The effect of multiscale morphology on electrical conduction characteristics of isotactic polypropylene ultrathin films

Abstract

With a modified solution-processing method, isotactic polypropylene (iPP) ultrathin films (∼160 nm) are fabricated and investigated, which reveals a multiscale morphological effect of intramolecular interaction, lamellar orientation and intra-spherulite structure on the macroscopic carrier transport process. The morphology of ultrathin films is tuned by annealing under various temperatures (Ta) from 140 °C to 170 °C. It is found that when Ta < 160 °C, the lamella has a more flat-on like orientation. The spherulite size grows steadily with Ta. However, when Ta ≥ 160 °C, the morphology varies dramatically from spherulite-like to seaweed-like with lamella oriented edge-on. Depending on the microstructure, the electrical conduction characteristics of iPP ultrathin film changes, correspondingly. The conductivity is estimated from the long-term polarization current, which follows a hopping mechanism with a hopping distance about 1.5 nm and a critical field about 100 MV/m. It is proposed that the morphological changes with Ta result in more structural disorders, smaller inter-spherulite weak regions, and well-developed microstructure, which greatly suppress the electrical conductivity of iPP ultrathin films over one order. The present work reveals the essential role of multiscale morphology on the electrical property, which is of help for establishing the structure-property relation.