3 years ago

Microstructural Evolution of Isotactic-Polypropylene during Creep: An In Situ Study by Synchrotron Small-Angle X-Ray Scattering

Microstructural Evolution of Isotactic-Polypropylene during Creep: An In Situ Study by Synchrotron Small-Angle X-Ray Scattering
Baobao Chang, Gert Heinrich, Konrad Schneider
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. The microstructural evolution during creep can be well distinguished into four stages. During secondary creep, lamellae thickening can be found. The lamellae thickening rate along the loading direction is higher than that perpendicular to the loading direction. After yielding, the fibrillary structure can be induced by lamellae breaking and reorganization. The length of the fibrillary structure increases from 364 to 497 nm, whereas its width stays at 102 nm as creep time increases.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/mame.201700152

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