Crystallite structure of cellulose

The crystallite structure of cellulose has been elucidated through analyses of the degradations of model compounds by gel-permeation chromatography. These compounds include single crystals of cellulose triacetate, regenerated cellulose from single crystals, precipitated celluloses from solutions, and commercial regenerated cellulose. The corresponding distribution profiles are found to follow the Keller transformation, which is a characteristic behavior of the folded crystals of synthetic polymers. It is also found that the leveling-off DP of cellulose is a first-order approximation of the corresponding fold length. A detailed folding chain model for the regenerated cellulose was constructed, and the various aspects of the structure are discussed. The kinetic data and the degradation products of native celluloses were also analyzed and found to obey the ruling of the chain fold model but not the conventional fringe-micellar model. In addition, an iodine-staining experiment pinpoints a minimum of 1.5% of true amorphous material; this is in quantitative agreement with the conformation analysis for the six-fold helical β-loop bonds. It is therefore suggested that the same chain-fold conformation is applicable to the native polymer. The hydrolysis of cellulose is found to be composed of a triple mode of degradation, i.e., a first-order random scission at the folds, a zero-order peeling reaction at the ends of the crystallites, and a first-order random scission on the lateral surfaces of the cellulose.     

Heterogeneous modification of various celluloses with fatty acids

Heterogeneous modification of various types of cellulose (microcrystalline cellulose, cellulose whiskers and regenerated cellulose) was performed with long-chain fatty acids by an esterification reaction. The differences in reactivity between the celluloses were studied as well as the influences of the chain length and double bond content of the fatty acids. The success of the modification reaction and the structure of modified samples were studied with diverse characterization methods. Surface modification changed the thermal stability of cellulose by decreasing the degradation temperature but also made the pyrolysis curve two-stepped due to the double bonds in the fatty acid chain. It was observed that the nature of the fatty acid affected the degree of substitution (DS). The longer the fatty acid chain was, the lower was the DS. Fatty acids with increased double bond content gave decreased DS. Regenerated cellulose seemed to have the highest surface reactivity due to the distinct morphological structure, which also led to a much lower quantity of fatty acids attached to the structure than for other modified cellulose particles. The mixture of tall oil fatty acids behaved in the same manner as the commercial fatty acids, proving to be an excellent “green” choice for this kind of application.     

SILYLATION OF CELLULOSE WITH TRICHLOROSILANE AND TRIETHOXYSILANE IN HOMOGENEOUS LiCl/N,N-DIMETHYLACETAMIDE SOLUTION*

Silyl celluloses (SiC) were prepared by reacting cellulose with chloropropyltrichlorosilane (CPTCSi) andchloropropyltriethoxysilane (CPTESi) in LiCl/N,N-dimethylacetamide (DMAc). The Si content in the silyl cellulose could becontrolled by adjustment of the molar ratio of silane and cellulose. FT-IR spectra showed that cellulose was readily reactedwith the above two silane reagents, and the reactivity of CPTCSi is higher than that of CPTESi. It was presumed that thereaction process belongs to graft-polymerization. The results of differential thermal analysis (DTA) indicated that thethermostability of the materials produced increased with the increase of Si content in the sample. The acid resistance of thesamples SiC in 1 mol/L HCl aqueous solution was improved significantly. When Si content was ca. 20%, the silyl cellulosehas excellent thermostability, hydrophobicity, low density and stability in 1 mol/L HCl aqueous solution, owing tocrosslinking of cellulose chain with silane.     

Dissolution of Cellulose in Aqueous NaOH Solutions

Dissolution of a number of cellulose samples in aqueous NaOH was investigated with respect to the influence of molecular weight, crystalline form and the degree of crystallinity of the source samples. A procedure for dissolving microcrystalline cellulose was developed and optimized, and then applied to other cellulose samples of different crystalline forms, crystallinity indices and molecular weights. The optimum conditions involved swelling cellulose in 8–9 wt % NaOH and then freezing it into a solid mass by holding it at –20°C. This was followed by thawing the frozen mass at room temperature and diluting with water to 5% NaOH. All samples prepared from microcrystalline cellulose were completely dissolved in the NaOH solution by this procedure. All regenerated celluloses having either cellulose II or an amorphous structure prepared from linter cellulose and kraft pulps were also essentially dissolved in the aqueous NaOH by this process. The original linter cellulose, its mercerized form and cellulose III samples prepared from it had limited solubility values of only 26–37%, when the same procedure was applied. The differences in the solubility of the celluloses investigated have been interpreted in terms of the degrees to which some long-range orders present in solid cellulose samples have been disrupted in the course of pre- treatments.     

Polyglucane derivatives with regular substituent distribution

6-O-trialkylsilyl celluloses, 2,3-O-carboxymethylcelluloses, cellulose-3-O-sulfate, and carboxymethylcellulose block copolymers were synthesized by regioselective functionalization of cellulose and of soluble cellulose intermediates like silyl and methoxy-substituted trityl ethers as well as formates and trifluoroacetates. The preparation and structure characterization (NMR, FTIR, HPLC after chain degradation) of those polyglucane derivatives with regular substituent distribution is of importance to design self-assembly systems and supramolecular structures (liquid-crystalline media, ultrathin films, recognition and bioactive materials).     

Molecular morphology of cellulose

Native celluloses of various biological origins, as well as regenerated celluloses were examined by electron microscopy after suitable dispersion. In all cases the specimens were found to be composed of a common filamentary unit which is rectangular in cross section and has the approximate dimensions 35 × 20 Å. It is suggested that these are the basic morphological units of cellulose; they are therefore called protofibrils. For protofibrils of regenerated cellulose it is shown that: (1) the molecular contour length greatly exceeds the protofibril length, (2) the mass of the protofibril corresponds to that of a single molecule, and (3) the protofibril length increases with molecular weight. Additionally, high resolution electron micrographs of native and regenerated protofibrils show an apparent axial texture with a periodicity of about 40 Å. From these observations and the knowledge that the molecular chain axis is aligned parallel to the protofibril axis, a model of the protofibril is deduced. The model consists of a ribbon which is pleated on itself so as to form a planar zigzag structure of rectangular cross section. This supersedes a previously proposed model of circular cross section. The structure is composed of a single folded, chain, arranged so that the short extended segments between the folds are parallel to the protofibril axis. The protofibril is thus regarded as the morphological expression of the cellulose molecule. Microfibrils and protofibrils often exhibit kinks, the angle between the kinked portions being 120°. This phenomenon is satisfactorily explained by the protofibril model and in fact provides good support for it. Finally, various properties of cellulose are considered in relation to the model. By contrast with the earlier crystalline–amorphous concepts of cellulose fine structure, it is suggested that protofibrils are completely crystalline structures, and that the properties of cellulose may be understood by considering processes that occur at the level of the protofibril as a unit.     

Mechanical properties of films made from dialcohol cellulose prepared by homogeneous periodate oxidation

2,3-Dialdehyde celluloses were prepared by homogeneous periodate oxidation in an aqueous solution of methylol cellulose. Since methylol cellulose stays dissolved in water for a certain time before decomposing gradually into regenerated cellulose, the oxidation reaction progressed homogeneously throughout the period. The resulting dialdehyde cellulose achieved an oxidation level of over 90 % in as little as 12 h. Reducing the dialdehyde celluloses with NaBH₄ resulted in water-soluble dialcohol celluloses, which have an open-ring structure at the C2–C3 position. The dialcohol celluloses were characterized using nuclear magnetic resonance spectrometry, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The T_g of the products decreased with increasing oxidation levels. The products might be processable, and unique tensile properties were obtained by cutting the C2–C3 bonds in the glucopyranose rings. The dialcohol celluloses prepared using a cast method yielded clear and transparent films which showed unique mechanical properties by tensile tests depending on the values of oxidation level.     

SEC-MALS analysis of cellouronic acid prepared from regenerated cellulose by TEMPO-mediated oxidation

The 4-amino-2,2,6,6-tetramethylpiperidine-1-oxy radical (4-amino-TEMPO)-mediated oxidation was applied to regenerated cellulose, and the obtained cellouronic acid was analyzed by size-exclusion chromatography attached with a multi-angle laser light scattering detector (SEC-MALS). Although the cellouronic acid filtered with the usual 0.1 μm membrane gave a bimodal SEC-elution pattern, the high-molecular-mass fraction was removed by micro filtration of the cellouronic acid solution with a 0.02 μm membrane. It is likely, therefore, that some colloidal particles formed from regenerated cellulose by the TEMPO-mediated oxidation are present as incompletely oxidized residues in the cellouronic acid sample and behave as the high-molecular-mass fraction in the SEC elution pattern. Then, the SEC-MALS analysis was applied to the 0.02 μm membrane-filtered cellouronic acid, and the accurate DPw value of 36 was obtained for cellouronic acid. This DPw value was far lower than that of carboxymethyl cellulose, hydroxypropyl cellulose or alginic acid, resulting from significant depolymerization of cellulose chains during the TEMPO-mediated oxidation. Because the value of DPw 36 for cellouronic acid is close to the leveling-off DP (about 40) of regenerated celluloses obtained by the dilute and heterogeneous acid hydrolysis, the DPw value of cellouronic acid must reflect the solid-state structure of the original regenerated cellulose used in the TEMPO-mediated oxidation.     

Quantitative analysis of cellulose-reducing ends

Methods for the quantification of total and accessible reducing ends on traditional cellulose substrates have been evaluated because of their relevance to enzyme-catalyzed cellulose saccharification. For example, quantification of accessible reducing ends is likely to be the most direct measure of substrate concentration for the exo-acting, reducing end-preferring cellobiohydrolases. Two colorimetric assays (dinitrosalicylic acid [DNS] and bicinchoninic acid [BCA] assay) and a radioisotope approach (NaB³H₄ labeling) were evaluated for this application. Cellulose substrates included microcrystalline celluloses, bacterial celluloses, and filter paper. Estimates of the number of reducing ends per unit mass cellulose were found to be dependent on the assay system (i.e. the DNS and BCA assays gave strikingly different results). DNS-based values were several-fold higher than those obtained using the BCA assay, with fold-differences being substrate specific. Sodium borohydride reduction of celluloses, using cold or radiolabeled reagent under relatively mild conditions, was used to assess the number of surface (solvent-accessible) reducing ends. The results indicate that 30–40% of the reducing ends on traditional cellulose substrates are not solvent accessible; that is, they are buried in the interior of cellulose structures and thus not available to exo-acting enzymes.     

Silylation of cellulose regiocontrolled by bulky reagents and dispersity in the reaction media

Reaction of cellulose (1) with the bulky thexyldimethylchlorosilane (TDMSCl) leads to regioselectively functionalized silyl celluloses with degree of substitution (DS) up to 2. Uniform 6-O-thexyldimethylsilyl cellulose (2) was obtained in ammonia-saturated aprotic dipolar media, which was unexpected in a heterogeneous reaction. On the contrary, the 6-O-selectivity is low under homogeneous conditions in a N,N-dimethylacetamide (DMA)/LiCl solution, and with an excess of TDMSCl 2,3-di-O-thexyldimethylsilyl cellulose (3) can be synthesized. The polymer structures were characterized by two dimensional NMR spectroscopy after subsequent methylation (polymers 4 and 5 ), desilylation (polymers 6 and 7 ) and acetylation (polymers 8 and 9 ) as well as by HPLC after chain degradation of 4 and 5 to the complementary methyl glucoses.     

Discoloration of celluloses treated with amino compounds

Degradation and discoloration of celluloses treated with different amino compounds, including primary amines and hexamethylenetetramine (HMTA), were studied using spectroscopic techniques. The colour generation was measured using standard colorimetry and the degradation and discoloration were characterized using diffuse reflectance FTIR spectroscopy and diffuse reflectance UV-visible spectroscopy. The colour of all the treated celluloses was due to the same chromophores, thus revealing that the discoloration mechanism was essentially the same. The most important reaction was the condensation between carbonyl and amino groups to form coloured imines, and secondary processes were detected at high temperatures and long heating times. The discoloration of celluloses modified with primary amines followed first-order kinetics. However, the discoloration of HMTA-treated cellulose showed an induction period which was related to the thermal decomposition of HMTA.     

Pyrolysis chemical ionization mass spectrometry of cellulose ethers

Pyrolysis ammonia chemical ionization (PyCI) mass spectrometry was performed on hy-droxyethyl-, hydroxypropyl-,methyl-, hydroxypropylmethyl-, and ethylhydroxyethyl cel-luloses. The mass peaks in the PyCI mass spectra of these cellulose ethers could be assigned to the ions of pyrolytic dissociation products which form via the [2 + 2 + 2] cycloreversion and the E_i elimination pyrolysis pathway. Structural information about the residual amount of nonderivatized cellulose, the relative chain length distributions of the substituents in hydroxyalkyl celluloses, and the end-capping of hydroxyalkyl substituents by alkyl groups in the mixed cellulose ethers is obtained. Interference of secondary pyrolysis products in the PyCI mass spectra is found to be of minor importance, especially in the lower mass regions. © 1995 John Wiley & Sons, Inc.     

Influence of hemicelluloses on the aggregation patterns of bacterial cellulose

Cellulose from the bacteriumAcetobacter xylinum was used as a model system for investigating the influence of other cell wall polysaccharides on the aggregation of cellulose. The patterns of aggregation of the bacterial cellulose were modified when the cellulose was produced in the presence of hemicellulose-like saccharides. The celluloses were found to be more like the I-type found in higher plant celluloses than the I-type in the control bacterial celluloses. The effects of isolation procedures on structure were also explored. It was found that the structures of isolated celluloses were influenced by the procedures used in isolation.Formerly The Institute of Paper Chemistry, Appleton, WI, USA.Retired     

Mechanism of structure formation of microbial cellulose during nascent stage

The structure of microbial cellulose (MC) produced by Acetobacter xylinum was studied in presence of Fluorescent Brightener, Direct Blue 1, 14, 15, 53, Direct Red 28, 75 and 79, as probe. X-ray diffraction pattern of the product showed that it was a crystalline complex of dye and cellulose. The product has the structure in which the monomolecular layer of the dye molecule is included between the cellulose sheets corresponding to the ( $ 1\bar{1}0 $ ) planes of microbial cellulose. As a result of dye inclusion, d-spacing of lower angle plane (100) of products becomes 8.0–8.8 Å instead of 6.1 Å of MC. The d-spacing for the higher angle plane must be (010) plane due to stronger van der Waals forces between the pyranose rings which reduced 5.3 Å space of (110) plane of MC to 3.9–4.5 Å in the product. However, cellulose regenerated from FB, DR28 products was cellulose I and IV, respectively, and that from each DB1, 14, 15, 53, DR75 and 79 products was cellulose II. Solid state ¹³C NMR and deuteration-IR showed the product was non-crystalline which was contrasted to X-ray results. The regenerated celluloses were cellulose I_β, IV_I and II, respectively. Thus the structure of the product depends on the characteristics of dye which affects the conformation of cellulose at the nascent stage by the direct interaction with cellulose chains. The different regenerated celluloses as well as different fine structure in the same cellulose allomorph were produced depending mainly on number and position of the sulfonate groups in the dye.     

Reducing end-specific fluorescence labeled celluloses for cellulase mode of action

Cellobiohydrolases (CBH-s) attack primarily from the cellulose chain ends. It is known that CBH-s can differ in their chain end preference. Here the cellulose reducing end-specific fluorescence labeling was studied with the aim to find the most suitable labeled cellulose for easy determination of CBH-s chain end preference. Anthranilic acid (AA) was used as fluorescence label. Bacterial cellulose, amorphous Avicel and filter paper were subjected to the labeling. Suitability of modified bicincoic acid method for determination of reducing groups on cellulose was also confirmed. AA labeled celluloses were hydrolysed with family 7 and 6 CBH-s from Trichoderma reesei and Phanerochaete chrysosporium. Hydrolysis data were plotted as released label (%) versus total degradation (%) (progress curves). Most characteristic progress curves were obtained with AA labeled regenerated amorphous Avicel where the family 6 CBH-s produced clearly upward curved progress curves confirming their preference for non-reducing ends.     

Variation of xyloglucan substitution pattern affects the sorption on celluloses with different degrees of crystallinity

The sorption of xyloglucan (XG) on cellulose is a basic feature of the supramolecular assembly of plant cell walls. The binding to cellulose of xyloglucan fractions from Rubus fruticosus suspension-cultured cells with different substitution patterns was assayed on celluloses having various degrees of crystallinity between 20 and 95%. The primary structure of XGs differing in their Xyl/Glc ratio affected their binding to cellulose. The less substituted XGs gave the highest binding yields. Selective removal of the terminal fucosyl residues of XGs differentially affected the binding depending on the crystallinity of cellulose. The results showed large variations on the way cellulose crystallinity affects the binding interaction of XGs. Interestingly, one of the highest binding capacities was exhibited by the primary cell wall cellulose isolated from the actual R. fruticosus cells which also had the lowest crystallinity. Differences in binding to primary wall cellulose appeared to be inversely related to the global substitution of the glucan main chain of XGs.     

Degrees of polymerization (DP) and DP distribution of cellouronic acids prepared from alkali-treated celluloses and ball-milled native celluloses by TEMPO-mediated oxidation

Various cellulose II samples, ball-milled native celluloses and ball-milled wood saw dust were subjected to 2,2,6,6-tetramethypyperidine-1-oxyl radical (TEMPO)-mediated oxidation to prepare cellouronic acid Na salts (CUAs). The TEMPO-oxidized products obtained were analyzed by ¹³C-NMR and size-exclusion chromatography (SEC). When the cellulose II samples with degrees of polymerization (DP) of 220–680 were used as the starting materials, the CUAs obtained had weight-average DP (DPw) values of only 38–79. Thus, significant depolymerization occurs on cellulose chains during the TEMPO-mediated oxidation. These DP values of CUAs correspond to the cellulose II crystal sizes along the chain direction in the original cellulose II samples, but not necessarily to their leveling-off DP values. CUAs can be obtained also from ball-milled native celluloses in good yields by TEMPO-mediated oxidation, although their DPw values are lower than about 80. On the other hand, CUA with DPw of about 170 was obtained from ball-milled wood saw dust.     

Physicochemical properties of maize cob cellulose powders reconstituted from ionic liquid solution

Suitable α-cellulose and cellulose II powders for use in the pharmaceutical industry can be derived from maize cob. α-Cellulose was extracted from an agricultural residue (maize cobs) using a non-dissolving method based on inorganic substances. Modification of this α-cellulose was carried out by its dissolution in the ionic liquid 1-butyl-3-methylimidazolium chloride ([C₄mim]Cl), and subsequent regeneration by addition of either water or acetone at room temperature, or of boiling water. X-ray diffraction and infrared spectroscopy results showed that the regenerated celluloses had lower crystallinity, and proved that the treatment with [C₄mim]Cl led to the conversion of the crystalline structure of α-cellulose from cellulose I to cellulose II. Thermogravimetric analysis and differential scanning calorimetry data showed quite similar thermal behavior for all cellulose samples, although with somewhat lower stability for the regenerated celluloses, as expected. The comparison of physicochemical properties of the regenerated celluloses and the native cellulose mainly suggests that the regenerated ones might have better flow properties. For some of the characterizations carried out, it was generally observed that the sample regenerated with boiling water had more similar characteristics to the α-cellulose sample, evidencing an influence of the regeneration strategy on the resulting powder after the ionic liquid treatment.     

The formylation of cellulose

The reaction between formic acid and disordered cellulose is described. It is shown that a degree of substitution as high as 2.5 can be readily obtained without any catalyst. There is no evidence of any difference between the formylation behavior of this cellulose up to the monoformate level and that beyond this level. Formic acid produces ordered, inaccessible regions in disordered cellulose; this behavior reduces the value of the latter material as a calibration standard in formylation studies and also throws doubt on the basic value of formylation as a method of measuring disorder in cellulose. There is evidence that native celluloses formylate less rapidly per unit amount of hydrogenbond-disordered material than do regenerated celluloses; this may be associated with the fibrillar nature of the native celluloses.     

Effect of Physicochemical Characteristics of Cellulosic Substrates on Enzymatic Hydrolysis by Means of a Multi-Stage Process for Cellobiose Production

The effect of two types of cellulose, microcrystalline cellulose and paper pulp, on enzymatic hydrolysis for cellobiose production was investigated. The particle size, the relative crystallinity index and the water retention value were determined for both celluloses. A previously studied multistage hydrolysis process that proved to enhance the cellobiose production was studied with both types of celluloses. The cellobiose yield exhibited a significant improvement (120% for the microcrystalline cellulose and 75% for the paper pulp) with the multistage hydrolysis process compared to continuous hydrolysis. The conversion of cellulose to cellobiose was greater for the microcrystalline cellulose than for the paper pulp. Even with high crystallinity, microcrystalline cellulose achieved the highest cellobiose yield probably due to its highest specific surface area accessible to enzymes and quantity of adsorbed protein.