Why do polyethylene crystals have sectors?

Area of Science:

• Materials Science
• Polymer Science
• Crystallography

Background:

• Understanding the crystallization behavior of n-paraffins is crucial for polymer science.
• Previous studies on n-paraffins have focused on chain folding and its impact on crystal morphology.
• The role of molecular packing versus chain folding in crystal sectorization remains an area of investigation.

Purpose of the Study:

• To investigate the relationship between molecular packing and crystal sectorization in n-paraffin monolamellar crystals.
• To determine the influence of chain length on crystal structure and molecular arrangement.
• To explore the effect of surface morphology on crystal growth and orientation.

Main Methods:

• High-resolution electron microscopy (3.7 Å optical diffraction) was employed to image n-paraffin crystals.
• Specimens with varying chain lengths (n-C36H74 to n-C82H166) were analyzed.
• Surface decoration with polyethylene crystallites was used to probe surface structure and preferred orientations.

Main Results:

• Higher molecular weight n-paraffins (>n-C60H122) form sectorized crystals with alternate bands of untilted and tilted chains.
• Lower molecular weight n-paraffins form nonsectorized crystals with generally untilted chains.
• Surface decoration revealed three preferred polyethylene crystal rod orientations on longer alkanes, suggesting a role for surface roughness.

Conclusions:

• Crystal sectorization in n-paraffins is primarily a function of chain-stem packing, not solely dependent on chain folding.
• Chain folds may play a secondary role, potentially influencing crystal collapse on surfaces.
• The study provides new insights into the fundamental principles governing polymer crystallization and morphology.