Microdeformation in thin films of 3FDA/PMDA polyimide and polyimide nanofoams

Transmission electron microscopy has been used to investigate the microdeformation behavior of thermally imidized thermoplastic pyromellitic dianhydride/1,1‐bis(4‐amino‐phenyl)‐1‐phenyl‐2,2,2 trifluoroethylene (3FDA/PMDA) polyimide films with a Tg of ∼ 440°C, prepared by solution casting of a polyamic ester precursor. Failure of the films at room temperature was by unstable cracking at about 5% strain, accompanied by homogeneous shear deformation at the crack tips. As the temperature was raised to above 100°C, zones of mixed shear and crazing were observed, and a stick‐slip mode of cracking. Above about 300°C shear was once again the dominant deformation mechanism and the films became fully ductile. In films containing porosity on a scale of a few nanometers, prepared by thermal degradation/imidization of a 3FDA/PMDA/poly α‐methyl styrene graft copolymer, film failure at room temperature was also by unstable cracking, but a zone of multiple craze‐like features was observed at crack tips, rather than a single shear deformation zone. The increase in extent of this zone of craze‐like features as the temperature was raised was again associated with an increase in crack stability. ©1995 John Wiley & Sons, Inc. Copyright © 1995 John Wiley & Sons, Inc.