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

• Published in: Polymer testing Vol. 25; no. 7; pp. 953 - 960
• Main Authors: Mano, J.F., Viana, J.C.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-10-2006; Elsevier
• Subjects: Applied sciences; DMA; Elastic modulus; Exact sciences and technology; Glass transition; Mechanical properties; Molecular dynamics; PET; Physical properties; Polymer industry, paints, wood; Properties and testing; Technology of polymers; Tensile mechanical tests; Tensile mechanical tests; Glass transition; Molecular dynamics; Elastic modulus; DMA; PET; Ethylene terephthalate polymer; Viscoelasticity; Stress strain relation; Ester polymer; Mechanical properties; Experimental study; Modeling; Phenomenological model; Dynamic mechanical properties; Measurement method; Activation energy
• ISSN: 0142-9418; 1873-2348
• DOI: 10.1016/j.polymertesting.2006.05.016

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.