Time-dependent uniaxial piezoresistive behavior of high-density polyethylene/short carbon fiber conductive composites
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Short carbon fiber (SCF) filled high-density polyethylene conductive composites were studied in terms of time-dependent piezoresistive behaviors. The time-dependent change of resistance under constant stress or strain was found to be the succession of the previous pressure-dependent piezoresistance. Depending on the filler volume fraction and the level of the constant stress or strain, resistance creep and resistance relaxation with different directions were observed. An empirical expression similar to the Burgers equation could be applied to fit the data for both the resistance creep and the resistance relaxation. The fitted relaxation time as a function of pressure showed that there exist two competing processes controlling the piezoresistive behavior and its time dependence. Mechanical creep and stress relaxation of the composites were also studied, and a comparison with the time-dependent resistance implied that there is a conducting percolation network attributed to the physical contacts between SCF and a mechanical network formed by the molecular entanglement or physical crosslinking of the polymer matrix and the interaction between the filler and the matrix. It is believed that the two networks dominate the electrical and the mechanical behaviors, respectively.
I. INTRODUCTION
Conductive polymer composites can be obtained by incorporating conductive fillers into an electrically insulating polymer matrix. For the past few decades, these materials have attracted a great deal of scientific and commercial interest because they exhibit unique electrical and mechanical properties in addition to some exclusive properties pertaining to polymeric materials such as light weight, low cost, ease of processing, and corrosion resistance.1 The resistivity of a conductive polymeric composite is critically determined by the volume fraction of the conductive filler.2,3 At low values of , is basically that of the polymer matrix; however, at a critical volume fraction c, namely, the percolation threshold, drops sharply by several orders of magnitude. This phenomenon is generally termed the percolation transition. According to the classical percolation theory,4 above c is only related to the variable ⳱ 0 ( − c)−t
,
(1)
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0355 J. Mater. Res., Vol. 19, No. 9, Sep 2004
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where t is the critical conductivity exponent and 0 a constant related to resistivity of the conducting phase. It is generally believed that a percolation network comes into appearance in the transition region and c coincides with the formation of the first continuous conducting filament through the polymer matrix. Because of the different responses of the matrix and the filler to an external stimulation, the percolation network can be further affected by the applied fields and environmental alternations such as mechanical force,1,3,5–8 solvent,2 electrical,9 magnetic,10 and ultrasonic11 fie
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