A modified method of reduced variables for stress relaxation

Abstract It has recently been shown that stress relaxation data may be conveniently interpreted using multiple box distribution of relaxation times. The multiple box distribution treatment suggested the possibility of modifying the method of reduced variables or time‐temperature superposition principle to avoid the difficulty of shifting curves in the rubbery region. This modification has now been applied to the experimental stress relaxation data for polyisobutylene. The results obtained verify the earlier observations that the logarithms of relaxation times and modulus constant are related to the reciprocal of temperature linearly, that the slope of these semilog linear relations changes when there is a phase transition, and that two types of mechanisms are operating in the relaxation of mechanical stress. The theorized stress‐time curves predicted by both methods fit the experimental data equally well, but the modulus at very short time predicted by the reduced variables method generally is higher than that by the multiple box distribution method. Relaxation data collected under high speed stretching condition are needed for better elucidation of the short time behavior of viscoelastic material.