The elastic behavior of a propylene-ethylene copolymer was investigated. An initial "conditioning" tensile extension up to 800% strain resulted in an elastomer with low initial modulus, strong strain hardening, and complete recovery over many cycles. Structural changes that occurred in the low crystallinity propylene-ethylene copolymer during conditioning, and that subsequently imparted elastomeric properties to the conditioned material, were investigated. Thermal analysis, wide and small angle X-ray diffraction, and atomic force microscopy measurements were performed at various strains during the conditioning process. Conditioning transformed crystalline lamellae into shish-kebab fibers by melting and recrystallization. The fibers, accounting for only 5% of the bulk, were interconnected by a matrix of entangled, amorphous chains that constituted the remaining 95%. It was proposed that the shish-kebab fibers acted as a scaffold to anchor the amorphous rubbery network. Entanglements of the amorphous chain segments acted as network junctions and provided the elastic response. The stress-strain response of materials conditioned to 400% strain or more was described by the classical rubber theory with strain hardening. The extracted value of M,, the molecular weight between network junctions, was intermediate between the entanglement molecular weights of polypropylene and polyethylene. (c) 2007 Wiley Periodicals, Inc.