On Viscoelastic Viscoelastic Viscoelastic Modelling of Poly Ethylene Terephthalate Behaviour during Multiaxial Elongations Slightly over the Glass Transition Temperature
The mechanical response of Polyethylene Terephthalate (PET) in elongation is strongly dependent on temperature, strain and strain rate. Near the glass transition temperature Tg, the stress-strain curve presents a strain softening effect vs strain rate but a strain hardening effect vs strain under conditions of large deformations. The main goal of this work is to propose a viscoelastic model to predict the PET behaviour when subjected to large deformations and to determine the material properties from the experimental data. To represent the non–linear effects, an elastic part depending on the elastic equivalent strain and a non-Newtonian viscous part depending on both viscous equivalent strain rate and cumulated viscous strain are tested. The model parameters can then be accurately obtained trough a comparison with the experimental uniaxial and biaxial tests.
Study of the molecular structure of PET films obtained by an inverse stretching process. Part I: Constant speed drawing of amorphous films
The constant rate stretching of amorphous poly(ethylene terephthalate) films is the first step of the industrial inverse process. To study this process, films deformed under uniaxial planar symmetry conditions have been produced on a laboratory stretching machine in order to discuss the influence of macroscopic parameters, such as draw ratio, temperature and melt viscosity of the amorphous film, on the deformation mechanism. The structure of oriented films has been characterized by combining X-ray diffraction and refractive index measurements. Stress strain curves have been simultaneously recorded. The draw temperature and the molecular weight of the polymer are determining factors controlling the development of molecular orientation and crystalline structure in the stretched films. The major influence of relaxation processes is described and a comparison with constant force drawn films is given.
Comparison of the rheology of polymer melts in shear, and biaxial and uniaxial extensions
The experimental properties of different polymer melts, polystyrene, high density polyethylene and low density polyethylene are compared for the first time in three different deformations: step shear, step biaxial extension and steady uniaxial extension. Properties of three other melts are also studied in step biaxial and shear experiments. For our comparative purposes some data of Laun and Winter from the literature are used, as well as new data reported here. In all the step strain experiments, the stresses can be factored into a time dependent relaxation modulus and a strain dependent damping function. The data are interpreted using a differential constitutive equation of Larson which satisfies this time-strain separability and has a single parameter that describes the strain softening character of the material. Results show that differences in the properties of the melts are most pronounced in uniaxial extension and least in biaxial extension. All melts follow the Doi-Edwards prediction relatively closely in biaxial extension. In uniaxial extension, the branched material shows a strong strain hardening effect although its shear and biaxial properties are similar to the other melts. The constitutive model gives a reasonably good fit to the data in all three deformations for unbranched materials for the same value of the adjustable parameter; the model, however, fails for the branched low density polyethylene.