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; PET5poly~ethylene terephthalate!, PHB5r-hydroxybenzoic acid ~LC!: 0.65the mole fraction of PHB. This is a multiphase system with PHB-rich islands in a PET-rich matrix. Tensile creep compliance was measured isothermally from 20 °C to 160 °C in 10 °C intervals. Master curves were determined using the time–temperature superposition for 20 °C and for the glass transition temperature of the PET-rich phase TgPET 562 °C. Experimental values of the temperature shift factor aT as a function of temperature T agree in the entire T range with those from Eq. ~7! relating aT to the reduced volume ṽ and the Hartmann equation of state Eq. ~10!. Values of aT(T) calculated from the Williams–Landel– Ferry ~WLF! formula give very large errors below Tg . A control 14 months creep experiment agrees with the theoretical predictions from Eq. ~7!. Stress relaxation experiments were performed under the constant strain of 0.5% from 20 °C to 120 °C, again master curves were determined for 20 °C and for TgPET and aT(T) values calculated. The stress relaxation aT(T) results agree with those from creep and with those from Eq. ~7!. © 1999 American Institute of Physics. @S0021-9606~99!51218-X#