By analyzing the deformation of —isotactic polypropylene through cyclic uniaxial compression at different temperatures—conclusions are drawn on the contribution of the crystalline phase and the amorphous phase to the hardening curve. The deformation of the crystalline phase, which deforms mainly by simple shear of the crystallites, strongly depends on the properties of the amorphous phase. A separation of strain in a relaxing and a quasipermanent part, as introduced by the work of Hiss (1999, “Network Stretching, Slip Processes and Fragmentation of Crystallites During Uniaxial Drawing of Polyethylene and Related Copolymers
,” Macromolecules, 32, pp. 4390–4403), is undertaken. By this experimental procedure it is possible to characterize the deformation dependence of several physical quantities such as Young’s modulus or the stored energy associated to each loading-unloading cycle. Furthermore specific transition strains, A, B, C, and D, can be determined where the recovery properties change. It is demonstrated that beyond point C the strain hardening can be described by the simple rubber hardening model of Haward (1987, “The Application of a Simplified Model for the Stress-Strain Curve of Polymers
,” Polymer, 28, pp. 1485–1488).