Abstract
Hydrothermal polymerization (HTP) is a technique to synthesize highly crystalline polyimides in solely water. The process involves monomer salt intermediates also able to undergo other polymerizations during the HTP experiment: sub-hydrothermal polymerization (sHTP) and solid-state polycondensation (SSP).
Both processes yield semicrystalline polyimides at best. This is widely believed to result from irreversible bond-formation and the incorporation of conformational defects during chain growth.
Here, microwave-assisted HTP is used, allowing for controlling the heating time to the reaction temperature, and thus to strongly reduce sHTP. Moreover, we show that highest crystallinity can be obtained in the higher hydrothermal regime (approximate to 200 degrees C), and synthesize two polyimides of sufficient crystallinity to refine their crystal structures from powder X-ray diffraction data.
This study brings us one step closer to a general picture and rational design of the hydrothermal synthesis of polyimides.