Time-resolved synchrotron X-ray diffraction and infrared spectroscopic studies of imidization and structural evolution in a microscaled film of PMDA-3,4 '-ODA poly(amic acid)

The imidization behavior and structural evolution in a microscaled film of poly(3,4'-oxydiphenylene pyromellitamic acid) precursor are studied by time -resolved synchrotron wide-angle X-ray diffraction and infrared spectroscop y to investigate the relationship between thermal imidization and structura l evolution in the precursor. The precursor film displays only short-range order, but its polyimide film shows a crystalline structure based on an ort horhombic crystal lattice unit. When the precursor is heated at 2.0 degrees C/min, it undergoes imidization over the temperature range 124-310 degreesC through a two-step process: (i) decomplexation of the amide linkage from r esidual solvent molecules and other intra- and intermolecular amic acid gro ups and (ii) imide-ring closure. The maximum rate of imidization occurs at 148.4 degreesC. Anhydride rings are found to form transiently over the rang e 93-310 degreesC, which are attributed to the nature of the equilibrium be tween the precursor and its constituent anhydride- and amino-terminated spe cies. The imidization reaction begins prior to the commencement of structur al evolution. The structural evolution takes place over 132-380 degreesC as a three-step process: initiation, primary growth, and secondary growth. In particular, the initiation step requires at least 3.2% imidization. The st ructural evolution is further influenced by the short-range ordered structu re formed in the precursor film in the process of film formation. However, the overall crystallinity in the fully imidized film is limited to only 21. 4%.