**2. Nanofillers and compatibilization of nanocomposites**

### **2.1. Carbon-based nanofillers**

### *2.1.1. Carbon nanotubes*

the last decades, it has been observed that the addition of low contents of these nanofillers into the polymer can lead to improvements in their mechanical, thermal, barrier and flammability properties, without affecting their processability [1, 2]. The ideal design of a nanocomposite involves individual nanoparticles homogeneously dispersed in a matrix polymer. The dispersion state of nanoparticles is the key challenge in order to obtain the full potential of properties enhancement [1, 2]. This uniform dispersion of nanofillers can lead to a large interfacial area between the constituents of the nanocomposites [2]. The reinforcing effect of filler is attributed to several factors, such as properties of the polymer matrix, nature and type of nanofiller, concentration of polymer and filler, particle aspect ratio, particle size, particle orientation and particle distribution [3]. Various types of nanoparticles, such as clays [3, 4], carbon nanotubes [5], graphene [6, 7], nanocellulose [8]

104 Nanocomposites - Recent Evolutions

and halloysite [9], have been used to obtain nanocomposites with different polymers.

between nanocomposites and the conventional composites [11].

nanocomposite and (c) exfoliated nanocomposite [10].

The evaluation of the nanofiller dispersion in the polymer matrix is very important, since the mechanical and thermal properties are strongly related to the morphologies obtained. Depending on the degree of separation of the nanoparticles, three types of nanocomposite morphologies are possible (**Figure 1**) [10]: conventional composites (or microcomposites), intercalated nanocomposites and exfoliated nanocomposites. When the polymer is unable to intercalate between the silicate layers, a composite of separate phases is obtained (**Figure 1(a)**), whose properties are in the same range as those observed in traditional composites [1]. An intercalated structure, in which a single (and sometimes more than one) extended polymer chain is intercalated between the layers of the silicate, results in a well-ordered multilayer morphology with intercalated layers of polymer and clay (**Figure 1(b)**). When the silicate layers are completely and uniformly dispersed in a continuous polymer matrix, an exfoliated structure is obtained (**Figure 1(c)**) [10]. Exfoliated nanocomposites have maximum reinforcement due to the large surface area of contact between the matrix and nanoparticles. This would be one of the main differences

**Figure 1.** Possible structures of polymer nanocomposites using layered nanoclays: (a) microcomposite, (b) intercalated

Carbon nanotubes (CNTs) are ultrathin carbon fibers with nanometer-size diameter and micrometer-size length. CNTs were discovered in 1991 by Sumio Iijima, and since then, these nanomaterials have been used in various applications [12]. The structure of CNT consists of enrolled graphitic sheet, which is a planar-hexagonal arrangement of carbon atoms distributed in a honeycomb lattice [12, 13]. The nanotubes can be classified into either multi-walled (MWCNT) or single-walled (SWCNT) depending on its preparation method [12, 14], as can be seen in **Figure 2**. MWCNTs consist of two or more concentric cylindrical shells of graphene sheets coaxially arranged around a central hollow core. On the other hand, SWCNT consists of a single graphene layer rolled up into a seamless cylinder [15, 16]. In addition to the exceptional electrical and conductive properties, the CNTs also present excellent mechanical properties, with an elastic modulus in the order of 1 TPa and maximum tensile strength can reach 300 GPa (for CNTs free of defects) [13, 17]. These properties are related to a strong covalent bond between carbons and its arrangement in cylindrical nanostructures [5].

Due to their excellent properties, considerable interest has been drawn on polymer nanocomposites with CNTs [18]. The incorporation of carbon nanotubes in polymer matrices has been explored as a strategy to obtain composite materials with electrical properties and with superior mechanical and thermal properties. However, these fillers are materials of difficult dispersion in polymeric matrices. Problems arising from agglomeration during processing are commonly observed due to the low polymer/CNT interaction (see **Figure 3**) [19]. The processing conditions may influence the dispersion state of these nanofillers in the resulting material. In addition, carbon nanotubes can be chemically modified to improve the interfacial interaction [19, 20].

**Figure 2.** Representation of SWCNT and MWCNT [14].

interest in many research groups around the world and has resulted in an abrupt increase in publications on the subject. This material consists of one atomic thick sheet of covalently sp<sup>2</sup>

The primitive cell of graphene is composed of two non-equivalent atoms, A and B, and these two sub-lattices are translated from each other by a carbon-carbon distance ac-c = 1.44 Å [25]. Graphene can be produced from graphite by different methods, such as thermal expansion of chemically intercalated graphite, micromechanical exfoliation of graphite, chemical vapor deposition and chemical reduction method of graphene oxide [27]. Graphene has Young´s modulus of 1 TPa, fracture strength of 125 GPa, thermal conductivity of 5000 W/m.fK and electrical conductivity up to 6000 S/cm [28]. These properties in addition to extremely high

potential for improving mechanical, electrical, thermal and gas barrier properties of polymer

The successful use of graphene depends on the exfoliation of bulk graphite into individual sheets. Several chemical-mechanical routes have been developed to produce individual exfoliated graphene sheets, for example, mechanical exfoliation, chemical exfoliation and chemical vapor deposition [30, 31]. Each method has its own advantages and drawbacks related to the purity and the presence of defects (oxygen and functional groups on the surface). The most common route to produce graphene involves the production of graphite oxide (GO) by oxidation chemistry followed by a reduction and mechanical exfoliation [6]. This is the basis of Hummers and Offeman's process [32]. GO is nonconductive, hydrophilic and can readily swell and disperse in water. Recently, several new methods of graphene functionalization were reported. Functionalized graphene sheets (FGS) demonstrate improved dispersibility in

Nanoclays belong to a class of materials generally made of layered silicates or clay minerals with traces of metal oxides and organic matter. Clay minerals are hydrous aluminum phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and others cations [33]. In the last decades, several published works have shown that the dispersion of exfoliated clays in polymer leads to a remarkable increase in stiffness, fire retardancy and barrier properties, beginning at a very low nanoparticle volume fraction [3]. Clays have been found to be effective reinforcing fillers for polymer due to lamellar structure

/g) and gas impermeability indicate graphene's great

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/g) [2]. Smectite clays are layered silicates, and they are a

bonded carbon atoms in a hexagonal arrangement [24, 25], as illustrated in **Figure 4**.

surface area (theoretical limit: 2630 m<sup>2</sup>

organic solvents and polymers [28, 30].

and high specific surface area (750 m<sup>2</sup>

**Figure 4.** Honeycomb lattice of graphene [26].

nanocomposites [28, 29].

**2.2. Layered nanoclays**


107

**Figure 3.** Micrographs obtained by transmission electron microscopy of nanocomposites with polyetherimide (PEI) and MWCNT [19].

The presence of functional groups on the side walls of CNTs increases the chemical reactivity between the filler and matrix, inducing to a better interface and consequently a better load transfer from the matrix to the reinforcement [5]. One route that has been considered for modification of carbon nanotubes is the surface treatment of these materials with a mixture of nitric and sulfuric acids (HNO<sup>3</sup> /H<sup>2</sup> SO<sup>4</sup> ), which results in the formation of carboxylic acid groups (-COOH) on the surface [5]. This treatment was developed by Goyanes et al [21] and can alter the nature of the CNTs surface making them more compatible with the polymer matrix. Besides the acid treatment, secondary particles, such as clay, have been used to improve the dispersion of carbon nanotubes and increase the electrical properties of composites containing these fillers [20]. In addition to obtaining nanocomposites using a single polymer matrix, the use of polymer/polymer blends as matrix has attracted the attention of researchers. It has been observed that polymer/polymer blends with carbon nanotubes have better electrical and thermal properties when compared to unfilled blends [18].

The electrical conductivity after the incorporation of CNTs in polymers occurs due to the formation of a three-dimensional network of CNTs inside the polymer matrix, which strength dependents on the distribution and dispersion of the CNTs. When the concentration of the nanofiller reaches a critical value, known as the limit of electrical percolation, the electrical conductivity of the nanocomposite increases unexpectedly. After this abrupt increase in electrical conductivity, it will show modest increases as the conductive additive increases inside the polymer matrix [18, 22]. In nanocomposites based on polymer blends, the amount of CNTs required to achieve electrical percolation may be even lower than in nanocomposites with a single polymer matrix, provided that a selective location of the CNTs occurs in the matrix phase or at the interface of the blend [18, 23]. In addition, it is especially desired the formation of blends with co-continuous morphology, where a double phenomenon of electric percolation can be found. Thus, the limit of electric percolation in polymer blends is strongly influenced by the concentration of nanotubes and also by the final morphology of the blends, which in turn is a function of the composition of the blend, the compatibilizer and the processing conditions [18].

#### *2.1.2. Graphene*

Graphene was discovered in 2004 by Andre. K. Geim and Konstantin S. Novoselov and has revolutionized the scientific frontiers in nanoscience and condensed matter physics due to its exceptional electrical, physical and chemical properties. Graphene has sparked enormous interest in many research groups around the world and has resulted in an abrupt increase in publications on the subject. This material consists of one atomic thick sheet of covalently sp<sup>2</sup> bonded carbon atoms in a hexagonal arrangement [24, 25], as illustrated in **Figure 4**.

The primitive cell of graphene is composed of two non-equivalent atoms, A and B, and these two sub-lattices are translated from each other by a carbon-carbon distance ac-c = 1.44 Å [25]. Graphene can be produced from graphite by different methods, such as thermal expansion of chemically intercalated graphite, micromechanical exfoliation of graphite, chemical vapor deposition and chemical reduction method of graphene oxide [27]. Graphene has Young´s modulus of 1 TPa, fracture strength of 125 GPa, thermal conductivity of 5000 W/m.fK and electrical conductivity up to 6000 S/cm [28]. These properties in addition to extremely high surface area (theoretical limit: 2630 m<sup>2</sup> /g) and gas impermeability indicate graphene's great potential for improving mechanical, electrical, thermal and gas barrier properties of polymer nanocomposites [28, 29].

The successful use of graphene depends on the exfoliation of bulk graphite into individual sheets. Several chemical-mechanical routes have been developed to produce individual exfoliated graphene sheets, for example, mechanical exfoliation, chemical exfoliation and chemical vapor deposition [30, 31]. Each method has its own advantages and drawbacks related to the purity and the presence of defects (oxygen and functional groups on the surface). The most common route to produce graphene involves the production of graphite oxide (GO) by oxidation chemistry followed by a reduction and mechanical exfoliation [6]. This is the basis of Hummers and Offeman's process [32]. GO is nonconductive, hydrophilic and can readily swell and disperse in water. Recently, several new methods of graphene functionalization were reported. Functionalized graphene sheets (FGS) demonstrate improved dispersibility in organic solvents and polymers [28, 30].

#### **2.2. Layered nanoclays**

The presence of functional groups on the side walls of CNTs increases the chemical reactivity between the filler and matrix, inducing to a better interface and consequently a better load transfer from the matrix to the reinforcement [5]. One route that has been considered for modification of carbon nanotubes is the surface treatment of these materials with a mixture of nitric

**Figure 3.** Micrographs obtained by transmission electron microscopy of nanocomposites with polyetherimide (PEI) and

(-COOH) on the surface [5]. This treatment was developed by Goyanes et al [21] and can alter the nature of the CNTs surface making them more compatible with the polymer matrix. Besides the acid treatment, secondary particles, such as clay, have been used to improve the dispersion of carbon nanotubes and increase the electrical properties of composites containing these fillers [20]. In addition to obtaining nanocomposites using a single polymer matrix, the use of polymer/polymer blends as matrix has attracted the attention of researchers. It has been observed that polymer/polymer blends with carbon nanotubes have better electrical and

The electrical conductivity after the incorporation of CNTs in polymers occurs due to the formation of a three-dimensional network of CNTs inside the polymer matrix, which strength dependents on the distribution and dispersion of the CNTs. When the concentration of the nanofiller reaches a critical value, known as the limit of electrical percolation, the electrical conductivity of the nanocomposite increases unexpectedly. After this abrupt increase in electrical conductivity, it will show modest increases as the conductive additive increases inside the polymer matrix [18, 22]. In nanocomposites based on polymer blends, the amount of CNTs required to achieve electrical percolation may be even lower than in nanocomposites with a single polymer matrix, provided that a selective location of the CNTs occurs in the matrix phase or at the interface of the blend [18, 23]. In addition, it is especially desired the formation of blends with co-continuous morphology, where a double phenomenon of electric percolation can be found. Thus, the limit of electric percolation in polymer blends is strongly influenced by the concentration of nanotubes and also by the final morphology of the blends, which in turn is a function of the

composition of the blend, the compatibilizer and the processing conditions [18].

Graphene was discovered in 2004 by Andre. K. Geim and Konstantin S. Novoselov and has revolutionized the scientific frontiers in nanoscience and condensed matter physics due to its exceptional electrical, physical and chemical properties. Graphene has sparked enormous

), which results in the formation of carboxylic acid groups

and sulfuric acids (HNO<sup>3</sup>

106 Nanocomposites - Recent Evolutions

MWCNT [19].

*2.1.2. Graphene*

/H<sup>2</sup> SO<sup>4</sup>

thermal properties when compared to unfilled blends [18].

Nanoclays belong to a class of materials generally made of layered silicates or clay minerals with traces of metal oxides and organic matter. Clay minerals are hydrous aluminum phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and others cations [33]. In the last decades, several published works have shown that the dispersion of exfoliated clays in polymer leads to a remarkable increase in stiffness, fire retardancy and barrier properties, beginning at a very low nanoparticle volume fraction [3]. Clays have been found to be effective reinforcing fillers for polymer due to lamellar structure and high specific surface area (750 m<sup>2</sup> /g) [2]. Smectite clays are layered silicates, and they are a

**Figure 4.** Honeycomb lattice of graphene [26].

required choice for the preparation of polymer nanocomposites due to their low cost, swelling properties and high cation exchange capacities. Some examples of these clays are montmorillonite, saponite, laponite, hectorite, sepiolite and vermiculite [16, 33]. Among these clays, montmorillonite is the most widely used clay in polymer nanocomposites, because of its large availability, well-known intercalation/exfoliation chemistry, high surface area and reactivity [33]. Montmorillonite (MMT) is composed of two tetrahedral silica sheets with an alumina octahedral sheet in the middle (2:1 layered structure), and the hydrated exchangeable cations occupy the spaces between lattices, as shown in **Figure 5**.

**2.3. Porous and hollow nanoparticles**

molecules, which is weakly held. It consists of sheets of SiO<sup>4</sup>

Halloysite nanotube (HNT) is an aluminosilicate with hollow micro- and nanotubular structure [38]. HNT is structurally much to kaolinite [39] and may intercalate a monolayer of water

of HNT is similar to nanoclays, while nanotubular geometry is similar to CNTs. Uniqueness of HNT exists in its tubular form with length up to few microns and diameter in nm range. It offers innovative possibilities for nanocomposite preparation [41]. Optimizing polymer properties by filler addition of low content has been the focus of industrial and academic research. HNT as nanofiller in polymeric materials has been found to significantly increase the mechanical, thermal, non-flammability and other physical properties of the nanocomposite [42]. Due to variety of characteristics, such as nanoscale size, shape, surface area and high

Zeolites are widely used as catalysts or catalyst supports in a variety of applications in refining and (petro)chemical industries [44]. Particularly, the faujasite-type framework is an aluminosilicate with cavities of 1.3 nm of diameter interconnected by pores of 0.74 nm, as illustrated in **Figure 7**. The cubic unit cell of these aluminosilicates contains around 192

of the particles has received special interest [46]. Micrometer-sized zeolites have a negligible external surface area compared with the large surface area in their internal microporous [47]. Zeolite nanoparticles lead to substantial changes in the material properties, increasing the intercrystalline space, the external and internal surface area and volume and pore mouths exposed. Therefore, the application of nanozeolites in some catalytic reactions can reduce diffusion path lengths and increase catalytic activity and selectivity, as well as improve reaction

tetrahedrons [45]. The development of zeolite synthesis methods to reduce the size

length to diameter ratio, HNT has been discovered for numerous applications.

octahedral [40], as shown in **Figure 6**. Consequently, the chemical composition

tetrahedra with sheets of edge

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*2.3.1. Halloysite*

sharing AlO<sup>6</sup>

*2.3.2. Zeolite*

(Si,Al)O<sup>4</sup>

medium stability [48].

**Figure 6.** Structure of halloysite nanotube [43].

The sheets have dimensions of 1 nm thickness and are 100–500 nm in diameter, resulting in platelets with high aspect ratio [35]. Stacking of the clay layers leads to a regular van der Waals gap between the layers called interlayer or gallery. Isomorphic substitution within the layers generates negative charges that are counterbalanced by alkali and alkaline earth cations (Li<sup>+</sup> , Na<sup>+</sup> or Ca2+) situated inside the galleries. The extent of the negative charge of the clay is characterized by the cation exchange capacity (CEC) [36]. Natural montmorillonite is hydrophilic and most polymers tend to be hydrophobic, so the clay surface must be modified to yield organophilic clay in these cases. This is often done by exchanging the cations in the gallery with alkylammonium or alkylphosphonium salts (for example, dioctadecyl dimethyl ammonium bromide), typically with chain lengths longer than eight carbon atoms (C<sup>8</sup> ). The clay that was previously hydrophilic becomes organophilic after modification [35]. The replacement of inorganic exchange cations with organic ions on the gallery surfaces of smectite clays is useful to expand the clay galleries. This facilitates the penetration into the gallery space (intercalation) by either the polymer chains. Other type of modification that has been used in nanoclays is the process known as silanization. The modification of the clay with organosilanes promoted covalent bonds between polymer and clay by reactive extrusion favoring strong interactions between clay and matrix. Examples of those silanes that have been used to modify nanofiller are 3-aminopropyltriethoxysilane (APTES) and vinyltrimethoxysilane (VTMS) [37].

**Figure 5.** Structure of 2:1 layered silicates [34].

#### **2.3. Porous and hollow nanoparticles**

#### *2.3.1. Halloysite*

required choice for the preparation of polymer nanocomposites due to their low cost, swelling properties and high cation exchange capacities. Some examples of these clays are montmorillonite, saponite, laponite, hectorite, sepiolite and vermiculite [16, 33]. Among these clays, montmorillonite is the most widely used clay in polymer nanocomposites, because of its large availability, well-known intercalation/exfoliation chemistry, high surface area and reactivity [33]. Montmorillonite (MMT) is composed of two tetrahedral silica sheets with an alumina octahedral sheet in the middle (2:1 layered structure), and the hydrated exchangeable cations

The sheets have dimensions of 1 nm thickness and are 100–500 nm in diameter, resulting in platelets with high aspect ratio [35]. Stacking of the clay layers leads to a regular van der Waals gap between the layers called interlayer or gallery. Isomorphic substitution within the layers generates negative charges that are counterbalanced by alkali and alkaline earth cations (Li<sup>+</sup>

 or Ca2+) situated inside the galleries. The extent of the negative charge of the clay is characterized by the cation exchange capacity (CEC) [36]. Natural montmorillonite is hydrophilic and most polymers tend to be hydrophobic, so the clay surface must be modified to yield organophilic clay in these cases. This is often done by exchanging the cations in the gallery with alkylammonium or alkylphosphonium salts (for example, dioctadecyl dimethyl ammo-

was previously hydrophilic becomes organophilic after modification [35]. The replacement of inorganic exchange cations with organic ions on the gallery surfaces of smectite clays is useful to expand the clay galleries. This facilitates the penetration into the gallery space (intercalation) by either the polymer chains. Other type of modification that has been used in nanoclays is the process known as silanization. The modification of the clay with organosilanes promoted covalent bonds between polymer and clay by reactive extrusion favoring strong interactions between clay and matrix. Examples of those silanes that have been used to modify nanofiller are 3-aminopropyltriethoxysilane (APTES) and vinyltrimethoxysilane (VTMS) [37].

nium bromide), typically with chain lengths longer than eight carbon atoms (C<sup>8</sup>

occupy the spaces between lattices, as shown in **Figure 5**.

**Figure 5.** Structure of 2:1 layered silicates [34].

Na<sup>+</sup>

108 Nanocomposites - Recent Evolutions

Halloysite nanotube (HNT) is an aluminosilicate with hollow micro- and nanotubular structure [38]. HNT is structurally much to kaolinite [39] and may intercalate a monolayer of water molecules, which is weakly held. It consists of sheets of SiO<sup>4</sup> tetrahedra with sheets of edge sharing AlO<sup>6</sup> octahedral [40], as shown in **Figure 6**. Consequently, the chemical composition of HNT is similar to nanoclays, while nanotubular geometry is similar to CNTs. Uniqueness of HNT exists in its tubular form with length up to few microns and diameter in nm range. It offers innovative possibilities for nanocomposite preparation [41]. Optimizing polymer properties by filler addition of low content has been the focus of industrial and academic research. HNT as nanofiller in polymeric materials has been found to significantly increase the mechanical, thermal, non-flammability and other physical properties of the nanocomposite [42]. Due to variety of characteristics, such as nanoscale size, shape, surface area and high length to diameter ratio, HNT has been discovered for numerous applications.

#### *2.3.2. Zeolite*

,

). The clay that

Zeolites are widely used as catalysts or catalyst supports in a variety of applications in refining and (petro)chemical industries [44]. Particularly, the faujasite-type framework is an aluminosilicate with cavities of 1.3 nm of diameter interconnected by pores of 0.74 nm, as illustrated in **Figure 7**. The cubic unit cell of these aluminosilicates contains around 192 (Si,Al)O<sup>4</sup> tetrahedrons [45]. The development of zeolite synthesis methods to reduce the size of the particles has received special interest [46]. Micrometer-sized zeolites have a negligible external surface area compared with the large surface area in their internal microporous [47]. Zeolite nanoparticles lead to substantial changes in the material properties, increasing the intercrystalline space, the external and internal surface area and volume and pore mouths exposed. Therefore, the application of nanozeolites in some catalytic reactions can reduce diffusion path lengths and increase catalytic activity and selectivity, as well as improve reaction medium stability [48].

**Figure 6.** Structure of halloysite nanotube [43].

the constitutive microfibrils. Different shearing types of equipment, such as a homogenizer, microfluidizer or ultra-fine friction grinder, are generally used. This material is usually called nanofibrillated cellulose (NFC) or cellulose nanofibrils (CNF) and is obtained as an aqueous suspension [49]. The width is 3–100 nm depending on the source of cellulose, defibrillation process and pretreatment, and the length is usually higher than 1 μm [53].

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The chemically induced destructuration strategy consists of applying a controlled strong acid hydrolysis treatment to cellulosic fibers, allowing dissolution of amorphous domains and therefore longitudinal cutting of the microfibrils. The ensuing nanoparticles are called cellulose nanocrystals (CNCs) and are obtained as an aqueous suspension [51]. These nanoparticles have high aspect ratio rod-like nanocrystals (whiskers). Their geometrical dimensions depend on the origin of the cellulose and hydrolysis conditions. Sulfuric acid is used for the preparation of CNC, and this process induces the formation of negatively charged sulfate groups at the surface. The average length is of the order of a few hundred nanometers and the width is of the order of a few nanometers [53]. An important parameter for CNCs is the aspect

The possibility of using metal hydrides (MH) alloys in hydrogen technology has being attracting interest [54]. These types of material react with hydrogen reversibly, thus being successfully utilized in the solid state storage of the gas. However, MH alloys under repeated hydriding/ dehydriding cycling suffer from a pulverization phenomenon due to a large volume mismatch between the hydride and the metal compound [55]. As a consequence, repeated hydrogen loading/unloading cycles produce free metal powder particles in nanoscale size. Particle fragmentation results in a considerable increasing of the metal surface area with a consequent enhancement of some properties. Among these, the hydriding kinetics is expected to improve even if a parallel increasing of undesired degradative phenomena (such as oxidation) can result in a detriment of the overall storage capacity of the material. Furthermore, from a technological point of view, the presence of unconfined nanoparticles inside the device can constitute an obstacle to the gas flow through the material [55]. Anyway, storing hydrogen in MH beds as a chemical compound appears to be a promising, cost-effective and safe method of hydrogen storage in the near future

[56]. An example of polymer nanocomposite with MH alloy is shown in **Figure 9**.

To obtain a polymer blend or nanocomposite with the desired properties, compatibilization is an important issue. Actually, the differences in chemical nature between the polymers or the polymer matrix and the nanoparticles give rise to systems with poor properties [58]. Compatibilization gains importance in order to improve the properties. The degradation, which must be minimized, involves the decomposition of the organomodifier and the interactions among the degradation products and the polymers. These, together with the processing conditions, influence the morphology and the properties of the material [59, 60] (**Figure 10**).

**2.6. Compatibilization in polymer nanocomposites**

ratio, which is defined as the ratio of the length to the width [49].

*2.4.2. Cellulose nanocrystals*

**2.5. Nanoparticles of metallic alloys**

**Figure 7.** (a) Zeolite A and (b) faujasite-type zeolites X and Y [45].

#### **2.4. Nanocellulose**

There is an interest in the use of biomass as a source of renewable energy and materials. A promising source of biomass is cellulose. By suitable chemical and mechanical treatments, it is possible to produce fibrous materials with one or two dimensions in the nanometer range from any naturally occurring sources of cellulose [49]. The term "nanocellulose" is used to cover the range of materials derived from cellulose with at least one dimension in the nanometer range. This material has been described as a new bionanomaterial [50]. Isolation of crystalline cellulosic regions, in the form of monocrystals, is done by an acid hydrolysis process [51]. The first report on the mechanical destructuration of cellulose fibers was published in 1983 in two companion papers [52]. Nanocellulose-based materials have a low carbon footprint and are sustainable, renewable, recyclable and nontoxic; they thus have the potential to be truly green nanomaterials with many useful and unexpected properties. **Figure 8** shows the illustration of the crystalline structure of cellulose.

#### *2.4.1. Cellulose nanofibrils*

The mechanically induced destructuration strategy consists of applying severe multiple mechanical shearing actions to a cellulosic fiber slurry to release more or less individually

**Figure 8.** Crystalline structure of cellulose [53].

the constitutive microfibrils. Different shearing types of equipment, such as a homogenizer, microfluidizer or ultra-fine friction grinder, are generally used. This material is usually called nanofibrillated cellulose (NFC) or cellulose nanofibrils (CNF) and is obtained as an aqueous suspension [49]. The width is 3–100 nm depending on the source of cellulose, defibrillation process and pretreatment, and the length is usually higher than 1 μm [53].

#### *2.4.2. Cellulose nanocrystals*

The chemically induced destructuration strategy consists of applying a controlled strong acid hydrolysis treatment to cellulosic fibers, allowing dissolution of amorphous domains and therefore longitudinal cutting of the microfibrils. The ensuing nanoparticles are called cellulose nanocrystals (CNCs) and are obtained as an aqueous suspension [51]. These nanoparticles have high aspect ratio rod-like nanocrystals (whiskers). Their geometrical dimensions depend on the origin of the cellulose and hydrolysis conditions. Sulfuric acid is used for the preparation of CNC, and this process induces the formation of negatively charged sulfate groups at the surface. The average length is of the order of a few hundred nanometers and the width is of the order of a few nanometers [53]. An important parameter for CNCs is the aspect ratio, which is defined as the ratio of the length to the width [49].

#### **2.5. Nanoparticles of metallic alloys**

The possibility of using metal hydrides (MH) alloys in hydrogen technology has being attracting interest [54]. These types of material react with hydrogen reversibly, thus being successfully utilized in the solid state storage of the gas. However, MH alloys under repeated hydriding/ dehydriding cycling suffer from a pulverization phenomenon due to a large volume mismatch between the hydride and the metal compound [55]. As a consequence, repeated hydrogen loading/unloading cycles produce free metal powder particles in nanoscale size. Particle fragmentation results in a considerable increasing of the metal surface area with a consequent enhancement of some properties. Among these, the hydriding kinetics is expected to improve even if a parallel increasing of undesired degradative phenomena (such as oxidation) can result in a detriment of the overall storage capacity of the material. Furthermore, from a technological point of view, the presence of unconfined nanoparticles inside the device can constitute an obstacle to the gas flow through the material [55]. Anyway, storing hydrogen in MH beds as a chemical compound appears to be a promising, cost-effective and safe method of hydrogen storage in the near future [56]. An example of polymer nanocomposite with MH alloy is shown in **Figure 9**.

#### **2.6. Compatibilization in polymer nanocomposites**

**Figure 8.** Crystalline structure of cellulose [53].

of the crystalline structure of cellulose.

**Figure 7.** (a) Zeolite A and (b) faujasite-type zeolites X and Y [45].

*2.4.1. Cellulose nanofibrils*

**2.4. Nanocellulose**

110 Nanocomposites - Recent Evolutions

There is an interest in the use of biomass as a source of renewable energy and materials. A promising source of biomass is cellulose. By suitable chemical and mechanical treatments, it is possible to produce fibrous materials with one or two dimensions in the nanometer range from any naturally occurring sources of cellulose [49]. The term "nanocellulose" is used to cover the range of materials derived from cellulose with at least one dimension in the nanometer range. This material has been described as a new bionanomaterial [50]. Isolation of crystalline cellulosic regions, in the form of monocrystals, is done by an acid hydrolysis process [51]. The first report on the mechanical destructuration of cellulose fibers was published in 1983 in two companion papers [52]. Nanocellulose-based materials have a low carbon footprint and are sustainable, renewable, recyclable and nontoxic; they thus have the potential to be truly green nanomaterials with many useful and unexpected properties. **Figure 8** shows the illustration

The mechanically induced destructuration strategy consists of applying severe multiple mechanical shearing actions to a cellulosic fiber slurry to release more or less individually

> To obtain a polymer blend or nanocomposite with the desired properties, compatibilization is an important issue. Actually, the differences in chemical nature between the polymers or the polymer matrix and the nanoparticles give rise to systems with poor properties [58]. Compatibilization gains importance in order to improve the properties. The degradation, which must be minimized, involves the decomposition of the organomodifier and the interactions among the degradation products and the polymers. These, together with the processing conditions, influence the morphology and the properties of the material [59, 60] (**Figure 10**).

Some of the benefits are controllable particle morphology [69], good interfacial adhesion of the nanofillers [70] and high transparency [71, 72]. When using this method, it is possible to [61] apply higher contents of nanofillers without agglomeration, have better performance of the final products, expand to the solvent-free form, have covalent bond among the nanoparticle functional groups and polymer chains and use both thermoset and thermoplastic polymers.

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This method is widely used for the production of polymer nanocomposites due to its simplicity. However, reaching a proper dispersion of the nanofiller in the polymer matrix can be

Solution blending is actually a system including the polymer and nanofiller, which are easily dispersed in an appropriate solvent [62]. Ultrasonic irradiation, magnetic stirring or even shear mixing can be used to disperse the nanofiller within the polymer [63]. In this method, when the solvent evaporates, the nanoparticle remains dispersed into the polymer chains, as shown in **Figure 12**. The produced nanocomposite can also be obtained as a thin film [61].

There are some problems for the solution blending from the economic and environmental point of view. A proper decision must be taken to choose a correct method according to the situation and the desired product [73]. Some of the benefits of using solution blending are [61]

One main limitation is the ease of agglomeration [63, 65].

**Figure 11.** Schematic illustration for the in situ polymerization method.

more difficult when compared to other methods [61, 62].

**Figure 12.** Schematic illustration for the solution blending method.

**3.2. Blending**

*3.2.1. Solution blending*

**Figure 9.** LaNi<sup>5</sup> /ABS after a mechanical-dry particle coating process in a tumbling mill [57].

**Figure 10.** Scheme of production of compatibilized nanocomposite of PVDF/SWCNT [60].
