# Poly(\(\varepsilon\)-caprolactone)-Ionic Liquid Composite as Piezoionic Mechanical Sensor
## 3 Result and Discussion
For this reason, we intended to study the viscoelastic properties of the devices in the range of 0.1-50 Hz at the working temperature of 25 \({}^{\circ}\)C. Measurements of the storage modulus (E\({}^{\prime}\)) and the loss modulus (E\({}^{\prime\prime}\)) were obtained and compared to determine the evolution of the mechanical properties from the crude PCL membrane to the final device. Obtained results at frequencies of 1, 10, and 50 Hz are reported in Table 1.  
From the data reported in Table 1, it can be observed that the values of E\({}^{\prime}\) and E\({}^{\prime\prime}\) remain almost constant when transitioning from PCL to the PCL/EMIMBF\({}_{4}\) composite. However, due to the stiffness of the electrodes, in the case of the final device both the values of E\({}^{\prime}\) and E\({}^{\prime\prime}\) show an increase of two orders in magnitude. Furthermore, with the increase in frequency, as expected, the values of E\({}^{\prime}\) and E\({}^{\prime\prime}\) tend to slightly increase in a linear trend, thereby indicating the absence of peaks related to the activation of dissipative phenomena. The comprehensive setup, described in the characterization section, enabled a thorough examination of the piezionic PCL sensor's response to mechanical vibrations, yielding valuable insights into its mechanoelectrical transduction behavior. The study focused on analyzing signals in the time domain, as shown in Figure 7. Specifically, it concentrated on the sensor's output voltage under a fixed RMS vibration level \begin{table}
\begin{tabular}{c c c c c} \hline  & & **PCL** & **PCL/EMIMBF\({}_{4}\)** & **PCL/EMIMBF\({}_{4}\)/Ag** \\ \hline E\({}^{\prime}\) (Pa) & 1 Hz & \(9.8\times 10^{5}\) & \(1.2\times 10^{6}\) & \(6.6\times 10^{8}\) \\  & 10 Hz & \(1.1\times 10^{6}\) & \(1.4\times 10^{6}\) & \(8.1\times 10^{8}\) \\  & 50 Hz & \(1.6\times 10^{6}\) & \(1.5\times 10^{6}\) & \(9.8\times 10^{8}\) \\ \hline E\({}^{\prime\prime}\) (Pa) & 1 Hz & \(8.8\times 10^{4}\) & \(1.3\times 10^{5}\) & \(8.7\times 10^{7}\) \\  & 10 Hz & \(8.9\times 10^{4}\) & \(1.9\times 10^{5}\) & \(1.3\times 10^{8}\) \\  & 50 Hz & \(3.3\times 10^{5}\) & \(3.3\times 10^{5}\) & \(1.6\times 10^{8}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Measurements of the storage modulus (E\({}^{\prime}\)) and the loss modulus (E\({}^{\prime\prime}\)) of the PCL membrane, PCL/EMIMBF\({}_{4}\), and PCL/EMIMBF\({}_{4}\)/Ag. of approximately 1.4 mm RMS. The frequency varied from 5 Hz to 9 Hz in the sinusoidal kinetic source induced by the shaker. The graph depicted the PCL's output voltage alongside a sinusoidal waveform generated using MATLAB(r)R2019a optimization. This process employed a nonlinear programming solver to minimize a quadratic deviation problem, executed through the fminsearch function. The waveforms illustrated significant attenuation of the output signal outside the resonance conditions. The mechanical resonant frequency, evident at around 7 Hz as in Figure 8, illustrates the sensor's frequency response. A sinusoidal excitation ranging from 5 to 9 Hz was applied using the shaker. The mean output voltage and standard deviation at each frequency were measured using the aforementioned digital oscilloscope. Figure 7: The evolution of the sensor’s output voltage, maintaining a constant RMS vibration level of approximately 1.4 mm while changing the frequency, which is detailed below, at the values as follows: (**a**) at 5 Hz, (**b**) at 6 Hz, (**c**) at 7 Hz, (**d**) at 8 Hz, (**e**) at 9 Hz. Each graph showcases the approximation using a sinusoidal waveform obtained through the MATLAB® function of fminsearch. In particular, the graph shows the mean value and standard deviation of the measurements of the output voltage. Ten repetitions were conducted, and a time window of 5 s was considered for each repetition. Notably, a prominent peak aligned with the mechanical resonant frequency, which closely resembled the natural frequency of the device. For clarity, the dots have been connected for the ease of the eye.