CURING STUDY OF A PREIMIDIZED PHOTOSENSITIVE POLYIMIDE

The effects of curing on removal of solvent and the development of che mical crosslinks were studied in a preimidized, intrinsically photosen sitive polyimide. The polymer, Probimide 412, was in solution of gamma -butyrolactone (GBL), and spun cast or doctor bladed to form samples f or analysis. The films were systematically studied from soft (100 degr ees C) to hard bake (400 degrees C), as the effects of cure environmen t (i.e., air vs. nitrogen) and UV exposure were monitored. The samples were characterized by thermogravimetric analysis (TGA), Fourier trans form infrared spectroscopy (FTIR), and dynamic mechanical analysis (DM A). The index of refraction was measured by waveguide propagation mode determination. TGA results show that there is a distribution of solve nt removal temperatures, probably due to polymer-solvent binding. Solv ent is removed from the polymer as it is thermally cured from similar to 150 degrees C to 300 degrees C, and UV curing aids in the removal o f solvent. Chemical crosslinks induced by both thermal and UV curing i ncrease the refractive index and darken the material. Crosslinking bro adens the glass transition and reinforces the rubbery modulus. FTIR re sults show that thermally induced crosslinking occurs in the polymer a t 400 degrees C in nitrogen and at 300 degrees C in air but does not o ccur appreciably at 300 degrees C in nitrogen or at any lower temperat ure. Ultraviolet curing also crosslinks the polymer and is caused by h ydrogen abstraction from alkyl substituents and subsequent crosslinkin g at the benzophenone carbonyl group. During thermal curing, although there is evidence from FTIR spectroscopy that a similar reaction may o ccur, there are probably multiple reactions simultaneously taking plac e.