Effect of Cobalt Oxide Nanoparticles on the Nano-scale Free Volume and Optical Properties of Biodegradable CMC/PVA Films

Chemically prepared cobalt oxide nanoparticles (NPs) were added to biodegradable carboxymethyl cellulose (CMC) (20%)/polyvinyl alcohol (PVA) (80%) blend films. The structural properties of these films were investigated by X-ray diffraction, Fourier transformation infrared spectroscopy (FTIR) and differential thermal analysis (DTA). Co₃O₄/(CMC + PVA) films show a semi-crystalline structure and the degree of crystallinity decreases with adding Co₃O₄ NPs. FTIR confirms the good interaction between the blend chains and Co₃O₄ NPs. The melting temperature T_m for CMC/PVA is about 212 °C then decreased with increasing Co₃O₄ content, due to some decrease in the ordered association of the blend which results in a decrease in the blend crystallinity. Positron annihilation lifetime spectroscopy (PALS) measurements were carried out with a conventional fast–fast coincidence spectrometer. It was found that ortho-positronium lifetime τ₃ and free volume V_f increase while o-Ps intensity I₃ still constant as Co₃O₄ concentration increases. This is because adding Co₃O₄ NPs encourages the formation of o-Ps. The swelling ratio SR% as a function of Co₃O₄ concentration was also studied and its value increases with Co₃O₄ addition. The optical study illustrated the decrease of the transmittance T of the films from 87 to 35.5% with increasing Co₃O₄ content from 0 to 0.9 wt%. Also, both the direct and indirect optical band gaps of the films red-shifted from 4.05 to 3.75 eV and 3.3 to 2.75 eV, respectively. The influence of Co₃O₄ on the refractive index of the films is also reported.