Unravelling crosslinking and molecular structure in complex polyurethanes via advanced multinuclear solid-state NMR

Abstract

A crucial aspect of creating cutting-edge chemical processes that enhance mechanical strength, stability, sustainability, and circularity is acquiring a more profound comprehension of the atomic-scale structure of native polymers. In this work, we present a multinuclear ¹³C and ¹⁵N solid-state NMR combined with fast magic angle spinning (fast-MAS) ¹H NMR at high magnetic field (22.3 T) in characterizing thermoplastic polyurethanes (PUs) chemistry. The study highlights the significance of performing atomic-scale characterization to obtain an accurate chemical picture of the polymers, especially when investigating crosslinking among polymer chains. This is because indirect confirmation of crosslinking, such as through molecular weight gains or losses, can only provide an incomplete picture, due to the complex chemical structure involved. Moreover, given that variations in synthesis parameters, such as the ratio of reactants, can significantly influence the properties of polyurethanes, it is essential to establish structure-function relationships in these polymers.

This research notably emphasizes the significance of fast-MAS 1D ¹H NMR at high-field in characterizing multiple polyurethane samples within a brief experimental duration of minutes. The study demonstrates that fast-MAS ¹H NMR facilitates straightforward chemical shift assignment and enables quantitative comparison of chemical compositions among three complex PU samples synthesized with distinct reactant proportions. Finally, this study examines the limitations of ¹H fast-MAS NMR, particularly in the context of characterizing low levels of crosslinking in polyurethanes. Despite the inherent low sensitivity of ¹⁵N nuclei, this research demonstrates that ¹⁵N Cross-Polarization (CPMAS) NMR experiment serves as robust method for the unambiguous detection of allophanates in polyurethanes.