Precise thickness control of solution-processed alpha-polypropylene thin film in sub-micron range and its morphological characterization

Isotactic polypropylene (iPP) is a widely used electrical material with excellent insulating, mechanical, thermal and eco-friendly properties, which has drawn much attention in recent years. Establishing the relationship between their multi-scale microscopic structures (from molecular level, lamella, to spherulite) and the electrical behaviour (carrier transport) plays a crucial role in the study and application of iPP-based materials. In the present paper, a solution-processed approach was proposed to fabricate very pure, thin and uniform iPP films. With the carefully optimized parameters, i.e., solution concentration, spin acceleration, spin speed and spin time, the film thickness is precisely controlled within submicron range and measured by using an ellipsometry. The quality of the prepared film is manipulated conveniently by the heat treatment at various temperatures (140°C-170°C) and durations (15 min-1 h). Their crystal structure, chemical groups and morphological evolution after heat treatments were studied in detail by XRD, infrared spectroscopy (FTIR) and atomic force microscopy (AFM), respectively, which further confirmed the versatility and reliability of the proposed approach. Compared with the conventional hot-pressed iPP films with a thickness over tens of microns, the advantages of sub-micron iPP films lie in 1) much less induced impurities, 2) precise thickness control, 3) clear dependence of morphology on annealing temperature and time, all of which indicates a promising opportunity for a deeper study of the iPP-based materials.