Toward Record-High Stiffness in Polyurethane Nanocomposites Using Aramid Nanofibers

Elastomers such as polyurethanes usually possess low stiffness, and the addition of traditional fillers typically results in a moderate improvement. Aramid nanofibers (ANFs) represent one of the most promising nanoscale building blocks for high-performance nanocomposites. In this work, waterborne polyurethanes (PUs) have been reinforced with ANFs using two solution processing methods, namely, layer-by-layer (LBL) assembly technique and the vacuum-assisted flocculation (VAF) method. Record-high modulus of 5.275 GPa and ultimate strength of 98.02 MPa are obtained among all the reported PU based nanocomposites. We attribute such achievement to the similar molecular structures of ANFs with PUs which ensures a high affinity made possible by the manifold interfacial interactions. The formation of multiple hydrogen bonds due to the presence of amide groups with appropriate spacing in both components is confirmed by the computer simulation. Compared with the VAF method, it is found that LBL assembly allows a better load transfer, resulting in higher ultimate strength and stiffness. The VAF method shows advantages in improving the ultimate strength at low loadings of ANFs. We believe our work may not only lead to a new practical combination within the field of composite materials but also provide important implications for the future design of nanocomposites based on the innovative nanofillers.