Solid-state structural evolution of poly(ethylene terephthalate) during step uniaxial stretching from different initial morphologies: An in situ wide angle x-ray scattering study

This work reports an in situ wide-angle Xray scattering (WAXS) study of the structural evolution of PET with distinct initial morphologies during step uniaxial stretching in the solid state. Two types of samples were analyzed under synchrotron X-ray radiation, namely quasi-amorphous (QA) and semicrystalline (SC) (with 2D and 3D order). Results show that initially different QA morphologies evolve following the same stages: (i) stage I (before neck), at almost constant orientation level the amorphous phase evolves into mesophase; (ii) stage II (neck formation), there is a rapid increase of polymer orientation and the appearance of a periodical mesophase from the highly oriented mesophase; (iii) stage III (necking propagation), there is a leveling off of the average polymer orientation together with partial conversion of the periodical mesophase and mesophase into highly oriented amorphous. The behaviors of the two SC morphologies arecompletely distinct. A 2D order crystalline morphology evolves with stretching likewise the QA through three stages: (i) at early stages of deformation the polymer orientation remains unchanged while the amorphous phase amount increases slightly, stage I; (ii) in stage II, a fast increase of polymer orientation is accompanied by large formation of mesophase; and (iii) in stage III there is the level off of polymer orientation as the chains approach their finite extensibility and the 3D crystalline order is achieved. Evolution of SC sample with 3D crystalline order mainly features constant orientation increase together with mesophase increment. Structure deformation models are suggested.Fundação para a Ciência e a Tecnologia (FCT

Structural development of poly(ethylene terephthalate) during uniaxial stretching above the glass-transition temperatura: study of the statistical influence of the stretching variables

In this article, we present an investigation of the structural development of poly(ethylene terephthalate) (PET) during uniaxial stretching above the glass-transition temperature; this followed a statistical design of experiment approach to determine the influence of the stretching variables on the structural development. Amorphous PET was submitted to a stretching program with variations in the stretching temperature, stretching rate, and stretching ratio. Stretched samples were rapidly quenched and characterized by wide-angle X-ray scattering, optical birefringence, and differential scanning calorimetry. The relevance and influence of the stretching variables on the obtained parameters (phase fraction, phase orientation, and thermal parameters) were analyzed. The strain-induced crystallinity was controlled by stretching temperature,stretching ratio and the interaction between them. Mesophase development was not dependant on the stretching temperature but on the interaction between the stretching rate and ratio. The molecular orientation was proportionally dependent on the stretching temperature, stretching ratio and their interaction.Fundação para a Ciência e a Tecnologia (FCT