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

Abstract

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