While the cellulose nanocrystal is the whisker form of nanocellulose, which is the amorphous part completely removed by acid hydrolysis with H2SO4 or HCL (Siqueira et al., 2010). High-intensity ultrasound can be considered as a mechanical method for producing cellulose nanofibers with hydrodynamic forces (Cheng et al., 2009). The parameters affecting the electrospinning process can be categorized into solution parameters (surface tension, concentration, viscosity and conductivity), processing conditions (voltage, needle-to-collector distance, collector type, flow rate) and ambient conditions ( humidity, pressure and temperature) (Cui et al., 2006; Biber et al., 2010).
The dimensions of CNC depend on the hydrolysis condition and the source of cellulose fibers (Habibi et al., 2010a). In 1927, Mason introduced the steam explosion method to defibrillate wood into fibers for the production of cardboard (Vignon et al., 1995). Nanocellulose particle size analysis can be performed using dynamic light scattering (DLS) and the surface charge can be measured by zeta potential (Zhou et al., 2012).
Morphological properties
The diameter of CNC is 10-20nm and its length is a few microns, while the diameter of NFC is about 5nm with much longer length. In this case, to avoid the increase of electrostatic charge, the samples are sputter coated with gold and then analyzed in the instruments (Qua et al., 2011). The diameter, length, and aspect ratio of CNC were 11 nm, 199 nm, and 18, respectively, with uniform orientation while NFC was 20 to 50 nm wide and more than 1 μm long with aggregate structure.
Drying of nanocellulose
Due to the hydrophilic nature of cellulosic materials, hydrogen bonds can form during drying of nanocellulose and lead to irreversible aggregation known as hornification (Eyholzer et al., 2010, Kalia et al., 2011) which can also alter size of nanocellulosic materials. as their unique characteristics. To address this obstacle, a wide variety of drying methods have been used and compared together by researchers. For example, solvent exchange of CNC from water to non-aqueous solvents with lower surface tension such as acetone was done by Ayuk et al.
Sanchez-Garcia and Lagaron (2010) compared freeze-drying and solvent exchange of CNC and concluded that freeze-dried CNC exhibits better dispersion, transparency, and morphology than solvent-exchanged counterparts. Production of CNC with ionic charge using H2SO4 hydrolysis can also be considered as another solution leading to relatively easy dispersion of this material after drying in aqueous media (Abitbol et al., 2011). In the case of NFC, Abe et al. 2007) used undried fibers to produce NFC and reduce their agglomeration.
The special feature of crystallization at temperatures lower than the freezing temperature of water is to increase the rate of freezing and thus prevent aggregation (Voronova et al., 2012). The suspension flow rate and hot air temperature are some of the main factors in this context (Quievy et al., 2010). Oven drying, freeze drying and atomization process (Quievy et al., 2010), spray drying and supercritical drying (Peng et al., 2012) have been used by researchers to evaluate their effect on the properties of the resultant NFC.
Modifications of nanocellulose
- Acetylation
- Silylation
- Application of coupling agents
- Grafting
Andresen, Johansson, Tanem, and Stenius (2006) reported that hydrophobizing MFC via partial surface silylation using the same silylation agent resulted in partial solubility of MFC and loss of nanostructure when silylation conditions were too harsh. The surface modification changed the character of MFC from hydrophilic to hydrophobic, while the crystalline structure of the cellulose microfibrils remained intact. The monoalkoxy- and neoalkoxy-type titanium-derived coupling agents react with the hydroxyl groups present on the surface of the substrate to form a monomolecular layer (Lu, Askeland & Drzal, 2008).
Nair and partners (2003) used phenylisocyanates and alkenylsuccinic anhydride to improve the quality of the interface between natural rubber and chitin whiskers in the presence of 3-isopropenyl-R,R¢-dimethylbenzylisocyanate. There are three methods reported by Stenstad, Andresen, Tanem, and Stenius (2008) for the modification of MFC by heterogeneous reactions in both water and organic solvents to produce cellulose nanofibers with a surface layer of moderate hydrophobicity. Succinic and maleic acid groups can be introduced directly onto the MFC surface as a monolayer by a reaction between the corresponding anhydrides and the surface hydroxyl groups of the MFC.
Andresen and Stenius (2007) also added extra functionality to MFC film by covalently grafting the cellulose with octadecyldimethyl (3-trimethoxysilylpropyl) ammonium chloride (ODDMAC). The chronological sequence of events for various modifications of nanocellulose can be seen in table1. Individualized Microfibrils from TEMPO-Catalyzed Oxidation of Native Cellulose Surface Modification of Bacterial Cellulose Nanofibers by Dependence on Acetyl Group DS.
Heterogeneous modification of different celluloses with fatty acids by liquid crystal esterification reaction of acrylamide-grafted nanocellulose whiskers. Modification of native cellulose nanofibers with functionalized small-walled carbon nanotubes for hybrid nanofiber/nanotube aerogels.
Nanocellulose-based polymer nanocomposites
Thermoplastic polymer-nanocellulose nanocomposites
For example, Cho and Park (2011) investigated the mechanical and thermal properties of PVA-based nanocomposites reinforced with CNC isolated from MCC. They stated that the highest modulus is obtained at a solution concentration of 4 up to 5% (w/w) of NFC. As they said, reinforcement effects and increase in dispersion viscosity are evident at 5% NFC loading, which means that the percolation phenomenon takes place at this loading.
Ramie's functionalized CNC grafted with organic acid chlorides reinforced low-density polyethylene (LDPE) nanocomposites studied by Junior de Menezes et al., (2009). Native and surface trimethylsilylated CNC were used as the particle phase in nanocomposites with a cellulose acetate butyrate matrix to improve the mechanical properties of polymers and to improve the adhesion between the particle and matrix phases in composites (Grunert and Winter, 2002). In this regard, Azizi Samir et al., (2004) evaluated the effect of CNC extracted from tunicates to improve the mechanical properties of poly(oxyethylene) (PEO) based nanocomposite electrolytes above the melting temperature.
Johnson et al., (2009) investigated a novel bio-based nanocomposite using TEMPO-oxidized NFC by high-intensity ultrasonication in a hydroxypropyl cellulose (HPC) matrix. In this context, Poly(ε-caprolactone) (PCL) as a semicrystalline biopolymer with a glass transition temperature around -60 oC and a melting temperature around 60 oC can be considered to prepare bionanocomposite. NFC was thus covalently grafted with PCL via ring-opening polymerization of ε-caprolactone introduced by Lonnberg et al., (2011).
They evaluated the effect of PCL graft length and ring-opening polymerization to the different molecular weights of the grafts on the mechanical properties of the bionanocomposite. Qu et al., (2010) added poly(ethylene glycol-1000) (PEG) as a compatibilizer to PLA to improve the interfacial interaction between the hydrophobic PLA and the hydrophilic NFC.
Thermoset polymer-nanocellulose nanocomposites
Lu et al. (2008) reported the surface treatment of NFC with three different coupling agents, including 3-aminopropyltriethoxysilane (APS), 3-glycidoxypropyl trimethoxysilane, and titanate coupling-reinforced epoxy resin, using an acetone solvent. The effect of silane-treated NFC in unsaturated polyester and epoxy resin matrices was studied by Abdelmouleh et al., (2005). In addition to epoxy and polyester, the fabrication of a nanocomposite film from TEMPO oxidized NFC and water-soluble phenol formaldehyde was the subject of research by Qing et al., (2012).
SEM micrographs of the film clearly showed the lamellar structure of NFC in cross-section of the neat film and of the composite. Sehaqui et al., (2011) reported the production of NFC-reinforced hydroxyethyl cellulose (HEC) film in a polystyrene petri dish under air atmosphere at room temperature with a thickness of 65-80 mm. SEM images of the fracture surface of the film showed some holes due to entrapped air in the film during the water evaporation step despite degassing the suspension.
Processing techniques have an important bearing on the final properties of nanocomposites based on nanocellulose. However, the use of nanocellulose as reinforcement is in its infancy, and the full reinforcement potential of nanocomposites has yet to be realized, in part due to issues related to scaling up manufacturing processes. Over decades, this particular field of study becomes more interesting, leading to advances in nanocellulose characteristics.
This is solely because many researches have found that properties of nanocellulose play an important role in nanocomposition. Better understanding of organic and polymer chemistries allows the current researcher to look deeply into interaction between polymer matrix and nanocellulose, leading to the advancement of nanocellulose values so that it can be universally used in various nanocomposites.
Conclusion
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