Induced crystallization and orientation of poly(ethylene terephthalate) during uniaxial and biaxial elongation

Stretching PET at a high strain rate above the glass transition temperature has a positive effect on the strength of the material. In a recent paper^[1], we presented the influence of stretch and blow molding parameters on the properties of the final product, especially on the crystallinity induced by stretching. In this paper, we focus on the effects of loading, temperature, elongation and strain rate on macromolecular orientation and crystallization kinetics. We present experimental results from uniaxial and biaxial elongation tests carried out on injected PET specimens. To minimize the effect of quiescent crystallization, specimens are quickly heated with infrared lamps before the test and temperature is regulated during the test. Both uniaxial and biaxial tests are analyzed using a cross correlation technique^[2] that compares a picture used as reference and the picture of the deformed specimen. This technique allows us to determine all strain components at each point of the specimen, even when the strain field is not homogeneous. In a second part, we present measurements of macromolecular orientation and crystallinity ratio performed after each test. The infrared dichroïsm technique is used to determine the orientation of the microscopic morphology of PET before and after the testing. DSC measurements and density measurements are carried out to calculate the crystallinity ratio. Influences of strain rate, temperature and strain path sequence are evaluated in order to build a database for recent models of induced crystallization^[3],[4],[5].