Molecular structure and thickness dependence of chain orientation in aromatic polyimide films

Degrees of orientation of main chains and imide rings were quantitatively estimated for spin‐coated films of six kinds of aromatic polyimides (PIs) using polarized attenuated total reflection (ATR)/Fourier transform infrared (FT‐IR) spectroscopy. The degrees of chain orientation parallel to the film planes are significantly larger for the PIs having rigid structures than those having flexible structures, and the introduction of side groups decrease the degrees of chain orientation. In contrast, the rotational orientations of imide rings are almost isotropic for all PI films. Moreover, the film thickness dependences of the degrees of orientation were investigated for two kinds of rigid‐rod PIs having bulky trifluoromethyl (CF3) side groups in their diamine moieties. The degrees of chain orientation slightly decrease as the film thickness increases, whereas the rotational orientation of imide rings is independent of the film thickness. The degrees of chain orientation on the substrate sides significantly differ from the atmospheric sides of PI films. This difference was generated during thermal imidization because of tensile stress originated from the mismatch in thermal expansion coefficients between the substrates and the PI films. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2109–2120, 2005