Stress relaxation behavior of biaxially oriented poly(ethylene terephthalate)

The stress relaxation behavior of biaxially oriented semicrystalline poly(ethylene terephthalate) was studied by thermomechanical analysis. Experimental techniques were developed for thin films. Relaxation moduli were measured as a function of stress, time, and temperature. The relaxation modulus was shown to be independent of stress over the range tested. Rate of loss of the relaxation modulus was found to be a nonlinear function of time and temperature up to about 100°C, encompassing the T g for the polymer. Over the temperature range of 100–120°C it was primarily temperature‐dependent. An empirical time—temperature superposition showed that significant losses in modulus can occur at very short times. At temperatures above the T g these losses can result in significantly reduced film physical properties.