Biaxial stretching of polymer films such as PET is a common process in the industry. Being able to monitor the stress-strain mechanical behavior of the films during stretching and linking this with the birefringence behavior provides a good tool to investigate stress and strain optical behavior of these films undergoing complex structural organization The results indicate that PET exhibits four stage stress optical behavior in simultaneous biaxial mode. The first stage is traditional linear stress optical behavior with stress optical constant (SOC) of 5.8 GPa-1 (5800 Brewsters). In the second stage birefringence rapidly increases and this is typically associate with the formation of poorly ordered crystals that are assumed to be at the nodes of the physical network. In the third stage the birefringence begins to level off with considerable negative slope variation with stress where the crystallinity further develops. The fourth stage was observed only at high stretching rates and in this distinct stage, birefringence become near constant while stress continues to increase as the chains have reached their full extensibilities.
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