Self-Nucleation Enables Polymorphic Selection in Thermoplastic Polyurethanes.

Area of Science:

• Polymer Science
• Materials Science
• Crystallization

Background:

• Thermoplastic polyurethanes (TPUs) exhibit complex crystallization behavior.
• Polymorphism in TPUs, specifically Form I and Form II, influences material properties.
• Controlling polymorphic outcomes is crucial for tailoring TPU performance.

Purpose of the Study:

• To investigate the impact of self-nucleation on the polymorphic crystallization of TPUs.
• To determine how self-nucleation affects crystallization kinetics and outcomes across varying hard segment (HS) content.
• To elucidate the mechanism by which self-nucleation induces the formation of specific polymorphs.

Main Methods:

• Differential Scanning Calorimetry (DSC) to monitor thermal transitions and crystallization.
• Ex situ Wide-Angle X-ray Diffraction (WAXD) for structural analysis.
• Polarized Light Optical Microscopy (PLOM) to visualize crystalline morphology.

Main Results:

• Self-nucleation significantly alters TPU crystallization, accelerating kinetics and promoting Form II formation.
• A distinct self-nucleation domain (Domain II) was identified, influencing polymorphic selection.
• Even low HS content TPUs, typically forming only Form I, were induced to crystallize in Form II via self-nucleation.
• Persistence of interurethane hydrogen bonds in the melt is proposed as the reason for Form II preference.

Conclusions:

• Self-nucleation provides precise control over TPU polymorphic selection across a broad compositional range.
• This thermal treatment strategy enables the formation of the more ordered Form II polymorph, even in challenging compositions.
• Self-nucleation offers a versatile method for tailoring TPU material properties through controlled thermal processing.