In situ infrared spectroscopy study on imidization reaction and imidization-induced refractive index and thickness variations in microscale thin films of a poly(amic ester)

Poly(amic ester) (PAE) is a soluble precursor of polyimide that has attracted interest from both the microelectronic and the flat-panel display industries because of its several important advantages, including excellent solubility, high hydrolytic stability, and solvent-free film formation, over the polyimide precursor, poly(amic acid), for which monomer-polymer equilibration always occurs in solution due to its carboxylic acid groups. In this study, poly(3,4'-oxydiphenylene pyromellitamic diethyl ester) (PMDA-3,4'-ODA PAE) was chosen as a PAE precursor, and its thermal imidization behavior in microscale thin films was investigated quantitatively for the first time using time-resolved infrared (IR) spectroscopy. In addition, the variations of the film refractive index and thickness with temperature and time were determined in detail from the time-resolved IR spectra and are fully interpreted in this paper by considering the imidization kinetics of the precursor.