Prediction of long-term service performance of polymeric materials from short-term tests: Creep and prediction of the stress shift factor of a longitudinal polymer liquid crystal

The material studied is a longitudinal polymer liquid crystal (PLC). The cr eep behavior of the PLC is examined in the region of nonlinear viscoelastic ity. The creep compliance D curves at nine different stress sigma levels, f rom 10 to 50 J.cm(-3) at a constant temperature are determined and shifted along the log time axis for sigma (ref) = 10 J.cm(-3) to produce the D vers us t/a(sigma) master curve. A fairly general formula for stress shift facto r a(sigma) based on free volume v(f) and the chain relaxation capability (C RC) derived by one of the authors is applied. The formula predicts values t hat agree with the experimental ones within the limits of the experimental accuracy. Thus, experiments at several stress levels can serve for predicti on of longterm behavior from short-term tests. The same value of the Doolit tle constant B is obtained separately from temperature shift and stress shi ft experiments for the PLC.