Nanocellulose-Based Functional Materials
Nanocellulose-based Films (Nanopaper)
The common method for paper production used in paper-making industry can also be used to prepare nanocellulose-based films. In brief, the nanocellulose suspension is filtered through a porous substrate (e.g., microporous membrane) and then the obtained wet cellulose film is dried and/or pressed to fabricate nanopaper (Wei et al. 2014). Since the pure CNF-based film was firstly reported (Nogi et al. 2009) in 2009, various pure nanocellulose-based films and functional composite films have been developed. As can be seen in Figure 3.4. the nanocellulose-based films with fascinating optical, thermal, electrical, and mechanical properties exhibit potential applications in emerging fields, such as flexible electronics, energy devices, and water treatment, etc.
The pure nanocellulose films made of nanocellulose like CNFs present excellent mechanical properties. This nanopaper with a tensile strength of 100-300 MPa and a modulus of 5-30 GPa is much stronger than the widely used printing paper and packaging paper (Zhu et al. 2014). To further improve the mechanical strength of nanocellulose films, lots of researchers prepared the films made of cellulose nanofibres with preferred alignment rather than random distribution (Zhu et al. 2017, Wang et al. 2018, Tang et al. 2015, Walther et al. 2011, Sehaqui et al. 2012). The as-fabricated nanocellulose film composed w'ith aligned BNCs exhibited extremely high strength (~1 GPa) and modulus (48.1 GPa) (Wang et al. 2018).
FIGURE 3.4 Schematic illustration of the nanocellulose-based films with excellent properties for various applications (Fang et al. 2019.)
The well-developed methods to prepare these anisotropic nanocellulose films include wet stretching (Tang et al. 2015), wet extrusion (Walther et al. 2011), and cold drawing (Sehaqui et al. 2012), etc. However, these down-top technologies have relatively high cost and complexity, which have limited the scalable production of the strong nanocellulose films. Recently, the top-down approach, combined with delignification and hot-press, to prepare wood-derived films with aligned cellulose nanofibres was developed by Hu group. This method is cost-effective, facile, and efficient, but the obtained nanocellulose films were a little weaker than the aforementioned films made by down-top technologies, which may have contributed to the degradation of cellulose during the delignification process (Fang et al. 2019). The nanocellulose films with excellent mechanical properties including high strength and toughness have been used as the substrate for various flexible electronics, such as transistors (Park et al. 2018, Huang et al. 2013), solar cells (Hu et al. 2013, Nogi et al. 2015), and organic light-emitting diodes (OLEDs) (Ummartyotin et al. 2012, Purandare et al. 2014), etc.
Unlike the common porous paper with ultralow transparency, the dense nanocellulose film always exhibited great optical properties, i.e., high transparency and/or high haze. The nanostructured cellulose fibres like CNFs have the diameter and interstice much smaller than the visible wavelength, w'hich would cause less light scattering compared with common paper. Furthermore, most of the light diffusions are forward scattering rather than back scattering. Therefore, the as-prepared nanocellulose films have both high transparency and high haze. As light-management layers, these nanopapers could significantly improve the working efficiency of solar cells (with an enhancement of 10.1-23.9%) by facile lamination (Jia et al. 2017, Fang et al. 2014. Ha et al. 2014). Another important nanocellulose films with advanced optical properties are CNC-based photonic films with structural colours. Compared to pigmentary colour, the structural colour generated from light interference is much more stable under cyclic stimulation. The prepared CNCs by acid hydrolysis are promising chiral nematic crystals for photonic films for various applications (Fernandes et al. 2017, Yao et al. 2017, Guidetti et al. 2016). For example, the self- assembled CNC films with tunable photonic properties and barrier capabilities can be utilized in the fields of sustainable consumer packaging products, as well as effective templates for photonic and optoelectronic materials and structures (Guidetti et al. 2016).
The nanocellulose-based composite films combined with functional materials, such as metal nanoparticles, advanced carbon materials, and conductive polymers, have also been extensively developed for applications in energy-storage devices and flexible electronics, etc. As can be seen in Figure 3.5a, a freestanding conductive film is fabricated by polypyrrole (PPy)/BNCs combined with graphene (RGO) through insitu polymerization and filtration (Ma et al. 2016). Based on this film electrode, the assembled flexible supercapacitor exhibited excellent performances, including high areal capacitance of 1.67 F cm-2, high areal energy density of 0.23 mWh cnr2, and a maximum power density of 23.5 mW cm-2. Besides, through vacuum filtration, an ultrathin and flexible MXene/CNFs composite film with a nacre-like lamellar structure has also been fabricated (Figure 3.5b) (Cao et al. 2018). This nanocellulose- based composite film exhibited high electrical conductivity (up to 739.4 S nr1) and excellent specific electromagnetic interference (EMI), shielding efficiency (up to 2647 dB cm2 g~‘), demonstrating its potential applications in various fields such as flexible electronics and smart robots.