Yixing Liu

Ultralight and highly flexible aerogels with long cellulose I nanofibers

Ultralight and highly flexible aerogels with long cellulose I nanofibers (CNFs) were produced viapurification and defibrillation of wood fibers into CNF hydrogels, followed by freeze-drying. The aerogels had a web-like entangled structure, low density, and high water uptake capability. With the increase of CNF content in hydrogels from 0.1 to 1.5%, the bulk density of the aerogels increased from 1.3 × 10−3 to 17.0 × 10−3 g cm−3, whereas the water uptake ratio (WUR) decreased from 155 to 54. The microstructure of the aerogels can be transformed from open 3D porous nanofibrillar network to 2D sheet-like skeletons by adjusting the CNF content of the hydrogels. A possible assembly mechanism was proposed based on this transformation. When the transparent supernatant fraction, which has ∼0.018% solid content obtained viacentrifugation of the hydrogels, was subjected to freeze-drying, ultra-low density aerogels (0.2 × 10−3 g cm−3) consisting of nanofibers with lengths above 1 mm and a width range of 40–180 nm, were successfully produced due to the self-assembled of the tiny CNFs and their bundles along the longitudinal direction.

Comparative Study of Aerogels Obtained from Differently Prepared Nanocellulose Fibers

This article describes the fabrication of nanocellulose fibers (NCFs) with different morphologies and surface properties from biomass resources as well as their self-aggregation into lightweight aerogels. By carefully modulating the nanofibrillation process, four types of NCFs could be readily fabricated, including long aggregated nanofiber bundles, long individualized nanofibers with surface C6 -carboxylate groups, short aggregated nanofibers, and short individualized nanofibers with surface sulfate groups. Free-standing lightweight aerogels were obtained from the corresponding aqueous NCF suspensions through freeze-drying. The structure of the aerogels could be controlled by manipulating the type of NCFs and the concentration of their suspensions. A possible mechanism for the self-aggregation of NCFs into two- or three-dimensional aerogel nanostructures was further proposed. Owing to web-like structure, high porosity, and high surface reactivity, the NCF aerogels exhibited high mechanical flexibility and ductility, and excellent properties for water uptake, removal of dye pollutants, and the use as thermal insulation materials. The aerogels also displayed sound-adsorption capability at high frequencies.

Fabrication and characterisation of α-chitin nanofibers and highly transparent chitin films by pulsed ultrasonication.

α-Chitin nanofibers were fabricated with dried shrimp shells via a simple high-intensity ultrasonic treatment under neutral conditions (60 KHz, 300 W, pH=7). The diameter of the obtained chitin nanofibers could be controlled within 20-200 nm by simply adjusting the ultrasonication time. The pulsed ultrasound disassembled natural chitin into high-aspect-ratio nanofibers with a uniform width (19.4 nm after 30 min sonication). The EDS, FTIR, and XRD characterisation results verified that α-chitin crystalline structure and molecular structure were maintained after the chemical purification and ultrasonic treatments. Interestingly, ultrasonication can slightly increase the degree of crystallinity of chitin (from 60.1 to 65.8). Furthermore, highly transparent chitin films (the transmittance was 90.2% at a 600 nm) and flexible ultralight chitin foams were prepared from chitin nanofiber hydrogels.

Concentration effects on the isolation and dynamic rheological behavior of cellulose nanofibers via ultrasonic processing

Chemical pretreatment combined with high-intensity ultrasonication was performed to disintegrate cellulose nanofibers from poplar wood powders. The cellulose content in each suspension was treated as the control variable because the suspension concentration significantly influences the properties of the resultant cellulose nanofibers via ultrasonic processing. The as-obtained cellulose nanofibers were characterized by fiber diameter distribution, crystal structure, and rheological analysis. An increase of not more than 1.2 % of the cellulose content resulted in finer nanofibers. Both storage modulus and loss modulus of cellulose nanofiber suspensions rapidly increased with increasing concentration because of the gradual formation of a stronger network structure. In addition, the dynamic mechanical behavior of suspensions with fiber contents lower than 0.8 % was affected by the frequency and temperature alteration in contrast with the suspension with higher fiber contents. The sol–gel transformation and the visco-elastic transition depend on the hydroxyl bonding and the cross-linking extent of cellulose nanofibers in various concentration environments.

Improvement of water resistance and dimensional stability of wood through titanium dioxide coating.

Abstract Moisture absorption and dimensional distortion are the major drawbacks of wood utilization as building material. In this study, poplar wood coated with a thin layer of titanium dioxide (TiO2) was prepared by the cosolvent-controlled hydrothermal method. Subsequently, its moisture absorption and dimensional stability were examined. Scanning electron microscopy analysis revealed that the wood substrate was closely and entirely covered with the TiO2 coat. Microscale features were visible despite masking of the ultrastructural features of the cell wall. To explore the effects of TiO2 coating on the water-repellency and dimensional stability of wood, a 90-day water immersion test was carried out. Results showed that water absorption and thickness swelling of TiO2-coated wood increase very slowly and minimally. Weight change after 90 days of water immersion was reduced to 20.5%, nearly one-tenth of untreated control wood, and that maximum cross-sectional relative swelling was only 1.2%. Specimens were conditioned for 3 months at 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% relative humidity (RH) to determine the effects of RH on moisture absorption and dimensional swelling of TiO2-coated wood. There was no change in weight after 3 months of being exposed to humidity conditions below 60%, whereas there was linear weight increase above 60% RH, but the maximum change was less than 6%. Cross-sectional relative swelling was less than 0.3% below 60% RH but increased as RH exceeded 60%. The maximum change was approximately 3%. Anisotropic thickness swelling of wood was almost eliminated after coating. It is obvious that TiO2 coating can act as a moisture barrier for wood and is an exceptionally strong water vapor-inhibiting shield under very humid conditions.

Improvement of dimensional stability of wood via combination treatment: swelling with maleic anhydride and grafting with glycidyl methacrylate and methyl methacrylate

Abstract A novel two-step combined treatment of poplar wood was developed to improve its dimensional stability. Maleic anhydride (MAN) was first employed to swell and bond to the wood cell wall, and then mixed monomers of glycidyl methacrylate/methyl methacrylate (GMA/MMA) were grafted to the cell wall through the chemical reaction with MAN within the wood cell lumen. The results of scanning electron microscopy and energy dispersive X-ray apparatus (SEM-EDX) and Fourier transform infrared spectroscopy (FTIR) analyses indicate that MAN penetrated and chemically bonded to the cell wall causing 9% volume swelling, and the copolymer from GMA/MMA monomers was grafted onto the wood cell wall, resulting in the improved interfacial compatibility between the polymer and wood matrix. The dimensional stability of poplar wood modified by the combined two-step treatment was remarkably improved compared with that of untreated poplar wood. The combination treatment of wood employed in this study proved to be more effective for improving the dimensional stability than treatment with PEG-1000 aqueous solution with 30% concentration.

Facile extraction of cellulose nanocrystals from wood using ethanol and peroxide solvothermal pretreatment followed by ultrasonic nanofibrillation

Cellulose nanocrystals (CNCs) were successfully extracted from wood flour by a two-step process that comprised ethanol and peroxide solvothermal pretreatment and an ultrasonic disintegration process. Characterization results showed that 97% of the total lignin and 70% of the hemicellulose could be fractionated in a single ethanosolv pretreatment step. Additional treatment with alkaline hydrogen peroxide removed the residual lignin and hemicellulose and resulted in high purity cellulose. The CNCs obtained after ultrasonication displayed a similar yield, size, morphology, and crystallinity but had better thermal stability and film forming properties than those produced by concentrated acid hydrolysis. Overall, the solvothermal treatment using ethanol and its combination with peroxide is an ideal substitute method for pretreatment of lignocellulose. Further integration of such pretreatments with ultrasonication provides a promising efficient process with low environmental impact for production of CNCs.

Highly Flexible and Conductive Cellulose-Mediated PEDOT:PSS/MWCNT Composite Films for Supercapacitor Electrodes

Recent improvements in flexible electronics have increased the need to develop flexible and lightweight power sources. However, current flexible electrodes are limited by low capacitance, poor mechanical properties, and lack of cycling stability. In this article, we describe an ionic liquid-processed supramolecular assembly of cellulose and 3,4-ethylenedioxythiophene for the formation of a flexible and conductive cellulose/poly(3,4-ethylenedioxythiophene) PEDOT:poly(styrene sulfonate) (PSS) composite matrix. On this base, multiwalled carbon nanotubes (MWCNTs) were incorporated into the matrix to fabricate an MWCNT-reinforced cellulose/PEDOT:PSS film (MCPP), which exhibited favorable flexibility and conductivity. The MCPP-based electrode displayed comprehensively excellent electrochemical properties, such as a low resistance of 0.45 Ω, a high specific capacitance of 485 F g-1 at 1 A g-1, and good cycling stability, with a capacity retention of 95% after 2000 cycles at 2 A g-1. An MCPP-based symmetric solid-state supercapacitor with Ni foam as the current collector and PVA/KOH gel as the electrolyte exhibited a specific capacitance of 380 F g-1 at 0.25 A g-1 and achieved a maximum energy density of 13.2 Wh kg-1 (0.25 A g-1) with a power density of 0.126 kW kg-1 or an energy density of 4.86 Wh kg-1 at 10 A g-1, corresponding to a high power density of 4.99 kW kg-1. Another kind of MCPP-based solid-state supercapacitor without the Ni foam showed excellent flexibility and a high volumetric capacitance of 50.4 F cm-3 at 0.05 A cm-3. Both the electrodes and the supercapacitors were environmentally stable and could be operated under remarkable deformation or high temperature without damage to their structural integrity or a significant decrease in capacitive performance. Overall, this work provides a strategy for the fabrication of flexible and conductive energy-storage films with ionic liquid-processed cellulose as a medium.

Revealing the structures of cellulose nanofiber bundles obtained by mechanical nanofibrillation via TEM observation.

To reveal the structures of cellulose naonfiber bundles extracted from lignocellulosic pulps and prepared by mechanical nanofibrillation methods, the samples were systematically investigated by transmission electron microscopy (TEM) observation. First, high magnification and high resolution TEM images were obtained starting from one end of the bundles. The imaging position was then carefully shifted along the length of the bundles until the other end was reached. Finally, a series of TEM images were integrated through image processing and analyzed. The cellulose nanofiber bundles displayed ribbon-like structures, which were organized with parallel aligned cellulose nanofibers 2-5 nm in width. The length of the bundles was >11 μm. The bundles were interconnected with other nanofibers and nanofiber bundles, forming entangled, web-like networks in suspension. Evidence demonstrating the existence of twisted bundle morphologies was also presented.

Efficient Flame-Retardant and Smoke-Suppression Properties of Mg–Al-Layered Double-Hydroxide Nanostructures on Wood Substrate

Improving the flame retardancy of wood is an imperative yet highly challenging step in the application of wood in densely populated spaces. In this study, Mg-Al-layered double-hydroxide (LDH) coating was successfully fabricated on a wood substrate to confer flame-retardant and smoke-suppression properties. The chemical compositions and bonding states characterized by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed the coating constituents of Mg-Al LDH. The coating evenly covered the sample wood surfaces and provided both mechanical enhancement and flame-retardancy effects. The limiting oxygen index of the Mg-Al LDH-coated wood increased to 39.1% from 18.9% in the untreated wood. CONE calorimetry testing revealed a 58% reduction in total smoke production and a 41% reduction in maximum smoke production ratio in the Mg-Al LDH-coated wood compared to the untreated wood; the peak heat release rate and total heat release were also reduced by 49% and 40%, respectively. The Mg-Al LDH coating is essentially hydrophilic, but simple surface modification by fluoroalkyl silane could make it superhydrophobic, with a water contact angle of 152° and a sliding angle of 8.6°. The results of this study altogether suggest that Mg-Al LDH coating is a feasible and highly effective approach to nanoconstructing wood materials with favorable flame-retardant and smoke-suppression properties.