Surface studies of polyimide thin films via surface-enhanced Raman scattering and second harmonic generation

Two polyimides having the same backbone chemical structure and different pe ndant side groups at the 2- and 2'-positions of the diamine, the six methyl ene units capped with 4-cyanobiphenyl end groups and trifluoromethyl, were synthesized (6FDA-6CBO and 6FDA-PFMB). Surface-enhanced Raman scattering an d surface optical second harmonic generation measurements show that after r ubbing the major change in 6FDA-PFMB surface appears in the orientation of the dianhydride, which was originally planar, but becomes tilted with respe ct to the surface plane. In the case of GFDA-6CBO, rubbing also causes the originally planar 4-cyanobiphenyls to tilt away from the surface and assume an azimuthally anisotropic distribution.