The failure processes, morphology and mechanical properties of a co-polyimide glass based on benzophenone tetracarboxylic acid dianhydride

Abstract

The phyiscal structure, failure processes and mechanical properties of solution-soluble copolyimide films based on benzophenone tetracarboxylic acid dianhydride are reported as a function of sample preparation. The failure processes and mechanical response are modified by the presence of residual solvent and microvoids, which are produced by the elimination of solvent clusters from the glass. The polyimide is amorphous, with the exception of a few isolated clusters of poorly formed spherulites and networks of 50 to 500 nm wide lamellae. The deformation modes observed when thin films were strained directly in the electron microscope were crazing, shear-band deformation and an edge-yielding phenomena. Edge-yielding, which has characteristics of both crazing and shear-banding, occurred in ∼1 μm wide bands which were 20 to 30° to the tensile stress direction. Shear-band deformation occurred in fine ∼-100 nm wide bands, which exhibited a sharp boundary between themselves and their surroundings. TEM indicated that the shear strain was uniform within these bands. Microvoids, 1.5 to 15 nm diameter, were found to initiate shear bands some of which were ∼ 1 nm wide. These bands increased in width by tearing at the microvoid initiation sites.