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Determining the maximum melting temperature of polymer crystals from a change in morphology of dewetting rims

Gang Wang, Guotao Shi, Anchenyi Yang, Binghua Wang, Changyu Shen, Jingbo Chen, Günter Reiter, Bin Zhang. Polymer 274 (2023) 125874

Dewetting experiments on molten spherulites, previously grown at T_c = 120 ◦C in thin films of isotactic polypropylene (iPP), revealed two kinds of instabilities of the dewetting rims, namely a fingering instability and a fracture instability for dewetting temperatures T_s above and below ca. 165 ◦C, respectively. For T_s ≥ 165 ◦C, the dynamics of growing dewetting holes on slippery substrates was governed by viscous dissipation, causing undulations leading to a fingering instability of the rims. However, for 160 ◦C < T_s < 165 ◦C, the dewetting rims exhibited a fracture instability, which was related to the release of elastic energy stored in the rim. From the dewetting dynamics of the molten polymers, we derived the non-equilibrium viscosity η_non-equ(T_s) and equilibrated viscosity η_equ(T_s) as a function of T_s. The values of η_non-equ(T_s) were significantly larger than ηequ(T_s), with differences decreasing for increasing T_s. Interestingly, extrapolation of ηnon-equ(T_s) to the cross-over point η_non-equ = η_equ yielded T_s,cross ≈ 166 ◦C. We relate T_s,cross with the maximum melting temperature of crystalline domains which existed within iPP spherulites. Below Ts,cross, not all crystallites were molten and the melt contained crystalline seeds. Above T_s,cross, the melt was homogeneous and free of any seeds. Our approach opens up a new possibility for determining the maximum melting temperature of polymer crystals.

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