A kinetic study of the mechanism of the solid-state reaction between pyromellitic dianhydride (PMDA) and oxydianiline (ODA)

A kinetic study of the solid-state reaction between the polyimide precursor monomers PMDA and ODA has been carried out in the temperature range between -50 and +23 degrees C, using molecular beam deposition (MBD) for film growth and transmittance infrared spectroscopy (FTIR) for reaction monitoring. The product of this reaction consists mainly of dimers. Only 50% of the anhydride groups (two per PMDA molecule), contained in the film resulting from the stoichiometric codeposition of the monomers PMDA and ODA, form amide links by the end of the low-temperature reaction. The very low mobility of the reaction products below room temperature results in cessation of the solid-state reaction, when all PMDA monomers have reacted with only one of their anhydride rings. Formation of amine carboxylate complexes proceeds in parallel with amic acid formation. The measured values for the activation energy of the solid-state reaction between PMDA and ODA lie between 48 and 53 kJ/mol, which indicates that diffusion of monomers through the bulk of the film is the rate limiting step of this reaction.