Stress‐optical coefficient of poly‐1,4‐butadienes

The stress‐optical coefficient of poly‐1,4‐butadiene is measured as a function of strain, cis/trans ratio, and degree of swelling. Deviations from the Kuhn‐Grün theory are found which are reduced upon swelling. Results are interpreted in terms of the Shindo‐Stein theory of birefringence of polymers containing statistical segments of different sizes. This interpretation for the unswollen polymer leads to the unreasonable conclusion that the cis segment is considerably longer than the trans. Reinterpretation for the swollen polymer leads to the more reasonable conclusion that the trans segment is slightly longer than the cis. The dependence of the stress‐optical coefficient on the nature of the swelling solvent is similar to the observations of Gent and Nagai and is believed to result from the effect of internal field from anisotropic solvent molecules. It is felt that the value of the stress‐optical coefficient for the dry rubber is modified by the internal field from somewhat locally ordered neighboring polymer molecules.