Stress-strain experiments as a mechanical spectroscopic technique to characterise the glass transition dynamics in poly(ethylene terephthalate)

In this work, further insights are presented to correlate results obtained from the modulus measurement obtained from conventional stress-strain experiments and the glass transition features of glass-forming materials. Data were obtained from injection moulded poly(ethylene terephthalate) samples, collected between 23 and 85 °C and covered four decades of strain rates, k, allowing the calculation of both tangent and secant moduli at different strain points, ε. The data represented as a function of temperature were consistent with dynamic mechanical analysis (DMA) results at 1 Hz, which showed a maximum in the loss modulus at 76 °C. It was possible to construct master curves along the log (k/ε) axis, from which a simple phenomenological model, assuming a Kohlrausch-Williams-Watts stretch exponential function for the modulus, allowed the extraction of the relevant viscoelastic properties of the material across the glass transition. The stretch parameter β was found to be around 0.22 and the characteristic times were consistent with the DMA results. The associated shift factors varied with temperature according to the WLF equation in the liquid region. However, a clear shift to an Arrhenius regime was detected in the glassy state, where an activation energy of 200 kJ mol−1 was obtained. This work suggests that stress-strain experiments can also be used to describe the dynamics of the glass transition in polymeric systems.