The combined influence of molecular weight and temperature on the physicalaging and creep compliance of a glassy thermoplastic polyimide

The effect of molecular weight on the viscoelastic performance of an advanc ed polymer (LaRC(TM)-SI) was investigated through the use of creep complian ce tests. Testing consisted of short-term isothermal creep and recovery wit h the creep segments performed under constant load. The tests were conducte d at three temperatures below the glass transition temperature of five mate rials of different molecular weight. Through the use of time-aging-time sup erposition procedures, the material constants, material master curves and a ging-related parameters were evaluated at each temperature for a given mole cular weight. The time-temperature superposition technique helped to descri be the effect of temperature on the timescale of the viscoelastic response of each molecular weight. It was shown that the low molecular weight materi als have higher creep compliance and creep rate, and are more sensitive to temperature than the high molecular weight materials. Furthermore, a critic al molecular weight transition was observed to occur at a weight-average mo lecular weight of (M) over bar (w) similar to 25,000 g/mol below which, the temperature sensitivity of the time-temperature superposition shift factor increases significantly. The short-term creep compliance data were used in association with Struik's effective time theory to predict the long-term c reep compliance behavior for the different molecular weights. At long times cales, physical aging serves to significantly decrease the creep compliance and creep rate of all the materials tested. Long-term test data verified t he predictive creep behavior. Materials with higher temperature and lower m olecular weights had greater creep compliance and higher creep rates.