Electron paramagnetic resonance spectroscopy of γ-irradiated cellulose and related compounds

EPR studies were carried out on different irradiated cellulosic materials. The shape of the EPR spectrum was found to depend upon the fine structure of cellulose. The peak-to-peak distances of the large peaks on the differential curves were found to be 23.3 and 24.8 gauss, respectively, for the amorphous and recrystallized cellulose. The number of free radicals present per unit weight in the recrystallized cellulose and in the amorphous cellulose were found to be in the ratio of 2.1:1. The EPR spectrum of a sample of irradiated cotton crystallites which was soaked in acetone before being dried was markedly symmetrical, in contrast to the EPR spectra of samples of cotton linters and of cotton crystallites dried from water, and was considered to be a reflection of the lower intermolecular bonding in the former. For corresponding doses, the free radicals on cotton crystallites have the highest g value, followed by cotton linters, followed by regenerated cellulose I. The g value decreased as the irradiation dose was increased.     

On the reactivity of cellulose free radicals in graft copolymerization reactions

The formation and behavior of photo-and mechanoinduced free radicals in cellulose were studied by ESR spectroscopy and the capability of these free radicals to initiate graft copolymerization reactions was demonstrated. Although an 11-line ESR signal was detected from cellulose irradiated with ultraviolet (UV) light, a higher-intensity ESR signal with a five-line pattern was detected from a sample mechanically milled at 77 K. The decay of photoinduced free radicals when heated took place monotonously, whereas mechanoradicals exhibited an anomalous behavior with an increased signal intensity at 150 K before decaying at a higher temperature. Mechanoradicals have been found to react more efficiently and rapidly with oxygen and methyl methacrylate (MMA) than photoinduced free radicals. The peroxy mechanoradicals, however, were mobile and decayed more rapidly than the peroxy photoinduced radicals. Simultaneous graft copolymerizations of MMA to cellulose demonstrated that mechano-and photoinduced free radicals are capable of initiating grafting reactions, but a higher degree of grafting efficiency was obtained from cellulose treated mechanically.     

γ-radiolysis of acetylated cotton cellulose: An ESR study

The nature and behavior of free radicals induced in acetylated cotton celluloses irradiated with γ-rays have been studied by electron spin resonance (ESR) spectroscopy. Dehydrogenation and deacetylation appear to be responsible for the free radicals observed from samples irradiated at 77°K. The degree of substitution enhanced the yield of acetyl radicals when the samples were irradiated at 77°K and adversely affected the overall radical concentration when irradiation was done at 300°K. In addition, the ESR spectra of samples irradiated under vacuum at 300°K were more intense than those obtained from samples irradiated in air. The nature, yield, and post-irradiation behavior of the primary radicals are discussed in the light of the ultimate chemical effects observed.     

The influence of fine structure on the pyrolysis of cellulose. I. Vacuum pyrolysis

The effects of crystallinity, orientation and degree of polymerization on the vacuum pyrolysis of cellulose have been investigated. Natural, modified and manmade celluloses were characterized and the kinetics of their pyrolysis at 251°C studied. The high-temperature pyrolysis of these samples was also investigated by means of DSC and TGA. At 251°C all of the samples showed a rapid, initial weight loss followed by a linear (weight loss)^1/2 versus time dependency. Both the initial weight loss and the subsequent reaction rate were dependent upon the crystallinity of the sample. The reaction rate was shown to be inversely proportional to (DP)^1/2 and related to the orientation. Effects of crystallinity and orientation could also be seen in the DSC thermograms and in the apparent energies of activation. Results are interpreted in terms of probable mechanisms of the uncatalyzed pyrolysis of cellulose.     

Hydrogen bonding on accessible surfaces of cellulose from various sources and relationship to order within crystalline regions

Celluloses from a variety of common sources were analyzed for availabilities of O(2)H, O(3)H, and O(6)H in order to estimate the extent of hydrogen bonding on accessible fibrillar surfaces. Celluloses from flax, ramie, sisal, and wood (both cellulose I and II from wood) together with liquid NH₃-swollen cotton and NaOH-swollen cotton (cellulose II) had relative availabilities similar to those of native cotton. Celluloses from Valonia centricosa and in rayon samples stood apart from each other and from the “cotton family.” The difference between Valonia and cotton celluloses appears to result, in addition to the accepted smaller, less perfect crystallites in cotton, from an O(2)H hydrogen bond which is likely the intramolecular bond between O(2)H and O(6′)H that is present in Valonia and absent in cotton. Rayon samples also showed evidence of similar bonds involving O(2)H on accessible surfaces. Since the regenerated rayons had relative availabilities different from those of mercerized cotton and wood cellulose samples, it is proposed that chain packing arrangements are not the same in these two types of cellulose II.     

Electron beam irradiated textile cellulose fibres.: ESR studies and derivatisation with glycidyl methacrylate (GMA)

Powdered samples of cotton, flax and viscose from textile fibres were subjected to electron beam irradiation (20-400 KGy). ESR signals were observed after irradiation. The intensity of the signals depended on the administered dose and exhibited an exponential decay with time. The ESR spectra, whose pattern depended drastically on the crystallinity of the samples, indicated the simultaneous presence of at least three carbon-centred radical species. When the whole irradiated textile fibres were quenched in a solution of glycidyl methacrylate (GMA) a significant increase of the weight was observed, the increase being strongly dependent on the administered dose and on the time interval elapsed between irradiation and quenching. Spectroscopic characterisation evidenced that GMA acted as cellulose radicals scavenger on the unsubstituted carbon of the double bond and was covalently linked to the fibre through a new C-C bond while maintaining unaltered the ester and the epoxide groups, available for further derivatisations. Because irradiation of the whole textile fibres in the range 20-200 KGy does not result in significant degradation of the material, GMA quenching of fibres irradiated in this dose range may represent an attractive route to their functionalisation.     

Formation of free radicals in photoirradiated cellulose. I. Effect of wavelength

Formation of free radicals in photoirradiated cellulose has been studied by means of ESR spectroscopy at 77°K. Three kinds of light sources with different wavelengths between 2500 and 4000 ÅR were employed. No radicals detectable by ESR were recorded when cellulose was irradiated with light of wavelength longer than 3300–3400 ÅR. Hydrogen atoms that generated a doublet spectrum (ΔH = 508 G) were observed when cellulose was irradiated with light longer than 2800 ÅR. Hydrogen atoms and formly radicals that generated doublet spectra with splitting constants of 508 and 129 G, respectively, were observed when irradiated with light shorter than 2800 ÅR. The scission of the polymer chain in cellulose is evident from decrease of the degree of polymerization, and the results of mass spectrometric analysis indicated H₂, CO, CO₂, and H₂O to be the main volatile products of cellulose upon photoirradiation.     

Measuring the crystallinity index of cellulose by solid state 13C nuclear magnetic resonance

The crystallinity index of cellulose is an important parameter to establish because of the effect this property has on the utilization of cellulose as a material and as a feedstock for biofuels production. However, it has been found that the crystallinity index varies significantly depending on the choice of instrument and data analysis technique applied to the measurement. We introduce in this study a simple and straightforward method to evaluate the crystallinity index of cellulose. This novel method was developed using solid state ¹³C NMR and subtraction of the spectrum of a standard amorphous cellulose. The crystallinity indexes of twelve different celluloses were measured and the values from this method were compared with the values obtained by other existing methods, including methods based on X-ray diffraction. An interesting observation was that the hydration of the celluloses increased their crystallinity indexes by about 5%, suggesting that addition of water increased cellulose order for all the cellulose samples studied.     

Synthesis and characterization of oxidized cellulose

Samples of oxidized cellulose (OC) with various carboxyl contents and degrees of crystallinity were obtained by the oxidation of native and mercerized cellulose with a solution of nitrogen(IV) oxide in CCl₄. A detailed characterization of these OC samples was performed. The effect of oxidation conditions (concentration of N₂O₄ in the solution and oxidation time) and starting cellulose material on OC characteristics (carboxyl, carbonyl and nitrogen content, degree of crystallinity and polymerization, surface area and swelling, and acidic properties) was investigated. Reactivity in the oxidation process was higher in mercerized cellulose than in native cellulose. The action of dilute solutions (10–15%) of N₂O₄ did not affect the degree of crystallinity of cellulose samples. Under these conditions, the oxidation took place mainly in amorphous regions and on the surface of crystallites. Oxidation in a concentrated (40%) N₂O₄ solution led to the destruction of crystallites, which increased the surface area and swelling of cellulose in water. The surface area and the swelling of OC samples increased with a decrease in the index of crystallinity. The acidic properties of OC were shown to increase with an increase of swelling in water. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4785–4791, 2004     

Mercerization and acid hydrolysis of bacterial cellulose

Structural changes in never- dried, disintegrated bacteria l cellulose by treatment with aqueous NaOH were examined by electron microscopy, X-ray diffractometry and acid hydrolysis behaviour and compared with those of cotton cellulose. The microfibril kept its fibrillar morphology after treatment with NaOH solutions of less than 9% (w/w), but changed into irregular aggregates when treated with NaOH above 12% (w/w), corresponding to the crystal conversion to cellulose II. The crystallinity of the resulting cellulose II was very low after a brief alkali treatment, but was increased significantly by elongated treatment (up to 10 days). In contrast, cotton cellulose was converted to cellulose II of fairly high crystallinity by alkali treatment of as little as 3 min duration, and the crystallinity did not change with longer treatments. The leveling-off degree of polymerization (LODP) of bacterial cellulose was decreased from 150 to 50 by 18% (w/w) NaOH treatment, while that of cotton linter decreased from 260 to 70. These characteristic differences between cotton linter cellulose and bacterial cellulose can be ascribed to a basic difference in microfibrillar organization in these materials: the microfibrils in cotton cellulose are in close contact with neighbouring microfibrils having opposite polarity, and in bacterial cellulose are isolated from each other and require chain folding to form the antiparallel cellulose II crystal     

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.     

Cellulose I crystallinity determination using FT–Raman spectroscopy: univariate and multivariate methods

Two new methods based on FT–Raman spectroscopy, one simple, based on band intensity ratio, and the other using a partial least squares (PLS) regression model, are proposed to determine cellulose I crystallinity. In the simple method, crystallinity in cellulose I samples was determined based on univariate regression that was first developed using the Raman band intensity ratio of the 380 and 1,096 cm^?1 bands. For calibration purposes, 80.5% crystalline and 120-min milled (0% crystalline) Whatman CC31 and six cellulose mixtures produced with crystallinities in the range 10.9–64% were used. When intensity ratios were plotted against crystallinities of the calibration set samples, the plot showed a linear correlation (coefficient of determination R ² = 0.992). Average standard error calculated from replicate Raman acquisitions indicated that the cellulose Raman crystallinity model was reliable. Crystallinities of the cellulose mixtures samples were also calculated from X-ray diffractograms using the amorphous contribution subtraction (Segal) method and it was found that the Raman model was better. Additionally, using both Raman and X-ray techniques, sample crystallinities were determined from partially crystalline cellulose samples that were generated by grinding Whatman CC31 in a vibratory mill. The two techniques showed significant differences. In the second approach, successful Raman PLS regression models for crystallinity, covering the 0–80.5% range, were generated from the ten calibration set Raman spectra. Both univariate-Raman and WAXS determined crystallinities were used as references. The calibration models had strong relationships between determined and predicted crystallinity values (R ² = 0.998 and 0.984, for univariate-Raman and WAXS referenced models, respectively). Compared to WAXS, univariate-Raman referenced model was found to be better (root mean square error of calibration (RMSEC) and root mean square error of prediction (RMSEP) values of 6.1 and 7.9% vs. 1.8 and 3.3%, respectively). It was concluded that either of the two Raman methods could be used for cellulose I crystallinity determination in cellulose samples.     

Photo-induced radical formation in cellulose

The characteristics of cellulose radicals produced by photo-irradiation were studied from their ESR spectra. All spectra of the irradiated samples contained a singlet spectrum with a line width of 28 gauss. Spectra consisting mainly of a single absorption line were observed in the initial stage of irradiation. With the elapse of irradiation time, two absorption lines with a line width of 8 gauss were added to the original singlet spectrum, leading to the three absorption lines widths of 8, 28, and 8 gauss, respectively, of which the middle line with a line width of 28 gauss showed the maximum intensity. This three-line spectrum was easily produced when hydrogen peroxide or metallic ions such as Fe³⁺, Ag⁺, Fe²⁺, and Ce⁴⁺ were used as sensitizers. The observed ESR spectra consisted of a singlet with a line width of 28 gauss and a triplet with a line width of 8 gauss and a splitting factor of 28 gauss. The triplet component was more unstable toward warming, as compared with the singlet component. Based on the experimental results, it was suggested that cellulose radicals due to the scissions of cellulose chains corresponding to the ESR spectra with a single line could be the most important factor for the initiation of graft copolymerization.     

Effect of structural and physico-chemical features of cellulosic substrates on the efficiency of enzymatic hydrolysis

Effects of major physicochemical and structural parameters of cellulose on the rate and degree of its enzymatic hydrolysis were tested with cellulosic materials from various sources. Some different pretreatments were: mechanical (milling), physical (X-ray irradiation), and chemical (cadoxen, H₃PO₄, H₂SO₄, NaOH, Fe²+/H₂O₂). The average size of cellulose particles and its degree of polymerization had little effect on the efficiency of enzymatic hydrolysis. For samples of pure cellulose (cotton linter, microcrystalline cellulose, α-cellulose), increase in the specific surface area accessible to protein molecules and decrease in the crystallinity index accelerated the enzymatic hydrolysis (the correlation coefficients were 0.89 and 0.92, respectively). In the case of lignocellulose (bagasse), a quantitative linear relationship only between specific surface area and reactivity was observed.     

Supramolecular Structure - A Key Parameter for Cellulose Biodegradation

Summary: Three different cellulosic substrata, like microcrystalline cellulose, cotton cellulose and spruce dissolving pulp, were chosen for biodegradation. The kinetics of the enzymatic hydrolysis of these celluloses by Trichoderma reesei, has been investigated. The experiments proved the fact that both the morphological structure and the crystalline one are crucial to the process and the ratio of the reactions. In addition, in order to obtain the most accessible cellulose substratum it was studied the biodegradation of cellulose allomorphs of spruce dissolving pulp. The insoluble cellulose fraction remaining after enzymatic hydrolysis was examined by X-ray diffraction method and it was established the degree of crystallinity and the average crystallite size. The enzymatic degradation is also proved by the decrease in the degree of polymerization of hydrolyzed samples.     

The oxidation and thermal transformations of macrooradicals in gamma irradiated cellulose

The secondary reactions of the oxidation and thermal transformations of gamma irradiated (at 77 K) and plasticized (with water) cellulose radicals were studied by 3 cm-and 2 mm-band EPR spectroscopy. The radiolysis of cotton cellulose was found to produce the H-C*=O formyl radical, and heating the irradiated samples to 190–200 K resulted in the formation of the ROO* peroxide radical. The EPR spectra of microcrystalline cellulose recorded at room temperature contained an individual triplet (α _β ^H = 2.5–2.7 mT) with an additional quadruplet structure (splitting 0.5–0.7 mT) from three γ-hydrogens. This triplet was interpreted as a signal of the primary radical at C4. The main direction of thermal transformations of primary radicals was synchronous reactions of the dehydration of the polycarbohydrate complex accompanied by the dissociation of the C-H, C-OH, and C-C bonds and elimination of H₂O, H₂, CO, and CO₂ with successive formation of allyl and then polyene radicals, which were a source of the growth of polyconjugated systems in macromolecules.     

Controlled production of spruce cellulose gels using an environmentally “green” system

Stable spruce cellulose suspensions were generated in NaOH/urea aqueous solutions and used to make thermally induced gels with various swelling ratios and compressive strengths. Wood cellulose cannot be easily dissolved in water or any common organic solvent due to its high molecular weight, which largely limits its applications. Spruce cellulose was hydrolyzed by diluted sulfuric acid of various concentrations and hydrolysis times. The dissolution of these partially degraded samples was investigated in a NaOH/urea aqueous solution system considered environmentally “green.” The effects of acid hydrolysis on the structure and properties of subsequent thermally induced gels were examined using scanning electron microscopy, swelling and re-swelling experiments, and mechanical testing. The molecular weight of spruce cellulose was significantly reduced by acid hydrolysis, whereas its crystallinity slightly increased because of the removal of amorphous regions. All samples could be partially dissolved in the NaOH/urea aqueous solution and formed stable suspensions. Hydrolyzed cellulose samples with lower molecular weight exhibited a higher solubility. Rheological experiments showed these cellulose suspensions could form gels easily upon heating. A porous network structure was observed in which dissolved cellulose was physically crosslinked upon heating and then regenerated to form a three-dimensional network, where the dispersed swollen cellulose fibers filled spaces to reinforce the structure. The swelling behavior and mechanical properties of these ‘matrix-filler’ gels could be controlled by varying the mild acid hydrolysis conditions, which adjusts their degree of solubility. This research provides several opportunities for manufacturing wood cellulose based materials.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.     

Physical and Chemical Characterization of Cellulose Based Textiles Modified by Periodate Oxidation

Conversion of dihydroxyl groups to dialdehyde by periodate oxidation is a useful method widely used in derivatization of cellulose to activate the polymer to further reactions as grafting polymerization. To investigate the cellulose behavior at different level of oxidation and to better understand the influence of the crystallinity on the effects induced by oxidative reactions on different cellulose materials, linen and cotton textiles have been oxidized with periodate solutions in different conditions. Oxidized cellulose samples have been characterized by several techniques: solid-state ¹³C NMR, Wide Angle X-Ray diffraction, and SEM. Moreover the mechanical properties of the untreated and oxidized yarns have been evaluated by means of tensile tests, the oxidation degree has been measured by means of the hydroxylamine hydrochloride method.     

Degradation processes in the cellulose/<Emphasis Type="Italic">N</Emphasis>-methylmorpholine-<Emphasis Type="Italic">N</Emphasis>-oxide system studied by HPLC and ESR. Radical formation/recombination kinetics under UV photolysis at 77 K

Degradation processes of N-methylmorpholine-N-oxide monohydrate (NMMO), cellulose and cellulose/NMMO solutions were studied by high performance liquid chromatography (HPLC) and electron spin resonance (ESR) spectroscopy. Kinetics of radical accumulation processes under UV (λ = 248 nm) excimer laser flash photolysis was investigated by ESR at 77 K. Beside radical products of cellulose generated and stabilized at low temperature, radicals in NMMO and cellulose/NMMO solutions were studied for the first time in those systems and attributed to nitroxide type radicals ∼CH₂–NO^•–CH₂∼ and/or ∼CH₂–NO^•–CH₃∼ at the first and methyl ^•CH₃ and formyl ^•CHO radicals at the second step of the photo-induced reaction. Kinetic study of radicals revealed that formation and recombination rates of radical reaction depend on cellulose concentration in cellulose/NMMO solutions and additional ingredients, e.g., Fe(II) and propyl gallate. HPLC measurements showed that the concentrations of ring degradation products, e.g., aminoethanol and acetaldehyde, are determined by the composition of the cellulose/NMMO solution. Results based on HPLC are mainly maintained by ESR that supports the assumption concerning a radical initiated ring-opening of NMMO.