Direct electrospinning of cellulose in the DBU-CO₂ switchable solvent system

Microcrystalline cellulose (MCC) was bought from Sigma-Aldrich. Cellulose pulp was provided from Rayonier Advanced Materials Company (Tartas Biorefinery, France) and was produced by ammonium sulphite cooking and bleached with an elementary chlorine free (ECF) process (purity in alpha-cellulose is 94 %). All cellulose samples were dried at 100 °C for 24 h under vacuum to remove free water before use. Dimethyl sulfoxide (DMSO) 99 % was purchased from VWR. Diazabicyclo [5.4.0] undec-7-ene (DBU, > 98 %) was bought from TCI and used without further purification. PVA 98–99 % hydrolyzed (molecular weight: 31,000–50,000 g/mol) was obtained from Sigma-Aldrich, while Polyethyleneoxide, PEO (MW 900,000 g/mol) was bought from Alfa Aesar.

Optical microscopy was used for the preliminary experiments, which is brought in supporting information part (S1 and S2). SEM analysis was done to further characterize the morphology of the particles. Preliminary investigations have been done using MCC. In the first set of experiments, the cellulose concentration was varied between 1 and 10 wt%. For all these concentrations, only particles with sizes ranging between 600 nm and 1.30 μm were obtained. The variation of the various electrospinning parameters (voltage, feed rate, electrospinning distance) did not make much difference. However, as expected, larger particle sizes were obtained at higher cellulose concentrations. It is important to point out that at 5 wt%, the electrospraying process was more stable, resulting in homogenous round-shaped particles with mean diameter about 1.2 μm (see Fig. 1). The obtained particles are smaller when compared to the report from Kyung et al. (2013) where slightly larger particles with mean diameter between 5 and 10 μm were obtained from electrospraying using ionic liquids. Whereas the initial objective of this work was to prepare cellulose fibers, such homogenous and well defined cellulose microparticles could also find applications in food industry, electronic and pharmaceuticals as reported by Chin et al. (2018).

Considering the previous reports on electrospinning that have shown increasing the viscosity of a solution is one way to transition from a particle to a fiber, (Agarwal 2016) we investigated higher concentrations of MCC. However, above 10 wt% MCC, the solution became much more unstable after solubilization, thus making it impossible to be used for electrospinning. As reported by Greiner et al. (2010) the use of additives such as PVA or PEO have been shown to improve the spinnability of a solution as well as to remove particles during an electrospinning process. These polymers additives can actually be removed after the fibers have been formed as seen from the works of Frenot and co-workers (Frenot et al. 2007). Considering the stability of the process at 5 wt% MCC, this concentration was employed for the trials with the both PVA and PEO as additive polymers.

For the investigation using PVA as an additive, the required PVA concentration was prepared in DMSO, to allow easier miscibility when added to the solubilized cellulose in the DBU/CO₂/DMSO system. Figure 2 shows the SEM image of the obtained beaded nanofibers. In these experiments, 5 wt% MCC solution was prepared as described previously (Onwukamike et al. 2017). Next, the PVA solution was added and allowed to become homogenous under mild stirring. Extra care was required at this stage to avoid the escape of CO₂ which will otherwise led to precipitation. Various ratios of MCC and PVA were investigated as shown in Table 1 and the corresponding optical images are shown in the Supporting Information (S1). Observations in supporting information are based on the results from optical microscopy. In the first sets of experiments, 5 wt% PVA was used, as a reference, electrospraying of neat 5 wt% PVA resulted only to formation of participles. When this concentration of PVA was mixed with 5 wt% MCC at 50/50 ratio, only particles were obtained. In like manner, only particles were obtained when the ratio was changed to 45:55 (MCC:PVA). In order to increase the chance of obtaining fibers, the next sets of experiments (1f and 1 g) were carried out using 10 wt% PVA. The ratio between 5 wt% MCC and 10 wt% PVA was kept at 50/50, resulting in a combination of particles and fibers. To our delight, when this ratio was slightly modified to 45 % MCC and 55 % PVA, a very stable electrospinning process was observed resulting to beaded fibers (See Fig. 2). The morphology of the obtained fibers was studied using SEM and showed well distributed and homogenous beaded fibers with mean diameter below 100 nm. Unlike what is commonly seen in the literature,(Jacobs et al. 2010; Liu et al. 2008) the beaded fibers obtained in this case appear to be continuous and evenly distributed along the fibers as shown in Fig. 2. It is noteworthy to mention that changing the electrospinning conditions did not affect the formation of this specific and uncommon structure.

Raman spectroscopy was used in order to further investigate, if the beads are comprised of MCC, PVA or both of them. A first hypothesis to explain this phenomenon was related to a potential phase separation between the MCC and PVA leading to the formation of such beads. If this was true, we would expect to observe either MCC or PVA enriched in these beads. Further investigations using Raman spectroscopy showed that in the mixture, the peak at 1440 cm ^-1 is attributed to symmetric bending mode of the CH₂ group of PVA and the one at 1380 cm^-1 is ascribed to O–H bending for MCC, which ascertains the presence of both MCC and PVA within the beads as seen in Fig. 3. Also, worthy to consider at this stage is the potential influence of CO₂, which is trapped within the solvent system. Zuo and colleagues have investigated on the origin of beads during electrospinning of poly(hydroxybutyrate-co-valerate) (Zuo et al. 2005). The outcome of their research showed that beads formation resulted from the instability of the electrospinning solution (axisymmetric instability) and was influenced by the viscosity, feed rate, applied voltage and surface tension. While their work explained the general mechanism of bead-in-fiber formation during electrospinning, it doesn´t explain the observed consistent and unique beads obtained in this study. So far, no previous report in the literature reported on a similar phenomenon.

Building on our observations using 10 wt% PVA, we decided to investigate higher PVA concentration (15 and 20 %). The motive here was to leverage on their higher viscosity at lower addition ratio in order to achieve fibers at higher MCC ratio. Upon using 15 wt% PVA, only particles were obtained when the ratio with MCC was kept at 90:10. Increasing the PVA ratio to 23 % (the highest we could achieve to reach a homogenous solution), a mixture of fibers with particles was obtained. However, the electrospinning process was very unstable leading to the formation of large droplets during process. When 20 wt% PVA was employed, it was possible to achieve a somewhat stable electrospinning process at the higher MCC ratio of 60 %. Notwithstanding, the most optimized electrospinning process was rather achieved when using 10 wt% PVA at 45:55 ratio between MCC and PVA (See supporting information S1 for optical microscopy images).

In addition to the use of PVA as an additive to improve the electrospinning process, we have equally employed the use of high molecular weight polyethyleneoxide (PEO). The various conditions investigated are summarized in Table 2. In order to have a reference, an electrospinning of neat 2 wt% PEO was carried out and resulted in a mixture of fibers and beads (See supporting information S2). In the subsequent experiments, the ratio of 5 wt% MCC was increased from 45 to 90 %. It was interesting to note that, irrespective of the incorporated MCC ratio, beaded fibers were obtained similar to those obtained when 10 wt% PVA was used as an additive at 45 % MCC incorporation (See supporting information S2 for optical microscopy images). These results are interesting in that we could achieve a high MCC loading ratio, while still maintaining a stable electrospinning process. One possible reason for still achieving such beaded fibers at relatively low PEO ratio could be attributed to the very high molecular weight of this polymer employed (9.10⁵ g/mol). In an attempt to further increase the viscosity of these systems in order to eliminate these beads, a higher weight concentration solution of PEO was prepared (4.6 wt%). With this much higher viscous solution, we could only achieve a stable electrospinning dope at a maximum MCC loading ratio of 45 % which still resulted to beaded-fibers (See Supporting information S2, 2e).

Considering that the molecular weight of a polymer is a key factor in the viscosity of the electrospinning dope and following the promising results obtained using MCC, we decided to employ a higher molecular weight cellulose type, i.e. CP. The assumption here was that by using CP, we could perhaps completely eliminate the beads in the beaded fibers obtained in the previous experiments with MCC, and that could not be eliminated by use of additives. Cellulose pulp (CP) was solubilized in the DBU-CO₂ solvent system in the presence of DMSO as previously described (Onwukamike et al. 2017). In the first sets of experiments, the concentration of CP was varied between 1 and 5 wt%. These experiments and resulting observations are summarized in Table 3.

From the performed experiments, mostly particles were obtained with few fibers when 1 wt% CP was used (See Support Information S3, 3a). When the CP concentration was increased to 2 wt% (3b), a mixture of particles and standing fibers were obtained (Fig. 4). These observations of standing fibers have been previously reported by Frenot et al. from a direct electrospinning of cellulose from a DMAc-LiCl solution (Frenot et al. 2007). The authors attributed this observation to the high conductivity (high charges present) of this electrospinning solution, resulting to a stronger electrostatic attraction towards the opposite charged collector plate. Therefore, it was not surprising for us considering that we are employing an equally ionically charged solvent system [Cellulose-OCO₂]^-[HDBU]⁺ (Onwukamike et al. 2017). Similar standing fibers, but with fewer beads, were observed when the CP concentration was increased from 2 to 3 wt%. As the cellulose concentration was increased to 4 %, only standing fibers were obtained as shown in Table 3.

The morphology of the obtained fibers was studied using SEM and the result is shown in Fig. 5. Increasing the CP concentration to 5 wt% still resulted in standing fibers with a mean diameter of 560 ± 30 nm.

As described above, the electrospinning of neat CP from the DBU-CO₂ solvent system resulted to standing fibers. The challenge this poses is that fibers easily collapse into much larger fibers. In addition, the non-uniform fibers resulting from this process are not interesting from an application point of view. As shown in the work on Frenot et al. (Frenot et al. 2007), where they obtained similar standing fibers, the phenomenon was attributed to the highly ionic solvent employed. Therefore, hypothesized that addition of a polymer additive might improve the process. This hypothesis was confirmed to be true when we obtained only beaded fibers when PVA as an additive was included (3 wt% CP and 5 wt% PVA at ratio of 56:44, see Expt. 3 g, optical image included in Supporting Information S3). The addition of PVA led to the disappearance of standing fibers and a very stable electrospinning process was obtained. Increasing the CP ratio to 67 % (experiment 3 h) still resulted to beaded fibers. SEM showed that the morphology of the obtained beaded fibers was very similar to those obtained when PVA was also used as an additive in MCC electrospinning (see Fig. 2). Similar beaded fibers were equally obtained when a higher weight concentration of CP (5 wt%) was employed. The SEM of beaded fibers obtained at 5 wt% CP at 67:33 ratio with 5 wt% PVA is shown in Fig. 6. This result further strengthens our earlier hypothesis that the observed unique beaded morphology might be strongly related to the interaction between the cellulose and polymer additive.