Ageing of natural rubber under stress

We report a dynamical-mechanical study of stress relaxation at small deform ation in a natural (polyisoprene) rubber well above its glass transition te mperature T-g. We find that an almost complete relaxation of stress takes p lace over very long relaxation periods, even though the elastic network is retained. The relaxation rate and the long-time equilibrium modulus are sen sitive functions of temperature which do not follow time-temperature superp osition. Many characteristic features of non-ergodic ageing response are ap parent at both short and very long times. We interpret the observed behavio ur in terms of the nature of rubber cross-links, capable of isomerisation u nder stress. and relate the results to recent models of slow glassy rheolog y.