STRESS-RELAXATION AND CHANGE IN ENTANGLEMENT STRUCTURE OF POLYISOBUTYLENE IN LARGE SHEARING DEFORMATIONS

Simultaneous measurements of stress relaxation and differential dynami c modulus were made at 20 degrees C for polyisobutylene in large shear ing deformations on which intermittent small oscillations were superpo sed perpendicularly. Static shear strain varied from 0.1 to 1.0 while the frequency of the superposed oscillations was 0.1 Hz, which falls i n the middle of the plateau zone of the polymer. The strain dependence of the differential dynamic modulus was found to coincide with the da mping function in relaxation modulus which is well explained by the Do i-Edwards theory. It was concluded that non-linear behaviour in relaxa tion modulus results from a change in entanglement structure.