*2.4. Characterization*

The surface morphologies of SiPs were observed under a 3D laser scanning confocal microscope (LSCM) model VK-X 100 (Keyence Corporation, Osaka, Japan) under prescribed conditions of laser: red semiconductor laser, λ = 658 nm, 0.95 mW, and pulse width of 1 ns, using a depth composition procedure.

The morphology of the fractured composites and dispersion state of the hybrid filler in the PP matrixes were investigated using a scanning electron microscope equipped with energy dispersive spectroscopy (SEM-EDS) (JCM 6000, JEOL Ltd., Tokyo, Japan). SEM images were obtained at 15 kV accelerating voltage and the PP composite samples were fractured under liquid nitrogen. Each sample was deposited on carbon tape and carbon-coated for 90 s before observation.

The optical properties of the PP composite films were determined by UV–Vis spectra. A rectangular piece of each film sample (4 cm × 4 cm) was directly mounted between two spectrophotometer magnetic cell holders. The transmittance spectra of the films were measured at selected wavelength ranges from 190 to 1000 nm using a UV–Vis spectrophotometer (GENESYS 50, Thermo Fisher Scientific, Waltham, MA, USA). The optical properties of the PP and PP composite films were characterized by the transmittance of visible (660 nm) regions.

Chemical analysis was carried out using a Fourier Transform Infrared (FT-IR) spectroscope Nicolet iD7 ATR (Thermo Fisher Scientific, Waltham, MA, USA). Each sample recording consisted of 16 scans recorded from 4000 to 400 cm<sup>−</sup>1.

The PP composite films were cut into a rectangular shape of 40 mm × 5 mm × 0.5 mm. The mechanical properties of each PP composite were determined by an IMC-18E0 model machine (Imoto Machinery Co., Ltd., Kyoto, Japan) at a rate of 10 mm/min crosshead speed at 23 ◦C. Each measurement was carried out with five replicates.

The thermal properties of PP composites were characterized using an EXSTAR TG/DTA 7200 (SII Nanotechnology Inc., Chiba, Japan). The samples were scanned at a range of 30 to 550 ◦C at a constant heating rate of 10 ◦C/min under a continuous nitrogen flow rate of 100 mL/min.
