The effects of solvent casting temperature and physical aging on polyhydroxybutyrate‐graphene nanoplatelet composites

Due to their unique set of properties, polymer composites reinforced with graphenic nanoparticles are materials of interest for applications such as actuators, sensors, and degradable electronic components. To implement polymer nanocomposites in such applications, it is necessary to understand how both processing conditions and aging affect their properties. This is especially important when the matrix is composed of a semicrystalline polymer susceptible to transformations due to aging. In this study, we investigate the physical properties of a biodegradable polymer nanocomposite, comprising a polyhydroxybutyrate (PHB) matrix loaded with graphene nanoplatelets (GNP) as a conductive filler. PHB/GNP nanocomposite films were prepared at different solvent casting temperatures ranging from 80 to 140°C. Results show that electrical resistivity decreased— from 42.3 Ω cm for 80°C to 3.01 Ω cm for 110°C and 1.5 Ω cm for 140°C—with increasing solvent casting temperature. Moreover, for nanocomposite films containing less than 10 wt% of GNP and processed at 80°C, we observed significant decrease in resistivity (>50%) over time when the sample was aged at room temperature. We postulate that this decrease in resistivity arises from the cold‐crystallization of PHB, as observed by X‐ray diffraction analysis, and the densification of the polymer matrix, which is a direct consequence of an increase in crystallinity of nearly 20% over 168 h of aging. These results show that understanding the aging behavior of nanocomposites made from semicrystalline polymers such as PHB is crucial when designing conductive polymer composites and active devices. The physical properties of polymer nanocomposites can continue to change after fabrication. These changes can impact both the stability and functionality of the nanocomposites. This study assesses the effect of room temperature aging on the electrical and physical properties of polymer nanocomposites comprised of graphene nanoplatelets in a semi‐crystalline polymer matrix, and explores the importance of microscopic changes in the polymer matrix on the final nanocomposite properties.