Highly Stretchable Printed Poly(vinylidene fluoride) Sensors through the Formation of a Continuous Elastomer Phase

Abstract

Stretchable piezoelectric stress/strain sensing materials have attracted substantial research interest in the fields of wearable health monitoring, motion capturing, and soft robotics. These sensors require operation under dynamic loading conditions with high strain range, changing strain/loading rates, and varying pre-stretch states, which are challenging conditions for existing sensors to produce reliable measurements. To overcome these challenges, an intrinsically stretchable poly(vinylidene fluoride) (PVDF) sensor is developed through the polymer blending of PVDF and acrylonitrile butadiene rubber (NBR). Through precipitation printing and vulcanization, the resulting PVDF/NBR blends exhibit strong β phase PVDF and a blend morphology with submicron-level phase separation, but also strains up to 544%. Both the blend morphology and the mechanical properties indicate that this PVDF/NBR blend can be considered as a continuous elastomer phase above micron scale. After electric poling and adding electrodes, the PVDF/NBR blends have excellent piezoelectric properties to be used as both stretching mode strain sensors and compression mode stress/force sensors. The stretching mode sensors can measure strain up to 70% without strain rate and pre-stretch dependence, while the compression mode sensors have a loading-rate-independent linear voltage-stress relationship up to 4.8 MPa stress and a negligible pre-stretch dependence. Therefore, the PVDF/NBR sensors can provide accurate and reliable stress/strain measurements when attached to soft structures, which paves the way for sensing and calibration of soft robots under dynamic loading conditions.