Optical and structural properties of thermally treated iPP fibers: Effect of strain rate

► Interferometry was used to investigate the annealed iPP fibers. ► A significant change in fiber shrinkage was observed and then levels off after 1.5 h long. ► Stretching improves molecular orientation, but annealing temperature spoils it. ► An empirical formula was suggested to correlate birefringence and the stretching strain. ► Birefringence profile exhibited distribution of molecular orientation across the fiber diameter.