Investigation of micromechanical properties and strain sensing behavior of electrically conducting and Piezoresistive flexible copolyester/Carbon Nanotubes Nanocomposites

Abstract

Various concentration of multiwalled carbon nanotubes (MWCNT) as conductive filler was incorporated into poly(butylene adipate-co-terephthalate) (PBAT), a flexible biodegradable copolyester by melt-mixing followed by compression moulding. Deformation behavior of electrically conductive nanocomposites was correlated with piezoresistivity, leading to their strain sensing behavior. Structural and morphological characterization of the materials were determined by Fourier transform infrared (FTIR) spectroscopy and microscopic techniques while thermal stability and crystallization behavior were analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. Comparative analysis of FTIR spectra of PBAT and PBAT/MWCNT nanocomposites suggested a physical matrix-filler binding force to form the composite microstructures. Microscopic techniques revealed an entangled CNT-network uniformly spread in the polymer matrix. Increased thermal stability of the materials was suggested by TGA, attributed to a good filler-matrix interfacial interaction. DSC results revealed the retarded crystallization process of the polymer with the formation of less perfect crystals. Increasing tensile modulus, Martens Hardness (HM) and indentation modulus (EIT ), and decreasing maximum indentation depth (hmax ) (by 50%, 50%, 100% and 32% on incorporation of 5 wt-% of fillers, respectively) confirmed the mechanical reinforcement of composites by MWCNT. Volume resistivity observed in the nanocomposites suggested the suitability of nanocomposites for strain sensing applications. An exponential-like increment of relative resistance change (ΔR/R0 ) of the nanocomposites as a function of strain confirmed their piezoresistivity. Applicability of nanocomposites as low-strain sensing materials was suggested by their reproducible ΔR/R0 values from 2% to 8% strain during cyclic strain test. Electron beam (EB) irradiation induced crosslinking of the nanocomposites was employed as a strategy to improve the strain sensing behavior of the nanocomposites. Cyclic strain test of irradiated samples (dose: 150 and 200 kGy) exhibited the improvement (up to 10% strain), attributed to their enhanced elastic deformability. The nanocomposites irradiated with the highest dose (300 kGy) exhibited no correlation between ΔR/R0 and strain, attributed to the formation of a 3D network of polymer crosslinks restricting the homogeneous deformation of the CNT-network. Moderate degree of EB irradiation induced crosslinking of polymer nanocomposites can be a strategy to improve their strain sensing behavior. Keywords: nanocomposites, biodegradability, electrical conductivity, piezoresistivity, strain sensing, electron beam irradiation, crosslinking