Application of the Interferometer to the Measurement of Dimensional Changes in Rubber

The interferometer method for the measurement of small dimensional changes is adapted here to measurements on soft rubber. A detailed dsecription is given concerning the preparation of specimens, the apparatus, the procedure and the calculations necessary for the application of this method. Particular emphasis is placed on the measurement of thermal expansivity and the dimensional changes involved in transitions between the different forms of rubber. The lower limit of temperature is set only by the refrigerating liquid, temperatures as low as −185° C. being utilized. The upper limit of temperature is set only by the rubber itself. Results obtained in a study of the expansivity and transitions of “stark” (frozen) rubber are given as an illustration of the application of the method. Other possible applications are the measurement of the swelling of rubber by vapors and the determination of stress-strain relations under compressive loads. The chief limitation of the application of this method to rubber arises from the plastic flow of unvulcanized rubber at elevated temperatures. The method also possesses the inconvenience of requiring continuous observation. Compared with alternative methods of measuring small dimensional changes, the method possesses advantages arising from its high sensitivity, from the small size of the specimens required, from the ease of obtaining temperature equilibrium, from the absence of a confining liquid and from the simplicity of the calculations.