In this study, we investigate what types of cellulosic materials are formed by soaking the cellulose/ionic liquid (1-butyl-3-methylimidazolium chloride, BMIMCl) solutions in various organic liquids. When the 5-wt% cellulose/BMIMCl solutions were soaked in organic liquids with high and moderate polarities (relative permittivities), the corresponding cellulosic solutions and gels were produced, respectively. On the other hand, soaking the cellulose/BMIMCl solutions in lower polar liquids resulted in aggregation of cellulose in the mixtures. As the gels with high boiling point media were stable, they were characterized by viscoelastic and compression measurements. Contents of organic media and BMIMCl in the gels were changed depending on the polarities, which affected the mechanical properties under compression mode. Furthermore, processes for production of the solution, gel, and aggregate were proposed.
Polysaccharides are widely distributed in nature and very important biomass resources . Of many kinds of natural polysaccharides, particularly, cellulose is one of the most abundant biomass resources and, accordingly, a useful organic substrate . Its β(1 → 4)-linked glycosidic arrangement among glucose repeating units leads to forming numerous intra- and intermolecular hydrogen bonds. Therefore, cellulose forms highly crystalline and fibrous chain packing, which can function as a structural material in biological systems, such as the component in cell wall. Accordingly, cellulose-based structural and fibrous materials, such as textiles, clothes, and furniture, have been practically developed. Besides, the stiff crystalline fashion among cellulose chains should be relaxed by incorporating some additives to provide new applications of cellulose, such as soft and plasticized materials. For example, cellulosic flexible materials, e.g., cellophane, have been prepared by adding plasticizers, such as glycerin . Cellulosic hydrogels have also been fabricated by a variety of swelling approaches [4,5,6,7].
On the other hand, we have successfully fabricated a flexible cellulosic film, which exhibits thermoplasticity and thermal processability, by composition with an ionic liquid, 1-butyl-3-methylimidazolium chloride (BMIMCl), through the ion gel formation , based on the fact that BMIMCl forms the cellulosic ion gel by standing a cellulose/BMIMCl solution at room temperature . Ionic liquids are molten salts, which show the liquid state at temperatures below a boiling point of water. Since BMIMCl was found to dissolve cellulose , ionic liquids have been known as useful solvents for cellulose [11,12,13,14,15,16,17,18,19].
Cellulosic solutions with ionic liquids, such as BMIMCl and 1-allyl-3-methylimidazolium chloride, can also be converted into hydrogels by simple operations [20,21,22,23,24]. For example, Peng et al. reported that a cellulosic hydrogel was facilely formed by soaking a 5-wt% cellulose/BMIMCl solution in water . We also reported that porous celluloses with mostly amorphous structure could be fabricated through the similar hydrogelation procedure from solutions with BMIMCl (2 wt%) and the subsequent regeneration . In the study, we also found unique re-swellable property of the porous celluloses, leading to reformation of hydrogels.
In the present study, we would like to reveal what types of cellulosic materials are formed by soaking the cellulose solution with BMIMCl in various organic liquids besides water. Consequently, we found that by soaking the cellulose/BMIMCl solutions in organic liquids with high and moderate polarities (relative permittivities), the corresponding cellulosic solutions and organogels were obtained, respectively (Fig. 1). By soaking the cellulose/BMIMCl solutions in lower polar liquids, on the other hand, cellulose aggregated in the mixtures.
Absorbent cotton of average DP of 2000–5000  was purchased from Hakujuji Corporation, Tokyo, Japan. An ionic liquid, BMIMCl (catalog number 94128, purity ≥ 98.0%), was purchased from Sigma-Aldrich, Germany. The following organic liquids were used as received: dimethylsulfoxide (DMSO, catalog number 043-07211, purity 99.0%), N,N-dimethylacetamide (DMAc, catalog number 043-31565, purity 98.0%), N,N-dimethylformamide (DMF, catalog number 045-02911, purity 99.5%), ethylene glycol (catalog number 058-00981, purity 99.5%), methanol (catalog number 137-01823, purity 99.8%), ethanol (catalog number 057-00451, purity 99.5%), benzyl alcohol (catalog number 027-01276, purity 99.0%), dichloromethane (catalog number 135-02441, purity 99.5%), chloroform (catalog number 038-02601, purity 99.0%), anisole (catalog number 016-15895, purity 99.0%), toluene (catalog number 204-01861, purity 99.5%), and liquid paraffin (catalog number 128-04375) were purchased from Wako Pure Chemical Co., Japan; N-methyl-2-pyrrolidone (NMP, catalog number 25897-08, purity 99.0%) was purchased from Kanto Chemical Co. Inc., Japan; and glycerin (catalog number 17017-35, purity 97.0%) and ethyl acetate (catalog number 14622-85, 99.0%) were purchased from Nacalai Tesque, Inc., Japan.
Production of cellulose solutions
A typical experimental procedure was as follows (entry 1; Table 1). A mixture of cotton cellulose (0.080 g) with BMIMCl (1.52 g, previously dried under reduced pressure at 100 °C for 3 h, water content; 3 wt% by thermal gravimetric analysis (TGA)) was left standing at room temperature for 24 h and subsequently heated at 115 °C in vacuum oven for 3 h to obtain a 5-wt% cellulose solution. By soaking the resulting media in DMSO (8.0 mL) for 3 days under ambient atmosphere, the mixture totally formed a solution (0.83 wt%).
Production of cellulose organogels
A typical experimental procedure was as follows (entry 9; Table 1). A mixture of cotton cellulose (0.080 g) with BMIMCl (1.52 g, previously dried under reduced pressure at 100 °C for 3 h) was left standing at room temperature for 24 h and subsequently heated at 115 °C in vacuum oven for 3 h to obtain a 5-wt% cellulose solution. The resulting solution was soaked in benzyl alcohol (8.0 mL) for 3 days under ambient atmosphere to give an organogel. The resulting gel was taken out from the media and wiped. A portion of the residual solution (ca. several 10 mg) was dissolved in D2O (0.5–1.0 mL), and the resulting solution was analyzed by 1H NMR measurement at room temperature to calculate contents of benzyl alcohol and BMIMCl in the gel. In the case using low boiling point liquids, such as chloroform (entry 11; Table 1), the resulting gel was dried under reduced pressure at room temperature for 24 h. From weights of the residual material and the gel, and a feed weight of cellulose, contents of organic media and BMIMCl in the gel were calculated.
TGA measurement was performed on a TG/DTA 6200 (Seiko Instruments Ins.) at a heating rate of 10 °C min−1. 1H NMR spectra were recorded on JEOL ECX400 and ECA 600 spectrometers. Dynamic viscoelastic measurement was conducted on a rheometer (Rheosol-G1000, UBM) with a cone and plate geometry (2° cone angle, 30 mm diameter). A dynamic frequency sweep was conducted by applying a constant strain of 0.06%, over a frequency range between 0.1 and 10 Hz. The stress–strain curves were measured on a tensile tester (Little Senstar LSC-1/30, Tokyo Testing Machine Co.) with cross head speed of 2 mm min−1.
Results and discussion
As previously reported, cellulosic hydrogels are facilely formed by soaking the solutions with BMIMCl in water [25, 26]. In this study, we have attempted to reveal what types of cellulosic materials are formed by soaking the cellulose/BMIMCl solutions in various organic liquids. Cotton cellulose was first dissolved in BMIMCl at 115 °C under reduced pressure according to literature procedure to form a 5-wt% clear solution . During dissolution of cotton cellulose, we maintained the vacuum condition, because the presence of even a small amount of water depolymerized cellulose in BMIMCl, due to strong hygroscopic nature of BMIMCl. The resulting solution after cooling to room temperature was then soaked in organic liquids as listed in Table 1, and the mixtures were left standing at room temperature. Soaking the solution in relatively high polar liquids for 3 days resulted in totally forming clear cellulosic solutions (0.83 wt%, entries 1–4; Table 1). Because the high polar organic liquids are miscible with BMIMCl well, BMIMCl molecules diffused with solvated cellulose chains in the organic liquids after soaking (Fig. 2(a)). The process resulted in the formation of the clear solutions.
In the case using organic liquids with moderate polarity, the corresponding organogels were gradually formed by soaking the cellulose/BMIMCl solution in them for 3 days (entries 9–11; Table 1). Miscibility of such organic liquids with BMIMCl was not well, leading to slow diffusion of BMIMCl molecules in the organic liquids. Therefore, miscible areas gradually formed in the mixtures, giving rise to partial desolvation and aggregation of cellulose chains on molecular level at the area (Fig. 2(b)). The aggregated parts acted as cross-linking points to form cellulose networks, leading to gelation. Interestingly, liquids having hydroxy groups, that is, glycerin, ethylene glycol, methanol, ethanol, and benzyl alcohol (entries 5–9; Table 1), resulted in the formation of organogels, regardless of their polarities, although the alcoholic liquids were miscible with BMIMCl. The gelation is probably owing to the possible formation of hydrogen bonds of hydroxy groups with cellulose chains and BMIMCl molecules, leading to slow diffusion of BMIMCl and desolvation of cellulose chains in the alcoholic media. The gels with low boiling point media were dried under reduced pressure to remain nonvolatile materials, that is, cellulose and BMIMCl. From the weights of the gels and residual materials, and a feed weight of cellulose, contents of the organic media and BMIMCl in the gels were calculated. On the other hand, this calculation method could not be applied to the gels with high boiling point media (higher than ca. 200 °C), due to impossibility of complete evaporation of the liquids even under reduced pressure. Alternatively, the residual solutions after gelation were subjected to 1H NMR analysis to estimate weights of the organic liquids and BMIMCl present in the solutions. From the data, contents of the organic media and BMIMCl in the gels were calculated. As shown in Table 1, the contents of the organic media decreased, whereas those of BMIMCl increased in the gels, depending on polarities of the organic liquids. This tendency is probably attributed to more rapid miscibility of the two liquids with the earlier formation of cellulose networks in the systems with increasing polarities, leading to remaining more amounts of BMIMCl in the gels.
As the organogels with high boiling point media were stable under environmental atmospheric conditions, they were subjected to viscoelastic and compression measurements. The frequency dependence of storage and loss moduli in viscoelastic measurement of the resulting material with glycerin (entry 5; Table 1) showed the signature of typical viscoelastic material with predominance of storage moduli on the whole frequency range (Fig. 3a), supporting the gelling state of the product. The viscoelastic measurement of the other materials with ethylene glycol and benzyl alcohol (entries 6 and 9; Table 1) also exhibited similar results, suggesting their gelling states (Fig. 3 b and c). The stress–strain curves under compression mode showed the decrease of fracture strain values with no significant change of fracture stress values with increasing the contents of BMIMCl in the gels (entries 5, 6, and 9; Table 1; Fig. 4). These data indicated the production of more brittle gels with increasing the contents of BMIMCl, probably owing to high viscosity of BMIMCl.
When lower polar liquids were used, cellulose aggregated in the mixtures (entries 12–15; Table 1). As interfaces were formed in the mixtures due to immiscibility of these liquids with BMIMCl, cellulose was gradually regenerated at the interfacial areas to form aggregates (Fig. 2(c)).
In this study, we developed the facile procedures for production of the cellulosic organic solutions and organogels by soaking 5 wt% cellulose/BMIMCl solutions in organic liquids with suitable polarities. On the other hand, cellulosic aggregates were produced by using organic liquids with lower polarities for soaking. Polarities of the liquids strongly influenced the contents of organic media and BMIMCl in the gels, which also affected mechanical properties under compression mode. The present cellulosic solutions and gels have potentials to be practically applied as reaction media for cellulose and cellulosic soft materials, respectively, in the future.
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Kadokawa, J., Ohyama, N., Idenoue, S. et al. Facile production of cellulosic organic solutions and organogels from ionic liquid media. Colloid Polym Sci (2020). https://doi.org/10.1007/s00396-020-04685-6
- Ionic liquid
- Organic solution