Study on the Diffusion Coefficients of Molecules in Precursor Varnishes and Solid Polyimides for Understanding the Polyimide Curing Process

Molecular diffusion in polyimides and their precursors, such as poly(amic acid) (PAA) varnishes, must be quantitatively analyzed to understand their reaction kinetics and improve their industrial production. Herein, the diffusion coefficients of dimethylacetamide (DMAc) and PAA in varnishes were measured using pulsed-field gradient nuclear magnetic resonance. The diffusion coefficient of DMAc decreased with increasing PAA concentration, and the dependency was stronger than expected, suggesting that DMAc diffusivity is influenced by the strong interaction between DMAc and PAA. The diffusion coefficient of PAA decreased with increasing PAA content due to the entanglement of polymer chains, as confirmed by viscosity measurements. The diffusion coefficient of DMAc in the polyimide matrix was measured (10–12–10–10 cm2 s–1) using capillary column inverse gas chromatography at 180–220 °C, which can be assumed as the infinite dilution condition. These fundamental data are crucial for understanding molecular diffusion during the imidization process.