Thermal cyclization of the poly(amic acid): thermal, mechanical, and morphological properties

Poly(amic acid) (PAA) was obtained by means of low-temperature solution polymerization of benzidine and pyromellitic dianhydride (PMDA). PAA exhibits a large endothermic peak between 110°C and 250°C, with a peak temperature around 164°C. This is attributed to the loss of water during the conversion of PAA to polyimide (PI). With increasing thermal cyclization temperatures from 25°C to 200°C, these endothermic peaks decrease drastically from 213.8 to 0.4 J/g. The endothermic peak finally became unobservable in the DSC thermogram for the heat treated sample at 250°C, due to the fully thermal heterocyclization. All the before- and after-heat-treated precursor films show a good thermal stability and a high weight residue % at 900°C in the range of 46–64%. The tensile strength and initial modulus of the precursor were 103 MPa and 5.40 GPa, respectively, and then gradually increased up to 178 MPa and 12.13 GPa with increasing thermal cyclization conditions. From SEM, we can see that the particle shape of the precursor changes from an irregular shape to a tetragonal crystalline phase with thermal cyclization above 150°C. This is due to the extension and crystallization of the rigid PI chain. The degree of crystallinity was determined to be between 1% and 43% with increasing thermal cyclization conditions. PAA showed a good solubility in strong acid as well as in aprotic solvents, such as DMAc, NMP, and DMSO, but heat-treated samples at higher temperatures showed a poor solubility in the same solvents.