Integrated real time studies to track all physical and chemical changes in polyimide film processing from casting to imidization.

摘要

Physical and chemical changes during the complex multi-step thermal imidization reaction were investigated including all processing steps (solution casting, drying and imidization), using newly developed highly instrumented measurement systems. These instruments allowed us to observe the dynamic relationship between the bound solvent evaporation that causes relaxation and chain orientation during the imidization.;Drying and imidization of PMDA-ODA solutions in NMP were investigated by a novel custom designed measurement system that tracks real time weight, thickness, surface temperature, in-plane and out-of-plane birefringence. At low temperature drying stage (T < 120°C), the weight and thickness reductions occurred rapidly as a result of solvent evaporation. All the parameters started leveling off while the out of plane birefringence steadily increased and reached a plateau at longer drying times. When the temperature was increased for imidization reaction (T=200°C), additional weight loss accompanied by temporary reduction of birefringence was observed due to evaporation of bound solvent as solvent molecules decomplexed from the polymer chains and plasticized the film. During the latter stage, out-of-plane birefringence rose rapidly as the polymer chains increasingly became oriented with their chain axes were preferentially oriented in the film plane. Throughout the whole process the in-plane birefringence remained zero. For the first time, these real time measurements allowed us to quantitatively show the dynamics between chain relaxation due to evaporation of the decomplexed solvent molecules, and orientation development due to decreased chain mobility caused by imidization reaction and increasing Tg for the PMDA-ODA/NMP solutions. In addition, the dynamics of this interplay was investigated by varying the processing conditions: initial casting thickness and drying temperature.;Chemical conversion, bound solvent and chain orientation that take place during thermal imidization of uniaxially constraint PMDA-ODA polyamic acid precursor film was investigated up to 200°C using real time measurement system that combines true stress, true strain, in-plane birefringence and temperature with polarized ultra-rapid scan polarized FT-IR spectrometry (URS-FT-IR). Upon heating, initially isotropic solution cast film developed stress and birefringence from the beginning while the solvent is decomplexed and evaporated. At a critical temperature (~130°C) onset of imidization reaction was observed as stress going through a maximum. Beyond this point, the evaporation and conversion took place simultaneously with steady increase in birefringence. At the end of the experiment, 80% conversion is achieved with 3% bound solvent remaining in the system.