Quasi-solid state uniaxial and biaxial deformation of PET/MWCNT composites: structural evolution, electrical and mechanical properties

Abstract

Composites of multi-walled carbon nanotubes (MWCNTs) and a poly(ethylene terephthalate)(PET), prepared by melt mixing, were uniaxially and biaxially deformed at 100 °C using a strain rate of 2 s−1 and at stretch ratios (SR) up to 2. A hierarchical organization of randomly well dispersed and distributed agglomerates, smaller bundles and individual MWCNTs was identified in the as extruded composite from extensive microscopic examination across the length scales using polarised optical microscopy (POM), field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). Evidence from X-ray diffraction (XRD) and differential scanning calorimetry (DSC) confirmed the MWCNTs induced crystallization via a heterogeneous mechanism and altered the crystallization behaviour of PET. Uniaxial deformation of the composite materials resulted in orientation of the MWCNTs in the direction parallel to the applied strain and for biaxial deformation at an angle approaching 45° to the vertical axis (extrusion direction), evidence for the latter obtained by HRTEM and from small angle X-ray scattering experiments (SAXS). In both instances, XRD and DSC confirmed the occurrence of significant strain induced crystallization resulting in large increases in tensile mechanical properties. Evidence from wide angle X-ray scattering experiments (WAXS), supported by XRD and DSC showed the overall crystalline content and degree of long-range ordering increased with MWCNT addition. The decrease in diffraction (SAXS) associated with lamella spacing implies the MWCNTs disrupt formation of lamella and reside in the inter-lamellar spacing. Annealing the composites, particularly at Tm −50 °C, results in an increase in PET crystalline content. Prior to deformation the MWCNTs formed an electrically conducting interconnected network throughout the PET matrix, however, this was partially destroyed when the composite was uniaxially or biaxially deformed, as the probability of conductor–conductor contacts decreased. It was possible to restore a MWCNT conducting network similar to that achieved prior to uniaxial deformation only by annealing the sample at Tm −50 °C.