Mechanically stretchable piezoelectric polyvinylidene fluoride (PVDF)/Boron nitride nanosheets (BNNSs) polymer nanocomposites

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Abstract

Mechanically stretchable piezoelectric materials are required by wearable devices, such as self-powering sensors and energy harvesters that can monitor human movement and vital biosignals. Polyvinylidene fluoride (PVDF) is a common choice for making flexible piezoelectric sensors but has limited stretchability (strain to failure). Herein we present a new method of significantly increasing the strain-to-failure of PVDF nanofibre membranes by incorporating amino-functionalised boron nitride nanosheets (BNNSs). The PVDF-BNNS nanocomposites were prepared by firstly dispersing a small amount of BNNSs, less than 1.5 wt%, into a PVDF solution and electrospinning. Compared to the un-modified PVDF, the addition of BNNSs yielded increases of eleven times, twice, and forty times in strength, stretchability, and in-plane thermal conductivity, while piezoelectric performance remains unaffected. The higher thermal conductivity offers significantly better heat dissipation ability of piezoelectric sensors than the un-modified PVDF material.

Introduction

Modern technologies such as soft robotics, wearable sensors and energy harvesters require flexible and stretchable electronics. Tremendous research endeavour has been made towards developing smart textiles and wearable or skin-attachable devices for broad applications such as activity tracker, navigation systems and health care. Poly(vinylidene fluoride) (PVDF) and its co-polymers are commonly applied for preparing flexible piezoelectric sensors and energy harvesting devices. However, the low strength and stretchability of PVDF polymer limit its applications in wearable or conformable technologies. Previous efforts to enhance the stretchability of piezoelectric PVDF sensors focused on geometrical design. For example, Kirigami cutting patterns was fabricated in PVDF film sensors to increase tensile strain [1]. It has been reported that with a Kirigami pattern, the elongation of nanocomposite conductors can be improved from 2% to over 100%. Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) nanofibre mats were sandwiched between elastomer sheets to be compliant with mechanical deformation so as to conform to skin motion [2]. These sandwiched piezoelectric sensors were shown to be durable under 1000 cycles of repetitive stretching and folding.

One-dimensional (1D) nanofibres, nanotubes and two-dimensional (2D) nanosheets have showed effective improvement in the mechanical, electrical, thermal and optical properties of polymers [3,4]. Among the various 2D nanosheets, monolayer hexagonal boron nitride (h-BN) has recently attracted strong interest due to their high thermal conductivity (∼2000 W/m K by calculation) [5], high mechanical strength (Young's modulus: 0.7–0.9 TPa, yield strength: ∼35 GPa) [6], superb chemical and thermal stability and strong oxidation resistance. Hexagonal boron nitride nanosheets (BNNSs) consist of alternating boron and nitrogen atoms in a honeycomb organisation, instead of carbon atoms only as in graphene [7,8]. The intrinsic properties made this nanomaterial a promising filler to enhance polymers for new multifunctional properties so that they can be used in more harsh and challenging environments [[9], [10], [11], [12]]. In our recent work on applying an environmental friendly route for preparing high temperature thermally conductive nanocomposite textiles, BNNSs were dispersed in a polyimide aqueous solution and electrospun into nanofibre membranes that exhibit high thermal conductivity of approximately 13 W/m K at 300 °C [13].

Due to its exceptional electric properties that can be utilised in active materials in energy harvesting and self-powered electronics [[14], [15], [16]] and strong resistance to chemicals such as aggressive reagents, PVDF has received resurgent interest recently. The electrospinning process, which provides in-situ mechanical stretching and electrical poling, can generate polymer fibres embedded with secondary nanofillers. Woo et al. prepared electrospun polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) membranes embedded with graphene nanosheets. They found that with 5 wt% of graphene nanosheets, the tensile strength of the nanofibre membrane enhanced 51% and the elongation increased 41% [17]. Zhang et al. have successfully fabricated PVDF nanofibre membranes with boron nitride fillers. Their results suggest that the tensile properties and thermal conductivity increase gradually with the loading of fillers, with the tensile strength reaching 24 MPa, elongation reaching 97% and in-plane thermal conductivity reaching 7 W/m K, when 30 wt% ultrasonicated boron nitride powder were used [18].

Herein we present a simple method of incorporating few-layer h-BN containing amino groups into PVDF nanofibres. Composite nanofibres were prepared by initially uniformly dispersing BNNSs into PVDF solution for electrospinning. Electro-thermo-mechanical tests were conducted to study the effects of BNNSs on the strength, ductility, thermal conductivity, piezoelectric property, and surface properties of PVDF nanofibre membranes. The distribution of BNNSs within the electrospun PVDF nanofibres was characterised by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX).

Section snippets

Materials and few-layer h-BN synthesis

PVDF pellets (Mw∼275,000) and solvents of N, N-dimethylformamide (DMF) and acetone were obtained from Aldrich and used as received. Few-layer h-BN containing amino groups were prepared by ball-milling. Commercial BN powder with urea as agent was processed using a Fritsch ball milling instrument (Pulverisette 5) operating at 400 rpm for 20  h at ambient temperature. The morphology of the synthesised BNNSs can be seen in our previous work [19]. The BN powder (PT110) was supplied by Momentive

SEM images and XRD pattern of the electrospun nanofibres

Firstly, the BNNSs were characterised by SEM and shown in Fig. 1. It can be seen that the prepared BNNSs have a lateral size of 50–200 nm. The SEM images of electrospun nanofibres are shown in Fig. 2. The bead-on-string morphology was observed for all the nanofibre types. The PVDF nanofibres had an average diameter of 245 nm; the diameter increased to 328 nm with 0.5 wt% addition of modified BNNSs, and then reduced to 206 nm and 242 nm for PVDF1.0BN (with 1.0 wt% of BNNSs) and PVDF1.5BN (with

Conclusion

This study has demonstrated a simple technique of significantly increasing the strength, ductility, and in-plane thermal conductivity of PVDF nanofibres by introducing amine-containing hexagonal boron nitride nanosheets (BNNSs) into the electrospinning process. Only a small amount (∼1.5 wt%) of BNNSs is needed to achieve increases of 11 times, twice, and forty times increases in strength, strain to failure, and in-plane thermal conductivity, without negatively affecting their piezoelectric

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