The isothermal shrinkage stress of highly oriented semi-crystalline poly（ethylene terephthalate） （PET） fiber was studied in the temperature range from 102℃ to 221℃.The data of the isothermal shrinkage stress on logarithmic time scales could be simply shifted to obtain a superimposed curve when the temperature was not above 186℃.The relationship between the shift factors and the experiment temperatures could be described satisfactorily by the Williams-Landel-Ferry equation with C1=71.9 and C2=511.7.Therefore,the increasing of shrinkage stress with treated time,resulted from the release of “frozen" stress in the oriented polymer materials,was resulted from the activation of the segments due to the glass transition.The much higher values of C1 and C2 than those of the non-crystalline materials were due to the higher stiffness of the molecules contributed to the stress increasing,which were constrained by the crystallization and the chain entanglement in a highly oriented semi-crystalline polymer.It was also proven by a wide α-relaxation peak up to 170℃ indicated by the tanδ-temperature curve.When the temperature was equal to or higher than 186℃,the decreasing of the isothermal shrinkage stress could be observed after it reached the maximum stress.According to the three-phase model of PET fiber provided by Prevorsek,such high temperature stress relaxation was due to the viscose flow of the extended non-crystalline molecules located between micro-fibrils,which was also indicated by the α′-relaxation from 180℃ to 250℃.
