Polymorphism of a long-chain cycloparaffin (CH2)120

Area of Science:

• Polymer Science
• Crystallography
• Materials Science

Background:

• Long-chain cycloparaffins exhibit complex crystalline structures.
• Understanding their polymorphism is crucial for materials applications.
• Previous studies have explored various aspects of their crystallization.

Purpose of the Study:

• To investigate the polymorphism of a long-chain cycloparaffin, specifically (CH2)120.
• To analyze the chain packing within monoclinic and orthorhombic crystal forms.
• To examine the influence of substrate interactions on crystal habit and system.

Main Methods:

• Transmission electron microscopy (TEM) was the primary technique.
• Isothermal crystallization from dilute p-xylene solutions.
• Decoration experiments using vapor-deposited polyethylene (PE).
• Growth of edge-on crystals on alkali halide substrates (NaCl, KBr, KCl).

Main Results:

• Both monoclinic and orthorhombic single crystals of (CH2)120 were observed, with orthorhombic forms being more frequent.
• Twin boundaries were identified in both crystal systems: (110) for orthorhombic and (100) for monoclinic.
• The crystal system of edge-on grown (CH2)120 depended on the substrate: monoclinic on NaCl, orthorhombic on KBr and KCl.
• Annealing on NaCl induced a transformation from the monoclinic to the orthorhombic form.
• Molecular orientation in edge-on crystals showed the molecular plane parallel to the substrate and the c-axis perpendicular to the crystal's longer dimension.
• Sub-cell dimensions closely matched those of monoclinic and orthorhombic polyethylene.

Conclusions:

• The study elucidates the polymorphism of (CH2)120, highlighting the coexistence of monoclinic and orthorhombic phases.
• Crystal habit, twin formation, and phase stability are sensitive to growth conditions and substrate interactions.
• The observed sub-cell structures suggest similarities in chain packing to polyethylene, providing insights into long-chain paraffin crystallization behavior.