An investigation into the microstructure and temperature-dependent evolutionary behavior of polyurea thickeners

Polyurea (PU) thickeners are extensively utilized in the formulation of high-performance lubricating greases due to their versatile structure and superior comprehensive properties. While it is well acknowledged that PU thickeners are formed through the thermally induced self-assembly of ureidyl molecules, the specific formation mechanisms of their diverse structures remain inadequately understood. This paper, by combining dynamic thermo-rheological analysis (DTRA) and in-situ infrared spectroscopy, reveals the formation processes of PU thickeners with typical structures such as nanotubes, helical fibers, and non-helical ribbons. The results demonstrate that helical fibers and non-helical ribbons originate from small-sized fibers with similar morphology, with increasing temperature leading to an increase in fiber size and localized aggregation. Nanotubes are progressively evolved from helical fibers under elevated temperature conditions. Notably, our study reveals that the relative proportion of disordered and ordered hydrogen bonds in the self-assembly system is highly correlated with its structure. Structures dominated by ordered hydrogen bonds tend to form non-helical ribbons, whereas an increase in disordered hydrogen bonds proportion promotes the formation of helical fibers, weakening the thermal stability of the structure and leading to its transition to nanotubes under solvating effects. The elucidated formation mechanisms of PU thickener structures, along with the structural evolution monitoring method based on DRTA, offer novel insights into the controlled synthesis of PU thickener with diverse structures.