During the past few decades, researchers have found that preparing some natural polymers into nanomaterials can give them new properties (Yang et al. 2020). The nanocellulose obtained via chemical or mechanical treatment can be divided into two categories: cellulose nanocrystals and cellulose microfibrils, also called cellulose nanofibrils (Hoeng et al. 2016). In addition to the natural advantages of abundant reserves and renewable recycling, nanocellulose has the advantages of fine nanostructure, good mechanical strength and low thermal expansion coefficient (Hu et al. 2016; Nogi 2013). Film material composed of nanocellulose fibrils has the advantages of fast ion diffusion and high temperature resistance. In addition, nanocellulose has a three-dimensional network porous structure, which can be combined with inorganic nanoparticles (Hokkanen et al. 2018), metal ions and their oxides (Alipour et al. 2020; Chang et al. 2020; Choi and Jeong 2019), carbon materials (Xing et al. 2019; Xu et al. 2020), and conductive polymers (Lay et al. 2016) to prepare multifunctional composite materials. It has been reported that nanocellulose films with high light transmittance and high haze have attracted increasing attention in the preparation of flexible electronic devices.
Zhu et al. (Zhu et al. 2013) focused on adjusting the diameter of the nanocellulose fibers (NFCs) and the bulk density of the nanocellulose film to obtain high light transmittance and high haze. They used TEMPO oxidant to treat cellulose and obtained nanocellulose films with various fiber diameter, and the density of the nanocellulose film was controlled by pressure and drying temperature. Finally, they prepared nanocellulose films with fiber diameter of 25 µm, 50 nm, and 10 nm, and nanocellulose films with thicknesses of 52 µm, 40 µm, and 36 µm and fiber diameter of 10 nm. The film made from 25 µm cellulose fibers has a lower specular transmittance (37%) and a lower diffuse transmittance (5%) at a wavelength of 550 nm. For the nanocellulose film made from 50 nm NFCs, the diffuse transmittance is 92% and the specular transmittance is 19%. The cellulose film made from 10 nm NFCs has a diffuse transmittance of 93% and a specular transmittance of 71%. For fibers with diameter of 50 nm and 10 nm, the optical transmittance can reach 92–93%. The haze of nanocellulose films with a fiber diameter of 25 µm, 50 nm and 10 nm at 550 nm is about 77%, 49% and 20%, respectively. It can be seen that they have higher light transmittance than plastics as well as extremely high haze, which makes them applicable in many fields. In addition to fiber diameter, the bulk density also affects its optical properties. When the total amount of cellulose is the same, as the bulk density increases, both the specular reflectance and the diffuse transmittance increase. With the bulk density increasing, the effective index of the cellulose film increases and the porosity of the paper decreases, which reduces scattering and increases light transmittance. The decrease in scattering results in the increase in transmittance and the decrease in haze value. Therefore, changing the bulk density can also adjust the optical properties of the nanocellulose film. These special optical properties of nanocellulose films provide unexampled possibilities for the development of green optoelectronic devices.
The optical properties of the nanocellulose film can be tailored by varying the diameter of the fibers and the bulk density of the film. Furthermore, adding other substances to the cellulose can also obtain materials with high light transmittance and high haze.
Fang et al. (Fang et al. 2014) investigated a facile method to regulate the optical properties of paper. The paper was prepared by vacuum filtration, and the haze could be adjusted from 18–60% by adjusting the weight ratio of the TEMPO-oxidied wood fiber to the NFCs, while retaining its transmittance over 90%. The total light transmittance of transparent films with different contents of NFCs exceeds 90%. In addition to excellent optical transparency, the transmission haze of transparent paper substrates in the visible wavelength range has also increased from 18–60%. Because the micron-sized TEMPO oxidized fibers have a wide range of light diffusion characteristics, the transparent paper without NFCs has the highest transmission haze (60%). With the NFCs content in the paper increasing, the haze gradually decreases. Such cellulose films with adjustable optical properties have great application potential. They can be widely used in light diffusion films and photovoltaic devices, such as outdoor displays and solar cells.
It can be seen that mixing TEMPO oxidized cellulose with NFCs can get a high light transmittance and high haze film, and the haze of the film can be adjusted by controlling the number of NFCs while maintaining a high light transmittance. Moreover, mixing spherical cellulose nanoparticles with NFCs can also produce films with the similar properties. Li et al. (Li et al. 2019) prepared a series of CNPs (cellulose nanoparticles) with various morphologies, such as fibrous (CNFs), short rods (CNCs-1), rods (CNCs-3) and spherical (CNCs-2 and CNCS-4). Cellulose nanocellulose films were prepared using CNFs to achieve an anti-trade-off between transparency (89% at 600 nm) and haze (77%) performance. Meanwhile, by mixing CNFs with CNCs-2 and adjusting their ratio, the optical haze of the film can be accurately controlled to any continuous adjustable value in the range of 5–77%. The nanocellulose film prepared in this way achieves both high transparency and high haze. Figure 1 shows the optical properties of nanopapers containing cellulose nanoparticles with different morphologies. The CNFs and CNCs-2 films show the highest and lowest optical haze, respectively. There are many voids and laminar gaps in the CNF cellulose film which can cause large light scattering. However, the size of CNCs-2 is small, it can fill these gaps to reduce light scattering. Therefore, by a simple mixing CNFs with CNCs-2, a nanocellulose film with continuously adjustable haze can be obtained. As the CNF/CNCs-2 molar ratio gradually increases from 1:4 to 4:1, the optical haze value increases from 23–60%. Therefore, the haze of the film can be easily adjusted in a range of 77–11% with this method while maintaining a high transparency. These nanocellulose films with high light transmittance and adjustable haze can be widely used in light diffusers and flexible optoelectronic fields.
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Direct mixing the solutions or suspensions of two substances would affects the uniformity and optical properties of the resulting film. A simple, effective and low-cost method to prepare a highly transparent and hazy film was reported by Yang et al. (Yang et al. 2019) by coating TEMPO-oxidized wood fiber suspension onto a TEMPO-oxidized cellulose nanofibril gel. The film exhibits up to 85% light transmission and 62% haze, while showing high tensile strength and excellent thermal stability. The microstructures of the two sides of the highly hazy transparent films were different: one was smooth, while the other was rough. As a reference, they directly mixed TEMPO-oxidized wood fibers and cellulose nanofibers to prepare a film sample. The final results show that the film prepared by the "coating" method presents more excellent optical properties than the film prepared by the "blending" method. This work provides a method for preparing green optoelectronic devices. In addition, ultrasonic treatment of TEMPO oxidized cellulose fibers (Lin et al. 2019) or surface modification of cellulose with hydrocarbons/dimethylformamide (Yu et al. 2018) can also increase light transmittance and haze.
Yang et al. (Yang et al. 2018) used a simple deposition process of micro-scale oxidized wood fibers to adjust the surface morphology of the cellulose nanofiber film. As the upper surface roughness increases, the haze of the film increases. The total transmittance of the obtained film is in the range of 83–88%, showing a low haze of 3.8% to a high haze of 62.3%. In addition, the lower surface of the cellulose nanofiber film is very smooth, which meets the requirement for electronic and optoelectronic applications.
The choice of raw materials may also affect the haze of the nanocellulose film. Zhou et al. (Zhou et al. 2018) systematically studied the optical properties of highly transparent paper made of micro-cellulose fibers with different fiber morphologies. They used the TEMPO-oxidation system to process different fibers, including Northwood fibers, Eucalyptus wood fibers and Manila hemp fibers. The fiber morphology before and after TEMPO oxidation was analyzed in detail. Film made of Northwood fibers, Eucalyptus wood fibers and Manila hemp fibers has a haze of 84%, 81% and 78%, respectively, at a wavelength of 550 nm. By analyzing the nanocellulose films prepared from different types of cellulose, it is found that the fiber diameter has an important influence on the transmission haze. The diameter of Northwood fibers, Eucalyptus wood fibers and Manila hemp fibers treated by TEMPO oxidation system is 45, 26 and 20 nm, and the fiber length is 0.62, 0.57 and 0.52 mm, respectively. It can be seen that as the average length and width of cellulose fibers increase, their haze increases accordingly. Meanwhile, the preparation method also has an impact on the optical properties of the transparent paper. They found that the haze of transparent film prepared by solution casting method is lower than the transparent film prepared by vacuum filtration method. Since the rougher surface causes stronger light scattering, the transparent film prepared by vacuum filtration presents higher haze. This research provides critical information for the future preparation of high light transmittance and high haze materials for electronic devices.
It is seen that nanocellulose films can be a good candidate as high transparent and high haze materials. One generally uses certain chemical or physical methods to disintegrate cellulose into a suitable size, and then vacuum filters the raw material suspension or casts it to prepare cellulose film with high haze and high light transmittance. Since it is difficult to prepare nano-level nanocellulose fibrils from direct high-pressure homogenization due to the strong intramolecular and intermolecular interaction, chemical treatment is generally required to reduce the hydrogen bond density before disintegrating. TEMPO is usually used to oxidize cellulose before the cellulose disintegrating Usually NFCs of different sizes need to be sieved. In this way one can finally obtain the film with properties of high light transmission and high haze.