THE PHOTOVISCOELASTIC INVESTIGATION OF HIGH POLYMERIC FILMS: (I) THE STRESS RELAXATION IN NATURAL RUBBER AND ITS BIREFRINGENCE AND CRYSTALLIZATION

Simultaneous measurement has been performed of the stress relaxation in rubber films and their optical properties, particularly their birefringence, which is the index to their molecular arrange ment, in order to determine what effect the mechanical behavior of rubber films will produce on their crystallization. Having natural rubber subjected to momentary stretch at various temperatures and at various rates, the different temporal changes of its stress and birefringence have been simultaneously measured with the use of a strain gauge and a photomultiplier, at different moments for an hour from the first 0.1 minute and on subsequent to its stretch. In this instance their X-ray diffraction patterns, respectively under the same situations, have been photographically preserved. The obtained results are as follows: When natural rubber films are stretched from three to four times their original length at room temperature, their stress and birefringence ordinarily begin to decrease with the lapse of time immediately after the stretch. This tendency holds with its behavior even when the stretch is performed to seven times its original length at higher temperature, say 60°C. The rubber deformation thus made by the stretch to over four times its original length will involve, however, subsequent decrease in stress but increases in birefringence at comparatively lower temperatures. The fall of temperature gives so conspicuously rise to this abnormal phenomenon as to develope it at 20°C when the stretch is made to five times as large length as the original. What is to account for this phenomenon. In effect it is attributed to the crystallization of the linear molecules in the amorphous region in the parallel direction to that of the stretch. This hypothesis is verified in the examination of its X-ray diffraction patterns.