Vitrification during cure produces anomalies and path-dependence in electrical resistance of conductive composites

Using a DGEBA/TETA epoxy filled with silver microflakes, we sought to investigate if the onset of conductivity would approximately obey the power-law as cure progresses, as reported in the open literature. We monitored the resistivity change as a function of cure and vitrification behaviour, using a set of concurrent experiments; in-situ four-wire electrical resistance measurements in a newly-developed probe-mold device, and differential scanning calorimetry. It was evident that the electrical conductivity evolved very differently depending on heating conditions, the composite glass transition temperature, and filler content. Periods were observed during cure where electrical conductivity was disrupted, attributed to stresses produced by vitrification. This produced as much as 1500-fold increases in final developed electrical resistance depending on heating conditions and composite glass transition properties, for the same conductive filler content. This discovery has far ranging implications on the industry practice of applying multi-step cure schedules that feature vitrification during cure.