Abstract: Periodic nano-lines in a large area have been formed on polyimide surface using polarized pulsed laser-inducing of the fourth harmonic Nd:YAG laser. The linear direction is parallel to the polarization direction and the periodical width of these lines is 200 nm.The dichroism due to the alignments of functional group in periodic nano-structure has been studied by recording polarized reflected FTIR spectra at 0° polarization parallel to the polymer lines and 90 °polarization lines. We found that C=Ostretching at 1722 cm^-1in the irradiated area has a stronger response than in the nonirradiated area when the IR polarization direction is parallel to the lines, while the band at 1231 cm^-1assigned to C-C-Cbetween phenylenes and the methylene groups has a stronger response when the polarized IRdirection is perpendicular to the lines. The dichroic spectrum indicates that in the irradiated area polyimide molecular chains are preferentially oriented perpendicular to the nano line direction. The orientation is caused by the directional electric field associated with the polarized laser beam during the melting and cooling process, while the polar carbonyl groups are subjected to the orientation in the electric field direction.

Key words: Polyimide, Na:YAG laser, Molecular alignment, Nano-meter microstructure