Benchmarking OPLS-AA and OpenFF for HDI-PEG Thermoplastic Polyurethanes with Varying Soft-Segment Length

Area of Science:

• Materials Science
• Computational Chemistry
• Polymer Science

Background:

• Thermoplastic polyurethane (TPU) properties depend on soft-segment mobility, hard-segment interactions, and morphology.
• Accurate atomistic prediction of TPU thermal and mechanical behavior requires robust force-field benchmarking.

Purpose of the Study:

• To benchmark two force-field frameworks (OPLS-AA and OpenFF/Sage) for predicting the thermal and mechanical properties of HDI-PEG polyurethanes.
• To investigate the impact of soft-segment (PEG) molecular weight on TPU properties using experimental and simulation methods.

Main Methods:

• Combined experimental techniques: Fourier-transform infrared spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and tensile testing.
• All-atom molecular dynamics simulations using OPLS-AA and OpenFF/Sage force fields across a range of PEG molecular weights (400-2000 g/mol).

Main Results:

• Both force fields reproduced composition-driven decreases in glass transition temperature (Tg) and density with increasing PEG molecular weight.
• OPLS-AA predicted higher absolute densities and Tg values compared to OpenFF.
• Simulations recovered composition-dependent stiffness differences, with OpenFF showing a more pronounced non-monotonic trend than OPLS-AA.

Conclusions:

• This work provides an experimental benchmark for assessing the robustness of composition-driven trends in HDI-PEG polyurethanes across different force fields.
• The study highlights which observables are sensitive to force-field choice and simulation scale in predicting TPU behavior.