Diffusion-limited kinetics modeling of one-step polyimide formation

A diffusion-limited kinetic model was developed to describe the imidization of one-step polythioetherimide formation based on an endgroup diffusion mo del. The changes of conversion and viscosity during the imidization were mo nitored with thermogravimetric analysis and dynamic stress rheometry, respe ctively. It was observed that the imidization rate began to decelerate afte r a fast early stage, whereas the viscosity in the system increased dramati cally after a period of low value. Amic acid and imide formations concurren tly take place in the one-step polyimide formation, but the formation of am ic acid is much slower than that of imide and is the rate-limiting step of imidization. When a second-order kinetic model was used to describe the imi dization, the effect of viscosity on the diffusion resistance of reactive g roups needed to be included. In order to predict the change of viscosity du ring the imidization, the Lipshitz-Macosko model was modified and introduce d into the diffusion-limited kinetic model by the Stokes-Einstein equation. The comparison of the modeled results with experimental data indicated tha t the diffusion-limited kinetic model and the modified Lipshitz-Macosko mod el were able to efficiently predict the changes of conversion and viscosity with temperature and time during the one-step polythioetherimide formation . (C) 2001 John Wiley & Sons, Inc.