The mechanism of polyimide pyrolysis in the early stage

Pyrolysis mechanism of Kapton-type polyimide was investigated with special attention to the early stage. Two approaches were employed to reveal the pyrolysis reactions: one is an analysis of evolved compounds, the other is a hydrolysis method that selectively converts polyimide char into low-molecular-weight compounds. In the initial step of pyrolysis, bond cleavage around imide ring and following hydrogen transfer generates various intermediates on aromatic segments in polymer chains. It was derived from the quantitative analysis by the hydrolysis method that new linkages between the intermediates are formed efficiently instead of the cleaved bonds. Solid C-13-NMR study showed that the packing structure of polyimide molecules is little changed by heat-treatment at the starting temperature of pyrolysis, though the structure is gradually disintegrated by reconstruction reactions at higher temperatures. It was also found that the thickness of sample films affects the weight loss and amounts of evolved compounds, while the difference of graphitizability has no appreciable effect.