MICRODEFORMATION IN THIN-FILMS OF 3FDA PMDA POLYIMIDE AND POLYIMIDE NANOFOAMS/

Transmission electron microscopy has been used to investigate the micr odeformation behavior of thermally imidized thermoplastic pyromellitic dianhydride/1,1-bis(4-aminophenyl)-1- phenyl-2,2,2 trifluoroethylene (SFDA/PMDA) polyimide films with a T-g of similar to 440 degrees C, pr epared by solution casting of a polyamic ester precursor. Failure of t he films at room temperature was by unstable cracking at about 5% stra in, accompanied by homogeneous shear deformation at the crack tips. As the temperature was raised to above 100 degrees C, zones of mixed she ar and crazing were observed, and a stick-slip mode of cracking. Above about 300 degrees C shear was once again the dominant deformation mec hanism and the films became fully ductile. In films containing porosit y on a scale of a few nanometers, prepared by thermal degradation/imid ization of a 3FDA/PMDA/poly alpha-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 th an a single shear deformation zone. The increase in extent of this zon e of craze-like features as the temperature was raised was again assoc iated with an increase in crack stability. (C) 1995 John Wiley and Son s, Inc.