Challenges in the experimental investigation of the caloric and thermomechanical behaviour of semi-crystalline polymers: A study on the example of polyethylene terephthalate (PET)

Abstract

Investigations on industrial produced polyethylene terephthalate (PET) were carried out in the temperature range from the glassy to the liquid state. In differential scanning calorimetry (DSC) experiments, the sample passed through this temperature range several times at different speeds, interrupted by isothermal segments. Thereby the caloric behaviour observed in earlier publications could be confirmed. A closer look reveals variations in the behaviour, depending on the initial state of the sample. The thermal and mechanical boundary conditions seem to have a long-term influence on the semi-crystalline state. Independent of the initial state, a fundamental decrease in the crystallization tendency could be observed after each temperature loop. The most likely explanation for that is an increase in molecular weight in the higher temperature ranges. This leads to an increase in viscosity, as it occurs in the solid poly-condensation (SSP) process. Mechanical experiments were also carried out in the temperature range investigated, but only at different isothermal levels due to experimental limits. For these experiments, theoretical considerations were made on the basis of the viscoelastic generalization of Hooke’s law. These show the influence of the significant decrease of the ratio between shear- and bulk stiffness in the glass-transition range. While the uniaxial tensile mode fail, the uniaxial compression mode with suppressed transversal expansion needs the fluid like behaviour to determine the bulk modulus instead of the longitudinal modulus. The experimental investigations confirm these theoretical results. Concerning the actual purpose of the mechanical investigation, Struik’s protocol is used at higher isothermal levels. As a result, crystallization-related changes in the mechanical behaviour could be observed.