Plastic deformation mechanisms in polyimide resins and their semi‐interpenetrating 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 yeilding; 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.