Nanoindentation and optical properties of Poly(4,4′-oxydiphenylene p-phenylene pyromellitimide) copolyimide thin films according to the p-Phenylene diamine content

Poly(p-phenylene pyromellitimide) (PMDA-PDA), poly(oxydiphenylene pyromellitimide) (PMDA-ODA), and poly(4,4′-oxydiphenylene-p-phenylene pyromellitimide) random copolyimide thin films with different p-phenylene diamine (PDA) contents were prepared. Nanoindentation was used to characterize the mechanical properties (hardness and modulus), and a prism coupler was used for measuring the optical properties (refractive index and birefringence). The hardness and modulus were calculated from curves of the nanoindentation load versus the displacement. The effect of the PDA content on the hardness and modulus was studied. The hardness of the polyimide thin films varied from 0.248 to 0.613 GPa, and the modulus varied from 3.78 to 6.75 GPa at a load of 0.127 mN. The hardness and modulus increased with increasing PDA content, whereas the penetration depth and plastic deformation decreased. As the load increased, the penetration depth increased. The hardness of PMDA-ODA films remained constant, whereas that of PMDA-PDA and PMDA-ODA/PDA films decreased with increasing load. The in-plane refractive index varied from 1.7219 to 1.8244, and the out-of-plane refractive index varied from 1.6390 to 1.5827, as a function of the PDA content. The birefringence varied with the PDA content from 0.0829 to 0.2417. The morphological structure of the prepared polyimide thin films was investigated with wide-angle X-ray diffraction. The mechanical properties and optical properties of the polyimide thin films were strongly dependent on the changes in the morphological structure, which originated from the variation of the composition.