As reported, for a given system, there is a critical value of the conductive particle loading, i.e., the percolation threshold (Φ∗) at equilibrium state, below which the effective conductive network will never be formed [17], [20] and [21]. Zhang et al. [20] proposed a thermodynamic percolation model to successfully illustrate the relationship between tp and Φ∗ as follows:
equation(3)
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where c is a constant, η is the viscosity of the matrix at the given temperature, and Φ is the experimental volume fraction of fillers in the composites. P(0) and P(∞) are the fractions of the conductive particles participating in the conductive network at t = 0 and at the equilibrium state (t = ∞), respectively. Fig. 4a and c displays the plots of Φ versus 1/tp for CNT/EVA10 and CNT/EVA25 samples, annealed at 140, 150, 160 and 180 °C. By extrapolating of 1/tp to zero, the values of Φ∗ can be obtained and listed in Table 1 for CNT/EVA10 and CNT/EVA25 systems. Moreover, the parameters P(0)/P(∞) and c/η are estimated as follows: