The birefringence of polyisobutylene

The variation of stress and birefringence occurring during the relaxation of elongated samples of polyisobutylene at constant length was observed at temperatures between −40° and +50°C. It is found that stress and birefringence relax in the same way throughout this region, and that the quantity (stress/birefringence × temperature) is a constant. This indicates that this material behaves in a manner consistent with the kinetic theories of elasticity and birefringence for an ideal rubber, and that the statistical segment size is fairly independent of temperature in this range. A value of 3.5 × 10−24 cm.3 is obtained for the difference between the principal polarizabilities of the statistical segment.