Effect of thermal dehydration and rehydration on Na-magadiite structure

The effect caused by dehydration and rehydration of the synthetic Na-magadiite was investigated by TGA, XRD, SEM, and ²⁹Si NMR. Thermal analysis of Na-magadiite presented two well-defined loss mass stages between 20 and 150 °C and another between 270 and 310 °C, both related to the removal of interlayer water. The swelling behavior of Na-magadiite was studied by thermal dehydration data obtained at 150 and 300 °C, and respective rehydration by water addition. X-ray patterns showed that the dehydration of Na-magadiite at 150 and 300 °C provoked the basal spacing decrease. The XRD also showed that only the material treated at 150 °C returned to the original structure with the rehydration. ²⁹Si NMR spectra showed that after rehydration, the Q₃/Q₄ relationship presented the same value for Na-magadiite treated at 150 °C. However, this Q₃/Q₄ value decreased when the treatment was done at 300 °C. Kinetic studies of thermal decomposition showed that the dehydration of magadiite is based on a phase boundary-controlled reaction, caused by contracting areas. The exfoliation of lamellas with thermal treatment can explain this behavior, as observed in SEM images.     

Structure of native and regenerated cellulose as revealed by proton NMR analysis

Differences in fiber structure between cotton and cuprammonium rayon are studied by a refined broad-line proton NMR analysis of samples swollen with deuterated dimethyl sulfoxide, which has no effect on the spectra but enhances differences in molecular mobility between crystalline and noncrystalline regions. The spectra obtained are decomposed into four components: broad, medium, narrow, and extremely narrow. These components are identified as contributions, respectively, from crystalline and rigid noncrystalline (frozen glassy) material, a noncrystalline glassy component exhibiting local segmental motion, a noncrystalline rubbery component exhibiting liquidlike molecular motion, and protons included in DMSO-d₆ as an impurity. The mass fraction of the narrow component in cotton was about 0.01, whereas it was as high as 0.18 in cuprammonium rayon. It is concluded that even in the swollen state, native cellulose is devoid of a liquidlike mobile component, but regenerated cellulose contains a considerable amount of a noncrystalline component involving liquidlike segmental motion of molecules.     

Anomalous effects in the thermal dehydration and rehydration of natrolite zeolites

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2121–2123, September, 1989.     

Microfibrillar carbon from native cellulose

Use of pyrolytic carbon from cellulose has been limited in practice to activated adsorbent carbon, but cellulose-derived carbon retaining the nanoscale microfibrillar morphology offers rich possibilities as an advanced material. Here we developed novel methods to prepare such materials by an improved drying of wet cellulose prior to pyrolysis. This procedure is an adaptation from electron microscopy techniques, i.e. rapid freeze drying of suspension and solvent exchange drying, both being effective in preventing coagulation of cellulose microfibrils/microcrystals. Pyrolytic carbon from such material has a large external surface area, with the graphitic carbon crystallites roughly aligned along the fiber axis. These features are potentially useful in developing novel carbon nanomaterials for electrodes, catalyst supports, or composite material elements.     

A native cellulose microfibril model

To aid in the understanding of cellulose ultrastructure, computer modelling has been employed to create a model of monoclinic (I) native cellulose. This was achieved by building a chain of cellulose, which was used in a two chain unit cell. An energy minimized microfibril model was created from several of these unit cells. A major advantage of this model is that it is a large scale unconstrained, isolated system. Thus, it facilitates the study of surface as well as central chains and provides a working model of a cellulose microfibril. An extensive analysis was carried out of intermolecular non-bond interactions and how they might contribute to the stability of the structure of crystalline native cellulose. 0969--0239 © 1998 Blackie Academic & Professional     

Modifying bacterial cellulose with gelatin peptides for improved rehydration

Bacterial cellulose (BC) is a nanoscale and useful biomaterial with a fine fiber network and high water holding capacity. However, dried BC exhibits poor rehydration ability. The present study investigated the rehydration ability of composites of hydrolyzed gelatin peptides (HGP) and hydroxypropylmethyl cellulose-modified BC (HBC). The HGP with molecular weights <9 kDa were obtained by hydrolyzing gelatin with a combination of 1 % alcalase and 1.5 % pronase E at 50 °C for 2 h. The HGP/HBC nanocomposites exhibited higher rehydration ratios than composites prepared with gelatin. According to SEM images, gelatin and HGP successfully penetrated the cellulose network in composite films prepared using both immersion and adsorption (DA) methods. The high hydrophilic property of HGP resulted in a rehydration ratio of approximately 180 % at a HGP/HBC ratio of 4.5:1 (W/W) in DA composites. The 1 min rehydrated HGP/HBC composites possessed similar mechanical properties to the original wet type composites. Overall, results indicated that the HGP/HBC composites prepared using the DA method demonstrated the highest rehydration ability among the composite films evaluated.     

Study of dehydration and rehydration processes of portlandite in mature and young cement pastes

The hydration process of the cements induces the formation of different kinds of hydration products. The main products of hydration are C?CS?CH gel and portlandite [Ca(OH)₂]. The C?CS?CH gel is an amorphous compound that is discomposed progressivity with the temperature until approximately 1,000?°C, while the portlandite is discomposed between 450 and 550?°C. Also, calcium carbonate can be formed as a consequence of the portlandite carbonation. All of these processes can be analysed and quantified by simultaneous differential thermal analysis and thermogravimetric analysis. And by X-ray diffraction it is possible to identify the crystalline phases. Some authors have corroborated that the portlandite can be rehydrated, after dehydration processes due to thermal exposition of the cement paste. But all of these experiments have been made with young cement pastes or at temperatures lower than 650?°C. In this work the behaviour of young and mature cement pastes have been studied in relation with the portlandite decomposition and the possibility of the rehydration of it in water presence. We found that young pastes and old pastes, stored at laboratory conditions, and later burned, show a certain grade of rehydration, specially the pastes burned at 650?°C (with ??80% of reformation of portlandite) with respect to the pastes burned at 1,000?°C (between 20 and 40%). It is corroborate that the rehydration process is directly related to the formation of CaO during the burning. Also, a formation of unstable portlandite is detected in young pastes burned at 650?°C, which can be rehydrated easily. Although, the mature pastes that have been burned initially and stored under laboratory conditions cannot be rehydrated, due to the formation of stable products during the storage.     

Grafting onto microfibrils of native cellulose

Attempts to modify the surface of oxidized cellulose microfibrils were made using amine terminated molecules. First, native cellulose was oxidized with catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), sodium hypochlorite and sodium bromide in water. The primary alcohol moieties were selectively oxidized into carboxyl groups. Then, the oxidized cellulose was coupled with amines derivatives by a peptidic reaction by using carbodiimide and hydroxysuccimide as catalyst and amidation agent. The obtained coupled cellulose showed low polarity, with stability in non-polar solvents. The products were characterized by FTIR, ¹³C NMR, rheology and conductometric titration as well as transmission electron microscopy. Their hydrophobic character was evaluated by observing their behavior in polar and non-polar solvents.     

Improvement of surface cementitious properties of coarse fly ash by dehydration and rehydration processes

Owing to poor bonding between coarse fly ash particles and hydration products, gap-graded blended cements with fly ash usually show lower compressive strengths than Portland cement. Surface cementitious properties of coarse fly ash were improved by dehydration and rehydration processes in the present study. The results show that during the calcination at 750?°C, C?CS?CH gel is mainly transformed into a new nesosilicate, which is similar to a less crystalline C₂S. The formation of melilite from hydration products is also noticed at 900?°C, however, this will not contribute to rehydration of calcined fly ash. Rehydration of new generated nesosilicate on the surface of coarse fly ash leads to a better bonding between coarse fly ash particles and hydration products. As a result, both early and late mechanical properties of gap-graded blended cements containing 25% cement clinker and 39% calcined coarse fly ash are higher than those of 100% Portland cements.     

Structure characterization of cellulose nanofiber hydrogel as functions of concentration and ionic strength

Carboxylated cellulose nanofibers (CNFs), having an average width of 7 nm and thickness of 1.5 nm, were produced by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation method. The fiber cross-sectional dimensions were determined using small-angle X-ray scattering (SAXS), transmission electron microscopy and atomic force microscopy techniques, where the rheological properties under different concentration and ionic strength were also investigated. The formation of hydrogel was evidenced by increasing the CNF concentration or ionic strength of the solvent (water), while the gel structure in ion-induced CNF hydrogels was found to be relatively inhomogeneous. The gelation behavior was closely related to the segmental aggregation of charged CNF, which could be quantitatively characterized by the correlation length (ξ) from the low-angle scattering profile and the scattering invariant (Q) in SAXS.     

Synthesis and characterization of hydroxypropyl cellulose from bacterial cellulose

Bacterial cellulose produced by Acetobacter xylinum has been reacted with propyleneoxide to synthesize hydroxypropyl cellulose(HPC) under different reaction conditions while diluted by toluene. The effects of mass ratio of bacterial cellulose to propyleneoxide, dilutability of toluene, reaction temperature(T) and time(t) were investigated by series of experiments. The degree of substitution(DS), hydroxypropyl content(A) and yield(η) were compared. The optimized product exhibited cold-water solubility and hot-water gelatinization in aqueous medium. Further study was carried out with FTIR, TGA, XRD, SEM and 13C-NMR for characterization. The water/air contact angle measurement reveals that it is a good hydrophobic material with good mechanical properties.     

Synchrotron-radiated X-ray and neutron diffraction study of native cellulose

Precise determination of d-spacings and compositional ratio of cellulose I_α and I_β in various native cellulose samples was successfully carried out by synchrotron-radiated X-ray diffraction and time-of-flight (TOF) neutron diffraction from quasi-powder specimens. X-ray diffraction peaks were separated by the deconvolution method using six types of profile function: Gaussian, Lorentzian, intermediate Lorentzian, modified Lorentzian, pseudo-Voigt, and Pearson VII. In terms of R-factors, the pseudo-Voigt function gave the best fit with the observation, and was used for determination of d-spacings. The numerical results for Valonia cellulose were: dI_α (1 0 0) = 0.613 nm; dI_β (1 1 0) = 0.603 nm; dI_β (1 1 0) = 0.535 nm; dI_α (0 1 0) = 0.529 nm; I_α content = 0.65. The differences determined between dI_α (1 0 0) and dI_β (1 1 0) and between dI_β (1 1 0) and dI_α (0 1 0) were similar to those previously reported. Comparison between unresolved peaks for the two types of cellulose samples revealed a small but definite difference between dI_α (1 1 0) and dI_β (2 0 0). The TOF neutron diffractometry using deuterated samples confirmed this difference. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modelling the crystalline deformation of native and regenerated cellulose

The molecular mechanics modelling of the deformation of a number of proposed structures for the crystalline regions of cellulose I- , I- and II are reported. The structures used are from coordinates that have recently been reported, which have made particular reference to the ability to locate the positions of hydrogen bonds. By comparison to previously reported structures of cellulose, where the emphasis on this has also been made, and in a diagnostic way, it is shown that it is possible to make some conclusions as to their validity. The effect of removing the intramolecular hydrogen bonding is also reported. All structures, with one exception, are shown to be sensitive to this operation. Two approaches to the molecular mechanics modelling are reported, wherein the structures are minimised under restraint, by altering the c-spacing, within the COMPASS^TM forcefield to obtain a simple chain stiffness value or alternatively by performing a full elastic constants determination for the unit cell.     

Extraction and characterization of cellulose single fiber from native Ethiopian Serte (Dracaena steudneri Egler) plant leaf

Biomaterials are renewable sources which are widely distributed, locally accessible, high possibility of recycling and biodegradation behavior. This investigation deals with the extraction and characterizing of new fiber obtained from Serte plant leaf that is found in Ethiopia. The physical, chemical and mechanical characters of the fiber had been tested and comparison with other plant fibers was done for the first time. Optimization of fiber extraction process has been done by varying the extraction variables like NaOH concentration, temperature and extraction time. Fibers possessing 56% cellulose content along with significant amount of hemicellulose, lignin and ash with tensile strength of 330?MPa were obtained. FTIR as well as X-ray diffraction analysis were also done to further analyze the fiber. This new plant leaf fiber can be another alternative resource in place of synthetic fibers depending on their application such as reinforcing polymer matrices.     

Synthesis and characterization of microcrystalline cellulose produced from bacterial cellulose

In this study, microcrystalline cellulose (MCC) was prepared from the acid hydrolysis of bacterial cellulose (BC) produced in culture medium of static Acetobacter xylinum. The MCC-BC produced an average particle size between 70 and 90 μm and a degree of polymerization (DP) of 250. The characterization of samples was performed by thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy (SEM). The MCC shows a lower thermal stability than the pristine cellulose, which was expected due to the decrease in the DP during the hydrolysis process. In addition, from X-ray diffractograms, we observed a change in the crystalline structure. The images of SEM for the BC and MCC show clear differences with modifications of BC fiber structure and production of particles with characteristics similar to commercial MCC.     

XRD study of the dehydration and rehydration behaviour of Al-pillared smectites differing in source of charge

The swelling properties of Al-pillared clays, obtained from five different smectites, were studied using X-ray diffraction. These clays, the dioctahedral beidellite and montmorillonite and the trioctahedral saponite, hectorite and laponite differ in source of isomorphic substitution and represent a series of decreasing basicity along the siloxane plane. An Al oxyhydroxy cation was inserted between the layers to form the respective pillared clays and these clays were heated incrementally to 600°C. The XRD peaks at each stage of heating were recorded as well as the same samples subsequently wetted. Basal spacings of each clay at each stage of dehydration d rehydration indicated that the swelling of tetrahedrally substituted saponite and beidellite was indeed restricted, compared with the other three clays. This was attributed to greater basicity of the oxygen plane of beidellite and saponite due to tetrahedral substitution of Si by Al, resulting in an increase in the strength of hydrogen bonds between either water or the interlayer polyhydroxy cation and the clay.The data from the XRD analyses helped in addition, to clarify the thermal transformations of the Keggin ion itself. According to the changes in thed-spacings of the pillared clays it was concluded that the Keggin ion lost its structural water at 200°C and dehydroxylated in a range beginning at 350°C. Between 500 to 600°C this polymer cation, which is thought to form the Al₂O₃ oxide, did not rehydrate.Dedicated to Prof. Menachem Steinberg on the occasion of his 65th birthdayThe authors wish to thank Laporte Industries, Inc., U.K. for the laponite sample.