Lamellar diffusion during heat treatment in meltcrystallized low-density polyethylene

The annealing behavior of low-density melt-crystallized polyethylene is discussed in terms of an unprecedented lamellar diffusion mechanism. For this purpose a combined small-angle x-ray diffraction (SAXS) and differential scanning calorimetry (DSC) study has been carried out. The presence of an initial double-lamellar population is directly evidenced by electron microscopy of freeze-cut and stained sections. The intercalation of thinner lamellae within the dominant wide-lamella population gives rise to an x-ray periodicity of 220 Å (structure I). The independent stacking of thinner lamellae within the material contributes to a second periodicity at 110 Å (structure II). During annealing at successively higher temperatures T_A, the low-molecular-weight components from the thin lamellae progressively diffuse out of the double population, reinforcing the stacks of thinner lamellae. At the melting temperature of the thinner crystals a complete segregation of these lamellae takes place, and a new periodicity (structure III) corresponding to stacks of collapsed thick lamellae emerges. The periodicity of structure III increases further with T_A. The thin lamellae of structure II can be extracted by a solvent, removing the corresponding SAXS peak and DSC maxima. The partial removal of thin lamellae from structure I by means of solvent extraction concurrently yields a decrease of the SAXS period. The present results suggest that molecular segregation of a low-molecular-weight fraction, contributing to the population of thinner lamellae, occurs during annealing.