Morphological effects on transparency and absorption edges of some semi-alicyclic polyimides

Abstract

Some novel semi-alicyclic polyimides based on epiclon and three aromatic diamines are investigated. The ellipsometry data reveal that the reduced polarizability provided by the alicyclic dianhydride moieties leads to polyimides with low refractive index, ranging from 1.64 to 1.72. The corresponding dielectric constants decrease from 2.97 to 2.71 as the polarization of the diamine part is lower. The transmittance is of appreciatively 90 % for polyimides containing methylene groups in the diamine residue. The cut-off wavelength decreases, as the backbone conjugation is smaller, being comprised between 305 nm and 317 nm — values comparable to that of a fully aliphatic PI. To obtain the optical energies describing absorption limits, the approach proposed by Tauc and Davis-Mott has been used, due to the similarity of the absorption edges. The values of optical band-gap are found from 3.51 to above 3.78 eV, indicating a low probability of optical absorption transitions and thus a good transparency. The Urbach and Tauc energies changed in the range of 480–806 meV and 1,453–2,786 meV, being smaller as the flexible sequence in the diamine moieties is longer. The novelty of this work consists in relating optical properties with morphological aspects, namely surface defects and entropy of morphology. These new results have a great impact in selecting the proper materials as liquid crystal alignment layers or as cell culture substrates.