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

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.