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

Dewetting experiments on molten spherulites, previously grown at Tc = 120 degrees 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 Ts above and below ca. 165 degrees C, respectively. For Ts >= 165 degrees 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 degrees C < Ts < 165 degrees 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 eta non-equ(Ts) and equilibrated viscosity eta equ(Ts) as a function of Ts. The values of eta non-equ(Ts) were significantly larger than eta equ(Ts), with differences decreasing for increasing Ts. Interestingly, extrapolation of eta non-equ(Ts) to the cross-over point eta non-equ = eta equ yielded Ts,cross approximate to 166 degrees C. We relate Ts,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 Ts,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.