Piezoresistive sensitivity tuning using polyelectrolyte as interface linker in carbon based polymer composites

Highlights

• Interface linker can bring the structural changes in the composite material.

• Long threads of CNF impacts tunneling resistance and improve piezoresistive sensitivity.

• Properties of the composites can be tailored by varying the concentration of PSSA/CNF in the PVDF.

• The PSSA enhanced dispersion of CNF and hence sensing performance of the composites.

Abstract

Poly (4-styrenesulfonic acid) (PSSA) solution with Carbon nanofiber (CNF) were mixed in polyvinylidene fluoride (PVDF) polymer matrix to prepare series of PVDF/PSSA/CNF blend membrane. The results demonstrate improvement in the gauge factor and piezoresistive sensitivity, with increasing concentration of CNF, in presence of PSSA. The morphology of the composite membranes was examined by scanning electron microscopy. The phase shift from α to β was observed by the XRD patterns and DSC curves confirmed no change in the crystallinity of the membranes. The elasticity was observed to be increased in thin membranes of ∼0.2 mm thickness, with increase in PSSA content. The detailed investigation revealed that, the tuning of PSSA in carbon-based polymer composite achieved significant increase in gauge factor up to 5.07, attributed to good interfacial linking characteristic of PSSA. The developed CPNC membranes may find their applications in prosthetic limbs, biomedical instruments and other wearable devices as pressure sensors.

Introduction

Polymers are widely being used as strain sensors, chemical sensors, biosensors, robotic arms, prosthetic limbs, energy devices and power sources in all domain of science and engineering [[1], [2], [3]]. The strain sensor is one of the extensively popular sensors and has wide applications to measure tensile strength, strain, shear, pressure, compression and other related quantities [[4], [5], [6]]. The sensitivity of the piezoresistive sensors is defined as the relative change in resistance with strain and measured in terms of gauge factor (GF) calculated by Eq. 1 [7]. Where ΔR is change in electrical resistance, R_i is initial electrical resistance and ε is mechanical strain.

$$GF=\frac{\left(\raisebox{1ex}{$\Delta R$}\!\left/ \!\raisebox{-1ex}{$R_i$}\right.\right)}{\epsilon }$$

Conductive polymer nanocomposite (CPNCs) owing to simple fabrication, good mechanical and electrical properties are drawing great attention of the researchers worldwide [[8], [9], [10], [11], [12], [13], [14], [15]]. However, controlling of piezoresistive sensitivity of the CPNCs depending upon the desired applications is contemplated to be a key issue [15,16].Carbon based nanocomposites membranes are much popular because of their low cost, high aspect ratio, low density, high strength, tunable morphology, environmental stability, chemical stability, high electrical conductivity and very good compatibility along with the availability of organic and inorganic solvents for chemical modification [[17], [18], [19], [20], [21], [22]]. Blending of carbon-based fillers, in the polymer matrix lead to the formation of conductive channels/ networks which can react to external stimuli like pressure, heat, light and many others [23,24].

Various researchers have investigated carbon based CPNC membranes for sensing applications. Georgousis et al. [25] fabricated PVDF/multi-walled carbon nanotubes (PVDF/MWCNT) membranes using melt mixing at different concentrations of filler. The strain sensing capabilities of the CPNCs were investigated by in-situ conductivity measurements. Ferreira et al. [26] prepared PVDF/carbon nanotubes (CNT) composites by hot pressing, which can measure the boundary pressure within prosthetic stumps/sockets. It was observed that the GF gradually increased up to 6.18 for 1.7 wt.% and then decreased with increase in the concentration of CNT. Yilmaz et al. [27] investigated the electromagnetic interference (EMI) shielding effectiveness and tensile strength of PVDF/ vapor grown CNF (PVDF/ VGCNF) CPNCs prepared by heat pressed compression molding. A strong relationship was detected between volume fraction of the fiber inside the polymer and tensile properties and found the composites suitable for EMI shielding and sensing purposes. Zhang et al. [28] worked with PVDF and used MWCNT as the conductive filler to enhance the sensitivity of the CPNC membrane with outstanding piezoresistivity and studied the microstructural behaviour of the CPNC membranes on deformation and their effect on sensitivity. The sensing characteristics and hydrolytic degradation of poly(lactic acid)/poly(ethylene oxide)/CNT (PLA/PEO/CNT) nanocomposites have been investigated by Zare et al. [29] and they observed that the size and number of pores after degradation depends on the composition of PEO and CNT. Wang et al. [30] reported the piezoresistive response of CNT composite film which was tested under laterally compressive strain. However, piezoresistive properties of the composites are very complicated and depend upon matrix interfacial bonding, modulus and the orientation of reinforcement [31]. Hence, it would be of great importance to analyze the effect of an efficient interfacial linker in carbon-based polymer nanocomposites.

Ke et al. [32] developed PVDF/IL/MWCNT nanocomposites having highly piezoresistive characteristics. The well disperse MWCNTs in PVDF matrix were attributed to presence of IL as an interface linker, which can further tune the strain sensing capabilities and electrical conductivity of the CPNCs. Prasad et al. [33] fabricated CPNC membranes using CNF/PVDF and Ionic Liquid/PVDF/CNF for pressure sensing applications. The interaction of IL and CNF produced a shift of PVDF from α phase to β phase which supported facile flow of the ions through membrane. Li et al. [34] obtained PVDF‐g‐PSSA/sulfonated graphene oxide (PVDF‐g‐PSSA/ SGO) nanocomposites with increased proton conductivity and anti-fouling properties. Zhang et al. [35] prepared poly(phenylene oxide)/polystyrene/ CNT (PPO/PS/CNT) membranes and the dispersion of CNT were tuned by varying the composition. Also, a direct relationship between the dispersion of CNT and electrical conductivity of the blend membranes was observed. Huslage et al. [31] studied about the effect of PSSA as interface linker to improve the mechanical properties of the membrane for fuel cell applications. He also observed that the sulphonic groups help in elongation of the membrane and maintaining high stability. Similar work proving the PSSA as interface linker was also studied by Panwar et al. [36] to enhance the actuation ability and proton conductivity using PVDF/PVP/PSSA blend membrane.

Despite the recent developments, there is still enormous room for further investigation on tuning the piezoresistive sensitivity or GF by regulating the interfacial interactions between the conducting fillers and polymer matrix. To achieve this objective, an efficient and low-cost interfacial linker is required which can bring the structural changes in the composite material [32].

In the present work, bendable, cheap but highly sensitive piezoresistive sensor have been prepared using solvent casting method. The PSSA solution has been used as an interfacial linker for dispersion of the conductive filler CNF in the PVDF. It was expected that longer threads of CNFs in comparison with CNTs will impact the tunneling resistance and as a result will improve the piezoresistive sensitivity of the material [37]. The results demonstrate substantial enhancement of GF from 1.88 to 5.07 without compromising other important mechanical and thermal properties. Also, the tuning of piezoresistive sensitivity is achieved using PSSA as an efficient low-cost interface linker in the nanocomposite film.