New Method for Determining the Degree of Cellulose I Crystallinity by Means of FT Raman Spectroscopy

A Raman crystallinity index – Xc_Raman – characterizing the degree of crystallinity of partially crystalline cellulose I samples was created, utilizing the crystallinity dependence of CH₂ bending modes. For calibration, physical mixtures containing different mass fractions of crystalline cellulose I and its amorphous form were prepared. Crystallinities from 0 to 60% were generated. Relative intensity ratios of the Raman lines I and I characterizing crystalline and amorphous parts of cellulose I correlated linearly with the mass fraction of crystalline cellulose I of the mixtures. Xc_Raman values of microcrystalline celluloses of different origins and varying degree of crystallinity correlated reasonably with results obtained from NMR spectroscopy (Xc_NMR values).     

The effect of N2O4 on the crystalline structure of cellulose

Depending on reaction conditions, the system cellulose–N₂O₄ may give two different unstable crystalline compounds, one being an ester (cellulose trinitrite), the second, an adduct of cellulose and HNO₃ (the Knecht compound). For these compounds, mechanisms of the formation of the crystalline phase as a result of topochemical reaction and self-organization are discussed. The different characteristics of structural transformations of the fiber under nitrosation and nitration are noted. The existence of polymorphic forms of the Knecht compound is suggested. These labile nitrogen-containing compounds make possible the regeneration of cellulose in its various modifications (cellulose I, II, IV, or amorphous cellulose) from the cellulose–N₂O₄ system. The formation of unstable compounds and their ability to crystallize in the reaction medium allows the passage from amorphous cellulose to its crystalline modifications II or IV under mild conditions. The causes of decrystallization of cellulose by N₂O₄ are established. © 1993 John Wiley & Sons, Inc.     

The Structure of Cellulose by Conformational Analysis. Part 5. The Cellulose Ii Water Sorption IsothermXS

Exact values of the sorption energies of single molecules of water on all available sorption sites of crystalline cellulose II have been obtained by conformational analysis. The sorption energies are equated to the total energy (E_tot ) of interaction between the water molecule and all the suitable atomic groups of the cellulose. E_tot is composed of van der Waals, H-bond, and electrostatic energies. The interferences of water molecules on vicinal sorption sites were obtained. Sites in which such interference can occur were identified for crystalline cellulose II. Sorption energy in crystalline cellulose II appears to depend only on the interaction of water with surface sorption sites of the crystal. There appears to be favorable sorption on 1) sites exerting high attractive forces, and 2) sites which are exposed and protrude from the crystal surface. Sites recessed from the crystal surface are generally repulsive due to strong interactions with neighboring groups. All the sorption energies of the “monolayer” were calculated. Very strong sorption sites cannot always form a second layer because of strong steric hindrance from vicinal groups. Sorption capacities of crystalline cellulose II were calculated, and the isotherm of the schematic five chain crystallite used was constructed by theoretical means. The results obtained were briefly compared with those for cellulose I crystallites and amorphous cellulose. The inflection points of the isotherm and the variability of Dent's k ₁ constant for the water monolayer with relative humidity for the cellulose I and II isotherms were also calculated by theoretical means.     

Investigation of the supramolecular structure of never dried bacterial cellulose

Supramolecular structure of initially wet bacterial cellulose of Acetobacter xylinum has been investigated by X-ray scattering including synchrotron radiation, transmission electron microscopy, and ¹³C-CP/MAS-NMR-spectroscopy. As a result a model is given of never dried swollen microfibrillar ribbons consisting of 5 to 12 waterfree I_α-crystalline subunits with a cross-section of about 7 nm × 13 nm and of water solvating the subunits. Lateral aggregation of these crystalline units was found along the smaller (110)-lattice planes with a layer of water between adjacent crystallites. The NMR-spectrum of wet bacterial cellulose exhibits an additional C-1 line component indicating cellulose-water interactions. During drying lateral dimensions of the microfibrillar ribbons, crystallite sizes, as well as the overall crystalline order decrease, whereas the Iα/Iβ-ratio of about 80/20 remains approximately unchanged. Conclusions were drawn with regard to the early states of structure formation of bacterial cellulose.     

Elastic modulus of the crystalline regions of cellulose polymorphs

The elastic modulus E_l of the crystalline regions of cellulose polymorphs in the direction parallel to the chain axis was measured by x-ray diffraction. The E_l values of cellulose I, II, III_I, III_II, and IV_I were 138, 88, 87, 58, 75 GPa, respectively. This indicates that the skeletons of these polymorphs are completely different from each other in the mechanical point of view. The crystal transition induces a skeletal contraction accompanied by a change in intramolecular hydrogen bonds, which is considered to result in a drastic change in the E_l value of the cellulose polymorphs. © 1995 John Wiley & Sons, Inc.     

X‐ray diffraction study of the thermal expansion behavior of cellulose triacetate I

Highly crystalline samples of cellulose triacetate I (CTA I) were prepared from highly crystalline algal cellulose by heterogeneous acetylation. X‐ray diffraction of the prepared samples was carried out in a helium atmosphere at temperatures ranging from 20 to 250 °C. Changes in seven d‐spacings were observed with increasing temperature due to thermal expansion of the CTA I crystals. Unit cell parameters at specific temperatures were determined from these d‐spacings by the least squares method, and then thermal expansion coefficients (TECs) were calculated. The linear TECs of the a, b, and c axes were α_a = 19.3 × 10^?5 °C^?1, α_b = 0.3 × 10^?5 °C^?1 (T < 130 °C), α_b = ?2.5 × 10^?5 °C^?1 (T > 130 °C), and α_c = ?1.9 × 10^?5 °C^?1, respectively. The volume TEC was β = 15.6 × 10^?5 °C^?1, which is about 1.4 and 2.2 times greater than that of cellulose I_β and cellulose III_I, respectively. This large thermal expansion could occur because no hydrogen bonding exists in CTA I. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 517–523, 2009     

Partial crystalline transformation of solvated cellulose IIII crystals, reproduced by theoretical calculations

In hot-water molecular dynamics simulation at 370 K, four cellulose III_I crystal models, with different lattice planes and dimensions, exhibited partial crystalline transformations of (1 ?1 0) chain sheets, in which hydroxymethyl groups were irreversibly rotated from gt into tg conformations, accompanied by hydrogen-bond exchange from the original O3–O6 to cellulose-I-like O2–O6 bonds. The final hydrogen-bond exchange ratio was about 95 % for some of the crystal models after 50 ns simulation. The corrugated (1 ?1 0) chain sheet was converted to a cellulose-I-like flat chain sheet with a slightly right-handed twist. The 3D structures of the three types of isolated chain sheet models were optimized using density functional theory calculations to compare their stabilities without crystal packing forces. The cellulose Iβ (1 0 0) models were more stable than the cellulose III_I (1 ?1 0) models. The optimized structure of cellulose III_I (1 0 0) models deviated largely from the initial sheet form. It was proposed to the crystalline transformation from cellulose III_I to Iβ that conversion of the chain sheet structure first take place, followed by sliding of the chain sheet along the fiber axis.     

The thermotropic and lyotropic liquid-crystalline properties of acetoacetoxypropyl cellulose

Acetoacetoxypropyl cellulose, formed by the acetoacetylation of hydroxypropyl cellulose using a diketene/acetone adduct at elevated temperature, forms both thermotropic and lyotropic liquid-crystalline phases. DSC and hot-stage polarized light microscopy confirmed the thermotropic nature of the bulk polymer. Thin layers showed green reflection colors at room temperature. The wavelength λ₀ of selective reflection was measured spectrophotometrically. The crystalline structure of the polymer was investigated using x-ray diffraction. A lyotropic mesophase formed in acetic acid at ≥ 40 wt% polymer. The value of λ₀ for the lyotropic cholesteric mesophase was determined by optical rotatory dispersion (ORD) and circular dichroism (CD) of a thin layer of a wholly anisotropic solution.     

The structural changes in crystalline cellulose and effects on enzymatic digestibility

The enzymatic hydrolysis of cellulose I achieves almost complete digestion when sufficient enzyme loading as much as 20 mg/g-substrate is applied. However, the yield of digestion reaches the limit when the enzyme dosage is decreased to 2 mg/g-substrate. Therefore, we have performed three pretreatments such as mercerization, dissolution into phosphoric acid and EDA treatment. Transformation into cellulose II hydrate by mercerization and dissolution into phosphoric acid were not sufficient because substrate changed to highly crystalline structure during saccharification. On the other hand, in the case of crystalline conversion of cellulose I to III_I by EDA, almost perfect digestion was achieved even in enzyme loading as small as 0.5 mg/g-substrate, furthermore, hydrolyzed residue was typical cellulose I. The structural analysis of substrate after saccharification provides an insight into relationships between cellulose crystalline property and cellulase toward better enzymatic digestion.     

Stoichiometry and stability of cellulose-hydrazine complexes

Highly crystalline cellulose samples from green algae (cellulose I) and mercerized ramie (cellulose II) were treated with anhydrous hydrazine and the resulting complexes were analyzed by synchrotron X-ray diffraction and thermogravimetry. Cellulose I-hydrazine complex could be fully described by a two-chain monoclinic unit cell, a = 0.879 nm, b = 1.076 nm, c = 1.038 nm, and γ = 122.0°, with space group P2₁. Cellulose II-hydrazine complex prepared from mercerized ramie gave a different two-chain monoclinic unit cell, a = 1.042 nm, b = 1.046 nm, c = 1.038 nm, γ = 129.7°, also with space group P2₁. Though having different crystal structures, the number of hydrazine molecules per glucopyranoside residue was 0.82 for cellulose I-complex and 0.93 for cellulose II-complex, probable stoichiometric value of 1.0. Hydrazine could be extracted from the complexes by organic solvents retaining the crystalline orders, resulting in the allomorphic conversion to cellulose III_I and cellulose III_II, both having non-staggered chain arrangements. These features are similar to those of cellulose-ethylenediamine complexes.     

Synthesis and acid catalysis of cellulose-derived carbon-based solid acid

SO₃H-bearing amorphous carbon, prepared by partial carbonization of cellulose followed by sulfonation in fuming H₂SO₄, was applied as a solid catalyst for the acid-catalyzed hydrolysis of β-1,4 glucan, including cellobiose and crystalline cellulose. Structural analyses revealed that the resulting carbon material consists of graphene sheets with 1.5 mmol g^?1 of SO₃H groups, 0.4 mmol g^?1 of COOH, and 5.6 mmol g^?1 of phenolic OH groups. The carbon catalyst showed high catalytic activity for the hydrolysis of β-1,4 glycosidic bonds in both cellobiose and crystalline cellulose. Pure crystalline cellulose was not hydrolyzed by conventional strong solid Brønsted acid catalysts such as niobic acid, Nafion^® NR-50, and Amberlyst-15, whereas the carbon catalyst efficiently hydrolyzes cellulose into water-soluble saccharides. The catalytic performance of the carbon catalyst is due to the large adsorption capacity for hydrophilic reactants and the adsorption ability of β-1,4 glucan, which is not adsorbed to other solid acids.     

The changes of crystalline structure of cellulose during dissolution in 1-butyl-3-methylimidazolium chloride

The morphology and crystalline structure changes of cellulose during dissolution in 1-butyl-3-methylimidazolium chloride [(BMIM)Cl] were investigated by optical microscopy and synchrotron radiation wide-angle X-ray diffraction (WAXD). Neither swelling nor dissolution of cellulose was observed under the melting point of [BMIM]Cl. While the temperature was elevated to 70 °C, the swelling phenomenon of cellulose happened with the interplanar spacing of ( _boxclose_boxclose_boxclose0 1\bar{1}0 ) and (020) planes increased slightly. With the temperature further going up to 80 °C, cellulose was dissolved gradually with the crystallinity (W _c,x) and crystalline index (CrI) of cellulose decreased rapidly, which indicated the crystalline structure of cellulose was destroyed completely and transformed into amorphous structure.     

Helical concept of the fine structure of cellulose

The helical concept of the fine structure of cellulose as proposed by Manley is discussed. The deuterium exchange experiments with cotton, cotton crystallites, and regenerated cellulose I, in which accessibility was determined by comparing the ratio of the infrared absorbance of the O? D peak to the O? H peak, revealed that the accessibility of cotton linters decreased on acid hydrolysis, whereas it increased on treatment with ethylamine followed by washing with water. This is in contrast to the finding of Manley, who had evaluated the accessibility by a gravimetric D₂O exchange method and had come to the conclusion that acid hydrolysis did not change the accessibility of cellulose and hence cellulose did not contain crystalline and amorphous phases but was all crystalline. On the basis of Manley's protofibril and the accessibility data obtained in this investigation, a concept of the fine structure of cellulose is proposed, in which the role of Manley's protofibril is analogous to the role of individual molecule in the fringe micellar model. This concept explains the properties of cellulose that are otherwise explainable on the currently accepted fringe micellar theory. In addition, it explains the marked shrinkage in the length of cotton and rayon fibers when placed in 16% sodium hydroxide solution.     

Carbon-13 NMR distinction between categories of molecular order and disorder in cellulose

Differences between values of proton rotating-frame spin relaxation time constants can be exploited to separate a solid-state¹³C NMR spectrum of cellulose into subspectra of crystalline and noncrystalline regions. Variations in chemical shifts and¹³C spin-lattice relaxation time constants can then be used to study variations in molecular order and disorder within each of the two broader categories. Mechanical damage during Wiley milling increases the content of noncrystalline cellulose and changes the nature of molecular disorder within that category. Resolution enhancement of the subspectrum assigned to crystalline cellulose reveals pairs of signals at 83.9 and 84.9 ppm (cellulose I) or 86.8 and 88.3 ppm (cellulose II) assigned to C-4 on well-ordered crystal surfaces. A broader peak in the subspectrum of crystalline cellulose I is assigned to poorly-ordered surfaces. Relative proportions in Avicel microcrystalline cellulose were estimated as: 54% in crystal interiors, 22% on well-ordered surfaces, 8% on poorly-ordered surfaces, 16% in domains of disorder extending more than a few nanometres.     

Phase transformations in microcrystalline cellulose due to partial dissolution

All-cellulose composites were prepared by partly dissolving microcrystalline cellulose (MCC) in an 8.0 wt% LiCl/DMAc solution, then regenerating the dissolved portion. Wide-angle X-ray scattering (WAXS) and solid-state ¹³C NMR spectra were used to characterize molecular packing. The MCC was transformed to relatively slender crystallites of cellulose I in a matrix of paracrystalline and amorphous cellulose. Paracrystalline cellulose was distinguished from amorphous cellulose by a displaced and relatively narrow WAXS peak, by a 4 ppm displacement of the C-4 ¹³C NMR peak, and by values of T₂(H) closer to those for crystalline cellulose than disordered polysaccharides. Cellulose II was not formed in any of the composites studied. The ratio of cellulose to solvent was varied, with greatest consequent transformation observed for c < 15%, where c is the weight of cellulose expressed as % of the total weight of cellulose, LiCl and DMAc. The dissolution time was varied between 1 h and 48 h, with only small additional changes achieved by extension beyond 4 h.     

Structural Parameters of Cellulose Produced by Acetobacter Xylinum and Their Variation in the Course of Drying of Gel Films

The cellulose-producing power of the VKM V-800 Acetobacter xylinum strain under conditions of static culture was studied. The culture medium was optimized with the aim to increase the cellulose yield and obtain highly crystalline cellulose I with molecular weight of about 5 × 10⁵.     

Chemical Synthesis of Native-Type Cellulose and Its Analogues via Enzymatic Polymerization

Abstract

The first successful chemical synthesis of cellulose was achieved by a polycondensation of β-cellobiosyl fluoride monomer catalyzed with cellulase, an extracellular hydrolytic enzyme of cellulose, in a mixed solvent of acetonitrile and acetate buffer. The product, synthetic cellulose, was the crystalline allomorph cellulose II, a thermodynamically more stable form. More detailed examinations of the polymerization conditions led to the formation of the native cellulose I, a metastable allomorph, for the first time. The key to the success was to use partially purified cellulase and an appropriate mixed solvent of acetonitrile/buffer. The formation of the two allomorphs of cellulose implies that the polarity of the glucan chain ordering can be controlled in a test tube. Based on these findings, a new concept “choroselectivity,” meaning spacial control in ordering the macromolecular chain, has been proposed. Cellulose analogues, 6-O-methylated cellulose and xylan, have been synthesized regio- and stereoselectively by using the enzymatic polymerization technique.     

Study of the Heterocoagulation Process in Aqueous System with Three-Component Dispersed Phase Microcrystalline Cellulose‒TiO<Subscript>2</Subscript>‒TiOSO<Subscript>4</Subscript>

Coagulating effect of titanyl sulfate on a mixed aqueous dispersion of microcrystalline cellulose and TiO₂ was examined. The particle retention on a filter and the filtration rate of the microcrystalline cellulose?TiO₂?TiOSO₄ ternary system were evaluated in a wide range of pH values.     

A multivariate characterization of crystal transformations of cellulose

A spectroscopic study of cellulose transformation processes, such as alkali treatment and annealing, showed that, in combination with multivariate data analysis techniques, a detailed understanding of the crystalline transformation processes could be reached.¹³C cross-polarization magic-angle spinning (CPMAS) NMR and near-infrared (NIR) spectroscopy of cotton linters and softwood pulps analysed during the processing revealed information, after data reduction using principal components data analysis, that could be connected to structural changes of the cellulose polymorphs. The data showed that alkali treatment of cotton linters led to a cellulose conversion from cellulose I to II, while annealing, both for linters and pulps, yielded a transformation from I to I.     

Quantum mechanical modeling of the structures, energetics and spectral properties of Iα and Iβ cellulose

Periodic planewave and molecular cluster density functional theory (DFT) calculations were performed on Iα and Iβ cellulose in four different conformations each. The results are consistent with the previous interpretation of experimental X-ray and neutron diffraction data that both Iα and Iβ cellulose are dominantly found in the tg conformation of the hydroxymethyl group with a H-bonding conformation termed “Network A”. Structural and energetic results of the periodic DFT calculations with dispersion corrections (DFT-D2) are consistent with observation suggesting that this methodology is accurate to within a few percent for modeling cellulose. The structural and energetic results were confirmed by comparison of calculated vibrational frequencies against observed infrared and Raman frequencies of Iα and Iβ cellulose. Structures extracted from the periodic DFT-D2 energy minimizations were used to calculate the ¹³C nuclear magnetic resonance chemical shifts (δ¹³C), and the tg/Network A conformations of both Iα and Iβ cellulose produced excellent correlations with observed δ¹³C values.