Role of relaxation on strain crystallization of uniaxially stretched poly (ethylene naphthalate) films: A mechanooptical study

The large deformation mechanooptical behavior of amorphous films of PEN in the rubbery temperature exhibits three-regime behavior. Regime I corresponds to classical stress−optical behavior of amorphous materials where PEN remains completely amorphous during stretching. Oriented crystallization and long-range connected network formation accompany the positive slope change in stress−optical behavior into regime II. In the last regime, III, the formation of “muscle bound” structure begins to level off birefringence as it approaches the finite extensibility of polymer chains. Relaxation of uniaxially stretched PEN was found to depend exclusively on the regime from which the relaxation is initiated. If the relaxation is initiated from within regime I, the stress and birefringence decrease traces back the linear stress−optical behavior while the material remains amorphous. The relaxation of stress and birefringence from the early stages of regime II deviates from the linear stress−optical behavior. At long enough relaxation times, birefringence eventually begins to increase while stress continues to decline. This marks the formation of highly localized highly oriented ordered regions with low translational ordering along the chain axis in the material. This unexpected behavior during relaxation was attributed to the formation of a long-range physical network that helps arrest the relaxation of orientations gained during stretching. If the relaxation is initiated toward the end of regime II, a fast increase of birefringence is observed while the stress decreases. This behavior is found to be the result of a spontaneous deformation process as observed by the increase of local true strain in the absence of macroscopic deformation. However, this mechanism is not as a result of relaxation alone but a combination of relaxation coupled with continuation of the deformation process that started during stretching and did not stop when the relaxation process started. In regime III, stress relaxation accompanies little or no increase in birefringence. In regime III other than establishment of three-dimensional order in crystalline regions, no other significant change is observed