X-rays to Probe Compression across Scales in Rigid Polyurethane Foams: Molecular Simulations and Synchrotron Experiments

Rigid polyurethane (PU) foams are widely used in engineering applications. This is underpinned by their multiscale and hierarchical structure, characterized by a nanometric segmented PU morphology and a closed-pore microstructure which determine the macroscopic material response. This work correlates the macroscopic behavior of rigid polyurethane foams derived from castor oil with their nanomorphological (in the crystalline domains) evolution upon compression. Molecular dynamics (MD) simulations were used to model the behavior of the PU chains in an ellipsoidal closed-pore structure, as well as the foam's mechanical properties and X-ray diffractogram. Macro-scale experimental and MD simulation results were correlated using the Nagy model to account for strain rate effects. Synchrotron wide-angle X-ray scattering (WAXS) measurements were performed on the foams before and after compression, with the results compared with simulated diffractograms. A shift in the interplane spacing in the semicrystalline phase of the hard PU segments was observed upon compression, indicating a change in the nanostructure resulting from macroscopic mechanical loading.