Vapor deposition and curing of polybenzoxazole precursors

Here we report on the vapor deposition polymerization of polybenzoxazole precursors. 3,3'-Dihydroxybenzidine and pyromellitic dianhydride were codeposited onto flat substrates to form poly(amic acid) films. As-deposited films contained poly(amic acid) species as well as unreacted dianhydride and diamine reactants. Quality films were obtained when reactant fluxes were well balanced. Coatings were cured under inert gas conditions and resulted in the conversion to polyimides (150-250 degrees C) and, at higher temperatures (500-550 degrees C), to semiaromatic polybenzoxazoles. Physical and chemical changes occurring during the curing process were studied with FT-IR, TGA, and nanoindentation experiments. The fully cured material is brittle and hard, and for thin films (< 1 mu m), films do not delaminate from glass substrates. Nanoindentation studies of both solution-prepared and vapor-deposited PBO precursors revealed that the elastic modulus improved upon conversion to the imide and benzoxazole forms, and polyimide showed the highest hardness value compared with poly(amic acid) and PBO.