Roles of Hard Nanophases in Dynamic Toughening of Self-Healing Structural Nanocomposites

Many traditional metallic alloys and structural polymers exhibit dynamic embrittlement. In contrast, nanophase-segregated copolymer, polyurea PU1000, is recently found dynamically tougher than many metallic high-strength alloys. However, it has been challenging to evaluate the dynamic toughness and identify associated dynamic toughening mechanisms of the self-organized nanostructured copolymer. Here, we present the discovery of nanoscale dynamic toughening mechanisms of the segmented block copolymer that can be generalized for designing advanced nanocomposites.<br/>To characterize the novel dynamic toughening mechanisms, firstly, we accurately measured the dynamic toughness of polyurea PU1000 by developing a framework of a big-data-generating experiment that uses a streak camera system. The toughness was measured by detecting the continuous-time history of displacement information along a line on the surface of a sample under plate-impact loading, employing a newly invented Dynamic Line-Image Shearing Interferometer (DL-ISI). Then, the dynamic cohesive-zone parameters of the running crack could be inversely extracted with a deep-learning framework based on a convolutional neural network (CNN) and a state-of-the-art conditional generative adversarial network (cGAN) model pre-trained with a corresponding FEM dataset. The dynamic fracture toughness of polyurea was measured 12100 ± 1100 J/m² under a very high crack-tip loading rate, ~10⁷ MPa√m/s. The local strength near the crack tip under a local strain rate of ~10⁸ s-1 was evaluated 294 MPa, which is nearly three times the 105 MPa spall strength at ~10⁶ s-1 strain rate.<br/>Secondly, we revealed molecular-level dynamic toughening mechanisms of polyurea through in-situ AFM experiments and mesoscale coarse-grain molecular dynamics simulations. For the experiment, we employed a novel in-situ AFM loading device with invariant observation sight to collect high-resolution in-situ AFM tapping-mode phase images of polyurea under stress-relaxation at various fixed strains ranging from 0% to 260%. From the in-situ AFM and coarse-grain molecular dynamics (MD) simulation studies, we found that the hard-domain fragmentation and subsequent self-healing processes of bidentate hydrogen bonds promote both ductility and strength of the copolymer in a broad range of strain rates. The mesoscale coarse-grain MD simulations showed the hydrogen-bond cross-linked hard domains fragments under high-strain-rate deformation in the timescale of nanoseconds, which contributes to the high toughness of crack-growth incubation. In the in-situ AFM study, we could also observe hard domain fragmentation under moderate strain rate, followed by self-healing process initially in lateral direction then relaxation process of self-healing in the loading direction. The nanoscale self-healing processes help recover the strength of the nanocomposite, increasing dynamic running-crack toughness.<br/>In this work, we found that nanoscale fragmentation and self-healing at the nanoscale foster the strengthening and toughening of the nanocomposite in a broad range of strain rates.<br/>The long rod-shaped hard nanophases (MDI) embedded in a soft matrix (polytetramethylene oxide) fragment primarily due to the shear-lag mechanism of interfacial load transfer between the hard and soft phases. As the matrix viscoelastically relaxes and the fragment size becomes smaller at higher strain rates, the nanoscale fragmentation and self-healing mechanisms enhance both strength and toughness under a high loading rate. We believe these newly revealed nanoscale strengthening and toughening mechanisms can be further capitalized for designing advanced polymeric nanocomposites and a particular class of high-entropy alloy nanocomposites.