Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis

The conditions required for the accurate measurement of the sulfur content of cellulose nanocrystals (CNCs) by conductometric titration are discussed. CNCs from sulfuric acid hydrolysis are electrostatically stabilized in aqueous suspension due to the introduction of charged sulfate ester groups onto the surface of the crystallites during reaction. The sulfur content thus largely reflects the surface charge of the crystals, and is crucial to the characterization and understanding of material properties. Conductometric titration is commonly used to quantify the sulfur content of CNCs, however, the exhaustive removal of free acid by dialysis and the necessity, type, quantity and duration of ion-exchange resin treatments are not always consistent. Here we explore the standard conditions of dialysis, ion-exchange, and the reproducibility of titration results. Extensive dialysis is found to be effective in the removal of free acid, but similar results are also achieved in shorter times and with less water using membrane ultrafiltration. It is also shown that the conditions of ion-exchange most commonly employed in the literature can lead to inaccurate sulfur contents. Finally, good agreement is obtained between the sulfur contents of different CNC batches prepared using the same hydrolysis conditions, and from titration and elemental analysis when thoroughly purified, well-dispersed samples, and appropriate resin conditions are used.     

Optimization of the isolation of nanocrystals from microcrystalline cellulose by acid hydrolysis

The objective of this work was to find a rapid, high-yield process to obtain an aqueous stable colloid suspension of cellulose nanocrystals/whiskers. Large quantities are required since these whiskers are designed to be extruded into polymers in the production of nano-biocomposites. Microcrystalline cellulose (MCC), derived from Norway spruce (Picea abies), was used as the starting material. The processing parameters have been optimized by using response surface methodology. The factors that varied during the process were the concentration of MCC and sulfuric acid, the hydrolysis time and temperature, and the ultrasonic treatment time. Responses measured were the median size of the cellulose particles/whiskers and yield. The surface charge as calculated from conductometric titration, microscopic examinations (optical and transmission electron microscopy), and observation of birefringence were also investigated in order to determine the outcome (efficiency) of the process. With a sulfuric acid concentration of 63.5% (w/w), it was possible to obtain cellulose nanocrystals/whiskers with a length between 200 and 400 nm and a width less than 10 nm in approximately 2 h with a yield of 30% (of initial weight).     

Effect of microfluidizing cycles after citric acid hydrolysis on the production yield and aspect ratio of cellulose nanocrystals

Cellulose - In recent years, a new strategy of using citric acid (CA) to produce carboxylic cellulose nanocrystals (CNCs) has received attractive interest. However, mainly due to the weak acidity...     

Effect of fiber drying on properties of lignin containing cellulose nanocrystals and nanofibrils produced through maleic acid hydrolysis

The effect of fiber drying on the properties of lignin containing cellulose nanocrystals (LCNC) and nanofibrils (LCNF) produced using concentrated maleic acid hydrolysis of a never dried unbleached mixed hardwood kraft pulp was evaluated. Two drying conditions, i.e., air drying and heat drying at 105 °C were employed. It was found that drying (both air and heat) enhanced acid hydrolysis to result in slightly improved LCNC yields and less entangled LCNF. This is perhaps due to the fact that drying modified the cellulose supermolecular structure to become more susceptible to acid hydrolysis and the enhanced hydrolysis severity at the fiber surface when using dried fibers. Drying substantially improved LCNC crystallinity and LCNF suspension viscoelastic behavior. The present study quantitatively elucidated the effect of pulp drying (either air or heat) on producing cellulose nanomaterials and has practical importance because commercial market pulp (heat dried) is most likely to be used commercially.     

Effect of pyran ring on acid hydrolysis of cellulose

Summary An effect of the pyran ring and its opening on the chemical behavior of the acetal bond was discovered.     

Octadecylamine as chemical modifier for tuned hydrophobicity of surface modified cellulose: toward organophilic cellulose nanocrystals

Cellulose - A novel, environmentally friendly and simple method for chemical functionalization of microcrystalline cellulose (MCC) to produce organophilic cellulose nanocrystals (CNC-ODA) is herein...     

Aligned cellulose nanocrystals and directed nanoscale deposition of colloidal spheres

Cellulose nanocrystals are aligned in wrinkled polydimethylsiloxane templates and transferred to polyethyleneimine-coated silica surfaces in a printing process similar to microcontact printing. The highly aligned nanorods were deposited onto the surfaces with a line-to-line distance of 225–600 nm without loss of alignment. It was also possible to repeat the transfer process on the same surface at a 90-degree angle to create a network structure. This demonstrates the versatility of the technique and creates more options for advanced multilayering of materials. To demonstrate that the surface properties of the anionic cellulose nanorods were unaffected by the transfer process and to prove the concept of functionalizing transferred particles, cationic latex particles were electrostatically self-assembled onto the cellulose nanorods. The directed deposition of these particles resulted in excellent site specificity and the highest resolution to date for controlled deposition of colloids on an electrostatically patterned surface.     

Cellulose nanocrystals obtained from microcrystalline cellulose by p-toluene sulfonic acid hydrolysis,NaOH and ethylenediamine treatment

Cellulose nanocrystals (CNCs) were first isolated from microcrystalline cellulose (MCC) by p-toluene sulfonic acid (p-TsOH) hydrolysis. Cellulose II nanocrystal (CNC II) and cellulose III nanocrystal (CNC III) were then formed by swelling the obtained cellulose I nanocrystal (CNC I) in concentrated sodium hydroxide solutions and ethylenediamine (EDA) respectively. The properties of CNC I, CNC II and CNC III were subjected to comprehensive characterization by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The results indicated that CNC I, CNC II and CNC III obtained in this research had high crystallinity index and good thermal stability. The degradation temperatures of the resulted CNC I, CNC II and CNC III were 300 °C, 275 °C and 242 °C, respectively. No ester bonds were found in the resulting CNCs. CNCs prepared in this research also had large aspect ratio and high negative zeta potential.

     

微波辅助的金属氯化物Lewis酸催化纤维素水解

研究了微波辐射下四种金属氯化物Lewis酸的催化纤维素酸水解反应性能,发现CuCl₂的催化性能最好。反应温度、反应时间、微波功率、催化剂用量和酸种类对纤维素水解转化率、葡萄糖和5-羟甲基糠醛(5-HMF)的选择性均有明显影响。与传统热反应相比,微波辐射明显加快纤维素酸水解速率,提高葡萄糖的选择性。0.5g纤维素和15g水,在微波功率800W,温度到达225℃时立即停止反应的条件下,当CuCl₂用量为0.05mmol时,纤维素转化率和葡萄糖选择性达72.6%和62.3%;当CuCl₂用量为0.15mmol时,5-HMF的选择性最高为13.2%;当CuCl₂用量为0.30mmol时,纤维素的转化率高达90.6%,但葡萄糖选择性只有6.7%。     

Understanding the effect of synthesis parameters on the catalytic ionic liquid hydrolysis process of cellulose nanocrystals

Conventional production of cellulose nanocrystals (CNC) normally encounters several problems including high energy consumption, toxicity and corrosion risk. Implementation of green solvent ionic liquid (IL) is an alternative to this conventional method. Production of CNC is known to be influenced by several factors such as reaction time and temperature. However, limited studies on the regeneration yield and other properties of CNC produced at both variables can be found. In this paper, CNC with desirable yield, crystallinity and particle size has been produced under catalytic hydrolysis process of IL. Two different parameters have been studied which are reaction temperature and time. It was found that, CNC with particle size of 9 nm and 73% crystallinity index has been produced at 30 min reaction time. Meanwhile, 100 °C reaction temperature manages to produce CNC with 90% yield and 76% crystallinity. In conclusion, reaction temperature and time affect the yield and thermal properties of hydrolysis process.     

TEMPO-mediated surface oxidation of cellulose nanocrystals (CNCs)

Cellulose nanocrystals were successfully oxidized with sodium hypochlorite using catalytic amounts of sodium bromide and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical at pH 10 in water. Carboxylate groups were selectively introduced at the surface of the crystals up to a total acid content of 1200 mmol kg^?1 without damaging the integrity of the crystals. The final acid content can easily be tuned by varying the amount of oxidant introduced. The effect of temperature, the quantity of oxidant and co-catalyst on the reaction kinetics were studied. Several methods were used for the characterization of the oxidized material like field emission scanning electron microscopy, diffuse reflectance infrared spectroscopy and thermogravimetric analysis.     

Mercerization and acid hydrolysis of bacterial cellulose

Structural changes in never- dried, disintegrated bacteria l cellulose by treatment with aqueous NaOH were examined by electron microscopy, X-ray diffractometry and acid hydrolysis behaviour and compared with those of cotton cellulose. The microfibril kept its fibrillar morphology after treatment with NaOH solutions of less than 9% (w/w), but changed into irregular aggregates when treated with NaOH above 12% (w/w), corresponding to the crystal conversion to cellulose II. The crystallinity of the resulting cellulose II was very low after a brief alkali treatment, but was increased significantly by elongated treatment (up to 10 days). In contrast, cotton cellulose was converted to cellulose II of fairly high crystallinity by alkali treatment of as little as 3 min duration, and the crystallinity did not change with longer treatments. The leveling-off degree of polymerization (LODP) of bacterial cellulose was decreased from 150 to 50 by 18% (w/w) NaOH treatment, while that of cotton linter decreased from 260 to 70. These characteristic differences between cotton linter cellulose and bacterial cellulose can be ascribed to a basic difference in microfibrillar organization in these materials: the microfibrils in cotton cellulose are in close contact with neighbouring microfibrils having opposite polarity, and in bacterial cellulose are isolated from each other and require chain folding to form the antiparallel cellulose II crystal     

Preparation and characterization of cellulose nanocrystals via ultrasonication-assisted FeCl3-catalyzed hydrolysis

Cellulose nanocrystals (CNC) was obtained from bamboo pulp via ultrasonication-assisted FeCl₃-catalyzed hydrolysis process, with parameters optimized by response surface methodology. The optimal parameters were reaction temperature: 107 °C, reaction time: 58 min, ultrasonication time: 186 min. The morphological, crystal structural, chemical structural and thermal features of the prepared cellulose nanocrystals were analyzed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), Fourier transfer infrared (FTIR) and thermogravimetric analysis. The results showed that the cellulose nanocrystals formed an interconnected network structure and CNC was rod-like with the length of 100–200 nm and the width of 10–20 nm. XRD result revealed that, compared with cellulose pulp, the crystallinity index of CNC increased from 69.5 to 79.4 %, while the cellulose I crystal structure remained. FTIR analysis demonstrated that CNC had the similar chemical structures to that of cellulose pulp, which indicated that the chemical structures of CNC remained unchanged in the presence of FeCl₃-catalyzed hydrolysis process and ultrasonication treatment. Thermogravimetric analysis revealed that the resulting CNC exhibited relatively high thermal stability. The research shows that ultrasonication-assisted FeCl₃-catalyzed hydrolysis could be a highly efficient method for preparing CNC.     

Selective hydrolysis of cellulose for the preparation of microcrystalline cellulose by phosphotungstic acid

Microcrystalline cellulose (MCC), prepared from natural cellulose through acid hydrolysis, has been widely used in the food, chemical and pharmaceutical industries because of its high degree of crystallinity, small particle size and other characteristics. Being different from conventional mineral acids, phosphotungstic acid (H₃PW₁₂O₄₀, HPW) was explored for hydrolyzing cellulose selectively for the preparation of MCC in this study. Various reaction parameters, such as the acid concentration, reaction time, temperature and solid-liquid ratio, were optimized. Rod-like MCC was obtained with a high yield of 93.62 % and also exhibited higher crystallinity and narrower particle diameter distribution (76.37 %, 13.77–26.17 μm) compared with the raw material (56.47 %, 32.41–49.74 μm) at 90 °C for 2 h with 58 % (w/w) HPW catalyst and a solid-liquid radio of 1:40. Furthermore, HPW can easily be extracted and recycled with diethyl ether for four runs without affecting the quality of the MCC products. The technology of protecting the crystalline region while selectively hydrolyzing the amorphous region of cellulose as much as possible by using HPW is of great significance. Due to the strong Brønsted acid sites and highest activity in solid heteropoly acid, the use of effective homogeneous HPW may offer an eco-friendly and sustainable way to selectively convert fiber resources into chemicals in the future.     

Effect of transient variation of temperature on acid hydrolysis of aspen hemicellulose

Thermal diffusivities of Aspen wood were experimentally determined and subsequently incorporated into a theoretical model to establish the effect of transient temperature variation within a solid substrate during acid catalyzed hydrolysis of hemicellulose. The simulation results were analyzed to evaluate the impact of particle size on hemicellulose hydrolysis. Total xylose yield was found to decrease for increasing chip size and this effect was intensified by increasing reaction temperature. Quantitative criteria were established for assessment of transient temperature effect on xylose yield and reaction time at various reaction conditions.     

Interfacial properties of cellulose nanoparticles obtained from acid and enzymatic hydrolysis of cellulose

Rod-shaped cellulose nanocrystals obtained by acid hydrolysis of eucalyptus fibers (CNCa) presented high aspect ratio (estimated length and width of 180 and 5 nm, respectively) and zeta potential of ?(17 ± 1) mV at pH 6. This typical morphology of cellulose nanocrystals was in contrast to nanoparticles obtained upon enzymatic hydrolysis of bacterial cellulose (CNCe), which were asymmetric and irregular due to surface-bound cellulases and presented a distinctive surface roughness. Interestingly, CNCe also displayed axial grooves, to yield a C-shape cross section that has not been reported before. The effect of the characteristic shape and surface chemistry of CNCa and of grooved CNCe was studied at oil/water interfaces and solid surfaces. Emulsions (20 % v/v oil) prepared with the CNCa were more stable than those prepared with CNCe, owing to their characteristic shape and surface chemistry. Hydrophilic (silica surfaces cationized by pre-adsorbed polycation) and hydrophobic (polystyrene films) solid surfaces were used as substrates for the adsorption of CNCe and CNCa for each type of surface. The ellipsometric data and AFM images indicated larger affinity of CNCe than CNCa for the hydrophobic surface. On the other hand, CNCa formed homogeneous monolayer on hydrophilic surfaces, whereas CNCe formed discontinuous films. Sequential adsorption behavior of CNCa on CNCe layers (or vice versa) suggested that the interaction between them is controlled by the orientation of enzymes bound to CNCe.     

Effects of cellulose nanocrystals on the vulcanization of natural rubber/cellulose nanocrystals nanocomposite and corresponding regulating strategies

Vulcanization is a vital process in rubber processing, it endows rubber with valuable physical and mechanical properties, making rubber a widely used engineering material. In addition to vulcanization agent, reinforcing fillers play a non-ignorable influence on the vulcanization of rubber nanocomposites. Herein, the effects of cellulose nanocrystals (CNCs) on the vulcanization of natural rubber (NR)/CNCs nanocomposite was studied. It was found that even though the addition of CNCs can effectively improve the dispersion of ZnO in NR matrix, the vulcanization of NR was inhibited. This may be attributed to the CNCs' adsorption of vulcanizing agents (DM, ZnO) and the acidic chemical environment on the surface of CNCs. In order to improve the vulcanization properties of NR/CNCs nanocomposite, tetramethyldithiochloram (TMTD) and triethanolamine (TEOA) were used as a combination accelerator and curing activator, respectively, and polyethylene glycol (PEG) was introduced to screen hydroxyl groups on the surface of CNCs to prohibit the CNCs' adsorption of vulcanizing agents. The results indicate that TMTD and TEOA effectively improved the vulcanization rate of NR/CNCs nanocomposite and increased the crosslink density by an order of magnitude. Subsequently, the tensile strength, tear strength, and so forth. of NR/CNCs nanocomposite were significantly improved. However, PEG hardly help to improve the vulcanization properties of NR/CNCs nanocomposite. In addition, the control samples without CNCs were prepared and characterized, the comparation between NR and NR/CNCs nanocomposite shows that the synergistic effect of crosslink density and CNCs' reinforcement more effectively improve mechanical properties of NR. This work not only elucidates the inhibiting mechanisms of CNCs on the vulcanization of NR, but also provides practical strategies for improving the vulcanization and properties of NR/CNCs nanocomposite. It may accelerate the application of CNCs as rubber reinforcing filler.