Theoretical and experimental studies on the mechanism of coloration of polyimides

The mechanism of coloration of polyimides is investigated theoretically and experimentally. Since light is considered to be absorbed by polyimides via charge-transfer excitation, we used the long-range-corrected time-dependent density functional theory recently developed by Tawada et al. [Y. Tawada, T. Tsuneda, S. Yanagisawa, T. Yanai, K. Hirao J. Chem. Phys.2004, 120, 8425] for the calculation of excitation energies and oscillator strengths. Classical molecular dynamic simulations for packed chain models of polyimides were also performed to analyze the structural information of polyimides in condensed phase. In order to predict the transparency of polyimide film, we developed a theoretical method by combining the results of electronic structure calculations and those of molecular dynamics simulations. We compare our theoretical results with experimental ones and discuss the difference between them. As a result, we clarify the new mechanism of coloration and obtain results for the theoretical UV/Visible spectra.