Boron Nitride Nanotube-Aligned Electrospun PVDF Nanofiber-Based Composite Films Applicable to Wearable Piezoelectric Sensors

In this research, piezoelectric and heat-dissipating boron nitride nanotube (BNNT)/polyvinylidene fluoride (PVDF) nanocomposite thin films having superior properties for wearable sensing applications are introduced. Neat and 2.5 wt % (BNNT2.5)- and 5.0 wt % BNNT (BNNT5.0)-embedded PVDF thin films were prepared by first producing gel-like electrospun nanofibers followed by hot-rolling, respectively. The piezoelectric performance of BNNT5.0 was as high as 128.0 ± 5.4 mV under 300 kPa, while Neat and BNNT2.5 showed 9.9 ± 0.1 and 22.7 ± 8.8 mV, respectively. Furthermore, BNNT2.5 also presents almost 9 times and BNNT5.0 presents more than 3 times higher thermal conductivities of 0.89 ± 0.40 and 0.32 ± 0.10 W/mK, respectively, compared to Neat (0.10 ± 0.01 W/mK) and both present very high thermal resistance with no phase change up to 180 °C and less than 4% shrinkage even at 200 °C, while Neat begins melting at 120 °C. BNNT2.5 and BNNT5 showed a high water-repelling property and mechanical strength as well. Tensile strengths and elongation for BNNT2.5 and BNNT5.0 were 18.1 MPa with 52.1% and 21.2 MPa and 34.0%, respectively. They both showed a low moisture absorption property with high water repellence with stable and hydrophobic water contact angles (92.2° with only 0.4° change for BNNT2.5 and 103.6° with 2.4° change for BNNT5.0 in 30 s), while Neat was in the hydrophilic region. Consequently, BNNT/PVDF films with highly enhanced piezoelectric-sensing and heat-dissipating properties and mechanical, thermal, and moisture resistance can be applicable for various wearable sensing environments.