Curing study of a preimidized photosensitive polyimide

The effects of curing on removal of solvent and the development of chemical crosslinks were studied in a preimidized, intrinsically photosensitive polyimide. The polymer, Probimide 412, was in solution of γ-butyrolactone (GBL), and spun cast or doctor bladed to form samples for analysis. The films were systematically studied from soft (100°C) to hard bake (400°C), as the effects of cure environment (i.e., air vs. nitrogen) and UV exposure were monitored. The samples were characterized by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA). The index of refraction was measured by waveguide propagation mode determination. TGA results show that there is a distribution of solvent removal temperatures, probably due to polymer-solvent binding. Solvent is removed from the polymer as it is thermally cured from ∼150°C to 300°C, and UV curing aids in the removal of solvent. Chemical crosslinks induced by both thermal and UV curing increase the refractive index and darken the material. Crosslinking broadens the glass transition and reinforces the rubbery modulus. FTIR results show that thermally induced crosslinking occurs in the polymer at 400°C in nitrogen and at 300°C in air but does not occur appreciably at 300°C in nitrogen or at any lower temperature. Ultraviolet curing also crosslinks the polymer and is caused by hydrogen abstraction from alkyl substituents and subsequent crosslinking at the benzophenone carbonyl group. During thermal curing, although there is evidence from FTIR spectroscopy that a similar reaction may occur, there are probably multiple reactions simultaneously taking place.