POSITRON LIFETIME AS A NANOPROBE FOR FREE-VOLUME DISTRIBUTION IN HIGH-DENSITY POLYETHYLENE CARBON-BLACK CONDUCTING COMPOSITES

The room-temperature electrical conductivity of High Density Polyethyl ene-Carbon Black (HDPE-CB) switching composites as a function of the c onductive filler (CB) content was studied by Positron Annihilation Lif etime Spectroscopy (PALS). The CB, highly structured and porous, impar ted high conductivity to its composite, starting at a low level of loa ding. Deconvolution of the lifetime spectra into 4 components reflecte d upon 3 different morphologies of the composite structure. The free v olume was probed using o-Ps pick-off annihilation lifetime parameter t au(4) as a measure of electron density and the mean free volume cavity radius. The probability of o-Ps formation and the concentration of fr ee volume cavities were probed by the o-Ps pick-off intensity I-4. In the blended composite of 29 phr (parts per hundred parts of resin), th e mean free volume size increased to 3.56 Angstrom from 3.27 Angstrom, in the pristine polymer with the corresponding increase in the free v olume to 204.5 Angstrom(3) from 146.8 Angstrom(3). The relative number of free volume cavities (I-4) in the composite remained fewer than th e host polymer over the CB concentration studied. The changes in the c avity size and concentration, particularly a decrease in the relative intensity I-4 by 10.5% resulted in the introduction of enhanced crysta llinity into the composite structure which was evidenced from a variat ion in crystallinity from approximate to 75% in the pristine to approx imate to 85% in the 29 phr CB-HDPE composite in the X-ray diffraction profiles.