Properties and applications of cellulose regenerated from cellulose/imidazolium-based ionic liquid/co-solvent solutions: A short review

2.1 Isolation of cellulose

The utilization of biopolymers for packaging, semi-structural, and structural applications is a promising means for environment protection. Among biopolymers, cellulose is the most extensively studied biopolymer because it is easily available, renewable, biodegradable, and cheap. Currently, there are many kinds of bioresources that can be used as a source of cellulose. Table 4 presents examples of bioresources and imidazolium-based ionic liquid/co-solvent mixtures employed for the isolation of cellulose. It can be seen that bioresources such as bamboo pulp (8), cotton linters (20), cotton pulp (22), cotton stalk (23), Eucalyptus sawdust (37), flax fibers (38), Napier grass (46), oil palm frond (33), and pine wood bits (45) have been employed as cellulose sources. Cellulose was successfully isolated by employing different types of imidazolium-based ionic liquid/co-solvent mixtures through the dissolution process. Other approaches with aqueous ionic liquids have involved swelling and hydrolysis of natural fibers into cellulose nanocrystals (47,48,49). In Table 4, it can also be observed that imidazolium-based ionic liquids with chloride and acetate counter anions are commonly employed because they are capable of interacting with hydroxyl groups of cellulose via hydrogen bonding interaction in the dissolution mechanism (6). Moreover, co-solvent such as DMSO is frequently used compared to other co-solvents mentioned earlier because DMSO is less toxic and cheaper than other co-solvents. Therefore, the isolation of cellulose from bioresources on a large scale can be achieved by utilizing the imidazolium-based ionic liquid/DMSO mixtures without neglecting the toxicity and the cost of the solvents.

2.2 Dissolution of cellulose

Table 5 presents imidazolium-based ionic liquid/co-solvent mixtures, the ratio of mixtures, cellulose contents, temperatures, and times for the dissolution of cellulose. It can be noticed that the ratios of co-solvent to ionic liquid are equal to or less than that of ionic liquid, and this is due to the fact that the co-solvent ratios are ideal for mixtures. Interestingly, this can decrease the total cost of the solvents because a lesser proportion of ionic liquid is required without sacrificing the solubility of cellulose in the solvents. Moreover, the solubility does not change significantly in the cellulose conformation. Conversely, the dissolution of cellulose in imidazolium-based ionic liquid/co-solvent mixtures can be up to 23.6 wt% of cellulose content. This content is hard to achieve when using only ionic liquid as a result of its very high viscosity. With the presence of co-solvent, the viscosity of the ionic liquid decreases, thus allowing a higher solubility quantity of cellulose since co-solvent enhances the interaction between the ionic liquid and cellulose without participating in the interaction. Besides that, the dissolution of cellulose in the imidazolium-based ionic liquid/co-solvent mixtures is normally conducted at elevated temperatures, regardless of the type of the mixtures. Hence, the dissolution temperature plays an important role in the dissolution of cellulose. With an increase in temperature, the dissolution kinetics accelerate, consequently permitting a faster dissolution. In addition, the maximum dissolution temperature is 110°C, which may prevent cellulose degradation.

2.3 Regeneration of cellulose

Cellulose can be regenerated from cellulose/ionic liquid/co-solvent solutions by immersing them in a coagulation agent. This agent is anti-solvent because of its ability to reduce the solvent quality of ionic liquids. In contrast to co-solvent, the coagulation agent induces cellulose regeneration by removing ionic liquid since it is completely miscible with the coagulation agent at any ratio. During the coagulation process, the anions and cations of the ionic liquid rapidly move away from the cellulose solution to the coagulation agent; this causes the regeneration of the cellulose gel. After the elimination of the coagulation agent, the formation of the solid cellulose occurs. Table 6 presents examples of imidazolium-based ionic liquid/co-solvent mixtures and coagulation agents used for the regeneration of cellulose. It can be observed that ethanol and water are regularly used as coagulation agents for cellulose regeneration. Generally, water is preferred as a coagulation agent because it is of low cost (6). Nonetheless, coagulation in water is clearly distinct from coagulation in alcohols with respect to the microstructures formed and the mechanisms that generate the microstructures (39). Conversely, the ionic liquid/co-solvent mixture can be recycled by evaporating a major quantity of coagulation agent under reduced pressure, followed by freeze-drying under vacuum pressure. In Table 6, it can also be seen that the coagulation agent such as carbon dioxide can be used to regenerate cellulose and remove ionic liquid/co-solvent mixtures (25). Long-chain cellulose with larger molecular weight is more easily to be surrounded by carbon dioxide molecules, leading to a poorer interaction with ionic liquid and thus resulting in an easier regeneration (50).

3.1 Properties of regenerated cellulose

Table 7 presents the crystalline, chemical, and thermal properties of cellulose regenerated from cellulose/imidazolium-based ionic liquid/co-solvent solutions. The regenerated cellulose has been prepared from bamboo pulp/AAeimOAc/DMSO solution (8). The crystalline, chemical, and thermal properties of the regenerated cellulose were characterized by using an X-ray diffractometer, Fourier transform infrared spectrometer, and thermal gravimetric analyzer. The crystalline property, such as the intensity of the X-ray diffraction peaks of the regenerated cellulose, is weaker compared to that of the original bamboo pulp cellulose. This indicated that the crystallinity of cellulose decreased because its crystalline structure changed from I to II. Furthermore, the chemical property, such as the infrared absorption band of the O–H stretching vibration, of the regenerated cellulose shifted to a higher wavenumber region compared to the O–H stretching vibration of the original bamboo pulp cellulose because the breaking of hydrogen bonding weakened the bonding force during the dissolution of cellulose. Conversely, the thermal property, such as the decomposition temperature of the regenerated cellulose, is lower than that of the original bamboo pulp cellulose. This was ascribed to the decreased number of hydrogen bonding between cellulose chains and crystallinity, which decreased the thermal stability of the regenerated cellulose (8). Hence, it can be deduced that the utilization of the AAeimOAc/DMSO mixture gives regenerated cellulose with a low crystallinity and low thermal decomposition behavior.

Meanwhile, the regenerated cellulose fibers have been prepared from cellulose powder/EmimOAc/DMF solutions (35). The crystalline, chemical, and thermal properties of the regenerated cellulose fibers were characterized by using an X-ray diffractometer, Fourier transform infrared spectrometer, and thermal gravimetric analyzer. The crystalline property, such as the degree of crystallinity of the regenerated cellulose fibers, is significantly lesser compared to that of the cellulose powder because the rapid coagulation of cellulose from the ionic liquid did not permit sufficient time to create hydrogen bonding. Besides, the chemical property such as the infrared absorption bands for EmimOAc and DMF is not detected in the regenerated cellulose fibers. This exhibited that the residue of the ionic liquid and the co-solvent in the fibers was fully removed by the coagulation agent. On top of that, the thermal property such as the decomposition temperature of the regenerated cellulose fibers is slightly smaller than that of the cellulose powder. This was caused by the influence of the cellulose crystallinity on the thermal stability of the microstructure of the regenerated cellulose fibers and consecutively resulted in the lower decomposition enthalpy (35). Therefore, it can be concluded that the utilization of the EmimOAc/DMF mixture grants regenerated cellulose fibers with a less degree of crystallinity and slightly low thermal stability.

The regenerated cellulose aerogels have been prepared from cotton stalk/AmimCl/DMSO solutions (23). The crystalline, chemical, and thermal properties of the regenerated cellulose aerogels were characterized by using an X-ray diffractometer, Fourier transform infrared spectrometer, and thermal gravimetric analyzer. The crystalline property such as the crystallinity index of the regenerated cellulose aerogels is lower than that of the raw cotton stalk. This was caused by the reorientation of cellulose chains in the form of parallel chains. Moreover, the chemical property, such as the infrared absorption bands of the O–H stretching vibrations, of the regenerated cellulose aerogels shifted to lower wavenumber regions compared to the O–H stretching vibration of the raw cotton stalk. This was attributed to the increase of hydrogen bonding between cellulose and other biopolymer components, resulting in the restriction of these biopolymers in the cellulose matrix. Besides that, the thermal property such as the decomposition temperature of the regenerated cellulose aerogels increased by up to 1.8% in comparison to the raw cotton stalk. This was due to the porous structure of the aerogels, which increased the thermal stability of the regenerated cellulose aerogels (23). Thus, it can be inferred that the utilization of the AmimCl/DMSO mixture provides regenerated cellulose aerogels with a low crystallinity index and high thermal decomposition behavior.

Besides that, the regenerated cellulose hollow fibers have been prepared from microcrystalline cellulose/EmimOAc/DMSO solutions (41). The crystalline, chemical, and thermal properties of the regenerated cellulose hollow fibers were characterized by using an X-ray diffractometer, Fourier transform infrared spectrometer, and thermal gravimetric analyzer. The crystalline property such as the degree of crystallinity of the regenerated cellulose hollow fibers is lower compared to that of the raw microcrystalline cellulose. This was because the initial crystalline structure was partly changed during the dissolving through interaction with ionic liquid. Moreover, the chemical property such as the infrared absorption bands of the C–O–C and C–O stretching vibrations, of the regenerated cellulose hollow fibers shifted to lower wavenumber regions in comparison to the C–O–C and C–O stretching vibrations of the raw microcrystalline cellulose. This was due to the cellulose I crystalline structure in the raw microcrystalline cellulose was transformed to cellulose II in the regenerated cellulose hollow fibers. In addition, the thermal property such as the decomposition temperature of the regenerated cellulose hollow fibers is lesser than that of the raw microcrystalline cellulose. This was because of the decreased crystallinity of cellulose II and the transition of the crystalline structure (41). So, it can be inferred that the utilization of the EmimOAc/DMSO mixture provides regenerated cellulose hollow fibers with a low degree of crystallinity and less thermal decomposition behavior.

Conversely, the regenerated cellulose film has been prepared from treated pine cellulose/EmimDEP/DMSO solution (45). The crystalline, chemical, and thermal properties of the regenerated cellulose film were characterized by using an X-ray diffractometer, Fourier transform infrared spectrometer, and thermal gravimetric analyzer. The crystalline property such as the X-ray diffraction peaks of the regenerated cellulose film exhibited the presence of broad crystalline peaks compared to that of the treated pine cellulose. This was because the cellulose was changed from cellulose I to cellulose II during the dissolution and regeneration. Furthermore, the chemical property, such as the infrared absorption band of the O–H stretching vibration, of the regenerated cellulose film shifted to a lower wavenumber region in comparison to the O–H stretching vibration of the treated pine cellulose. This was because the connection between the hydroxyl groups was decreased, and subsequently, the force of the intermolecular hydrogen bonding was reduced. In addition, the thermal property such as the decomposition temperature of the regenerated cellulose film is lower than that of the treated pine cellulose. This was attributed to the decrease in crystallinity of the regenerated cellulose film, which lowered its thermal stability (45). Consequently, it can be deduced that the utilization of the EmimDEP/DMSO mixture gives broad crystalline peaks and low thermal stability to the regenerated cellulose film.

3.2 Properties of regenerated cellulose blends

Table 8 displays the crystalline, chemical, and thermal properties of cellulose blends regenerated from cellulose blend/imidazolium-based ionic liquid/co-solvent solutions. The regenerated cellulose/agar blend aerogels have been prepared from microcrystalline cellulose/agar/BmimCl/DMSO solutions (34). The chemical and thermal property of the regenerated cellulose/agar blend aerogels were characterized by using Fourier transform infrared spectrometer and differential scanning calorimeter. The chemical property, such as the infrared absorption band of the O–H stretching vibration, of the regenerated cellulose/agar blend aerogel shifted to a higher wavenumber region in comparison to the O–H stretching vibration of the raw microcrystalline cellulose. This was due to the formation of intermolecular hydrogen bonding between the hydroxyl groups of cellulose and the hydroxyl groups of agar. This indicated the presence of intermolecular interaction between cellulose and agar, which increased the miscibility between them. Nonetheless, the thermal property such as the melting point temperature of the regenerated cellulose/agar blend aerogel is considerably lower compared to that of the pristine agar. This was ascribed to the thermodynamic factor such as polymer–polymer interaction, which exhibited the depression in the melting temperature (34). Thus, it can be inferred that the utilization of the BmimCl/DMSO mixture provides regenerated cellulose/agar blend aerogels with a good interaction of their components and low melting temperature.

Meanwhile, the regenerated cellulose/chitin blend films have been prepared from microcrystalline cellulose/chitin/BmimOAc/GVL solutions (30). The crystalline and chemical properties of the regenerated cellulose/chitin blend films were characterized by using an X-ray diffractometer and Fourier transform infrared spectrometer. The crystalline property such as the X-ray diffraction peaks of the regenerated cellulose/chitin blend films showed only flat and broad diffraction peaks compared to those of the raw microcrystalline cellulose and chitin. This was attributed to the formation of cellulose II in the amorphous state as well as the transition from the α-chitin to the β-chitin polymorph after regeneration. Moreover, the crystallinity index and crystallite size of the regenerated cellulose/chitin blend films are lower than those of the raw microcrystalline cellulose and chitin. This was due to the dissolution process, whereby crystalline structures of cellulose and chitin were destroyed. Besides that, the chemical property such as the intensity of the infrared absorption bands of the regenerated cellulose/chitin blend films is lesser in comparison to those of the raw microcrystalline cellulose and chitin. This also showed the change of the crystalline structures after regeneration (30). Hence, it can be deduced that the utilization of the BmimOAc/GVL mixture gives regenerated cellulose/chitin blend films with a low crystallinity index and crystallite size.

The regenerated cellulose/polyamide blend films have been prepared from microcrystalline cellulose/polyamide-6/EmimOAc/DMSO solutions (42). The chemical and thermal properties of the regenerated cellulose/polyamide blend films were characterized by using Fourier transform infrared spectrometer and differential scanning calorimeter. The chemical property, such as the infrared absorption bands of the C–O stretching vibrations, of the regenerated cellulose/polyamide blend films shifted to higher wavenumber regions compared to the C–O stretching vibration of the raw microcrystalline cellulose. Furthermore, the infrared absorption bands of the N–H deformation vibrations of the regenerated cellulose/polyamide blend films shifted to higher wavenumber regions in comparison to the N–H deformation vibration of the pure polyamide-6. This was because of the interactions of the N–H group with the cellulose backbone. Nevertheless, the thermal property such as the melting point temperature of the regenerated cellulose/polyamide blend films is substantially lower than that of the pure polyamide-6. This was owing to the intermolecular interactions between the cellulose and polyamide chains (42,51). So, it can be inferred that the utilization of the EmimOAc/DMSO mixture provides regenerated cellulose/polyamide blend films with good interactions of their components and low melting temperature.

Besides that, the regenerated cellulose/polylactic acid blend films have been prepared from microcrystalline cellulose/polylactic acid/BmimOAc/DMF solutions (26). The crystalline, chemical, and thermal properties of the regenerated cellulose/polylactic acid blend films were characterized by using an X-ray diffractometer, Fourier transform infrared spectrometer, and thermal gravimetric analyzer. The crystalline property, such as the X-ray diffraction peaks of the regenerated cellulose/polylactic acid blend films, disappeared than those of the raw microcrystalline cellulose and polylactic acid film. This was attributed to the amorphous state present in the blend films. On top of that, the chemical property, such as the infrared absorption bands of the O–H stretching vibrations, of the regenerated cellulose/polylactic acid blend films shifted to higher wavenumber regions in comparison to the O–H stretching vibration of the pure cellulose film. This was due to the presence of a hydrogen bonding interaction between the hydroxyl groups of cellulose and the carbonyl groups of polylactic acid, which decreased the hydrogen bonding interaction between the cellulose chains. In addition, the thermal property such as the decomposition temperature of the regenerated cellulose/polylactic acid blend films increased by up to 33% compared to the pure cellulose film. This was ascribed to the presence of interaction between them (52), which induced the good thermal stability of the blend films (26). Consequently, it can be deduced that the utilization of the BmimOAc/DMF mixture gives regenerated cellulose/polylactic acid blend films with a good interaction of their components and high thermal decomposition behavior.

Conversely, the regenerated cellulose/polyacrylonitrile blend fibers have been prepared from raw cotton/polyacrylonitrile/BmimCl/DMSO solutions (15). The chemical and thermal properties of the regenerated cellulose/polyacrylonitrile blend fibers were characterized by using Fourier transform infrared spectrometer and thermal gravimetric analyzer. The chemical property, such as the intensity of the infrared absorption bands of the hydroxyl stretching vibrations, of the regenerated cellulose/polyacrylonitrile blend fibers is higher in comparison to the hydroxyl stretching vibration of the pure polyacrylonitrile. This was caused by the addition of the natural biopolymer, which has a substantial band in this wavenumber region. However, the intensity of the infrared absorption bands of the amide stretching vibrations, of the regenerated cellulose/polyacrylonitrile blend fibers, is lower compared to the amide stretching vibration of the pure polyacrylonitrile. Conversely, the thermal property such as the decomposition temperature of the regenerated cellulose/polyacrylonitrile blend fibers is significantly lower than that of the pure polyacrylonitrile. This phenomenon is similar to lignin/polyacrylonitrile blends that showed a decrease in the decomposition temperature (15). Therefore, it can be concluded that the utilization of the BmimCl/DMSO mixture grants regenerated cellulose/polyacrylonitrile blend fibers with a high hydroxyl stretching vibration intensity and low thermal decomposition behavior.

4.1 Applications of regenerated cellulose

Table 9 presents examples of applications of cellulose regenerated from cellulose/imidazolium-based ionic liquid/co-solvent solutions. It can be seen that the imidazolium-based ionic liquid/DMSO mixtures are often utilized in the development of regenerated cellulose applications because DMSO is less toxic and cheaper than other co-solvents, as previously mentioned. On top of that, the regenerated cellulose was applied in the manufacturing of continuous and sustainable filaments (44), and the mechanical properties of the filaments are higher than other commercial regenerated cellulose fibers. Moreover, the regenerated cellulose was applied in the preparation of nanofibers (35) that have finer fiber diameters, better web uniformity, higher crystallinity, and better thermal stability. In addition, the regenerated cellulose was applied in the fabrication of aerogels (23), and this approach is not only environmentally friendly but also advantageous in creating a more commercially viable ionic liquid-based system. Besides that, the regenerated cellulose was also applied in the preparation of hollow fibers (41), and these fibers can be used as a precursor to develop hollow fiber carbon membranes for gas separation. Furthermore, the regenerated cellulose was applied in the production of high-performance fibers (11) by using high concentrations of cellulose with low molecular weight, and the processing time and cost are also significantly reduced. In addition, the regenerated cellulose was applied in the production of 3D-printed structures (29); therefore, this renewable biopolymer can replace the synthetic nonbiodegradable polymers in the 3D additive manufacturing process for producing single and multiple components.

4.2 Applications of regenerated cellulose blends

Table 10 presents examples of applications of cellulose blends regenerated from cellulose blend/imidazolium-based ionic liquid/co-solvent solutions. It can be observed that cellulose can be blended with natural biopolymers and synthetic polymers in imidazolium-based ionic liquid/co-solvent mixtures for a wide variety of applications. Conversely, the regenerated cellulose/agar blends were applied in the fabrication of blend aerogels (34), and the bulk density of the aerogels is lower compared to their individual components because of the small pore size diameters. Furthermore, the regenerated cellulose/chitin blends were applied in the preparation of biodegradable films (30) that can be used in packaging, medical, or textile uses substituting fossil resource-based substances. In addition, the regenerated cellulose/chitosan blends were applied in the production of porous macrospheres (28), and these macrospheres are able to remove significant amounts of copper ions contaminants from water. Besides that, the regenerated cellulose/polyacrylonitrile blends were also applied in the creation of blend fibers (15) that can be employed as low-cost environmentally friendly precursors for carbon fiber. Moreover, the regenerated cellulose/polyamide blends were applied in the preparation of blend films (42), and these films have the potential to be used in textiles or technical usages owing to their homogeneous morphology. In addition, the regenerated cellulose/polylactic acid blends were applied in the fabrication of biocompatible films (26) that have possible functions in the biomedical field. The films are appropriate for large-scale production due to their very simple preparation procedure and quite mild manufacturing condition.