Non-universal behavior well above the percolation threshold and thermal properties of core-shell-magnetite-polymer fibers

Aligned nanocomposite electrospun fibers are investigated for electrical (dc, ac, and dielectric) and thermal properties (10-40 wt.). This nanocomposite consists of poly(methylmethacrylate) (PMMA) grafted core-shell magnetite (CSM) nanoparticles and PMMA. Electrical properties as a function of tunnel gap (in between the CSM nanoparticles) are studied as the polymer shell stays intact and the cores do not touch each other well above the percolation threshold. The depleted improvement in dc conductivity ( dcwt) with increasing wt. (the improvement percentage: dc10 to dc20 ≈ 100; dc20 to dc30 ≈ 40 and dc30 to dc40 ≈ 1.2) affirms the non-touching cores of CSM. Interestingly, the observed ac conductive behavior (0.1-13 MHz) in the high end of the frequency range is in clear contrast to that of a typical percolating system, in fact it does not explicitly follow the universal power law. Some of the obtained critical exponents are not accommodated by the universal theory and significantly different from the present theoretical/experimental predictions. Additionally, an improvement in thermal stability of ∼30 C and an overall increase in glass transition temperature are reported. © 2011 American Institute of Physics.