Modification of Nanocellulose to Improve Properties

CHAPTE R 7

Modification of Nanocellulose to Improve Properties The most serious issues which need to be solved are incompatible natures, the difficult dispersion of cellulose in polymer matrix, and also lack of good interfacial adhesion between cellulose fibers as polar materials with nonpolar mediums, such as polymeric materials (Heux et al., 2000; Hubbe et al., 2008; Kalia et al., 2009). Because of the hydrophilic nature of cellulose, nanocellulose cannot be uniformly dispersed in several nonpolar polymer media, its suspension is a gel-like structure at very low concentration, and once dried, the nanocellulose forms films or aggregates. As a result, nanocellulose modification is of interest to limit this phenomena and open up new possibilities. Compatibility with a variety of matrices used in coating colors or in extrusion can be tried. Nanocellulose surface modification can also help to produce “active” nanocellulose and introduce new functionalities. Surface modification of fibers or modification of matrix is one method to overcome these problems (Akil et al., 2011). Identification of optimum surface modification is extremely important for nanofibrillar cellulose (NFC) quality (Hubbe et al., 2008). Table 7.1 presents the advantages. Several authors have reviewed research related to chemical modifications, particularly those by which macroscopic cellulosic fibers can be made less hydrophilic and more mixable with oleophilic matrices (Mohanty et al., 2001; Bledzki et al., 1998; Lu et al., 2000, 2008; Eichhorn et al., 2001; Lindström and Wågberg, 2002; Jacob et al., 2005; Rodionova et al., 2011; Missoum et al., 2012a,b; Goussé et al., 2004; Andresen et al., 2006; Andresen and Stenius, 2007; Johansson et al., 2011 Siqueira et al., 2010; Pahimanolis et al., 2011; Ma­tsumura and Glasser, 2000; Matsumura et al., 2000; Belgacem and Gandini, 2005). It is often advantageous to increase the effective surface area or to remove waxy or loosely bound materials from the fibers (George et al., 2001; Mohanty et al., 2001). The free energy of the surface can also be increased by the use of corona discharge (Belgacem and Gandini, 2005) or by other chemical treatments. Table 7.1: Advantages of Surface Modification Improve fiber distribution and fiber matrix interfacial adhesion Reduce the hydrophobic tendency of the fibers Adding new properties to the material Pulp and Paper Industry. http://dx.doi.org/10.1016/B978-0-12-811101-7.00007-1 Copyright © 2017 Elsevier Inc. All rights reserved.

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92  Chapter 7 Cellulosic surfaces can be derivatized by several direct reactions, which involve the hydroxyl groups. Esterification and silanation reactions are most commonly used in producing c­ellulosic materials for use in composites; many other treatments have been used less commonly (Mohanty et al., 2001; Belgacem and Gandini, 2005; George et al., 2001). Further options include:   1. use of bifunctional reagents. These reagents provide another reactive functionality, in addition to the part of the molecule, which reacts with the fiber surface; 2. organometallic chemistry; and 3. activation of the surface followed by polymerization, such that bonds are produced between the phases.

7.1 Formation of Ionic Groups Carboxymethylation, oxidation, sulfonation, and grafting are the routes, which introduce ionic groups on the surface of cellulose fibers. These are discussed in the following sections.

7.1.1 Carboxylation Cellulosic surfaces can be rendered much more hydrophilic by using treatments, which form carboxylic acids. The surfaces also are very much higher in negative charge as long as the pH is above 3.5 so that the groups are in their conjugate base forms (as carboxylates). The process of carboxymethylation was cited as a method to promote the breakup of cellulosic fibrous material to its nanoelements in addition to promoting a stable suspension in aqueous solution (Wågberg et al., 2008). These researchers observed that the colloidal stability of the resulting suspensions of nanofibers was sensitive to adjustments in pH and salt c­oncentrations. The highly negatively charged nanofibers were found to interact strongly with polyelectrolytes (oppositely charged), and it was possible to produce polyelectrolyte multilayers on the nanofibers. An effective method to induce controlled oxidation of cellulosic surfaces, for creating carboxyl groups, involves treatment with the 2,2,6,6-tetramethylpiperidine-loxyl radical (TEMPO) (Saito et al., 2005, 2007; Montanari et al., 2005; Saito and Isogai, 2005, 2006, 2007). Habibi et al. (2006) performed TEMPO-mediated oxidation of cellulose whiskers that were produced from the animal cellulose tunicin using hydrochloric acid hydrolysis. The authors showed that with a degree of oxidation of up to 0.1, the samples maintained their initial morphological integrity as well as native crystallinity, but at their surface, the hydroxyl methyl groups were selectively converted to carboxylic groups, thus giving a negative surface charge to the whiskers. These oxidized whiskers did not flocculate when dispersed in water; their suspensions appeared birefringent. Saito et al. (2007) found that the cellulose fibers derivatized using a similar method could be readily converted into nanofibers by using mechanical treatment. Strong electrostatic repulsion between the resulting negatively charged nanofibers caused the aqueous nanofibers suspensions to be highly stable.

Modification of Nanocellulose to Improve Properties  93 Eyholzer et al. (2010) evaluated the effect of carboxymethylation before (route 2) and after homogenization (route 1) on refined, bleached beech pulp. Results of sedimentation test revealed that NFC from route 2 easily dispersed compared to route 1. The crystallinity index of materials produced using route 2 was also lesser as compared to route 1. Furthermore, Fourier transform infrared spectroscopy peak at 1595 cm−1 showed the carboxymethylation of fibers. Additionally, scanning electron microscopy (SEM) images showed less agglomeration and lower diameter of fibers from route 2 as compared to route 1. The untreated refinedbleached beech pulp (RBP) formed large aggregates, which were not dispersed in the suspension. RBP by mechanical process also produced some aggregates. The carboxymethylation after mechanical process did not result in significant changes in the morphology of the freeze-dried cellulose, but only carboxymethylation produced a network of cellulose fibrils with overall diameters below 1 μm. The SEM image of freeze-dried RBP-cm showed a coherent system of cellulose nanofibrils having overall diameters below 100 nm. Siró et al. (2011) examined the impact of multiple homogenization on the properties of carboxymethylated softwood pulp. The morphological analysis of the study showed a diminution in fiber aggregates by increasing homogenization steps. Additional two or three homogenization increased elasticity, modulus of elasticity, and tensile strength of films, whereas oxygen permeability did not change significantly. Carboxymethylated NFC gels were found to have the potential for converting extremely transparent and oxygen barrier films. Maleic anhydride (MAH) and succinic anhydride also have been utilized to treat cellulosic materials. This method has been used for inducing negative charges on the surfaces of microfibrillated cellulose (Stenstad et al., 2008; Kamel et al., 2008). Although it is possible to carry out such reactions by heating cellulose fibers in the presence of dry MAH, conditions may need to be controlled carefully for avoiding undesired embrittlement of such fibers (Hubbe et al., 1999).

7.1.2 Oxidation TEMPO oxidation is a kind of pretreatment that facilitates isolation of nanofibers; it selectively introduces carboxyl acidic groups at the C6 of glucose unit (Xhanari et al., 2011; Iwamoto et al., 2011). In the TEMPO oxidation process, an additional catalyst (sodium bromide and primary oxidant (sodium hypochlorite) are used at pH 9–11) (Saito et al., 2009). Mechanism of TEMPO oxidization of cellulose has been studied by Isogai and Kato (1998), and rheological properties of TEMPO-oxidized NFC were examined by Lasseuguette et al. (2008). The suspensions exhibited thixotropic and pseudoplastic behavior. Up to 0.23% as a critical concentration, the viscosity of suspension was proportionate to the concentration. The suspension was more Newtonian below that level, whereas it revealed shear thinning behavior above the concentration. Besbes et al. (2011) conducted an investigation in order to study the effect of carboxyl content on homogenization of TEMPO-oxidized eucalyptus. TEMPO oxidation reduced passing cycles to obtain gel, facilitated defibrillation, and was also able to

94  Chapter 7 prevent the blockage of the homogenizer. Those mentioned effects became obvious when the carboxyl content was up to 300 μmol/g, but the yield of NFC over 500 μmol/g carboxyl content surpassed 90% at 60 MPa. In case of this oxidation reaction, the nature of resultant materials was very much dependent upon the initial materials. When native cellulose is used, even under extreme conditions, oxidation takes place only at the surface, and it becomes negatively charged. By using mercerized and regenerated cellulose, water-soluble salt could be obtained as the oxidized product (Siró and Plackett, 2010). Besbes et al. (2011) used TEMPO oxidization under neutral conditions for pine, alfa, and eucalyptus fibers to find out the effects of raw materials in the TEMPO oxidation process. The carboxyl content was up to 500 μmol/g. Field emission scanning electron microscope (FE-SEM) observations showed that width of NFC was around 5–20 nm in the case of all the samples. Viscosity measurement and light transmission studies of gels revealed 90% yield in nanoscale fibers for eucalyptus and pine fibers (after several cycles at 60 MPa) in comparison to alfa fibers. Saito et al. (2006) used TEMPO oxidization and homogenization for producing individual NFC from bleached sulfite cotton, wood pulp, bacterial cellulose, and tunicin. Using 1 g cellulose and 3.6 mmol sodium hypochlorite, almost entire sulfite cotton and wood pulp produced long individual nanofibers and yielded transparent and highly viscous suspensions. The restrictive degree of oxidation reduced in the following order: wood pulp > cotton pulp > bacterial cellulose and tunicin. Sequential periodate–chlorite oxidation was used as an effective and new pretreatment method for increasing the nanofibrillation of hardwood by using homogenization (L­iimatainen et al., 2012). The oxidization process included oxidizing hydroxyl groups to aldehyde groups followed by oxidizing the aldehyde groups to carboxyl groups. The o­xidization of cellulose with 0.38–1.75 mmol/g carboxyl contents was able to produce transparent and highly viscous gels without blockage of the homogenizer having 85% to 100% yields in 1–4 passes having width of approximately 25 ± 6 nm.

7.1.3 Sulfonation Sulfonation is considered to induce anionic charge on the NFC surface. Liimatainen et al. (2013) used periodate and bisulfite to promote nanofibrillation, and obtained sulfonated NFC having a width of 10–60 nm from hardwood pulp. They found that only 0.18 mmol/g of sulfonated groups was required to enable nanofibrillation and for obtaining highly transparent and viscous gel. The aqueous nanofibrils existed as highly viscous and transparent gels and possessed cellulose I crystalline structures with crystallinity indexes of approximately 40%. A transparent film was obtained from sulfonated nanofibrils having tensile strength of 164 ± 4 MPa and Young’s modulus of 13.5 ± 0.4 MPa. Oxidative sulfonation was found to be a potential green method to promote nanofibrillation of cellulose. This method avoids the production of halogenated wastes, because the periodate used can be efficiently regenerated and recycled. Treatment of cellulose fibers with moderately concentration sulfuric acid is usually a common

Modification of Nanocellulose to Improve Properties  95 step in the production of microcrystalline cellulose (MCC). This typically results in partial sulfonation of the cellulosic surfaces (Beck-Candanedo et al., 2005; Lima and Borsali, 2004). The colloidal stability of aqueous suspensions of MCC produced in this way (Beck-Candanedo et al., 2005) has been attributed to double-layer repulsion forces, which were induced by sulfonic acid groups at the surfaces of the particles (Lima and Borsali, 2004).

7.1.4 Grafting Grafting reactions can be also used to attach ionic groups to cellulosic surfaces. Cai et al. (2003) attached quaternary ammonium groups onto macroscopic cellulose fibers for use in composites. Stenstad et al. (2008) conducted the preparation of a wide range of treatments of microfibrillated cellulose. Each started with oxidation by cerium (IV), followed by a grafting reagent. Positive ionic charges were obtained by grafting with hexamethylene diisocyanate followed by amines. Dou et al. (2006) produced cellulose based nanoparticles having negative charges. The colloidal stability of their materials displayed very high and reversible responses to temperature. Three methods for modification of microfibrillated cellulose (MFC) by using heterogeneous reactions in both water and organic solvents for producing cellulose nanofibers with a surface layer of moderate hydrophobicity were reported (Stenstad et al., 2008). Epoxy functionality was introduced by oxidation with cerium (IV) followed by grafting with glycidyl me­thacrylate on the MFC surface. The reactive epoxy groups served as a starting point for further functionalization with ligands, which typically unreacted with the surface hydroxyl groups present in the native MFC. This reaction is conducted in aqueous media so the use of organic solvents and laborious solvent exchange procedures can be avoided, which is a major advantage of this technique. In the same study by these authors, grafting of hexamethylene diisocyanate followed by reaction with amines produced a far more hydrophobic MFC surface. Succinic and maleic acid groups can be easily introduced directly onto the MFC surface as a monolayer by a reaction between the corresponding anhydrides and the surface hydroxyl groups of the MFC. Also, N-octadecyl isocyanate has been utilized as the grafting agent for improving MFC compatibility with polycaprolactone (Siqueira et al., 2008). Besides this, five different chemicals: styrene MAH, ethylene acrylic acid, guanidine hydrochloride, and Kelcoloids HVF and LVF stabilizers (propylene glycol alginate) were utilized to prepare bionanocomposites from polylacic acid and polyhydroxybutyrate as matrices by Wang et al. (2007) in order to study the potential use of hemp nanofibers (chemically coated) as reinforcing agents for biocomposites. Nanofibers were only partially dispersed in the polymers and therefore resulted in low mechanical properties compared to those predicted by theoretical calculations. M­orphological analyses of sisal whiskers by Siqueira et al. (2008) by using N-octadecyl isocyanate (C18H37NCO) as the grafting agent showed the homogeneity and nanometric dimensions of sisal whiskers. Although the purpose of nanocellulose modification is generally to improve compatibility with nonpolar polymers, thus to improve mechanical properties, chemical modification adds extra functionality to nanocellulosic materials. For instance, positively charged amine-functionalized MFC is found to be antimicrobially active in biomedical applications (Thomas et al., 2005).

96  Chapter 7 Extra functionality was also added to microfibrillated cellulose film by covalently grafting the cellulose with octadecyldimethyl(3-trimethoxysilylpropyl) ammonium chloride (ODDMAC) (Andresen and Stenius, 2007). The surface-modified MFC films displayed antibacterial activity against both gram-negative and gram-positive bacteria at very low concentrations of antimicrobial agent on the surface. More than 99% of Escherichia coli and Staphylococcus aureus were killed when the atomic concentration of ODDMAC nitrogen on the film surface was 0.14% or higher.

7.2 Generation of Hydrophobic Surfaces Modifying the surface of NFC for making it more hydrophobic is a suitable method for reducing agglomeration of these materials, which has been mentioned earlier. Reactions to change the surface of cellulose from hydrophilic to hydrophobic are discussed in this section.

7.2.1 Acetylation/Alkylation Hydrophobic nature to cellulosic surfaces can be imparted by the way of ester formation. Matsamura and Glasser (2000) and Matsamura et al. (2000) were the first to esterify the surfaces of cellulosic nanoparticles. They obtained high strength development. The results were attributed to a high compatibility at the macromolecular level between cellulose I domains in a matrix of partially esterified cellulose. Ifuku et al. (2007) and Nogi et al. (2006a,b) were first to use acetylated cellulosic nanofibers in the production of reinforced clear plastic. The use of alkenyl succinic anhydride by Caulfield et al. (1993) is interesting, since the same chemical is being used in paper machine systems to impart hydrophobicity to paper during the drying process. A similar process was used by Yuan et al. (2006) for the treatment of cellulosic whiskers. Isocyanate can be an alternative material for esterifying agents since it can produce covalent bonds with surface hydroxyl groups (Hubbe et al., 2008). Siqueira et al. (2009, 2010) made a comparison of surface modification of cellulose nanocrystals (CNC) and NFC by using N-octadecyl isocyanate on mechanical and thermal properties of polycaprolactone composite. The degree of substitution for CNC and NFC was 0.07 and 0.09, respectively. The average diameter of NFC was about 52 ± 15 nm, and CNC width was about 5 ± 1.5 nm. The isocyanate grafting was found to improve the dispersion of both CNC and NFC in organic solvents. These results showed that this modification method improved the final properties of the composites. Girones et al. (2007) reported the use of such a system for improving the compatibility of cellulose in polystyrene-based composites. In this system, the reaction to the cellulosic surfaces took place during the compounding of the composite. Gou et al. (2004) reported a two-step process. In the first step, the cellulose surface was esterified with methacrylic anhydride. The unsaturated groups grafted onto the surface were subsequently able to participate in a polymerization reaction of styrene to produce grafted polystyrene. Vilaseca et al. (2005) observed a similar

Modification of Nanocellulose to Improve Properties  97 effect, starting with producing ester bonds between jute fibers and unsaturated fatty acids. The unsaturated groups were able to participate in subsequent free radical polymerization reactions. Treatment of cellulosic surfaces with MAH-modified polyolefins is another method that has been shown for various sizes of cellulosic filler elements. By proper selection of the polyolefin, the cellulosic surfaces produced can be designed for near ideal compatibility with a wide range of matrix polymers. Successful compounding with the chemicals just described requires that the cellulosic elements get well mixed with the matrix polymer, but without excessive temperature heating or duration for avoiding thermal degradation. Based on these principles, maleated polyolefin can be added to a dry mixture of unsubstituted polyolefin and cellulosic material. The reaction with the cellulosic surfaces can then take place during compounding. Qui et al. (2004) reported that it was possible to achieve a higher density of ester bonds and stronger interfacial adhesion by ball milling the cellulose and the maleated polyolefin materials together before heating and extrusion. Maldas and Kotka (1991) reported a method in which unmodified MAH was added to a mixture of polystyrene and sawdust before extrusion of a composite. The improved compatibility with unsaturated polyolefin might be attributed to a free radical reaction with the C]C double bonds in the carboxylated ester groups, which result from reaction of MAH with hydroxyl groups at the cellulosic surfaces (Marcovich et al., 1996). The improvements in composite properties were attributable to improved wettability of the cellulosic surfaces by an unsaturated matrix polymer. Some researchers have reported the use of MAH-modified polypropylene and related chemicals during production of matrix copolymers for improving their compatibility with cellulosic filler material.

7.2.2 Silylation Silane-based chemicals can be also used to attach a wide range of functional groups onto the surfaces of cellulosic fibers. Several studies have reported the modification of cellulosic materials with silanes to improve their performance when used in composites. Lu et al. (2000) described 40 different types of coupling agents that might be considered for such a­pplications. Castellano et al. (2004) has reported the mechanism of silanation coupling reactions. In the absence of water, SiOR groups do not react with cellulosic hydroxyl groups, although they do react with lignin’s more acidic phenolic hydroxyls. Moisture can lead to partial hydrolysis of the silane, making it reactive with the cellulosic hydroxyl groups as long as the temperature is high. Roman and Winter (2006) reported that the presence of silylated ce­llulosic nanocrystals influenced the crystallization of the matrix polymer, increasing the composite stiffness and reducing the heat capacity. Silane based surface modification is a good method to change the fiber surface from h­ydrophilic to hydrophobic. In the absence of water, even at high temperature, no reaction

98  Chapter 7 occurs between SidOR and hydroxyl groups of cellulose, whereas SidOR reacts with lignin’s phenolic hydroxyl groups. Addition of moisture is found to initiate a reaction between silanol groups and hydroxyl groups of cellulose at high temperature (Hubbe et al., 2008). Surface silylation of NFC from bleached softwood pulp using chlorodimethyl isopropylsilane was studied by Andresen et al. (2006). They found that the degree of surface substitution was about 0.6–1, which showed that silylated NFC could be dispersed in a polar solvent. Derivatization became negligible due to the competitive hydrolysis of silane agent when the molar ratio of silane agent of repeating glucose unit turned into less than 3:1. Goussé et al. (2004) examined the rheological properties of mild silylation of NFC by isopropyl di­methylchlorosilane. The morphology of these nanofibers was found to be similar to un­derivatized ones and produced stable suspensions without fluctuation. The suspension exhibited shear thinning effect and thickening characteristics but had no significant yield stress point. They observed that NFC obtained inherent flexibility, and their suspensions’ rheological behavior was similar to a polymer solutions by silylation process. Qua et al. (2011) made a comparison of the effect of three different types of pretreatments including acid, alkaline, and silane in combination with high pressure homogenization (HPH) on flax fibers. Silane pretreatment inhibited agglomeration and produced finer fibers in comparison with alkaline and acid pretreatments. For alkaline and acid pretreatments, thermal stability of NFC increased by increasing the number of cycles through HPH, but thermal stability of NFC after silane pretreatment showed significant increase without HPH. According to these authors, a combination of alkali and acid pretreatment would be more effective for flax fibers, which contain higher amounts of pectins and hemicelluloses.

7.3 Surface Modification by Adsorption The surface of cellulose nanoparticles can be improved by using surfactants or polyelectrolyte adsorption (Missoum et al., 2013).

7.3.1 Surfactants Surfactants are mostly amphiphilic organic compounds. These compounds contain hydrophobic groups and hydrophilic groups. Cellulose films produced from carboxymethylated NFC were modified by Aulin et al. (2008) by coating with varying amounts of a fluorosurfactant, such as perfluorooctadecanoic acid. These authors noted a strong reduction of dispersive surface energy after adsorption as compared to carboxymethylated NFC, from 54.5 mN/m to 12 mN/m, respectively. The anionic surface of TEMPO–NFC can be modified by using a cationic surfactant. N-hexadecyl trimethylammonium bromide (also called cetyltrimethylammonium bromide, CTAB) dissolved in water was deposited on the surface of NFC films (Syverud et al., 2011; Xhanari et al., 2011). The adsorbed layer of CTAB was found to increase the hydrophobicity of the film without significantly affecting its

Modification of Nanocellulose to Improve Properties  99 mechanical properties. Xhanari et al. (2011) used CTAB and didodecyl and dihexadecyl ammonium bromide for controlling the water repellency of cellulose nanofibrils. The surfactant was added directly to NFC in an aqueous suspension. Contact angle values were found to be higher for TEMPO–NFC film dipped in CTAB solution as compared to neat TEMPO–NFC film (60 degree and 42 degree, respectively). The treated material was not fully hydrophobic, but it was rendered more water repellent (lower adhesion with water). FE-SEM characterization was conducted on a covered filter paper using the mixture NFC–Tempo + CTAB. The easiest method for modifying the characteristics of cellulosic surfaces suspended in water is using the water-soluble substances, which have an affinity for surfaces, ie, “surfactants.” Addition of surfactants improves the compatibility between cellulosic solids and matrix polymers in the fabrication of composites (Kim et al., 2009). The hydrophilic head group of the surfactant adsorbs on the cellulose surface, whereas its hydrophobic tail finds proper solvency conditions in the matrix, thus preventing aggregation of the cellulose inclusions via steric stabilization. In such cases, the reasons for improved composite properties may include better wettability and adhesion between the phases as well as the possibility of more uniform distribution of the cellulosic materials within the matrix. The surfactant treatments are often considered as being inexpensive. However, very high surface area per unit mass of nanocellulosic material can imply a rather high addition level, significantly resulting in cost increase (Dufresne, 2006).

7.3.2 Treatment With Polyelectrolytes Irreversible adsorption onto cellulosic surfaces can be obtained by using cationic polyelectrolytes of high molecular weight (Wågberg, 2000). Renneckar et al. (2006) described methods based on polyelectrolyte adsorption as being one of the three methods of improving the properties of cellulose-reinforced composites (in addition to surface derivatization of the cellulose and chemical reactions designed to take place during the extrusion of composites). The method using polyelectrolytes was called “bottom-up,” as it can involve the selfassembl­y of polyelectrolytes onto the cellulose; in other words, depending on their charge interactions, the charged macromolecules arrange themselves into a contiguous layer. Related strategies could be considered for cellulosic nanocomposites. de la Orden et al. (2007) treated cellulose fibers first with polyethylenimine (PEI) (a well-known, highly cationic p­olyelectrolyte). Then the treated fibers were compounded into a polypropylene matrix in the presence of pressure and heat. Infrared spectroscopic analysis of the resulting composites revealed that the amines of the PEI had reacted with carbonyl and carboxyl groups, producing amide linkages under the conditions of extrusion. Ahola et al. (2008) reported that in the formation of a paper-like composite, it can be beneficial to add cellulosic nanofibers and a cationic polymer se­quentially, forming a “bilayer” on cellulosic fibers instead of premixing the nanofibers and cationic polyelectrolyte.

100  Chapter 7 Particularly impressive gains in bonding properties and unique optical effects can be achieved by careful use of oppositely charged polyelectrolytes, gradually building up multilayers on surfaces of interest. Such treatments have been used for the treatment of cellulosics. Preliminary work in this area was conducted by Aksberg and Ödberg (1990). These researchers reported the adsorption of an anionic polyacrylamide on cellulosic fibers with preadsorbed cationic polyelectrolytes. Ding et al. (2005) used this method for the first time in the case of cellulosic fibers having fiber widths in nanometer range. Cranston and Gray (2006), Podsiadlo et al. (2005, 2007), and Holt et al. (2007) used a similar approach in which cellulosic nanofibers played the role of “anionic polyelectrolyte” in a multilayer deposition scheme. A new technique was proposed by Martins et al. (2012) for producing nanopaper with antimicrobial activity using polyelectrolytes as binder between NFC and silver nanoparticles. They reported a layer-by-layer assembly onto NFC with cationic polyelectrolytes (ie, PDDA, PHA, and PEI (cationic polyelectrolytes)) and anionic polyelectrolyte (ie, PSS). The adsorption of a first layer of cationic polyelectrolyte was performed on NFC. This was followed by a second layer deposited using PSS as anionic polyelectrolyte and finally recovered with a last layer of the same cationic polyelectrolyte. Then the silver colloidal suspension colloidal suspension was mixed with this modified NFC. This method was successfully used to impart antibacterial properties to NFC. The antibacterial activity was observed for NFC/Ag materials against different types of bacteria. The activity can be adjusted by varying the amount and characteristics of NFC/Ag used as nanofiller in the papers. Physical adsorption can be easily performed on charged NFCs for achieving more hydrophobic behavior. However, this procedure can induce some migrations phenomena of physically adsorbed moieties. That is why processes aiming at modifying NFCs chemically were developed.

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