Controlled accessibility Lewis acid catalysed thermal reactions of regenerated cellulosic fibres

A combination of techniques have been used to characterise lyocell regenerated cellulose fibre subjected to low-moisture thermal-catalytic reactions with zinc chloride Lewis acid. Application from non-swelling ethanol reduces catalyst accessibility, but at high temperatures migration takes place through the internal fibre morphology. The extent of chain scission is reduced at lower temperatures, leading to a higher leveling-off degree of polymerisation (LODP). In contrast, application of zinc chloride from water results in a lower LODP, due to the more even distribution of catalyst. The weights of extractable polymer material increase according to two separate rate constants, following established semicrystalline models. A higher Arrhenius activation energy for chain scission is seen for zinc chloride application from ethanol, which may be due to the physical mobilisation of the cellulose polymer at high temperature, associated with a cellulose T_g. This may also aid recrystallisation. Cellulose dehydration endotherms and pyrolysis exotherms are shifted to lower temperature for application of zinc chloride from ethanol compared to water, which may be the result of a higher local concentration of catalyst and a faster reaction onset.     

微波辅助的金属氯化物Lewis酸催化纤维素水解

研究了微波辐射下四种金属氯化物Lewis酸的催化纤维素酸水解反应性能,发现CuCl₂的催化性能最好。反应温度、反应时间、微波功率、催化剂用量和酸种类对纤维素水解转化率、葡萄糖和5-羟甲基糠醛(5-HMF)的选择性均有明显影响。与传统热反应相比,微波辐射明显加快纤维素酸水解速率,提高葡萄糖的选择性。0.5g纤维素和15g水,在微波功率800W,温度到达225℃时立即停止反应的条件下,当CuCl₂用量为0.05mmol时,纤维素转化率和葡萄糖选择性达72.6%和62.3%;当CuCl₂用量为0.15mmol时,5-HMF的选择性最高为13.2%;当CuCl₂用量为0.30mmol时,纤维素的转化率高达90.6%,但葡萄糖选择性只有6.7%。     

Electrokinetic studies of cellulosic fibres

Results on surface conductivity of cotton fibres dyed with reactive dyes have been reported. In general the surface conductivity values were higher in alkaline pH, but lower in acidic pH as compared to the neutral pH. This observation has been attributed to the contribution of the functional groups present on the dye molecule reacted with the fibre. Results on the surface conductivity of fibres dyed with mono- and bifunctional reactive dyes suggest that dyes react with the hydroxyl groups of the fibre, and that the bifunctional dye molecule forms a crosslink between two cellulosic chains.     

Study of cellulosic fibres morphological features and their modifications using hemicelluloses

The microfibrilar structure and the morphology of the fibre are some of the most important characteristics that determine fibre performance during yarn making. This article is focused on understanding of morphological features of manmade cellulosic fibre and exploring an alternate way to alter fibre morphology. It is observed that though the chemical nature of different types of cellulose fibres viz. viscose, modal and lyocell is same; different processing routes lead to different cross-section and morphologies of fibres which leads to their characteristic properties and spinning behavior. A novel way is attempted to alter the fibre morphology of viscose fibre by changing the fibre regeneration kinetics and molecular weight distribution through addition of low molecular weight and branched structured hemicelluloses in spinning dope. Addition of hemicelluloses in the spinning dope prior to spinning and regeneration of viscose fibres is found to alter the morphology of the fibres without affecting tensile properties of the fibres.     

Nanoindentation of regenerated cellulose fibres

Nanoindentation was performed on cross sections of regenerated cellulose fibres with different structure and properties. Same as in single-fibre tensile tests, the elastic modulus of lyocell was higher than the elastic modulus of viscose. However, in spite of its tensile strength being twice as high as viscose, the hardness of lyocell was 15% lower than viscose. The overall degree of orientation of cellulose chains being higher in lyocell than in viscose, it is proposed that reduced lateral bonding in lyocell is the reason for the low hardness measured by nanoindentation compared to viscose.     

The surface chemical modification of cellulosic fibres in view of their use in composite materials

The surface modification of various cellulosic materials was studied in heterogeneous conditions using different grafting agents bearing anhydride or isocyanate reactive groups. Some of these reagents were oligomeric, others polymeric and most were chosen so as to provide a non-polar character to the surface of the fibres in view of their possible use as reinforcing agents in composites based on polymeric matrices. The success of these chemical modifications was assessed by Fourier-transform Infrared Spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and elemental analysis. The accessibility of the superficial OH groups varied as a function of the coupling agent in terms of both molecular size and chemical nature and ranged between 1 and 3% of the total hydroxy groups borne by the initial cellulose sample. This revised version was published online in August 2006 with corrections to the Cover Date.     

Biointeractive antibacterial fibres using polyelectrolyte multilayer modification

Contact-active antibacterial surfaces are a novel tool in the antibacterial battle. The preparation of such surfaces usually involves harsh reaction conditions and organic solvents. A more sustainable alternative would involve physical adsorption of water-soluble polyelectrolytes using a renewable substrate. Here, highly charged cationic polyvinylamines (PVAm), with or without hydrophobic modifications, have been adsorbed onto the naturally anionic cellulosic wood-fibres. To increase the amount of PVAm, polyelectrolyte multilayers were prepared using polyacrylic acid as the anionic polyelectrolyte. The modified fibres were characterised for PVAm content, water retention and antibacterial properties. The use of multilayers increased the total polymer content without notably reducing the water swelling. The fibres were shown to have excellent bioactive properties and reduced waterborne Escherichia coli and Bacillus subtilis by more than 99.9 %, which is a generally accepted definition of an antibacterial material. A large reduction in bacterial growth was observed upon addition of nutrients, although minor growth was detected after 24 h. The results further show that one adsorbed polymer layer was sufficient to obtain a contact-active surface, which makes the PVAm multilayer system seemingly unique. No polymer leaching from any of the samples was detected, indicating that the fibres work via a contact-active antibacterial mechanism. The results show the feasibility of constructing a sustainable antibacterial material using a renewable substrate and water-based solutions in the material construction process.     

Single-site catalyst immobilization using magnesium chloride supports

Immobilization and activation of a broad range of titanium-, chromium-and nickel-based single-site catalysts for ethylene polymerization has been carried out using supports of type MgCl₂/AlR_n(OEt)_{3 − n} , prepared by reaction of AlR₃ with adducts of magnesium chloride and ethanol. The spherical particle morphology of the support is retained and replicated during catalyst immobilization and polymerization, yielding polyethylenes with controlled particle size and morphology. The single-site nature of these catalysts is also retained, giving polymers with narrow molecular weight distribution. Furthermore, very high catalyst activities can be obtained as a result of a stabilizing effect of the support, which prevents the rapid decay in activity often observed in homogeneous polymerization with these catalysts. The text was submitted by the authors in English.     

The surface charge of regenerated cellulose fibres

In this paper the influence of charged species on the sheet strength of viscose fibres was investigated. Four samples of chemical modified viscose fibres, as well as a reference fibre were studied. The swelling of these viscose fibres and the breaking length of hand sheets have been determined. Comparing the results, the influence of both, swelling and surface charge on the bonding force, is evident. The allocation of the charges, induced by cationic starch and Carboxmethylcellulose, has been analyzed by Titration, attenuated total reflection spectroscopy (ATR) and X-ray photoelectron spectroscopy (XPS). Titration was used to make a first estimation of the charge distribution within the fibre. Using ATR and XPS, more detailed information about the surface charge has been achieved. All measurement methods showed a significant amount of charge on the fibre surface.     

The hydrolysis and recrystallisation of lyocell and comparative cellulosic fibres in solutions of mineral acid

Regenerated cellulosic fibres undergo a process described as scission-reordering during hydrolysis in solutions of mineral acid. This occurs within disordered polymer regions at lateral crystal interfaces, which are accessible to aqueous agents through the pore spaces and polymer free volume. This process is distinct from that of oligomer-solubilsation, which occurs within disordered polymer regions in series between crystal domains, where no effective template exists for recrystallisation. The degradation of series disorder will have the greatest influence on fibre tensile properties, which fall dramatically even at low levels of hydrolysis. The mechanics of fibrillation are most sensitive to the degradation of lateral disorder, which occurs at a higher rate constant. Soft-touch fabric processing may therefore be possible under conditions where there is a reduced influence on tensile performance. A kinetic model has been proposed to describe the hydrolysis and recrystallisation pathways, which shows that lyocell has longer but thinner crystal domains than viscose or modal fibres, and also a tighter distribution of lateral crystal sizes. Lyocell also has a lower proportion of series disorder and also thinner regions of lateral disorder. This is consistent with the overall greater crystallinity of the original lyocell fibre and the also of the final microscrystalline product.     

Interpretation of relaxation and swelling phenomena in lyocell regenerated cellulosic fibres and textiles associated with the uptake of solutions of sodium hydroxide

The uptake of solutions of sodium hydroxide by lyocell fibre results in a phenomenon in textiles described as swelling–shrinkage. The response of woven fabrics in a tensile stress–relaxation experiment shows two time-dependent processes, corresponding to different mechanisms of pressure development. Rapid diffusion has been assigned to osmotic swelling through the interconnected pore structure of the fibre (D = 6–15 × 10⁻¹² m²/s), which is influenced by the extent of ionization of hydroxyl groups at the pore surfaces. A ratio for the cellulose and water dissociation constants (K_cell/K_w) of 70 provides best agreement with experimental data. A second slower diffusion process (D = 2–10 × 10⁻¹⁴ m²/s) is assigned to transport through the cellulose polymer structure, associated with the Na-cellulose transition. This can be modeled assuming an ion-exchange equilibrium, where the cellulose gel converts reversibly between compact hydrogen and expanded sodium forms, with K = 1.04 × 10¹⁴, in favour of the hydrogen form. The model successfully predicts the concentration dependence of the transition and the movement to higher concentration with external constraint. The slow diffusion process only becomes apparent at high alkali concentrations, as the pores in the fibre collapse due to the expansion of the gel. Continued gel-diffusion is only possible through the polymer phase, which then dominates over fast pore-diffusion.     

Aluminum chloride as a solid is not a strong Lewis acid

Aluminum chloride is used extensively as Lewis acid catalyst in a variety of industrial processes, including Friedel-Crafts and Cl/F exchange reactions. There is a common misconception that pure AlCl3 is itself a Lewis acid. In the current study, we use experimental and computational methods to investigate the surface structure and catalytic properties of solid AlCl3. The catalytic activity of AlCl3 for two halide isomerization reactions is studied and compared with different AlF3 phases. It is shown that pure solid AlCl3 does not catalyze these reactions. The (001) surface of crystalline AlCl(3) is the natural cleavage plane and its structure is predicted via first principles calculations. The chlorine ions in the outermost layer of the material mask the Al3+ ions from the external gas phase. Hence, the experimentally found catalytic properties of pure solid AlCl3 are supported by the predicted surface structure of AlCl3.     

Electrokinetic studies of cellulosic fibres I. Zeta potential of fibres dyed with reactive dyes

Summary The influence of reaction between a reactive dye molecule and cotton cellulose on the zeta potential of the cellulosic fibres has been studied by streaming potential method. As a result of such a reaction the negative value of zeta potential was decreased and the extent of this decrease was proportional to the amount of dye entered into the reaction. With respect to the influence of various structural features of a reactive dye molecule on the zeta potential of dyed fibres, it was observed that the zeta potential was independent of the type of reactive system but was very much dependent on the number of –SO₃H groups in the dye molecule and the chemical constitution of the chromophoric system in the dye structure.A bifunctional reactive dye (Remazol Black B —C. I. Reactive Black 5) has been shown to form crosslinks between cellulose chains and bring about considerable lowering of negative zeta potential of the dyed fibres as compared to the equivalent amount of same dye but in the modified form to act as a monofunctional reactive dye. This bifunctional dye was also observed to induce H-bonding in the vicinity of the sites of cross-links. Trichloropyrimidine type of dyes have been found to be neither completely monofunctional nor fully bifunctional but seem to lie in between the monofunctional and bifunctional types of dyes.

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Zusammenfassung Der Einfluß der Reaktion zwischen einem reaktiven Farbstoffmolekül und Baumwollzellulose auf das-Potential der Zellulosefasern wurde mit der Strömungspotential-Methode untersucht. Als Ergebnis einer solchen Reaktion sank der negative-Potentialwert ab, und die Abnahme war proportional dem Betrag an Farbstoff, der in Reaktion trat. Im Hinblick auf den Einfluß verschiedener struktureller Eigenschaften eines reaktiven Farbstoffmoleküls auf das-Potential der gefärbten Faser wurde festgestellt, daß letzteres unabhängig vom Typ des Reaktions-systems war, abet sehr abhängig von der Zahl der –SO₃H-Gruppen im Farbstoffmolekül und der Konstitution des chromophoren Systems.Ein zweifunktionaler reaktiver Farbstoff (Remazol Black B — C. I. Reactive Black 5) bildet Vernetzungen zwischen Zelluloseketten und erniedrigt beträchtlich das negative-Potential der gefärbten Fasern, verglichen mit einem Farbstoff in modifizierter Form, so daß er als monofunktioneller Farbstoff wirkt. Dieset zweifunktionale Farbstoff induziert auch Wasserstoffbrücken in der Nachbarschaft der Vernetzungsstellen. Farbstoffe von Trichlorpyrimidin-Typ sind weder streng monofunktionell noch voll bifunktionell. Sie liegen in der Mitte.

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With 11 figures     

The structure of different cellulosic fibres characterized by Raman spectroscopy

Different samples of cellulosic materials were analyzed by Raman spectroscopy and wood chips from Pinus elliottii, treated with acidic and alkaline aqueous solutions, were used to evaluate diagnostic signatures of the chemical structure of the cellulosic fibres. Cotton and whiskers synthesized from cotton, ancient Egyptian linen from a mummy wrapping, and five different paper sheets used in museum handling were compared. The complementarity of the Raman spectroscopic and scanning electron microscopic data facilitated the evaluation of the crystallinity, the level of organization and chain sizes of the fibres and the identification of different oxidation products. Intensity ratios measured from pairs of key bands were used to characterize the crystallinity, chain lengths and presence of oxidative decomposition in the range of the studied samples. Finally, the Raman spectra of the ancient Egyptian linen specimen indicated a potential future application of the proposed analysis for the characterization of archaeological pieces composed of linen.     

Electrostatic assembly and growth of gold nanoparticles in cellulosic fibres

Synthetic studies of nanocomposites containing gold nanoparticles attached onto wood or bacterial cellulosic fibres have been performed in situ in the presence of the fibres or by polyelectrolyte-assisted deposition. The optical properties of the final nanocomposites could be tailored not only by the starting Au nanoparticles characteristics but also by the preparative method associated to the type of cellulosic fibres used as the substrate. Thus, gold nanoparticles assembled or generated in situ within cellulosic fibres, are excellent components for long term optical and chemically stable nanocomposites, which appear particularly interesting for security paper applications.     

Chemical modification of the surface of cellulosic fibres. 2. Introduction of alkenyl moieties via condensation reactions involving isocyanate functions

Various cellulosic substrates including powders, long fibres and sheets were treated with isocyanates bearing an alkenyl function in a non-swelling medium. These heterogeneous reactions introduced a small but significant number of polymerizable moieties at the surface of the cellulose. In a second step, the free radical polymerization of styrene or methyl methacrylate was carried out in the presence of these modified cellulosics. It was shown that the alkenyl functions appended onto their surface took part in the chain growth thus giving a continuous path of covalent bonds between the solid substrate and the polymer matrix. 0969--0239 © 1998 Blackie Academic & Professional     

Lewis acid initiated or high pressure promoted reactions of isoprene with phenylsulphinyl chloride

Lewis acid initiated reaction of isoprene with PhSOCl proceeds in a ene fashion with the formation of 2-phenylsulphinylmethyl-1,3-butadiene. High pressure promoted addition produces Z-1-phenylsulphinyl-4-chloroadduct presumably $\text{via}$ [4+2]cycloaddition.     

Enantioselective catalytic ring expansion of methylenecyclopropane carboxamides promoted by a chiral magnesium Lewis acid

A catalytic enantioselective ring expansion of monoactivated methylenecyclopropanes (MCP) in the presence of N-tosyl aldimines was developed using a chiral bis(oxazoline) ligand-MgI2 complex. After evaluation of ligands and optimization of the reaction conditions, the reaction has been applied to a variety of aromatic and heteroaromatic aldimines providing the corresponding trans-C2,C3-disubstituted methylenepyrrolidines in generally good yields (greater than 52%) and up to 86% ee. [reaction: see text].     

The effect of hydrophilic modification of expanded graphite on the thermophysical properties of magnesium chloride hexahydrate

Journal of Thermal Analysis and Calorimetry - Magnesium chloride hexahydrate, as a phase change materials, is investigated usually to store solar energy. Here, the effect of hydrophilic...