Online monitoring of birefringence development in heat-setting polymer films with a fast dual-wavelength optical technique. I. Uniaxially oriented poly(ethylene naphthalate)

The purpose of this article is to study the fast structural changes that ta ke place during the deformation and heat setting of uniaxially preoriented poly(ethylene 2,6-naphthalate) films. For this purpose, an online birefring ence measurement system coupled with a heat-setting oven was used. In the o nline birefringence system, whose details were reported previously (H. Venk atesvaran and M. Cakmak, SPE ANTEC Tech Pap 1993, 39, 257; H. Venkatesvaran and M. Cakmak, Polym Eng Sci 2001, 41, 341), a green and red laser is pass ed through the films being annealed in a heat-setting chamber; the resultin g data are analyzed for the details of structural mechanisms that take plac e during this process. This technique is shown to be capable of detecting r eversals in birefringence development; as a result, complex sequences of st ructural evolution can be tracked. The design of this system allows one to observe not only the increase or decrease in birefringence during the cours e of the heat-setting process but also the rate dependencies on processing variables. In this study, we establish that the rate of structural rearrang ement, as detected by birefringence, depends strongly on the state of the p re-existing chain orientation and the level of crystallinity. Increased lev els of preorientation in the films results in decreases in the rate of bire fringence change during heat; setting. This is primarily attributed to the increased levels of steric hindrance developed with the increase in orienta tion and crystallinity in the preoriented samples. (C) 2001 John Wiley & So ns, Inc.