Mesostructure, electron paramagnetic resonance, and magnetic properties ofpolymer carbon black composites

Electron paramagnetic resonance (EPR) has now become firmly established as one of the methods of choice for analyzing the carbon network over a range of different volume fraction of the carbon black in the composite, i.e., be low and above the respective conduction threshold concentration. In the pre sent article, two types of carbon blacks, having very different primary str uctures, surface areas, and percolation thresholds, were used; Raven 7000 ( of high surface area and high percolation threshold volume fraction) and Y5 0A (of low surface area and low percolation threshold volume fraction). A s emiquantitative image analysis of the microstructure from transmission elec tron microscopy reveals information about the spatial distribution of the c arbon aggregates and agglomerates inside the composite. We observe that the apparent surface of agglomerates increases significantly with increasing c arbon black content for the two types of blacks investigated. Adsorbed oxyg en on the carbon black cristallites and dynamic coalescence under mixing co nditions can be responsible for the broadening of the dispersed phase surfa ce distribution. The interagglomerate distance in two samples of concentrat ions f < f(c) and f congruent to f(c) of Raven 7000 are nearly identical in dicating that the dc condition threshold can therefore be almost entirely a ttributed to the coalescence of smaller aggregates. Line shape simulation s howed that the changes in the absorption EPR spectra, at temperatures betwe en 105 and 300 K, of the composite samples containing Raven 7000 can be des cribed by a linear superposition of two distinct Lorentzian (one broad and the other narrow) resonance lines and a single (narrow) Lorentzian resonanc e line for composite samples containing Y50A. The spins giving rise to the EPR signal reside in the carbon black particles. In Raven 7000, the signifi cant difference in linewidth between the two signals demonstrates a differe nt environment where the restriction of the motion of the paramagnetic cent ers varies. The narrower line was assigned to spin probes with high mobilit y (carbon black aggregates) and the broad one to probes with restricted mob ility incorporated in carbon black agglomerates. In Y50A, only the sites wi th high mobility were detected. When the temperature is increased the data demonstrate that the EPR signal intensity, which is the double integral in arbitrary units divided by the mass of the carbon black contained in the sa mple, decreases slowly in the temperature range 105-300 K. The various phen omena observed are attributed mainly to the aggregates and agglomerates str ucture in the composite samples. The temperature dependence of the paramagn etic susceptibility deduced from the EPR integrated intensity is discussed in terms of Adriaanse 's model [L. J. Adriaanse, J. A. Reedijk, P. A. A. Te unissen, H. B. Brom, M. A. J. Michels, and J. C. M. Brokken-Zijp, Phys. Rev . Lett. 78, 1755 (1997)]. The magnetic susceptibility of the composite samp les is also measured with a superconducting quantum interference device mag netometer, operating at an applied magnetic field of 0.5 T, from 2 K to roo m temperature. The observed temperature dependence of the spin susceptibili ty is discussed and suggests that morphology heterogeneity is of overwhelmi ng importance to understand the magnetic properties of these materials. (C) 2001 American Institute of Physics.