Transparent Cellulose Nanopaper Research Articles

High-Speed Fabrication of Clear Transparent Cellulose Nanopaper by Applying Humidity-Controlled Multi-Stage Drying Method.

As a renewable nanomaterial, transparent nanopaper is one of the promising materials for electronic devices. Although conventional evaporation drying method endows nanopaper with superior optical properties, the long fabrication time limits its widely use. In this work, we propose a multi-stage drying method to achieve high-speed fabrication of clear transparent nanopaper. Drying experiments reveal that nanopaper’s drying process can be separated into two periods. For the conventional single-stage evaporation drying, the drying condition is kept the same. In our newly proposed multi-stage drying, the relative humidity (RH), which is the key parameter for both drying time and haze, is set differently during these two periods. Applying this method in a humidity-controllable environmental chamber, the drying time can be shortened by 35% (from 11.7 h to 7.6 h) while maintaining the same haze level as that from single-stage drying. For a conventional humidity-uncontrollable oven, a special air flow system is added. The air flow system enables decrease of RH by removing water vapor at the water/air interface during the earlier period, thus fabricating clear transparent nanopaper in a relatively short time. Therefore, this humidity-controlled multi-stage drying method will help reduce the manufacturing time and encourage the widespread use of future nanopaper-based flexible electronics.

Sustainable valorization of paper mill sludge into cellulose nanofibrils and cellulose nanopaper

As a kind of agro-industrial wastes, paper mill sludge (PMS) has posed serious environmental and economic challenges for disposal due to the more stringent regulations and diminishing land availability in recent years. The present study is aimed at providing a sustainable approach to efficiently convert PMS to cellulose nanofibrils (CNFs) and cellulose nanopaper (CNP) by formic acid (FA) hydrolysis pretreatment and the followed microfluidization. It is found that FA hydrolysis (4–6 h) could swell and shorten PMS fibers, and only two-pass microfluidization is sufficient to get uniform CNFs from the collected cellulose residual. Results indicate that the obtained CNFs show high thermal stability and crystallinity index, surface functionality (ester groups), as well as a high yield of over 75 wt.%. Notably, more than 90 % FA can be recovered and the hydrolyzed sugars could be potentially used to produce platform chemicals (e.g. lactic acid, furfural). Finally, transparent CNP is prepared from the CNFs suspension via a simple vacuum filtration technique. The resultant CNP shows good mechanical properties with the maximum tensile strength and toughness of 106.4 MPa and 6.62 MJ/m3, respectively. Therefore, the current work provides a green and sustainable method to valorize PMS for the production of valuable CNFs and CNP.

Clear transparent cellulose nanopaper prepared from a concentrated dispersion by high-humidity drying.

Optically transparent cellulose nanopaper is a promising candidate for flexible device substrates because of its light weight, surface smoothness, and high dimensional stability with respect to temperature. Conventionally, clear transparent nanopaper has been fabricated from cellulose nanofiber dispersions with quite low concentration: less than 0.5 wt%. However, this diluteness leads to several problems, such as huge energy consumption and long operation time for drying. Therefore, nanopaper should be fabricated from a concentrated dispersion to mitigate these problems. In this study, transparent nanopaper was fabricated from cellulose nanofiber dispersions with various concentrations (0.24–1.81 wt%). Optical experiments revealed that the haze of the transparent nanopaper increased monotonically with cellulose nanofiber dispersion concentration, when the cellulose nanofiber dispersion was prepared from holocellulose pulp and conventional over-drying was applied. Based on our insight into the origin of this increase in the haze of transparent nanopaper, we developed high-humidity drying, which successfully produced clear transparent nanopaper from a concentrated dispersion without prolonged drying time.

Deposition of amorphous carbon nitride films on flexible substrates by reactive sputtering for applications in light-driven active devices

Amorphous carbon nitride (a-CNx) thin films deposited by reactive sputtering have great potential for driving source applications of light-driven active devices. We demonstrate, for the first time, the photoinduced deformation of a-CNx deposited on flexible substrates, namely, poly(ethylene naphthalate) (PEN) films and transparent cellulose nanopaper. a-CNx films without delamination were obtained on both substrates. By decreasing the thickness of PEN films, the photoinduced deformation became extremely large. A light-driven pump was fabricated using a-CNx-coated PEN films, and then the pumping motion was observed up to 10 Hz. When a He–Ne laser traced the surface of a-CNx films deposited on the nanopaper, the sample moved to the opposite side of the laser spot. The motion involved repeated expansions and contractions similar to the motion of caterpillars occurring owing to the temporary photoinduced deformation of a-CNx films.

Acetylation of optically transparent cellulose nanopaper for high thermal and moisture resistance in a flexible device substrate

Optically transparent cellulose nanopaper is a promising candidate for flexible device substrates because it is lightweight, has a smooth surface and high dimensional stability against temperature. However, the moisture and thermal resistance should be improved, while maintaining high transparency for application. In a transparent polymer composite reinforced with cellulose nanofibers, these properties were improved by cellulose acetylation, which were fabricated by starting from acetylated pulps or rinsing acetylated nanofiber film with water. However, these acetylation procedures reduce the transparency of cellulose nanopaper because the hydrophobic nature of acetyl groups produce inhomogeneous aggregations of cellulose nanofibers, which cause light scattering. Therefore, we propose a new procedure of acetylated cellulose nanopaper by rinsing with low polarity solvents such as toluene. The acetylated cellulose nanopaper obtained shows improved moisture and thermal resistance, and maintained optical transparency. The moisture and thermal resistance increase with the degree of acetyl group substitutions. The total transmittance, haze, crystallinity, coefficient of thermal expansions, dielectric constant, and dielectric loss are constant for varying DSs. These findings promote the application of transparent cellulose nanopaper in flexible device substrates.

Hazy Transparent Cellulose Nanopaper

The aim of this study is to clarify light scattering mechanism of hazy transparent cellulose nanopaper. Clear optical transparent nanopaper consists of 3–15 nm wide cellulose nanofibers, which are obtained by the full nanofibrillation of pulp fibers. At the clear transparent nanopaper with 40 μm thickness, their total transmittance are 89.3–91.5% and haze values are 4.9–11.7%. When the pulp fibers are subjected to weak nanofibrillation, hazy transparent nanopapers are obtained. The hazy transparent nanopaper consists of cellulose nanofibers and some microsized cellulose fibers. At the hazy transparent nanopaper with 40 μm thickness, their total transmittance were constant at 88.6–92.1% but their haze value were 27.3–86.7%. Cellulose nanofibers are solid cylinders, whereas the pulp fibers are hollow cylinders. The hollow shape is retained in the microsized cellulose fibers, but they are compressed flat inside the nanopaper. This compressed cavity causes light scattering by the refractive index difference between air and cellulose. As a result, the nanopaper shows a hazy transparent appearance and exhibits a high thermal durability (295–305 °C), and low thermal expansion (8.5–10.6 ppm/K) because of their high density (1.29–1.55 g/cm3) and crystallinity (73–80%).

Natural Materials Assembled, Biodegradable, and Transparent Paper-Based Electret Nanogenerator.

Developing eco-friendly and low-cost electronics is an effective strategy to address the electronic waste issue. In this study, transparent cellulose nanopaper (T-paper) and polylactic acid (PLA) electret were used to construct a biodegradable and transparent paper-based electret nanogenerator. The nanogenerator could be assembled with paper products to form a self-powered smart packaging system without impairing the appearance, due to the high transparency and desirable output performance. Furthermore, the self-degradation property in the natural soil of the nanogenerator is demonstrated, indicating that the nanogenerator is recycled and will not pollute the environment. We anticipate that this study will provide new insights to develop eco-friendly power source and paper-based electronics.

Chemical Modification of Cellulose Nanofibers for the Production of Highly Thermal Resistant and Optically Transparent Nanopaper for Paper Devices.

Optically transparent cellulose nanopaper is one of the best candidate substrates for flexible electronics. Some types of cellulose nanopaper are made of mechanically or chemically modified cellulose nanofibers. Among these, nanopapers produced from chemically modified cellulose nanofibers are the most promising substrate because of their lower power consumption during fabrication and higher optical transparency (lower haze). However, because their thermal durability is as low as plastics, paper devices using chemically modified nanopaper often do not have sufficiently high performance. In this study, by decreasing the carboxylate content in the cellulose nanofibers, the thermal durability of chemically modified nanopaper was drastically improved while maintaining high optical transparency, low coefficient of thermal expansion, and low power consumption during fabrication. As a result, light-emitting diode lights illuminated on the chemically modified nanopaper via highly conductive lines, which were obtained by printing silver nanoparticle inks and high-temperature heating.

Fast Preparation Procedure for Large, Flat Cellulose and Cellulose/Inorganic Nanopaper Structures

Nanostructured materials are difficult to prepare rapidly and as large structures. The present study is thus significant because a rapid preparation procedure for large, flat, smooth, and optically transparent cellulose nanopaper structures is developed using a semiautomatic sheet former. Cellulose/inorganic hybrid nanopaper is also produced. The preparation procedure is compared with other approaches, and the nanopaper structures are tested in uniaxial tensile tests. Optical transparency and high tensile strength are demonstrated in 200 mm diameter nanopaper sheets, indicating well-dispersed nanofibrils. The preparation time is 1 h for a typical nanopaper thickness of 60 μm. In addition, the application of the nanopaper-making strategy to cellulose/inorganic hybrids demonstrates the potential for "green" processing of new types of nanostructured functional materials.