Voltage response of three ionic polymer pressure sensors based on ion migration at different ambient humidities

Besides bending sensor, ionic polymer materials can work as pressure sensor based on ion migration and show more potential in engineering application. To explore a better ionic polymer material for pressure sensor, three typical ionic polymer materials, Nafion, poly(vinylidene fluoride)/ionic liquid (IL/PVDF) composite, thermoplastic polyurethane/ionic liquid (IL/TPU) composite, were selected to comparatively investigate the sensing properties under an applied pressure. Ionic polymer pressure sensors with circular truncated cone structure were developed by casting method. Their voltage responses were evaluated by a self-made test platform under a series of step force. The voltage sensitivity and the stability were obtained by performing the tests at different ambient humidity levels. The sensitivity of Nafion sensor was influenced by the ambient humidity heavily, which increased from 8.97 mV N-1 (28.17 mV MPa-1) to 31.11 mV N-1 (97.69 mV MPa-1) as the relative humidity (RH) increasing from 30% RH to 90% RH. In comparison, the voltage responses of IL/TPU sensor and IL/PVDF sensor were rarely influenced by the humidity because of the stability of IL, but strongly related to the content of IL. As the IL content increasing from 30 wt.% to 50 wt.%, the sensitivity of IL/TPU sensor increased from 5.65 mV N-1 (17.74 mV MPa-1) to 16.55 mV N-1 (51.97 mV MPa-1), that of IL/PVDF sensor increased from 14.12 mV N-1 (44.33 mV MPa-1) to 32.23 mV N-1 (101.20 mV MPa-1) monotonously. The underlying mechanisms were revealed based on water exchange in air and ion transport theory qualitatively. IL/PVDF sensor with 50 wt.% IL content shows the best sensing performance in air and is promising for practical application.

Automated summary

Three typical ionic polymer materials, Nafion, poly(vinylidene fluoride)/ionic liquid (IL/PVDF) composite, and thermoplastic polyurethane/ionic liquid (IL/TPU) composite, were compared for their sensing properties under applied pressure. Nafion sensor's sensitivity was highly influenced by ambient humidity, while IL/TPU and IL/PVDF sensors were stable in humidity but sensitive to IL content. IL/PVDF sensor with 50 wt.% IL content showed the best sensing performance in air and is promising for practical applications.