Absorption, fluorescence, and thermal properties of transparent polyimides based on cyclobutanetetracarboxylic dianhydride

Ultraviolet/visible absorption spectra and fluorescence spectra of transparent polyimides derived from 1.2: 3,4-cyclobutanetetracarboxylic dianhydride (CBDA) with an aromatic diamine, 4,4'-diaminodiphenylmethane (DPM), a fluorinated diamine, 2,2-bis(4-diaminodiphenyl)hexafluoropropane (6FdA), and an alicyclic diamine, 4,4'-diaminodicyclohexylmethane (DCHM) were investigated, compared with their model compounds. Both monomer and excimer fluorescence were observed in concentrated CH2Cl2 solution of model compounds, suggesting head to tail intermolecular overlap structures at the excited state. All polyimide films derived from CBDA showed high transparency in the visible region, i.e., no absorption tail was found above 375 nm. Especially, PI(CBDA/DCHM) showed the highest transparent property with almost no absorption above 310 nm. Absorption shoulders at 235 nm of PI(CBDA/DPM) and PI(CBDA/6FdA) are due to the absorption of phenylene groups in the diamine moieties. The fluorescence spectra of all polyimide films studied showed the existence of ground-state complexes. Their excitation wavelength dependence would be due to the existence of various conformations of packing structure. The glass transition temperatures, T-g, of these polyimides based on CBDA lie in the region of 277 degrees C to 360 degrees C. These polyimides have decomposition temperatures around 450 degrees C under deaerated conditions.