A study on the rheological and electrical properties of poly(lactic acid)/graphene nanoplatelet composites by application of electric field

Abstract

The goal of this thesis is to examine effect of electric field-induced graphene nanoplatelet (GNP) networks on rheological and electrical properties of poly(lactic acid) (PLA)/GNP composites and correlating between the composite properties and the GNP networks depending on concentration of GNPs or application intensity/time of the electric field. For melt-compounded PLA composites containing GNPs at various, low GNP volume fractions ≤ 0.34 vol%, this thesis focused on correlation between the composite properties and a large-scale structure of GNPs formed by a fixed, strong AC electric field (with the frequency and intensity of 60 Hz and 1.75 kV/mm, respectively). Optical microscopy, transmission electron microscopy, and 2D wide-angle X-ray diffraction measurements revealed that almost randomly oriented, thin stacks of GNPs in the as-fabricated composites were aligned by the electric field in the field direction and further organized into a column of stacks. This columnar structure bridged two parallel plate electrodes when the GNP volume fraction was above a threshold value ~ 0.17 vol%. Corresponding to this structural change, the liquid-like rheological response of the as-fabricated composites became the solid-like response (for the composites having GNP volume fractions ≥ 0.17 vol%), after application of the electric field. In contrast, the loss modulus G" was insensitive to the electric field and remained almost proportional to low angular frequency, suggesting that the columnar structure formed by the field was an elastic structure bridging the parallel plates. This bridge also served as an electrically conductive path between the plates so that an insulator-conductor transition occurred when the composites with GNP volume fractions ≥ 0.17 vol% were subjected to the electric field. Quantitative analysis of the equilibrium modulus and conductivity of the composites suggested that the field-induced columnar structure was not a rigid single slab but included the junctions being much softer and less conductive compared to the body of GNP. Moreover, analysis of the GNP volume fraction dependence of the equilibrium modulus and static electrical conductance after the transition (GNP volume fraction ≥ 0.17 vol%) suggested that an elastically inert secondary structure was formed by the GNP stacks remaining out the columnar structure (primary structure). This secondary structure appeared to be in soft contact with the primary structure to provide an extra conducting path. Meanwhile, for a melt-mixed composite of PLA with GNPs having a fixed, low volume fraction = 0.34 vol%, this thesis also examined growths of mechanical and electrical properties under an AC electric field of various intensities E for various times tE, focusing on field-induced GNP structures governing those properties. A fraction of randomly oriented GNPs was aligned by the field and then connected into columns, as suggested from optical microscopy. This structural evolution led to qualitatively similar growths of low-frequency storage modulus and static electrical conductivity. The key quantity for understanding this growth was a time tE* for occurrence of short circuit that detected formation of GNP columns conductively bridging the electrodes. The growths of both modulus and conductivity for various E were summarized as functions of a reduced variable, tE/tE*, confirming the growths commonly reflected the evolution of the GNP columns. However, the modulus grew fast and leveled off by tE/tE* ~ 1, whereas the conductivity kept growing gradually even at tE/tE* > 1. This difference was discussed in relation to the matrix chains and residual GNPs out the column. This thesis, which focused on the interrelation between the composites properties and the field-induced GNP structure, highlights the importance of controlling the microstructure of GNPs particularly for their orientation in order to realize the excellent intrinsic properties of GNPs still in the composites.