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

Abstract

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 degrees C and 250 degrees C, with a peak temperature around 164 degrees C. This is attributed to the loss of water Juring the conversion of PAA to polyimide (PI). With increasing thermal cyclization temperatures from 25 degrees C to 200 degrees 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 degrees 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 degrees 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 degrees C. This is due to the extension and crystallization of the rigid P1 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. (C) 2000 Elsevier Science Ltd. All rights reserved.