1999 Volume 27 Issue 1 Pages 25-30
The physical aging behavior of amorphous poly (ethylene terephthalate) (PET) has been studied as a function of aging time and temperature. Volume recovery measurements are performed for samples quenched from an equilibrium temperature of 80°C to aging temperatures between 20°C and 75°C. The isothermal volume curves are plotted in the form of normalized fractional free volume (=(V-V∞)/V∞, where V∞ is the equibilium value of specific volume V) versus logarithmic aging time. All the data can be superimposed by appropriate horizontal and vertical shifts of the individual curves. The horizontal shift factors show the WLF type of temperature dependence, while the vertical ones correspond to the volume change due to thermal expansion. The specific volume V∞conf of the fully relaxed glass estimated from the reduced master curve is different from that calculated from linear extrapolation of the equilibrium specific volume V∞ in the liqiud state to temperature below the glass transition temperature (Tg). The equilibrium Tg of the amorphous PET is determined to be 57°C from the estimated value of V∞conf versus temperature curve.
