Rheological studies of the interactions in cellulose/ethylene diamine/salt systems

Degree of polymerization (DP) of cellulose was measured to confirm that aging time and salt concentration did not cause cellulose degradation. Dynamic rheological studies of cellulose solutions were carried out to probe the evolving interactions between cellulose and ethylene diamine (EDA)/salt solvent system. Potassium thiocyanate (KSCN) was used as the salt in these studies. Steady shear studies indicated that all solutions exhibited shear‐thinning behavior. The empirical Cox‐Merz rule did not hold true for the cellulose system with weak gel microstructure. The shear viscosities at the shear rates explored decreased with aging time. The zero‐shear viscosity, however, increased with increasing salt concentration. Oscillatory shear studies were investigated and the time temperature superposition (TTS) method was used to extend the experimental frequency range of the instrument. The results showed that the average relaxation time of the cellulose system decreased as the sample aged and increased with increasing salt concentration, indicative of dynamic interactions between cellulose and the solvent system in solution. The conformations of cellulose chains were constantly changing over time. The system gelled when the salt concentration was increased to a critical point. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2326–2334, 2008     

Study of cellulose/ethylene diamine/salt systems

To better understand the complex interactions leading to dissolution of cellulose in ethylene diamine (EDA)/salt solvents, studies of interactions in sub systems of solution components and a model system based on cellobiose were conducted. Interaction between EDA and salt cation was investigated through comparison of solvation of K⁺, Na⁺ and Li⁺ in the EDA/H₂O binary solvent system. The least degree of solvation of K⁺ in EDA increased its availability for direct interaction with cellulose. Wide angle X-ray diffraction was utilized to study the interaction between EDA and cellulose. The effect of various solvents on cellulose crystalline polymorph was compared. The results indicated that cellulose was easily accessible to EDA and 1,3-diaminopropane, but was not affected by water or ethanolamine. The effect of salt concentration was investigated using cellobiose as a model compound through HSQC (Heteronuclear Single Quantum Coherence) NMR spectroscopy. Solid state CP/MAS (cross polarization/magic angle spinning) ¹³C NMR spectroscopy was employed to characterize changes in the conformation of the CH₂OH group of cellulose during dissolution. A mechanism scheme of cellulose dissolution in EDA/KSCN systems was proposed based on the information gathered.     

The role of salt on cellulose dissolution in ethylene diamine/salt solvent systems

Ethylene diamine (EDA)/salt solvent systems can dissolve cellulose without any pretreatment. A comparison of the electrical conductivity of different salts in EDA was made at 25 °C, and conductivity decreased in the order of KSCN>KI>NaSCN at the same molar concentration. Among the salts tested, potassium thiocyanate (KSCN) was capable of dissolving both high molecular weight (DP>1000) and low molecular weight (DP = 210) cellulose, and this was confirmed by polarized light microscopy. ³⁹K and ¹⁴N NMR experiments were conducted at 70 °C as a function of cellobiose concentration with EDA/KSCN as the solvent. The results showed that the K⁺ ion interacts with cellobiose more than the SCN⁻ ion does. Recovered cellulose was studied by infrared spectroscopy (FTIR) and wide angle X-ray diffraction (WAXD). Changes in the FTIR absorption bands at 1,430 and 1,317 cm⁻¹ were associated with a change in the conformation of the C-6CH₂OH group. The changes in positions and/or intensities of absorption bands at 2,900, 1,163, and 8,97cm⁻¹ were related to the breaking of hydrogen bonds in cellulose. X-ray diffraction studies revealed that cellulose, recovered by precipitating cellulose solutions with water, underwent a polymorphic transformation from cellulose I to cellulose II.     

Rheology of cellulose/KSCN/ethylenediamine solutions and coagulation into filaments and films

Dissolution of cellulose in ethylenediamine/potassium thiocyanate (KSCN) was studied as a function of cellulose and KSCN concentration. Concentrations of up to 9% (w/w) cellulose were obtained. Large variations in solution rheology with salt and cellulose concentration were observed, and phases including flowing solutions and gels were identified visually. Rheological data indicated that viscosity decreased with increasing salt or cellulose concentration in certain composition ranges. Viscosity decrease with concentration increase is associated with either onset of liquid crystalline ordering or phase separation in the system. Both of these are quite likely in the cellulose/ethylenediamine/KSCN system, depending on composition. Additionally, comparison of loss (G′′) and storage (G′) moduli confirmed that compositions that exhibited gel behavior at zero shear became liquid at shear rates as low as 1 Hz. Solutions were coagulated into filaments and films using ethanol (CH₃CH₂OH) and methanol (CH₃OH). Infrared spectroscopy (FTIR) indicated that significant quantities of KSCN salt remained in the fibers and films after coagulation. Subsequent washing removed residual KSCN and improved physical properties. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2013–2022, 2005     

Cellulose/chitosan composites prepared in ethylene diamine/potassium thiocyanate for adsorption of heavy metal ions

Cellulose/chitosan composites were successfully prepared in a new and basic-based solvent system, ethylene diamine/potassium thiocyanate (EDA/KSCN), by dissolving cellulose and chitosan in 70/30 (w/w) EDA/KSCN at ?19 °C, and then coagulating in methanol. Wide angle X-ray diffraction studies revealed that the EDA/KSCN solvent system is capable of disrupting the hydrogen bonds in both cellulose and chitosan and increase the amorphous regions. Stability tests proved that the composites are stable in acidic aqueous solution due to the hydrogen bonds formed between cellulose and chitosan. This is the first time to dissolve chitosan in a basic-based solvent system and prepare cellulose/chitosan composites in a straightforward way. The adsorption of heavy metal ions (Cu²⁺, Cd²⁺, and Pb²⁺) onto the cellulose/chitosan composites was investigated. The adsorption capacity is highly dependent on pH and the maximum metal uptake was obtained at pH 5.0. Increasing initial metal concentration enhanced the diffusion of metal ions to the composite surface and therefore the metal removal efficiency. Higher percentage of chitosan in the composites also led to higher metal adsorption. The results indicated that the prepared cellulose/chitosan (1:1) composite can adsorb 0.53 mmol/g Cu²⁺, 0.28 mmol/g Cd²⁺ and 0.16 mmol/g Pb²⁺ ions at pH 5.0. The Freundlich model and the pseudo-second-order model were in good agreement with the adsorption isotherms and kinetics, respectively. X-ray photoelectron spectroscopy studies indicated that the binding of heavy metal ions is attributed to the nitrogen atoms of amino groups in chitosan. The composites can be reused for metal removal.     

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     

Rapid dissolution of cellulose in LiOH/urea and NaOH/urea aqueous solutions

Rapid dissolution of cellulose in LiOH/urea and NaOH/urea aqueous solutions was studied systematically. The dissolution behavior and solubility of cellulose were evaluated by using (13)C NMR, optical microscopy, wide-angle X-ray diffraction (WAXD), FT-IR spectroscopy, DSC, and viscometry. The experiment results revealed that cellulose having viscosity-average molecular weight ((overline) M eta) of 11.4 x 104 and 37.2 x 104 could be dissolved, respectively, in 7% NaOH/12% urea and 4.2% LiOH/12% urea aqueous solutions pre-cooled to -10 degrees C within 2 min, whereas all of them could not be dissolved in KOH/urea aqueous solution. The dissolution power of the solvent systems was in the order of LiOH/urea > NaOH/urea > KOH/urea aqueous solution. The results from DSC and (13)C NMR indicated that LiOH/urea and NaOH/urea aqueous solutions as non-derivatizing solvents broke the intra- and inter-molecular hydrogen bonding of cellulose and prevented the approach toward each other of the cellulose molecules, leading to the good dispersion of cellulose to form an actual solution.     

Celluloses in an ionic liquid: the rheological properties of the solutions spanning the dilute and semidilute regimes

The ionic liquid of 1-allyl-3-methylimidazolium chloride ([amim]Cl) was used as the good solvent to dissolve celluloses. Cellulose concentration covers the range of 0.1-3.0 wt %, spanning both the dilute and semidilute regimes. The rheological properties of the cellulose ionic liquid solutions have been investigated by steady shear and oscillatory shear measurements in this study. In the steady shear measurements, all the cellulose solutions show a shear thinning behavior at high shear rates; however, the dilute cellulose solutions show another shear thinning region at low shear rates, which may reflect the characteristics of the [amim]Cl solvent. In the oscillatory shear measurements, for the dilute regime, the reduced dimensionless moduli are obtained by extrapolation of the viscoelastic measurements for the dilute solutions to infinite dilution. The frequency dependences of the reduced dimensionless moduli are intermediate between the predictions from the Zimm model and elongated rodlike model theories, while the fitting by using a hybrid model combining these two model theories agrees well with the experimental results. For the semidilute regime, the frequency dependences of moduli change from the Zimm-like behavior to the Rouse-like behavior with increasing cellulose concentration. In the studied concentration range, the effects of molecular weight and temperature on solution viscoelasticities and the relationship between steady shear viscosity and dynamic shear viscosity are presented. Results show that the solution viscoelasticity greatly depends on the molecular weight of cellulose; the empirical time-temperature superposition principle holds true at the experimental temperatures, while the Cox-Merz rule fails for the solutions investigated in this study.     

Rapid dissolution of spruce cellulose in H<Subscript>2</Subscript>SO<Subscript>4</Subscript> aqueous solution at low temperature

Dissolution of cellulose is the key challenge in its applications. It has been discovered that spruce cellulose with high molecular weight (4.10 × 10⁵ g mol^?1) can be dissolved in 64 wt% H₂SO₄ aqueous solution at low temperature within 2 min, and the cellulose concentration in solution can reach as high as 5 % (w/v). FT-IR spectra and XRD spectra proved that it is a direct solvent for cellulose rather than a derivative aqueous solution system. The cold H₂SO₄ aqueous solution broke the hydrogen bonds among cellulose molecules and the low temperature dramatically slowed down the hydrolysis, which led to the dissolution of cellulose. The resultant cellulose solution was relatively stable, and the molecular weight of cellulose only slightly decreased after storage at ?20 °C for 1 h. Due to the high molecular weight of cellulose, cellulose solution could form regenerated films with good mechanical properties and transparency at low concentration (2 % w/v). This work has not only provided the new evidence of cellulose dissolution which facilitated the development of cellulose solvent, but also suggested a convenient way to directly transfer cellulose with high molecular weight into materials without structure modifications.     

Cellulose gel and aerogel from LiCl/DMSO solution

Recently-discovered lignocellulosic solvent, 8%(w/w) lithium chloride/dimethyl sulfoxide (LiCl/DMSO), was found to dissolve cellulose of varied crystal forms and degree of polymerization. Cellulose samples could be activated for dissolution by complexation with ethylenediamine (EDA), giving EDA contents of 20–23% (w/w) in the complex irrespective of the cellulose type. The cellulose solution gave well-resolved ¹³C NMR spectrum, confirming molecular dispersion. Cellulose could be coagulated by ethanol to give translucent cellulose gels, which could be converted to highly porous aerogels via solvent exchange drying. Nitrogen adsorption analysis gave their specific surface areas of 190–213 m²/g, with typical mesopore sizes of 10–60 nm. Scanning electron microscopy revealed the network structure of aerogel composed of relatively straight fibril segments, approx. 20 nm wide and 100–1,000 nm long. X-ray diffraction showed that the material is poorly crystalline cellulose II.     

Coordination equilibria in transition metal based cellulose solvents

Taking into account the coordination chemistry of divalent palladium and nickel, two new cellulose solvents have been developed. Pd-en is an aqueous solution of the ethylenediamine palladium complex [(en)Pd(OH)₂]. Low molecular diols are deprotonated twice in Pd-en and form a five membered chelate ring with the transition metal. Cellulose dissolves in Pd-en to give a yellow solution, which allows light scattering analysis. NMR investigation of the solutions are simplified due to the diamagnetism of palladium(II) complexes. — The analysis of solution equilibria is essential in the development of complexing polysaccharide solvents. This feature is demonstrated by means of the new cellulose solvent Ni-tren, which is an aqueous solution containing tris(2-aminoethyl)amine, nickel salt and hydroxide in a molar ratio of ≥1:1:2. The difference in the cellulose solving mechanism between Ni-tren and the long known solvents ‘nioxen’ and ‘nioxam’ developed by Jayme in the 1950ths is outlined. The question, wether or not the dissolution of cellulose in a complexing solvent is a derivatization, is discussed.     

The solubility of cellulose in liquid ammonia/salt solutions

The dissolution of cellulose in solutions of liquid ammonia and ammonium thiocyanate is discussed. Viscosity measurements on dilute solutions of cellulose in this solvent over a range of shear rates and shear stresses are reported. A four-bulb Ubbelohde suspended level viscometer was used for the measurements. Plots of log [η] versus log M gave Mark-Houwink coefficients of a = 0.95 and K = 6.686 × 10^?5 at 25°C for [η] as dl/g. The Bloomfield equation was used to calculate effective bond lengths (b) from limiting viscosity numbers of cellulose in solutions of ammonia/ammonium thiocyanate and Cuene, respectively. Results indicate that cellulose may have similar configurations in both solvents and also that the ammonia solutions are true cellulose solutions. Miscibility of the cell- ulose/ammonia/ammonium thiocyanate solutions with organic solvents, such as glycerol, is also reported. Further, a few interesting characteristics of the liquid ammonia/ammonium salt solutions, discussed briefly, are the convenient boiling point, the rheological behavior, and the relatively high concentration of cellulose obtainable.     

Effects of nitren extraction on a dissolving pulp and influence on cellulose dissolution in NaOH–water

A commercial dissolving pulp was treated with aqueous solutions containing 3, 5 and 7 % of an organometalic complex (nitren) with the aim to selectively extract xylan and study its impact on the conventional physical–chemical properties of the pulp. The influence of these treatments on the pulp dissolution in a moderate solvent (8 % NaOH aqueous solution) was assessed by measuring the dissolution yields and the dissolution mechanisms. The results of this study show that nitren treatment has the effect of removing a large part of the xylan present in a dissolving pulp. It is also removing mannans and most important, it is influencing cellulose in two ways, (1) extracting it with more intensity when the nitren concentration increases, and (2) decreasing its mean molecular mass, also more evident with nitren concentration increase. The nitren extractions are favourable for the dissolution in cold NaOH–water, being more effective with higher concentrations. This chemical modification of the fiber surface leads to the disassembly of the primary wall. This allows an easier access of the NaOH reagent to regions not accessible on the initial fibres, which with the decrease of the cellulose molecular weight allows an easier dissolution and gives different dissolution mechanisms.     

Inorganic Molten Salts as Solvents for Cellulose

Inorganic molten salts can be used as efficient solvents for cellulose in a wide range of degrees of polymerization. Furthermore, molten salts can be applied as reaction medium for the derivatization of cellulose. For both dissolution and derivatization of cellulose, knowledge of the solution state as well as information about chemical interactions with the solvent system is essential. Using the melts of LiClO₄·3H₂O, NaSCN/KSCN/LiSCN·2H₂O and LiCl/ZnCl₂/H₂O as cellulose solvents, factors which determine the dissolving ability will be discussed. Besides the specific structure of the molten salt hydrate, the cation and the water content of the melt are the most important factors for the dissolving capability of a molten salt hydrate system. FT-Raman spectroscopy, ⁷Li and ¹³C NMR spectroscopy were applied to describe solvent–cellulose interactions and the state of cellulose dissolved in the molten salts. Using Raman and solid state NMR spectroscopy it was proved that cellulose is amorphous in the frozen solvent system. The application of inorganic molten salts as a medium for cellulose functionalization is demonstrated for cellulose carboxymethylation and acetylation.     

SEC–MALLS analysis of ethylenediamine-pretreated native celluloses in LiCl/<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethylacetamide: softwood kraft pulp and highly crystalline bacterial,tunicate, and algal celluloses

Softwood and hardwood bleached kraft pulps (SBKP and HBKP, respectively) and highly crystalline native celluloses such as algal, tunicate, bacterial and cotton lint celluloses were dissolved in 8 % (w/v) LiCl/N,N-dimethylacetamide (DMAc) after ethylenediamine (EDA) pretreatment. Complete dissolution of SBKP and other highly crystalline native celluloses in 8 % LiCl/DMAc was achieved after solvent exchange from EDA to DMAc through methanol. Neutral sugar composition analysis showed no significant differences between the original and EDA-treated pulps. A combination of size-exclusion chromatography and multi-angle laser light scattering (SEC–MALLS) was used to analyze the cellulose solutions after dilution to 1 % (w/v) LiCl/DMAc. The 0.05 % (w/v) solutions of highly crystalline cellulose in 1 % (w/v) LiCl/DMAc contained entangled molecules, and therefore 0.025 % (w/v) cellulose solutions in 1 % (w/v) LiCl/DMAc were used in the SEC–MALLS analysis to obtain reliable conformation plots (or double-logarithmic plots of molecular mass vs. root-mean-square radius). All the cellulose samples except SBKP gave conformation plots with slope values of 0.56–0.57, showing that these cellulose molecules had random-coil conformations. In contrast, SBKP gave a slope value of 0.35, indicating that some branched structures were present in the high-molecular-mass fraction. Double-logarithmic plots of the reduced viscosities of the cellulose solutions in 1 % (w/v) LiCl/DMAc versus the molecular mass were linear, except for SBKP, also suggesting the presence of anomalous cellulose structures in SBKP.     

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.     

Cellulose in NaOH–water based solvents: a review

The article is a critical review of all aspects of the dissolution of cellulose in NaOH-based aqueous solutions: from the background properties of the solvent itself, to the mechanisms of cellulose fibre swelling and dissolution, solution structure and properties and influence of additives and, finally, to the properties of various materials (fibres, films, aerogels, composites and interpenetrated networks) prepared from these solutions. A historical evolution of the research on this topic is presented. The pros and cons of NaOH-based aqueous solvent for cellulose are summarised and some prospects are suggested.     

共溶剂存在下N-烯丙基吡啶氯盐离子液体对纤维素的溶解性能研究

采用一步法合成N-烯丙基吡啶氯盐离子液体([APy]Cl),考察其对纤维素的溶解性能.结果发现,在120℃下对棉浆粕(聚合度(DP)=556)的溶解度可高达19.71%,但再生后聚合度为223,热降解严重.通过添加不同种类共溶剂的方法克服此缺点.结果表明,有机溶液(DMSO,DMAc,DMF或吡啶)作为[APy]Cl的共溶剂时,[APy]Cl/DMAc复合溶剂对棉浆粕的溶解效果最佳,100℃下溶解度为15.03%,再生后聚合度为403.此外降低了溶剂成本.但70℃下,溶解度仅为1.36%,溶解能力较弱.继续探讨了[AMIM]Cl作为[APy]Cl的共溶剂时对纤维素的溶解性能,结果表明,70℃下,[APy]Cl/[AMIM]Cl复合溶剂对棉浆粕的溶解度为8.78%,再生后聚合度为516.可知添加上述2种共溶剂均使[APy]Cl在低于自身熔点下形成液体并能够溶解一定量纤维素,拓宽了溶解温度区间及应用平台.对FTIR,XRD和TGA谱图分析,结果表明上述为纤维素的直接溶剂,可将其晶型由Ⅰ型转变成Ⅱ型,再生后热稳定性稍有降低.通过照片和SEM表明再生膜无色透明,结构致密.     

Room temperature dissolution of cellulose in tetra-butylammonium hydroxide aqueous solvent through adjustment of solvent amphiphilicity

The amphiphilicity of solvent systems is realized for adjusting the dissolution of natural cellulose by making use of tetra-butylammonium hydroxide (TBAH) as an example. TBAH aqueous solution is found to have an obvious effect on adjusting its amphiphilicity, along with a flexible concentration ranging from 40 to 60 wt% for dissolving cellulose. With a suitable amphiphilic property, cellulose can be dissolved by a TBAH aqueous system . The experimental results demonstrate that with the introduction of urea (more than 0.2:1, w:v) into a TBAH aqueous system, the dissolution process of cellulose can be dramatically promoted, leading to a transparent solution of cellulose. Herein, a complex solvent of TBAH/urea has been proposed for mild and effective dissolution of cellulose under ambient conditions. In the TBAH/urea complex solvent, the structure of the hybrid hydrate of TBAH and urea formed. Urea served as a hydrophobic contributor adjusting the amphiphilicity of the solvent system, allowing interfacial resistance between the amphiphilic crystal surfaces of the natural cellulose and solvent to be reduced. After that, the crystal of natural cellulose could be fully infiltrated and subsequently dissolved by the TBAH/urea aqueous solvent. The performances of the aqueous solvent and ambient temperature dissolution make aqueous TBAH/urea a potential and green solvent of cellulose for broad applications, such as composites, films or wet spinning of cellulose, in laboratories or industries.     

Role of urea in alkaline dissolution of cellulose

To elucidate the role of urea in dissolution of cellulose in aqueous alkali-urea solvent, the dissolution process was monitored by differential scanning calorimetry and X-ray diffractometry. Urea had no direct interaction with cellulose in dissolution process, but promoted the decrease of crystallinity. Moreover, the addition of urea increased the dissolved fraction of cellulose in the solvent by 1.5–2.5 times and improved the thermal stability of the solution. Urea might help alkali hydrate to penetrate into crystalline region of cellulose by stabilizing the alkali-swollen cellulose molecules, leading to an increase in dissolved fraction of cellulose. This stabilization may be due to the local accumulation of urea on the hydrophobic surface, preventing the hydrophobic association of dissolved cellulose molecules.