Sum-frequency generation (SFG) vibrational spectroscopy of side alkyl chain structures of polyimide surfaces

Sum-frequency generation (SFG) vibrational spectroscopy was used to study how side alkyl chains of a polyimide are oriented at the air–polymer interface and how they are affected by mechanical rubbing and adsorption of a liquid crystal monolayer. The spectra of polyimides with various alkyl chain lengths all indicate that the chains protrude out of the polyimide surface with a very broad distribution. Longer alkyl chains appear to contain more gauche defects. While the polyimide backbones are aligned by rubbing, the side chain orientation is hardly affected. Adsorption of a liquid crystal monolayer on the surface significantly reduces the gauche defects in the alkyl chains.