Real time development of structure in partially molten state stretching of PP as detected by spectral birefringence technique

Our main focus in this study is to investigate the deformation behavior of PP in temperature range where the PP is partially molten using a newly developed uniaxial stretching system. This system allows the real time study of the structural reorganization processes as reflected in birefringence coupled with true stress and true strain at temperature and deformation rates used in industrial film process. This instrument revealed that birefringence–stress relationship is linear beyond an initial yielding point until the onset of strain hardening beyond which negative deviation from this linearity is observed. At lower strains the films were found to continue to change significantly during holding stage leading to increase in birefringence. These changes observed in holding stage decreases with decrease of stretching speed and temperature and with increase of total strain. Increased strain rates result in destruction of crystallites that involves block rotations of these regions leading to observation of lower birefringence at faster rates. The latter process generates large amount of stretched amorphous chains that gradually convert to crystalline state during holding.