Carbonization and graphitization of BPDA/PDA polyimide films: effect of structure of polyimide precursor

Abstract

Poly(amide acid) was prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) and was then reacted with NaH and various kinds of alkyl iodides to transform it into its alkyl esters. The cast films were imidized as fixed on glass substrates to give polyimide films. The tensile modulus of polyimide films prepared from the methyl ester precursor was lower than that from poly(amide acid), but it increased with increasing size of the alkyl group. The polyimide films were carbonized by heating to 900°C, and then further heated to 2800°C for graphitization. Their degrees of graphitization and orientation of graphite crystallites as a function of weight loss at imidization were studied by measurements of X-ray diffraction at room temperature and magnetoresistance at liquid nitrogen temperature. Both measurements clearly indicate that the graphitized films prepared from poly(amide ester) have high degrees of graphitization. It was also made clear that the orientation of the graphitized films prepared from poly(amide ester) with small alkyl groups is lower than that from poly(amide acid), but the orientation increased with increase in the size of the alkyl group.