Influence of molecular weight on the mechanical performance of a thermoplastic glassy polyimide

Mechanical testing of an advanced thermoplastic polyimide (LaRC(TM)-SI) wit h known variations in molecular weight was performed over a range of temper atures below the glass transition temperature. The physical characterizatio n, elastic properties and notched tensile strength were all determined as a function of molecular weight and test temperature. It was shown that notch ed tensile strength is a strong function of both temperature and molecular weight, whereas stiffness is only a strong function of temperature. A criti cal molecular weight (M-c) was observed to occur at a weight-average molecu lar weight ((M) over bar (w)) of similar to 22000 g/mol below which, the no tched tensile strength decreases rapidly. This critical molecular weight tr ansition is temperature-independent. Furthermore, inelastic analysis showed that low molecular weight materials tended to fail in a brittle manner, wh ereas high molecular weight materials exhibited ductile failure. The micros tructural images supported these findings. (C) 2000 Kluwer Academic Publish ers.