Investigation of dynamics and structure development during uniaxial stretching of PET films at different molecular weights using fully automated spectral birefringence technique

In order to develop non-intrusive automated control protocols to tailor final properties during film processing, an understanding of structural development is essential. Recently developed real-time the Spectral Birefringence system fulfills all these requirements to couple optical properties with structural details of polymer films through its measure of optical anisotropy.; The Spectral Birefringence method is optimized for noise reduction to monitor rapid optical changes during uniaxial stretching of different molecular weight PET films. A tensile stretcher is designed and built in our laboratory that keeps the center of the dumbbell-shaped sample at a constant position for optical measurements. A laser width measurement system is also incorporated in this stretching system to assess the true mechanical response of the material at the middle portion of the film.; The relationship between birefringence and true stress development of various molecular weight PET films uniaxially stretched at 2, 20, and 200 mm/min at 90°C shows that there are three stress-optical regimes with different characters.; In the first well-known stage, birefringence develops linearly with true stress. In the second stage, the linear relationship gains a nonlinear character close to the strain hardening point. Finally in the third stage, birefringence reaches a plateau at the higher values of true stress.; Structural studies performed by WAXS, DSC and Raman are used to elucidate these three stages of birefringence development during stretching: in the first stage, the deformation of polymer chains takes place while they remain in amorphous state. At the end of this stage, chains start to orient to form oriented crystals that are responsible for the breakdown of linear birefringence development. After the breakdown, in the second stage, the deformation causes the chains to orient and crystallize substantially as it is shown by crystallinity and trans conformer fraction increase. The second stage is identified by an accelerated increase of birefringence. Finally, in the third stage, birefringence increase slows down and almost reaches a plateau; crystallinity conversion becomes almost negligent; and the trans conformer fraction continues to increase, indicating that chains are still responding to deformation by becoming oriented and taut.