Introduction
Bio-based raw material sources have gained global interest as a potential replacement for their fossil-based counterparts. In this regard, cellulose, which is the most abundant renewable carbon based material of our planet offers huge valorization opportunity as a viable alternative (Klemm et al.
2005). In addition, cellulose offers a competitive advantage compared to other bio-based resources, as it does not compete with food or feed, and constitutes 35–50 % of the more than 170 × 10
9 tons of lignocellulosic biomass produced annually (Delidovich et al.
2016; Putro et al.
2016; Rose and Palkovits
2011). Apart from being the key raw material for the pulp and paper industry, (Klemm et al.
2005) well known cellulose derivatives such as cellulose esters, cellulose ethers or more recently cellulose carbonates, find applications in various areas ranging from optical films, textile, coating, cigarette filters, non-woven fibers, drug release and as viscosity modifiers (Klemm et al.
2005; Dwyer and Abel
1986; Edgar et al.
2001; Elschner and Heinze
2015; Law
2004; Doshi and Reneker
1995). Electrospinning involves applying a high voltage between a polymer solution and a metal collector, which causes the charged polymer solution to move towards the collector (electrostatic force), leading to formation of sub-micron (nano-) fibers upon reaching the collector as the solvent evaporates (Doshi and Reneker
1995; Fong and Reneker 2018;, Colman et al.
2008), which are of interest for filtration, catalysis or biomedical applications (Huang et al.
2003; Heidari et al.
2017).
One of the challenges of for cellulose elctrospinning remains the difficulty of solubilizing cellulose in a sustainable fashion. Due to the strong intra- and inter-molecular hydrogen bonds, cellulose does not solubilize in common organic solvents including water (Klemm et al.
2005). Thus, special solvents able to break this intra-inter hydrogen bonds are required to solubilize cellulose. Examples such as N,N-dimethylacetamide-lithium chloride (DMAc-LiCl), (McCormick and Dawsey
1990) N-methylmorpholine N-oxide (NMMO), (Fink et al.
2001) dimethyl sulfoxide-tetramethyl ammonium fluoride (DMSO-TBAF), (Heinze et al.
2000) or trifloroacetic acid (TFA) (Hasegawa et al.
1992). These solvents suffer from limitations including difficult recovery and/or toxicity and are thus not considered sustainable. However, such solvents have been explored for direct electrospining of cellulose. As an example, Ohkawa and colleagues reported the direct electrospining of cellulose in trifloroacetic acid (TFA) leading to formation of nanocellulose fibers with mean diameter of 40 nm (Ohkawa et al.
2009). It is noteworthy that the use of TFA in the solubilization of cellulose proceed through a derivative step in which the cellulose is modified to cellulose trifloroacetate (Heinze and Koschella
2005). Whereas the direct electrospinning of cellulose offers benefits i.e. avoiding any pre-derivatization. The use of TFA (acute toxicity LD50, rat 200–400 mg/kg-rat) raises environmental and safety concerns. In a similar fashion, DMAc-LiCl as well as NMMO/H
2O was employed by Kim et al. (
2005,
2006) for direct electrospinning of cellulose to obtain sub-micron fibers with diameters ranging from 150 to 750 nm.
Another class of solvents that offer a more sustainable option for direct cellulose solubilization are ionic liquids. They are generally defined as low melting molten salts with melting temperatures below 100 °C (Welton
1999; Hallett and Welton
2011) and are classified as “greener” solvents due to their very low vapor pressures with a potential to be recycled and reused, which are all important aspects for sustainability (Swatloski et al.
2002). This solvent have been explored for various chemical modification (El Seoud et al.
2007; Mormann and Wezstein
2009; Wu et al.
2004). Coutinho and co-workers employed a mixture of the ionic liquids 1-ethyl-3-methylimidazolium acetate and 1-decyl-3-methylimidazolium to electrospin cellulose fibers, obtaining nanofibers with average diameters ranging between 165 and 185 nm (Freire et al.
2011). The cellulose fibers were collected via coagulation in water and the ionic liquid could be recovered and re-used. Similarly, Linhard et al. reported the direct electrospinning of cellulose and cellulose-heparin in RTIL (room-temperature ionic liquids, using 1 N-butyl-3-methylimidazolium chloride and 1 N-ethyl-3-methylimidazolium benzoate) (Viswanathan et al.
2006). Unlike the previous report, the cellulose fibers were obtained by extraction of the RTIL using ethanol. However, larger micron-sized cellulose fibers were obtained and this was attributed to the non-volatility of the solvents. Finally, 1-allyl-3-methylimidazolium chloride (AMIMCl) in DMSO was used by Han et al. (Xu et al.
2008). Though ionic liquids might be “greener” solvents compared to the traditional ones, they are not without limitations and challenges (Clough et al.
2015). Among their limitations are toxicity (example [C4mim]
+ Cl
-, LD50, rat = 50–300 mg/kg) (Gericke et al.
2012) and non-inertness to cellulose especially those with acetate anion, (Clough et al.
2015; Ebner et al.
2008), which makes their recovery more challenging and expensive compared to other cellulose solvents.
To overcome the difficulty and challenges in solubilizing cellulose, researchers have employed the use of cellulose derivatives like cellulose acetate (CA), methyl cellulose and carboxymethylcellulose (CMC), which can be more easily solubilized in common solvents for electrospinning (Formhals
1938a,
b). It is noteworthy that a post-derivatization step is necessary to recover the native cellulose fibers. This extra step creates more wastes, making the approach less sustainable. In 2003, Park and co-workers reported the electrospinning of cellulose acetate (CA) in an acetone/water mixture containing between 10 and 15 wt% water (Son et al.
2004). CA fibers with mean diameter of 2.3 μm were obtained under acidic conditions, while much thinner fibers with a mean diameter of 460 nm were obtained under basic conditions. Frenot et al. reported the electrospinning of CMC (carboxy methyl cellulose), HPMC (hydroxy propyl methylcellulose), and MC (methyl cellulose). CMC was electrospun in a 1:1 ratio with PEO using water as solvent (Frenot et al.
2007). Nanofibers with mean diameter between 200 and 250 nm were obtained. Morphology of the fibers was studied using SEM and showed that the obtained morphology was independent on the MW and DS but rather depended on the substitution pattern. In addition, extraction of PEO after the electrospinning process resulted in a change in fiber morphology and depended on the substitution pattern in the CMC. Similar to CMC, the obtained fiber morphology was not influenced by molecular weight (MW) or degree of substitution (DS). In the case of CMC using same solvent system as for HPMC, a more wet fiber was collected showing a more coarse and collapse fiber morphology (Frenot et al.
2007).
While cellulose remains an important and versatile renewable resource, it is important to consider beyond the raw material source to ensure sustainability (Llevot et al.
2016; Onwukamike et al.
2018a). In this regard, and to ensure a more sustainable electrospinning of cellulose, the use of a more sustainable solvent system with a potential of recyclability will enable us to improve the sustainability of this process. A very good example of a sustainable solvent system based on CO
2 in the presence of an organic super base was developed by Jessop et al. in (
2005). This system allows the possibility to switch the polarity of a solvent simply by adding/removal of CO
2. The application of this solvent system for cellulose solubilization was independently and simultaneously demonstrated by Zhang et al. (
2013) and Xie et al. (
2014).
Recently, we reported a thorough optimization of the solvent system and proved the presence of a cellulose carbonate intermediate, which introduces the solubility of cellulose in the DMSO solvent (Onwukamike et al.
2017). We were able to achieve complete cellulose solubilization within 10–15 min at 30 °C under very low CO
2 pressure (2–5 bar). Such mild solubilization conditions allowed us to explore this switchable solvent system to show a more sustainable succinylation of cellulose, (Söyler et al.
2018) transesterification of cellulose using plant oils directly, (Onwukamike et al.
2018c) and in multicomponent reaction, in which the CO
2 was employed as a C1-carbon source (Onwukamike et al.
2018b). In addition, we employed this solvent system to produce cellulose aerogels with porosity higher than 95 % (Onwukamike et al.
2019). These examples showed the high versatility of the CO
2 switchable solvent system and equally open up their potential for other cellulose-based applications.
In this report and based on our previous experiences working with the CO2-switchable solvent system for various cellulose valorization, we investigated the potential of this solvent system in the direct electrospinning of cellulose. The challenge was the difficulty to keep the cellulose solution stable over electrospinning process. Two different cellulose were investigated, i.e. microcrystalline cellulose (MCC) and cellulose pulp (CP). Parameters investigated include the cellulose concentration, electrospinning conditions as well as use of various additives to improve the electrospinning process.
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