement of the electrical and mechanical performances of MWNT fibers through the combined treatments of mechanical densification and epoxy infiltration. Compared to the mechanical performances of CNT fibers produced by different post-treatments, the combined post-treatments employed in their study showed better effects, with enhancement factors of more than 13.5 for tensile strength and 63 for stiffness. After the first mechanical treatment, their condensed CNT ribbons achieved a tensile strength much greater than that of the best CNT fibers spun with wet-spinning and arrayspinning methods, as shown in **Figure 12**. When further combined with epoxy infiltration, the CNT/epoxy ribbons reached significantly greater strength (up to 5.2 GPa) and stiffness (up to 444 GPa), which are very comparable to those of commercial PAN carbon fibers as shown in **Figure 12**. Furthermore, while the strength of their CNT/epoxy ribbons was comparable to that of the best double-walled CNT (DWNT) ribbons produced by the floating-catalyst method [40], their stiffness was much higher. The results suggest that by using a polymer infiltration treatment, the performance of MWNT fibers with low electrical and mechanical properties could achieve the performance of many other high-strength fibers.

**4. Mechanical and physical properties of aligned CNT polymer composites**

The outstanding physical and mechanical properties of aligned CNT assemblies make them promising for the research and development of high-performance composites. The final properties of the composites are affected by many factors, such as, morphology of individual

Pure CNT assemblies including fibers and films have an ineffective load transfer between CNTs as the CNTs interact with each other via weak van der Waals forces. Among all methods used to enhance load transfer between CNTs, polymer impregnation is one of the most effective treatments in enhancing the mechanical properties of CNT assemblies. This rein‐ forcement mainly stems from the enhanced inter-tube load transfer and the crystallinity of the impregnated polymer. Several impregnating methods are used to fabricate aligned CNT

Dip coating or soaking is widely used to impregnate polymer into CNT assemblies. In this method, CNT assembles are immerged into polymer solution for sufficient infiltration and then taken out for curing. Liu et al. [32] reports the mechanical properties of PVA impregnated fibers spun from CNT arrays. **Figure 11** shows the schematic of the CNT/polymer fiber manufacturing process and compares the tensile properties of a CNT/PVA fiber with two types of pure CNT fibers. As can be seen, the CNT/PVA composite fiber with 19 wt.% PVA possesses a tensile strength of 1.95 GPa. This result is 255% higher than that of simply twisting the CNT fiber and 103% higher than that of a CNT fiber subjected to twisting and shrinking by acetone. The greater strength of the CNT/PVA fiber stems from the decrease in fiber diameter due to the high wettability between dimethyl sulfoxide (DMSO) and CNTs, and the increase in tensile load due to improved load transfer efficiency between CNTs after PVA impregnation

**Figure 11.** (a) Schematic of the CNT/polymer fiber manufacturing process; and (b) stress-strain curves of a typical

nanotubes and impregnating method.

58 Carbon Nanotubes - Current Progress of their Polymer Composites

**4.1. Impregnating method**

polymer composites.

(**Figure 11**(**b**)).

SWNT/PVA yarn and two types of pure SWNT yarns. [32]

*4.1.1. Dip coating/soaking*

**Figure 12.** Comparisons of mechanical properties of the best CNT fibers from array-spinning [15] and wet-spinning [13], ribbons from aerogel spinning, and PAN carbon fibers [40].

Liu et al. [23] fabricated CNT/polyimide aerogel (CNT/PIA) composite fibers by dip-coating the CNT fibers in a sol solution, and then drying them using the supercritical CO2 drying process. In the CNT/PIA composite fibers, CNT fibers are tightly wrapped by porous polyi‐ mide aerogel, showing a core-shell structure. This core-shell structure resulted in the light weight, low density and high surface area of the composite fiber. Owing to the superior properties of the CNT fibers (stiffness of ~450 MPa, tensile strength of ~83 MPa and electrical conductivity of ~419 S/cm), the CNT/PIA composite fibers achieve significant enhancements in mechanical and electrical properties (stiffness of ~68.1 MPa, strength of ~11.6 MPa and electrical conductivity of ~418 S/cm), compared with the pure PIA and other CNT/PIA composite [42–46]. It was also found that the mechanical and electrical properties of CNT/PIA composite fibers decline with an increase in the diameter of CNT fibers.

#### *4.1.2. Resin transfer molding*

Resin transfer molding (RTM) is a very common and cost-effective method to fabricate composites in industries, in which the liquid resins are first injected to the preforms and then cured to be solid. Given its capability of making composites with large sizes and complex shapes, RTM is expected to be appropriate for preparing CNT/epoxy composites in large scale. Liu et al. [5] developed aligned CNT/epoxy composite films by combining layer-by-layer and vacuum-assisted RTM (VA-RTM) method using direct chemical vapor deposition (CVD)-spun CNT plies. The CNTs in the plies are well-condensed during the VA-RTM process (**Fig‐ ure 13**(**a**)), leading to much higher mass fractions of CNTs (up to 24.4 wt.%) compared with those obtained from the conventional dispersion methods. Due to good alignment of the condensed CNTs in the plies, the CNT/epoxy composite with 24.4 wt.% CNTs achieves ~5 and ~10 times enhancements in their Young's modulus and strength, respectively. A high tensile toughness of up to 6.39 × 103 kJ/m3 was also obtained in the composite films, making them promising candidates for protective materials, as shown in **Figure 13**(**b**). The electrical conductivity of the aligned CNT/epoxy composites reaches as high as 252.8 S/cm for the composite with 24.4 wt.% CNTs, which is ~20 times greater than that of the CNT/epoxy composites obtained using dispersion methods [47–49].

**Figure 13.** (a) Experimental set-up of the RTM process for preparing CNT/epoxy films; and (b) The CNT weight frac‐ tions and the thickness of CNT/epoxy composite films as a function of CNT plies. [5].

#### *4.1.3. Spray winding and layer-by-layer deposition*

The aforementioned methods are mostly regarded as off-line methods in which highly packed CNT assembles are used as preforms. Because the preforms are already tightly packed, however, these methods often have the problem of the uniformity of infiltration. As a result, the un-infiltrated part may become defects and limit the overall performance of the composites [50]. In order to control the uniformity of infiltration and avoid over-infiltration, Liu et al. [50, 51] developed a one-step approach of "spray winding" to fabricate high-performance CNT composites. In this on-line infiltration method, CNT sheets, drawn out from CNT arrays, were continuously collected (wound) onto a rotating mandrel under the spray of a polymer solution, as shown in **Figure 14**(**a**). The spray-wound CNT/PVA composite films, the CNT fraction is tunable, and could be as high as 65 wt.% to reach the best mechanical properties. The best film had the tensile strength, stiffness and toughness up to 1.8 GPa, 45 GPa, and 100 J/g, respectively, much better than the fibers made by the same CNT and PVA and many other CNT/polymer composites. The high performance can be attributed to the long CNTs, highly-aligned tube morphology, and good interfacial bonding between CNT and PVA, which were obtained simultaneously. In order to control the exact layers of the composite films in large scale, Zhang et al. [52] reported a "layer-by-layer (LBL) deposition" method to produce CNT polymer composites, as shown in **Figure 12**(**b**). This on-line deposition method allowed PVA to infiltrate into the CNT film efficiently, resulting in a remarkable improvement in the mechanical property of CNT/PVA composite. The composite film possessed a tensile strength of 1.7 GPa, which is almost one order of magnitude and 20 times higher than those of the pure CNT and PVA films, respectively.

**Figure 14.** (a) Schematic view of spray winding [50]; and (b) Schematic illustration of the LBL deposition process [52].

### **4.2. Effect of CNT morphology**

in mechanical and electrical properties (stiffness of ~68.1 MPa, strength of ~11.6 MPa and electrical conductivity of ~418 S/cm), compared with the pure PIA and other CNT/PIA composite [42–46]. It was also found that the mechanical and electrical properties of CNT/PIA

Resin transfer molding (RTM) is a very common and cost-effective method to fabricate composites in industries, in which the liquid resins are first injected to the preforms and then cured to be solid. Given its capability of making composites with large sizes and complex shapes, RTM is expected to be appropriate for preparing CNT/epoxy composites in large scale. Liu et al. [5] developed aligned CNT/epoxy composite films by combining layer-by-layer and vacuum-assisted RTM (VA-RTM) method using direct chemical vapor deposition (CVD)-spun CNT plies. The CNTs in the plies are well-condensed during the VA-RTM process (**Fig‐ ure 13**(**a**)), leading to much higher mass fractions of CNTs (up to 24.4 wt.%) compared with those obtained from the conventional dispersion methods. Due to good alignment of the condensed CNTs in the plies, the CNT/epoxy composite with 24.4 wt.% CNTs achieves ~5 and ~10 times enhancements in their Young's modulus and strength, respectively. A high tensile

promising candidates for protective materials, as shown in **Figure 13**(**b**). The electrical conductivity of the aligned CNT/epoxy composites reaches as high as 252.8 S/cm for the composite with 24.4 wt.% CNTs, which is ~20 times greater than that of the CNT/epoxy

**Figure 13.** (a) Experimental set-up of the RTM process for preparing CNT/epoxy films; and (b) The CNT weight frac‐

The aforementioned methods are mostly regarded as off-line methods in which highly packed CNT assembles are used as preforms. Because the preforms are already tightly packed,

tions and the thickness of CNT/epoxy composite films as a function of CNT plies. [5].

*4.1.3. Spray winding and layer-by-layer deposition*

was also obtained in the composite films, making them

composite fibers decline with an increase in the diameter of CNT fibers.

*4.1.2. Resin transfer molding*

60 Carbon Nanotubes - Current Progress of their Polymer Composites

toughness of up to 6.39 × 103 kJ/m3

composites obtained using dispersion methods [47–49].

Individual nanotube morphologies, such as length and alignment, have great influence on mechanical and physical properties of CNT polymer composites. Wang et al. [53] reported an ultrastrong and stiff CNT/composite using a stretch-winding process. The unstretched composites exhibited strength of 2.0 GPa, Young's modulus of 130 GPa and electrical conduc‐ tivity of 820 S/cm. After stretching the strength, Young's modulus and electrical conductivity were increased to as high as 3.8GPa, 293 GPa and 1230 S/cm, respectively. These remarkable improvements can be ascribed to the enhancement of CNT alignment and decreasing of waviness. The alignment of the CNTs was characterized by polarized Raman. Specifically, the shift of the intensity ratio (IG‖/IG⊥) of G band peaks was measured. Following the stretchwinding process, the intensity ratio for the 12%-stretched sheet is increased to 7.6 from 1.6, indicating that alignment of the CNTs in the nanocomposites was significantly improve via the stretching process. Therefore, the improved CNT alignment is correlated with the observed improvements in mechanical and electrical properties of the composites.

Park et al. [54] studied the effects of nanotube length and alignment on thermal conductivity of MWNT/epoxy composites. It was found that the long-MWCNT composites exhibited higher thermal conductivity than the short-MWCNT composites with the same weight percentages. At room temperature, 10 wt.% short-MWCNT/epoxy composite showed thermal conductivity of 0.35 W/mK, while the long-MWCNT composites showed 2.6 W/mK even at lower concen‐ tration of 6.38 wt.%. To improve the in-plane thermal conductivity, CNT sheets (60 wt.%) were stretched mechanically. The thermal conductivity increased up to 83 W/mK (25% stretched) and 103 W/mK (40% stretched) along the alignment direction compared to 55 W/mK of the random sample.

### **5. Mesoscopic modeling of thermal conduction in aligned CNT composites**

Due to the unique morphology of aligned CNT composites, it is difficult to directly measure their thermal conductivity, especially for composites in thin film and long fiber structures. Proper computational modeling is required to accurately predict the thermal conductivity of aligned CNT composites. The widely-used effective medium theories (EMTs) can well predict the thermal conductivity of CNT composites obtained using dispersion methods. However, the EMTs generally fail to predict the thermal conductivity of aligned CNT composites, since they cannot take into account the complex morphology of CNTs and the thermal boundary resistances (TBRs) at both CNT-CNT and CNT-matrix interfaces [55]. The TBRs are the resistances to the heat flow at interfaces, which have been regarded as the bottleneck of thermal conduction in CNT composites [56].

### **5.1. Thermal conduction model for two-phase aligned CNT composites**

In order to accurately predict the thermal conductivity of aligned CNT composites, Duong et al. [57] developed an off-lattice Monte Carlo (MC) approach by quantifying thermal energy through a large quantity of random thermal walkers. Thermal walkers have a random Brownian motion in the polymer matrix, which is described by the position changes of thermal walkers in each direction. The position changes take values from a normal distribution with a zero mean and a standard deviation, as expressed as below:

$$
\sigma = \sqrt{2D\_{\rm m} \Delta t} \tag{1}
$$

where *D*<sup>m</sup> is the thermal diffusivity of polymer matrices, and *∆t* is the time increment of the simulation [58]. When a walker jumps to the CNT-polymer matrix interface, it may jump into the CNT with a probability *f*m-CNT, or remain within the polymer matrix with a probability (1*f*m-CNT). The probability is a function of the TBR between polymer and CNT, *R*m-CNT, obtained using the acoustic mismatch theory (AMT) [59]:

winding process, the intensity ratio for the 12%-stretched sheet is increased to 7.6 from 1.6, indicating that alignment of the CNTs in the nanocomposites was significantly improve via the stretching process. Therefore, the improved CNT alignment is correlated with the observed

Park et al. [54] studied the effects of nanotube length and alignment on thermal conductivity of MWNT/epoxy composites. It was found that the long-MWCNT composites exhibited higher thermal conductivity than the short-MWCNT composites with the same weight percentages. At room temperature, 10 wt.% short-MWCNT/epoxy composite showed thermal conductivity of 0.35 W/mK, while the long-MWCNT composites showed 2.6 W/mK even at lower concen‐ tration of 6.38 wt.%. To improve the in-plane thermal conductivity, CNT sheets (60 wt.%) were stretched mechanically. The thermal conductivity increased up to 83 W/mK (25% stretched) and 103 W/mK (40% stretched) along the alignment direction compared to 55 W/mK of the

**5. Mesoscopic modeling of thermal conduction in aligned CNT composites**

Due to the unique morphology of aligned CNT composites, it is difficult to directly measure their thermal conductivity, especially for composites in thin film and long fiber structures. Proper computational modeling is required to accurately predict the thermal conductivity of aligned CNT composites. The widely-used effective medium theories (EMTs) can well predict the thermal conductivity of CNT composites obtained using dispersion methods. However, the EMTs generally fail to predict the thermal conductivity of aligned CNT composites, since they cannot take into account the complex morphology of CNTs and the thermal boundary resistances (TBRs) at both CNT-CNT and CNT-matrix interfaces [55]. The TBRs are the resistances to the heat flow at interfaces, which have been regarded as the bottleneck of thermal

In order to accurately predict the thermal conductivity of aligned CNT composites, Duong et al. [57] developed an off-lattice Monte Carlo (MC) approach by quantifying thermal energy through a large quantity of random thermal walkers. Thermal walkers have a random Brownian motion in the polymer matrix, which is described by the position changes of thermal walkers in each direction. The position changes take values from a normal distribution with a

m

where *D*<sup>m</sup> is the thermal diffusivity of polymer matrices, and *∆t* is the time increment of the simulation [58]. When a walker jumps to the CNT-polymer matrix interface, it may jump into the CNT with a probability *f*m-CNT, or remain within the polymer matrix with a probability (1-

= D 2*D t* (1)

improvements in mechanical and electrical properties of the composites.

62 Carbon Nanotubes - Current Progress of their Polymer Composites

**5.1. Thermal conduction model for two-phase aligned CNT composites**

s

zero mean and a standard deviation, as expressed as below:

random sample.

conduction in CNT composites [56].

$$f\_{\rm m-CNT} = 4 \mid (\rho C\_{\rm p} \nu R\_{\rm m-CNT}) \tag{2}$$

where *ρ* is the density of polymer, *C*p is the specific heat of polymer, and *v* is the sound velocity in the polymer matrix. Due to the ballistic phonon transport and ultrahigh thermal conduc‐ tivity in the CNT [60], thermal walkers are assumed to travel at an infinite speed inside the SWNT [61]. The walker is allowed to exit from a CNT to the matrix by using another probability *f*CNT-m, which is related to *f*m-CNT in a way that satisfies the second thermodynamic theorem [62, 63]:

$$\left.V\_{\text{CNT}}f\_{\text{CNT}-\text{m}}=C\_{f-\text{CNT}}\sigma\_{\text{m}}A\_{\text{CNT}}f\_{\text{m}-\text{CNT}}\tag{3}$$

wherein *V*CNT and *A*CNT are the volume and surface area of a CNT, and *σ*<sup>m</sup> is the standard deviation of Brownian motion in the polymer matrix. *Cf*-CNT is the thermal equilibrium factor at the polymer-CNT interface, which is dependent on the geometry of the CNTs, and the interfacial area between the CNT and the matrix.

By using the developed MC approach, Duong et al modeled SWNT-epoxy and SWNTpolymethyl methacrylate (PMMA) composites [64]. The thermal conductivity of SWNT-epoxy and SWNT-PMMA composites were accurately predicted. The effects of the SWNT orientation, weight fraction and TBRs on the thermal conductivity of composites were quantified. The quantitative findings showed that in SWNT-PMMA composites with 1.0wt.% of SWNT loading, aligned SWNTs achieved enhanced thermal conductivity 15 times higher than that of PMMA, whereas, the randomly dispersed SWNTs only resulted in thermal conductivity ~5 times higher than that of PMMA [64]. This indicated the superiority of aligned CNT compo‐ sites.

Since CNTs are normally grown into forests or spun into fibers, the contacts between CNTs may play a significant role to modify the thermal conductivity of composites. Duong et al. then modified their model to study the effect of CNT-CNT contacts on the thermal conductivity of both SWNT-epoxy and MWNT-epoxy composites [65,66]. A representative volume element (RVE) was built based on the real CNT-epoxy composites, as shown in **Figure 15**. In MWNTepoxy composites with 20 vol % of MWNT loading, aligned MWNTs without contacts achieved a thermal conductivity nearly 40 times higher than that of epoxy, while, aligned MWNTs with contacts induced a thermal conductivity only 20 times higher than that of epoxy. This indicated that CNT-CNT contacts in aligned CNTs may reduce the thermal conductivity of composites. The anisotropic thermal conductivity of aligned CNT composites was also quantified. In both SWNT-epoxy and MWNT-epoxy composites, the thermal conductivity parallel to the aligned CNTs was much higher than that perpendicular to the aligned CNTs. The SWNT-epoxy composites had more significantly anisotropic thermal conduction than MWNT-epoxy composites.

**Figure 15.** Schematic drawing of CNT reinforced composites, and aligned CNTs in a polymer as a representative vol‐ ume element [65].

Bui et al. modified Duong's approach to investigate the thermal behavior of the SWNTpolystyrene (PS) composites at different volume fractions and at various temperature [67]. It was found that the thermal conductivity of SWNT-PS composites increased with the temper‐ ature rise. By validating with experimental data [68], the TBRs at both SWNT-PS and SWNT-SWNT interfaces were estimated by using the MC approach. The calculations at various temperature showed that the TBR between SWNT and PS increased with the rise of tempera‐ ture. A TBR value of SWNT-SWNT was estimated to be 12.15×10-8 m2 K/W at 300K, which was higher than that between SWNT and PS (2.21×10-8 m2 K/W). Bui et al. also conducted the comparable study between graphene-polymer composites and SWNT-polymer composites [69]. The quantitative results showed that graphene nanosheets were more effective than SWNTs to enhance the thermal conduction in polymer composites.

#### **5.2. Thermal conduction model for multi-phase aligned CNT composites**

Recently, multiphase polymer composites, which contain more than one type of additive in the matrix, have attracted much attention [70,71]. The multiphase composites can combine the merits of all the components, inducing advanced multifunctional properties. Diverse multi‐ phase polymer composites have been developed, such as CNT/nanoparticle/polymer compo‐ sites [70,71], CNT/graphene/polymer composites [72,73], CNT/fiber/polymer composites [74,75] and CNT-stabilized polymer blends [76]. Since there is no effective approach to study thermal properties of CNT multiphase composites, Gong et al. [77] extended Duong's MC approach to investigate heat transfer phenomena in CNT multiphase composites. In Gong's model, a three-phase poly(ether ether ketone) (PEEK) composite containing SWNTs and tungsten disulfide (WS2) nanoparticles was chosen as a case study, as shown in **Figure 16**(**a**) [78]. The TBRs at all interfaces (i.e. SWNT-PEEK, WS2-PEEK and SWNT-SWNT) were taken into account in their model. The results showed that the thermal conductivity of multiphase composites increased when the CNT concentration increased, and when the TBRs of CNT-PEEK and WS2-PEEK interfaces decreased. The thermal conductivity of composites with CNTs aligned parallel to the heat flux was enhanced ~2.7 times relative to that of composites with randomly-dispersed CNTs.

The model could also quantitatively study the effect of the complex morphology and disper‐ sion of SWNTs, (e.g., individual and bundled SWNTs, the number of SWNT bundles, and the number of SWNTs per bundle), on the thermal conductivity of multiphase composites. It was found that the TBR at the SWNT-SWNT interface played a significant role in the thermal conduction of the composite with SWNT bundles. As presented in **Figure 16**(**b**), the thermal conductivity of the three-phase composite decreased with the rise of SWNT-SWNT TBR. A critical SWNT-SWNT TBR was found to be 0.155×10-8 m2 K/W, which dominated the heat transfer mechanism in the three-phase composite. Proper treatment may be used to reduce the SWNT-SWNT TBR, such as the condensation of SWNT fibers, to enhance the thermal con‐ ductivity of multiphase composites with aligned SWNTs [78]. Besides the CNT multiphase composites, Gong et al. also modified the MC model to study the thermal conduction mecha‐ nisms in graphene composites [79] and CNT aerogels [80], as well as the multiphase biological systems containing CNTs [77,81,82], which indicated that the MC approach may be applicable for modeling heat transfer in diverse aligned CNT composite systems.

**Figure 16.** (a) Schematic plot of the computational model of the SWNT/WS2/PEEK composites with SWNT bundles; (b) Effect of the SWNT-SWNT TBR on the thermal conductivity of the SWNT/WS2/PEEK composites [78].

### **6. Conclusions and outlook**

**Figure 15.** Schematic drawing of CNT reinforced composites, and aligned CNTs in a polymer as a representative vol‐

Bui et al. modified Duong's approach to investigate the thermal behavior of the SWNTpolystyrene (PS) composites at different volume fractions and at various temperature [67]. It was found that the thermal conductivity of SWNT-PS composites increased with the temper‐ ature rise. By validating with experimental data [68], the TBRs at both SWNT-PS and SWNT-SWNT interfaces were estimated by using the MC approach. The calculations at various temperature showed that the TBR between SWNT and PS increased with the rise of tempera‐

comparable study between graphene-polymer composites and SWNT-polymer composites [69]. The quantitative results showed that graphene nanosheets were more effective than

Recently, multiphase polymer composites, which contain more than one type of additive in the matrix, have attracted much attention [70,71]. The multiphase composites can combine the merits of all the components, inducing advanced multifunctional properties. Diverse multi‐ phase polymer composites have been developed, such as CNT/nanoparticle/polymer compo‐ sites [70,71], CNT/graphene/polymer composites [72,73], CNT/fiber/polymer composites [74,75] and CNT-stabilized polymer blends [76]. Since there is no effective approach to study thermal properties of CNT multiphase composites, Gong et al. [77] extended Duong's MC approach to investigate heat transfer phenomena in CNT multiphase composites. In Gong's model, a three-phase poly(ether ether ketone) (PEEK) composite containing SWNTs and tungsten disulfide (WS2) nanoparticles was chosen as a case study, as shown in **Figure 16**(**a**) [78]. The TBRs at all interfaces (i.e. SWNT-PEEK, WS2-PEEK and SWNT-SWNT) were taken into account in their model. The results showed that the thermal conductivity of multiphase

K/W at 300K, which was

K/W). Bui et al. also conducted the

ture. A TBR value of SWNT-SWNT was estimated to be 12.15×10-8 m2

SWNTs to enhance the thermal conduction in polymer composites.

**5.2. Thermal conduction model for multi-phase aligned CNT composites**

higher than that between SWNT and PS (2.21×10-8 m2

64 Carbon Nanotubes - Current Progress of their Polymer Composites

ume element [65].

Both the CNT fibers and their polymer composites fabricated using the methods outlined in this article attain superior mechanical, electrical and thermal properties compared with CNT composites fabricated using the conventional methods. Their advanced properties make them promising candidates for diverse applications, such as protective materials in airplanes and electrode materials in energy storage devices. For the diverse industrial applications of the aligned CNT composites, more studies should be carried out to fabricate the composites on a large scale and at low cost. New synthesis approaches can be developed to control the diameter of composite fibers and the size of composite films. To enhance their mechanical properties, cross linking should be created within CNT fibers through proper post-treatments. Chemical compositions and fabrication conditions require optimization for better polymer infiltration into the aligned CNT fibers, to achieve enhanced properties of the aligned CNT composites.

### **Author details**

Hai M. Duong1\*, Feng Gong1,2, Peng Liu1 and Thang Q. Tran1

\*Address all correspondence to<EMAIL_ADDRESS>
1 Department of Mechanical Engineering, National University of Singapore, Singapore

2 School of Chemical, Biological and Materials Engineering, University of Oklahoma, USA

### **References**

[8] Roberto Guzmán de V, Namiko Y, Antonio M, Brian LW. Multi-physics damage sensing in nano-engineered structural composites. Nanotechnology. 2011;22(18): 185502.

promising candidates for diverse applications, such as protective materials in airplanes and electrode materials in energy storage devices. For the diverse industrial applications of the aligned CNT composites, more studies should be carried out to fabricate the composites on a large scale and at low cost. New synthesis approaches can be developed to control the diameter of composite fibers and the size of composite films. To enhance their mechanical properties, cross linking should be created within CNT fibers through proper post-treatments. Chemical compositions and fabrication conditions require optimization for better polymer infiltration into the aligned CNT fibers, to achieve enhanced properties of the aligned CNT composites.

and Thang Q. Tran1

1 Department of Mechanical Engineering, National University of Singapore, Singapore

2 School of Chemical, Biological and Materials Engineering, University of Oklahoma, USA

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## **Bio-inspired Design and Fabrication of Super-Strong and Multifunctional Carbon Nanotube Composites**

Xiaohua Zhang, Xueping Yu, Jingna Zhao and Qingwen Li

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62810

#### **Abstract**

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72 Carbon Nanotubes - Current Progress of their Polymer Composites

Carbon nanotubes (CNTs) are ideal scaffolds to design and architect high-perform‐ ance composites at high CNT volume fractions. In these composites, the CNT align‐ ment determines the level of aggregation and the structure morphology, and thus the load transfer efficiency between neighboring CNTs. Here, we discuss two major solutions to produce high-volume fraction CNT composites, namely the layer-bylayer stacking of aligned CNT sheets and the stretching of entangled CNT webs (networks). As inspired by the growth procedure of natural composites, the aggrega‐ tion of CNTs can be well controlled during the assembling process. As a result, the CNTs can be highly packed, aligned, and importantly unaggregated, with the impregnated polymers acting as interfacial adhesion or mortars to build up the composite struc‐ ture. The CNT/bismaleimide composites can yield a super-high tensile strength up to 6.27–6.94 GPa and a modulus up to 315 GPa.

**Keywords:** carbon nanotube composite, high volume fraction, super-strong, multi‐ functionality, bio-inspired

### **1. Introduction**

Since their discovery in 1991 [1], carbon nanotubes (CNTs) have generated huge activity in most areas of science and engineering due to their unprecedented mechanical, electrical, and thermal properties. For example, lightweight multifunctional composites with enhanced properties can be produced by effectively incorporating individual CNTs into polymer matrices [2, 3]. The first polymer nanocomposites using CNTs as a filler were reported in 1994, where the CNTs were

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

aligned within the epoxy matrix by the shear forces induced by cutting with a diamond knife [4]. In the following decades, tremendous research has been done to develop CNT-reinforced composites with a high strength and modulus. However, as compared to individual CNTs, the composites produced by these "conventional" fabrication methods usually do not exhibit significantlyimprovedmechanicalandelectricalperformances.This ismainlyduetothelimited content of CNT (usually under 5 wt%). Furthermore, it is also important to introduce a uniform CNT dispersion in polymer matrix and adhesion between different constituents in improving the composite performances.

Recently, a new type of CNT composite material has been developed using macroscopic CNT assemblies as raw materials, namely high-volume fraction CNT composites, where the volume fraction of CNT is usually much larger than 50%. To obtain these composites, the macroscopic forms of entangled or aligned CNTs, that is, fibers [5–7], forests [8–12], and membranes [13– 16] are used as scaffolds, and the polymer is impregnated into the free pores of these CNT network. Of great importance, the super-aligned CNT sheets drawn out from spinnable CNT forests [17] and the entangled CNT webs (networks) grown with an injection chemical vapor deposition method [18] have superb advantages in obtaining super-strong high-volume fraction CNT composites.

Furthermore, to design the structure for high-performance composites at high CNT fractions, nature can offer us with scientific and technological clues from the formation process of biological composites. The natural composite materials are usually built up by common organic components via the naturally mild approaches [19], such as super-tough spider fibers [20], strong hard nut skins [21], and wear-resistant molluscan shells [22]. In these composites, the major components such as proteins, cellulose molecules, and nanometer-sized crystals of carbonated calcium phosphates or calcium carbonates are homogeneously distributed and orientated along with other co-existing components [20, 23]. This provides us new approaches to obtain super-strong CNT composites [24]. For CNT composites, due to the strong tendency to agglomerate between CNTs [25–29], it is still difficult to uniformly disperse CNTs within polymer matrix at a high-volume fraction and thus to mimic the natural composites. Fortu‐ nately, CNTs can be treated as linear macromolecules, and thus, the processing on them can be dealt with in a biomimic way. Therefore, it becomes possible to mimic the formation process of biological composites to design new type of CNT composites.

This chapter thus aims at the structural design of super-strong and multifunctional CNT composites at high-volume fractions. Various processing methods are presented in the following sections, namely the layer-by-layer stacking of aligned CNT sheets, stretching on entangled CNT webs, and, most importantly, bio-inspired aggregation control to optimize the composite structure.

### **2. Conventional composite processing**

Solution processing and melt processing have been widely used to prepare composites, where CNTs and polymer matrix can be well mixed together. For the solution treatment, CNT and polymer are mixed in a suitable solvent before evaporating the solvent to form a composite film. One benefit of such method is that agitation of the CNT powder in a solvent facilitates CNT de-aggregation and dispersion. In general, agitation of CNT or of CNT/polymer mixture is provided by magnetic stirring, shear mixing, reflux, and, as widely used, ultrasonication.

aligned within the epoxy matrix by the shear forces induced by cutting with a diamond knife [4]. In the following decades, tremendous research has been done to develop CNT-reinforced composites with a high strength and modulus. However, as compared to individual CNTs, the composites produced by these "conventional" fabrication methods usually do not exhibit significantlyimprovedmechanicalandelectricalperformances.This ismainlyduetothelimited content of CNT (usually under 5 wt%). Furthermore, it is also important to introduce a uniform CNT dispersion in polymer matrix and adhesion between different constituents in improving

Recently, a new type of CNT composite material has been developed using macroscopic CNT assemblies as raw materials, namely high-volume fraction CNT composites, where the volume fraction of CNT is usually much larger than 50%. To obtain these composites, the macroscopic forms of entangled or aligned CNTs, that is, fibers [5–7], forests [8–12], and membranes [13– 16] are used as scaffolds, and the polymer is impregnated into the free pores of these CNT network. Of great importance, the super-aligned CNT sheets drawn out from spinnable CNT forests [17] and the entangled CNT webs (networks) grown with an injection chemical vapor deposition method [18] have superb advantages in obtaining super-strong high-volume

Furthermore, to design the structure for high-performance composites at high CNT fractions, nature can offer us with scientific and technological clues from the formation process of biological composites. The natural composite materials are usually built up by common organic components via the naturally mild approaches [19], such as super-tough spider fibers [20], strong hard nut skins [21], and wear-resistant molluscan shells [22]. In these composites, the major components such as proteins, cellulose molecules, and nanometer-sized crystals of carbonated calcium phosphates or calcium carbonates are homogeneously distributed and orientated along with other co-existing components [20, 23]. This provides us new approaches to obtain super-strong CNT composites [24]. For CNT composites, due to the strong tendency to agglomerate between CNTs [25–29], it is still difficult to uniformly disperse CNTs within polymer matrix at a high-volume fraction and thus to mimic the natural composites. Fortu‐ nately, CNTs can be treated as linear macromolecules, and thus, the processing on them can be dealt with in a biomimic way. Therefore, it becomes possible to mimic the formation process

This chapter thus aims at the structural design of super-strong and multifunctional CNT composites at high-volume fractions. Various processing methods are presented in the following sections, namely the layer-by-layer stacking of aligned CNT sheets, stretching on entangled CNT webs, and, most importantly, bio-inspired aggregation control to optimize the

Solution processing and melt processing have been widely used to prepare composites, where CNTs and polymer matrix can be well mixed together. For the solution treatment, CNT and polymer are mixed in a suitable solvent before evaporating the solvent to form a composite

of biological composites to design new type of CNT composites.

**2. Conventional composite processing**

the composite performances.

74 Carbon Nanotubes - Current Progress of their Polymer Composites

fraction CNT composites.

composite structure.

In an early study [30], multiwalled CNTs were dispersed in chloroform by sonication. Then, polyhydroxyaminoether (PHAE) was dissolved in the CNT/chloroform dispersion and mixed by additional sonication. The suspension was poured into a Teflon mould and dried in ambient conditions to obtain the CNT composites. In the following decade, various similar methods have been reported. For example, CNTs were first chemically modified and then dispersed in water [31]. The dispersion was blended with poly(vinyl alcohol) (PVA)/water solution to give composite solutions which were used to prepare the composites.

As pristine CNTs cannot be well-dispersed in most solvents, surfactants such as sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDSA), polyvinylpyrrolidone (PVP), and dodecyl tri-methyl ammonium bromide (DTAB) were also used to assist the CNT dispersion before mixing with the polymer solution [32–38]. This technique results in excellent dispersion with no derogatory effects on film properties observed. However, the existence of surfactant could affect the interfacial strength between CNT and polymer.

For those polymers which are insoluble in the designed solvent, melt processing has become a common alternative. For example, amorphous polymers can be processed above their glass transition temperature, and semi-crystalline polymers can be heated above the melt temper‐ ature to induce sufficient softening [39–43]. In general, melt processing involves the melting of polymer pellets to form a viscous liquid. CNTs can be mixed into the melt by shear mixing. Bulk samples can then be fabricated by techniques such as compression molding, injection molding, or extrusion. In such approach, the optimized processing conditions depend on not only the types of CNT, but also the whole range of polymer–nanotube combinations. This is because nanotubes can effect melt properties such as viscosity, resulting in unexpected polymer degradation under conditions of high shear rates [39].

The dispersion and melting treatments have been widely used to obtain the composite structure by well mixing polymer and CNTs. Besides the structure design, interfacial covalent bonding is often applied in these methods in order to increase the composite performances. For example, in situ polymerization enables grafting of polymer molecules onto the wall of CNT and thus allows the preparation of composites with high nanotube loading. It is also particularly important for the preparation of insoluble and thermally unstable polymers, which cannot be processed by solution or melt processing. For example, due to their π-bonds, CNTs can participate in the polymerization of poly(methyl methacrylate) (PMMA), which resulted in a strong interface between the CNT and the PMMA matrix [44–46]. Poly(pphenylene benzobisoxazole) (PBO)/CNT composites were also obtained with in situ polymer‐ ization by introducing CNTs in poly(phosphoric acid) (PPA) [47]. For CNT composites using polyamide, in situ polymerization can be realized between carboxylated CNTs and ε-capro‐ lactam monomer [48].

In situ polymerization has been applied for the preparation of composites with CNT and various types of polymer. In this approach, covalent functionalization of CNT surface plays a special role for CNT processing and applications [49–55]. "Grafting from" and "grafting to" are the two main strategies to introduce covalent bonding between polymers and CNTs [2]. In the former approach, the initiators are immobilized onto CNT surfaces and then, the monomers are in situ polymerized with the formation of covalent bonding between polymer and CNT. The latter approach is based on attachment of already preformed end-functionalized polymer molecules to functional groups on CNT surface via different chemical reactions. There have been many reports on the grafting techniques, where the grafted polymers include PMMA [56–60], polyethylene (PE) [61–64], polystyrene [65–67], chitosan [68, 69], and so on.

There have been tremendous development on CNT composites using these conventional processing methods over the past 20 years. However, the dispersion of long CNTs is still hindered by their entanglement and aggregation, and the CNTs are limited to a low fraction and randomly orientated. Consequently, the final composite strength is usually below 400 MPa [70, 71]. Further, in situ polymerization or hot stretching is also not very effective in improving the mechanical properties [2, 28, 72]. Therefore, further development is still of great necessity, especially on the design of composite structure.

### **3. Layer-by-layer stacking of aligned CNTs**

The large mass scale production of CNT has pushed the rapid development of CNT in various applications [73, 74]. However, the random orientation and entanglement hinder the structure design of high-volume fraction and aligned CNT composites. The development on spinnable CNT forests opened a new way towards such composite structure. CNT forests (also called CNT carpets) superficially resemble bamboo forests, except that the CNT "trees" in these forests can be over 50,000 times longer than their diameter, and this very high aspect ratio is useful for optimizing the mechanical and electrical properties. As a unique type of CNT forests, known as spinnable forests, one can continuously transform the vertically aligned CNTs into a horizontally aligned CNT sheet by a simple dry drawing or spinning method [17, 75–79]. Thus, the spinnability (or processability/drawability) is defined by the stable width and continuous length of the spun-out CNT sheets and the available spinning rates [80].

### **3.1. CNT/polymer composite films**

The spinnable CNT forests have been successfully used to fabricate high-performance composite films. At first, the spun-out aligned CNTs were stacked together at different orientations to make a CNT preform [81]. After a resin transfer molding process, homogene‐ ously dispersed CNT/epoxy composites with a CNT loading up to 16.5 wt% were obtained. The Young's modulus and tensile strength of the composites reached 20.4 and 231.5 MPa, corresponding to 716 and 160% improvement compared to pure epoxy.

To further improve the CNT volume fraction, the CNT sheets were stacked in a continuous winding way with the aid of solution spraying [12]. **Figure 1A** shows the schematic of spray winding, where a CNT sheet is continuously wound onto a rotating mandrel on which micrometer-sized polymer solutions are deposited. With the long-chain polymer PVA, the aligned composite film exhibited a tensile strength up to 1.8 GPa at a CNT content of 65 wt% [12]. Due to the alignment, the composite film was also stiff (modulus of 40–96 GPa), tough (energy absorbed before fracturing of 38–100 J g−1), and electrically conductive (conductivity of 780 S cm−1). By following this method, high-performance CNT composites with epoxy, polyimide (PI), and bismaleimide (BMI) were produced [82–85].

special role for CNT processing and applications [49–55]. "Grafting from" and "grafting to" are the two main strategies to introduce covalent bonding between polymers and CNTs [2]. In the former approach, the initiators are immobilized onto CNT surfaces and then, the monomers are in situ polymerized with the formation of covalent bonding between polymer and CNT. The latter approach is based on attachment of already preformed end-functionalized polymer molecules to functional groups on CNT surface via different chemical reactions. There have been many reports on the grafting techniques, where the grafted polymers include PMMA [56–60], polyethylene (PE) [61–64], polystyrene [65–67], chitosan [68, 69], and so on. There have been tremendous development on CNT composites using these conventional processing methods over the past 20 years. However, the dispersion of long CNTs is still hindered by their entanglement and aggregation, and the CNTs are limited to a low fraction and randomly orientated. Consequently, the final composite strength is usually below 400 MPa [70, 71]. Further, in situ polymerization or hot stretching is also not very effective in improving the mechanical properties [2, 28, 72]. Therefore, further development is still of great necessity,

The large mass scale production of CNT has pushed the rapid development of CNT in various applications [73, 74]. However, the random orientation and entanglement hinder the structure design of high-volume fraction and aligned CNT composites. The development on spinnable CNT forests opened a new way towards such composite structure. CNT forests (also called CNT carpets) superficially resemble bamboo forests, except that the CNT "trees" in these forests can be over 50,000 times longer than their diameter, and this very high aspect ratio is useful for optimizing the mechanical and electrical properties. As a unique type of CNT forests, known as spinnable forests, one can continuously transform the vertically aligned CNTs into a horizontally aligned CNT sheet by a simple dry drawing or spinning method [17, 75–79]. Thus, the spinnability (or processability/drawability) is defined by the stable width and

continuous length of the spun-out CNT sheets and the available spinning rates [80].

corresponding to 716 and 160% improvement compared to pure epoxy.

The spinnable CNT forests have been successfully used to fabricate high-performance composite films. At first, the spun-out aligned CNTs were stacked together at different orientations to make a CNT preform [81]. After a resin transfer molding process, homogene‐ ously dispersed CNT/epoxy composites with a CNT loading up to 16.5 wt% were obtained. The Young's modulus and tensile strength of the composites reached 20.4 and 231.5 MPa,

To further improve the CNT volume fraction, the CNT sheets were stacked in a continuous winding way with the aid of solution spraying [12]. **Figure 1A** shows the schematic of spray winding, where a CNT sheet is continuously wound onto a rotating mandrel on which micrometer-sized polymer solutions are deposited. With the long-chain polymer PVA, the aligned composite film exhibited a tensile strength up to 1.8 GPa at a CNT content of 65 wt%

especially on the design of composite structure.

76 Carbon Nanotubes - Current Progress of their Polymer Composites

**3.1. CNT/polymer composite films**

**3. Layer-by-layer stacking of aligned CNTs**

**Figure 1.** (A) Schematic view of spray winding. A CNT sheet is drawn out from a spinnable forest and continuously wound onto a rotating mandrel on which micrometer-sized droplets of polymer solution are deposited [12]. (B) Nor‐ malized intensity of G′-band peak as a function of the angle between the sample's longitudinal direction and the polar‐ ization axis of the incident laser beam. (C) Typical stress–strain curves of pristine CNT sheet, unstretched and stretched composites, demonstrating a significant improvement on mechanical properties through aligning and straightening of CNTs [82]. (D) Schematic presentation of heat conduction mechanism in high-volume fraction CNT composites [83].

The layer-by-layer stacking allowed a high level of CNT alignment in the final composites. The alignment can be quantitatively measured with polarized Raman spectroscopy [8, 86]. **Figure 1B** shows the normalized intensity of Raman G′-band peak as a function of the angle between the sample's longitudinal direction and the polarization axis of the incident laser beam; according to the results, we have reported previously [12]. The normalized G′ peak was strongest at 0° (along the CNT orientation) and monotonically decreased from 1 to 0.493 at 90° (perpendicular to the orientation).

To further improve the CNT alignment, a pair of stationary rods was placed between the CNT forest and the rotating mandrel. Due to the surface friction, the rods induced a certain shear stretching on the CNT sheet [82]. As a result, the mechanical, thermal, and electrical properties of the composites were simultaneously improved; the film exhibited a strength of 3.8 GPa, Young's modulus of 293 GPa, electrical conductivity of 1230 S cm−1, and thermal conductivity of 41 Wm−1 K−1. **Figure 1C** shows the stress–strain curves of pristine CNT sheet without matrix and CNT composite sheets stretched to various stretch ratios, where the effect of CNT alignment and straightness can be clearly seen.

In these composite films, the longer the CNTs the higher the thermal and electrical conduc‐ tivities are. However, the mechanical properties including the strength and modulus exhibited no CNT length dependency [83]. (A) Setup for layer-by-layer stacking of aligned CNT sheets. (B) As-prepared CNT film on a mandrel. (C) Flexible CNT film strip. (D,E) Photographs of a shining CNT strip and a strip coated with a 12 nm gold layer. Inset in (D) is the shape of a water droplet on the strip. (F) SEM image of a woven fabric consisting of CNT strips. **Fig‐ ure 1D** shows a schematic of thermal (electrical) conducting mechanism for the high-volume fraction CNT composites. Due to the long tube length, thermal conduction across the interfaces is as not dominant as that along the CNTs. On the contrary, the thermally insulating polymer restricts the phonon mobility at the interface in short-CNT-reinforced composites. This means that the high interfacial thermal resistance can strongly limit the total thermal conductivity. However, with increasing the tube length, several issues might affect the mechanical perform‐ ance. First, the average tube diameter and wall number increased with the tube length, leaving the aspect ratio remaining nearly unchanged. Second, the increased tube length usually causes more aggregation between the CNTs. These two issues both hinder the efficiency of interfacial stress transfer.

### **3.2. Super-strong CNT assembly film**

As inspired by the forest-based fiber spinning where macroscopic one-dimensional assembly fiber with pure (neat) CNTs can be produced [75, 76], the layer-by-layer stacking can be also used to fabricate pure CNT films with high mechanical performances. Although no polymer molecules are impregnated, various solvents such as ethanol, acetone, and *N*,*N*-dimethylme‐ thanamide (DMF) are used to densify the CNTs. The solvent can be either sprayed onto the mandrel during the winding process or dip onto the CNT film after the winding [87]. **Figure 2**

**Figure 2.** Pure CNT assembly films can be produced by layer-by-layer stacking of as-drawn CNT sheets with the aid of solvent densification [87] (A) Setup for layer-by-layer stacking of aligned CNT sheets. (B) As-prepared CNT film on a mandrel. (C) Flexible CNT film strip. (D,E) Photographs of a shining CNT strip and a strip coated with a 12 nm gold layer. Inset in (D) is the shape of a water droplet on the strip. (F) SEM image of a woven fabric consisting of CNT strips.

shows the CNT films obtained using 2–3-walled 4–6-nm-diameter CNTs. The CNT film had a smooth and shinning surface, was flexible, and exhibited tensile strengths of 1.1–1.9 GPa and Young's modulus of 40–90 GPa. Obviously, the alignment allowed a high degree of utilization of the intrinsic properties of CNT.

In these composite films, the longer the CNTs the higher the thermal and electrical conduc‐ tivities are. However, the mechanical properties including the strength and modulus exhibited no CNT length dependency [83]. (A) Setup for layer-by-layer stacking of aligned CNT sheets. (B) As-prepared CNT film on a mandrel. (C) Flexible CNT film strip. (D,E) Photographs of a shining CNT strip and a strip coated with a 12 nm gold layer. Inset in (D) is the shape of a water droplet on the strip. (F) SEM image of a woven fabric consisting of CNT strips. **Fig‐ ure 1D** shows a schematic of thermal (electrical) conducting mechanism for the high-volume fraction CNT composites. Due to the long tube length, thermal conduction across the interfaces is as not dominant as that along the CNTs. On the contrary, the thermally insulating polymer restricts the phonon mobility at the interface in short-CNT-reinforced composites. This means that the high interfacial thermal resistance can strongly limit the total thermal conductivity. However, with increasing the tube length, several issues might affect the mechanical perform‐ ance. First, the average tube diameter and wall number increased with the tube length, leaving the aspect ratio remaining nearly unchanged. Second, the increased tube length usually causes more aggregation between the CNTs. These two issues both hinder the efficiency of interfacial

As inspired by the forest-based fiber spinning where macroscopic one-dimensional assembly fiber with pure (neat) CNTs can be produced [75, 76], the layer-by-layer stacking can be also used to fabricate pure CNT films with high mechanical performances. Although no polymer molecules are impregnated, various solvents such as ethanol, acetone, and *N*,*N*-dimethylme‐ thanamide (DMF) are used to densify the CNTs. The solvent can be either sprayed onto the mandrel during the winding process or dip onto the CNT film after the winding [87]. **Figure 2**