  The microscopic model of semi-crystalline polymer in high-elastic state is
proposed. The model is based on the assumption that, below the melting
temperature, the semi-crystalline polymer comprises crystal nuclei connected by
stretched chain segments (SCS) with random configuration of monomers. The key
factor that stalls the phase transition below the melting temperature is the
tension of the SCS. External stress applied to the polymer also shifts the
equilibrium and causes unfolding of the nuclei, which enables large reversible
deformation of the polymer without loss of integrity. The simple 1D model
predicts plateau in stress-strain curve of high-elastic polymer above the yield
stress, which agrees with experimental observations. The model prediction for
the temperature dependence of polytetrafluoroethylene (PTFE) yield stress in
high-elastic state is in satisfactory agreement with experiment.
