Effects of Prior Aging at 288°C in Air and in Argon Environments on Creep Response of PMR-15 Neat Resin

The creep behavior of PMR-15 neat resin, a polyimide thermoset polymer, aged in air and in argon environments at 288 degrees C for up to 1000 h was evaluated. Creep tests were performed at 288 degrees C at creep stress levels of 10 and 20 MPa. Creep periods of at least 25-h in duration were followed by 50-h periods of recovery at zero stress. Prior isothermal aging increased the elastic modulus and significantly decreased the polymer's capacity to accumulate creep strain. The aging environment had little influence on creep and recovery behaviors. However, aging in air dramatically degraded the tensile strength of the material. Dynamic mechanical analysis revealed an increase in the glass transition temperature from similar to 330 degrees C to similar to 336 degrees C after 1000 h in argon or in air at 288 degrees C. The rise in the glass transition temperature with aging time is attributed to an increase in the crosslink density of the PMR-15 polyimide. Increase in the crosslink density due to aging in both air and argon environments is likely behind the changes in the elastic modulus and the decreased capacity for inelastic straining. A visibly damaged surface layer of similar to 0.16 mm thickness was observed in specimens aged in air for 1000 h. Results indicate that the unoxidized core material governs the overall mechanical response, whereas the oxidized surface layer causes a decrease in tensile strength by acting as a crack initiation site and promoting early failures. (C) 2008 Wiley Periodicals, Inc.(dagger) J Appl Polym Sci 111: 228-236, 2009