Changes of mechanical properties in cold-crystallized syndiotactic polypropylene during aging

Many semicrystalline polymers undergo a process of aging when they are stor ed at temperatures higher than their glass-transition temperature (T,). Syn diotactic polypropylene was quenched from the melt to -40 degreesC, crystal lized from the glassy state at 20 or 40 degreesC and stored at the respecti ve temperature for different aging times up to 7200 h. A significant increa se in the tensile modulus and stress at yield and a decrease in strain at y ield were observed for both aging temperatures. Differential scanning calor imetry (DSC) scans of aged material showed an endothermic annealing peak 15 -30 degreesC above the previous aging temperature, the maximum temperature and enthalpic content of which increased with aging time. The position and the shape of the melting peak were not affected by aging. Scans of the stor age modulus obtained from dynamic mechanical analyser measurements indicate d a softening process starting at about 20 degreesC above the aging tempera ture and correlating with the annealing peak detected by DSC. Density measu rements: and wide-angle X-ray scattering investigations revealed that neith er the crystallinity increased significantly nor did the crystal structure change. So the observed property changes induced by aging are attributed to microstructural changes within the amorphous phase. Furthermore, it could be shown by anncaling experiments carried out at 60 degreesC, that aging ab ove T-g is, analogous to aging below T-g (physical aging), a thermoreversib le process.