The conformation of a rubbed polyimide

We report second harmonic generation (SHG) measurements of rubbed and unrub bcd surfaces of the fluorinated main-chain polyimide PMDA/BDAF made from py romellitic dianhydride (PMDB) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluo ropropane (BDAF). Such rubbed polymers are commonly used as alignment mater ials for the nematic phases of liquid crystal display cells (LCD), although the rubbing process itself is not vet well understood. The SHG results are used to construct the full orientational distribution function of the nonc entrosymmetric part of the polymer. This allows us to determine the preferr ed conformations of the polyimide at the surface and gives insight into the conformational changes of the polymer during rubbing.