Polyimide molecular chains alignment in polarized pulsed laser-induced periodic surface structures

Periodic nano-lines in a large area have been formed on polyimide surface u sing polarized pulsed laser-inducing of the fourth harmonic Nd : YAG laser. The linear direction is parallel to the polarization direction and the per iodical width of these lines is 200 nm. The dichroism due to the alignments of functional group in periodic nano-structure has been studied by recordi ng polarized reflected FTIR spectra at 0 degrees polarization parallel to t he polymer lines and 90 degrees polarization lines. We found that C = O str etching at 1 722 cm(-1) in the irradiated area has a stronger response than in the nonirradiated area when the IR polarization direction is parallel t o the lines, while the band at 1 231 cm(-1) assigned to C-C-C between pheny lenes and the methylene groups has a stronger response when the polarized I R direction is perpendicular to the lines. The dichroic spectrum indicates that in the irradiated area polyimide molecular chains are preferentially o riented 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 ar e subjected to the orientation in the electric field direction.