Electrical conductivity recovery in carbon nanotube–polymer composites after transient shear

Abstract

Investigations on electric conductivity have been performed on polycarbonate (PC) containing 0.6 vol% of multi-walled carbon nanotubes (MWNT) in a rheometer, where the plates act as capacitor plates for dielectric spectroscopy. The time dependent recovery of the DC-conductivity was recorded in the rest time after a transient shear deformation. The DC-conductivity shows a tremendous increase with the time after shearing. This conductivity recovery is explained by the reorganization of the conducting MWNTnetwork, which was destroyed by the shear. The reformation kinetics of MWNT-clusters is considered as a cooperative cluster-cluster aggregation coupled to electrical percolation of conductive MWNT.

Superposition approach for description of electrical conductivity in sheared MWNT/polycarbonate melts

“…This is valid at least for moderate shear rates, less than 10 3 s -1 (see e.g. [12,13,41]). The superposition approach is tested on polycarbonate composites filled with multi-walled CNTs at different temperatures.…”

“…For the conductivity measurements round samples of 25 mm diameter and of 2 mm thickness were cut from the mould injected plates. The time-dependent dielectric measurements were performed using a Novocontrol impedance analyser coupled with a laboratory rheometer (Ares, Rheometric Scientific), in which the rheometer plates are equipped with ring electrodes (inner diameter 19 mm, outer diameter 25 mm) [12,13,35,36]. The setup with ring electrodes ensures a relatively narrow distribution of the shear rates in the relevant region of electrical field during steady rotation.…”

“…For steady shear conditions constant values for the electrical conductivity and the dynamic shear modulus were found, indicating a stationary state of the filler network due to the competition of shear-induced build up and destruction [35,36]. In the quiescent state of melt the destroyed agglomerates were found earlier to be rebuilt in the process of quiescent agglomeration [12][13][14][15]. As stated already in [36], for a more realistic description of the filler network and its dependence on the thermo-rheological history a 'superstructure' has to be taken into account, e.g.…”

“…Increasing shear rate has a negative effect on conductive network formation: when shear rate exceeds a critical value the electrical conductivity was found to decrease to the matrix conductivity [31]. For lack of transient percolation theories, we proposed recently a phenomenological approach, in which time and shear-dependence of electrical conductivity is accounted for by coupling percolation theory with an appropriate kinetic equation for buildup and destruction of conductive agglomerates [12,13]. To our knowledge, agglomeration of nanoparticles in a polymer matrix was first discussed by Schueler et al [32,33] for carbon black in uncured epoxy.…”

“…Heinrich et al [34] proposed agglomeration of layered nanofillers in a thermoplastic or rubber matrix along with a kinetic equation for description of the time evolution of the shear modulus. In our first simplified approach [12,13] we assumed, that the 'percolation' path is formed by sphere-like conductive agglomerates, containing loosely packed CNTs. These agglomerates can be formed or destroyed in shear flow [35,36].…”