Polymer Nanotubes Nanocomposites: Synthesis, Properties and Applications

By Vikas Mittal

Since the publication of the successful first edition of the book in 2010, the field has matured and a large number of advancements have been made to the science of polymer nanotube nanocomposites (PNT) in terms of synthesis, filler surface modification, as well as properties. Moreover, a number of commercial applications have been realized. The aim of this second volume of the book is, thus, to update the information presented in the first volume as well as to incorporate the recent research and industrial developments.

 

This edited volume brings together contributions from a variety of senior scientists in the field of polymer nanotube composites technology to shed light on the recent advances in these commercially important areas of polymer technology. The book provides the following features:

 

• Reviews the various synthesis techniques, properties and applications of the polymer nanocomposite systems

• Describes the functionalization strategies for single walled nanotubes in order to achieve their nanoscale dispersion in epoxy matrices

• Provides insights into the multiscale modeling of the properties of PNT

• Provides perspectives on the electron microscopy characterization of PNT

• Presents an overview of the different methodologies to achieve micro-patterning of PNT

• Describes the recent progress on hybridization modifications of CNTs with carbon nanomaterials and their further applications in polymer nanocomposites

• Provides details on the foams generates with PNT

• Provides information on synthesis and properties of polycarbonate nanocomposite.

• Describes the advanced microscopy techniques for understanding of the polymer/nanotube composite interfaces and properties. 

• Language: English
• Publisher: Wiley
• Release date: Sep 2, 2014
• ISBN: 9781118945933

Vikas Mittal

Dr. Vikas Mittal worked as Associate Professor in the Department of Chemical Engineering at The Petroleum Institute (part of Khalifa University of Science and Technology), Abu Dhabi, UAE. Before, he was employed at BASF, Germany as polymer engineer and at SunChemical, UK as materials scientist. Dr. Mittal received his PhD degree in 2006 from Department of Materials and Department of Chemistry and Applied Biosciences at Swiss Federal Institute of Technology (ETH) Zurich, Switzerland. He has been an active researcher in the field of polymer nanotechnology and its applications in various streams. He has published more than 125 peer reviewed papers on these subjects, along with 35 edited and authored books. His research accomplishments have also resulted in many patents. In addition, he has published many book chapters and has also delivered numerous keynote and invited lectures.

Book preview

Preface

It is a pleasure to write this preface for the 2nd edition of Polymer Nanotubes Nanocomposites. Since the release of the 1st edition in 2010, a large number of advancements have been made in the science of polymer nanotube nanocomposites in terms of synthesis and filler surface modification, as well as properties. Furthermore, a number of commercial applications have been realized. Thus, the aim of this second volume is to update the information presented in the first volume, as well as to incorporate recent findings.

Chapter 1 reviews various synthesis techniques and properties, as well as applications, of the polymer nanocomposite systems. Chapter 2 focuses on the functionalization strategies for single-walled nanotubes in order to achieve their nanoscale dispersion in epoxy matrices. Chapter 3 provides insights into the multiscale modeling of the properties of the polymer nanotube nanocomposites. Chapter 4 provides perspectives on the electron microscopy characterization of the polymer nanotube nanocomposites. In Chapter 5, the use of polymer nanotube nanocomposites for transfemoral sockets is described. Chapter 6 presents an overview of the different methodologies to achieve micro-patterning of polymer nanotube nanocomposites. An overview of recent progress on hybridization modifications of CNTs with carbon nanomaterials and their further applications in polymer nanocomposites is given in Chapter 7. Chapter 8 provides details on the foams generated with polymer nanotube nanocomposites and concludes that hybrid materials based on metallic honeycombs filled with polymer-carbon nanotube foams and sheets built from different layers of polymer foams display excellent electromagnetic absorption, confirming their high potential for EMI shielding. Chapter 9 provides information on the synthesis and properties of polycarbonate nanocomposites. Chapters 10 and 11 focus on the advanced microscopy techniques used for understanding polymer/nanotube composite interfaces and properties. Chapter 12 concludes the volume by summarizing the latest challenges as well as perspectives for the future of polymer nanotube nanocomposite materials.

Dr. Vikas Mittal

Abu Dhabi

May 2014

Chapter 1

Polymer Nanotube Nanocomposites: A Review of Synthesis Methods, Properties and Applications

Joel Fawaz and Vikas Mittal*

Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE

*Corresponding author: [e-mail]

Abstract

Owing to their high mechanical and electrical properties, nanotubes are ideal fillers for the generation of composites. Polymer nanotube nanocomposites are synthesized after achieving suitable surface modifications of the nanotubes using different synthesis methods like melt mixing, in-situ polymerization and solution mixing. All these methods have their own advantages and limitations and varying degrees of success in achieving nanoscale dispersion of nanotubes in addition to achieving significant property enhancement in the composite properties. The tensile modulus is generally reported to be significantly enhanced on the incorporation of even small amounts of nanotubes. Though the tensile strength and elongation at break in many cases are reported to improve, they are more likely dependent on the morphology of the nanocomposites. The glass transition temperature as well as degradation temperature are also observed to significantly increase mostly owing to the reinforcing effect of nanotubes. Many other properties like electrical conductivity, heat deflection temperature, etc., also increase on the addition of nanotubes to the polymer.

Keywords: Nanocomposites, dispersion, aspect ratio, in situ, melt, morphology, tensile properties, glass transition temperature, degradation, functionalization, electrical conductivity, resistivity

1.1 Introduction

Many experimental and theoretical studies have reported the modulus of the nanotubes to be in the same range as graphite fibers and even the strength at least an order of magnitude higher than the graphite fibers [1–11]. In any case, even if the real mechanical properties of nanotubes are actually somewhat lower than the estimated values, nanotubes still represent high potential filler materials for the synthesis of polymer nanocomposites. The surface area per unit volume of nanotubes is also much larger than the other filler fibers, leading to much larger nanotube/matrix interfacial area in the nanotube-reinforced composites than in traditional fiber-reinforced composites. Figure 1.1 represents such an interface polymer fraction in nanotube-reinforced polymers where the ratio of the thickness t of the interphase versus the inclusion radius rf is plotted with respect to the volume fraction of the inclusion [1]. Owing to the interfacial contacts with the nanotubes, the interfacial polymer has much different properties than the bulk polymer. The conversion of a large amount of polymer into interface polymer fraction due to the nanoscale dispersion and high surface area of nanotubes generates altogether different morphology in the nanotube nanocomposites, which results in the synergistic improvement in the nanocomposite properties. In order to achieve optimized interfacial interactions between the polymer and nanotubes, nanoscale dispersion of the filler is required, which necessitates compatibilization of the polymer and inorganic phases. Therefore, the nanotubes need to be surface modified before their incorporation into the polymer matrix. Therefore, as CNTs agglomerate, bundle together and entangle, it may lead to defect sites in the composites, subsequently limiting the impact of CNTs on nanocomposite properties. Salvetat et al. [12] studied the effect of CNTs dispersion on the mechanical properties of nanotube-reinforced nanocomposites, and it was observed that poor dispersion and rope-like entanglement of CNTs caused significant weakening of the composites. Thus, alignment of CNTs is also equally important to enhance the properties of polymer/CNT composites [13,14]. Stress transfer property of the nanotubes in the composites is another parameter which controls the mechanical performance of the composite materials. Many studies using tensile tests on nanotube/polymer nanocomposites have reported the bonding behavior between the nanotubes and the matrix [15,16], in which there was an interfacial shear strength ranging from 35 to 376 MPa. The range of values was due to the different diameters of the nanotubes and the number of wall layers. However, other behaviors have also been reported based on interfacial compatibility. In their study, Lau and Hui [17] observed that most of the nanotubes were pulled out during the tensile testing owing to no interaction at the interface.

Figure 1.1 Fraction of interphase polymer as a function of volume fraction of fiber inclusion, where t is the interphase thickness and rf is the radius of the nanotube/fiber inclusion.

Reproduced from [1] with permission from Elsevier.

It has also been reported that in the case of multiwalled nanotubes, the inner layers of nanotubes cannot effectively take any tensile loads applied at both ends owing to the weak stress transferability between the layers of the nanotubes [8,18]. This results in the outmost layer of the nanotubes taking the entire load. As a result, the failure of the multiwalled nanotubes could start at the outermost layer by breaking the bonds among carbon atoms.

Nanotube nanocomposites with a large number of polymer matrices have been reported in recent years. The composites were synthesized in order to enhance mechanical, thermal and electrical properties of the conventional polymers so as to expand their spectrum of applications. Different synthesis routes have also been developed in order to achieve nanocomposites. The generated morphology in the composites and the resulting composite properties were reported to be affected by the nature of the polymer, nature of the nanotube modification, synthesis process, amount of the inorganic filler, etc. This chapter reviews nanocomposite structures and properties reported in a few of these reports and also stresses the future potential of nanotube nanocomposites by mentioning some of their reported applications. Recent reviews were published and can be found in [19–21].

1.2 Methods of Nanotube Nanocomposites Synthesis

1.2.1 Direct Mixing

This method, unlike the others, is used only for thermoset polymers. The carbon nanotubes are dispersed into a low viscosity thermosetting resin, usually epoxy, by mechanical mixing or sonication [22]. Afterwards, the mixture is cured to produce the nanocomposite. Another direct mixing technique involves the use of solvent to lower the viscosity of the epoxy resin [23]. The CNTs are first exfoliated in ethanol under sonication before mixing them with the epoxy resin. Once dispersion is obtained, the solvent is evaporated and hardener is added to trap the CNTs in the polymer matrix.

1.2.2 Solution Mixing

Solution mixing method has the advantage that the viscosity of the system can be controlled to be low so as to achieve higher extents of nanotube dispersion in the polymer systems. Both thermoset and thermoplastic polymers can be employed using this approach to achieve nanocomposites. The disadvantage associated with this method is, however, the requirement of a large amount of solvent for the nanocomposite synthesis, which for industrial applications may not be environmentally friendly or cost effective. For thermoset nanocomposites, one can also use the prepolymer to disperse the nanotubes and the prepolymer can then be crosslinked during the evaporation of solvent. Suhr et al. [19] reported the solution mixing approach as shown in Figure 1.2 for the synthesis of polycarbonate nanocomposites. The nanotubes were first oxidized in nitric acid before dispersion as the acidic groups on the sidewalls of the nanotubes can interact with the carbonate groups in the polycarbonate chains. To achieve nanocomposites, the oxidized nanotubes were dispersed in THF and were added to a separate solution of polycarbonate in THF. The suspension was then precipitated in methanol and the precipitated nanocomposite material was recovered by filtration. From the scanning electron microscopy investigation of the fracture surface of nanotubes, the authors observed a uniform distribution of the nanotubes in the polycarbonate matrix [24].

Figure 1.2 Schematic of synthesis of CNT polycarbonate nanocomposites by solution mixing approach.

Reproduced from [24] with permission from American Chemical Society.

Similarly, Biercuk et al. [25] reported the use of a solution mixing approach for the synthesis of epoxy nanocomposites. Epoxy prepolymer was dissolved in solvent in which the CNTs were also uniformly dispersed. The solvent was subsequently evaporated, and the epoxy prepolymer was crosslinked. The resulting nanocomposite was reported to have a good dispersion of nanotubes. In other studies, multiwalled nanotubes were mixed in toluene in which polystyrene polymer was dissolved [26, 27] to generate polystyrene nanocomposites. The nanocomposites were generated both by film-casting and spin-casting processes. The solution mixing method has also been used to attain alignment of the nanotubes in the composites [28,29]. Aspect ratio and rigidity of the nanotubes were reported to be the two factors which affect the alignment of the nanotubes. If the nanotubes were longer and more flexible, the alignment of the nanotubes in the composites was observed to deteriorate [30,31]. Stretching the cast film of the nanocomposite synthesized by the solution-mixing method resulted in the improvement of the nanotube alignment [30].

Liu and Choi [32] reported high quality dispersion of MWNTs at concentrations up to 9 wt% in poly(dimethylsiloxane) (PDMS) matrix using solution mixing. For better dispersion, a systemic study was conducted to determine the optimal solvent for both CNTs and PDMS. Chloroform was selected over the other common solvents, such as THF and DMF, due to its high solubility of the components and stability of the mixture. Moreover, functionalization of the CNTs by carboxyl groups further enhanced dispersion. The nanocomposite synthesis entailed the initial dispersion of fMWNTs in chloroform which was then sonicated for 1 hour. Meanwhile, PDMS base resin was dissolved in chloroform and magnetically stirred for 15 minutes. The separate mixtures were mixed together and sonicated for 1–2 hours. Solvent evaporation was efficiently performed by applying vacuum at elevated controlled temperatures. This process enabled the retainment of the initial dispersion as can be seen in the SEM images in Figure 1.3.

Figure 1.3 SEM micrographs of fracture surfaces of PDMS nanotube composities containing 7 wt% filler at (a) 160x, (b) 1000x, (c) 3000x and (d) 10,000x.

Reproduced from [32] with permission from Multidisciplinary Digital Publishing Institute.

Polyvinyl alcohol (PVA)/MWCNT nanocomposite membranes were reported by Shirazi et al. [33] as a means of dehydrating isopropanol. The nanocomposites were prepared by solution mixing in which PVA was dissolved and stirred in deionized water at 90°C followed by filtration and removal of bubbles by vacuum. CNTs are then added to the solution and ultrasonicated for 4 hours followed by the use of crosslinker and a catalyst. Figure 1.4 illustrates the procedure followed to prepare the membrane. Good dispersion of MWCNTs was achieved up to 2 wt% loading; whereas, increasing the loading above 2 wt% tended to cause agglomeration. Moreover, when measuring the outer diameter of the nanotubes in the 2 wt% loading nanocomposite, it was found to be similar to that of the neat CNT. On the other hand, in the 4 wt% loading nanocomposites, the diameter was measured to be higher, signaling the formation of CNT bundles.

Figure 1.4 Flow diagram describing the procedure of PVA nanocomposite membranes.

Reproduced from [33] with permission from Elsevier.

Martone et al. [34] compared solution and direct mixing in terms of dispersion. Different solvents (ethanol, acetone and sodium dodecyl sulfate aqueous surfactant) and dispersion techniques (magnetic, mechanical and sonication) were used to disperse the CNTs in an epoxy matrix. It was observed that direct mixing using sonication yielded submicron and more uniform texture compared to other methods, as seen in Figure 1.5.

Figure 1.5 Optical micrographs for epoxy nanocomposites prepared via solution mixing in (a) acetone, (b) surfactant, (c) ethanol and (d) via direct mixing using sonication.

Reproduced from [34] with permission from BMP-PT.

1.2.3 In-Situ Polymerization

This mode of nanocomposite synthesis is beneficial owing to the fact that the nanotube dispersion can be achieved in a solvent in which monomer is also dissolved or suspended. The low viscosities encountered during this process lead to better dispersion of nanotubes. The subsequent polymerization of monomer then leads to the uniform intercalation of polymer around the nanotubes. In many instances, the generated polymer can also be chemically grafted to the nanotube surfaces either by the acidic functionalities generated on the surface by chemical treatment or by direct grafting of polymer chains from the surface of the nanotubes by using surface immobilized initiators. Barazza et al. used miniemulsion approach to achieve polystyrene nanocomposites [35]. Cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) were used to functionalize the nanotubes. Hexadecane was used as a costabilizer and oil-soluble initiator AIBN was used for the polymerization. After the polymerization, the whole reaction contents were poured into a large volume of pure isopropyl alcohol to recover the nanocomposite. The incorporation of nanotubes in the polymer matrix was successfully achieved as demonstrated in Figure 1.6. The incorporation of nanotubes resulted in the black coloration of the nanocomposites materials as well as significant reduction in the electrical resistivity of the composite material. Raman spectra for the composite material also indicated a reduced vibrational freedom of the polymer chains as a consequence of the nanotube incorporation. An adsorbed polymer layer on the nanotube bundles was achieved as shown in Figure 1.6 which was observed to contribute to a better dispersion of the nanotubes.

Figure 1.6 TEM micrographs showing nanotube bundles with an adsorbed polystyrene layer in a 8.5% weight SWNT-PS composite.

Reproduced from [35] with permission from American Chemical Society.

Velasco-Santos et al. [36] also reported the in-situ polymerization of methyl methacrylate with both the treated and untreated nanotubes to generate polymer nanocomposites. The amount of initiator AIBN, reaction time and temperature were controlled to tune the molecular weight of polymer in the composites. The treated nanotubes had COOH and COO- functionalities on the sidewalls as well as tips and resulted in better property enhancement of the composites as compared to the untreated nanotubes. The authors suggested that the use of in-situ polymerization as well as functionalization of the nanotubes lead to the synergistic reinforcement of the organic and inorganic components of the composite.

Some studies on the grafting of the polymer chains from the surface of the nanotubes have also been reported. Qin et al. reported the polymerization of n-butyl methacrylate from the surface of nanotubes by using controlled living polymerization method [37]. Gao et al. also followed the similar grafting from the surface approach and polymerized methyl methacrylate on the surface by using atom transfer radical polymerization [38]. Figure 1.7 shows the schematic of the process in which an atom transfer radical polymerization initiator was covalently immobilized on the surface of the nanotubes which was subsequently used to graft polymer brushes from the surface. The use of controlled polymerization methods allow the benefits to control the molecular characteristics of the polymer grafts thus allowing to tune the properties of the hybrids. A variety of polymer architectures like block copolymers, multi-arm brushes, etc., can also be grafted by using the controlled polymerization mode.

Figure 1.7 Schematic of grafting of PMMA chains from the surface of nanotubes using atom transfer radical polymerization.

Reproduced from [38] with permission from American Chemical Society.

Bai et al. [39] reported the synthesis of grafted poly(3,4-ethylenedioxythiophene) (PEDOT)/MWNT composite using in-situ oxidative polymerization. This polymerization was conducted in a ternary phase system. The MWCNTs were sonicated with AOT surfactant dissolved in p-xylene, followed by the addition of FeCl3 solution. Eventually, the monomer EDOT was added drop-wise to the suspension with a reaction time of 24 hours. Lastly, the product was obtained through thorough washing and vacuum drying. The use of AOT surfactant allowed the uniform dispersion and stability of the nanotubes. The composition of the nanocomposite was found to have 8 wt% MWCNTs. Moreover, it was determined that the PEDOT grafted itself on the walls of MWCNTs creating a 3-dimensional network that gives rise to excellent capacitor properties.

Mansourpanah et al. [40] synthesized polycaprolactone-modified MWCNTs (PCL-MWCNTs) followed by the fabrication of polyethersulfone (PES)/PCL-MWCNTs. This was carried out using a variation of in situ polymerization and solution mixing. PCL-MWCNTs were first prepared by activating the CNTs in acid medium of H2S and HNO3 under reflux for 10 hours and later cooled, cleaned and dried. ε-caprolactone and modified CNTs were added together and sonicated at controlled elevated temperatures. PCL-MWCNTs were extracted by precipitation. On the other hand, PES was dissolved in DMA and PVP; whereas different concentrations of PCL-MWCNTs were dissolved in choloform. The two mixtures were mixed and stirred at 50°C and 200 rpm for 5 hours. However, using a film applicator to prepare PES/PCL-MWCNT membranes, the evaporation step was cancelled and instead immersed in a water bath to remove the solvent and other water-soluble polymer. This procedure allowed for good dispersion as well as enhanced porosity.

Dash et al. [41] reported the synthesis of Poly(anthranilic acid) (PAA)/MWCNT composites via in situ chemical oxidative polymerization. The CNTs were first functionalized using H2SO4 and HNO3 to provide carboxylic acid groups at the surface. Then, the functionalized MWCNTs were sonicated in a 1.2 M HCL solution for 2 hours before adding aniline and anthranilic acid to the suspension. APS reagent in HCL solution was added to the mixture and mechanically stirred. The co-polymer products obtained were filtered, washed and vacuum dried. SEM analysis showed that the diameter of the nanocomposite increased with increasing MWNT loading as PAA coats itself on the outer surface of the nanotubes, as shown in Figure 1.8. Dash et al. [41] stated that the coating arises from the strong interactions between the comonomer (i.e., aniline) and the functionalized MWNTs.

Figure 1.8 SEM micrographs of (a) MWCNT, (b) c-MWCNT, (c) neat anthranilic acid, (d) PAA/c-MWCNT at 2 wt%, (e) PAA/c-MWCNT at 5 wt% and (f) PAA/c-MWCNT at 10 wt% filler content.

Reproduced from [41] with permission from Springer.

Wu and Liu [42] prepared PS/MWCNTs via solution free radical in situ polymerization. Without any pretreatment of MWCNTs, they were combined with styrene monomers, toluene and AIBN initiators and the mixture was heated at 90°C for 11 hours. The product was precipitated and vacuum dried. FTIR analysis concluded the successful grafting of PS onto the walls of CNTs. Moreover, qualitative relationships between initiator and temperature with monomer conversion and polymer grafting were established by the authors. It was noted that with increase of AIBN initiator, monomer conversion increases. However, the highest grafting% was achieved with 0.05 g AIBN. Increasing the polymerization temperature increases both grafting and conversion, as shown in Table 1.1.

Table 1.1 Effect of polymerizing conditions on monomer conversion and polymer grafting percentages for PS nanocomposites. Reproduced from [42] with permission from Taylor & Francis.

Li and Kim [43] reported the synthesis of polyaniline (PANI)/MWCNT composites for sensor application. The synthesis was conducted via in situ oxidation polymerization in which the aniline monomers and MWCNTs were added to 1 M HCL followed by the addition of the ammonium per-sulfate (APS) reagent solution. The mixture was stirred for 2 hours at room temperature then the product obtained was filtered and washed. Core and shell structures were visible in SEM images signaling the typical structure of polymer grafted nanocomposites and the diameter increased with increasing MWCNT.

1.2.4 Melt Mixing

Melt mixing of polymer with the inorganic filler is a very attractive technique to synthesize nanocomposites using a large variety of polymers. This technique has also been exploited in great details for the polymer clay systems and the generated knowledge and experience is applicable also to polymer nanotube nanocomposites in many ways. The advantage of this technique is the direct mixing of the polymer at high temperature with the filler thus requiring no solvent which makes this process more industrially attractive as well as environmentally friendly. The nanotubes have also been reported to have a lesser extent of fiber breakage during compounding in melt [44, 45]. Alig et al. [46] discusses in depth the relation between processing conditions, CNT dispersion and filler network morphology with the properties generated. It is stated that dispersion of CNTs involves several steps:

1. Wetting of initial agglomerates by the polymer.

2. Infiltration of polymer chains into the initial agglomerates to weaken them.

3. Dispersion of agglomerates by rupture and erosion.

4. Distribution of individualized nanotubes into the matrix.

The dispersion is generally improved because of the presence of high extents of shear in melt compounding equipments. Longer processing times also lead to better mixing of organic and inorganic phases and alignment of the nanotubes in the composites can also be improved when elongational flow is additionally applied. However, melt mixing may also lead to serious degradation of the polymer if the compounding temperature is too high or very long processing times are used. The organic surface modifications immobilized on the sidewalls of the nanotubes are also prone to thermal damage during the compounding thus requiring an optimal mixing temperature and mixing time which do not cause the thermal damage but are also high and long enough to ensure homogenous mixing. Increasing the mixing temperature would lower the viscosity of the polymer matrix and this in turn makes the dispersion worse [46]. It is important to note that this depends on the screw speed as well as the polymer grade. Moreover, high loadings of nanotubes limit wetting and infiltration and result in bigger agglomerates. Figures 1.9 and 1.10 illustrate the effects of mixing speed and mixing temperature on area ratio, degree of dispersion and distribution coefficient; respectively for MWNT/PC nanocomposites [46]. The higher distribution coefficient reflects an increase in agglomeration tendency.

Figure 1.9 Area ratio vs mixing speed for PC nanocomposites with different viscosities and at 1 wt% filler.

Reproduced from [46] with permission from Elsevier.

Figure 1.10 Mixing temperature (TM) vs (a) degree of dispersion (b) distribution coefficient for PC nanocomposites at 1 wt% prepared via melt mixing at 50 rpm and 5 min.

Reproduced from [46] with permission from Elsevier.

Pötschke et al. [47] reported the polycarbonate nanocomposites by melt mixing method using twin-screw co-rotating intermeshing extruder. Compounding temperature of 240°C, screw speed of 280 rpm and a feed rate of 980 g/h were used for the composite generation. The SEM investigations of the polymer nanotube masterbatches revealed random orientation of nanotubes and formation of interconnecting structures. The authors also reported that it was not possible to estimate fiber length from the micrographs owing to the complex nanotube network. The diameters of the nanotubes in the composites were observed to in the range of 10 to 50 nm which is higher than the other studies reporting the diameter in the range of 10 to 15 nm. It was suggested that a thick polycarbonate layer existed on the surface of nanotubes thus increasing the diameter as well as indicating some extents of interphase mixing or phase adhesion. Maiti et al. [48] reported the synthesis of PC/PCL-MWCNT nanocomposites using melt mixing. A masterbatch of PCL-MWCNT was first prepared via melt mixing using internal mixer at 65°C and 60 rpm for 10 min. Then, the masterbatch was melt blended with pure PC at 280°C and 60 rpm for 10 min. This procedure yielded a homogeneous dispersion of CNTs at low loadings as analyzed in SEM.

CNT/LLDPE nanocomposite fibers were synthesized using melt extruder as reported by Mezghani et al. [49]. The melted LLDPE pellets were mixed with aligned MWNTs using 24 mm diameter Thermo Haake twin screw extruder of length 40D. The temperature was maintained 160°C whereas the last zone of the extruder was maintained at 180°C. The spinneret die was used to produce the fibers with average extruded fiber diameter of 620 μm which were air-cooled and drawn (6x) at room temperature. Good distribution of the CNTs with no agglomeration in the LLDPE matrix was noted due to its passing through 3 mixing stages. Moreover, alignment of CNTs was present due to drawing of the fibers.

Shih et al. [50] reported biodegradable poly(butylene succinate) nanocomposites through melt blending in a counter-rotating internal mixer with a rotation speed of 60 rpm for 5 min at 120°C. The authors observed that the generated composites consisted of well-dispersed nanotubes and exhibited enhanced thermal and mechanical properties. Kim et al. [51] reported the thermotropic liquid crystalline polymer (TLCP) nanocomposites prepared by a melt blending process in a Haake rheometer equipped with a twin-screw which was operated in intermeshing co-rotating mode. The temperatures of the heating zone from the hopper to the die were set to 290, 300, 305, and 295°C, and the screw speed was fixed at 40 rpm. The polymer and nanotubes were physically mixed before feeding them to the extruder. The microscopic investigation of the resulting composites, as shown in Figure 1.11, revealed that nanotubes were embedded in the polymer from both the ends, though some of them were pulled out from the matrix. As shown in the image, some nanotubes also broke while still remaining strongly embedded in the polymer matrix thus indicating that CNTs had good interfacial mixing with TLCP matrix owing to positive interfacial interactions. Synthesis of poly(butylene terephthalate) nanotube nanocomposites was also reported using the melt blending in the rheometer equipped with twin-screw [36].

Figure 1.11 SEM image of the fracture surfaces for the nanocomposites containing 0.5 wt% of CNTs. The arrows indicate that the nanotubes were to be broken with their ends still embedded in the polymer matrix or were bridging the local microcracks in the nanocomposites.

Reproduced from [51] with permission from Elsevier.

Combination of solution mixing and melt mixing methods has also been reported [53]. The nanotubes were dispersed in chloroform to which polyethylene powder was also mixed. The solvent was then evaporated and the mixture was dried in oven. The dry mixture was subsequently melt mixed using a twin-screw extruder with a 30 g bowl. The mixture was blended at a rate of 75 rpm using compounding temperature of 110°C and processing time of 10 min. Giraldo et al. [54] reported the synthesis of nylon nanocomposites using the co-rotating twin screw extruder using for heating zones from 230 to 250°C and a screw speed of 100 rpm. PA 11 nanocomposites incorporating different amounts of nanotubes were also reported by Huang et al. [55] using a twin screw extruder at 220°C and a screw speed of 80 rpm. The cryo-fracture SEM analysis of the composites indicated homogeneous dispersion of nanotubes throughout the PA11 matrix. The authors reported that upon failure, most of the MWNTs were broken apart, while many of them were still in the matrix. This behavior indicated a strong interfacial adhesion between the organic and inorganic components and a sufficient load transfer from the polymer to the nanotubes.

PA6/PMMA/SWCNT nanocomposites were reported by Madhukar et al. [56] in order to determine the suitability of SWCNTs as compatibilizers. They were prepared via melt mixing in Brabender-Mixer with mixing speed of 50 rpm at 230°C. The neat PA6/PMMA composites were reported to have a two phase system with dispersed particles. However, upon the addition of functionalized of 1 wt% SWCNTs containing carboxylic groups, uniform mixing of the polymers was reported with much smaller agglomerates compared to the neat composite. This was considered to be due to reduction in interfacial tension between the blends which reduced coalescence.

Ferguson et al. [57] and Schwartz and Bryant [58] also reported combined use of kneader and injection molding for the better dispersion of nanotubes in the composites. The authors reported that the CNTs did not break or orient because of their size and geometry, and therefore, conductivity of the nanocomposite was retained even after subsequent processing. The physical properties of the polymer were also retained due to small amount of CNTs present in the matrix. Alignment of nanotubes in the composites was also reported to be tunable by using several melt-mixing methods. Spinning of extruded melt samples was demonstrated as a method to generate the well aligned polypropylene nanocomposites [59].

One novel approach of improving the dispersion in melt mixing is by using masterbatches that are prepared by in situ polymerization as reported for PMMA, PS [60] and phthalocyanine (Pc) nanocomposites [61]. For Pc/MWCNT nanocomposites synthesis, masterbatches were prepared by first melting the Pc monomer, which is 4,4’-bis(3,4-dicyanophenoxy) biphenyl (BPH), at 250°C while stirred mechanically for 10 min and further 15 min when CNTs were slowly added [61]. The mixture was cooled at room temperature before smashed to produce the powder. The masterbatch was then placed in a preheated mold at 250°C and cured at controlled elevated temperatures for 4 hours until the reaction was complete. Good dispersion with smoother surface was achieved using masterbatch in comparison with direct mixing technique; as shown in Figure 1.12.

Figure 1.12 SEM micrographs of 2 wt% MWCNT/Pc nanocomposites prepared by (a) masterbatch and (b) melt mixing; (c) zoomed area of the rectangular box in ‘b’ showing agglomeration.

Reproduced from [61] with permission from Springer.

Annala et al. [60] used in situ polymerized masterbatches in co-rotating twin screw miniextruder under nitrogen blanket to synthesize PS and PMMA/MWCNT nanocomposites. Different screw speeds and mixing times were used to determine the optimum conditions for better properties. A screw speed of 120 rpm and a mixing time of 10 min were used as a result. Two polymerization process were used to obtain different molecular weights. Masterbatches for PMMA and high molecular weight PS nanocomposites were prepared by emulsion polymerization using KPS as an initiator. On the other hand, masterbatch for low molecular weight PS nanocomposites was prepared by using emulsion/suspension method using AIBN initiator. Viscosity of masterbatch was adjusted by adding a plasticizer to the emulsion before drying it to form the powder. Direct melt mixing was done by adding the carbon nanotubes directly to the extruder. It was noted by the authors that depending on the intrinsic properties of the nanocomposites and the interaction between the nanotubes and polymer matrix, the method of feeding CNTs can affect the properties.

Shim and Park [62] reported the synthesis of PP/PS-MWCNT via melt mixing using a bravender mixer at 30 rpm and 200°C for 1 hour. However, the grafting of PS on MWCNT was done via in situ polymerization. A core-shell structure was reported using TEM with a thickness of 2–4 nm of the grafted polymer; whereas using SEM, the PS-CNTs were uniformly dispersed in the PP matrix. Moreover, a interconnected network structure in the matrix.

1.3 Properties of Polymer Nanotube Nanocomposites

It is stated that the elastic modulus and tensile strength of CNTs may exceed 1.0 TPa and lie in the range of 10–50 GPa; respectively [22]. Moreover, the thermal conductivity of the CNTs can be as high as 3 kW/m-K [22]. These properties increase the attractiveness of CNTs as nanocomposite for polymer materials. This is further demonstrated in the coming sections.

1.3.1 Mechanical Properties

Hou et al. [63] reported the poly(vinyl alcohol) nanocomposites using single walled (SWNT), few walled (FWNT) and multi walled (MWNT) nanotubes. The nanotubes were covalently functionalized to generate acid functionalities on the sidewalls. The incorporation of nanotubes even in the amount of 0.2 wt% in the polymer was observed to enhance the Young’s modulus and tensile strength of the polymer significantly. Though all the different types of nanotubes resulted in higher tensile properties, but the FWNTs were observed to be particularly beneficial. The Young’s modulus of the composites with 0.2 wt% of the functionalized FWNTs was observed to be 6.33 GPa, which was 1.99 GPa higher than the pure polymer. Similarly, the tensile strength was also much better for the composites with FWNTs as demonstrated in Figure 1.13a. Increasing the amount of nanotubes in the composites also correspondingly enhanced the tensile strength as shown in Figure 1.13b. The authors reported that the FWNTs had diameters in the range of 3–8 nm and length in the range of 20 μm. The higher diameter and thicker wall FWNTs were reported to be much easier to be individually dispersed in solvent or polymer than SWNTs (also confirmed by the electron microscope images of the composite materials).

Figure 1.13 Stress-strain curves of nanotube nanocomposites (a) containing 0.2 wt% of different types of functionalized CNTs and (b) containing different concentrations of fFWNTs.

Reproduced from [4] with permission from American Chemical Society.

In polyamide nanocomposites [55], the storage modulus of the composites was reported to increase steadily with increasing the loading of MWNTs. At 2 wt% concentration of the nanotubes, the storage modulus of the nanocomposite was measured to be 1.97 GPa, which is an increase of 54% than the storage modulus of 1.28 GPa for the pure polyamide matrix.

Cao et al. [64] reported nanotube incorporation in Chitosan with medium molecular weight. The polymer, MWNTs and the composites with different fractions of MWNTs were characterized by X-ray diffraction. The MWNTs exhibited a sharp diffraction peak at about 2θ of 25.8°, which is caused by the regular arrangement of the concentric cylinders of graphitic carbon [65]. However, the diffraction peak associated with nanotubes was absent in chitosan nanocomposites which indicated the effective dispersion of nanotubes in the polymer matrix. A small amount of nanotubes were reported to significantly affect the mechanical performance of the nanocomposites. With 3 wt% nanotubes in the composites, the tensile strength and Young’s modulus increased from 39.6 MPa and 2.01 GPa for the pure polymer to 105.6 MPa and 4.22 GPa for the composite respectively. The elongation at break was also observed to be only slightly reduced owing to the incorporation of 1 wt% of filler. However, this decrease was much more significant when the filler content was higher than 1 wt% owing to some extent of aggregation of the nanotubes on increasing filler content. On the other side, Marroquin et al. [66] discussed the integration of Fe3O4 to the MWNT/Chitosan nanocomposite films. A 5wt% loading of Fe3O4/MWNT greatly enhanced tensile strength and modulus by a factor of 70% and 155% compared to a 5wt% MWNT/chitosan films; 159% and 179% compared to the neat chitosan. The reason is contributed to the antiplasticating nature of Fe3O4 that restricts chain movements and enhances crystallinity. %Crystallinity was determined using XRD to be 77% for 5wt% Fe3O4/MWNT/chitosan and 70% for MWNT/chitosan. It is important to note that even without incorporating Fe3O4, the properties were enhanced,. It is noted that the elongation at break was greatly reduced at 5 wt% loading as discussed above.

Zhang et al. [67] reported the polyimide nanocomposites with nanotubes containing CH3(CH2)17NHCO functional groups on the surface. The microscopic investigation of the composites confirmed the uniform distribution of the nanotubes in the polymer matrix. The nanotubes appeared as an interconnected structure at a loading of 7 wt% or higher indicating a nanotube network. The tensile strength of the pure polymer was measured to be 89 MPa, which increased to 130 MPa at a filler loading of 7 wt%.

However, further increase in the filler fraction in the composites led to a reduction in the tensile strength. The improvement in the tensile strength was suggested to be due to strong interactions between the polymer matrix and the nanotubes. Also, the elongation at break was only gradually decreased with increasing the nanotube fraction. The tensile modulus of the composite was also observed to linearly increase with increasing nanotube content and the modulus of the nanocomposite with 7 wt% filler loading was more than twice the modulus of the pure polymer. The modulus was slightly reduced on further increasing the nanotube fraction.

Polymer nanocomposites with medium density polyethylene were reported with a variety of fluorinated and un-fluorinated nanotubes [53]. The nanocomposites consisting of 1 wt% F-SWNT-C11FxHy (fluorinated and surface treated nanotubes) nanotubes showed an increase in tensile strength by 52.4%, modulus by 15.9% and elongation by 18.9% as compared to the pure polymer. The composites with 1 wt% F-SWNT-C11H23 (fluorinated and surface treated nanotubes) had an increase of 28.3% in modulus as compared to the pure polymer. The tensile strength also increased from 4.33 MPa for the pure polymer to 5.01 Mpa for the nanocomposite, the elongation at break was however observed to decrease. The 1 wt% SWNT (pristine nanotubes) nanotube nanocomposite showed an 11% increase in tensile strength as compared to polymer. The modulus also increased from 637 MPa for the pure polymer to 763 MPa for the nanocomposite. The 1 wt% F-SWNT (fluorinated nanotubes) nanotube composites however showed a decrease in both the tensile strength as well as tensile modulus of the composites as compared to pure polymer.

MWCNT/HDPE composites were synthesized by Tang et al. [22] using melt processing technique. The mechanical properties, such as stiffness and peak load, were investigated using punch tests. It was reported that with increasing the concentration of the MWCNTs, these properties were enhanced. Table 1.2 summarized the results obtained.

Table 1.2 Mechanical performance of HDPE nanocomposites. Reproduced from [22] with permission from Pergamon.

Mezghani et al. [49] investigated the effect of MWNT concentration on the mechanical properties of MWNT/LLDPE nanocomposite fibers, prepared by melt mixing. It was recorded that at 1wt% loading of MWNTs, 38% higher tensile strength was observed compared to the neat LLDPE. However, ductility and toughness records were the highest at 0.3 wt% CNT with 122% and 105% increase compared to neat LLDPE; respectively. Uniform distribution and aligned CNTs were determined to be the cause of the enhanced properties.

Poly(butylene terephthalate) (PBT) nanocomposites incorporating different fractions of nanotubes were reported by Kim et al. [52]. The storage modulus of the nanocomposites was observed to increase on the incorporation of nanotubes into the PBT matrix which was attributed to the physical interactions between PBT matrix and nanotubes. The nanocomposites also exhibited higher tensile strength and tensile modulus as compared to the pure polymer. At 2 wt% of the filler content in the composites, the tensile strength and tensile modulus were significantly increased by 35.1 and 21.7%, respectively. The enhancement in the mechanical properties was much more significant at the low filler fractions than at higher fractions. The authors suggested that nanotubes at higher concentrations tended to bundle together because of their intrinsic van der Waals attractions between the individual nanotubes in combination with high aspect ratio and large surface area.

Ji et al. [68] reported the poly(acrylonitrile) nanotubes composite nanofiber sheets with high mechanical performance. The tensile strength and modulus of the pure poly(acrylonitrile) terpolymer nanofiber sheet (without nanotubes) were reported to be 71.9 MPa and 2.2 GPa, respectively. On the incorporation of 2.0 wt % functionalized multi walled nanotubes, the tensile strength and modulus was observed to enhance to about 114.8 MPa and 3.2 GPa. The properties were also significantly enhanced after hot stretching processes owing to the alignment and orientation of macromolecular chains. The tensile strength and modulus of the pure terpolymer fiber sheet increased to about 215.9 MPa and 3.6 GPa, respectively. The composite samples with 2 wt % functionalized nanotubes also exhibited marked increases in tensile strength from 114.8 to 302.5 MPa and in modulus from 3.2 to 6.7 GPa owing to the improved alignment of nanotubes. Figure 1.14 shows the mechanical performance of the composites. The experimental values have also been compared to the calculated values of the mechanical properties as a function of increasing the content of nanotubes in the composites. The experimental and theoretical values were observed to match only at low concentrations of the filler, whereas at high concentrations of filler, these values have significant discrepancies owing to the aggregation of the nanotubes in the composites.

Figure 1.14 (a,b) Tensile modulus and strength of PAN nanocomposites. Curves ‘a’ and ‘c’ represent theoretical values of hot stretched and original electrospun composites, whereas curves ‘b’ and ‘d’ represent the experimental values of these composites.

Reproduced from [68] with permission from American Chemical Society.

Kim et al. [51] studied the thermotropic liquid crystalline polymer (TLCP) nanocomposites with varying extents of nanotubes. The mechanical performance of the nanocomposites has been demonstrated in Table 1.3. Increasing the nanotube extent, an increase in both the strength and modulus of the composites as compared to pure polymer was observed. However, these values were always smaller than the calculated values using the Halpin Tsai equations probably owing to incomplete exfoliation and the misalignment of the nanotubes.

Table 1.3 Mechanical performance of the TLCP nanocomposites. Reproduced from [51] with permission from Elsevier.

Geng et al. [69] reported epoxy nanocomposites with pristine, silane treated and surfactant treated (Triton) nanotubes as a function of increasing the content of nanotubes. The authors observed that both the flexural strength and modulus of the nanocomposites were enhanced with incorporation of nanotubes using differently modified nanotubes. The surfactant treated nanotubes were observed to be much effective than the silane treated or pristine nanotubes in enhancing the composite properties. It was also observed that the flexural properties attained an optimum at roughly 0.1% after which the incorporation of nanotubes was either not effective or the properties even started to degrade. The authors explained that the mechanical performance of the nanotubes was a result of two interrelated factors: interfacial adhesion and dispersion of nanotubes in the polymer matrix.

Razin et al. [70] reported the storage moduli (E’) of PMMA grafted nanotubes. It was noted that with slight addition of 0.7 wt% of grafted MWCNTs, E’ doubled at 40°C. Moreover, E’ increases with increasing MWCNT loading. At higher temperatures such as 120°C, there was a slight enhancement to the moduli values. That was considered to be due to PMMA maintaining its mechanical properties with the addition of MWCNT and that grafting CNTs help in distributing local stresses among the matrix.

Fragneaud et al. [71] reported polystyrene nanocomposites using as received nanotubes (a-CNx) and polystyrene grafted nanotubes (PS-g-CNx). The morphology of the generated composites containing 2.5 vol% of the nanotubes was analyzed through the fracture surfaces. The nanocomposites containing polystyrene grafted nanotubes had better dispersion of the nanotubes within the polymer matrix. Only small extent of aggregation was observed, whereas in the composites containing untreated nanocomposites, larger agglomerations (up to 10 μm) were observed. This inhomogeneous dispersion of nanotubes in