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Designing Sensory and Adaptive Composite Materials

Newly developed computational approaches aid in the design of composites that exhibit remarkable abilities to both sense external cues and adapt to these cues in controllable, 'programmable' ways.

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

Topics:
Aerospace Biological sciences Coating/Surface Modification Coatings & Adhesives Composite materials Composites Data Acquisition Design processes Detectors Joining Joining & Assembly Manufacturing & Prototyping Manufacturing processes Materials Materials properties Mechanical Components Nanomaterials Nanotechnology Sensors Smart materials
This Brief includes a Technical Support Package (TSP).
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Designing Sensory and Adaptive Composite Materials

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    Aerospace & Defense Technology Magazine

    This article first appeared in the December, 2020 issue of Aerospace & Defense Technology Magazine (Vol. 5 No. 7).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "Designing Sensory and Adaptive Composite Materials" presents the findings from a research project funded by the Air Force Office of Scientific Research (AFOSR) from June 2015 to June 2018. The research focuses on developing new computational approaches to design composite materials that can sense external stimuli and adapt in programmable ways. The study emphasizes two primary classes of materials: polymer-grafted nanoparticles (PGNs) and stimuli-responsive, fiber-reinforced gels.

    The first class, PGNs, involves hard nanoparticles that are grafted with polymers and cross-linked into extensive networks. The goal is to create PGN networks that enhance their strength and toughness in response to applied forces, mimicking the natural process of bone remodeling, which improves mechanical properties after deformation. The research highlights the use of biomimetic catch bonds, which become stronger under deformation, leading to composites with improved mechanical properties compared to those using conventional slip bonds. The findings indicate that networks with catch bonds exhibit greater ductility, toughness, and faster recovery from strain.

    The second class of materials studied consists of stimuli-responsive, fiber-reinforced gels. These gels can change their properties in response to environmental cues such as light and heat. The incorporation of fibers enhances the mechanical robustness of the gels and allows for a broader range of responses to external signals. A notable feature of this system is the use of spirobenzopyran (SP) chromophores, which enable manipulation of the material using light. By exposing the gels to blue light, researchers can induce structural changes that affect the material's functionality, showcasing a novel form of photo-chemo-mechanical transduction.

    The document also discusses the computational models developed during the research, which allow for the prediction and optimization of the performance and mechanical integrity of these hybrid materials in varying environmental conditions. The research provides fundamental guidelines for designing new materials that could have significant applications in various fields, particularly for the Air Force.

    Overall, the findings from this research contribute to the understanding of how to create advanced composite materials that can adapt to their surroundings, offering potential for innovative applications in technology and engineering.

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