Supramolecular Structural Variations in iPP/ZnO Nanocomposites: Effect on Optical and Dielectric Performance

This study investigates the impact of melt-cooling rate on the structural, optical, and electrical properties of isotactic polypropylene (iPP) nanocomposites containing wurtzite-type zinc oxide (ZnO) nanocrystals. The nanocomposites were synthesized via a solution-blending and hot-pressing method, followed by cooling under three distinct modes: slow cooling, ice-water quenching, and liquid-nitrogen quenching. X-ray diffraction (XRD) analysis revealed that slow cooling resulted in the formation of both α- and γ-phases in the iPP matrix, whereas rapid cooling produced only the α-phase. Atomic-force-microscopy analysis showed pronounced morphological differences among samples, with slow-cooled films exhibiting rougher surfaces. The findings demonstrate that controlled melt-cooling rate governs supramolecular structural evolution and interfacial ordering in iPP/ZnO nanocomposites. The increased photoluminescence (PL) in slow-cooled samples is linked to the presence of the γ-phase, which promotes interactions between polymer structural defects and deep-level defects in ZnO crystals. Conversely, dielectric results showed that slow cooling lowered interfacial polarization owing to restricted chain mobility, consistent with the Maxwell-Wagner-Sillars (MWS) theory. These results highlight a distinct process-structure-property relationship and indicate that precise regulation of cooling rate can simultaneously adjust optical and dielectric performance. This work highlights the innovative role of processing-induced supramolecular modification and its importance in the design of tunable materials for optoelectronics, flexible-dielectric components, UV-active coatings, sensing, and advanced packaging applications.