Non-Linear Mechano-Optical Behavior and Structural Transitions in PEN/PEI Blends Under Uniaxial Deformation: Dynamic Phase Behavior.

The mechanooptical behavior and structural organization processes in melt miscible PEN−PEI blends have been investigated in the rubbery state as influenced by blend composition, stretching temperature, and deformation rate. This was accomplished using a spectral birefringence system integrated in a specially built uniaxial stretching machine1,2 where real-time birefringence, true stress, and true strain are monitored during the course of the deformation. Three distinct stress−optical regimes have been observed with an additional glassy component. The final structure and deformation behavior of the blends have been mapped out in a dynamic phase diagram showing that the material undergoes three critical structural transitions. At low temperatures near Tg the polymer remains in a nematic-like state, and orientation-induced crystallization occurs only above a certain stretching temperature. At intermediate temperatures the liquid−liquid (Tll) transition occurs wherein the material transforms from a “structured liquid” to a “true liquid” state at (1.08Tg (K)) exhibited by the disappearance of the initial glassy component as the material becomes devoid of the segmental correlations. At higher temperatures, where the relaxation process dominates and where the thermal induced crystallization is still suppressed, the material was found to remain in the amorphous state even after being stretched to large deformation levels