Stress-Optical Behavior of Rubber Networks at Large Deformations

Abstract

The birefringence of natural rubber networks at large deformations has been investigated experimentally and compared with the simultaneously determined stress-strain behavior. Our data is analyzed using a statistical theory of flexibly jointed chains, derived herein, which is believed to be more significant for the particular range of deformation used than the theories of Treloar and of Kuhn and Grün. In addition, the experimental data of Saunders is commented on in light of our theoretical development. We find that for network extensions exceeding those of the Gaussian region there is little correlation between the observed and theoretical behavior of the stress and birefringence (based upon the theory of flexibly jointed chains) and this lack of agreement is attributed to the fact that the statistical parameters needed for the description of the optical chain properties differ in magnitude from those required for the mechanical properties. Furthermore, by considering the points of incipient crystallization the strain behavior of the stress-optical coefficient is highly indicative of non-Gaussian behavior rather than crystallization, and therefore yields strong support for the position that non-Guassian behavior does not exist in rubber networks.