Uniaxial deformation behavior of PEN films in the rubbery state is investigated as a function of processing temperature, draw rate, and molecular weight (M-w). Real-time measurement of true stress-true strain and birefringence of the films during the process of stretching is obtained by using a novel instrumented uniaxial stretching machine that couples the spectral birefringence system with the real-time strain detection system. This enables quantitative measurement of true stress-true strain and birefringence, enabling quick and nonstop measurement of stress-optical behavior (SOB) of PEN. A three-regime SOB was obtained for PEN films: (i) regime I-stress optical rule applies with a stress optical constant (SOC) of 27.5 GPa(-1); following this regime the behavior become nonlinear in (ii) regime II and birefringence rises steeply; finally in (iii) regime III, birefringence reaches a plateau while stress increases. All regimes are affected by the temperature, rate, and M-w Decreasing temperature and increasing stretching rate and M-w extend the range of regime I over which the material remains amorphous. Regime II and III slopes were found to be dependent on rate and M-w. Measurement of strain rate indicates that affine deformation holds until the end of regime I, rising sharply in regime II, as a result of spontaneous deformation. The strain rate rapidly decreases to zero when regime III starts. The peak in strain rate coincides with the fast increase of birefringence during regime II. According to WAXD and DSC studies, the material remains amorphous in the regime I. Right after the transition to the regime II, the amorphous chains are extremely oriented but possess low translational order, which prevents them from crystallizing. Further stretching the structure evolves very rapidly to a crystalline structure, with approximately 40% crystallinity that is maintained throughout the regime III at the end of which finite chain extensibility prevents further changes in the structure. A model representing the structural mechanisms followed from the amorphous to the crystalline state is suggested. Both rate and temperature have a significant effect on the structural development of PEN: at intermediate to high rates and low temperatures a semicrystalline structure is obtained whereas at very high temperatures and low rates the structure remains completely amorphous despite the large deformation that has been applied. On the basis of the these data, a dynamic phase diagram was constructed.