Introduction
Cellulose is a versatile raw material which can be converted into numerous products applicable in various fields. However, its full potential is not fully met, partly due to the difficulty in dissolving it. Cellulose dissolution is a prerequisite for manufacturing value-added cellulosic products, such as regenerated fibers, filaments and films (Wawro et al.
2009; Yang et al.
2011a), and producing functional cellulose-based materials in a homogenous environment (Gericke et al.
2013; Wang et al.
2016). Functionalizing cellulose in the dissolved state is desired due to the full availability of hydroxyl (OH) groups, control of both the degree of substitution (DS) and the distribution of the functional groups as well as minimal chain degradation and high yield due to small consumption of reagent by side reactions (Heinze et al.
2000). Yet, as the load-bearing structure in nature, cellulose is a recalcitrant polysaccharide organized in the plant cell wall in a hierarchical structure that resists degradation. The stiffness of cellulose chains and their close packing through numerous hydrogen bonds make dissolving cellulose difficult. Cellulose solubility is affected by strong intra- and intermolecular hydrogen bonding as well as hydrophobic interactions between the molecules (Lindman et al.
2010; Medronho and Lindman
2015). Therefore, it is insoluble in water and in the most common organic solvents.
Cellulose can, however, be dissolved with certain solvents or solvent systems which are able to diffuse in and disrupt the crystalline structure as well as dissociate cellulose chains from each other (Ghasemi et al.
2017a,
b). Cellulose dissolution typically begins with swelling of the fibers. When cellulose is placed in contact with a solvent, the solvent molecules permeate into the structure of the biopolymer changing its volume and physical properties (Medronho and Lindman
2015). If the cellulose-solvent interactions are stronger than the intra- and intermolecular interactions between cellulose chains, the crystalline network of cellulose is gradually destroyed increasing the possibility of conformational chain movement of individual chains (Rabideau et al.
2013). The weakening interactions between cellulose molecules and the increasing chain movement result in the transformation of solid cellulose fibers to swollen gel-like medium (Ghasemi et al.
2017a; Lu et al.
2011). With increasing swelling, the individual cellulose chains start to disintegrate from each other and diffuse into the bulk solution (Ghasemi et al.
2017a). In complete dissolution, the supramolecular structure of cellulose is expected to be completely destroyed resulting in a solution where cellulose chains are separated from each other. Generally, dissolution is favored by an increase of entropy in solution (e.g. the entropy of mixing and the entropy of conformation mobility) and thus, due to the long polymer chains, dissolving polymers is more challenging than dissolving their parent monomers (Budtova and Navard
2016). The former is true also for cellulose. However, the dissolution of cellulose is additionally hindered by two properties, namely, (1) the rigidity of cellulose chains limiting the degrees of freedom when the chain is dissolving and (2) the tendency of cellulose molecules to self-aggregate in solution (Budtova and Navard
2016). Thus, a good cellulose solvent is not only able to disturb the interaction between cellulose chains but also to prevent them from self-aggregating in solution.
Aqueous sodium hydroxide solution is a promising choice for cellulose dissolution due to the rapid, non-toxic, low cost and environmentally friendly dissolution process. The process for dissolving cellulose in aqueous NaOH was discovered already in the early 1900s (Davidson
1934,
1936) and it is still attracting a lot of interest (Cai et al.
2008; Egal et al.
2007; Hagman et al.
2017; Huh et al.
2020; Medronho and Lindman
2015). Cellulose dissolves rapidly in aqueous NaOH, however, the dissolution is limited to low temperatures (< 0 °C), moderately low degree of polymerization (DP), low concentration of cellulose and narrow concentration range of NaOH (7–10%) (Budtova and Navard
2016; Cai et al.
2008; Egal et al.
2007; Hagman et al.
2017; Isogai and Atalla
1998; Medronho and Lindman
2015). Furthermore, there is a tendency for gelation of semidilute solutions (e.g. 5% cellulose in 9% NaOH solution) with increased time and temperature (Liu et al.
2011; Pereira et al.
2018; Roy et al.
2003). One of the leading theories explaining cellulose dissolution in aqueous NaOH at low temperature is the formation of hydrates with water that permeate into the structure of the cellulosic material and detach individual chains from each other (Cai et al.
2008; Egal et al.
2007). According to Egal et al. (
2007) the structure of aqueous NaOH–cellulose solution is highly dependent on the concentrations of the species present in the solution and at the optimal concentrations, there is an unstable equilibrium between NaOH hydrates bonded to each other and to cellulose. On the other hand, charging up polymers is expected to increase their solubility. In the strongly alkaline conditions typically used for cellulose dissolution, cellulose is in a dissociated form which can play a significant role in the dissolution of this polymer (Kihlman et al.
2013). This phenomenon was studied by Bialik et al. (
2016) who have shown that there is a correlation between the dissolving power of alkali hydroxides and their pK values. As possible explanations for the counter-intuitive effect of temperature to cellulose dissolution in aqueous NaOH, it has been proposed that the network of NaOH hydrates is stronger at low temperatures preventing cellulose chains from interacting with each other (Egal et al.
2007). Alternatively, more polar conformations of cellulose are favored at low temperature promoting interactions with the polar solvent (Medronho and Lindman
2014).
The dissolution of cellulose in aqueous NaOH and the stability of the solution against gelation can be enhanced with certain additives, such as a small amount of zinc oxide (Kihlman et al.
2013; Liu et al.
2011; Yang et al.
2011b). Despite active research, the role of ZnO has remained to be an open question. ZnO forms zincate (Zn(OH)
4
2−
) in strongly alkaline solution, and it has been suggested that this species enhances the dissolution of cellulose by forming even stronger hydrogen bonds with it (Yang et al.
2011b). On the other hand it has also been suggested that Zn(OH)
4
2−
enhances cellulose dissolution by associating to it and thus further charging up the molecules in the alkaline conditions (Kihlman et al.
2013). Moreover, Liu et al. (
2011) have suggested that ZnO acts as water “binder” stabilizing cellulose solutions in aqueous NaOH.
The work presented here contributes to the understanding of cellulose dissolution in aqueous NaOH–ZnO and elaborates on the role of ZnO. Moreover, the reactivity of cellulose dissolved in the NaOH–ZnO system is discussed. Two samples, microcrystalline cellulose consisting of pure, low molecular weight cellulose and never-dried, bleached softwood pulp containing cellulose of high molecular weight were dissolved in a solvent system containing 9 w-% NaOH and 1 w-% ZnO. The structure of the samples was studied with Raman spectroscopy and the reactivities were assessed based on oxidation with 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxo-piperidinium cation (4-AcNH-TEMPO
+) as a probe reaction with the method published by Khanjani et al. (
2017). 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) -catalyzed oxidation in alkaline pH has been widely studied in the production of oxidized cellulose nanofibrils because of the high reaction rate, yield and selectivity (Isogai et al.
2011; Okita et al.
2010; Saito et al.
2007,
2009; Salminen et al.
2017). The actual oxidizing agent in these reactions is the TEMPO
+-cation (Isogai et al.
2011). It is able to rapidly and selectively convert the hydroxymethyl groups of cellulose into carboxylate groups in mild conditions (pH 9, room temperature) (De Nooy et al.
1994). In addition, the reaction can be followed and quantified with iodometry (Bichsel and von Gunten
1999; Pääkkönen et al.
2015; Khanjani et al.
2017). Thus, oxidation with 4-AcNH-TEMPO
+ provides a means to study cellulose reactivity quantitatively. Raman spectroscopy was chosen for the structural studies as it enables analysis of chemical bonds at the molecular level. Moreover, it is non-destructive and enables analysis of samples in their native state since minimal sample preparation is required and also wet samples can be analyzed (Agarwal
2014). Raman spectroscopy is especially useful in studying cellulosic samples as, in general, the materials are inherently not fluorescent and thus the obtained Raman spectra have good signal-to-noise ratios (Agarwal
2019). In the current work, oxidation with 4-AcNH-TEMPO
+ as a means to study the reactivity of cellulose in the dissolved state is reported for the first time, and, based on the reactivity studies as well as the Raman spectroscopic analyses, a new structure for cellulose dissolved in aqueous NaOH–ZnO is proposed.
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