Thermal and current noise in carbon black-filled polystyrene and polyethylene in the vicinity of Tg and Tm

Abstract

Thermal and current noise in carbon black-filled polystyrene and polyethylene was studied in the vicinity of Tg and Tm. A highly conductive grade of carbon black was used (Ketjenblack EC): 4% for PS and 6.5% for PE. Pronounced maxima in noise voltage were found at Tg for PS and Tm for PE. Prolonged storage at T lower than Tg enhanced the intensity of the noise peak at Tg for PS. A hysteresis occurred on cooling after a heating cycle, with the current and thermal noise being lower on the cooling cycle (no maximum in the case of PS). The peak temperature was lower for PE on the cooling cycle. Good agreement was found between experimental and calculated thermal noise values. The thermal noise spectra were white within the temperature range of 20° to 140°C. The appearance of the current noise peaks could be associated with a conduction model where a decrease in the number of contact points in the carbon black network occurs in the transition regions. This is in accordance with the concept proposed earlier that the carbon black network existing in a polymer undergoes a rearrangement when the polymer melts (softens) or solidifies.