Abstract
In this study, multi-walled carbon nanotube (MWCNT) was modified by surface grafted conjugated polymer, poly(3-hexylthiophene) (P3HT) to improve the dispersion of MWNCT. By controlling the reaction condition, different coating thickness of P3HT on MWCNT was synthesized. The functionalized MWCNT was blended with polystyrene (PS) to make nano-composites for electric resistivity measurement with 4-point probe and parallel electrodes. The resistivity of composites for the vertical direction to film surface was three orders higher than it for horizontal direction, and the percolation threshold of vertical direction was also higher than it of horizontal direction. Due to the horizontal orientation of MWCNTs in composite film, electron needs to overcome more polymer barriers than it transports along horizontal direction. Also because of the orientation of MWCNTs, the percolation cluster is more easily formed in the horizontal direction than vertical direction. Therefore, the percolation threshold of vertical direction is higher than it of horizontal direction. Boltzmann distribution and unit projected length were used to describe the orientation status of MWCNTs and estimate the number of contact points. By approximation from perpendicular and parallel direction of composite with percolation model, the contact resistance and the resistivity of grafted P3HT for different coating thickness can be estimated. From the calculation result, the resistivity of P3HT coating exponentially decreased with reducing coating thickness and the resistivity of grafted P3HT was lower than it of bulk. Different thickness of P3HT thin film was also prepared by spin coating and measured the resistivity by parallel plane electrodes. For the spin-coated P3HT thin film, the resistivity also exponentially decreased with reducing thickness from 100nm to 5nm and the resistivity was also lower than it of bulk. The low resistivity of P3HT thin film is because that the electric conductivity will be mainly dominated by the microscopic electron transport mechanism, the Miller-Abraham hopping theory, in the thin polymer film. The possibility of forming a percolation route for electron transport exponentially increases with reducing transport distance, thus the resistivity will exponentially decrease with reducing film thickness and the resistivity is lower than it of bulk. Since there are an interaction force between MWCNT and grafted P3HT film, the conformation or arrangement of P3HT may be affected and thus the localization situation of grafted P3HT will be different from spin-coated P3HT film. By studied the thickness effect of resistivity, the mechanism which dominates the conduction behavior can be figured out. This will aid the photoelectric research of applied CNT composites such as solar cell and light-emitting diode, and improve the efficiency of composite device.