Real-Time Crystallization and Melting Study of Ethylene-Based Copolymers by SAXS, WAXD, and DSC Techniques

Abstract

Isothermal crystallization and subsequent melting experiments of metallocene-based polyethylenes with different degrees of octene branching were carried out as a function of crystallization temperature using real-time simultaneous synchrotron small-angle x-ray scattering (SAXS) and wide-angle x-ray diffraction (WAXD) techniques and differential scanning calorimetry (DSC). The variation of crystallinity was determined by WAXD; the crystal long period, lamellar thickness, interlayer amorphous thickness and invariant were determined by SAXS. Results from these studies were interpreted using the model of branch exclusion which affects the ability of the chain-reentry into the crystal phase and retards the ability to undergo a lamellar isothermal thickening process during prolonged annealing. The short chain branching turns out to have little effect on the equilibrium melting temperature (Tm0≈144.1°C), which is determined by the Thomson-Gibbs equation. The surface free energy of the fold lamellar surface is found to increase with the branch content. Furthermore, our results indicate that a samll fraction of chain branch may be incorporated in the crystals lowering the heat of fusion.