Molecular Weight Changes of Poly(Amic Acid)‐Polyimide Copolymer During Thermal Imidization Detected by Gel Permeation Chromatography

ABSTRACT Poly(amic acid) (PAA) is the precursor of polyimide (PI), and the imidization from PAA to PI is a highly complex process involving the formation of a series of PAA‐PI copolymers. The molecular weight changes during this process critically impact the resulting product performance. This study applied two distinct thermal imidization methods, that is, multi‐step and isothermal heating, to (6FDA‐DMB) PAA. The structural evolution, imidization degree (ID) and molecular weight variations were monitored by Fourier transform infrared (FT‐IR) spectroscopy and gel permeation chromatography (GPC). During multi‐step thermal imidization from 80°C to 350°C, ID gradually increased to 94.6%. Specifically, ID experienced the highest rate of increase between 120°C and 200°C. Regarding the isothermal imidization at 200°C, 250°C, and 350°C, the ID initially increased over time and then stabilized. Moreover, the higher the temperature, the faster the ID reached plateau, and complete imidization was achieved by curing at 350°C for 5 min. This study revealed a two‐stage evolution of molecular weight under both multi‐step and isothermal heating conditions. The molecular weight exhibited initial decrease followed by an increase, attributed to the decomposition and recombination of PAA chains. These findings provide deeper insights into molecular weight dynamics during PAA‐PI thermal conversion.