Chapter 1
Cellulose-Nanofiber-Based Materials

Antonio Norio Nakagaito and Hiroyuki Yano

1.1 Introduction

Cellulose is the main constituent of the structural framework of the fibrous cell wall in higher plants, and it is the most abundant polysaccharide in nature. As an organic substance, a considerable fraction of the available carbon in the Earth is sequestered in the cellulose molecules. Among the primary sources of cellulose is wood, with the usual benefits that satisfy the current needs, as being a renewable, sustainable, and carbon-neutral source of biofuels and monomers [1] in addition to cellulose nanofibers. These nanofibers, mostly known as cellulose microfibrils by the wood science community, are found embedded in a matrix of hemicelluloses and lignin in the cell wall. The tubular cell wall structure comprises a helically wound arrangement of cellulose microfibrils, nanofibers (4 nm × 4 nm) [2] consisting of semicrystalline cellulose molecular chains parallel to their axes. In the crystalline domains, the cellulose chains are arranged in a way such that each long molecule is connected by hydrogen bonds to the neighboring chains forming a highly ordered crystalline form. Every molecule of these chains is made of glucose rings joined together without foldings, just as the benzene rings are joined in aramid. Even the density and the modulus of the two materials are very similar [3]. The cellulose microfibril possesses a Young's modulus close to that of a perfect cellulose crystal, 138 GPa [4], and considering that the strength of a single kraft pulp fiber can reach a tensile strength of 1.7 GPa [5], the estimated tensile strength lays well beyond 2 GPa. That is to say that we can easily find in nature a renewable equivalent of a strong synthetic fiber currently used in aerospace and military applications. It can be obtained from any cellulose source, be it trees, agricultural crops, or even agricultural waste, and if combined with a proper bio-based matrix resin, it has the potential to replace petroleum-based plastics.

This chapter does not intend to be a thorough review of the research activities concerning cellulose nanocomposites, but just aims to introduce the reader to an ebullient field that promises to bring alternatives to the oil-based materials that we became so used to in the past century. More comprehensive surveys can be found in recent review articles by Hubbe et al. [6], Siro and Plackett [7], and Moon et al. [8].

1.2 The Percolation and Entanglement Phenomena of Cellulose Nanofibers

The reinforcing effect of cellulose nanofibers was extensively studied during the past decade, and as reviewed by Berglund [9], the research concentrated basically on attempts to understand the cellulose microfibril or cellulose whisker reinforcement mechanisms in film composites analyzed in the rubbery state.

The most probable first report on cellulose nanocomposites is attributed by Berglund [9] to Boldizar et al. [10]. In 1987, the production of thermoplastics reinforced with hydrolyzed pulp fibers was described. The embrittlement brought by the hydrolytic treatment was intended as a means to facilitate the disintegration of the original fiber into fibrillar entities, or nanofibers, suggesting the possibility to exploit their unusually high modulus and strength values to make composites. Prehydrolyzed cellulose was treated mechanically by a beater or a high-pressure homogenizer, compounded with a thermoplastic matrix (PS (polystyrene)-latex, PP (polypropylene)), and injection molded. The modulus of the composites increased up to three times relative to the pure matrix at a 40 wt% cellulose content, but the tensile strength practically did not change, and in some cases even decreased. The achieved reinforcement was not as high as anticipated because of the possible agglomeration of the fibrils resulting in a poor dispersion inside the matrix. Notwithstanding, PVAC (polyvinyl acetate)-latex mixed with microfibrillated cellulose (MFC) films prepared by casting method revealed the inherent stiffening properties of cellulose microfibrils. Young's modulus of PVAC was improved from 63 MPa to 1.6–2.9 GPa at a 40 wt% cellulose content.

Extensive works involving cellulose microfibrils and whiskers have been carried out by researchers at the Centre de Recherches sur les Macromolécules Végétales–Centre National de la Recherche Scientifique (CERMAV-CNRS), France. In 1995, Favier et al. [11, 12] reported the production of polymer films reinforced with cellulose whiskers extracted from sea animals, tunicates. Whiskers are very thin single-crystal fibrils having a nearly perfect crystalline structure. An aqueous suspension of latex obtained by copolymerization of styrene and butyl acrylate was mixed with aqueous suspension of tunicin whiskers and the water was let to evaporate slowly at room temperature. In this method, whiskers were well dispersed throughout the composite. Films up to 6 wt% of cellulose exhibited an increase in shear modulus in the rubbery state of more than two orders of magnitude. Moreover, while the modulus of the matrix decreased with temperature, the modulus of the composites remained constant up to the temperature at which cellulose started to decompose. The unusually large reinforcing effect was explained assuming that a strong interaction between whiskers occurs and is governed by a percolation mechanism, forming a rigid network linked by hydrogen bonds. Helbert et al. [13] used the same latex reinforced with whiskers extracted from wheat straw. Water suspensions of latex and whiskers were mixed and freeze dried and then hot pressed. Above the glass-transition temperature (Tg), a 30 wt% whisker composite had a storage modulus of almost two orders of magnitude higher than the matrix. The higher extent of reinforcement was again attributed to the formation of a whisker network. Chazeau et al. [14–16] produced plasticized polyvinyl chloride (PVC) reinforced with tunicin whiskers. Aqueous suspensions of whiskers and microsuspensions of PVC were mixed and freeze dried. Then the freeze-dried powder plus plasticizer were hot mixed and compression molded into sheets. The shear elastic modulus at 380 K (above Tg) for a sample with whisker volume content of 12.4% increased almost two orders of magnitude relative to the modulus of the matrix. However, the modulus did not stabilize over Tg, having a decreasing slope similar to that of the matrix materials. In this case, the formation of a flexible whisker network connected by an interphase of immobilized matrix was assumed, instead of a rigid network linked by hydrogen bonds, as a consequence of the processing method by hot mixing and compression. Dufresne et al. [17] produced elastomeric Mcl-PHA (medium-chain-length poly(hydroxyalkanoate)) latex reinforced with tunicin whiskers. The storage tensile modulus of a 6 wt% whisker content composite above Tg increased almost an order of magnitude compared to the matrix. Similar to the previous case, the reinforcing effect was attributed to the formation of a whisker network connected by transcrystalline layers grown on cellulose surface instead of a rigid network because of the semicrystalline nature of the matrix.

Other research reporting the production of composites reinforced with tunicin whiskers using different matrix materials, such as PHO (poly(β-hydroxyoctanoate)) [18], resulted in the storage modulus drop above Tg being reduced from 3 GPa for the matrix to 0.5 GPa for a film reinforced by 6 wt% whiskers; and epoxy [19], where storage modulus above Tg for a 2.5 wt% whisker was 38 MPa compared to that of the matrix, 1.9 MPa. In both cases, the formation of a rigid network of hydrogen-bond-linked whiskers reinforcing the composites was observed. Further research was reported with reinforcing whiskers of chitin instead of cellulose, which showed varied results regarding the formation of whisker networks. Chitin is another abundant polysaccharide found in the exoskeleton tissue of marine crustaceans and insects. The chemical structure of chitin is identical to that of cellulose except that a hydroxyl group on each glucose ring is replaced with an acetamido group [20]. Poly(caprolactone) matrix composites [21] exhibited a partial formation of whisker networks, while latex matrix composites [22] showed formation of rigid networks. The reinforcement by chitin whiskers of natural rubber [23, 24] showed that only the casting method leads to the formation of whisker networks while freeze drying and hot pressing does not. Only the slow evaporation process gives enough time for whiskers to move and form a rigid network within the matrix. Chemical modification of the surface of chitin whiskers [25] improved their adhesion to the natural rubber matrix but led to a decrease in mechanical properties of the composites, indicating a partial or complete avoidance of the chitin whisker network formation. Yet, all these studies attribute the reinforcing effect of the whisker-filled composites to the formation of a percolated network.

In a percolated system, all the reinforcing elements are connected in a way such that there are paths linking one element to the next forming an unbroken cluster spanning the whole material from edge to edge. In other words, the reinforcing phase of the composite forms some sort of a stiff skeleton that firmly supports the matrix, rather than a multitude of individual reinforcing elements. These elements could be strongly connected by hydrogen bonds or less strongly by other means depending on the processing...