Dissolution of cellulose in TBAA/DMSO
The dissolution of cellulose in the binary solvent of TBAA/DMSO with a molar ratio of 10/1 was conducted at 50oC. The solubility and dissolution rate were determined and the results are shown in Figs. 1 and 2.
They indicate that a large amount of cellulose can be dissolved in the binary solvent of TBAA/DMSO regardless to the crystalline degree, shape or size of cellulose. The solubility and dissolution rate increases with a decrease in the degree of polymerization (DP) of cellulose. In the case of microcrystalline cellulose, the cellulose solution became optically unclear if the cellulose concentration was higher than the solubility shown in Fig. 1. However, in the case of NBKP or absorbent cotton, the solubility was limited not due to the optical opacity but the excessive viscosity which makes it impossible to stir.
The effect of temperature on the dissolution time was evaluated and the results are shown in Fig. 3. Even cellulose could be dissolved at room temperature, the dissolution rate increased considerably with increasing temperature up to 50oC, and then dissolution rate becomes less dependent on temperature. It is suggested that an increase in temperatures do not only promote the solvent diffusivity or accessibility but also affect the solvation of TBAA.
Effect of DP or temperature on viscosity of cellulose solution are investigated and the results are shown in Figs. 4 and 5. It can be seen that viscosity increases with an increase in DP or a decrease in temperature, revealing an opposite tendencies to those of solubility and dissolution rate. This result is suggested to be attributed to the fact that an increase in viscosity could reduce the diffusion and in turn hinder the cellulose dissolution.
The above results indicates that the dissolution of cellulose in the binary solvent seems to be a diffusion-determining process. Therefore, any factor increasing the viscosity can extend the dissolution time, such as, the concentration and DP of cellulose, as well as temperature etc.
Dissolution of NBKP in various binary solvents
In order to understand what roles the polar aprotic solvent and TBAA play in dissolution, the dissolution of cellulose in various binary solvents containing 15wt% TBAA was conducted and the results are shown in Table 1.
It can be seen that a large amount of cellulose could be dissolved in all the polar aprotic solvents (PAS) in Group A which are large in both donor number (DN) and accepter number (AN), and slightly dissolved in the polar aprotic solvents in Group B which DN are large but AN are small. However, cellulose could not be dissolved but swelled in the polar aprotic solvents in Group C which have are low in both DN and AN. It is interesting to find that cellulose could neither be dissolved nor swelled in all the protic solvents such as water and alcohol, etc. The results suggest that solubility be affected not only the donor ability but also the acceptor ability of PAS.
In order to verify the cellulose solubility in various binary solvents containing the PAS in Group A and Group B, the relationship between the molar ratio (MR) of PAS/TBAA and solubility was investigated and result is shown in Fig. 6.
It demonstrates that solubility increase with an increase in MR of binary solvents to a maximum value and then turn to decrease regardless the species of PAS. The MR for obtaining the max solubility is expressed as the optimal molar ratio (OMR) and was plotted with DN as shown in Fig. 7. It reveals an increasing tendency with an increase in DN. One except is only pyridine, being due probably to its basicity which may diminish the solvation of TBAA.
On the other hand, the max solubility did not simply increase with an increase in the DN value, suggesting that there exist other factors affecting the solubility.
Solvent retention value (SRV)
Usually, the dissolution of a polymer into a solvent involves two transport processes, namely solvent diffusion and chain disentanglement. SRV could provide a measure of solvent diffusivity or accessibility for the cellulose, which in turn enables prediction of the contribution of polar aprotic solvents to dissolution of cellulose.
The SRV of NBKP sheet in various PAS was measured and the results are shown in Table 3 and Fig. 8.
It is seen that SRV of DMSO increased to 500% within 15 minutes, approximately five times of other PAS in Group A and ten times of those in Group B and C. It means that PAS of large DN and AN has a high SRV. Furthermore, by plotting SRV with maximum solubility, it is interesting to see that the solubility has a good relation with SRV, as shown in Fig. 9, revealing an increasing tendency with an increase in SRV.
A proposal for the dissolution mechanism
The results mentioned above indicate that solubility is affected by SRV and the optimal molar ratio (OMR) is determined by donor number. In fact, both of SRV and OMR decisively affect the valid amount of TBAA that really access to cellulose. The higher SRV or the lower OMR, the larger the valid amount of TBAA.
It is known that the dissolution of cellulose is quite different from the common homopolymer because it consists of a huge amount of intermolecular and intramolecular H-bonds. A good solvent for cellulose involves not only the solvent diffusion, but also the capability of disrupting the H-bonds.
Cellulose could not dissolve in DMSO, even though it has an exceedingly high diffusivity. Therefore, TBAA is suggested to be the determinant factor of H-bond disruption, and a comprehensive understanding of TBAA solvation phenomena in the binary solvent system is essential for investigating the disruption of H-bonds of cellulose.
Solvation or ionization is mostly commonly found in electrolyte. There are many experimental and computational studies (Cox et al 1974; Mayer 1975; Sebastien 2015; Izutsu 1980) have been reported for the solvation of alkali cations in various organic solvents especially the polar aprotic solvents.
It have been indicated that solvent with stronger donor ability has higher solvation ability. That is, the solvation ability increases with an increase in DN. It was also reported that tetraalkylammonium cation (R4N+) is stable in polar aprotic solvents but unstable in water.
Based on the reports, it was suggested that in those binary solvents that contain PAS of high DN, TBAA could be solvated or ionized to cation TBA+ and anion Ac−. Anion Ac− could form strong interactions with hydroxyl groups of cellulose, leading to the disruption of H-bonds between inter or intra cellulose chains. In order to verify the suggestion, pH of various TBAA solution was measured regarding with the effect of DN. In this experiment, the TBAA concentration was fixed to 30wt%, and the mass ratio of PAS/water is faxed to 3/1 to facilitate the pH testing. The results are shown in Fig. 10.
It can be seen that pH of TBAA solution in water and methanol are respectively 6.5 and 7.5, being close to neutral. However, it is interesting to see that pH value increase remarkably after adding a polar organic solvents and showing an increasing tendency with an increase in DN. The results indicate that in the polar aprotic solvent, TBAA could be solvated into TBA+ and Ac− and the solvation ability increase with an increase in DN.
Based on these results, it is proposed that dissolution of cellulose in the binary solvent of PAS/TBAA involves three processes, namely solvent diffusion, solvation of TBA + to give rise anions Ac−, and disruption of H-bonds. The higher the solvation ability, the easier the disruption of H-bonds. DMSO is the optimal solvent for cellulose dissolution, being attributed to its high diffusivity and strong solvation of TBA+.