Effect of thermal history on the deformation and failure of polyimides

Thermal-processing structure-property relationships for polyetherimide (PEI ), poly(4,4'-oxydiphenylene pyromellitimide) (POPPI), and phenylethynyl-ter minated imide (PETI-5) composite matrices are reported from a fundamental p erspective. For thermoplastic PEI, deformation and failure depend primarily on free volume as evidenced by moisture-absorption, mechanical-property, a nd mass-density changes as a function of annealing. The deformation of POPP I can be divided into the following three regimes as a function of annealin g temperature: (1) physical aging-induced glassy state free-volume decrease s, (2) thermally activated microvoid collapse, and (3) chemical degradation . In the case of PETI-5, macroscopic defects, free volume, and polymer morp hology control deformation. The effects of residual crystallinity on deform ation are reported, and it is shown that mechanical toughness can be signif icantly decreased upon annealing below the glass-transition temperature. (C ) 2001 John Wiley & Sons, Inc.*.