Changes in the Electrical Resistivity of Thin-Films for Polymer Composites Induced by Heating-Cooling Cycles

This paper is focus on the analysis of the electrical resistivity of polymer composites respect to the temperature when samples are under heating-cooling cycles in two type of experiments namely: (1) at three different heating rates and (2) an isothermic one. Three different films S1, S2 and S3 of Polystyrene + 22 wt% CB were submitted to several heating-cooling cycles until a reversing processof the resistivity behavior was reached for each film. The heating was performed from room temperature, RT (24 °C) to 120 °C at rates of 1, 2 and 5 °C/min, respectively. A fourth film S4 was exposed to isothermic-heating-cooling cycles, consisting on heat from RT to 120 ºC (at 2 °C/min) and keep the filmat 120 °C for 2.5 hours. The cooling rate was 0.2 °C/min for all samples. Results show that the cycle’s number for reaching the electrical stability of films were strongly dependent on the heating rate more than the kind of process (continuous or isothermic). Main analysis shows that as a lower the heating rate (1 °C/min) the number of heating-cooling cycles for reaching the reversibility (electrical stability) are less (6 cycles in our case) than those for higher heating rates (9 and 12 cycles). Results are explained in terms of the redistribution of carbon black particles due to the composite´s structural and thermal relaxation; in other words the dynamic polymer chain mobility during the heating process and the chain relaxation process during the cooling one