With the increasing depletion of petroleum resources, the exploitation of low-cost biorenewables, bioresource has received great attention [1
]. Cellulose, as the most abundant renewable resource in nature, has been regarded as a potential alternative to fossil resource. However, cellulose is extremely difficult to dissolve in water and most conventional organic solvents due to the close packing by numerous inter- and intra-molecular hydrogen bonds. Therefore, many possible applications of this cheap and abundant natural resource are limited. To tackle the issue, great efforts have been made in the scientific community in recent years, and a series of cellulose solvents have been developed such as N
-oxide, lithium chloride + N
-dimethylacetamide, tetrabutyl ammonium fluoride + dimethyl sulfoxide DMSO, NaOH/Thiourea, etc. [3
In recent years, ionic liquids (ILs), especially imidazolium-based carboxylate ILs have received considerable attention due to their powerful capacity for cellulose dissolution [7
]. Recently, some efforts have been made to develop more efficient cellulose solvent systems by adding co-solvents to ILs. For example, Rinaldi developed solvent systems (1-butyl-3-methylimidazolium chloride + aprotic polar solvent including dimethyl sulfoxide DMSO, N
-dimethylformamide DMF, etc.) which have lower viscosity and higher dissolving rate than neat ILs [13
]. Later on, Xu et al. found that 1-butyl-3-methylimidazolium acetate [Bmim][CH3
COO] + DMSO (DMF or N
-dimethylacetamide DMAc) solvents could effectively dissolve cellulose at ambient temperature without any heating [14
]. Sun et al. developed tetrabutylammonium acetate/dimethyl sulfoxide (TBAA/DMSO) solvent which could dissolve cellulose and spin cellulose fibers by a wet spinning system [17
]. Rein et al. investigated the structure of cellulose in 1-ethyl-3-methylimidazolium acetate [Emim][CH3
COO] + DMSO/DMF solvent and found that cellulose was dissolved molecularly in the solvents [18
It has been reported that 1-allyl-3-methylimidazolium acetate [Amim][CH3
COO] displays powerful dissolution capacity for cellulose even at ambient temperature [8
]. Nevertheless, neat [Amim][CH3
COO] is slightly viscous which is not conducive to cellulose dispersion despite less viscosity than the chloride/bromide-based IL counterparts [19
]. As mentioned above, the addition of aprotic polar solvent to IL can significantly not only improve cellulose solubility but reduce solvent viscosity and thus improve the mass transport and kinetics of dissolution. However, aprotic polar solvents are volatile organic compounds which are easily lost to the environment and cause environmental pollution. Moreover, the aprotic polar solvents + IL solvents are difficult to recover and recycle. To tackle the issue, adding a cosolvent with negligible vapor pressure instead of a volatile organic solvent is a promising strategy.
Therefore, in the present work, novel [Amim][CH3
COO]/PEG solvents were developed by adding PEG-200 to [Amim][CH3
COO]. The selection of PEG-200 is based on the fact that, PEG-200 is a nonvolatile, biodegradable, corrosion inhibiting, cheap and easily obtained molecular solvent, which has been approved by the FDA for internal consumption. Moreover, PEG was reported to be a hydrogen-bonding acceptor that prevents the re-association of hydroxyl groups of cellulose forming gel, and the chains of the PEG make its repeat units difficult to be extruded out from solution compared with small molecules such as urea or thiourea, by the self-association of cellulose molecules, which results in the stability of the cellulose solution [20
]. At the same time, the solubilities of cellulose in [Amim][CH3
COO]/PEG solvents were determined. 13
C NMR technique was employed to investigate the possible dissolution mechanism of cellulose in [Amim][CH3
COO]/PEG solvent. Additionally, the regenerated cellulose films from [Amim][CH3
COO]/PEG solvent were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and thermogravimetric analysis (TGA).
2. Experimental Section
Microcrystalline cellulose (MCC) with a 270 viscosity-average degree of polymerization (DP) was purchased from Sigma Aldrich Company (Shanghai, China). Polyethylene glycol (PEG) with a number-average molecular weight of 200 was purchased from Shanghai Jingchun Biotechnology Co. Ltd. (Shanghai, China). Deuterated DMSO (DMSO-d6
) (>99.9%) used for NMR samples was purchased from Qingdao Weibo Tenglong Technology Co., Ltd. (Qingdao, China). The above materials were used as received without further purification. [Amim][CH3
COO] was synthesized and purified by using the procedure described in the literature [14
2.2. Dissolution of Cellulose in [Amim][CH3COO]/PEG Solvents
COO]/PEG-10 and [Amim][CH3
COO]/PEG-20 containing 10 wt % and 20 wt % PEG in [Amim][CH3
COO]/PEG solvent, respectively were prepared by adding PEG in dried [Amim][CH3
COO] under stirring. Cellulose was added into a 25 mL colorimetric tube which contained 2.0 g of [Amim][CH3
COO]/PEG, and the tube was sealed with parafilm. The tube was then immersed in an oil bath (DF-101S, Gongyi Yingyu Instrument Factory, Gongyi, China), and the instability of the bath temperature was estimated to be ±0.5 °C. After the cellulose was fully dissolved, the solution became completely clear and no cellulose particles were observed under a polarization microscope (Nanjing Jiangnan Novel Optics Co. Ltd., Nanjing, China). Then, additional cellulose was added. When the cellulose solution became saturated to the point that no more cellulose could be dissolved, its solubility (expressed by gram per 100 g of solvent) at a given temperature was calculated from the amount of the solvent and cellulose added. The solubilities of cellulose in [Amim][CH3
COO]/PEG solvents are summarized Table 1
2.3. Measurements 13C NMR Spectra
Measurements of 13C NMR spectra for [Amim][CH3COO] in [Amim][CH3COO]/PEG-10 solvent, [Amim][CH3COO]/PEG-20 solvent, [Amim][CH3COO]/PEG-10/cellulose solution were performed on a Bruker DMX 300 spectrometer (Bruker Corporation, Rheinstetten, German) at room temperature. DMSO-d6 was used as an external standard. Chemical shifts were given in ppm downfield from TMS. An [Amim][CH3COO]/PEG-10/cellulose solution with 8.0% solubility was obtained by dissolving cellulose in [Amim][CH3COO]/PEG-10 solvent.
2.4. Preparation and Characterization of Regenerated Cellulose Film
Five percent of cellulose solution was prepared by dissolving cellulose in [Amim][CH3COO]/PEG-10 solvent at 40 °C. The solution was cast onto a glass plate to give a thickness of about 2 mm, the air bubbles removed in a vacuum oven for 30 min, and then immediately coagulated in water to obtain a transparent regenerated cellulose gel film. The regenerated cellulose gel film was washed with running distilled water followed by drying at 60 °C in a vacuum oven. The dried cellulose film was coded as RC-A.
The above as-prepared cellulose gel film was frozen for 2 h in a refrigerator, then freeze-dried using a FD-10 freeze-dryer (Henan Xiongdi Instrument Equipment Co. Ltd., Zhengzhou, China). The cold trap temperature was below −45 °C and the vacuum pressure was below 0.1 MPa during the freeze-drying process. The freeze-dried cellulose film was coded as RC-F.
Five percent of as-prepared cellulose solution was cast onto a glass plate to give a thickness of about 2 mm, air bubbles removed in a vacuum oven for 30 min. Then, the cellulose solution film was frozen for 2 h at –80 °C and subsequently immersed in distilled water for regeneration-gelation. The gel was repeatedly washed by distilled water to remove [Amim][CH3COO]/PEG. After being frozen for 2 h in a refrigerator, the hydrogel was freeze-dried using a FD-10 freeze-dryer to obtain cellulose aerogel. The cold trap temperature was below −45 °C and the vacuum pressure was below 0.1 MPa during the freeze-drying process. The freeze-dried cellulose film was coded as RC-FF.
The RC-A, RC-F, and RC-FF films were employed for the measurements of SEM, XRD, ATR-FTIR spectroscop, and TGA. Scanning electron micrographs (SEM) of the regenerated cellulose films in the dry state were frozen in liquid nitrogen, and immediately snapped. The fractured surfaces of the films were sputtered with gold, and then photographed. The XRD patterns were collected on a BrukerD8Advance diffraction spectrometer with Cu-Ka radiation (λ = 1.54 Ǻ) over the range 3°–60° (2θ) at a scan speed of 2° (2θ) per minute. An ATR-FTIR (Nicolet iN10, Thermo Fisher Scientific, USA) system with Ge crystal ATR accessory, MCT (mercury–cadmium telluride) detector, and OMINC picta workstation (Thermo Fisher Scientific, Waltham, MA, USA) were employed for IR observation. Spectra were collected in high-resolution mode (4 cm–1 resolution and 64 scans) under ATR 5% maximum pressure. Background was subtracted for every measurement. Triplicate tests were performed at different sites for every sample. Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA 449 C thermal analyzer (Netzsch Corporation, Freistaat Bayern, German) using alumina crucibles. The measurements were carried out under flowing N2 at a heating rate of 10 °C·min−1.
Low viscous and efficient solvents ([Amim][CH3COO]/PEG) have been developed, which are advantageous for processing/fabricating cellulose materials in practical applications. Moreover, the solvents have negligible vapor pressures and can be recycled, which can enable sustainable production, lower energy consumption and costs, as well as reducing possible harm to the environment. The possible dissolution mechanism for cellulose is suggested to be that the [CH3COO]– anion and [Amim]+ cation of [Amim][CH3COO] in [Amim][CH3COO]/PEG solvent mainly contribute to cellulose dissolution. By changing cellulose processing strategies, the architecture structure of the regenerated cellulose material can be modified. Moreover, there are no chemical reactions between lignin and [Amim][CH3COO]/PEG. In addition, TGA findings indicate that the regenerated cellulose exhibits good thermal stability compared to the original cellulose.