  Three series of tensile relaxation tests are performed on isotactic
polypropylene at room temperature in the vicinity of the yield point. In the
first series of experiments, injection-molded samples are used without thermal
pre-treatment. In the second and third series, prior to testing the specimens
are annealed at 130 C for 4 and 24 hours, respectively. Constitutive equations
are derived for the time-dependent response of semicrystalline polymers at
isothermal loading. A polymer is treated as an equivalent temporary network of
macromolecules bridged by junctions (physical cross-links, entanglements and
crystalline lamellae). Under loading, junctions slip with respect to their
positions in the bulk material (which reflects the viscoplastic behavior),
whereas chains separate from their junctions and merge with new ones at random
times (which reflects the viscoelastic response). The network is thought of as
an ensemble of meso-regions (MR) with various activation energies for
detachment of chains from temporary nodes. Adjustable parameters in the
stress-strain relations are found by fitting observations. Experimental data
demonstrate that the shape of the relaxation spectrum (characterized by the
distribution of MRs with various potential energies) is independent of
mechanical factors, but is altered at annealing. For specimens not subjected to
thermal treatment, the growth of longitudinal strain does not affect the volume
fraction of active MRs and the attempt rate for detachment of chains from their
junctons. For annealed samples, the concentration of active MRs increases and
the attempt rate decreases with strain. These changes in the time-dependent
response are attributed to broadening of the distribution of strengths of
crystalline lamellae at annealing.
