Creep and stress relaxation in a longitudinal polymer liquid crystal: Prediction of the temperature shift factor

The polymer liquid crystal PLC is the PET/0.6PHB copolymer; PET=poly(ethyle ne terephthalate), PHB=mu-hydroxybenzoic acid (LC): 0.6=the mole fraction o f PHB. This is a multiphase system with PHB-rich islands in a PET-rich matr ix. Tensile creep compliance was measured isothermally from 20 degrees C to 160 degrees C in 10 degrees C intervals. Master curves were determined usi ng the time-temperature superposition for 20 degrees C and for the glass tr ansition temperature of the PET-rich phase T (gPET) =62 degrees C. Experime ntal values of the temperature shift factor a(T) as a function of temperatu re T agree in the entire T range with those from Eq. (7) relating a(T) to t he reduced volume <(nu)over tilde> and the Hartmann equation of state Eq. ( 10). Values of a(T)(T) calculated from the Williams-Landel-Ferry (WLF) form ula give very large errors below T (g). A control 14 months creep experimen t agrees with the theoretical predictions from Eq. (7). Stress relaxation e xperiments were performed under the constant strain of 0.5% from 20 degrees C to 120 degrees C, again master curves were determined for 20 degrees C a nd for T gPET and a T (T) values calculated. The stress relaxation a T (T) results agree with those from creep and with those from Eq. (7). (C) 1999 A merican Institute of Physics. [S0021-9606(99)51218-X].