3 years ago

Influence of Annealing on Mechanical αc-Relaxation of Isotactic Polypropylene: A Study from the Intermediate Phase Perspective

Influence of Annealing on Mechanical αc-Relaxation of Isotactic Polypropylene: A Study from the Intermediate Phase Perspective
Baobao Chang, Gert Heinrich, Konrad Schneider, Roland Vogel
The influence of annealing on mechanical αc-relaxation of isotactic polypropylene (iPP) is investigated. In the sample without annealing, polymer chains in the intermediate phase are constrained by crystallites with a broad size distribution, leading to one αc-relaxation peak with activation energy (Ea) of 53.39 kJ mol−1. With an annealing temperature between 60 and 105 °C imperfect lamellae melting releases a part of the constraining force. Consequently, two αc-relaxation peaks can be observed (αc1- and αc2-relaxation in the order of increasing temperature). Both relaxation peaks shift to higher temperatures as annealing temperature increases. Ea of αc1-relaxation decreases from 38.43 to 35.55 kJ mol−1 as the intermediate phase thickness increases from 2.0 to 2.2 nm. With an annealing temperature higher than 105 °C, a new crystalline phase is formed, which enhances the constraining force on the polymer chains. So the αc1-relaxation peak is broadened and its position shifts to a higher temperature. Moreover, the polymer chains between the initial and the newly formed crystalline phase are strongly confined. Therefore, the αc2-relaxation is undetectable. Ea of αc1-relaxation decreases from 23.58 to 13.68 kJ mol−1 as the intermediate phase thickness increases from 2.3 to 3.0 nm. Mechanical αc-relaxation of isotactic polypropylene can be greatly influenced by the constraint force on polymer chains in the intermediate phase. Imperfect lamellae melting caused by annealing leads to a nonhomogeneous intermediate phase, giving rise to two mechanical αc-relaxation. Lamellae thickening enhance the constraint force on polymer chains in the intermediate phase, resulting in an increase of the mechanical αc-relaxation temperature.

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

DOI: 10.1002/mame.201700291

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