On-line analysis of stress and birefringence development in simultaneous biaxial elongation of poly(ethylene terephthalate)

On-line measurements of elongational stress and birefringence of poly(ethylene terephthalate) film were performed under simultaneous equal-biaxial elongation at various strain rates and temperatures. The strain hardening started to occur at a true strain of about unity, which was practically independent of drawing condition. Even though the drawing was done under the equal-biaxial condition, the PET films started to show optical anisotropy in the film plane before the strain hardening. The optical anisotropy was more significant under the drawing conditions of high strain rate and low temperature, however it only appeared in the limited range of strain and disappeared again in aft the samples when strain was high enough. The starting point of the optical anisotropy corresponds to the off-plane birefringence, which is defined as the refractive index difference in the directions parallel and normal to the film plane, of about 0.03. This birefringence also corresponds to the critical point above which the stress-optical law is not applicable because of the starting of spontaneous molecular orientation. A similar spontaneous orientation behavior was also observed in the uniaxial elongation process.