# Poly(\(\varepsilon\)-caprolactone)-Ionic Liquid Composite as Piezoionic Mechanical Sensor
## 3 Result and Discussion  
The aim of our research was to develop a device capable of detecting vibrations by harnessing the piezoelectric effect generated through the creation of a composite PCL/ionic liquid. The produced three-layer device was composed of a PCL electrospun scaffold soaked in EMIMBF\({}_{4}\) and placed between two silver-based electrodes (Figure 2). We focused our research on a widely used biodegradable polymer such as PCL. Concerning the choice of the IL, we are aware that there are ILs that are more environmentally friendly than EMIMBF\({}_{4}\). But, we have chosen EMIMBF\({}_{4}\) in this preliminary phase to put ourselves in conditions where the ionic liquid ensures high ionic conductivity. Our recent studies on a bacterial cellulose/IL composite [43] have demonstrated that, compared with other imidazolium-based ionic liquids or choline-derived ones, EMIMBF\({}_{4}\) provides a better performance. This is attributed to the fact that, as reported in the literature, its specific conductivity values are among the highest, reaching 13 mS/cm [44]. The weight percentage abundance of each component, calculated by weighing the samples after each stage of their preparation, was found to be 39% PCL by weight, 58.5% EMIMBF\({}_{4}\) by weight, and 2.5% conductive varnish by weight. As is known, electrospinning results depend on many different factors, which, among others, can usually be divided into solution properties such as polymer concentration, molecular weight, viscosity, surface tension, solution vapor pressure, electrical conductivity, and dielectric constant. They can also be divided into process parameters like flow rate, spinning voltage, configuration of electrodes, and distance between the electrodes, as well as environmental parameters such as temperature and humidity. These factors affect the initiation and stability of the electrospinning process and, ultimately, the morphology and properties of the fibers. In our experiment the combination of the used solvent mixture (CHCl\({}_{3}\)/DMF), the distance between the syringe tip and the grounded drum collector, and the flow rate of 3 mL/h produced a scaffold where fibers in the presence of flat polymer ribbons are present (SEM micrograph of Figure 3 on the left). The soaking of the membrane with EMIMBF\({}_{4}\) resulted in a modification of membrane morphology, as shown through the SEM micrograph (Figure 3 on the right) in which the IL, filling the pores of the PCL scaffold, is present as a continuous phase that wets the fibers and ribbons. To determine the actual presence of EMIMBF\({}_{4}\) on the sample surface, we analyzed the composition of the soaked PCL surface through X-ray photoelectron spectroscopy (XPS). The wide scan shown in Figure 4 evidenced that carbon, oxygen, boron, nitrogen, and fluorine are present on the sample surface. The peaks centered at about 187 (B\({}_{18}\)), 400 (N\({}_{18}\)), and 685 eV (F\({}_{18}\)), which derive from emerging on the surface of the IL. Figure 2: Structure and photograph of the device. The ATR FTIR registered in the range 4000-650 cm\({}^{-1}\) (Figure 5) showed that the device presents the IR characteristic peak of PCL and EMIMBF\({}_{4}\) on its surface. The broad peak centered at about 2945 cm\({}^{-1}\) due to the alkyl chains, and the peak centered at about 1721 cm\({}^{-1}\) due to the stretching of carbonyl group of the ester function, derive from the presence of PCL. Regarding EMIMBF\({}_{4}\) presence, the bands at 3165 and 3124 cm\({}^{-1}\) are assigned to the C-H of imidazole ring, while the bands at 1576 and 1457 cm\({}^{-1}\) are due to the imidazole ring skeleton's stretching vibration, and the broad peak at 1017 cm\({}^{-1}\) is due to the C-H of the imidazole ring stretching vibration. TGA of the PCL membrane confirmed that electrospinningning did not produce any significative degradation of the polymer structure.