Revealing molecular mechanisms of colorless transparent polyimide films under photo-oxidation

Colorless transparent polyimide (CPI) films with high thermal stability, superior mechanical and dielectric properties are widely used in optoelectronic devices, in which the properties are degraded in service. In this work, the degradation in properties and molecular mechanisms of the CPI-L film with the trifluoromethyl group and CPI-T film with the alicyclic group under photo-oxidation are revealed. It is found that for CPI-L and CPI-T films after 168 h photo-oxidation, the mechanical properties and thermal stability are decreased, especially the yellowness index is increased by 740% and 530%, respectively. Consequently, the degradation of the CPI-L film is more severe than the CPI-T film after photo-oxidation. This is because that the transmission at 365 nm of CPI-L and CPI-T films before photo-oxidation is 51% and 64%, respectively. After photo-oxidation, the ether bond, imide ring and phenyl of both CPI films are broken, and the rearrangement of CN in the CPI-L film is observed. And eroded voids and/or microcracks caused by off-gassing appear at 120 h and 72 h, respectively. Furthermore, possible molecular mechanisms of CPI films during photo-oxidation are proposed, and the relationship between molecular mechanisms and properties degradation is elucidated. Moreover, the content of the CO is decreased with the increased depth of CPI-L and CPI-T films, indicating the enrichment of the CO on the surface. It is mainly because free radicals generated during photo-oxidation can combine with the atmospheric O2.