纤维素醋酸酯的均相合成

纤维素的非均相反应取代不均匀、产率低。本文用纤维素在LiCl／DMAc溶液中的均相反应。制得了纤维素三醋酸酯(CTA)、纤维素二醋酸酯(CDA)、纤维素—醋酸酯(CMA),并对产品结构性能进行了表征。     

Cellulose dissolution in aqueous lithium bromide solutions

A new all-aqueous process of the dissolution/regeneration of cellulose was developed. Cellulose was completely dissolved in the 54–60 wt% lithium bromide aqueous solutions in the temperature range of 110–130 °C within a dissolution time of 1 h. Then, the cellulose was directly regenerated from the solution by cooling down to approximately 70 °C and removing the salts with water, yielding a translucent gel. The cellulose gel was not significantly chemically decomposed even though some decrease of the degree of polymerization occurred during the dissolution/regeneration of cellulose. The X-ray diffraction analysis demonstrated that the dissolution/regeneration of cellulose induced a crystalline structural change from cellulose I to cellulose II, confirming the complete loss of the original cellulose structure. The cellulose gel had highly porous three-dimensional networks composed of fairly long cellulose fibrils interconnected with one another. The dissolution/regeneration of cellulose in aqueous lithium bromide solutions offers new and important options for cellulose-based materials.     

New solvents for cellulose: dimethyl sulfoxide/ammonium fluorides

New solvents based on DMSO in combination with alkylammonium fluorides, in particular TBAF . 3H(2)O and BTMAF . H(2)O, were established as media for homogeneous functionalization of cellulose. Even DMSO in combination with freshly prepared, anhydrous TBAF, obtained by the reaction of tetrabutylammonium cyanide and hexafluorobenzene, dissolves cellulose. In contrast, a mixture of DMSO and tetramethylammonium fluoride does not dissolve cellulose. The solvents were characterized by capillary viscosity, which showed that a cellulose solution of DMSO/BTMAF . H(2)O possesses a lower viscosity at comparable cellulose concentrations compared with DMSO/TBAF . 3H(2)O. The determination of the degree of polymerization of the starting cellulose (microcrystalline cellulose, spruce sulfite pulp, and cotton linters), and of the regenerated samples, shows that degradation of the polymer depends on the dissolution time, temperature and on the ammonium fluoride used. The results of different homogeneous reactions including acylation and carbanilation of cellulose in the solvents were compared with those of the most-commonly-applied solvent N,N-dimethylacetamide/LiCl. The products were characterized by elemental analysis, (1)H- and (13)C NMR spectroscopy (additionally after perpropionylation) and FTIR spectroscopy.     

Investigation of the structure of cellulose in LiCl/DMAc solution and its gelation behavior by small-angle X-ray scattering measurements

Cellulose gels were prepared from cellulose in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) solution. When the cellulose concentration in the solution is above the one at which cellulose molecules overlap, cellulose gels were formed. While the gel prepared by the addition of water was turbid, the one prepared by the ion exchange was colorless, transparent, and optically anisotropic. In order to explain this gelation behavior of cellulose, small-angle X-ray scattering (SAXS) measurements of the cellulose solutions and the gels were performed. The SAXS profiles of the cellulose solutions and the gels suggested that the large-scale fluctuation of the molecular chain density in the solution can be the origin of the molecular aggregates formed in the gel. Furthermore, the differences in the structure of the gels at the macroscopic and the molecular level were discussed in terms of the phase separation and the molecular association.     

Gas chromatographic determination of fatty acids contained in different lipid classes after their separation by solid-phase extraction

Enthalpic phenomena were shown to contribute to the size exclusion separation mechanisms during chromatographic analysis of solutions of pullulan and cellulose in LiCl-N,N-dimethylacetamide (LiCl-DMAc) solvent and eluent. The effect of LiCl concentration in the sample solutions and the effect of temperature were of the same order of magnitude for both pullulan and cellulose samples. This led to systematic errors in the determination of mean molecular mass in the range of tens of percent, depending on the chromatographic conditions and on the molecular mass of the analyte. The systematic error is much higher than the random errors; the typical values of the latter being up to a few percent (RSD). Low column temperature and a higher content of LiCl in the sample solution led to lower determined mean molecular mass values. This can be explained by a decrease in the interactions between dissolved macromolecules, although polymer-stationary phase interactions should also be taken into account. Furthermore, the cellulose stability in solution was determined: the zero order random degradation constant being k = 6.9 x 10(-8) mol mol-1 monomer day-1.     

Comparative evaluation of size-exclusion chromatography and viscometry for the characterisation of cellulose

The analysis using size-exclusion chromatography (SEC) with multi-angle light scattering (MALS) and differential refractive index (DRI) detection of cellulose dissolved in lithium chloride/N,N-dimethylacetamide (LiCI/DMAc) is evaluated and compared to two other methods currently used for cellulose analysis. These are SEC with low-angle light scattering (LALS) and ultra-violet detection of cellulose derivatised to tricarbanilates (CTC), and viscometry in cadmium triethylene diamine dihydroxide (cadoxen). The cellulose source is Whatman No. 1 paper, unaged or artificially aged with a combination of heat and humidity. The values of the molar mass (Mr) averages of cellulose obtained with the different methods resulted quite different for both aged and unaged paper. SEC of cellulose in LiCl/DMAc provided the highest Mr averages values, followed by SEC of CTC, while viscometry yielded the lowest values. These differences were more or less pronounced depending on the initial degradation state of the paper. Several hypotheses are presented in order to explain these discrepancies and each method is discussed on the basis of its suitability to characterise the aging-induced degradation.     

Enhancement of cellulose dissolution in water-based solvent via ethanol–hydrochloric acid pretreatment

Cellulose dissolution in water-based solvents is essential for processing of regenerated cellulose products such as fibres, films and particles. Cellulose dissolution in NaOH–urea aqueous solution has emerged as a simple and attractive alternative for processing cellulose solutions. However, this solvent requires energy intensive pretreatments such as milling or refining. In this paper we investigate a one step chemical pretreatment method using ethanol–hydrochloric acid prior to the dissolution of cellulose in NaOH–urea–water. The dissolution mechanism of the pretreated sample was initially examined in diluted cupri-ethylenediamine and 7% NaOH–12% urea–water solvent using optical microscopy methods and field emission scanning electron microscopy. The apparent energy of activation for the viscous flow of ethanol–acid pretreated pulp in NaOH–urea–water was calculated using rheological methods. Our results showed that the dissolution of pretreated pulp was achieved up to 4% cellulose concentration. We suggest that the enhancement of dissolution was due to a combination of degradation of remnant primary fibre wall layer and reduction of degree of polymerization of cellulose.     

The role of transition metals in oxidative degradation of cellulose

The role of transition metals in oxidative degradation of cellulose has been studied. Degradation experiments with model papers and studies of hydroxyl radical production in solution have been performed with Fe, Cu, Mn, Co, Cr, Ni, and Zn. Rates of production of hydroxyl radicals in solution have been estimated using the radical scavenger N,N′-(5-nitro-1,3-phenylene)bisglutaramide in the pH interval 7-9. Hydroxyl radical production during degradation of Cu-containing cellulose has been studied. To gain a better insight into chemistry behind degradation processes, chemiluminometric experiments were also performed.The experiments provide strong evidence that the role of transition metals during the oxidative degradation of cellulose is catalytic. A correlation between the behaviour of transition metals in solution and in paper was established at low contents of transition metal in paper.     

纤维素溶剂研究进展

概述了纤维素溶剂的重要研究进展,主要包括N-甲基吗啉-N-氧化物(NMMO)在85℃以上高温可破坏纤维素分子间氢键,导致溶解;氯化锂/二甲基乙酰胺(LiCl/DMAc)在100℃以上可溶解纤维素;1-丁基-3-甲基咪唑盐酸盐([BMIM]Cl)和1-烯丙基-3-甲基咪唑盐酸盐([AMIM]Cl)离子液体,含强氢键受体Cl-离子,通过它们与纤维素羟基作用而引起溶解.氨基甲酸酯体系则是通过尿素与纤维素在100℃以上反应转变为纤维素氨基甲酸酯,然后再溶解于NaOH水溶液中;氢氧化钠/水体系,只能溶解结晶度和聚合度较低的纤维素;NaOH/尿素、NaOH/硫脲和LiOH/尿素水溶液体系,它们预冷至-5~-12℃后可迅速溶解纤维素.主要是通过低温产生小分子和大分子间新的氢键网络结构,导致纤维素分子内和分子间氢键的破坏而溶解,同时尿素或者硫脲作为包合物客体阻止纤维素分子自聚集使纤维素溶液较稳定.低温溶解技术不仅突破了加热溶解的传统方法,而且可推进化学"绿色化"进程.共引用参考文献50篇.     

The effect of water on cellulose solutions in DMAc/LiCl

The solution state of cellulose in the system N,N-dimethylacetamide/lithium chloride (DMAc/LiCl) depends on various factors such as cellulose concentration, provenience (cotton, hardwood, softwood) and chemical history (pulping, pretreatment, bleaching) of cellulose, LiCl concentration, activation method, dissolution conditions (time, shaking), and water content. In particular the influencing of the latter has been intensively investigated in our present studies. Working in anhydrous conditions is not practicable for routine size exclusion chromatography (SEC) analysis. Especially in solutions diluted to SEC levels (0.9 wt% LiCl), an aggregation induced by water was observed. Depending on the time of dissolution and on the amount of water, changes in the solution state were observed. In some cases the amount of aggregates increases within a few minutes. This is reflected by a time-dependent increase in the scattering intensity and quantitatively proved by an increase in the aggregation peak in the calculated intensity distributions. With less soluble pulps, traces of water (lower than 0.01 M) can already suffice to induce and promote aggregation. To disturb a “good” stock solution, the concentration of water must be higher than 0.05 M. The aggregates formed correspond to the model of the fringed micelle.     

Tensile Properties and Morphology of Oil Palm Empty Fruit Bunch Regenerated Cellulose Biocomposite Films

The regenerated cellulose (RC)biocomposite films were prepared using casting method where oil palm empty fruit bunch (OPEFB) and microcrystalline cellulose (MCC) were dissolved in N-dimethylacetamide/lithium chloride (DMAc/LiCl)solution. The increasing of OPEFB contents up to 2 wt% increased the tensile strength and modulus of elasticity of RC biocomposite films while the elongation at break decreased. However, at 3 and 4 wt% of OPEFB content, the tensile strength and modulus of elasticity decreased with increases OPEFB content, but elongation at break increased. The increment of tensile strength and modulus of elasticity at 2 wt% is due to the OPEFB fiber that partially dissolved and dispersed with the OPEFB matrix. The morphology studies illustrate that at 2 wt% of OPEFB content of biocomposite films surface consists less voids and agglomerations than at 4 wt%. This can be considered the RC filler was partially dispersed with the RC matrix in the biocomposite films.     

The influence of alkaline reserve on the aging behavior of book papers

Degradation of cellulose under alkaline conditions is involved either involuntarily or deliberately in many different cellulose processing steps, such as pulping, bleaching, or aging within the viscose process, and the underlying chemistry has been the topic of numerous studies. When it comes to aging under alkaline conditions—either natural or accelerated (artificial)—the degradation processes are by far less investigated and understood. A prominent example of moderately alkaline cellulosic material is deacidified book paper from libraries which had undergone a mass-deacidification treatment. We studied their aging behavior under accelerated conditions in comparison to non-deacidified duplicates in order to better understand how the alkaline reserve, which was introduced by the deacidification treatment, affects the stability of the books on the long run. GPC analysis of cellulose and determination of carbonyl functionalities were performed, which were critical parameters to achieve a deeper insight into hydrolytic and oxidative changes of cellulose structure upon deacidification treatment and subsequent aging. Also, model book papers impregnated with different amounts of alkaline reserve were used to support the findings from the original book samples. Hydrolytic degradation rates of the original book papers were significantly reduced after mass deacidification compared to the non-deacidified duplicates. The beneficial effect of mass deacidification on cellulose stability was found to be strongly related to the amount of alkaline reserve deposited, independent of varying parameters of book papers. Although some indication of alkali-induced β-elimination was found (a minor decrease of the along-chain carbonyl content in the original deacidified book papers during aging), it did not occur to an extent that significantly influenced the molar mass of cellulose. The beneficial effect of retarded hydrolytic degradation by mass deacidification thus clearly outweighed possible negative alkalinity effects of the deposited alkaline reserve.     

棉浆粨纤维素的超声波处理

氢键;结晶程度;棉浆粨纤维素的超声波处理     

Thermal behavior of cellulose fibers with enzymatic or Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> treatment

Summary New regenerated cellulose fibers were developed during the last decades as environmentally friendly systems. In this work, three fibers: lyocell, modal and viscose were subjected to an enzymatic treatment. Likewise, different lyocell fibers were washed in a Na₂CO₃ solution under severe conditions. Analysis was performed by means of differential scanning calorimetry, thermogravimetry and scanning electron microscopy. In all samples, at low temperature, water desorption was detected. Furthermore, thermal analysis shows wide exothermic processes that began between 250 and 300°C corresponding to the main thermal degradation and it is associated to a depolymerization and decomposition of the regenerated cellulose. It is accompanied with mass more than 60% mass loss. Kinetic analysis was performed and activation energy values 152-202 kJ mol^-1 of the main degradation process are in agreement with literature values of cellulose samples.     

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.     

The thermal behaviour of cellulose samples with different structure

In this work the thermal characteristics of cellulose samples with different structure were investigated. The samples were prepared by reacting the cellulose with ethanolic hydroxide solution. Depending on the time of alkaline treatment, the intensity of cellulose transformation differed. Starting from cellulose I structure, with the highest degree of crystallinity, the other samples consisted of mixed structures of cellulose I and II, or were completely transformed to cellulose II structure with the lowest degree of crystallinity. The thermal behaviour of the samples was studied by using a Perkin Elmer TGS-2 and DSC-2 instruments. The kinetic parameters of dehydration and degradation were determined from non-isothermal TG-data (Nitrogen-inert atmosphere and a heating rate of 20 deg/min). The thermal effects of water evolution (heating rate of 80 deg/min) of the cellulose samples were found to depend on the structural characteristics and the crystallinity of the samples. The activation energy and frequency factor were in correlation with the structural changes.     

Solvation of lithium chloride in aqueous and mixed solutions of an aprotic solvent

The dynamic hydration and solvation numbers of lithium chloride are estimated on the basis of experimental data on the limiting electrodialysis concentration of an electrolyte from aqueous and aqueousorganic solutions containing aprotic solvent N,N-dimethylacetamide. It is established that the dependence of the hydration numbers of the salt on the volume fraction of the aprotic solvent is of an extreme character, and its solvation number on N,N-dimethylacetamide does not depend on the composition of the mixed solution.     

Dissolution of cellulose in ethylene diamine/salt solvent systems

Investigation of the dissolution of cellulose in Ethylene Diamine (EDA)/Potassium thiocyanate (KSCN) solutions by infrared spectroscopy (FTIR) and thermal analysis (DSC) indicated that changes to the solvent during freeze thaw cycling of mixtures was consistent with increased interaction between cellulose and solvent. Thermal transitions in the system, however, occurred at temperatures outside the range used in thermal cycling to promote dissolution. Further exploration of the dissolution and mixing process indicated that mixing was the limiting step in solution formation. The dissolution of two types of cellulose with different molecular weights (Degree of Polymerization (DP)=210 and >1000) was studied using EDA/KSCN solution as the solvent. The solubility and the dissolution rate of cellulose depended on both the solvent composition and cellulose molecular weight. Cellulose could dissolve faster in the solvent with lower salt concentration but the highest cellulose concentration was obtained in the solvent with 30~35% KSCN. Rheological measurements showed that cellulose solutions exhibited viscous solution behavior at low KSCN concentration but primarily elastic behavior at high salt concentration.     

New solvents for cellulose: Hydrazine/thiocyanate salt system

The hydrazine/thiocyanate system was found to be an excellent solvent for cellulose. The solubility and solution properties were investigated. Even at room temperature, the combinations of hydrazine and lithium, sodium, and potassium thiocyanate had high dissolution power for cellulose, up to an 18% (w/w) maximum, unrelated to the polymorph, whereas a combination with ammonium thiocyanate exhibited a solubility difference among celluloses I, II, and III. The effect of the temperature cycling of the system for the rapid dissolution of cellulose was investigated thermodynamically. In these systems, a high concentration of salts was necessary to effect the cellulose dissolution; this suggested that an undissociated salt–solvent complex played an important role in the cellulose dissolution as implied by electroconductivity measurements of the hydrazine/salt system. Gel and liquid‐crystal formation was observed in all systems above 4 and 6% (w/w) cellulose concentrations, respectively. The values of both critical concentrations were quite similar to those observed in the ammonia/ammonium thiocyanate system studied earlier in our laboratories. The gelation temperature was between approximately 10 and 50 °C, depending on the salt and cellulose concentration. The dependence of the cellulose solubility on the degree of polymerization was also examined. It is suggested that these solvent systems have great potential for the fiber and film formation of cellulose. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 601–611, 2002; DOI 10.1002/pola.10135