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.
Development of texture in poly(ethylene terephthalate) by plane-strain compression
Abstract Morphological alterations induced by plane-strain compression of semicrystalline poly(ethylene terephthalate) (PET) at 190°C were studied using small-angle and wide-angle X-ray diffraction techniques, polarized light microscopy and differential scanning calorimetry. Based on the observations, a scheme of texture development is outlined. An initially spherulitic morphology transforms into one that comprises stacks of fragmented crystalline lamellae with lamellar normals oriented towards the flow direction. After some initial deformation by interlamellar sliding in the amorphous material (100)[001] chain slip operated throughout the remainder of the deformation. Up to a compression ratio of 2.6 the (100)[001] chain slip mechanism orients lamellar normals towards the compression axis. Further deformation caused fragmentation of the thinned-out lamellae and subsequent reorientation of lamellar normals towards the flow direction. Pole figure analysis indicated cooperative activity of the (100)[001] chain slip and (100)[010] transverse slip during the later stages of texture development. The (100)[010] transverse slip mechanism controlled the orientation of the crystal structure in a plane orthogonal to the flow direction. There was no evidence of (010)[001] chain slip during texture development. At a compression ratio of 3.3 pole figure analysis revealed the existence of a dual unit cell orientation that renders an overall orthotropic symmetry to plane-strain compressed PET.
Optical properties of simultaneous biaxially stretched poly(ethylene terephthalate) films
We offer a detailed study on the anisotropic optical properties in uni and simultaneous biaxially stretched Poly(ethylene terephthalate) (PET) films. Cast amorphous sheets of PET were stretched to a series of extension ratios in two mutually perpendicular directions at 80, 90, and 100°C. Additionally, 0selected films were subsequently “heat-set” by annealing with their width and lengths constrained. The principal refractive indices at sodium D wavelength of these, asstretched and heat-set films were obtained using a modified Abbe refractometer. The changes in the principal refractive indices with the processing history were correlated with the orientation of PET chains and phenyl plane normals, which were determined independently by wide angle X-ray (WAXS) pole figure technique.