Structure variations through the thickness of poly (ethylene terephthalate) films: characterization and mechanism of formation

An exploratory study was conducted to determine the existence, magnitude and techniques required to characterize through the thickness inhomogeneity in oriented poly(ethylene terephthalate) (PET) films. Variations in through the thickness orientation and morphology are the result of a temperature gradient existing within the film during deformation. A review of experimental techniques was made in order to determine the most suitable and sensitive techniques currently available for characterizing through the thickness inhomogeneity.

A series of single and multilayer oriented films was prepared from cast sheet samples having superimposed temperature gradients present through the thickness prior to stretching. Density, thermal analysis, birefringence and microscopy were used to characterize the stretched samples. Films were characterized both in terms of average properties and in relation to through the thickness variations. The effect of temperature and strain rate on bulk properties of homogeneous (nongradient) and heterogeneous (gradient) films was investigated.

Laminate films were used as a means of characterizing gradient films and to demonstrate the magnitude of through thickness structural gradients. Small-angle light scattering (SALS) patterns of the individual layers of laminate films revealed significant differences in superstructure present within the layers. Chemical etching of heterogeneous oriented films with n-propylamine showed major differences in surface-to-surface etch patterns, whereas nongradient films exhibited similar etch patterns on both surfaces.

The present study considers the use of critical angle refractometry and the interferometry as practical techniques for the detection and characterization of through the thickness inhomogeneity in oriented PET films. A detailed description of the general principles of measurement, the instruments and the experimental procedure associated with each of the techniques is presented. The advantages and limitations of each technique are also discussed.