Temperature-Dependent Crystalline-Amorphous Structures in Linear Polyethylene - Surface Melting and the Thickness of the Amorphous Layers

Abstract

SAXS experiments on isothermally crystallized polyethylene reveal a clear temperature dependence of the crystalline-amorphous structure over a broad range of temperatures below the crystallization temperature. This phenomenon is analyzed using SAXS in combination with dilatometry, which allows for a quantitative determination of structure parameters. Reversible surface melting is shown to be the process responsible for the phenomenon. The thickness of the amorphous layers reflects a local equilibrium within the kinetically controlled amorphous-crystalline superstructure. Existing models which try to explain this behavior are reviewed and compared to the experimental results. After a slight modification is introduced to the theoretical considerations, the data can be adequately represented by a model which describes the amorphous layers as an entanglement network. To the deformations caused by surface melting the network responds with forces opposing further crystallization, thus leading to a temperature-dependent equilibrium.