Influence of Annealing on Mechanical α c ‐Relaxation of Isotactic Polypropylene: A Study from the Intermediate Phase Perspective

The influence of annealing on mechanical α c ‐relaxation of isotactic polypropylene ( i PP) 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 α c ‐relaxation peak with activation energy ( E a ) 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 α c ‐relaxation peaks can be observed (α c1 ‐ and α c2 ‐relaxation in the order of increasing temperature). Both relaxation peaks shift to higher temperatures as annealing temperature increases. E a of α c1 ‐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 α c1 ‐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 α c2 ‐relaxation is undetectable. E a of α c1 ‐relaxation decreases from 23.58 to 13.68 kJ mol −1 as the intermediate phase thickness increases from 2.3 to 3.0 nm.