Photodegradation of Polyimides .5. An Explanation of the Rapid Photolytic Decomposition of a Selected Polyimide via Anhydride Formation

The photolytic decomposition in air of a polyimide film based on a dianhydride and a diarylamine with hexafluoroisopropylidene 6F bridging groups is extremely rapid, leading to efficient chain cleavage and subsequent photooxidative decomposition. Only relatively short photolysis times with an unfiltered medium-pressure mercury lamp are required to give a clean photoablation of the 6F-6F polyimide films in air. IR difference spectroscopy shows the appearance of anhydride groups on photolysis in air, indicating a photooxidation process via decomposition of the arylimide linkage. A photophysical and photochemical analysis of several model N-arylphthalimides indicates that both solvent polarity and electron-withdrawing/donating substituents can greatly alter the efficiency of the photolysis process. The formation of the triplet state is also dependent on the substituents and solvent polarity. In general, an increase in the CT character of the excited state by either substituent or solvent effects leads to a reduction in both the photolysis and intersystem crossing quantum yields