  A constitutive model is developed for the mechanical response of elastomers
at finite strains. A polymer is treated as a network of linear chains linked by
permanent (chemical crosslinks) and temporary (entanglements and van der Waals
forces) junctions. Temporary junctions are assumed to be in two states: loose
(passive) when they impose only topological constrains on available
configurations of chains, and tight (active) when their effect is tantamount to
that for crosslinks. Stretching of a specimen implies that some loose junctions
become active, which decreases the average length of a chain.
  A long chain is treated as an ensemble of inextensible strands connected in
sequel. Two neighboring strands are bridged by a bond which may be in two
conformations: flexed (trans) and extended (cis). A bond in the flexed
conformation is modeled as a linear elastic solid, whereas the mechanical
energy of a bond in the extended conformation (two rigid rods directed along a
straight line) is disregarded. For a virgin specimen, all bonds are in the
flexed conformation. Under loading some bonds are transformed from flexed to
extended conformation.
  Stress-strain relations for a rubbery polymer and kinetic equations for the
trans-cis transition are derived using the laws of thermodynamics. Governing
equations are determined by 5 adjustable parameters which are found by fitting
experimental data in uniaxial tensile tests on natural rubber vulcanizates with
various amounts of crosslinks. Fair agreement is demonstrated between results
of numerical simulation and observations with the elongation ratio up to $k=8$.
We analyze the effects of cyclic loading, thermal annealing and recovery by
swelling on the material constants.
