The crystal-form transition behaviours and morphology changes in a polyamide 6 cyclic dimer

Area of Science:

• Materials Science
• Chemical Crystallography
• Physical Chemistry

Background:

• Cyclic dimers, specifically 1,8-diazacyclotetradecane-2,9-dione, exist in distinct polymorphic forms (α and β).
• Understanding solid-state transformations is crucial for material properties and applications.

Purpose of the Study:

• To systematically investigate the characteristics and phase transitions of pure α- and β-forms of 1,8-diazacyclotetradecane-2,9-dione.
• To elucidate the transition behaviors between these polymorphic forms under various conditions.

Main Methods:

• Solubility tests in methanol, water, and caprolactam solutions.
• Thermal analysis using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC).
• Optical microscopy with polarized light microscopy (PLM) and video recording.

Main Results:

• The α-form dissolves and transforms into the β-form in methanol.
• Co-precipitates exhibiting iridescence form in caprolactam solution at lower temperatures.
• Aggregation in water leads to multi-slice layers and compact structures.
• Two endothermic transitions were observed via DSC: β to α at ~242°C, and melting of α above 345°C.
• Sublimation of the β-form occurs post-transition, while the α-form may sublimate below 150°C with volatile matter.

Conclusions:

• The α- and β-forms of 1,8-diazacyclotetradecane-2,9-dione exhibit distinct behaviors in different solvents and thermal conditions.
• Phase transitions, including solid-state conversion and sublimation, are temperature-dependent and solvent-influenced.
• Polymorphism significantly impacts the aggregation, structure, and thermal properties of this cyclic dimer.