## 3 Results and Discussion
### Film-Forming Process of Electrospun Pectin-Based Fibers
In Figure 4c it can be observed that some transparent regions along the pectin-based materials were successfully attained when the mats were subjected to temperatures above 165 \({}^{\circ}\)C with an applied pressure of 12-24 kN for 1 min. Nevertheless, the resultant materials also exhibited an intense brownish color, partially lost their integrity and also became too brittle to be applied as packaging materials. The above-reported morphological change was further confirmed in Figure 5 by comparison of the SEM images of the cross-sections of the electrospun pectin-based mats from S3 prior to annealing, shown in Figure 5a, and post-processed at 220 \({}^{\circ}\)C for 20 s, shown in Figure 5b. This phenomenon can be explained by the oxidation and degradation of pectin compounds at high temperatures. Nevertheless, interestingly, it can be observed that a compact packing rearrangement of the electrospun fibers was observed by a process of fibers coalescence. This phenomenon successfully resulted in the formation of a continuous film. However, some voids were also formed that could result from the evaporation of gases during thermal decomposition. Figure 4: Effect of the post-treatment conditions on the pectin-based materials obtained from S3. (**A**) Different temperatures were applied for 1 min without pressure. (**B**) Different times were applied at 190 \({}^{\circ}\)C without pressure. (**C**) Different pressures were applied at 165 \({}^{\circ}\)C for 60 s.  
Based on the results shown above, the effect on the film-forming process of the two plasticizers, that is, glycerol and PEG\({}_{900}\), was analyzed. The SEM images of the pectin-based films obtained from the fibers produced with the solutions containing the plasticizers are gathered in Figure 6. The electrospinningning of the PEO\({}_{2000}\)-containing pectin solutions with 20-30 wt% glycerol, that is, S5, S6, and S7, resulted in electrospun mats that, after annealing at 150 \({}^{\circ}\)C, produced softer and more flexible films. A similar improvement was attained for the electrospun PEO\({}_{2000}\)-containing pectin mats with 20-30 wt% PEG\({}_{900}\), labelled as S8, S9, and S10, after annealing at 155 \({}^{\circ}\)C, though the films were less homogenous. As mentioned earlier, the role of plasticizers in pectin is based on increasing the chain mobility and free volume by creating H-bond interactions with the biopolymer chains [21]. The use of plasticizers thus successfully opened up the post-processability of the electrospun mats by annealing. However, temperatures around 160 \({}^{\circ}\)C were still needed, which are certainly close to the T\({}_{\text{deg}}\) values previously measured for pectin and are also responsible for darkening the film samples. Also, for all the formulations, the annealed mats still comprised some porosity, as one can observe in the SEM images gathered in the left column of Figure 6.  
To increase the homogeneity of the resultant pectin materials, the electrospun pectin-based fibers were washed with dichloromethane prior to annealing. As seen in the SEM images shown in the middle column of previous Figure 6, the washed fibers partially coalesced, which could potentially enable to reduce the energy requirement for annealing. As mentioned earlier, dichloromethane is a solvent that does not dissolve pectin, but it dissolves glycerol whereas PEG\({}_{900}\) and PEO\({}_{2000}\) are slightly soluble. When the pectin fiber mats containing glycerol were immersed in dichloromethane, they did not lose their integrity but, due to the removal of glycerol, the electrospun fibers partially coalesced. Thereafter, annealing of the washed fibers was successful when applied at 150 \({}^{\circ}\)C and 140 \({}^{\circ}\)C for the fibers obtained from the solutions containing glycerol, that is, S5, S6, and S7, and PEG\({}_{900}\), that is, S8, S9, and S10, respectively.