2D Nanosheets-Polymer Composites: Mechanical, Electrical Properties Evaluation and Applications

Abstract

The emergence of graphene and other related 2-dimensional materials like hexagonal boron nitride (hBN) and molybdenum disulfide etc., have boosted the polymer nanocomposites (PNCs) particularly with their mechanical & electrical properties. In this work, mechanical and electrical properties of both liquid exfoliated hBN and graphene nanosheets (GNS) are evaluated in polyvinyl chloride (PVC), polyvinyl alcohol (PVA) and thermoplastic polyurethane (TPU). The effects of filler aspect ratio, alignment, and dispersion have particularly been focused. It is found that the misalignment of hBN and GNS inside polymer matrices yields relatively low levels of reinforcement. A post treatment technique of uniaxial drawing helped to align the nanosheets inside polymers. In case of hBN-PVC composites, the modulus reinforcement levels of dY/dVf ~800±200 GPa was achieved with the 300% drawing, matching the theoretically predicted values. A considerable increase in the Young's modulus and strength with maximum values ×~3 higher than the neat polymer is achieved at 0.001 Vf hBN. Such an enhancement in the mechanical properties is superior to any other hBN-PNCs reported in literature. The level of increase in the mechanical characteristics could not be explained solely on the basis of strain induced alignment of hBN nanosheets inside PVC using Halpin-Tsai theory. A hypothetical claim of strain induced exfoliation of hBN inside composites validated the level of reinforcement. The claim of strain induced exfoliation was particularly looked upon in GNS-PVA composites via X-ray diffraction measurements. Drawing 200% enticed the GNS to align along the composite films, both maximum Young's modulus and maximum strength are ~×4 and ~×2 higher respectively than that of neat polymer at 0.0002~0.0006 Vf GNS. Drawing 0.006 Vf GNS-PVA composites to 200%~350% strains fully oriented and exfoliated the GNS inside polymer. Study of electrical properties in GNS-TPU composites revealed percolation threshold of 0.0055 Vf GNS; with 3 orders of magnitude rise in electrical conductivity. The dielectric spectroscopy for same filler fraction values indicated that improved dispersion of high aspect ratio (~1272) GNS instigated the enhancement in dielectric constant (ε~5 times), dielectric tangent loss (tanδ~4 times) and AC conductivity (σAC~10-1000 order) for 100 Hz at room temperature. The increase in dielectric characteristic parameters were more pronounced at elevated temperature-473 K (ε~105, tanδ~90 and σAC~25 S/m at 100 kHz). While further rise in GNS loading 0.19 Vf indicated rise in electrical properties (ε~107, tanδ~103& σAC~105) at room temperature & 25 kHz. The DC conductivity~53 S/m at 0.19 Vf GNS and stiff, strong yet tough nature was enough to test these composites for EMI shielding and strain sensing applications. A ~14 dB shielding effectiveness in the frequency range 6~12 GHz for 0.12 Vf GNS-TPU thin films (0.35~0.50 mm) was observed. The same GNS-TPU composite was then utilized successfully for the health monitoring of composites under stress.