  We report a dynamical-mechanical study of stress relaxation at small
deformation in a natural (polyisoprene) rubber well above its glass transition
temperature Tg. We find that an almost complete relaxation of stress takes
place over very long periods of time, even though the elastic network integrity
is fully retained. The relaxation rate and the long-time equilibrium modulus
are sensitive functions of temperature which do not follow time-temperature
superposition. Many characteristic features of non-ergodic ageing response are
apparent at both short and very long times. We interpret the observed behaviour
in terms of the properties of rubber crosslinks, capable of isomerisation under
stress, and relate the results to recent models of soft glassy rheology.
