Introduction
Cellulose is the most abundant and renewable natural biopolymer on earth and due to its particular molecular structure has unique chemical and physical properties, such as high thermostability and excellent mechanical capabilities (Nishino et al.
1995; Zhang et al.
2017). Therefore, cellulose has gained increasing attention as a replacement material for a number of finite fossil fuel products (Jiang et al.
2019; Sayyed et al.
2019). Due to the highly crystalline structure, abundant intra and intermolecular hydrogen bonds and the hydrophobic interactions between rings, cellulose is insoluble in water and in most organic solvents. It can only be dissolved via the breaking down of the hydrogen bond network (Glasser et al.
2012; Lindman et al.
2010).
Cellulose can be dissolved via two key ways, either with prior chemical modification or without (Sen et al.
2013). For the former, the hydroxyl groups on the cellulose molecules are firstly functionalized, by nitration for example (Barbosa et al.
2005), esterification (Ratanakamnuan et al.
2012), etherification (Fox et al.
2011) or xanthation (Polyuto et al.
2000). In general, these processes are environmentally harmful (Zhang et al.
2017). For example, the first industrialized procedure for fabricating rayon fibres, known as the viscose process, uses carbon disulphide and aqueous sodium hydroxide; both of which are known to be environmentally damaging. The production of sodium cellulose xanthate involves harmful chemicals such as sulphuric acid, H
2S and SO
2 (Clotworthy
1928). For the later method (without chemical modification), solvents such as N-methylmorpholine-N-oxide monohydrate (NMMO) (Rosenau et al.
2002), Lithium Chloride/N, N-Dimethylacetamide (LiCl/DMAc) (Matsumoto et al.
2001), Sodium Hydroxide (NaOH) Aqueous Solution (Cai et al.
2007), etc. can dissolve cellulose by breaking the hydrogen bonds directly. These solvents however also have several disadvantages, such as: high vapour pressure; low thermostability; and a high level of toxicity. The only commercialized solvent, NMMO, cannot be considered environmentally-friendly due to the combination of the oxidative side reactions, difficulty to recycle and high temperatures needed for dissolution (Rosenau et al.
2002).
As a result of the increasing demand for ‘green’ cellulose solvents, ionic liquids (ILs) containing anions and cations have gained attention as promising cellulose solvents due to their high thermostability, negligible vapor pressure, broad liquid applied range, designable properties and potential recyclability (Seoud et al.
2007; Moon et al.
2011). The history of the application of ILs for dissolving cellulose can be traced back to a patent in 1934, (Graenacher
1934) though their popularity as green solvents was initiated more recently when Swatloski et. al. (
2002) discovered the efficacy of dialkylimidazolium-based ILs (with melting points below 100 °C) as suitable solvents for cellulose (Swatloski et al.
2002). This original work has led to an increasing interest in ILs in relation to cellulosic dissolution. To date, cellulose with solubility up to 39% dissolving in 3-methyl-N-butylpyridinium chloride has been reported (Heinze et al.
2005). Once dissolved, cellulose can then be coagulated from the viscous cellulose-ILs solution by coagulation in agents such as water, ethanol and acetone. This is followed by a drying process to evaporate the coagulants (Huber et al.
2012). The mechanism by which coagulation occurs can be explained as follows: coagulants stop the dissolving process via the preferential breaking of hydrogen bonds between solvent molecules and cellulose molecules, forming new hydrogen bonds between the solvent and coagulant molecules (Gupta et al.
2013).
It is important to understand and quantify the cellulose dissolution behaviour in ILs for both chemical derivatization and other cellulose processing methods. The mechanism by which ILs dissolve cellulose is still not fully understood, but the most widely accepted hypothesis is that the anions in the ionic liquids play a predominant role—by competing with the hydroxyl groups on the cellulose to form new hydrogen bonds. Additionally, it is thought the cations play a minor role in the solvation of cellulose (Cho et al.
2011; Lu et al.
2014; Zhang et al.
2017). As reported by Cuissinat et al. (
2008), there are four possible states, observable via optical microscopy, by which cellulose fibres can exist during dissolution in ionic liquids. These states are as follows: state 1, balloon formation (due to the swelling of cellulose); state 2, bursting of this balloon; state 3, dissolution of the unswollen areas; state 4, dissolution of the balloon membrane.
A number of authors have reported on the kinetics of cellulosic dissolution in ionic liquids (Budtova and Navard
2015; Druel et al.
2018; Gericke et al.
2009). Within these studies, a rheological activation energy, ranging from 46 kJ/mol to approximately 70 kJ/mol, is documented. This energy is shown to be dependent on the IL used and the concentration of cellulose. Additionally, a recently published paper showed that the dissolution rate of cellulose fibers was in linear correlation with the viscosity of 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) when both are expressed in natural logarithmic form. Furthermore, the dissolution process was found to follow Arrhenius behaviour (Chen et al.
2020). A number of other authors have studied the thermodynamics of dissolution, as well as the dissolution enthalpies of cellulose in multiple ILs; which were found to be dependent on the water content of the ILs, anion-cation composition and cellulose source (Parviainen et al.
2014). Moreover, the enthalpy of solvation of a cellulose pentamer in [C2mim][OAc] was reported to be 96.4 kJ/mol, which was obtained from force-field molecular dynamics simulations (Brehm and Pulst
2019). Even though there have been some research on the cellulose/ILs solution, the cellulose dissolution behaviour in ILs still needs to be further quantified.
In this current work, investigation of the dissolution behaviour of cellulose in ILs is conducted using a combination of X-ray diffraction (XRD) and optical microscopy (OM). Both techniques allow for the growth of the dissolved and coagulated fraction (CF) at various temperatures and times to be quantitatively measured. Cotton was chosen to be the source of cellulose, displaying the highest cellulose I content of all plants, at around 90%. The IL chosen was [C2mim][OAc], which is a liquid at room temperature and has been widely studied within our research group (Green et al.
2017; Lovell et al.
2010; Ries et al.
2014; Sescousse et al.
2010). The effect of different cotton fibre arrangements including single fibres, arrays and bundles on dissolution in [C2mim][OAc] is studied with time–temperature superposition (TTS). The cellulose dissolution behaviour is also quantified by the determination of an activation energy of dissolution for each cotton fibre arrangement. Additionally, the effect of cotton fibre arrangement on dissolution speed is also investigated. Apart from providing quantitative information on the physics of cellulose dissolution, we believe that the results of this study will aid in future research in relation to the production of all-cellulose composites (Gindl and Keckes
2005; Nishino et al.
2004). Within these materials, any of the fibre arrangements used in this study may be utilised.
Conclusions
The dissolution behaviour of cotton fibres in the ionic liquid [C2mim][OAc] has been studied using X-ray diffraction and optical microscopy. Different arrangement of cotton fibres were processed with [C2mim][OAc] at different temperatures for a range of times, after which XRD and optical microscopy were used for characterizing the crystalline structure and micromorphology of the samples. The amount of dissolution was quantified by calculating the coagulation fraction from either optical microscopy pictures of processed fibre cross sections, or X-ray diffraction deconvolution by which the cellulose I fraction was obtained. The growth of the coagulated fraction was found to follow time–temperature superposition and an Arrhenius behaviour was found for the shift factors used α
T. OM pictures show that the cotton single fibre was dissolved from the outer layer with a coagulation layer forming around the undissolved central core. As either time and/or temperature was increased the coagulation layer became larger. For arrays and bundles, the dissolution also happens in-between different single fibres, which makes the quantifying method for single fibres from OM pictures unsuitable to arrays and bundles. Therefore, the decrease of Cellulose I measured from XRD was chosen to calculate the coagulation fraction for both cotton bundles and arrays, see Eq. (
2). XRD results show that the raw cotton samples are native cellulose with a Cellulose I fraction of 71% which transforms to Cellulose II after processing, in other words the outer coagulation layer of processed cotton single fibre can be considered to be Cellulose II and amorphous cellulose. Compared with cotton bundles, cotton arrays dissolve at a faster rate as shown in Fig.
9. The kinetics of cellulose dissolution is quantified by an activation energy calculated from an Arrhenius equation. For single fibres, arrays and bundles, the dissolution activation energies are 96 ± 8 kJ/mol, 101.3 ± 0.2 kJ/mol and 90 ± 5 kJ/mol, respectively. These activation energies show an independence of the cotton fibre arrangement and are all in a good agreement with an average value of 96 ± 3 kJ/mol (23 ± 1 kcal/mol). In contrast, the dissolution speed is influenced by cotton fibre arrangement significantly, with a ranking of: CA≈CF > CB. The methods and results presented in this work could be instructive for the further study of the dissolution of cellulose-based materials and the fabrication of cotton-based all cellulose composites, which could be made from woven cotton fibres or bundles.
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