Influence of orientation on the relaxation and ultimate properties of polypropylene

As is known, the relaxation maxima of polymers are often shifted within the time or temperature scale if the materials are stretched or drawn. This behavior is due to changes of the free volume and/or of the crystallization and orientation of the stretched specimens. Since the relations between these parameters and the shift of the maxima are not yet known very well, an examination was made using polypropylene as a suitable example. The knowledge of these relations is of technical importance, too, because the shift of the maxima is often accompaned by a shift of corresponding brittle‐tough transitions. The relaxation maxima were determined in a good approximation by measuring the time‐dependent moduli of elasticity and by differentiating the modulus curves. It was found that the main maximum of polypropylene shifts with increasing draw ratio to longer times. At the same time the activation energy of the maximum decreases. Tensile tests, carried out on the unstretched and on the stretched specimens, yielded the same shift of the brittle‐tough transition as that of the main maximum. The molecular causes of this behavior are discussed.