Biological systems have developed structural motifs that allow these systems to resist mechanical deformation. On the molecular scale, biological catch bonds play a vital role in this functionality since these bonds effectively become stronger under deformation. Inserted into hybrid materials, biomimetic catch bonds could lead to composites that exhibit improved mechanical properties in response to an applied force.

Computer simulations were used to investigate the mechanical properties of a network of polymer-grafted nanoparticles (PGNs) that are interlinked by labile “catch” bonds. In contrast to conventional “slip” bonds, the lifetime of catch bonds can potentially increase with the application of force (i.e., the rate of rupture can decrease). Subjecting the PGN networks to a tensile deformation (Figure 1), it was found that the networks encompassing catch bonds exhibit greater ductility and toughness than the networks interconnected by slip bonds. Moreover, when the applied tensile force is released, the catch bond networks exhibit lower hysteresis and faster relaxation of residual strain than the slip bond networks. The effects of the catch bonds on the mechanical behavior are attributed to transitions between two conformational states, which differ in their sensitivity to force. These findings provide guidelines for creating nanocomposite networks that are highly resistant to mechanical deformation and show rapid strain recovery.

Figure 2. Schematic representation of 3D PGN network with the various types of bonding interactions.

While the calculations for the above model were formulated for a 3D system, the simulations were carried out in 2D, presenting a slice through the sample. This approach was extended by developing the full 3D simulations and introducing a method for systematically varying the fraction of catch bonds in the network (Figure 2). The ability to precisely tune the mechanical properties of polymeric composites is vital for harnessing these materials in a range of diverse applications.

Polymer grafted nanoparticles (PGNs) that are cross-linked into a network offer distinct opportunities for tailoring the strength and toughness of the material. Within these materials, the free ends of the grafted chains form bonds with the neighboring chains. Tailoring the nature of these bonds could provide a route to tailoring the macroscopic behavior of the composite. Using computational modeling, the behavior of three-dimensional PGN networks that encompass both high-strength “permanent” bonds and weaker, more reactive labile bonds was simulated. The labile connections are formed from slip bonds and biomimetic catch bonds. Unlike conventional slip bonds, the lifetime of the catch bonds can increase with an applied force, and hence, these bonds become stronger under deformation.

With the 3D model, the mechanical response of the composites to a tensile deformation was examined, focusing on samples that encompass different numbers of permanent bonds, different bond energies between the labile bonds, and varying numbers of catch bonds. It was found that at the higher energy of the labile bonds, the mechanical properties of the material could be tailored by varying both the number of permanent bonds and catch bonds. Notably, as much as a two-fold increase in toughness could be achieved by increasing the number of permanent bonds or catch bonds in the sample (while keeping the other parameters fixed).

In contrast, at the lower energy of the labile bonds considered here, the permanent bonds played the dominant role in regulating the mechanical behavior of the PGN network. The findings from the simulations provide valuable guidelines for optimizing the macroscopic behavior of the PGN networks and highlight the utility of introducing catch bonds to tune the mechanical properties of the system.

This work was done by Anna Balazs of the University of Pittsburgh for the Air Force Research Laboratory. For more information, download the Technical Support Package below. AFRL-0297

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This article first appeared in the December, 2020 issue of Aerospace & Defense Technology Magazine.

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