PLASTIC-DEFORMATION MECHANISMS IN POLYIMIDE RESINS AND THEIR SEMIINTERPENETRATING NETWORKS

The plastic deformation mechanisms in both thermoplastic and thermoset polyimide resins and their semi-interpenetrating networks (semi-IPNs) were investigated. The fundamental tendency to undergo strain localization (crazing and shear banding) as opposed to a more diffuse (or homogeneous) deformation in these polymers was evaluated. In situ polarized light microscopic observation of crack-tip deformation mechanisms in solvent-cast films was conducted with a miniature testing device attached to the microscope stage. NASA LaRC TPI, a thermoplastic polyimide, was found to exhibit combined shear yielding and crazing under plane-stress loading conditions. Contrarily, NR-150B2 thermoplastic polyimide exhibited diffuse shear yielding; no shear banding or crazing was found near the crack tip. Adding a small amount of thermoplastic component, either NR-150B2 or LaRC TPI, was found to raise considerably the fracture toughness of PMR-15 thermoset PI. PMR-15 thermoset films also showed diffuse shear yielding, albeit with a much smaller deformation zone. This was consistent with its low toughness and could be understood on the basis of a limited extensibility of a highly crosslinked network. Numerical calculations were performed to confirm this low value of network chain draw ratio. The dimensions of the deformation zone ahead of the crack tip in a semi-IPN with a thermoset matrix were increased as a higher weight fraction of the thermoplastic component was added. The deformation zone size of a thermoplastic matrix was found to decrease with an increasing amount of thermoset PMR-15. Deformation also became more diffuse with a higher PMR-15 content in LaRC TPI. Fracture toughness variations can be correlated with deformation size changes in these semi-IPNs.