Melting and van der Waals Stabilization of PE Single Crystals Grown from Ultrathin Films

Zhang, Fajun [U Tuebingen] Baralia, George G. [Zeiss Germany] Nysten, Bernard [UCL] Jonas, Alain M. [UCL]

We have obtained a series of single- and multilayered extended chain polyethylene (PE) crystals resting on a silicon dioxide substrate by crystallizing a low molar mass oligomer fraction of low polydispersity from an ultrathin film. Crystal thicknesses were determined from the height histograms of atomic force microscopy (AFM) images. For crystallization temperatures above 108 °C, the thickness of each layer was 16.9 ± 1.4 nm, close to the computed average extended length of the oligomer chain. The crystal melting was followed by a hot-stage optical microscopy (OM) in reflection. Different layers melt at different temperatures, due to differences in interfacial free energy depending on whether they are in contact with the substrate or another layer or have a free interface. The interfacial free energies of the different interfaces were obtained by the Gibbs–Thompson equation, and the difference between them was quantitatively related to the van der Waals stabilization energy resulting from placing a layer in contact with the substrate or with another layer. Our study demonstrates the importance of interfaces when considering the thermal behavior of ultrathin films of crystalline polymers and illustrates that a simple energetic concept can be used to explain the stabilization of crystals resting on a surface.