Microstructural evolution of isotactic-polypropylene during creep: An in situ study by synchrotron small-angle X-ray scattering

The structure evolution of isotactic-polypropylene during creep is investigated by in situ synchrotron small-angle X-ray scattering. During primary creep, strain grows nonlinearly to a value less than 15%. The long period in loading direction (L∥) increases with time, whereas the long period perpendicular to the loading direction (L⊥) decreases slightly. During the secondary creep, strain increases linearly with time. L∥ and L⊥ show the same tendency with strain. The increase of the long period is caused by lamellae thickening, which is a kind of cooperative motion of polymer chains with their neighbors at the lamellae surface. Moreover, the growth rate of L∥ is larger than that of L⊥, indicating that the orientation of molecular chains along the loading direction decreases the energy barrier of the cooperative motion. During tertiary creep, strain grows dramatically in a short time. In this step, the lamellae are tilted, rotated, and then disaggregated. In addition, a fibrillar structure is formed during lamellae breaking. The length of the fibrillar structure increases from 364 to 497 nm while its width stays at 102 nm with increasing creeping time