Graphene nanoplatelet-based nanocomposites: electromagnetic interference shielding properties and rheology

Kashi, S 2017, Graphene nanoplatelet-based nanocomposites: electromagnetic interference shielding properties and rheology, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Graphene nanoplatelet-based nanocomposites: electromagnetic interference shielding properties and rheology
Author(s) Kashi, S
Year 2017
Abstract Polymer nanocomposites, produced by embedding nano-sized particles in polymeric matrices, are a new class of materials. These materials have attracted world-wide attention due to their superior mechanical, electrical, thermal and barrier properties as well as their outstanding microstructures over the conventional composites.

A variety of nanofillers have been produced and used in fabricating polymer nanocomposites over the past two decades. Graphene nanoplatelets (GNPs) are a new type of carbonous nanofiller with extraordinary physical properties, which can be used for reinforcing polymers and developing novel materials with multifunctional properties such as electrical conductivity. GNP-based polymeric nanocomposites can be used in different areas including electrostatic discharge protection, lightening-protection panels, and electromagnetic interference shielding (EMI) applications.

The focus of this research is on the development of electrically conductive biodegradable nanocomposites with EMI shielding application. For this purpose, two biodegradable polymers, poly (lactic acid) (PLA) and poly (butylene adipate-co-terephthalate) (PBAT), were chosen as the polymeric matrices, and GNP was used as the conductive nanofiller. PLA/GNP and PBAT/GNP nanocomposites with 0-15 wt% GNPs were produced and characterised via different techniques and the properties of the two systems were systematically analysed and compared.

While the glass and melting temperatures of the matrices did not vary considerably with GNP incorporation, their crystallisation temperatures exhibited significant increase. Furthermore, their crystallinity was affected. In particular, crystallinity of PLA was enhanced significantly. Different trends were detected in variations of the Young's moduli of the two polymers with GNP loading; Modulus of PBAT increased continuously with increasing GNP content to 15 wt% while modulus of PLA reached a maximum at 9 wt% GNPs.

Thermal stability of the nanocomposites was extensively studied by thermogravimetric analysis under both air and nitrogen atmospheres and at different heating rates. Results showed that GNP embedding enhanced thermal stability of the polymers effectively. In particular, PLA thermal degradation was significantly delayed in the presence of the platelets.

While magnetic permeability of the polymers was not affected by GNP incorporation, their electrical properties were significantly enhanced. Dielectric constants of PLA and PBAT increased with increasing GNPs, obtaining comparable values for the same GNP content. On the other hand, dielectric loss of PLA nanocomposites with 9–15 wt% GNPs was markedly higher than that of PBAT nanocomposites. Sihvola's unified mixing rule of complex permittivity was used to model the behaviour of dielectric constants and losses of GNP-based nanocomposites for the first time.

As the GNP concentration increased from 6 to 9 wt% (3.5 to 5.3 vol%), an abrupt increase was detected in both AC and DC conductivities of PLA and PBAT, indicating the formation of conductive pathways of GNPs within the matrices. At 15 wt% GNPs, AC conductivities of 7.4 and 3 S/m were obtained for PLA and PBAT nanocomposites respectively despite the higher conductivity of pure PBAT compared to that of pure PLA. This difference was attributed to the better dispersion of GNPs in PBAT, also observed in SEM images. Relatively poor dispersion of GNPs in PLA appeared to facilitate their physical contacts, leading to higher conductivity.

EMI shielding effectiveness (SET) of the nanocomposites as well as contributions of reflection and absorption mechanisms to their radiation attenuation were extensively investigated. Enhancement of the electrical properties of PLA and PBAT with GNP embedding resulted in higher SET. For samples with a thickness of 1 mm, SET of PLA and PBAT increased with increasing GNP concentration. PLA and PBAT nanocomposites showed comparable values of SET with reflection being the primary shielding mechanism. However, they exhibited considerably different potentials for radiation absorption due to their different dielectric loss values.

Evaluation of shielding performance of the nanocomposites with other thicknesses (1.5 and 2.8 mm) demonstrated that their performance was a function of thickness and radiation frequency in addition to the GNP concentration. It was observed that a greater thickness might not necessarily lead to higher SET and therefore, in designing a nanocomposite for EMI shielding application, material's electromagnetic properties and thickness should be chosen based on the radiation frequency.

Another significant part of the present project is its contribution to the knowledge on rheology of GNP-based nanocomposites. Variations of the viscoelastic properties of PLA/GNP and PBAT/GNP nanocomposites, obtained from frequency sweeps, were investigated under simultaneous effects of GNP loading and temperature for the first time. Although it is well-known that temperature can affect microstructure of the materials and consequently alter their viscoelastic properties, the rheological measurements of GNP-based nanocomposites in previous studies have been performed at one single temperature so far.

GNP embedding resulted in significant increments in the viscoelastic properties of PLA and PBAT. Solid-like flow behaviour was observed for highly-filled samples while pure polymers and nanocomposites with low GNP loadings showed liquid-like behaviour. However, changing the measurement temperature revealed that rheological percolation threshold of GNPs in these nanocomposites is temperature sensitive. Winter-Chambon gelation criterion was used to determine the percolation threshold in the two systems at different temperatures. The percolation threshold in PLA was found to drop from 8.5 wt% at 180 °C to 5.2 wt% at 220 °C. Similarly, PBAT/GNP nanocomposites exhibited a decreasing trend in percolation threshold from 11.5 wt% at 160 °C to 7 wt% at 220 °C. Furthermore, in contrast to the ideal melts, viscoelastic properties of some of the PLA/GNP and PBAT/GNP nanocomposites increased with increasing temperature.

In contrast to the dynamic rheological properties, shear viscosity of all PBAT/GNP nanocomposites decreased with increasing temperature. Temperature dependency of the nanocomposites' shear viscosity was described by Arrhenius equation. The flow activation energy (Ea) was found to decrease with increasing GNPs at low shear rates. Consequently, temperature sensitivity of nanocomposites' viscosity decreased with increasing GNPs. Many previous studies reported an increasing trend in Ea with increasing filler loading. Detailed analysis of these works showed that Ea calculations in these studies were carried out at high shear rates while Ea in the present study was determined in the non-shear-thinning region where the original structure of the nanocomposites is not significantly disturbed.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Composite and Hybrid Materials
Functional Materials
Keyword(s) Graphene
Poly lactide
poly butylene adipate-co-terephthalate
Electromagnetic interference shielding
Thermal properties
Thermal stability
Mechanical properties
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Created: Fri, 04 Aug 2017, 12:20:36 EST by Denise Paciocco
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