Multiblock Thermoplastic Polyurethanes: In Situ Studies of Structural and Morphological Evolution under Strain

Area of Science:

• Materials Science
• Polymer Chemistry
• Solid-State Physics

Background:

• Thermoplastic polyurethane ureas (TPUs) are versatile polymers with tunable properties.
• Understanding the structure-property relationships in TPUs is crucial for advanced material design.
• The influence of soft and hard block chemistry on TPU morphology and deformation behavior requires further investigation.

Purpose of the Study:

• To investigate the in situ structural evolution of multiblock thermoplastic polyurethane ureas during deformation.
• To elucidate the role of different polydiols (poly(1,4-butylene adipate) - PBA and poly-ε-caprolactone - PCL) and diisocyanates (2,4-toluylene diisocyanate - TDI and 1,6-hexamethylene diisocyanate - HMDI) on material behavior.
• To correlate structural changes with mechanical responses under tensile stress.

Main Methods:

• In situ deformation experiments combined with small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS).
• Synthesis of multiblock thermoplastic polyurethane ureas using PBA and PCL as soft blocks and TDI and HMDI as hard blocks.
• Analysis of crystalline domain formation, structural reorganization, and phase transitions during stretching.

Main Results:

• The urethane environment dictates the soft block's crystal structure, leading to two crystalline domain populations.
• Aromatic TDI hard blocks create rigid domains, limiting polydiol crystallization and contributing to elastic modulus.
• Linear HMDI hard blocks allow higher crystallization rates and more regular morphology, resulting in thermoplastic behavior (PBA) or elastic deformation with stress-induced crystallization (PCL).

Conclusions:

• The chemical nature of hard blocks (TDI vs. HMDI) profoundly impacts the soft block's crystallization and the overall deformation mechanism of TPUs.
• TDI-based TPUs exhibit enhanced elasticity due to constrained polydiol lamellae, forming an additional physical network.
• HMDI-based TPUs display varied responses, from plastic flow (PBA) to elastic deformation and strain-induced crystallization (PCL), highlighting the interplay between soft block crystallinity and hard block structure.