Real time development of orientation in PP during stretching as as detected by spectral birefringence technique
The true stress- true strain and birefringence development in a series of PP films with varying levels of tacticity has been measured using a newly developed uniaxial stretching system. This system allows the real time study of the structural reorganization processes at industrially meaningful temperature and deformation rates. The system is fast enough to follow the deformation at high rates of strains and allows us see their deformation at temperatures well below the melting temperature of the samples provided that severe necking does not occur. It has been found that birefringence increases steadily with true strain. Certain stress is required to start to increase birefringence at lower temperature, meanwhile, the birefringence changes almost linearly with stress essentially following stress optical law behavior, as long as the temperatures are near melt temperature.