3.5.1 Polymer latexes

The first publication reporting the preparation of cellulose nanocrystals reinforced polymer nanocomposites was carried out using a latex obtained by the copolymerization of styrene and butyl acrylate, poly(S-co-BuA), and tunicin whiskers (Favier et al., 1995a). The same copolymer was used in association with wheat straw (Helbert et al., 1996) or sugar beet (Azizi samir et al., 2004b) cellulose nanocrystals. Other latexes such as poly(β- hydroxyoctanoate) (PHO) (Dubief et al., 1999; Dufresne, 2000; Dufresne et al., 1999), polyvinylchloride (PVC) (Chazeau et al., 1999a; 1999b; 1999c; 2000), waterborne epoxy (Matos Ruiz et al., 2001), natural rubber (NR) (Bendahou et al., 2009; 2010; Siqueira et al., 2010a), and polyvinyl acetate (PVAc) (Garcia de Rodriguez et al., 2006) were also used as matrix. Recently, stable aqueous nanocomposite dispersions containing cellulose whiskers and a poly(styrene-co-hexyl-acrylate) matrix were prepared via miniemulsion polymerization (Ben Elmabrouk et al., 2009). Addition of a reactive silane was used to stabilize the dispersion. Solid nanocomposite films can be obtained by mixing and casting the two aqueous suspensions followed by water evaporation. Alternative methods include freeze-drying and hot-pressing or freeze-drying, extruding and hot-pressing the mixture.

3.5.2 Hydrosoluble or hydrodispersible polymers

The preparation of cellulosic particles reinforced starch (Anglès and Dufresne, 2000; Kvien et al., 2007; Mathew and Dufresne, 2002; Mathew et al., 2008; Orts et al., 2005; Svagan et al., 2009), silk fibroin (Noishiki et al., 2002), poly(oxyethylene) (POE) (Azizi Samir, 2004a; 2004c; 2004f; 2005b; 2006), polyvinyl alcohol (PVA) (Lu et al., 2008; Paralikar et al., 2008; Roohani et al., 2008; Zimmermann et al., 2004; 2005), hydroxypropyl cellulose (HPC) (Zimmermann et al., 2004; 2005), carboxymethyl cellulose (CMC) (Choi and Simonsen, 2006), or soy protein isolate (SPI) (Wang et al., 2006) has been reported in the literature. The hydrosoluble or hydrodispersible polymer is first dissolved in water and this solution is mixed with the aqueous suspension of cellulose nanocrystals. The ensuing mixture is generally evaporated to obtain a solid nanocomposite film. It can also be freeze-dried and hot-pressed.

3.5.3 Nonaqueous systems

In addition to the use of an aqueous polymer dispersion, or latex, an alternative way to process nonpolar polymer nanocomposites reinforced with cellulose nanocrystals involves their dispersion in an adequate (with regard to matrix) organic medium. Coating with a surfactant or surface chemical modification of the nanoparticles can also be considered. The global objective is to reduce their surface energy in order to improve their dispersibility/compatibility with nonpolar media.

Coating of cotton and tunicin whiskers by a surfactant such as a phosphoric ester of polyoxyethylene (9) nonyl phenyl ether was found to lead to stable suspensions in toluene and cyclohexane (Heux et al., 2000) or chloroform (Kvien et al., 2005). Coated tunicin whiskers reinforced atactic polypropylene (aPP) (Ljungberg et al., 2005), isotactic polypropylene (iPP) (Ljungberg et al., 2006), or poly(ethylene-co-vinyl acetate) (EVA) (Chauve et al., 2005) were obtained by solvent casting using toluene. The same procedure was used to disperse cellulosic nanoparticles in chloroform and process composites with polylactic acid (PLA) (Kvien et al., 2005; Petersson and Oksman, 2006). Nanocomposite materials were also prepared by dispersing cellulose acetate butyrate (CAB) in a dispersion of topochemically trimethylsilylated bacterial cellulose nanocrystals in acetone and subsequent solution casting (Grunert and Winter, 2002).

Surface chemical modification of cellulosic nanoparticles is another way to decrease their surface energy and disperse them in organic liquids of low polarity. It generally involves reactive hydroxyl groups from the surface. Experimental conditions should avoid swelling media and the peeling effect of surface-grafted chains inducing their dissolution in the reaction medium. The chemical grafting has to be mild in order to preserve the integrity of the nanoparticle. Goussé et al. (2002) stabilized tunicin microcrystals in tetrahydrofuran (THF) by a partial silylation of their surface. Grunert and Winter (2002) reported the preparation of bacterial cellulose nanocrystals topochemically trimethylsilylated. Resulting nanoparticles were dispersed in acetone to process nanocomposites with a cellulose acetatebutyrate matrix. Araki et al. (2001) prepared original sterically stabilized aqueous rod-like cellulose microcrystals suspensions by the combination of HCl hydrolysis, oxidative carboxylation and grafting of poly(ethylene glycol) (PEG) having a terminal amino group on one end using water-soluble carbodi-imide. The PEG-grafted microcrystals displayed drastically enhanced dispersion stability evidenced through resistance to addition of 2 M sodium chloride. They also showed ability to redisperse into either water or chloroform from the freeze- dried state. Alkenyl succinic anhydride (ASA) can be used for acylating the surface of cellulose nanocrystals. Surface chemical modification of tunicin whiskers with ASA was reported by Yuan et al. (2006). The acylated whiskers were found to disperse in medium- to low-polarity solvents. It was shown that by controlling the heating time, whiskers with different dispersibility could be obtained. Nogi et al. (2006a) and Ifuku et al. (2007) were among the first to use acetylated cellulosic nanofibers in the preparation of reinforced clear plastic.

Preparation of stable cellulose whiskers suspensions in dimethylformamide (DMF) (Azizi Samir et al., 2004e; Marcovich et al., 2006), and dimethyl sulfoxide (DMSO) or N-methyl pyrrolidine (NMP) (van den Berg et al., 2007) without either addition of a surfactant or any chemical modification was also reported. From DMF, tunicin whiskers reinforced POE plasticized with tetraethylene glycol dimethyl ether (TEGDME) were prepared by casting and evaporation of DMF (Azizi Samir et al., 2004f). Cross-linked nanocomposites were also prepared by dispersing cellulose nanocrystals in a solution of an unsaturated linear polycondensate, addition of a photoinitiator, casting, evaporating the solvent and UV-curing (Azizi Samir et al., 2004d).

3.5.4 Long-chain grafting

Long-chain surface chemical modification of cellulosic nanoparticles involving grafting agents bearing a reactive end group and a long ‘compatibilizing’ tail was also reported. The general objective was to increase the apolar character of the nanoparticle. In addition, the surface modifications can act as binding sites for active agents in drug delivery systems or for toxins in purifying and treatment systems. These surface modifications may also be able to interdiffuse, upon heating, to form the polymer matrix phase. The covalent linkage between reinforcement and matrix results in near-perfect stress transfer at the interface with exceptional mechanical properties of the composite as a result.

Nanocomposite materials were processed from polycaprolactone (PCL)- grafted cellulose whiskers using the grafting ‘onto’ (Habibi and Dufresne, 2008) and grafting ‘from’ (Habibi et al., 2008) approaches. The ensuing nanoparticles were used to process nanocomposites using PCL as matrix and a casting/evaporation technique from dichloromethane. A co-continuous crystalline phase around the nanoparticles was observed. Cellulose whiskers were also surface-grafted with PCL via microwave-assisted ring-opening polymerization yielding filaceous cellulose whisker-graft-PCL nanocrystals which were incorporated into PLA as matrix (Lin et al., 2009). Epoxy functionality was introduced onto the surface of cellulosic nanoparticles by oxidation by cerium (IV) followed by grafting of glycidyl methacrylate (stenstad et al., 2008). The length of the polymeric chain was varied by regulating the amount of glycidyl methacrylate. The surface of cellulose whiskers was also chemically modified by grafting organic acid chlorides presenting different lengths of the aliphatic chain by an esterification reaction (de Menezes et al., 2009). These functionalized nanoparticles were extruded with low density polyethylene (LDPE) to prepare nanocomposite materials. Cellulose whiskers reinforced waterborne polyurethane nanocomposites were synthesized via in situ polymerization using casting/evaporation technique (Cao et al., 2009). The grafted chains were able to form a crystalline structure on the surface of the nanoparticles and induce the crystallization of the matrix. Cellulose nanoparticles were modified with n-octadecyl isocyanate (C₁₈H₃₇NCO) using two different methods with one consisting of an in situ solvent exchange procedure (siqueira et al., 2010c). Phenol was also enzymatically polymerized in the presence of TEMPO-oxidized cellulosic nanoparticles to prepare nanocomposites under ambient conditions (Li et al., 2010).

3.5.5 Extrusion and impregnation

Very few studies have been reported concerning the processing of cellulose nanocrystals reinforced nanocomposites by extrusion methods. The hydrophilic nature of cellulose causes irreversible agglomeration during drying and aggregation in nonpolar matrices because of the formation of additional hydrogen bonds between amorphous parts of the cellulose nanoparticles. Therefore, the preparation of cellulose whiskers reinforced PLA nanocomposites by melt extrusion was carried out by pumping the suspension of nanocrystals into the polymer melt during the extrusion process (Oksman et al., 2006). An attempt to use PVA as a compatibilizer to promote the dispersion of cellulose whiskers within the PLA matrix was reported (Bondenson and Oksman, 2007). Organic acid chlorides-grafted cellulose whiskers were extruded with LDPE (de Menezes et al., 2009). The homogeneity of the ensuing nanocomposite was found to increase with the length of the grafted chains (Fig. 3.4).

Another possible processing technique of nanocomposites using cellulosic nanoparticles in the dry state involves the filtration of the aqueous suspension to obtain a film or dried mat of particles followed by immersion in a polymer solution. The impregnation of the dried mat is performed under vacuum. Composites were processed by filling the cavities with transparent thermosetting resins such as phenol formaldehyde (Nakagaito and Yano, 2004; 2008; Nakagaito et al., 2005), epoxy (Shimazaki et al., 2007), acrylic (Iwamoto et al., 2008; Nogi et al., 2005; Yano et al., 2005) and melamine formaldehyde (Henriksson and Berglund, 2007). Nonwoven mats of cellulose microfibrils were also used to prepare polyurethane composite materials using a film stacking method (Seydibeyoğlu and Oksman, 2008).

Water-redispersible nanofibrillated cellulose in powder form was recently prepared from refined bleached beech pulp by carboxymethylation and mechanical disintegration (Eyholzer et al., 2010). However, the carboxymethylated sample displayed a loss of crystallinity and strong decrease in thermal stability limiting its use for nanocomposite processing.

3.5.6 Electrospinning

Electrostatic fiber spinning or ‘electrospinning’ is a versatile method for preparing fibers with diameters ranging from several micrometers down to 100 nm through the action of electrostatic forces. Bacterial cellulose whiskers were incorporated into POE nanofibers with a diameter of less than 1 mm by the electrospining process to enhance the mechanical properties of the electrospun fibers (Park et al., 2007). The whiskers were found to be globally well embedded and aligned inside the fibers, even though they were partially aggregated. Electrospun polystyrene (PS) (Rojas et al., 2009), PCL (Zoppe et al., 2009) and PVA (Peresin et al., 2010) microfibers reinforced with cellulose nanocrystals were obtained by electrospinning. Nonionic surfactant sorbitan monostearate was used to improve the dispersion of the particles in the hydrophobic PS matrix.

3.5.7 Multilayer films

The use of the layer-by-layer (LBL) technique is expected to maximize the interaction between cellulose whiskers and a polar polymeric matrix, such as chitosan (de Mesquita et al., 2010). It also allows the incorporation of high amounts of cellulose whiskers, providing a dense and homogeneous distribution in each layer.

Podsiadlo et al. (2005) reported the preparation of cellulose whiskers multilayer composites with a polycation, poly(dimethyldiallylamonium chloride) (PDDA), using the LBL technique. They concluded that the multilayer films presented high uniformity and dense packing of nanocrystals. Orientated self-assembled films were also prepared using a strong magnetic film (Cranston and Gray, 2006a) or spin coating technique (Cranston and Gray, 2006b). The preparation of thin films composed of alternating layers of orientated rigid cellulose whiskers and flexible polycation chains was reported (Jean et al., 2008). Alignment of the rod-like nanocrystals was achieved using anisotropic suspensions of cellulose whiskers. Green composites based on cellulose nanocrystals/xyloglucan multilayers have been prepared using the nonelectrostatic cellulose-hemicellulose interaction (Jean et al., 2009). The thin films were characterized using neutron reflectivity experiments and AFM observations. More recently, biodegradable nanocomposites were obtained by the LBL technique using highly deacetylated chitosan and cellulose whiskers (de Mesquita et al., 2010). Hydrogen bonds and electrostatic interactions between the negatively charged sulfate groups on the nanoparticles surface and the ammonium groups of chitosan were the driving forces for the growth of the multilayered films. A high density and homogeneous distribution of cellulose nanocrystals adsorbed on each chitosan layer, each bilayer being around 7 nm thick, were reported. Self-organized films were also obtained using only charge-stabilized dispersions of cellulose nanoparticles with opposite charges (Aulin et al., 2010) from the LBL technique.

3.6.1 Microstructure

A visual examination is the simplest way to assess the dispersion of cellulosic nanoparticles within the host polymeric matrix. Because of the nanoscale dimensions of the reinforcing phase the transparency of the nanocomposite film should remain if initially observed for the unfilled matrix. Opacity suggests the presence of aggregates of micrometric sizes (Ljungberg et al., 2005). Optical properties of UV-cured acrylic resin impregnated bacterial cellulose nanofibers were studied by Nogi et al. (2006b). Polarized optical microscopy was used by Azizi Samir et al. (2004c) to observe and follow the growth of POE spherulites in tunicin whiskers reinforced films. For the unfilled POE matrix, birefringent spherulites were clearly identified through the characteristic Maltese cross pattern indicating a spherical symmetry. For a 10 wt% tunicin whiskers reinforced material, the supermolecular structure was found to be quite different. It was observed that the spherulites exhibited a less birefringent character, most probably owing to a weakly organized structure. It was supposed that the cellulosic filler most probably interfered with the spherulite growth and that during growth the whiskers are ejected and then occluded in interspherulitic regions. The high viscosity of the filled medium most probably restricts this phenomenon and limits the size of the spherulites.

Scanning electron microscopy (SEM) is generally employed for the more extensive morphological inspection of cellulose nanocrystals reinforced polymers by observation of the cryofractured surfaces. By comparing the micrographs showing the surface of fracture of the unfilled matrix and of the composites, the nanoparticles can be easily identified (Fig. 3.5). In fact, they appear as white dots, the concentration of which is a direct function of the particles content in the composite. These shiny dots correspond to the transversal sections of the cellulose whiskers, but their diameter determined by SEM microscopy is much higher than the whiskers diameter. This results from a charge concentration effect owing to the emergence of cellulose whiskers from the observed surface (Anglès and Dufresne, 2000).

The dispersion of nanoparticles in the nanocomposite film strongly depends on the processing technique and conditions. SEM comparison between either cast and evaporated or freeze-dried and subsequently hot-pressed composites based on poly(S-co-BuA) reinforced with wheat straw whiskers, demonstrated that the former were less homogeneous and displayed a gradient of whiskers concentration between the upper and lower faces of the composite film (Dufresne et al., 1997; Helbert et al., 1996). It was suggested that the casting/evaporation technique results in films that are more homogeneous and in which the whiskers have a tendency to orient randomly into horizontal planes. A two-dimensional in-plane random network of tunicin whiskers reinforced epoxy was also reported from Raman spectroscopy experiments (Šturcová et al., 2005).

TEM observation can also be performed to investigate the microstructure and dispersion of the nanoparticles in the nanocomposite film. Casting/evaporation was reported to be an efficient processing technique to obtain a high dispersion level (Favier et al., 1995a; 1995b; Kvien et al., 2005; Matos Ruiz et al., 2001; Noishiki et al., 2002; Zimmermann et al., 2005). small angle x-ray scattering (sAXs) and small angle neutron scattering (SANS) are other ways to monitor the dispersion of cellulosic whiskers in the matrix. This latter technique was used to identify an isotropic dispersion of tunicin whiskers in plasticized PVC (Chazeau et al., 1999a). Atomic force microscopy (AFM) imaging has been used to investigate the microstructure of cellulose nanocrystals reinforced polymer nanocomposites (Kvien et al., 2005; Zimmermann et al., 2005). A comparison between field emission (FE) SEM, AFM and bright-field (BF) TEM for structure determination of cellulose whiskers and their nanocomposites with PLA was carried out (Kvien et al., 2005). It was found that AFM overestimated the width of the whiskers owing to the tip-broadening effect. FESEM allowed for a quick examination giving an overview of the sample but with limited resolution for detailed information. Detailed information was obtained from TEM, but this technique requires staining and suffers in general from limited contrast and beam sensitivity of the material.

3.6.2 Thermal properties

The glass–rubber transition temperature, T_g, of cellulose whiskers filled polymer composites is an important parameter, which controls different properties of the resulting composite such as its mechanical behavior, matrix chains dynamics and swelling behavior. Its value depends on the interactions between the polymeric matrix and cellulosic nanoparticles. These interactions are expected to play an important role because of the huge specific area inherent to nanosize particles. For semicrystalline polymers, possible alteration of the crystalline domains by the cellulosic filler may indirectly affect the value of T_g.

No modification of T_g was reported for cellulose whiskers reinforced poly(s-co-BuA) (Dufresne et al., 1997; Favier et al., 1995a; 1995b; Hajji et al., 1996), PHO (Dubief et al., 1999; Dufresne, 2000; Dufresne et al., 1999), PVC (Chazeau et al., 1999a), POE (Azizi Samir et al., 2004a; 2004c), PP (Ljungberg et al., 2005) and NR (Bendahou et al., 2009; Siqueira et al., 2010a). In glycerol plasticized starch based composites, peculiar effects of tunicin whiskers on the T_g of the starch-rich fraction were reported depending on moisture conditions (Angles and Dufresne, 2000). For low loading level (up to 3.2 wt%), a classical plasticization effect of water was reported (Fig. 3.6). However, an antiplasticization phenomenon was observed for higher whiskers content (6.2 wt% and up). These observations were discussed according to the possible interactions between hydroxyl groups on the cellulosic surface and starch, the selective partitioning of glycerol and water in the bulk starch matrix or at whiskers surface, and the restriction of amorphous starch chains mobility in the vicinity of the starch crystallite coated filler surface. For glycerol plasticized starch reinforced with cellulose crystallites prepared from cottonseed linter (Lu et al., 2005), an increase of T_g with filler content was reported and attributed to cellulose/starch interactions. For tunicin whiskers/sorbitol plasticized starch (Mathew and Dufresne, 2002), T_g increased slightly up to about 15 wt% whiskers and decreased for higher whiskers loading. Crystallization of amylopectin chains upon whiskers addition and migration of sorbitol molecules to the amorphous domains were proposed to explain the observed modifications. When using a PVA matrix, an increase in T_g was reported when the cellulose whiskers content increased (Garcia de Rodriguez et al., 2006; Roohani et al., 2008). A similar observation was reported for CMC reinforced with cotton cellulose whiskers (Choi and Simonsen, 2006).

The melting temperature, T_m, was reported to be nearly independent of the filler content in plasticized starch (Anglès and Dufresne, 2000; Mathew and Dufresne, 2002) and in POE-based materials (Azizi Samir et al., 2004a; 2004c; 2004e) filled with tunicin whiskers. The same observation was reported for CAB reinforced with native bacterial cellulose whiskers (Grunert and Winter, 2002). However, for the latter system, T_m increased when the amount of trimethylsilylated whiskers increased. This difference was attributed to the stronger filler–matrix interaction for chemically modified whiskers. A decrease in both T_m and degree of crystallinity of PVA was reported when adding cellulose nanocrystals (Roohani et al., 2008). However, for electrospun cellulose whiskers reinforced PVA nanofibers, the crystallinity was found to be reduced upon filler addition (Peresin et al., 2010).

A significant increase in crystallinity of sorbitol plasticized starch (Mathew and Dufresne, 2002) was reported when increasing the cellulose whiskers content. This phenomenon was attributed to an anchoring effect of the cellulosic filler, probably acting as a nucleating agent. For POE-based composites the degree of crystallinity of the matrix was found to be roughly constant up to 10 wt% tunicin whiskers (Azizi Samir et al., 2004a; 2004c; 2004e) and to decrease for higher loading level (Azizi Samir et al., 2004c). The nucleating effect of cellulosic nanocrystals appears to be mainly governed by surface chemical considerations (Ljungberg et al., 2006). It was shown from both x-ray diffraction and DSC analysis that the crystallization behavior of films containing unmodified and surfactant-modified whiskers displayed two crystalline forms (α and β), whereas the neat matrix and the nanocomposite reinforced with nanocrystals grafted with maleated polypropylene only crystallized in the α form. It was suspected that the more hydrophilic the whisker surface, the more it appeared to favor the appearance of the β phase. Grunert and Winter (2002) observed from DSC measurements that native bacterial fillers impede the crystallization of the CAB matrix whereas silylated ones help to nucleate the crystallization. The crystallinity of N-octadecyl isocyanate-grafted sisal whiskers reinforced PCL was found to increase upon filler addition, whereas no influence of N-octadecyl isocyanate-grafted sisal MFC was reported (Siqueira et al., 2009). This difference was attributed to the possibility of entanglement of MFC that tends to confine the polymeric matrix and restrict its crystallization.

For tunicin whiskers filled semi-crystalline matrices such as PHO (Dufresne et al., 1999) and glycerol plasticized starch (Anglès and Dufresne, 2000) a transcrystallization phenomenon was reported. For glycerol-plasticized starch-based systems, the formation of the transcrystalline zone around the whiskers was assumed to be caused by the accumulation of plasticizer in the cellulose/amylopectin interfacial zones improving the ability of amylopectin chains to crystallize. This transcrystalline zone could originate from a glycerol–starch V structure. In addition, the inherent restricted mobility of amylopectin chains was proposed to explain the lower water uptake of cellulose/starch composites for increasing filler content. Transcrystallization of PP at cellulose nanocrystal surfaces was identified and found to result from enhanced nucleation owing to some form of epitaxy (Gray, 2008).

The presence of sulfate groups introduced at the surface of the whiskers during hydrolysis with H₂SO₄ promoted their thermal decomposition (Roman and Winter, 2004; Li et al., 2009). Thermogravimetric analysis (TGA) experiments were performed to investigate the thermal stability of tunicin whiskers/POE nanocomposites (Azizi Samir et al., 2004a; 2004c). No significant influence of the cellulosic filler on the degradation temperature of the POE matrix was reported. Cellulose nanocrystals content appeared to have an effect on the thermal behavior of CMC plasticized with glycerin (Choi and Simonsen, 2006) suggesting a close association between the filler and the matrix. The thermal degradation of unfilled CMC was observed from its melting point (270 °C) and it had a very narrow temperature range of degradation. Cellulose nanocrystals degraded at a lower temperature (230 °C) than CMC, but showed a very broad degradation temperature range. The degradation of cellulose whiskers reinforced CMC was observed between these two limits, but of interest was the lack of steps: the composites were reported to degrade as a unit.

3.6.3 Mechanical properties

Nanoscale dimensions resulting in a very high surface area-to-volume ratio and impressive mechanical properties of rod-like cellulose whiskers have attracted significant interest in the last fifteen years. These characteristics along with the remarkable suitability for surface functionalization make them ideal candidates for improving the mechanical properties of the host material.

The first demonstration of the reinforcing effect of cellulose whiskers in a poly(S-co-BuA) matrix was reported by Favier et al. (1995a; 1995b), who measured by dynamic mechanical analysis (DMA) a spectacular improvement in the storage modulus after adding tunicin whiskers even at low content into the host polymer. This increase was especially significant above the T_g of the thermoplastic matrix because of its poor mechanical properties in this temperature range. Figure 3.7 shows the isochronal evolution of the logarithm of the relative storage shear modulus (log G′ _T/G′₂₀₀, where G′₂₀₀ corresponds to the experimental value measured at 200 K) at 1 Hz as a function of temperature for such composites prepared by water evaporation. In the rubbery state of the thermoplastic matrix, the modulus of the composite with a loading level as low as 6 wt% is more than two orders of magnitude higher than the one of the unfilled matrix. Moreover, the introduction of 3 wt% or more cellulosic whiskers provides an outstanding thermal stability of the matrix modulus up to the temperature at which cellulose starts to degrade (500 K). Since this pioneer work, many studies have reported the reinforcing capability of cellulose nanoparticles.

This outstanding reinforcing effect was ascribed to a mechanical percolation phenomenon (Favier et al., 1995a; 1995b). A good agreement between experimental and predicted data was reported when using the series–parallel model of Takayanagi modified to include a percolation approach. It was suspected that all the stiffness of the material was caused by infinite aggregates of cellulose whiskers. Above the percolation threshold the cellulosic nanoparticles can connect and form a three dimensional continuous pathway through the nanocomposite film. For rod-like particles, such as tunicin whiskers with an aspect ratio of 67, the percolation threshold is close to 1 vol% (Favier et al., 1997a). The formation of this cellulose network was supposed to result from strong interactions between whiskers, such as hydrogen bonds (Favier et al., 1997b). This phenomenon is similar to the high mechanical properties observed for a paper sheet, which result from the hydrogen-bonding forces that hold the percolating network of fibers. This mechanical percolation effect explains both the high reinforcing effect and the thermal stabilization of the composite modulus for evaporated films. Any factor that affects the formation of the percolating whiskers network or interferes with it changes the mechanical performances of the composite (Dufresne, 2006). Three main parameters were reported to affect the mechanical properties of such materials, viz. (i) the morphology and dimensions of the nanoparticles, (ii) the processing method, and (iii) the microstructure of the matrix and matrix/filler interactions.

3.6.4 Swelling properties

Swelling or kinetics of solvent absorption is a method that can highlight specific interactions between the filler and the matrix. It usually involves first drying and weighing the sample, and then immersing it in the liquid solvent or exposing it to the vapor medium. The sample is then removed at specific intervals and weighed until an equilibrium value is reached. The swelling rate of the sample can be calculated by dividing the gain in weight by the initial weight. Generally, the short-term behavior displays a fast absorption phenomenon whereas in the long term, the kinetics of absorption is low and leads to a plateau, corresponding to the solvent uptake at equilibrium. The diffusion coefficient can be determined from the initial slope of the solvent uptake curve as a function of time. For cellulosic particles reinforced composites, it is generally of interest to investigate the water absorption of the material because of the hydrophilic nature of the reinforcing phase. When a nonpolar polymeric matrix is used, the absorption of a nonaqueous liquid can be investigated.

A higher resistance of thermoplastic starch to water was reported when increasing the cellulose nanoparticles content (Anglès and Dufresne, 2000; Lu et al., 2005; Svagan et al., 2009). Both the water uptake and the diffusion coefficient of water decreased upon nanoparticles addition. These phenomena were ascribed to the presence of strong hydrogen bonding interactions between particles and between the starch matrix and cellulose whiskers. The hydrogen bonding interactions in the composites tend to stabilize the starch matrix when it is submitted to a highly moist atmosphere. Moreover, the high crystallinity of cellulose might also be responsible for the decreased water uptake at equilibrium and diffusion coefficient of the material. A lower water uptake and dependence on cellulose whiskers content were reported when using sorbitol rather than glycerol as plasticizer for the starch matrix (Mathew and Dufresne, 2002). An explanation was proposed based on the chemical structure of both plasticizers, more accessible end hydroxyl groups in glycerol being about twice those of sorbitol. Similar results were reported for cellulose whiskers reinforced SPI (Wang et al., 2006) and CMC (Choi and Simonsen, 2006). Sisal whisker addition was found to stabilize PVA-based nanocomposites with no benefit seen when increasing the whisker content beyond the percolation threshold (Garcia de Rodriguez et al., 2006). A lower water uptake was observed when using MFC instead of cellulose whiskers as a reinforcing phase in NR (Bendahou et al., 2010). This observation was explained by the difference in the structure and composition of both nanoparticles and, in particular, by the presence of residual lignin, extractive substances and fatty acids at the surface of MFC that limits, comparatively, the hydrophilic character of the filler. In addition, assuming that the filler/matrix compatibility was consequently lower for whiskers-based nanocomposites, one can imagine that water infiltration could be easier at the filler/matrix interface. For MFC-based nanocomposites, despite higher amorphous cellulose content, the higher hydrophobic character of the filler favors the compatibility with NR and therefore restricts the interfacial diffusion pathway for water.

Swelling experiments of PVC reinforced with tunicin whiskers were conducted in methyl ethyl ketone (MEK) (Chazeau et al., 1999a). A significant decrease in swelling was observed when increasing the cellulose whiskers content, which was assumed to be caused by the existence of an interphase making a link between nanoparticles, thus allowing the formation of a flexible network. The swelling behavior in toluene of poly(S-co-BuA) reinforced with cellulose fibrils from Opuntia ficus-indica cladodes was reported by Malainine et al. (2005). They observed a strong toluene resistance even at very low filler loading. Although the unfilled matrix completely dissolved in toluene, only 27 wt% of the polymer was able to dissolve when filled with only 1 wt% of cellulose microfibrils. This phenomenon was attributed to the presence of a three-dimensional entangled cellulosic network which strongly restricted the swelling capability and dissolution of the matrix. For higher microfibrils content, no significant evolution was observed because of both the overlapping of the microfibrils restricting the filler–matrix interfacial area and the decrease of the entrapping matrix fraction owing to the densification of the microfibrils network. Similar experiments were conducted with cellulose microfibrils obtained from sugar beet pulp reinforced poly(s-co-BuA) (Dalmas et al., 2006). Toluene resistance of nanocomposites was found to be less significant than for microfibrils from Opuntia ficus-indica cladodes and to evolve with filler content. It was observed that the cohesion of composites prepared by evaporation was higher than the one of freeze-dried/hot-pressed materials. This difference was attributed to the presence of a H-bonded network in the former samples. It was concluded that the solvent did not have any effect on the hydrogen bonds of the cellulose network present in evaporated composites. On the contrary, for freeze-dried/hot-pressed materials, the lower interactions created between fibrils and polymer chains meant that they were able to be more easily disentangled and dissolved by the solvent. The swelling behavior of cellulose whiskers reinforced NR in toluene as reported by Bendahou et al. (2010) was found to strongly decrease even with only 1 wt% of cellulose nanoparticles and to be almost independent of filler content and nature (MFC or whiskers).

3.6.5 Barrier properties

Petrochemical-based polymers predominate in the packaging of foods because of their ease of processing, excellent barrier properties and low cost. However, there is currently an increasing interest in replacing conventional synthetic polymers by more sustainable materials. One promising application area of cellulosic nanoparticles is barrier membranes, where the nano-sized fillers impart enhanced mechanical and barrier properties. Research in this area is burgeoning and evolving rapidly to enhance the barrier properties and to overcome certain limitations. In addition, it is well known that molecules penetrate with difficulty in the crystalline domains of cellulose microfibrils. Moreover, the ability of cellulosic nanoparticles to form a dense percolating network held together by strong inter-particle bonds suggest their use as barrier films.

The oxygen permeability of paper was considerably reduced when coated with MFC layer (Syverud and Stenius, 2009). The water vapor transmission rate of cotton nanocrystals reinforced CMC (Choi and Simonsen, 2006) and PVA (Paralikar et al., 2008) films were reported to decrease. Cellulose nanoparticles prepared by drop-wise addition of ethanol/HCl aqueous solution into a NaOH/urea/H₂O suspension of MCC were found to decrease the water vapor permeability of glycerol plasticized-starch films (Chang et al., 2010). The water vapor barrier of mango puree based edible (Azeredo et al., 2009) and chitosan films films (Azeredo et al., 2010) were successfully improved by adding cellulose whiskers

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