Applications of CNF-/BCNF-Based Nanocomposites

Nanocomposites always offer superior and enhanced properties compared to the properties possessed by their individual polymers. The blending of polymers vanquishes the limitations of individual components. Among cellulose-based polymers, the CNFs and BCNFs majorly serve as the reinforcing fibers and provide mechanical strength and stiffness to the nanocomposites. There are two limitations associated with CNFs and BCNFs when producing nanocomposites.

Firstly, as they are hydrophilic polymers, it gets difficult to blend them with hydrophobic polymers and hence additional surface modifications are required. Some commonly practiced methods of surface modification are acetylation, silylation, application of a coupling agent, and grafting [45]. These methods make the surface hydrophobic and improve the dispersion of the polymers in nonpolar polymeric media.

Secondly, the processing temperature should always be lower than 200°C because CNFs and BCNFs may degrade and emit volatile components at higher temperatures and affect the final properties of the nanocomposite [19]. By considering these limitations, a vast range of nanocomposites are being synthesized. Figure 15.2 gives a picturesque overview of different applications of CNFs and BCNFs.

Figure 15.2 Applications of cellulose nanocomposites.

In the following section, we specifically discuss the contributions of CNFs and BCNFs in the synthesis of nanocomposites for desired applications.

Nanopaper (Tapes, Laminae, Transparent Films)

"Nanopaper is defined as a sheet made completely of cellulose nanofibers" [47]. Compared to the ordinary paper, the nanopaper offers excellent physical and mechanical properties. The nanodimension reduces the scattering of light; hence, it is optically transparent, unlike ordinary paper. Compared to plastic substrates, nanopapers can tolerate high processing temperatures and have better thermal stability [62]. The coefficient of thermal expansion for nanopaper is in the range of 12-28.5 ppmk^-1, whereas for plastic it is in the range of 20-100 ppmk^-1 [63]. The excellent mechanical and thermal properties, high flexibility, transparency, and biodegradability of nanopapers help to replace the plastic substrates in a multitude of applications [62]. Table 15.4 lists the synthesis of various kinds of nanopapers with their applications.

Table 15.4 denotes some of the examples of nanopapers synthesized using different techniques rather than the conventional paper making process. The thermal stability, barrier ability, and mechanical properties of nanopapers can be tuned by varying the amounts of CNFs.

Intelligent Clothes

Protective clothing or intelligent clothing is designed for the protection of the individual from harmful attacks of chemicals, toxic vapors, radiation, fire, etc. Few attempts have been made to produce smart textiles. Some examples are mentioned below.

Gashti and Almasian synthesized lightweight, flame-retardant composite cellulose fabrics for civil application. Carbon nanotubes (CNTs) were used as a fire-retarding material and were successfully stabilized over bleached 100% pure cotton fabric using a vinylphosphonic acid monomer. Later, UV irradiation was done to coat multiwalled carbon nanotubes (MWCNTs) and flame-retardant cross-linking agents over cotton fabric. The protective clothing

showed high heat resistance, heat insulation, and a mass transport barrier created by the MWCNT coating [69]. Thorvaldsson et. al. synthesized nonwetting textile by coating cellulose micro fiber fabric with electrospun cellulose nanofibers and treating it with fluorine plasma. The composite fabric showed superhydrophobicity with a contact angle >150° [70].

Protective clothing is mainly designed for military personnel or emergency responders and should provide full barrier protection by absorbing or blocking toxic agents [71].

Electronic Devices and Sensors

The ability of cellulose nanofibrils to easily become functionalized by chemical treatment and physical blending or incorporation has opened a multitude of avenues in their applicability in sensing technology and electroactive devices [72]. A sensor is a compact device composed of a receptor and a transducer. When a receptor receives any kind of stimuli, the transducer converts it to an electric signal. Generally, CNFs and BCNFs form an insulating phase in conducting nanocomposites. Literature surveys indicate interesting abilities of cellulose nanocomposites in electric devices.

Wang et al. synthesized supercapacitor electrodes out of in situ polymerization of aniline onto a BCNF scaffold. Polyaniline is known for its high electrical conductivity and has been found to enhance the mass specific capacitance of this nanocomposite to a value as high as 273 F/g at 0.2 Ag^-1 current density [73]. In a similar way, Sasso et al. synthesized polypyrrole cellulose nanofiber and cellulose nanocrystal-based nanocomposite films with excellent mechanical properties [74]. Kumar et al. synthesized a flexible strain sensing device by growing piezoelectric ZnO rods over a cellulose paper matrix. The piezoelectric ZnO harnesses mechanical and thermal energy from the ambient environment and can further process it into electrical energy [75]. The output voltage and power of the nanocomposite are 80 mV and 50 nWcm^-2, respectively.

Coming to gas sensing, Mun et al. produced a hybrid nanocomposite of cellulose-titanium dioxide MWCNTs as a potential ammonia gas sensor at room temperature [76]. The NH₃ molecule replaces the preadsorbed oxygen on the MWCNT surface and facilitates the gas sensing mechanism. The sensor formed is flexible and cheap, with good sensitivity (50-500 ppm) and repeatability.

Kafy et al. synthesized a cellulose/graphene nanocomposite that shows extremely sensitive responses to organic solvents [77]. Grafting of functionalized graphene oxide enhanced mechanical, dielectric, and electric performances of the nanocomposite. This work inspires the invention of more such liquid sensors functioning on the basis of the diffusion mechanism of solvents through the nanocomposite membranes.

Tissue Engineering Scaffold

Tissue engineering involves the synthesis and use of a 3D scaffold on which healthy cells are seeded and grown into a tissue. It is mainly designed to repair and regrow injured or broken tissue. Tissue engineering scaffolds should be biocompatible, should anchor and enhance cell proliferation, and should slowly degrade with time.

Nasri-Nasrabadi et al. prepared a hydrogel kind of scaffold from porous starch and CNFs by applying three different techniques: freeze diying, salt leaching, and film casting. CNFs were added in order to improve mechanical properties, enhance porosity, add hydrophilicity, and control the degradation rate [78]. The CNFs 40- 90 nm in diameter were extracted from rice straw, and the addition of 15 wt% CNFs increased Young’s modulus by 287%. An MTT assay showed growth of fibroblast cells over the nanocomposite.

Zhang et al. electrospun a biocompatible scaffold by using PLA- and PEG-grafted cellulose nanocrystals. The advantages of adding a CNC-grafted PEG polymer as a filler to the nanocomposite are improved tensile strength, decreased glass transition temperature, enhanced cell viability and cell proliferation count, and biocompatibility with human mesenchymal cells [79].

In yet another example, a CNF nanocomposite has been synthesized as a substitute for bone ligaments or tendons. Mathew et al. prepared a partially dissolved CNFs network by using ionic liquids at 80°C. Due to partial dissolution, the partially dissolved CNFs formed the reinforcing phase while the dissolved CNFs formed the matrix phase [80]. The mechanical characterizations and biocompatibility assay showed that the nanocomposite possesses strain and strength comparable to that of natural ligaments (strength = 25-30 MPa; strain = 20%-28%) and also the composite showed adhesion and differentiation of human ligament and endothelial cells [80].

Coming to another application, Wan et al. developed a novel vascular tissue engineering scaffold that mimics natural extracellular matrix and promotes blood compatibility. They hybridized heparin and BCNF and prepared the scaffold by a cosynthesis process. Hybridized heparin provided anticoagulant sulfate groups to the BCNF and imparted anticoagulant property to the scaffold [81]. The BCNF-heparin scaffold has interconnected pores, which helped in cellular attachment and vascularization.

Food Packaging

A food packaging must protect the food from spoilage and maintain the quality and safety of the food in storage or transportation. Food may spoil due to exposure to light, oxidation, external forces, permeation of moisture, or a microbial attack [82]. An ideal food packaging film will have good mechanical strength, excellent barrier properties, and antimicrobial activity. Several attempts have been made to synthesize biodegradable food packaging films from CNF composites, as explained below.

Ghaderi et al. produced food packaging film by partial dissolution of CNFs obtained from sugarcane bagasse. The low-value agricultural waste showed high tensile strength (140 MPa), better interfacial adhesion, stress transfer capability, and good barrier potential [82]. It has the potential to be used as a fully biodegradable food packaging material. De Moura et al. incorporated 41 nm silver nanoparticles into hydroxyl propyl methyl cellulose. The cast film showed good tensile strength (51 MPa), decreased water vapor pressure, bactericidal activity against Escherichia Coli and Staphylococcus Aureus species, and a low barrier potential [83].

An interesting attempt in food packaging was made by Azeredo et al. They produced an edible food packaging film by adding CNFs as a reinforcement to mango puree using a film casting technique. Addition of CNFs increased the tensile properties and decreased the glass transition temperature. The decreased glass transition temperature shows a plasticizing effect of sugars in the puree [84].

Coating Additives

A common problem persisting with the synthesis of nanocomposites is the incompatibility between a hydrophobic matrix and hydrophilic fillers, leading to the inferior performance of the composite. Coating is preferred in order to enhance the interaction between the matrix phase and the fibers. Mostly, inorganic nanoparticles are embedded into the matrix for various applications, such as textile coatings, ceramics, catalysts, and adsorbents [85]. Coating aids in the modification of fibers for multiple uses. There are numerous ways of coating, such as grafting of particles, layer-by-layer deposition, sol- gel coating, and gamma and UV irradiation [85]. A few such examples are discussed below.

Cady et al. deposited a 5 nm coating of copper nanoparticles over chemically treated cotton fibers by using a layer-by-layer electrostatic self-assembly process. The nanocomposite showed excellent bactericidal property against the pathogen Acinetobacter baumannii by a partial contact killing mechanism [86]. The biocompatible metal-based nanocomposite can be used in wound care. Gashti et al. deposited hydrophobic silica nanoparticles over a cotton substrate by using 1,2,3,4-butanetetracarboxylic acid as a cross-linking agent and sodium hypophosphite as a catalyst. The nanocomposite showed improved thermal property with super hydrophobicity of the cellulose composite (a water contact angle of 132°) [87].

Thus, the coating has the ability to modify the surface properties of the nanocomposite and also to impart specific properties to the nanocomposite, such as antimicrobial activity.

Filtration

Generally, a nanofiltration membrane has a pore size 0.5-2 nm in diameter, through which a small-sized positively charged metal ion can pass [88]. The anions formed on the membrane precipitate out these metal ions from the flowing water. Thus, filtration takes place through the nanofiltration membrane. The various factors affecting the efficiency of nanofiltration are temperature, pressure, cross flow velocity, pH, salinity, etc. [88].

The cellulose-based nanocomposites offer controlled fiber morphology, high porosity and interconnectivity, good thermal stability, mechanical flexibility, and excellent tunability with other polymers; which makes them interesting candidates for membrane filtration technology. In one such attempt, Badawi et al. deposited randomly oriented CNTs on a cellulose acetate fiber mat and studied its use as a filtration membrane. CNTs are usually added to accomplish desalination and increase the permeation rate of the solvent since they offer the desired geometry. The authors concluded that the addition of a small wt% of CNTs (0.0005-0.005 wt%) to the cellulose acetate matrix significantly improved the water permeation rate and decreased salt retention [89]. Thus, the properties of a cellulose nanocomposite can be easily tailored so it can be used as a filtration membrane.

So far, numerous techniques have been reviewed to form cellulose nanocomposites for a wide range of applications. The size, mechanical properties, thermal stability, and tunability of CNFs and BCNFs open new possibilities toward the synthesis of highly efficient engineering materials and devices.