# Poly(\(\varepsilon\)-caprolactone)-Ionic Liquid Composite as Piezoionic Mechanical Sensor
## 3 Result and Discussion
In fact, as shown in Figure 6a where the temperature dependence of the PCL's weight loss is plotted, the DTG curve displays one main degradation process with a maximum weight loss rate observed at 425 \({}^{\circ}\)C and a shoulder at a lower temperature at ca. 335 \({}^{\circ}\)C, which are in agreement with what was reported in the literature [45]. These peaks in the DTG curve are the result of two consecutive steps in the degradation process, with the first one being due to the production of H\({}_{2}\)O, CO\({}_{2}\), and 5-hexanoic acid, and the second one being due to there being \(\epsilon\)-caprolactone (cyclic monomer) evolution in addition to the preceding compounds. Comparing the TGA and DTG curves of the PCL membrane (Figure 6a), EMIMBF\({}_{4}\) (Figure 6b), and the PCL membrane soaked with EMIMBF\({}_{4}\) (Figure 6c), it can be noted that the interaction of the IL with PCL determines a change in the maximum weight loss rate temperature of both PCL and the IL. Figure 4: Wide scan **of the** XPS spectrum of the surface of the PCL/EMIMBF\({}_{4}\) sample. Figure 3: SEM micrographs showing the morphology of the membrane as electrospun (**left**) and soaked with the EMIMBF\({}_{4}\) IL (**right**). Figure 5: ATR FTIR spectra of PCL (black), EMIMBF\({}_{4}\) (green), PCL/EMIMBF\({}_{4}\) (red), and PCL/EMIMBF\({}_{4}\)/Ag (yellow) samples. Figure 6: TGA (continuous line) and DTG (dashed line) plots of the PCL membrane as electrospun (**a**), EMIMBF\({}_{4}\) (**b**), the PCL membrane soaked with the EMIMBF\({}_{4}\) (**c**), and the soaked membrane coated with silver-based electrodes (**d**), EMIMBF\({}_{4}\) in static air with a ramp of 10 \({}^{\circ}\)C min\({}^{-1}\) (**e**), and the PCL membrane soaked with the EMIMBF\({}_{4}\) in static air with a ramp of 10 \({}^{\circ}\)C min\({}^{-1}\) (**f**). In fact, in the case of PCL, there are still two main decomposition steps, but their maximum weight loss rates occur at 357 \({}^{\circ}\)C and 415 \({}^{\circ}\)C, respectively, while the maximum weight loss rate for EMIMBF\({}_{4}\) is at 497 \({}^{\circ}\)C instead of at 507 \({}^{\circ}\)C. The coating of the PCL/EMIMBF\({}_{4}\) composite through the deposition of the silver-based electrodes determines a modification of the DTG curve profile (Figure 6d) that is the result of the superposition of different contributions. There are the ones deriving from PCL degradation at lower temperatures and the one deriving from the maximum weight loss rate of EMIMBF\({}_{4}\) at higher temperatures, and between them a peak due to the maximum weight loss rate of the matrix in the silver varnish. TGA can also provide an indication of the IL's stability under the specific conditions in which the device will be used. Our device is designed for use in ambient air and, accordingly, we conducted a TGA experiment in static air with a ramp of 10 \({}^{\circ}\)C min\({}^{-1}\). Upon comparing the resulting TGA curve with the preceding ones, as illustrated in Figure 6e, it becomes evident that the IL's weight loss occurs in a single step. However, the maximum weight loss rate is observed at a lower temperature (460 \({}^{\circ}\)C) compared with the previous condition (507 \({}^{\circ}\)C). When examining the PCL/EMIMBF\({}_{4}\), the resulting DTG curve continues to display two main peaks in weight loss rates, which are centered at 400 \({}^{\circ}\)C and 472 \({}^{\circ}\)C, respectively. Notably, these peaks occur at lower temperatures compared with the observations made within a nitrogen atmosphere (at 415 \({}^{\circ}\)C and 497 \({}^{\circ}\)C, respectively). Moreover, from the observation of the thermogravimetric analysis curves, it can be deduced that the ionic liquid used contains a negligible percentage of water, which is approximately 0.04% by weight. On the other hand, the final device contains a water content of about 3% by weight. The obtained mechanical sensor is forecast to work in the low frequency domain.