Electrical conductivity and fiber orientation of poly(methyl methacrylate)/carbon fiber composite sheets with various thickness

Abstract Poly(methyl methacrylate) (PMMA) composites containing carbon fibers (CFs) with the controlled aspect ratio (AR) at various concentrations were produced and hot pressed into rectangular sheets with a variety of thickness. The orientation of CFs on the surface and inside the specimen was obtained, and the conductivity σ in three different directions (X‐length, Y‐width, Z‐thickness) were investigated. A preferential orientation of CFs in XY plane was found inside the samples, resulting in higher σ_X and σ_Y. The orientation of CFs in Z direction increases with the sample thickness increasing, leading to an increasing σ_Z. Thus the anisotropic conductivity between σ_X (or σ_Y) and σ_Z reduces from three orders of magnitude to one order of magnitude. The percolation thresholds (Φ_c) in three directions of the samples were revealed by fitting with McLachlan GEM equation, which lead to Φ_c,X ≈ Φ_c,Y ≈ Φ_c,Z, independent of the thickness and the measuring direction. The values of exponent t in GEM equation were found to be t_X > t_Y > t_Z, while t_Z increases with the sample thickness. For the first time, the relationship between the exponent t and the sample thickness was revealed and discussed.