Cellulose samples are routinely analyzed by X-ray diffraction to determine their crystal type (polymorph) and crystallinity. However, the connection is seldom made between those efforts and the ...crystal structures of cellulose that have been proposed with synchrotron X-radiation and neutron diffraction over the past decade or so. In part, this desirable connection is thwarted by the use of different conventions for description of the unit cells of the crystal structures. In the present work, powder diffraction patterns from cellulose Iα, Iβ, II, IIII, and IIIII were calculated based on the published atomic coordinates and unit cell dimensions contained in modified “crystal information files” (.cif) that are supplied in the Supplementary Information. The calculations used peak widths at half maximum height of both 0.1 and 1.5° 2θ, providing both highly resolved indications of the contributions of each contributing reflection to the observable diffraction peaks as well as intensity profiles that more closely resemble those from practical cellulose samples. Miller indices are shown for each contributing peak that conform to the convention with c as the fiber axis, a right-handed relationship among the axes and the length of a < b. Adoption of this convention, already used for crystal structure determinations, is also urged for routine studies of polymorph and crystallinity. The calculated patterns are shown with and without preferred orientation along the fiber axis. Diffraction intensities, output by the Mercury program from the Cambridge Crystallographic Data Centre, have several uses including comparisons with experimental data. Calculated intensities from different polymorphs can be added in varying proportions using a spreadsheet program to simulate patterns such as those from partially mercerized cellulose or various composites.
Despite nomenclature conventions of the International Union of Pure and Applied Chemistry and the International Union of Biochemistry and Molecular Biology, the repeating unit of cellulose is often ...said to be cellobiose instead of glucose. This review covers arguments regarding the repeating unit in cellulose molecules and crystals based on biosynthesis, shape, crystallographic symmetry, and linkage position. It is concluded that there is no good reason to disagree with the official nomenclature. Statements that cellobiose is the repeating unit add confusion and limit thinking on the range of possible shapes of cellulose. Other frequent flaws in drawings with cellobiose as the repeating unit include incorporation of O-1 as the linkage oxygen atom instead of O-4 (the O-1 hydroxyl is the leaving group in glycoside synthesis). Also,
n
often erroneously represents the number of cellobiose units when
n
should denote the degree of polymerization i.e., the number of glucose residues in the polysaccharide.
•The X-ray diffraction patterns of control and mercerized cotton fibers were simulated with perfect cellulose Iβ and II crystals, respectively, as well as amorphous cellulose.•The simulated ...crystalline fraction was larger for control cotton but smaller for mercerized cotton as compared with their respective Segal CI value. This discrepancy was explained by the dependence of the Segal amorphous intensity on crystal size, cellulose polymorph, and the degree of polymorphic conversion.
The Segal method estimates the amorphous fraction of cellulose Iβ materials simply based on intensity at 18° 2θ in an X-ray diffraction pattern and was extended to cellulose II using 16° 2θ intensity. To address the dependency of Segal amorphous intensity on crystal size, cellulose polymorph, and the degree of polymorphic conversion, we simulated the diffraction patterns of cotton celluloses (Iβ and II) and compared the simulated amorphous fractions with the Segal values. The diffraction patterns of control and mercerized cottons, respectively, were simulated with perfect crystals of cellulose Iβ (1.54° FWHM) and cellulose II (2.30° FWHM) as well as 10% and 35% amorphous celluloses. Their Segal amorphous fractions were 15% and 31%, respectively. The higher Segal amorphous fraction for control cotton was attributed to the peak overlap. Although the amorphous fraction was set in the simulation, the peak overlap induced by the increase of FWHM further enhanced the Segal amorphous intensity of cellulose Iβ. For cellulose II, the effect of peak overlap was smaller; however the lower reflection of the amorphous cellulose scattering in its Segal amorphous location resulted in smaller Segal amorphous fractions. Despite this underestimation, the relatively good agreement of the Segal method with the simulation for mercerized cotton was attributed to the incomplete conversion to cellulose II. The (1−10) and (110) peaks of cellulose Iβ remained near the Segal amorphous location of cellulose II for blends of control and mercerized cotton fibers.
The X-ray diffraction-based Segal Crystallinity Index (CI) was calculated for simulated different sizes of crystallites for cellulose Iβ and II. The Mercury software was used, and different ...crystallite sizes were based on different input peak widths at half of the maximum peak intensity (pwhm). The two cellulose polymorphs, Iβ and II, gave different CIs despite having the same pwhm values and perfect periodicity. The higher CIs for cellulose II were attributed to a greater distance between the major peaks that are closest to the recommended 2-θ value for assessing the amorphous content. That results in less peak overlap at the recommended 2-θ value. Patterns calculated with simulated preferred orientation had somewhat higher CIs for cellulose Iβ, whereas there was very little effect on the CIs for cellulose II.
Cellulose is often described as a mixture of crystalline and amorphous material. A large part of the general understanding of the chemical, biochemical and physical properties of cellulosic materials ...is thought to depend on the consequences of the ratio of these components. For example, amorphous materials are said to be more reactive and have less tensile strength but comprehensive understanding and definitive analysis remain elusive. Ball milling has been used for decades to increase the ratio of amorphous material. The present work used 13 techniques to follow the changes in cotton fibers (nearly pure cellulose) after ball milling for 15, 45 and 120 min. X-ray diffraction results were analyzed with the Rietveld method; DNP (dynamic nuclear polarization) natural abundance 2D NMR studies in the next paper in this issue assisted with the interpretation of the 1D analyses in the present work. A conventional NMR model’s paracrystalline and inaccessible crystallite surfaces were not needed in the model used for the DNP studies. Sum frequency generation (SFG) spectroscopy also showed profound changes as the cellulose was decrystallized. Optical microscopy and field emission-scanning electron microscopy results showed the changes in particle size; molecular weight and carbonyl group analyses by gel permeation chromatography confirmed chemical changes. Specific surface areas and pore sizes increased. Fourier transform infrared (FTIR) and Raman spectroscopy also indicated progressive changes; some proposed indicators of crystallinity for FTIR were not in good agreement with our results. Thermogravimetric analysis results indicated progressive increase in initial moisture content and some loss in stability. Although understanding of structural changes as cellulose is amorphized by ball milling is increased by this work, continued effort is needed to improve agreement between the synchrotron and laboratory X-ray methods used herein and to provide physical interpretation of the SFG results.
•Core-shell structured hydrogels were prepared and CNFs were incorporated into the hydrogel system.•The shell was micro-porous and the core was macro-porous.•The hydrogels could be used in multiple ...adsorption–desorption cycles.•The mechanical strength and adsorption capacity of hydrogel was modified by incorporating cellulose nanofibers.
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Core-shell structured hydrogels consisting of a flexible interpenetrating polymer network (IPN) core and a rigid semi-IPN shell were prepared through chemical crosslinking of polyvinyl alcohol (PVA) and sodium alginate (SA) with Ca2+ and glutaraldehyde. Short cellulose nanofibers (CNFs) extracted from energycane bagasse were incorporated in the hydrogel. The shell was micro-porous and the core was macro-porous. The hydrogels could be used in multiple adsorption–desorption cycles for dyes, and the maximum methyl blue adsorption capacity had a 10% increase after incorporating CNFs. The homogeneous distribution of CNFs in PVA-SA matrix generated additional hydrogen bonds among the polymer molecular chains, resulting in enhanced density, viscoelasticity, and mechanical strength for the hydrogel. Specifically, the compressive strength of the hydrogel reached 79.5kPa, 3.2 times higher than that of the neat hydrogel.
The hazards of handling
N
-methylmorpholine-
N
-oxide (NMMO) cannot be described often enough, although it is a rather common (co-)solvent for cellulose. Inaccuracies in the literature regarding the ...chemistry of NMMO, such as its role in solvent systems and in redox systems involving transition metal ions, increase the risk of dangerous failure of the intended reactions which might result in uncontrolled exothermicities, damage of laboratory equipment or even accidents and work safety issues. Therefore, suitable cautionary and mitigation strategy statements regarding the hazards of handling NMMO are encouraged for future publications regarding NMMO and cellulose.
Cellulose serves as a skeleton for many of the useful products upon which we rely on each day. When we want to learn about a skeleton, it makes sense to think about X-ray methods. The same can be ...said when it comes to learning about the crystallinity of cellulose. Over the past six decades, the Segal X-ray diffraction (XRD) method has been popular for judging the percent crystallinity of powder samples. However, XRD patterns for ideal cellulose crystals can be easily simulated, and limitations of the Segal and other methods become obvious. Calculated patterns for model 100% crystalline powder particles are predicted to be less crystalline by the Segal method. Except for the Rietveld method, current approaches do not account for particle orientation or different shapes of crystallites. The Rietveld method has so many variables that it can easily overfit the data. The take-away message is that routine XRD examination is important for showing sample characteristics, but fractional crystallinity values are affected by constraints related to simplifications required for the analysis.
•The revised highlights are as below: Cellulose I/II hybrid fibers were obtained during delignification process.•Defibrillation effect with reduced fiber width was observed.•The crystallinity for ...cellulose fibers decreased after NaClO2 treatment.•XRD, 13C NMR and FTIR results showed the same trend in the degree of crystallization.
Cellulose I, cellulose II and cellulose I/II hybrid fibers were prepared from energycane bagasse using NaOH and NaClO2 treatments. The definitive defibrillation effect with an average width of 12±5μm was observed for the fibers treated with 20wt% NaOH for 10h and NaClO2 for 2h. The ribbon shaped cellulose I fibers were converted to a swollen state with a rougher surface by 20wt% NaOH treatment for 10h. The percentage of cellulose I decreased from 100% to 5%, and the corresponding CI values increased from 58.2% to 68.8% during the conversion from cellulose I to II. After further NaClO2 treatment, the CI values were decreased because of partial destruction of hydrogen bond network. XRD, NMR and FTIR results present the same trend in the degree of crystallization for all the samples.
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