Effect of Stretching on Crystalline Structure, Ferroelectric and Piezoelectric Properties of Solution-Cast Nylon-11 Films

Compared to polyvinylidene fluoride (PVDF) and its copolymers, castor-oil-derived nylon-11 has been less explored over the past decades, despite its excellent piezoelectric properties at elevated temperatures. To utilize nylon-11 for future sensor or vibrational energy harvesting devices, it is important to control the formation of the electroactive δ′ crystal phase. In this work, nylon-11 films were first fabricated by solution-casting and were then uniaxially stretched at different stretching ratios (SR) and temperatures (Ts) to obtain a series of stretched films. The combination of two-dimensional wide-angle X-ray diffraction (2D-WAXD) and differential scanning calorimetry (DSC) techniques showed that the fraction of the δ′ crystal phase increased with the stretching ratio and acquired a maximum at a Ts of 80 °C. Further, it was found that the ferroelectric and piezoelectric properties of the fabricated nylon-11 films could be correlated well with their crystalline structure. Consequently, the stretched nylon-11 film stretched at an SR of 300% and a Ts of 80 °C showed maximum remanent polarization and a remarkable piezoelectric coefficient of 7.2 pC/N. A simple piezoelectric device with such a nylon-11 film was made into a simple piezoelectric device, which could generate an output voltage of 1.5 V and a current of 11 nA, respectively.