Exfoliation of graphite by dry ball milling with cellulose

Dry ball milling of graphite with cellulose and related polysaccharides was found effective for exfoliation-dispersion of graphene-like carbon. The exfoliation behavior was found to depend strongly on the polymer species; namely, polysaccharides are much more effective than thermoplastic polymers. The compression-molded slabs from co-milled powder with cellulose and carboxymethylcellulose showed an electrical percolation threshold of 1.0 % (w/w) or lower. The carbon fraction isolated from milling with carboxymethylcellulose was water-dispersible, containing single- to few-layer graphenes. This method can provide facile and solventless graphene exfoliation and mechanical alloying with polymers.     

Biological pretreatment of lignocellulose for enzymatic hydrolysis of cellulose

Pretreatment of lignocellulosic materials is considered as the rate-limiting step in an economically feasible process for enzymatic hydrolysis of cellulose. Biological delignification techniques have not been developed as intensively as physical and chemical methods. However, white-rot fungi are effective degraders of lignin, and some of them even preferentially remove lignin from wood compared with carbohydrates, and therefore might be suitable for biological pretreatment of lignocellulose. White-rot fungi were cultivated on wheat straw and the residue was hydrolyzed withTrichoderma reesei cellulase. Of nineteen fungi examined,Pleurotus ostreatus, Pleurotus sp. 535,Pycnoporus cinnabarinus 115,Ischnoderma benzoinum 108,Phanerochaete sordida 37,Phlebia radiata 79, and two unidentified fungi were found suitable for pretreatment of straw: the yields of reducing sugars and glucose based on original straw were markedly better compared with uninoculated straw, and these fungi also gave better results thanPolyporus versicolor, a nonselective reference fungus (Cowling, 1961). In the best cases the efficiency of the biological pretreatment was comparable with that of alkali treatment (2% NaOH, 0.4 g NaOH/g straw, 10 min at 115‡C), but the fungal treatment resulted in a higher proportion of glucose in the hydrolyzates. Combined fungal and (strong) alkali treatment did not give better results than alkali or fungal treatment alone. When culture flasks were periodically flushed with oxygen the treatment time could be reduced by about 1 wk with the two fungi,P. sordida 37 andP. cinnabarinus 115, tested. The effect of oxygen in pretreatment reflected the effect of oxygen in the degradation of¹⁴C-lignin of poplar wood to¹⁴CO₂ by these fungi (Hatakka and Uusi-Rauva, 1983). The economic feasibility of the biological pretreatment process is poor due to the long cultivation times needed. The best results were obtained with the longest treatment time studied, which was 5 wk. However, the rapid progress in the field of biological lignin degradation may help to accelerate the delignification process, and also find factors that favor lignin degradation, but suppress the utilization of carbohydrates.     

Monolayers of reactive cellulose derivatives

Functional cellulose derivatives are very versatile materials for the creation of mono- and multilayer systems. Hydrophobic alkyl and trimethylsilyl celluloses form highly ordered Langmuir-Blodgett multilayers on hydrophobic substrates. Cellulose thiosulfates and methyl thio ethers were self-assembled on gold and silver surfaces to form hydrophilic monolayers. Cellulose layer systems are capable for chemical transformations under conservation of the structural order. They are suitable platforms for the investigation of molecular recognition at surfaces and the construction of sensor devices. Both biological ligands, e.g. biotin, and enzymes, e.g. horse radish peroxidase, could be attached to cellulose under conservation of their biological function.     

Synthesis, structural and hydrogenation properties of Mg-rich MgH2-TiH2 nanocomposites prepared by reactive ball milling under hydrogen gas

MgH(2)-TiH(2) nanocomposites have been obtained by reactive ball milling of elemental powders under 8 MPa of hydrogen pressure. The composites consist of a mixture of β-rutile MgH(2), γ-orthorhombic high pressure MgH(2) and ε-tetragonal TiH(2) phases with nanosized crystallites ranging from 4 to 12 nm. In situ hydrogen absorption curves on milling reveal that nanocomposite formation occurs in less than 50 min through the consecutive synthesis of the TiH(2) and MgH(2) phases. The abrasive and catalytic properties of TiH(2) speed up the formation of the MgH(2) phase. Thermodynamic, kinetic and cycling hydrogenation properties have been determined for the 0.7MgH(2)-0.3TiH(2) composite and compared to nanometric MgH(2). Only the MgH(2) phase desorbs hydrogen reversibly at moderate temperature (523 to 598 K) and pressure (10(-3) to 1 MPa). The presence of TiH(2) does not modify the thermodynamic properties of the Mg/MgH(2) system. However, the MgH(2)-TiH(2) nanocomposite exhibits outstanding kinetic properties and cycling stability. At 573 K, H-sorption takes place in less than 100 s. This is 20 times faster than for a pure nanometric MgH(2) powder. We demonstrate that the TiH(2) phase inhibits grain coarsening of Mg, which allows extended nucleation of the MgH(2) phase in Mg nanoparticles before a continuous and blocking MgH(2) hydride layer is formed. The low crystallinity of the TiH(2) phase and its hydrogenation properties are also compatible with a gateway mechanism for hydrogen transfer from the gas phase to Mg. Mg-rich MgH(2)-TiH(2) nanocomposites are an excellent media for hydrogen storage at moderate temperatures.     

The initial structure of cellulose during ammonia pretreatment

A protocol was developed to freeze-trap (at 150 K) cellulose as it is undergoing liquid ammonia pretreatment, and then to collect X-ray diffraction data from the freeze-trapped reactants as the reaction is allowed to proceed and ammonia is allowed to melt and then evaporate, leaving ammonia-cellulose I. Cellulose adopts a new two-chain crystal form, which we call low temperature phase ammonia-cellulose I (two-chains and ~ten ammonia molecules within a unit cell of a = 15.49 Å, b = 11.35 Å, c = 10.42 Å and γ = 143.5°). A schematic model was developed that is characterized by sheets of hydrophobically stacked cellulose-chains with hydrophilic channels between them that are filled with ammonia molecules. Neighboring chains in these sheets have either different conformations or are staggered with respect to each other. As ammonia is allowed to evaporate, the unit cell size is reduced by a factor of two as the two independent chains become identical.     

Clean and reactive nanostructured cellulose surface

A simple, solvent-free and low cost method to activate the surface of nanofibrillated cellulose films for further functionalization is presented. The method is based on the oxidative properties of UV radiation and ozone, to effectively remove contaminants from nanocellulosic surface, which remains clean and reactive for at least a week. The efficiency of the method is demonstrated by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. In clear contrast to previous results on nanoscaled cellulose the relative atomic concentration of non-cellulosic carbon atoms was only 4 %, and water completely wetted the surface within seconds. After activation, neither chemical degradation nor morphological changes on cellulose were observed. This surface activation is essential for further functionalization of the film in dry state or nonpolar media. The surface activation was confirmed by silylation and a four times higher degree of substitution was achieved on the activated sample compared to non-activated reference film, as monitored with XPS.     

The glass transition and crystallization of ball milled cellulose

Samples of ball milled cellulose were prepared by ball milling pulps from eucalyptus and softwood (spruce/pine). Water sorption isotherms were obtained by both dynamic vapor sorption and equilibration over saturated salt solutions, in the water content range of 5–42% db (db = dry basis; water as a % age of total solids). Dynamic mechanical analysis using a pocket technique showed a water content dependent thermal transition occurring at the same temperature for the two pulp samples, which was interpreted as a glass transition. Fitting the data to a Couchman–Karasz relationship predicted a value for T _g of the dry cellulose of approximately 478 K, which was similar to values previously reported for other dry polysaccharides. No clear glass transition could be observed calorimetrically, although an endotherm at approximately 333 K was measured, which in polymers is normally attributed to enthalpic relaxation, however the lack of dependence of this endotherm on water content suggests that the melting of some weak associations, such as residual hydrogen bonds, could be a more credible explanation. An exotherm was also observed on heating, which was dependent on water content and which was attributed to partial crystallization of the cellulose. This was confirmed by Wide angle X-ray diffraction and cross polarization magic angle spinning ¹³C NMR (CPMAS NMR). The recrystallisation was predominantly to form I of cellulose. This was thought to be caused by a small amount of residual form I (probably less than 5%) acting as a template for the crystallizing material. Differential scanning calorimetry reheat curves showed the appearance of freezable water for water contents higher than 20%, as a result of a transfer of water to the amorphous phase following crystallization. The increase in cellulose rigidity following crystallization was also confirmed by CPMAS NMR relaxation. Low resolution proton NMR T ₂ relaxation suggested the presence of proton water/cellulose exchange, which was active at water contents of 20% and above.     

Structural and electrochemical properties of TiFe alloys synthesized by ball milling for hydrogen storage

The binary TiFe alloy was synthesized by mechanical alloying (MA) under argon atmosphere at room temperature. The effect of ball to powder weight ratio on the microstructures was characterized by X-ray diffraction (XRD). The effect of milling time on the electrochemical and activation properties was investigated by scanning electron microscope (SEM), galvanostatic charging and discharging, constant potential discharge, and potentiodynamic polarization techniques. Relationships between electrochemical properties, such as polarization, variation of electrochemical discharge capacity, \( \frac{D_{\mathrm{H}}}{a^2} \) ratio exchange current density, and Nernst potential and alloy compositions were evaluated. XRD results showed that with increasing ball to powder weight ratio, the amorphization process is accelerating and powders milled with a ratio of 1:8 have the highest conversion rate to TiFe. SEM observations reveal that particles show cleavage fracture morphology and size distribution is generally normalized. TiFe milled during 40 h was easily activated within 5 cycles and showed the best discharge capacity equal to 147 mAh g^?1. A good cycling was observed after 20 cycles at ambient temperature for the alloy milled for 30 h. A correlation between alloy composition, \( \frac{D_{\mathrm{H}}}{a^2} \) report, exchange current density, and Nernst potential on one hand and the variation of the electrochemical discharge capacity during cycling for different milling times on the other hand was observed.     

Enhancing the enzymatic hydrolysis of cellulosic materials using simultaneous ball milling

One of the limiting factors restricting the effective and efficient bioconversion of softwood-derived lignocellulosic residues is the recalcitrance of the substrate following pretreatment. Consequently, the ensuing enzymatic process requires relatively high enzyme loadings to produce monomeric carbohydrates that are readily fermentable by ethanologenic microorganisms. In an attempt to circumvent the need for larger enzyme loadings, a simultaneous physical and enzymatic hydrolysis treatment was evaluated. A ball-mill reactor was used as the digestion vessel, and the extent and rate of hydrolysis were monitored. Concurrently, enzyme adsorption profiles and the rate of conversion during the course of hydrolysis were monitored. α-Cellulose, employed as a model substrate, and SO₂-impregnated steam-exploded Douglas-fir wood chips were assessed as the cellulosic substrates. The softwood-derived substrate was further posttreated with water and hot alkaline hydrogen peroxide to remove >90% of the original lignin. Experiments at different reaction conditions were evaluated, including substrate concentration, enzyme loading, reaction volumes, and number of ball beads employed during mechanical milling. It was apparent that the best conditions for the enzymatic hydrolysis of α-cellulose were attained using a higher number of beads, while the presence of air-liquid interface did not seem to affect the rate of saccharification. Similarly, when employing the lignocellulosic substrate, up to 100% hydrolysis could be achieved with a minimum enzyme loading (10 filter paper units/g of cellulose), at lower substrate concentrations and with a greater number of reaction beads during milling. It was apparent that the combined strategy of simultaneous ball milling and enzymatic hydrolysis could improve the rate of saccharification and/or reduce the enzyme loading required to attain total hydrolysis of the carbohydrate moieties.     

Tailoring materials by high-energy ball milling: TiO2 mixtures for catalyst support application

We carried out a rational design of catalyst supports by high-energy ball milling. Tailored mixtures of TiO₂ crystalline phases were obtained using rotational speed and milling time as variable parameters. Polymorphic transformation from anatase to rutile through high-pressure TiO₂ (II) as intermediate was confirmed by X-ray Diffraction (XRD), Raman Spectroscopy and Transmission Electron Microscopy (TEM). Also, starting material doubled its specific surface area due to particle fragmentation, as confirmed by surface area of Brunauer-Emmet-Teller (S_BET) and Scanning Electron Microscopy (SEM). Defects introduced during milling process generated oxygen vacancies in the surface and bulk of supports, as evidenced by X-ray Photoelectron Spectroscopy (XPS) and Electron Paramagnetic Resonance (EPR). Furthermore, longer milling time increased reducibility and oxygen mobility of supports, as observed by H₂ Temperature Programmed Reduction (H₂-TPR) and O₂ Temperature Programmed Desorption (O₂-TPD). Phase composition remained unchanged even under extreme conditions, highlighting the stability of unusual TiO₂ (II) phase. Properties achieved in present materials could benefit metal-support interactions and play a major role in supported catalysts.     

A green protocol for the one-pot multicomponent Petasis boronic Mannich reaction using ball milling

Solvent-free reactions of salicylaldehyde with various boronic acids and amines were accomplished under ball-milling conditions to achieve the corresponding alkyl- and arylaminophenols in good to excellent yields. This simple protocol offers advantages such as high yields, shorter reaction times, simple workup procedure and eco-friendliness.     

Reducing end-group of cellulose as a reactive site for thermal discoloration

Thermal discoloration of cellulose (Avicel PH-101 and Whatman No. 42 filter paper) was studied in N₂ at 160-280 °C with glycerol-treated and NaBH₄-reduced samples, to understand the role of the reducing end. Thermal discoloration of glycerol-treated Avicel PH-101, in which some of the reducing ends were converted into glycosides (non-reducing ends), was suppressed compared with the original cellulose, and the level of suppression was directly related to the extent of glycosylation of the reducing ends. The stabilization efficiency of glycerol-treated Whatman No. 42 filter paper suggested that the reducing ends newly formed by reduction of the degree of polymerization (DP) (to about 200) during heat treatment contributed to the discoloration. The important role of the reducing ends in thermal discoloration was supported by the stabilization of Avicel PH-101 by reduction with NaBH₄ (giving a reducing end content that was 2% of that of the original cellulose). Thermally induced discoloration was also inhibited by heating cellulose in suspension in the polyether tetraethyleneglycol dimethylether, which has been reported to inhibit the thermal degradation of reducing sugars.     

Length-controlled cellulose nanofibrils produced using enzyme pretreatment and grinding

The length of cellulose nanofibrils (CNFs) is a significant parameter for various applications. The goal of this research was to employ a fabrication method to produce length-controlled CNFs; the chosen technique was enzy-grinding (enzyme pretreatment followed by mechanical grinding). Here, we presented the results of the optimization of the diameter and length, the characterization of the properties of CNFs and nanofilms prepared using these fibrils. The cellulose morphology, crystallinity index (CrI), chemical structure, and thermal stability were investigated as functions of the enzyme loading and hydrolysis time. The results showed that enzy-grinding could effectively reduce the diameter and length of cellulose fibrils. The average diameter was about 8.6 ± 3.6 nm, and the length could be controlled over the range from 0.76 ± 0.38 μm to ≥ 4 μm (i.e. aspect ratios from 43 to ≥ 328). After the grinding process, the CNFs maintained high thermal stability and no change in the chemical structure compared to the original pulp. The transmittance and mechanical properties of the CNF films were strongly dependent on the fibril length. The fabrication of length-controlled CNFs using the enzy-grinding process is meaningful and significant research which could be relevant to the optimization of such materials for various applications.     

Preparation and characterization of biodegradable sugarcane bagasse nano reinforcement for polymer composites using ball milling operation

The present work includes the processing and characterization of nano-based natural reinforcement for polymer composite materials. Sugarcane bagasse has been collected and the fibers were extracted using manual striping process. Undesirable materials present in the extracted fibers were removed by 1% NaOH-based chemical treatment. The macrofibers were reduced to nano scale by using high-energy ball milling process. Nanoparticles from bagasse fibers were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The degree of crystallinity of nano bagasse is 55.2% and it was reported by using XRD. A FTIR spectrum confirms the presence of cellulose functional groups in nano bagasse. The nano bagasse dimensions and morphology were investigated using SEM. The average length and diameter of the nano bagasse is 51.2 and 46.1 nm, respectively. Thermal stability of the nano bagasse was revealed by TGA analysis. The chemical composition of cellulose, lignin, and hemicellulose contents was also investigated.     

Nanocrystalline CaF2 particles obtained by high-energy ball milling

Structural changes in mechanically treated CaF₂ powders have been studied by X-ray diffraction, transmission electron microscopy, thermal analysis, ¹⁹F and ¹H MAS Nuclear Magnetic Resonance and Electron Paramagnetic Resonance methods.Applying the same methods, the results could be compared with those of mechanochemically synthesized CaF₂ samples, prepared for the first time in this study by high-energy ball milling. The applied methods indicate that the mechanically treated samples become, under the applied conditions, nanocrystalline. Unexpectedly, the mechanochemically synthesized samples show the same effects, i.e., nanocrystalline samples were formed. In contrast to many oxide compounds, a weak amorphization takes place only after a strong mechanical impact, and essentially in grain boundaries of spherical particles. Observed effects after the application of mechanical impact like broadening of XRD reflections, broadening of ¹⁹F resonances, or the decreasing ability for H trapping at ambient temperature, are mainly due to decreasing particle sizes as well as very small structural changes in the bulk of the particles. Surprisingly, the spin relaxation of both nuclear spins (¹⁹F) and electron spins (H) appears to be the most sensitive tool for the investigation of mechanically and chemically induced changes indicating the increasing surface to bulk ratio with increasing mechanical impact.     

Fabrication of carbon quantum dots via ball milling and their application to bioimaging

Carbon quantum dots (CQDs) with an average diameter of 3 nm, exhibiting blue photoluminescence, have been obtained from commercial conductive carbon black by a cost-effective and straightforward exfoliation method using dry ball milling in the presence of sodium carbonate. As a secondary abrasive medium, sodium carbonate provides effective exfoliation of carbon black with a high degree of CQD graphitization and plays an essential role in the functionalization of CQDs with oxygen groups. Due to the low toxicity of CQDs against HeLa cancer cells (cell viability above 90% at a CQD concentration of 200 μg cm⁻³) and the ability to penetrate cells and emit blue light, CQDs are possibly suitable for biological imaging of cells.