Quantitative Correlation between Mechanical Behaviors and Free Volume of Oriented Polyethylene Terephthalate: an <i>In Situ</i> PALS Study

With a high-counting-rate positron annihilation lifetime spectrometer, the current work achieves an in situ measurement of free volume in the amorphous layer of oriented polyethylene terephthalate (PET) film during tensile deformation. Tuning the amorphous structure with different annealing temperatures, we discovered that stretch in the linear deformation region enlarges the hole size for the samples annealed at low temperatures (25 and 150 °C), while it increases the number density of free volume for the samples annealed at high temperatures (190 and 230 °C). Interestingly, for all samples, the fraction of free volume increases linearly with stress with the same slopes in linear elastic and plastic deformation regions, respectively. The linear relation between stress and fraction of free volume stimulates us to propose a new concept of the free volume modulus Kf, which is about 0.77 and 0.25 GPa in linear elastic and plastic deformation regions, respectively. A quantitative mechanic model accounting for the contribution of free volume is established for the amorphous layer in a semicrystalline polymer.