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 uC using a strain rate of 2 s 21 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 45u 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 T m 250 uC, 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 T m 250 uC.

Structure evolution in poly(ethylene terephthalate) (PET) – Multi-walled carbon nanotube (MWCNT) composite films during in-situ uniaxial deformation

“…Mayoral [11] confirmed the formation of different crystal polymorphs when composites of PET and MWCNTs in extruded sheet form were uniaxial and biaxial deformed.…”

“…This is observed during thermal analysis as an increase in the crystallization temperature of the polymer, although, there is often no increase in melting temperature or change in glass transition temperature [11,[13][14][15][16][17]. Following the structure evolution in polymer-CNT composites during processing is not straightforward and requires techniques that allow timeresolved data acquisition ranging from less than a second to several minutes depending on the processing conditions.…”

“…in fuel cells and bipolar plates. Recently, Mayoral [11] and McCrossan [12], reported on the structural morphology, electrical and mechanical properties of composites of PET and MWCNTs. The mechanical properties (stiffness, strength and toughness) were vastly modified on addition MWCNTs into the PET matrix: the PET-MWCNT composites showed an enhancement in Young's modulus of 79% and electrical conductivity by 14 orders of magnitude, compared to neat PET.…”

“…Each composite is denoted with respect to its MWCNT loading: PET1% and PET2%. Table 1, lists the glass transition temperature (T g ), melting temperature (T m ), crystallization temperature (T c ) and crystallinity (X c ) of the PET-CNT cast composite materials [11]. The dispersion and distribution of the MWCNTs in the polymer matrix were investigated using various microscopy techniques and it was found that the MWCNTs were largely randomly dispersed in the PET matrix, but a small fraction were aligned in the extrusion flow direction due to the design of the die used in the extrusion process [11].…”

Smarter–lighter–greener: research innovations for the automotive sector

“…Figure 9 is a high-resolution transmission electron microscope (HRTEM) image showing the concentric arrangement of a multi-walled carbon nanotube (MWCNT) embedded in a poly(ethylene) matrix. A key aspect is the ability to effectively process the nanocomposites [47]. Research builds on a strong legacy of international research into the drawing (or extrusion) of the polymer, polyethylene terephthalate [48][49][50][51][52][53][54].…”