Kinetics of structural evolution during crystallization of preoriented PETas followed by dual wavelength photometric birefringence technique

A two-color laser photometric measurement system was used to follow birefri ngence changes in annealing of prestretched PET films. This technique was s hown to be capable of detecting large retardation (and thus birefringence) changes beyond one wavelength of the fight. The use of green and red lasers in the photometric system allows the detection of the reversals in birefri ngence during the course of annealing. Unless critical orientation/crystall inity levels are developed by stretching the films, heat setting the films at temperatures close to glass transition temperature results in complete e limination of preferential orientation. At intermediate stretch ratios, cry stalline regions form and the act as anchor points establishing a network f or the oriented amorphous chains. These films exhibit partial relaxation fo llowed by rapid increase in birefringence due to the accelerating effect of orientation on crystallization. At high stretch ratios, where substantiall y oriented and strain crystallized structures are obtained, the initial rel axation stage disappears and birefringence continue to increase throughout the heat setting process even at temperatures very close to glass transitio n temperature. In these films, however. the total change of birefringence d ecreases as more of the chains are oriented and crystallized in the stretch ing stage, leaving a smaller fraction of polymer chains to rearrange during the annealing stage. The kinetics of the structural change exhibit a compl ex behavior and the largest rates of structural changes were observed in fi lms exhibiting intermediate birefringence levels.