Polymer Nanocomposites Polymer nanocomposites are polymer matrix composites in which the reinforcement has at least one of its dimensions in the nanometer range (1 nanometer (nm)=10-3 um (micron)=10-9m).These composites show great promise not only in terms of superior mechanical properties,but also in terms of superior thermal,electrical,optical,and other properties,and,in general,at relatively low-reinforcement volume fractions.The principal reasons for such highly improved properties are (1)the properties of nano-reinforce- ments are considerably higher than the reinforcing fibers in use and(2)the ratio of their surface area to volume is very high,which provides a greater interfacial interaction with the matrix. In this chapter,we discuss three types of nanoreinforcements,namely nanoclay,carbon nanofibers,and carbon nanotubes.The emphasis here will be on the improvement in the mechanical properties of the polymer matrix.The improvement in other properties is not discussed in this chapter and can be found in the references listed at the end of this chapter. 8.1 NANOCLAY The reinforcement used in nanoclay composites is a layered silicate clay min- eral,such as smectite clay,that belongs to a family of silicates known as 2:1 phyllosilicates [1].In the natural form,the layered smectite clay particles are 6-10 um thick and contain >3000 planar layers.Unlike the common clay minerals,such as talc and mica,smectite clay can be exfoliated or delaminated and dispersed as individual layers,each ~1 nm thick.In the exfoliated form,the surface area of each nanoclay particle is ~750 m-/g and the aspect ratio is >50. The crystal structure of each layer of smectite clays contains two outer tetrahedral sheets,filled mainly with Si,and a central octahedral sheet of alumina or magnesia(Figure 8.1).The thickness of each layer is ~1 nm,but the lateral dimensions of these layers may range from 200 to 2000 nm.The layers are separated by a very small gap,called the interlayer or the gallery.The negative charge,generated by isomorphic substitution of Al+with Mg2+or Mg2+with Li+within the layers,is counterbalanced by the presence of hydrated alkaline cations,such as Na or Ca,in the interlayer.Since the forces that hold the layers together are relatively weak,it is possible to intercalate small organic molecules between the layers. 2007 by Taylor&Francis Group.LLC
8 Polymer Nanocomposites Polymer nanocomposites are polymer matrix composites in which the reinforcement has at least one of its dimensions in the nanometer range (1 nanometer (nm) ¼ 103 mm (micron) ¼ 109 m). These composites show great promise not only in terms of superior mechanical properties, but also in terms of superior thermal, electrical, optical, and other properties, and, in general, at relatively low-reinforcement volume fractions. The principal reasons for such highly improved properties are (1) the properties of nano-reinforcements are considerably higher than the reinforcing fibers in use and (2) the ratio of their surface area to volume is very high, which provides a greater interfacial interaction with the matrix. In this chapter, we discuss three types of nanoreinforcements, namely nanoclay, carbon nanofibers, and carbon nanotubes. The emphasis here will be on the improvement in the mechanical properties of the polymer matrix. The improvement in other properties is not discussed in this chapter and can be found in the references listed at the end of this chapter. 8.1 NANOCLAY The reinforcement used in nanoclay composites is a layered silicate clay mineral, such as smectite clay, that belongs to a family of silicates known as 2:1 phyllosilicates [1]. In the natural form, the layered smectite clay particles are 6–10 mm thick and contain >3000 planar layers. Unlike the common clay minerals, such as talc and mica, smectite clay can be exfoliated or delaminated and dispersed as individual layers, each ~1 nm thick. In the exfoliated form, the surface area of each nanoclay particle is ~750 m2 =g and the aspect ratio is >50. The crystal structure of each layer of smectite clays contains two outer tetrahedral sheets, filled mainly with Si, and a central octahedral sheet of alumina or magnesia (Figure 8.1). The thickness of each layer is ~1 nm, but the lateral dimensions of these layers may range from 200 to 2000 nm. The layers are separated by a very small gap, called the interlayer or the gallery. The negative charge, generated by isomorphic substitution of Al3þ with Mg2þ or Mg2þ with Liþ within the layers, is counterbalanced by the presence of hydrated alkaline cations, such as Na or Ca, in the interlayer. Since the forces that hold the layers together are relatively weak, it is possible to intercalate small organic molecules between the layers. 2007 by Taylor & Francis Group, LLC.
8 -O.OH -Al,Mg.Fe Silicate layer ~1 nm -0,0H Si Basal spacing -0 Na" Inter layer ○：0：OH O.●：Si(A) ●：Al,Fe,Mg FIGURE 8.1 Crystal structure of smectite clay.(From Kato,M.and Usuki,A., Polymer-Clay Nanocomposites.T.J.Pinnavai and Beall,eds.,John Wiley Sons, Chichester,U.K.,2000.With permission.) One of the common smectite clays used for nanocomposite applications is called montmorillonite that has the following chemical formula Mx(Al4-xMgx)SisO20(OH)4, where M represents a monovalent cation,such as a sodium ion,and x is the degree of isomorphic substitution (between 0.5 and 1.3).Montmorillonite is hydrophilic which makes its exfoliation in conventional polymers difficult.For exfoliation,montmorillonite is chemically modified to exchange the cations with alkyl ammonium ions.Since the majority of the cations are located inside the galleries and the alkyl ammonium ions are bulkier than the cations,the exchange increases the interlayer spacing and makes it easier for intercalation of polymer molecules between the layers. When modified smectite clay is mixed with a polymer,three different types of dispersion are possible.They are shown schematically in Figure 8.2.The type of dispersion depends on the polymer,layered silicate,organic cation,and the method of preparation of the nanocomposite. 1.Intercalated dispersion,in which one or more polymer molecules are intercalated between the silicate layers.The resulting material has a well-ordered multilayered morphology of alternating polymer and silicate layers.The spacing between the silicate layers is between 2 and 3 nm. 2007 by Taylor Francis Group,LLC
One of the common smectite clays used for nanocomposite applications is called montmorillonite that has the following chemical formula Mx(Al4xMgx)Si8O20(OH)4, where M represents a monovalent cation, such as a sodium ion, and x is the degree of isomorphic substitution (between 0.5 and 1.3). Montmorillonite is hydrophilic which makes its exfoliation in conventional polymers difficult. For exfoliation, montmorillonite is chemically modified to exchange the cations with alkyl ammonium ions. Since the majority of the cations are located inside the galleries and the alkyl ammonium ions are bulkier than the cations, the exchange increases the interlayer spacing and makes it easier for intercalation of polymer molecules between the layers. When modified smectite clay is mixed with a polymer, three different types of dispersion are possible. They are shown schematically in Figure 8.2. The type of dispersion depends on the polymer, layered silicate, organic cation, and the method of preparation of the nanocomposite. 1. Intercalated dispersion, in which one or more polymer molecules are intercalated between the silicate layers. The resulting material has a well-ordered multilayered morphology of alternating polymer and silicate layers. The spacing between the silicate layers is between 2 and 3 nm. O Basal spacing Inter layer Silicate layer ~1 nm Si Si O, OH O, OH Al, Mg, Fe O Na+ :O :OH . :Si (AI) :Al, Fe, Mg FIGURE 8.1 Crystal structure of smectite clay. (From Kato, M. and Usuki, A., Polymer–Clay Nanocomposites, T.J. Pinnavai and Beall, eds., John Wiley & Sons, Chichester, U.K., 2000. With permission.) 2007 by Taylor & Francis Group, LLC.
Layered silicate Polymer (a) (b) (c) FIGURE 8.2 Three possible dispersions of smectite clay in polymer matrix.(a)phase- separated (microcomposite):(b)intercalated (nanocomposite):and (c)exfoliated (nanocomposite).(From Alexandre,M.and Dubois,P.,Mater.Sci.Eng.,28,1,2000.With permission.) 2.Exfoliated dispersion,in which the silicate layers are completely dela- minated and are uniformly dispersed in the polymer matrix.The spacing between the silicate layers is between 8 and 10 nm.This is the most desirable dispersion for improved properties. 3.Phase-separated dispersion,in which the polymer is unable to intercal- ate the silicate sheets and the silicate particles are dispersed as phase- separated domains,called tactoids. Following are the most common techniques used for dispersing layered silicates in polymers to make nanoclay-polymer composites. 1.Solution method:In this method,the layered silicate is first exfoliated into single layers using a solvent in which the polymer is soluble.When the polymer is added later,it is adsorbed into the exfoliated sheets,and when the solvent is evaporated,a multilayered structure of exfoliated sheets and polymer molecules sandwiched between them is created. The solution method has been widely used with water-soluble polymers, such as polyvinyl alcohol(PVA)and polyethylene oxide. 2.In situ polymerization method:In this method,the layered silicate is swollen within the liquid monomer,which is later polymerized either by heat or by radiation.Thus,in this method,the polymer molecules are formed in situ between the intercalated sheets. 2007 by Taylor&Francis Group.LLC
2. Exfoliated dispersion, in which the silicate layers are completely delaminated and are uniformly dispersed in the polymer matrix. The spacing between the silicate layers is between 8 and 10 nm. This is the most desirable dispersion for improved properties. 3. Phase-separated dispersion, in which the polymer is unable to intercalate the silicate sheets and the silicate particles are dispersed as phaseseparated domains, called tactoids. Following are the most common techniques used for dispersing layered silicates in polymers to make nanoclay–polymer composites. 1. Solution method: In this method, the layered silicate is first exfoliated into single layers using a solvent in which the polymer is soluble. When the polymer is added later, it is adsorbed into the exfoliated sheets, and when the solvent is evaporated, a multilayered structure of exfoliated sheets and polymer molecules sandwiched between them is created. The solution method has been widely used with water-soluble polymers, such as polyvinyl alcohol (PVA) and polyethylene oxide. 2. In situ polymerization method: In this method, the layered silicate is swollen within the liquid monomer, which is later polymerized either by heat or by radiation. Thus, in this method, the polymer molecules are formed in situ between the intercalated sheets. Layered silicate Polymer (a) (b) (c) FIGURE 8.2 Three possible dispersions of smectite clay in polymer matrix. (a) phaseseparated (microcomposite); (b) intercalated (nanocomposite); and (c) exfoliated (nanocomposite). (From Alexandre, M. and Dubois, P., Mater. Sci. Eng., 28, 1, 2000. With permission.) 2007 by Taylor & Francis Group, LLC
The in situ method is commonly used with thermoset polymers,such as epoxy.It has also been used with thermoplastics,such as polystyrene and polyamide-6 (PA-6),and elastomers,such as polyurethane and thermoplastic polyolefins(TPOs).The first important commercial appli- cation of nanoclay composite was based on polyamide-6,and as dis- closed by its developer,Toyota Motor Corp.,it was prepared by the in situ method [2].In this case,the montmorillonite clay was mixed with an o,@-amino acid in aqueous hydrochloric acid to attach carboxyl groups to the clay particles.The modified clay was then mixed with the caprolactam monomer at 100C,where it was swollen by the mono- mer.The carboxyl groups initiated the ring-opening polymerization reaction of caprolactam to form polyamide-6 molecules and ionically bonded them to the clay particles.The growth of the molecules caused the exfoliation of the clay particles. 3.Melt processing method:The layered silicate particles are mixed with the polymer in the liquid state.Depending on the processing condition and the compatibility between the polymer and the clay surface,the polymer molecules can enter into the interlayer space of the clay particles and can form either an intercalated or an exfoliated structure. The melt processing method has been used with a variety of thermoplastics, such as polypropylene and polyamide-6,using conventional melt processing techniques,such as extrusion and injection molding.The high melt viscosity of thermoplastics and the mechanical action of the rotating screw in an extruder or an injection-molding machine create high shear stresses which tend to delaminate the original clay stack into thinner stacks.Diffusion of polymer molecules between the layers in the stacks then tends to peel the layers away into intercalated or exfoliated form [3]. The ability of smectite clay to greatly improve mechanical properties of polymers was first demonstrated in the research conducted by Toyota Motor Corp.in 1987.The properties of the nanoclay-polyamide-6 composite prepared by the in situ polymerization method at Toyota Research are given in Table 8.1. With the addition of only 4.2 wt%of exfoliated montmorillonite nanoclay,the tensile strength increased by 55%and the tensile modulus increased by 91% compared with the base polymer,which in this case was a polyamide-6. The other significant increase was in the heat deflection temperature (HDT). Table 8.1 also shows the benefit of exfoliation as the properties with exfoliation are compared with those without exfoliation.The nonexfoliated clay-PA-6 composite was prepared by simply melt blending montmorillonite clay with PA-6 in a twin-screw extruder. Since the publication of the Toyota research results,the development of nanoclay-reinforced thermoplastics and thermosets has rapidly progressed. 2007 by Taylor Francis Group,LLC
The in situ method is commonly used with thermoset polymers, such as epoxy. It has also been used with thermoplastics, such as polystyrene and polyamide-6 (PA-6), and elastomers, such as polyurethane and thermoplastic polyolefins (TPOs). The first important commercial application of nanoclay composite was based on polyamide-6, and as disclosed by its developer, Toyota Motor Corp., it was prepared by the in situ method [2]. In this case, the montmorillonite clay was mixed with an a,v-amino acid in aqueous hydrochloric acid to attach carboxyl groups to the clay particles. The modified clay was then mixed with the caprolactam monomer at 1008C, where it was swollen by the monomer. The carboxyl groups initiated the ring-opening polymerization reaction of caprolactam to form polyamide-6 molecules and ionically bonded them to the clay particles. The growth of the molecules caused the exfoliation of the clay particles. 3. Melt processing method: The layered silicate particles are mixed with the polymer in the liquid state. Depending on the processing condition and the compatibility between the polymer and the clay surface, the polymer molecules can enter into the interlayer space of the clay particles and can form either an intercalated or an exfoliated structure. The melt processing method has been used with a variety of thermoplastics, such as polypropylene and polyamide-6, using conventional melt processing techniques, such as extrusion and injection molding. The high melt viscosity of thermoplastics and the mechanical action of the rotating screw in an extruder or an injection-molding machine create high shear stresses which tend to delaminate the original clay stack into thinner stacks. Diffusion of polymer molecules between the layers in the stacks then tends to peel the layers away into intercalated or exfoliated form [3]. The ability of smectite clay to greatly improve mechanical properties of polymers was first demonstrated in the research conducted by Toyota Motor Corp. in 1987. The properties of the nanoclay–polyamide-6 composite prepared by the in situ polymerization method at Toyota Research are given in Table 8.1. With the addition of only 4.2 wt% of exfoliated montmorillonite nanoclay, the tensile strength increased by 55% and the tensile modulus increased by 91% compared with the base polymer, which in this case was a polyamide-6. The other significant increase was in the heat deflection temperature (HDT). Table 8.1 also shows the benefit of exfoliation as the properties with exfoliation are compared with those without exfoliation. The nonexfoliated clay–PA-6 composite was prepared by simply melt blending montmorillonite clay with PA-6 in a twin-screw extruder. Since the publication of the Toyota research results, the development of nanoclay-reinforced thermoplastics and thermosets has rapidly progressed. 2007 by Taylor & Francis Group, LLC
TABLE 8.1 Properties of Nanoclay-Reinforced Polyamide-6 Tensile Tensile Charpy Impact Wt% Strength Modulus Strength HDT (C) of Clay (MPa) (GPa) k/m2) at 145 MPa Polyamide-6 0 69 1.1 2.3 65 (PA-6 PA-6 with exfoliated 4.2 107 2.1 2.8 145 nanoclay PA-6 with 5.0 61 1.0 2.2 89 nonexfoliated clay Source:Adapted from Kato,M.and Usuki,A.,in Polymer-Clay Nanocomposites,T.J.Pinnavai and G.W.Beall,eds.,John Wiley Sons,Chichester,UK,2000. The most attractive attribute of adding nanoclay to polymers has been the improvement of modulus that can be attained with only 1-5 wt%of nanoclay. There are many other advantages such as reduction in gas permeability and increase in thermal stability and fire retardancy [1,4].The key to achieving improved properties is the exfoliation.Uniform dispersion of nanoclay and interaction between nanoclay and the polymer matrix are also important factors,especially in controlling the tensile strength,elongation at break,and impact resistance. 8.2 CARBON NANOFIBERS Carbon nanofibers are produced either in vapor-grown form [5]or by electro- spinning [6].Vapor-grown carbon nanofibers(VGCNF)have so far received the most attention for commercial applications and are discussed in this section. They are typically 20-200 nm in diameter and 30-100 um in length.In com- parison,the conventional PAN or pitch-based carbon fibers are 5-10 um in diameter and are produced in continuous length.Carbon fibers are also made in vapor-grown form,but their diameter is in the range of 3-20 um. VGCNF are produced in vapor phase by decomposing carbon-containing gases,such as methane(CH4),ethane(C2H6),acetylene (C2H2),carbon mon- oxide(CO),benzene,or coal gas in presence of floating metal catalyst particles inside a high-temperature reactor.Ultrafine particles of the catalyst are either carried by the flowing gas into the reactor or produced directly in the reactor by the decomposition of a catalyst precursor.The most common catalyst is iron, which is produced by the decomposition of ferrocene,Fe(CO)s.A variety of other catalysts,containing nickel,cobalt,nickel-iron,and nickel-cobalt com- pounds,have also been used.Depending on the carbon-containing gas,the 2007 by Taylor&Francis Group.LLC
The most attractive attribute of adding nanoclay to polymers has been the improvement of modulus that can be attained with only 1–5 wt% of nanoclay. There are many other advantages such as reduction in gas permeability and increase in thermal stability and fire retardancy [1,4]. The key to achieving improved properties is the exfoliation. Uniform dispersion of nanoclay and interaction between nanoclay and the polymer matrix are also important factors, especially in controlling the tensile strength, elongation at break, and impact resistance. 8.2 CARBON NANOFIBERS Carbon nanofibers are produced either in vapor-grown form [5] or by electrospinning [6]. Vapor-grown carbon nanofibers (VGCNF) have so far received the most attention for commercial applications and are discussed in this section. They are typically 20–200 nm in diameter and 30–100 mm in length. In comparison, the conventional PAN or pitch-based carbon fibers are 5–10 mm in diameter and are produced in continuous length. Carbon fibers are also made in vapor-grown form, but their diameter is in the range of 3–20 mm. VGCNF are produced in vapor phase by decomposing carbon-containing gases, such as methane (CH4), ethane (C2H6), acetylene (C2H2), carbon monoxide (CO), benzene, or coal gas in presence of floating metal catalyst particles inside a high-temperature reactor. Ultrafine particles of the catalyst are either carried by the flowing gas into the reactor or produced directly in the reactor by the decomposition of a catalyst precursor. The most common catalyst is iron, which is produced by the decomposition of ferrocene, Fe(CO)5. A variety of other catalysts, containing nickel, cobalt, nickel–iron, and nickel–cobalt compounds, have also been used. Depending on the carbon-containing gas, the TABLE 8.1 Properties of Nanoclay-Reinforced Polyamide-6 Wt% of Clay Tensile Strength (MPa) Tensile Modulus (GPa) Charpy Impact Strength (kJ=m2 ) HDT (8C) at 145 MPa Polyamide-6 (PA-6) 0 69 1.1 2.3 65 PA-6 with exfoliated nanoclay 4.2 107 2.1 2.8 145 PA-6 with nonexfoliated clay 5.0 61 1.0 2.2 89 Source: Adapted from Kato, M. and Usuki, A., in Polymer–Clay Nanocomposites, T.J. Pinnavai and G.W. Beall, eds., John Wiley & Sons, Chichester, UK, 2000. 2007 by Taylor & Francis Group, LLC
decomposition temperature can range up to 1200C.The reaction is conducted in presence of other gases,such as hydrogen sulfide and ammonia,which act as growth promoters.Cylindrical carbon nanofibers grow on the catalyst particles and are collected at the bottom of the reactor.Impurities on their surface, such as tar and other aromatic hydrocarbons,are removed by a subsequent process called pyrolitic stripping,which involves heating them to about 1000C in a reducing atmosphere.Heat treatment at temperatures up to 3000C is used to graphitize their surface and achieve higher tensile strength and tensile modulus.However,the optimum heat treatment temperature for maximum mechanical properties is found to be close to 1500C [5]. The diameter of carbon nanofibers and the orientation of graphite layers in carbon nanofibers with respect to their axis depend on the carbon-containing gas,the catalyst type,and the processing conditions,such as gas flow rate and temperature [7,8].The catalyst particle size also influences the diameter. Several different morphologies of carbon nanofibers have been observed [8,9]:platelet,in which the graphite layers are stacked normal to the fiber axis; hollow tubular construction,in which the graphite layers are parallel to the fiber axis,and fishbone or herringbone (with or without a hollow core),in which graphite layers are at an angle between 10 and 45 with the fiber axis(Figure 8.3).Single-wall and double-wall morphologies have been observed in heat- treated carbon nanofibers [10].Some of the graphite layers in both single-wall and double-wall morphologies are folded,the diameter of the folds remaining close to I nm. Table 8.2 lists the properties of a commercial carbon nanofiber(Pyrograf III)(Figure 8.4)as reported by its manufacturer (Applied Sciences,Inc.).The tensile modulus value listed in Table 8.2 is 600 GPa;however it should be noted that owing to the variety of morphologies observed in carbon nanofibers,they exhibit a range of modulus values,from as low as 110 GPa to as high as 700 GPa.Studies on vapor-grown carbon fibers (VGCF)[11],which are an order of (a) (b) (c) (d) (e) (0 FIGURE 8.3 Different morphologies of carbon nanofibers.(a)Graphite layers stacked normal to the fiber axis;(b)Hollow tubular construction with graphite layers parallel to the fiber axis;(c)and (d)Fishbone or herringbone morphology with graphite layers at an angle with the fiber axis;(e)Fishbone morphology with end loops;and (f)Double- walled morphology. 2007 by Taylor Francis Group,LLC
decomposition temperature can range up to 12008C. The reaction is conducted in presence of other gases, such as hydrogen sulfide and ammonia, which act as growth promoters. Cylindrical carbon nanofibers grow on the catalyst particles and are collected at the bottom of the reactor. Impurities on their surface, such as tar and other aromatic hydrocarbons, are removed by a subsequent process called pyrolitic stripping, which involves heating them to about 10008C in a reducing atmosphere. Heat treatment at temperatures up to 30008C is used to graphitize their surface and achieve higher tensile strength and tensile modulus. However, the optimum heat treatment temperature for maximum mechanical properties is found to be close to 15008C [5]. The diameter of carbon nanofibers and the orientation of graphite layers in carbon nanofibers with respect to their axis depend on the carbon-containing gas, the catalyst type, and the processing conditions, such as gas flow rate and temperature [7,8]. The catalyst particle size also influences the diameter. Several different morphologies of carbon nanofibers have been observed [8,9]: platelet, in which the graphite layers are stacked normal to the fiber axis; hollow tubular construction, in which the graphite layers are parallel to the fiber axis, and fishbone or herringbone (with or without a hollow core), in which graphite layers are at an angle between 108 and 458 with the fiber axis (Figure 8.3). Single-wall and double-wall morphologies have been observed in heattreated carbon nanofibers [10]. Some of the graphite layers in both single-wall and double-wall morphologies are folded, the diameter of the folds remaining close to 1 nm. Table 8.2 lists the properties of a commercial carbon nanofiber (Pyrograf III) (Figure 8.4) as reported by its manufacturer (Applied Sciences, Inc.). The tensile modulus value listed in Table 8.2 is 600 GPa; however it should be noted that owing to the variety of morphologies observed in carbon nanofibers, they exhibit a range of modulus values, from as low as 110 GPa to as high as 700 GPa. Studies on vapor-grown carbon fibers (VGCF) [11], which are an order of (a) (b) (c) (d) (e) (f) FIGURE 8.3 Different morphologies of carbon nanofibers. (a) Graphite layers stacked normal to the fiber axis; (b) Hollow tubular construction with graphite layers parallel to the fiber axis; (c) and (d) Fishbone or herringbone morphology with graphite layers at an angle with the fiber axis; (e) Fishbone morphology with end loops; and (f) Doublewalled morphology. 2007 by Taylor & Francis Group, LLC
TABLE 8.2 Properties of Vapor-Grown Carbon Nanofibers Carbon Nanofibers" Properties Pyrotically Stripped Diameter(nm) 60-200 Density (g/cm) 1.8 Tensile Modulus(GPa） 600 Tensile Strength(GPa) 7 Coefficient of thermal expansion(10C) -1.0 Electrical resistivity (n cm) 55 a Pyrograf III,produced by Applied Sciences,Inc. magnitude larger in diameter than the VGCNF,have shown that tensile modulus decreases with increasing diameter,whereas tensile strength decreases with both increasing diameter and increasing length. Carbon nanofibers have been incorporated into several different thermo- plastic and thermoset polymers.The results of carbon nanofiber addition on the mechanical properties of the resulting composite have been mixed. 0.2um FIGURE 8.4 Photograph of carbon nanofibers.(Courtesy of Applied Sciences,Inc. With permission.) 2007 by Taylor Francis Group.LLC
magnitude larger in diameter than the VGCNF, have shown that tensile modulus decreases with increasing diameter, whereas tensile strength decreases with both increasing diameter and increasing length. Carbon nanofibers have been incorporated into several different thermoplastic and thermoset polymers. The results of carbon nanofiber addition on the mechanical properties of the resulting composite have been mixed. TABLE 8.2 Properties of Vapor-Grown Carbon Nanofibers Carbon Nanofibersa Properties Pyrotically Stripped Diameter (nm) 60–200 Density (g=cm3 ) 1.8 Tensile Modulus (GPa) 600 Tensile Strength (GPa) 7 Coefficient of thermal expansion (106 =8C) 1.0 Electrical resistivity (mV cm) 55 a Pyrograf III, produced by Applied Sciences, Inc. FIGURE 8.4 Photograph of carbon nanofibers. (Courtesy of Applied Sciences, Inc. With permission.) 2007 by Taylor & Francis Group, LLC.
In general,incorporation of carbon nanofibers in thermoplastics has shown modest to high improvement in modulus and strength,whereas their incorpor- ation in thermosets has shown relatively smaller improvements.An example of each is given as follows. Finegan et al.[12]conducted a study on the tensile properties of carbon nanofiber-reinforced polypropylene.The nanofibers were produced with a variety of processing conditions (different carbon-containing gases,different gas flow rates,with and without graphitization).A variety of surface treatments were applied on the nanofibers.The composite tensile specimens with 15 vol%nanofi- bers were prepared using melt processing (injection molding).In all cases,they observed an increase in both tensile modulus and strength compared with poly- propylene itself.However,the amount of increase was influenced by the nanofiber production condition and the surface treatment.When the surface treatment involved surface oxidation in a CO2 atmosphere at 850C,the tensile modulus and strength of the composite were 4 GPa and 70 MPa,respectively,both of which were greater than three times the corresponding values for polypropylene. Patton et al.[13]reported the effect of carbon nanofiber addition to epoxy. The epoxy resin was diluted using acetone as the solvent.The diluted epoxy was then infused into the carbon nanofiber mat.After removing the solvent,the epoxy-soaked mat was cured at 120C and then postcured.Various nanofiber surface treatments were tried.The highest improvement in flexural modulus and strength was observed with carbon nanofibers that were heated in air at 400C for 30 min.With~18 vol%of carbon nanofibers,the flexural modulus of the composite was nearly twice that of epoxy,but the increase in flexural strength was only about 36%. 8.3 CARBON NANOTUBES Carbon nanotubes were discovered in 1991,and within a short period of time, have attracted a great deal of research and commercial interest due to their potential applications in a variety of fields,such as structural composites, energy storage devices,electronic systems,biosensors,and drug delivery sys- tems [14].Their unique structure gives them exceptional mechanical,thermal, electrical,and optical properties.Their elastic modulus is reported to be >1 TPa,which is close to that of diamond and 3-4 times higher than that of carbon fibers.They are thermally stable up to 2800C in vacuum;their thermal con- ductivity is about twice that of diamond and their electric conductivity is 1000 times higher than that of copper. 8.3.1 STRUCTURE Carbon nanotubes are produced in two forms,single-walled nanotubes (SWNT)and multiwalled nanotubes (MWNT).SWNT is a seamless hollow cylinder and can be visualized as formed by rolling a sheet of graphite layer, 2007 by Taylor Francis Group,LLC
In general, incorporation of carbon nanofibers in thermoplastics has shown modest to high improvement in modulus and strength, whereas their incorporation in thermosets has shown relatively smaller improvements. An example of each is given as follows. Finegan et al. [12] conducted a study on the tensile properties of carbon nanofiber-reinforced polypropylene. The nanofibers were produced with a variety of processing conditions (different carbon-containing gases, different gas flow rates, with and without graphitization). A variety of surface treatments were applied on the nanofibers. The composite tensile specimens with 15 vol% nanofibers were prepared using melt processing (injection molding). In all cases, they observed an increase in both tensile modulus and strength compared with polypropylene itself. However, the amount of increase was influenced by the nanofiber production condition and the surface treatment. When the surface treatment involved surface oxidation in a CO2 atmosphere at 8508C, the tensile modulus and strength of the composite were 4 GPa and 70 MPa, respectively, both of which were greater than three times the corresponding values for polypropylene. Patton et al. [13] reported the effect of carbon nanofiber addition to epoxy. The epoxy resin was diluted using acetone as the solvent. The diluted epoxy was then infused into the carbon nanofiber mat. After removing the solvent, the epoxy-soaked mat was cured at 1208C and then postcured. Various nanofiber surface treatments were tried. The highest improvement in flexural modulus and strength was observed with carbon nanofibers that were heated in air at 4008C for 30 min. With ~18 vol% of carbon nanofibers, the flexural modulus of the composite was nearly twice that of epoxy, but the increase in flexural strength was only about 36%. 8.3 CARBON NANOTUBES Carbon nanotubes were discovered in 1991, and within a short period of time, have attracted a great deal of research and commercial interest due to their potential applications in a variety of fields, such as structural composites, energy storage devices, electronic systems, biosensors, and drug delivery systems [14]. Their unique structure gives them exceptional mechanical, thermal, electrical, and optical properties. Their elastic modulus is reported to be >1 TPa, which is close to that of diamond and 3–4 times higher than that of carbon fibers. They are thermally stable up to 28008C in vacuum; their thermal conductivity is about twice that of diamond and their electric conductivity is 1000 times higher than that of copper. 8.3.1 STRUCTURE Carbon nanotubes are produced in two forms, single-walled nanotubes (SWNT) and multiwalled nanotubes (MWNT). SWNT is a seamless hollow cylinder and can be visualized as formed by rolling a sheet of graphite layer, 2007 by Taylor & Francis Group, LLC
whereas MWNT consists of a number of concentric SWNT.Both SWNT and MWNT are closed at the ends by dome-shaped caps.The concentric SWNTs inside an MWNT are also end-capped.The diameter of an SWNT is typically between 1 and 1.4 nm and its length is between 50 and 100 um.The specific surface area of an SWNT is 1315 m2/g,and is independent of its diameter [15].The outer diameter of an MWNT is between 1.4 and 100 nm. The separation between the concentric SWNT cylinders in an MWNT is about 3.45 A,which is slightly greater than the distance between the graphite layers in a graphite crystal.The specific surface area of an MWNT depends on the number of walls.For example,the specific surface area of a double-walled nanotube is between 700 and 800 m2/g and that of a 10-walled nanotube is about 200 m2/g [15]. The structure of an SWNT depends on how the graphite sheets is rolled up and is characterized by its chirality or helicity,which is defined by the chiral angle and the chiral vector(Figure 8.5).The chiral vector is written as Ch na1 ma2, (8.1) Zigzag direction (0,0) (3,0) (4,0) (6,0) (8,0) (9,0) (1,1) Ch (8,1) (2,2) (4,2) (5,2) (7,2) (8,2) (3,3) (6,3) a2 (4,4) (74) (5,5) Armchair direction FIGURE 8.5 Chiral vector and chiral angle.(From Govindaraj,A.and Rao,C.N.R., The Chemistry of Nanomaterials,Vol.1,C.N.R.Rao,A.Muller,and A.K.Cheetham, eds.,Wiley-VCH,KGaA,Germany,2004.With permission.) 2007 by Taylor&Francis Group.LLC
whereas MWNT consists of a number of concentric SWNT. Both SWNT and MWNT are closed at the ends by dome-shaped caps. The concentric SWNTs inside an MWNT are also end-capped. The diameter of an SWNT is typically between 1 and 1.4 nm and its length is between 50 and 100 mm. The specific surface area of an SWNT is 1315 m2 =g, and is independent of its diameter [15]. The outer diameter of an MWNT is between 1.4 and 100 nm. The separation between the concentric SWNT cylinders in an MWNT is about 3.45 A8, which is slightly greater than the distance between the graphite layers in a graphite crystal. The specific surface area of an MWNT depends on the number of walls. For example, the specific surface area of a double-walled nanotube is between 700 and 800 m2 =g and that of a 10-walled nanotube is about 200 m2 =g [15]. The structure of an SWNT depends on how the graphite sheets is rolled up and is characterized by its chirality or helicity, which is defined by the chiral angle and the chiral vector (Figure 8.5). The chiral vector is written as Ch ¼ na1 þ ma2, (8:1) Zigzag direction (0,0) (1,1) Ch (3,0) (2,2) a1 a2 (3,3) (4,4) (5,5) Armchair direction (7,4) (6,3) (4,2) (5,2) (7,2) (8,2) (4,0) θ (6,0) (8,0) (8,1) (9,0) FIGURE 8.5 Chiral vector and chiral angle. (From Govindaraj, A. and Rao, C.N.R., The Chemistry of Nanomaterials, Vol. 1, C.N.R. Rao, A. Mu¨ller, and A.K. Cheetham, eds., Wiley-VCH, KGaA, Germany, 2004. With permission.) 2007 by Taylor & Francis Group, LLC
where a and az are unit vectors in a two-dimensional graphite sheet and(n,m) are called chirality numbers.Both n and m are integers and they define the way the graphite sheet is rolled to form a nanotube. Nanotubes with n0,m 0 are called the zigzag tubes(Figure 8.6a)and nanotubes with n=m0 are called armchair tubes(Figure 8.6b).In zigzag tubes,two opposite C-C bonds of each hexagon are parallel to the tube's axis, whereas in the armchair tubes,the C-C bonds of each hexagon are perpen- dicular to the tube's axis.If the C-C bonds are at an angle with the tube's axis, the tube is called a chiral tube(Figure 8.6c).The chiral angle 0 is defined as the angle between the zigzag direction and the chiral vector,and is given by [3/2m1 =tan (8.2) 2n+m a (b) (c) FIGURE 8.6 (a)Zigzag,(b)armchair,and (c)chiral nanotubes.(From Rakov,E.G., Nanomaterials Handbook,Y.Gogotsi,ed.,CRC Press,Boca Raton,USA,2006.With permission.) 2007 by Taylor Francis Group,LLC
where a1 and a2 are unit vectors in a two-dimensional graphite sheet and (n, m) are called chirality numbers. Both n and m are integers and they define the way the graphite sheet is rolled to form a nanotube. Nanotubes with n 6¼ 0, m ¼ 0 are called the zigzag tubes (Figure 8.6a) and nanotubes with n ¼ m 6¼ 0 are called armchair tubes (Figure 8.6b). In zigzag tubes, two opposite C–C bonds of each hexagon are parallel to the tube’s axis, whereas in the armchair tubes, the C–C bonds of each hexagon are perpendicular to the tube’s axis. If the C–C bonds are at an angle with the tube’s axis, the tube is called a chiral tube (Figure 8.6c). The chiral angle u is defined as the angle between the zigzag direction and the chiral vector, and is given by u ¼ tan1 31=2m 2n þ m (8:2) (a) (b) (c) FIGURE 8.6 (a) Zigzag, (b) armchair, and (c) chiral nanotubes. (From Rakov, E.G., Nanomaterials Handbook, Y. Gogotsi, ed., CRC Press, Boca Raton, USA, 2006. With permission.) 2007 by Taylor & Francis Group, LLC.
