Transparency and absorption edges of disiloxane modified copolyimides

Optical characteristics of two novel copolyimides prepared from an alicyclic or a fluorine-based dianhydride combined with an aliphatic siloxane-based diamine and an aromatic one containing ether linkages, are compared. Ellipsometry data show that the reduced polarizability, provided by the aliphatic moieties, leads to copolyimides with low refractive index (1.53–1.57). The corresponding dielectric constants decrease from 2.71 to 2.57 as the polarization of the dianhydride unit is reduced by the presence of CF3 groups. The transmittance is of approx. 90% and the cut-off wavelengths are comprised between 336 and 380 nm. The values of optical band-gap are higher for the fluorine-based copolyimide, namely 3.00 eV comparatively to 2.34 eV for the sample with alicyclic dianhydride sequences, indicating a lower probability of optical absorption transitions and thus a better transparency. The Urbach and Tauc energies changed in the range of 2295–2809 meV and 893–1161 meV, being smaller as the polarizability is diminished by the presence of CF3 groups. Optical properties are correlated with morphological aspects, namely surface roughness parameters and entropy of morphology, which varies in agreement with the refractive indices of copolymers.