Abstract
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.
Highlights
Piezoelectric materials having non-centrosymmetric point groups can develop net deformation from their equilibrium positions with the application of an electric field or mechanical stress, allowing for the conversion of a mechanical stress into an electrical charge or an electrical field into a mechanical strain, known as the “direct” effect and the “converse” effect, respectively [1]
A castor-oil-derived nylon-11 has been less explored over the past decades, it possesses the advantage δ’ of despite its excellent piezoelectric properties at elevated temperatures compared to polyvinylidene fluoride (PVDF) and its copolymers [15]
Nylon-11 is crystallized from the molten state into the triclinic α- or α0 form, which does not exhibit piezoelectricity due to the random polarity of the crystalline domains that cannot be rearranged by electric poling because of the strong interchain hydrogen bonding [19,20]
Summary
Piezoelectric materials having non-centrosymmetric point groups can develop net deformation from their equilibrium positions with the application of an electric field or mechanical stress, allowing for the conversion of a mechanical stress into an electrical charge or an electrical field into a mechanical strain, known as the “direct” effect and the “converse” effect, respectively [1]. Since the discovery of piezoelectricity in polyvinylidene fluoride (PVDF) by Kawai in 1969, the piezoelectric effect has been found in other polymers, including PVDF copolymers, odd nylons, cellulose and poly (lactic acid) [9,10,11,12]. They have a relatively lower piezoelectric coefficient than their ceramic counterparts, piezoelectric polymers typically provide flexibility and processability, allowing integration into small, lightweight devices for applications such as human body movement monitoring and human-based or low-frequency ambient energy harvesting [13,14]. Researches on understanding of the crystalline structure of solution-cast nylon-11 films are still quite limited, and especially its the crystalline structural evolution upon post-treatment like uniaxial stretching has been less focused on
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