The measurement of molecular orientation in fibers by acoustic methods

In the present work, the sound velocity in polymeric fibers or film strips is related to a quantitative measure of molecular orientation in the sample. It is shown that this measure of orientation can be calculated from two experimental quantities: the sound velocity in an oriented specimen and the velocity in a randomly oriented sample. Both the oriented and unoriented samples must have the same chemical composition, and both measurements must be made at the same temperature, below the glass transition temperature, T g . Sound velocities were measured by a pulse technique. This method permitted a continuous measurement of orientation changes during conventional stress‐strain tests without interrupting the tests. In addition, the uniformity of orientation along the length of fibers was measured continuously during windup. Examples of these applications are given, and a comparison is drawn between acoustic and x‐ray measures of orientation.