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

The influence of annealing on mechanical $\alpha_c$-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 $\alpha_c$-relaxation peak with activation energy (Ea) of 53.39 kJ/mol$^{-1}$. With an annealing temperature between 60 oC~105 C$^{\circ}$ imperfect lamellae melting releases a part of the constraining force. Consequently, two $\alpha_c$-relaxation peaks could be observed ($\alpha_{c1}$- and $\alpha_{c2}$-relaxation in the order of increasing temperature). Both relaxation peaks shift to higher temperatures as annealing temperature increases. $E_a$ of $\alpha_{c1}$-relaxation decreases from 38.43 to 35.55 kJ/mol$^{-1}$ as the intermediate phase thickness increases from 2.0 nm to 2.2 nm. With an annealing temperature higher than 105 C$^{\circ}$, a new crystalline phase is formed, which enhances the constraining force on the polymer chains. So the $\alpha_c$1-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 $\alpha_{c2}$-relaxation is undetectable. $E_a$ of $\alpha_{c1}$-relaxation decreases from 23.58 to 13.68 kJ/mol$^{-1}$ as the intermediate phase thickness increases from 2.3 nm to 3.0 nm