Novel true stress-true strain-birefringence measurement systems for real time measurement during multiaxial deformation and heat setting of polymer films: 'Application on PET films'

The spectacular growth in the number and variety of polymers has generated materials with a wide range of physical properties and applications. Being able to identify, modify and characterize these materials is not just of academic interest but also of commercial and environmental concern. Many of the important structural and microstructural features of polymers have no counterparts in other classes of materials.; The aim of this study is to develop and validate a highly instrumented system capable of stretching and heat setting the polymer films, in the mean time being able to monitor the true mechanical and optical behavior during such processes in real time.; The nature of the very fast structural changes occurring during the film stretching and the subsequent heat setting processes requires measuring techniques that are capable of monitoring these fast changes at comparable speeds. After-the-fact or off-line techniques can not provide accurate information about these structural changes.; PET is one of the most commonly used polymers in the form of fibers and films. During the course of this study, PET films were stretched in different biaxial stretching modes, relaxed at the stretching temperature and subsequently heat set, simulating the tenter frame process most widely used for the production of the biaxially oriented films. During these processes, on line measurements of true stress-true strain in plane and out of plane birefringences were carried out in real time.; The structural changes occurring in the films were investigated using Laser Raman spectroscopy, Differential Scanning Calorimetery (DSC) and Wide Angle X-ray Scattering (WAXS) techniques.; The study revealed mainly three regimes of behavior in the relationship between the stress and birefringence during the simultaneous and sequential stretching modes. In a first linear stage (Regime I) the stress optical rule holds, from which the stress optical constant (SOC) for PET was found to be 5.8 GPa-1 (5800 Brewsters). In a second stage (Regime II) the birefringence increases faster than the stress with the introduction of a poorly ordered low population crystalline sites. Once sufficient "slack" is taken from the system, Regime III is reached when finite extensibility brings about saturation of birefringence while stress continues to increase. (Abstract shortened by UMI.)...