7.2.1 Nanocomposites Based on Matrix Types

Among different types of nanocomposites, ceramic matrix nanocomposites present a special group. Ceramic materials exhibit good level of resistance to wear, temperature, and chemicals. The downside of these materials is brittleness, which prevents wide use in various applications. To overcome this obstacle, nanofillers, which have energy-dissipating capability including nanofibers, whiskers, nanoparticles, and platelets, are included in ceramic matrices. These agents act as load carrying and/or bridging components. The reinforcements deflect the crack and/or provide load bridging effect, and protects nano- and microcracks from transforming into macrocracks [2, 25]. Ceramic matrix nanocomposites based on matrices including SiN, SiC, and Al₂O₃. Al₂O₃ are widely used for reinforcing ceramic-matrix nanocomposites to increase strength and toughness. Ceramic matrix materials can be produced via different methods including conventional powder technique, polymer precursor method, spray pyrolysis, chemical and physical vapor deposition techniques, sol–gel technique, and template synthesis techniques [2, 26, 27].

Metal matrix nanocomposites include metal or alloy matrices, which are inherently ductile, and nanofillers incorporated within them. Metal matrix composites comprise matrices of Al, Fe, Cr, Mg, and W. They show high toughness, strength, and modulus. Metal matrix nanocomposites can be prepared via spray pyrolysis, chemical and physical vapor deposition techniques, electro-deposition, and sol–gel technique. Metal matrix nanocomposites are durable to high temperatures. They are promising for various application areas such as aerospace, transportation, and structural applications [2, 20].

Compared to the other two nanocomposite types, polymer matrix nanocomposites form a special class. Polymer matrix composites are based on matrix materials such as polyolefins (polyethylene, polypropylene), condensation polymers (polyester, polyamid), vinyls, and other special polymers [2, 28]. Polymers generally present some advantages such as lightweight, easy processibility, and corrosion resistance. On the other hand, their shortcomings include low strength and low thermal resistance. Nevertheless, there are some properties that are desired for certain applications while being considered as weakness for others. An example of this is low modulus. It is a drawback for components in structural elements [2]. However, it is a must for textile applications that necessitate flexibility, elasticity, and resilience such as smart textile applications [29]. Whereas metal and ceramic matrix nanocomposites have the potential to cater various industries such as transportation, aerospace, and defense, polymer matrix nanocomposites show promise for use in sensors, batteries, microelectronics, and wearables [2, 30, 31]. The remaining of this chapter will focus mainly on polymer matrix nanocomposites.

7.5.1 Carbon Nanotubes

Roger Bacon was the first to find multiwalled carbon nanotubes in the 1950s [35]. The first nanotubes to be observed were multiwalled carbon nanotubes as shown in Figure 7.2. Multiwalled carbon nanotubes are composed of two or more coaxial cylindrical graphene sheets, while single-walled carbon nanotubes include only one graphene cylinder sheet. Both multi- and single-walled carbon nanotubes are microcrystals and exhibit solid physical properties, while the diameters fall in the range of molecular dimensions. Invention of carbon nanotubes carried nanocomposite research to the next level. Carbon nanotubes present outstanding mechanical, thermal, and electrical characteristics [2, 3].

As the carbon atoms show a helical arrangement and the dimensions are in the nano range, electrical properties are remarkably affected. Thus, even though graphite exhibits semimetal properties, carbon nanotubes can show metallic or semiconducting characteristics. The invention of carbon nanotubes has contributed to the attention on fullerenes. Despite their outstanding properties, fullerenes have found limited use in composites. The structure of carbon nanotubes shows difference in comparison to conventional carbon fibers used as reinforcement in the composite industry. However, in fact, carbon nanotubes exhibit the most perfect ordered carbon fiber structure investigated at the atomic level [3].

Single-walled carbon nanotubes are generally found in bundles. Here, individual single-walled carbon nanotubes present stiffness values substantially higher than those of the bundle they belong to. Incorporation of carbon nanotubes into polymers leads to improvements in electrical and thermal properties [3]. Single-walled carbon nanotubes exhibit a density level corresponding to 1/6 of steel, and that of multiwalled carbon nanotubes is around 1/2 of the density of aluminum. Tensile strength values of carbon nanotubes are higher than high-strength steel, whereas rigidity is close to diamond. They present substantial resilience; in other words, they can endure flexing in great angles and high strains before getting damaged. In this aspect, they are superior to carbon fibers, which suffer brittleness. Moreover, carbon nanotubes possess theoretical electrical and thermal conductivity levels, which are close to those of diamond. Their thermal expansion factor is nearly zero. Their thermal stabilities in vacuum and air conditions are higher than those of metal wires. Their specific surface areas are around 3000 m² g^–1. Whereas the theoretical values are as mentioned, the measured values differ based on experimental conditions [2].

Whereas carbon nanotubes are generally produced via chemical vapor deposition methods, carbon nanotube-reinforced polymer nanocomposites can be produced via conventional blending, solution mixing, melt blending, and in situ polymerization [2]. Incorporation of carbon nanotubes in nanocomposites leads to remarkably enhanced mechanical properties such as toughness, strength, and stiffness [1, 2].

Carbon nanotubes also exhibit antibacterial activity. Direct contact results in killing E. coli bacteria, which may be due to puncture of the cells and leaking of their cellular material. Nevertheless, adverse effects on humans may also be present, which would be of concern to people involved in production processes of carbon nanotubes rather than the final consumers [1]. Nanocomposites of carbon nanotubes were used for pressure-, optical-, thermal-, and biosensors, self-cleaning, antibacterial, energy storing, and energy-harvesting applications, among others [22, 36–38]. Even though carbon nanotubes pose some challenges during handling, their outstanding characteristics show promise for a wide array of applications [2].

7.5.2 Carbon Nanofiber

Carbon nanofibers (CNF) form a unique structure falling between carbon fibers with diameters between 5 and 10 μm and carbon nanotubes of diameters 1–10 nm. Carbon nanofibers exhibit greater surface areas as compared to carbon fibers and offer advantages entailed with this. The diameter of carbon nanofibers is around 100–200 nm, whereas the length can vary from 100 μm to several centimeters. They exhibit aspect ratios typically greater than 100. They can exhibit different shapes including truncated cones, whole cones, and stacked coins [2, 3]. Carbon nanofiber-based nanocomposites have been utilized in nanocomposites for use in sensor applications [39].

7.5.3 Nanocellulose

Cellulose is one of the two most abundant materials available on earth, the other one being starch [40]. Cellulose is a biodegradable and biocompatible component of composites based on renewable resources [41]. Cellulose, in nano form, has found use in sensors (ion sensors, strain sensors, metal ion sensors), biosensors, energy harvesting [42], and actuator applications [11, 43].

Nanocellulose, besides common natural cellulose, exhibits high strength, low thermal expansion coefficient, transparency [17], hydrophilic nature, and dispersibility in aqua [43, 44]. Naturally, cellulosic fibers exhibit a composite structure including bundles of microfibrils showing highly ordered crystalline and irregular amorphous structures [43, 45]. After removal of the amorphous parts via acid hydrolysis processes, nanocrystals are left behind, which are long cellulose crystals exhibiting very high stiffness and strength [1]. Different self-assembled nanocrystal forms of cellulose include cellulose nanowhiskers and microfibrils [11].

The key factor for use of cellulose in smart applications is the interaction between the negative charge of cellulose and the charged analytes. This has strong influence on ion transfer as well as selective permeability. The possibility of modifying cellulose suggests flexibility to cater different demands [43]. A difficulty in working cellulose with conventional polymers is the incompatibility between the hydrophilic polar cellulose structure and hydrophobic apolar polymers. In order to solve this issue, chemical surface treatments have been developed [45].

In a relevant study, Thielemans et al. [46] developed membranes of cellulose nanowhiskers exhibiting selective permeability based on the charge of species. In the preparation stage, aqueous cellulose nanocrystal solutions were produced via esterification of surface hydroxyl groups, which render cellulose highly reactive, by using sulfuric acid solutions.

The surface charges of nanocellulose affect the mediator performance of sensors [47]. Nanocellulose was reported to show high permeability to cationic analytes, while low permeability to negatively charged ions [46]. Another nanocomposite was produced by polymerization of pyrrole on microfibrillated cellulose to give a polypyrrole-microfibrillated system presenting open porous hydrogel structure. This composite shows promise for use in ion exchange and energy-storing applications [48].

A nanocellulose component used in composites is bacterial (microbial) cellulose, that is, cellulose produced by bacteria rather than the widely known cellulose found in plant structures. Bacterial cellulose exhibits nanostructure [44]. A glucose biosensor comprising bacterial cellulose was reported by Lv et al. [42] exhibiting flexibility and self-powering as shown in Figure 7.3. Nanocomposites of bacterial cellulose and gold were developed for use in biosensors [44].

In a strain sensor application, nanocomposites of bacterial cellulose—double-walled carbon nanotubes—were produced. Higher content of carbon nanotubes resulted in higher strain sensitivity [49].

Nanocellulose have also found use in sensors detecting pH and heavy metal ions. Here, detector molecules or colorimetric reagents are immobilized on cellulose surfaces [43, 50].

Cellulose has also found use in actuator and strain sensor applications. In 2002, interestingly, an actuation mechanism in cellulose paper was discovered [16]. Then, this paper was referred to as electroactive paper. The electroactive paper underwent a bending motion as electric voltage was applied. Voltage, frequency and paper type are the major factors affecting the bending motion [17]. Ion migration and piezoelectric property impart electroactivity. Piezoelectric function is obtained when applied mechanical strain leads to electric generation or polarization. This effect can be observed in some non-conducting materials including ceramics and quartz crystals. Piezoelectric effect can be obtained from cellulose due to its dipolar orientation as well as monoclinic crystal form. This property can be further enhanced through application of electric and magnetic fields, and mechanical strain [43]. Yang et al. [18] reported improved piezoelectric properties and stiffness as a result of applied mechanical stretching on electroactive paper. Stretching leads to orientation in randomly oriented cellulose chains in amorphous regions.

Electroactive paper based on cellulose can be prepared as composite structures with carbon nanotubes, metal oxides, and polymers. Electroactive papers can be used in strain sensors, biosensors, chemical sensors and actuators, and energy-harvesting applications [17]. Applying polypyrrole on the electroactive paper improved actuating performance. Introduction of single-walled carbon nanotubes and multiwalled carbon nanotubes to cellulose electroactive paper leads to improved force and actuation frequency. Metal oxide incorporation in the cellulose improves chemical durability, mechanical strength, conducting property, and sensitivity to light. As seen from the mentioned studies, applicability of electroactive paper in smart applications can be greatly extended via introduction of nanofillers to overcome its drawbacks [17].

7.5.4 Conducting Polymers

An important subgroup of nanocomposites is conducting polymer-based composites. Via rational design and effective optimization of conductive polymer-based nanocomposites’ parameters, advancement in research and feasibility of commercialization can be achieved [2, 3]. These conducting polymer nanocomposites may exhibit magnetic susceptibility, dielectric, piezoresistive, catalytic, or energy-storage and -harvesting capabilities [1, 2].

Conducting polymers including polypyrrole and polyaniline have found common use in smart applications. Conducting polymers generally exhibit low strength in comparison to conventional polymers. Moreover, they exhibit low solubility and dispersibility in common organic solvents. Thus, it is not very feasible to coat them via common methods. Hence, it has become advantageous to use them in a nanocomposite composition with additional components [43].

In a study, a conductive paper has been prepared from a nanocomposite of polypyrrole and cellulose. They obtained improved absorption for larger anions. The results show promise for use in sensors for biomedicine applications where biomarker, DNA, and other protein extraction are necessary. The nanocomposites were prepared by using one of two oxidizing agents: Fe (III) chloride or phosphomolybdic acid. Use of Fe (III) chloride led to better ion transport, surface area, conductivity, and higher current [51].

A nanocomposite of cellulose nanocrystal–polypyrrole–glucose oxidase enzyme was developed as a biosensor in diabetes treatment. Here, the cellulose nanocrystal–polypyrrole structure was used to immobilize the enzyme, whereas the nanosized pores facilitated electron transfer. Esmaeili et al. [52] obtained high glucose sensitivity and good repeatability. The limit of detection was 50 +10 μΜ, and exclusion of interfering species such as uric acid, ascorbic acid, and cholesterol was also achieved.

7.5.5 Nanoparticles

Nanoparticles are often defined by their diameters, which are less than 100 nm. Nanoparticles can be of different organic or inorganic materials. Different nanoparticles that have found use in nanocomposites can be given as [1, 3]:

• Metals (aluminum, iron, gold, silver, nickel, palladium, platinum, …)
• Metal oxides (titanium oxide, zinc oxide, Al₂O₃, …)
• Nonmetal oxide (silica—SiO₂)
• Other (silicon carbide SiC).

The type of nanoparticle included in the nanocomposite determines the resultant electrical, thermal, and mechanical characteristics. To give examples, aluminum nanoparticles impart conductivity, silicon carbide improves mechanical strength and resistance to corrosives, and silica enhances tensile and impact properties [3].

Silica, titanium, and aluminum nanoparticles have found use in forming barrier property against gases as well as to attain self-cleaning and antimicrobial effects [1].

Nanoparticles of metals can be used in catalysts, and inert metals can be used in biomedical applications such as tumor treatments. Wang et al. [53] produced polymer solar cells incorporating aluminum-doped ZnO nanoparticles with surfactant agents including ethanolamine, ethylenediamine, diethylenetriamine, and triethylenetetramine and reached super power conversion efficiency levels for similar systems by exceeding 10%.

Metal oxide nanoparticles impart electrical and thermal conductivity, mechanical strength, and barrier effect, besides other functions including UV protection, antibacterial, and photocatalytic (self-cleaning) activity. Metal oxide nanoparticles include MnO₂, SnO₂, WO₃, SiO₂, Fe₂O₃, ZrO₂, PtO₂, and TiO₂ [1, 2].

Titania or titanium dioxide (TiO₂) has common use for photocatalytic, UV protection, and antibacterial functions. Silver has found use for antibacterial effects [1]. Inclusion of silver nanoparticles at very low loadings as low as 1 wt% leads to increased mechanical strength and thermal durability [2]. As with other materials, high surface area resulted from nano dimensions increases effectivity of silver [1].

7.5.6 Nanoclays

Clay is generally defined as materials composed of very fine inorganic components including hydrated aluminum silicates. Phyllosilicate, also referred to as the clay mineral, includes hydrate silicates of aluminum and magnesium and shows layered structure. Each clay mineral includes two forms of sheets, which are tetrahedral and octahedral [3].

Among layered silicates, hectorite, saponite, and montmorillonite are the ones that enjoy the widest use in nanocomposites. Montmorillonite, the most popular one among them, exhibits high surface reactivity. Each layered sheet exhibits thickness around 10 Å (1 nm), whereas the remaining dimensions fall within the range between 30 nm and several microns. Their aspect ratio varies between 10 and 1000, while the specific surface area is approximately 750 m²/g. The aspect ratio falls as the layers break during nanocomposite production. On the other hand, if good dispersion cannot be achieved, the effective aspect ratio falls as well [3].

Nanocomposites of montmorillonite were reported to show enhancements in ionic conductivity of PEO. This was attributed to the fact that the PEO cannot crystallize due to intercalation, as crystallites are naturally not conductive. The conductivity is slightly affected by the ambient temperature [2]. Another nanocomposite of montmorillonite exhibited decreased flammability [15].

Polymer matrix nanocomposites containing silicate layers can be prepared by intercalative methods. Via this technique, various nanocomposites presenting intercalated or exfoliated structures can be obtained. The structure is determined by the penetration level of polymer matrix into the galleries of the silicate. In case of the intercalated nanocomposites, slight entrance of polymer chains between the sheets is observed, where the stacked order is maintained. On the other hand, in terms of the exfoliated form, a high affinity between the polymer and clay leads to separation of sheets and loss of stacked structure [1, 2].

In their natural state, silicate layers may not form nanocomposites. Normally, silicate layers are not miscible with polymers as they include hydrate sodium or potassium ions. In order to increase affinity of layered silicates to polymers, the chemical structure should be changed from hydrophilic to organophilic. This can be achieved by organic modification. Such modified nanolayers of silicates are named as organosilanes or nanoclays [3]. Hence, the ability of forming nanocomposite structure depends on the components, production technique, and the affinity between the layered silicate and the matrix polymer; consequently, the type of the composite is determined: tactoid, intercalated, or exfoliated [2].

If efficient separation of sheets cannot be achieved, the tactoid form cannot be broken down and no nanocomposite structure can be achieved, as the resultant composite exhibits a phase-separated structure as in the case of conventional microcomposites. In this form, the nanocomposite structure is not formed as the polymer and the clay are immiscible. It is crucial issue to break the tactoid structure in order to achieve nanocomposite structure. In the other case, in the nanocomposites of intercalation, the polymer enters into the silicate layers in a crystallographical order. Generally, intercalation occurs as a few polymer molecule arrays in each layer. Ordered clay sheets result in very good properties, which are more pronounced in high aspect ratios [1–3].

Polymer-silicate layer nanocomposites have received intensive attention based on their outstanding characteristics such as high stiffness, strength, thermal durability, and resistivity against flammability. Silicate layers show flexibility as their properties can be modified via ion exchange reactions with cations. Very low loadings of layered silicates can lead to substantial increases in properties. Moreover, the possibility of melt-mixing polymers with silicate layers without the need for organic solvents is another positive aspect of intercalation chemistry. A very strong interface interaction can be achieved between polymers and the silicate layer, much higher compared to that observed in conventional composites. High aspect ratio results in high stiffness in the resulting nanocomposite [2].

7.5.7 Nanowires

Nanowires can also be incorporated in nanocomposites for use in smart textile applications including energy harvesting as in flexible solar cells. In a related example, utilization of silver nanowires in supercapacitors [19] leads to high power conversion efficiency related to low resistance due to high conductivity of silver nanowires, which promotes charge mobility [54]. Catenacci et al. [55] produced a core-shell copper–silver nanowire welded composite including a silicon elastomer, which presented a serpentine pattern. Based on the pattern and silicon matrix, the composite system showed 300% stretchable. As expected, the capability of stretching is very important for textile applications.

In another application of nanowires, copper nanowires were vertically aligned in polydimethylsiloxane matrix to act as thermal interface materials, which are of high importance for heat management of electronic devices [56]. A transparent electrode was produced via embedding copper nanowires in polymethyl methacrylate matrix. The matrix did not only provide transparency; it also imparted copper nanowire chemical stability. When the fashion aspect of textiles is considered, it becomes apparent that transparency is an important parameter of smart components to be used in smart textile applications [29].

A different nanowire example is that of polyaniline. Polyaniline is a conducting polymer. Nanocomposites of polyaniline nanowire–polyamide nanofiber–cellulose acetate film systems were developed. First, polyamide nanofibers were produced via electrospinning. The nanowires were then infiltrated with cellulose acetate to form a transparent film. Followingly, polyaniline nanowires were grown on the substrate through an in situ polymerization technique. The produced electrode presented good durability against cyclic bending [57].

7.5.8 Others

A number of other components have found use in nanocomposites for use in smart textile applications. These include common natural fibers including flax and wood fibers [34, 58], and synthetic polymers including but not limited to polyamide, polypropylene, polyurethane, polyethylene glycol, polyacrylate, polydimethylsiloxane, epoxy, polyvinyl alcohol, and polycaprolactone [27, 34, 56, 59].

On natural animal-based fibers (wool, silk), limited studies are present in terms of use in nanocomposite smart textile applications. Whereas wool has been found as a textile substrate rather than a nanocomposite component, few works are present for silk [60, 61]. Silk fibroin is a very interesting material. It is bio-based and biodegradable and shows properties similar to synthetic polymer fibers. It has applications in nanocomposites with target uses rather limited to biomedicine applications due to its cost [61]. In a study, nanocomposite of silk-gold was produced through electrospinning and tested in vivo for replacing nerves including nerve conducting velocity, compound muscle action potential, and motor unit potential [60]. In another study, nanofibrous webs of silk fibroin and silver nanoparticles were produced for antibacterial function [62]. It would be interesting to see applications of silk fibers, especially the fibroin section, in nanocomposites for smart textile applications. Some nanocomponents used in smart nanocomposites and the attained functionalities are listed in Table 7.1.

7.6.1 Laminated Nanocomposites

Some composites exhibit laminated/layered composition. Some of these layered composites may contain nanostructured layers. A common example to this is composite filtration systems. Nanofibrous web layers commonly find place in these filters. Nanofibrous webs are generally produced via the electrospinning method. High porosity ratios and small pore dimensions exhibit advantages in terms of filtration efficiency [22].

Laminate composites have found use in moisture management applications. Moisture management is very critical in terms of ensuring body comfort, especially for sportswear. A dual-layer approach is generally adopted. In the first layer close to the body, hydrophobicity is preferred, whereas hydrophilic content is located in the outer layer. This results in the push–pull effect where the inner layer pushes moisture to the outer layer via capillary force and in turn to wicking action [22]. In a related study, the inner layer was a hydrophobic polydopamine-treated nonwoven layer, and the outer was a hydrophilic electrospun nanofiber membrane of polyacrylonitrile–SiO₂ [14]. Janus fabrics are another example of superamphiphilicity, which shows superhydrophobicity at one side and superhydrophilicity at the other [84].

7.6.2 Nanocomposite Fibers

A different form of nanocomposites is the fiber form. In this form, the fiber exhibits a nanocomposite structure itself. In this way, different functionalities can be incorporated into the fiber. An example is fire protection. Montmorillonite clay and intumescent flame-retardant agents can be used as nanofillers in a fiber. It was reported that addition of carbon nanotubes also enhanced fire protection capability [22].

In a nanofiber exhibiting nanocomposite structure, electrical conduction and magnetism have been simultaneously achieved. In the nanofiber, polyvinyl pyrrolidone was selected as the matrix, whereas varying amounts of polyaniline and Fe₃O₄ nanoparticles were incorporated to fine-tune conductivity and magnetism properties. The prepared nanofiber shows promise for use in electromagnetic shielding and microwave absorption applications [71].

7.6.3 Nanocomposite Membranes

Membranes of electrospun nanofibers are also used for different applications. They take place as lightweight breathable layers in protective clothing. They form breathable, flexible membranes with high filtration efficiency and low pressure drop. They may be used with other porous layers such as polyurethane foams with open cells, that is, interconnected pores. Electrospun nanofibrous webs are also used in rechargeable lithium ion batteries [33].

Various methods can be found for manufacturing nanofibers such as chemical vapor deposition, composite spinning, drawing, template synthesis, melt blowing, self-assembly, electrostatic spinning, and so on [22]. Among these methods, electrostatic spinning has been the most popular one depending on its versatility and ease of setup [85].

In its own, electrospun nanofiber webs present low durability against compression and other mechanical effects. Thus, electrospun nanofiber membranes are generally used in composite systems with other components or layers, such in laminated composites. In an application, electro-spun nanofibrous webs are utilized as carrier matrices of phase change materials for thermoregulation applications [59]. Phase change materials are materials that can store heat energy when the ambient temperature is higher than a critical point and vice versa, while keeping their temperature nearly constant. Advantages of electrospun nanofibrous webs can be given as high porosity, high surface area, and ease of encapsulation in terms of acting as carriers or phase change materials [12, 22, 59].

Composite systems are preferred for filtration applications in order to achieve high efficiency levels. Nanocomposites are especially promising for filtering small structures while maintaining the necessary pressure drop, system integrity, and strength performances. Electrospun nanofiber membranes offer high porosity, low pore dimensions, which are necessary for filtration applications. So, electrospun nanofiber membranes are utilized as layers of composite filtration systems. In a close field, electrospun nanofiber webs may be utilized as a membrane of protective clothing. They present air and water vapor permeability, elasticity, and good filtration performance [33, 65].

7.6.4 Nanocomposite Coatings

Coating of textiles with nanoparticles does not result in durable coatings most of the time. One of the effective means of preparing nanoparticle coating with washing fastness is producing nanocomposites where the functional nanoparticle is embedded in a polymer matrix. Improvement of bonding results in durable coating effect with better human—and environmental and ecological—safety. Via use of such nanocomposites, functions including UV resistance, antimicrobial effect, conductivity, fire protection, and self-cleaning can be attained. The performance characteristics of nanocomposites may surpass those of each component. Appropriate selection of the optimum polymer system may lead to reduced agglomeration [12].

Via adoption of a nanocomposite system, chemical species showing affinity to textile substrates can be introduced, besides the main functional groups. This way, coatings durable to washing and other effects may be produced. The prepared nanocomposites may be coated onto the textiles via different methods such as dip–dry or blade coat–dry with subsequent cure processes. Nanocomposite systems offer durable thin coatings presenting transparency and multifunctionality. Nanocomposites used in textile coatings include metal nanoparticles, metal oxides, titania and zinc oxide, graphene, carbon nanotubes, as well as phase change materials [12, 33].

In order to improve bonding of nanoscale materials on substrates for achieving continuation of the designed functions, nanofibers and/or nanofillers may be treated with physical or chemical surface modification procedures. Another means of ensuring durable nano-effects is embedding functional nanomaterials in polymer matrices, which have the affinity to the textile substrate [6, 33].

In a nanocomposite coating application, silver nanowires and fluorosilane were coated on a cotton fabric. Silver nanowires contributed to conductivity as well as antibacterial effect of the fabric. Superhydrophobicity was obtained with a contact angle of 156° from the fluorosilane–silver nanowire-coated fabric as well [82].

7.6.5 Nanocomposite Hydrogels

It should not be considered that composite and nanocomposite materials are all rigid materials. Hydrogels constitute a very special interesting stimuli-responsive material group, which presents composite structures [9]. As hydrogels are covered in another chapter of this book [86], they are not investigated in this chapter.

7.7.1 Sensors

Sensors, a very important component of smart devices, are capable of measuring a physical quantity and converting it into a signal that can be detected by a human observer or an electronic device. Electrode takes a critical part in the sensor determining the sensitivity and efficiency. Mediators facilitate ion transfer between the analyte and the electrode. Mediators can be composed of different materials such as conducting polymers, metallocenes, and conductive nanocellulose composites [43]. Mediators are generally covered on surfaces of electrodes, taking place between the analyte and the electrode. The properties of the mediators heavily influence the response rate of the sensor. Low response rate is a very critical issue in terms of sensor applications. Response rates in bulk polymers are rather low due to the time necessary for target molecules to penetrate into the polymer. However, in the case where nanostructuring is achieved via nanofibers or nanotubes, response rate can be highly increased. Furthermore, as known, nanoscale materials exhibit a very high surface area per volume, which enhances sensitivity as well as the response rate. These render nanocomposites ideal materials for use in sensors [11, 43].

Sensors, as key elements of smart devices, can function based on electrical, mechanical, or optical mechanisms. They can be subgrouped as electrochemical, piezoelectric, acoustic, fluorescent, and colorimetric sensors, etc. [11]. Biosensors constitute an important class of sensors capable of sensing biochemicals including glucose, estrogen, and urea. A very important type of biosensors is the electrochemical sensor, which can produce an electrical signal from a concentration of chemical species by use of enzymes. Nanocellulose has found use in nanocomposites for biosensor applications [11, 43]. Glucose biosensors attract extensive effort due to interest in diabetes treatment research. Here, glucose oxidase enzyme is used [40]. An active layer of this enzyme is used to react with glucose and transmits current into the electrode. Here, the strength of the electric signal produced at the electrode is proportional to the concentration of glucose. Another biosensor application of nanocellulose is in wound care via detecting elastase, a biomarker for inflammatory diseases [11].

Nag et al. [10] developed a flexible wearable sensor for monitoring respiration and other body motions. The multilayered sensor comprises a polydimethylsiloxane layer and a membrane of nanocomposite including functionalized multiwalled carbon nanotubes. The polydimethylsiloxane and the carbon nanotubes acted as electrodes. The sensor functioned based on strain sensitivity. The thickness was selected to give the best strain and conductivity behavior. Polydimethylsiloxane was selected based on its cost efficiency, hydrophobicity, inert structure, and nontoxic nature. Carbon nanotube was preferred based on thermal stability and tensile strength.

In the mentioned study, multiwalled carbon nanotubes were functionalized by introduction of carboxylic (–COOH) groups. Incorporation of these groups resulted in better dispersion in the polymer matrix. This, in turn, causes stronger interfacial bonding and, subsequently, better conductivity [10]. The electrodes were produced as patterns on the substrate. The selected method was laser ablation. Other techniques that can be used include inkjet printing, 3-D printing, and photolithography. The laser method is advantageous in terms of its ease of use as no template or extra material consumption is necessary [10].

The functionalized multiwalled carbon nanotubes were used as a conductor. They were embedded in the flexible substrate based on polydimethylsiloxane. The resultant structure acted as a sensor monitoring multiple physiological variables. The advantages of multiwalled carbon nanotubes include high electrical conductivity and aspect ratio. Incorporation of functional sites such as carboxyl (–COOH) groups leads to enhanced conducting and dispersing performance. Polydimethylsiloxane was selected for its low stiffness, which is required for wearable use. Polydimethylsiloxane is also more cost efficient in comparison to polyethylene naphthalate and polyethylene terephthalate, which have also been used for flexible sensor manufacturing. The hydrophobic nature of polydimethylsiloxane prevents interfering effect of sweat on sensor functionalization [10].

The mechanism behind the function of the sensor is the change in the capacitance induced by deformation of electrodes due to body motions. The capacitance of parallel-plate capacitor can be given as

(7.1)

where C is the capacitance in Farads, ε_o is the dielectric constant of free space, ε_r is the relative dielectric coefficient of the material, A is the area of one plate, and d is the distance between the plates. Difference in the area or the distance leads to change in the capacitance [10].

7.7.2 Antibacterial Activity

Another function that is expected from textiles is antibacterial activity. In a related study, nanocomposites of cellulose–Cassia alata leaf extract–copper nanoparticles were developed. Presence of copper nanoparticles improved thermal stability and tensile strength of cellulose. The nanocomposite exhibited good antibacterial activity [13].

7.7.3 Defense Applications

Nanocomposites are investigated for use in defense applications. One related study concerns body armor improvement. Shear thickening fluids are used in body armors. Shear thickening fluids exhibit rapid stiffening response when under shear. Recent work has been reported to improve shear stiffening effect using silica particles in polyethylene glycol fluid matrix. This fluid was applied on Kevlar (para-aramid) fabric and led to higher energy absorption under impact [87].

Nanocomposites also have found use in microwave protection. In a related study, a two-ply nanocomposite was prepared to have a total thickness of as low as 1 mm. One layer is polyaniline (absorbing layer) and the other is polyaniline–magnetite (Fe₃O₄; matching layer). The nanocomposite was reported to have better microwave absorption in comparison to single-layer absorbers [72]. In another work, composite nanofibers were produced from polyaniline–magnetite–polyvinyl pyrrolidone. In this nanocomposite structure, polyvinyl pyrrolidone acted as the matrix. The nanocomposite exhibits double functionality such as electrical conductivity and magnetic effect. The developed system may find use in electromagnetic interference shielding and microwave absorption [71].

7.7.4 Fire Protection

Polymers generally exhibit poor resistance to fire. Upon ignition, polymers mostly burn quickly emitting heat and toxic fumes. Incorporation of nanoclay leads to substantial increase in flame retardancy. Nanoclay addition results in substantial reduction in burning rate and hinders diffusion of volatiles and air [3].

7.7.5 Actuators

Nanocomposites enabled development of actuators that minimize material and energy consumption. Lower particle dimensions, higher surface areas, and lower nanofiller loading rates allow production of cost-efficient actuators [3]. Actuators have the capability to function in hostile environments that humans cannot withstand [9].

Enhanced piezoelectric performance can be achieved from composites of cellulose with nanotubes, chitosan, ionic liquid, polypyrrole, and polyaniline. An actuator of nanocomposites including cellulose, polypyrrole, and ionic liquid was developed. Here, wet cellulose films were produced from cotton pulp via spin coating. Then, pyrrole was polymerized and adsorbed on cellulose; further, activation was carried out in ionic liquid of 1-butyl-3-methylimidazolium chloride solution. The nanocomposite resulted in higher conductivity and mobility compared to pristine cellulose, conventional electroactive paper, and conducting polymer [70].

7.7.6 Self-Cleaning

Another function that nanocomposites can serve is self-cleaning. Self-cleaning mechanism can be induced by both UV and visible light via use of nanocomposites. TiO₂ presents promising effectivity for photocatalytic stain removal. However, long exposure to UV irradiation is necessary, whereas success under visible light is very limited. Thus, titania is imparted with nanoparticles of noble metals, dyes, and compounds including SiO₂ [12, 37, 81].

Nanocomposite systems of porphyrin and TiO₂ were developed to achieve photocatalytic self-cleaning. Self-cleaning effect is induced by visible light due to presence of porphyrin. Porphyrin dye molecules are excited under visible light and electrons are injected to the conduction band of titania. This leads to formation of superoxide anions, which take part in stain decomposing. The durability of porphyrin against light was improved via use of metals in the coating layer [33].

Superhydrophobicity also contributes to self-cleaning. As the water contact angle on a surface exceeds 150°, the surface is considered as superhydrophobic. Superhydrophobicity is attained via nano- and micro-roughness on the surface mimicking lotus leaves as well as using components of hydrophobic chemistry such as fluorosilane [27, 81].

7.7.7 Energy Harvesting

Energy harvesting is another advanced function that nanocomposites can serve. A group of smart textile components that can scavenge and store energy is referred to as nanogenerators [21]. Nanogenerators may be produced by printing nanocomposite inks on textile substrates. In such an example, a piezoelectric textile nanogenerator was developed by utilization of a nanocomposite film incorporating silver nanoparticles and polymers. The nanocomposite ink can be screen-printed on textile or plastic surfaces. Addition of silver nanoparticles improved piezoelectrical property. Through bending and compression motions, tens of microwatts were scavenged by this piezoelectrical nanogenerator [88].

Nanogenerators can be produced by integrating two electrodes in a single yarn even in a single fiber. In an example, a fiber-based nanogenerator was prepared by Zhong et al. [69]. In this example, two cotton threads were used, one of which was coated with carbon nanotubes, whereas the other was coated first with carbon nanotubes then with polytetrafluoroethylene. The treated cotton threads were twisted in a double helix. The twisted threads were woven into fabrics. These fiber-based nanogenerators can convert body motions and vibrations into electricity.