Influence of sacrificial bonds on the mechanical behaviour of polymer chains

Abstract

A growing focus in modern materials science is the attempt to transfer principles found in nature into new technological concepts with the goal of producing novel materials with tailored mechanical properties. One of these principles used in nature is the concept of sacrificial bonding (i.e. non-covalent cross-links that rupture prior to the protein backbone), which is associated with increased toughness in many biological materials. In the present work, the influence of the number and distribution of sacrificial bonds (SBs) on three main mechanical parameters-strength, work to fracture and apparent stiffness-is investigated in a simple model system using computer simulations. The results show that the work to fracture is mainly determined by the number of SBs present in the system, while the apparent stiffness and, to a lesser extent, the strength is altered when the distribution of SBs is changed. IntroductionIn contrast to engineered materials, biological organisms utilize a relatively limited selection of building blocks to synthesize materials (e.g. proteins, sugars, environmentally abundant ions). In spite of this, however, natural materials span an extremely wide range of mechanical properties, which is achieved by hierarchical structuring of the material over multiple length scales and by a combination of materials with opposing mechanical properties. One common and successful strategy to increase the toughness of protein-based biological materials is to use so-called sacrificial bonds (SBs).2 These non-covalent cross-links are weaker than the covalent bonds that comprise the protein backbone, and consequently, upon loading, they rupture before the covalent bonds fail. By doing so, SBs reveal hidden length (i.e. the length increase associated with unfolding of folded proteins) providing an efficient energy dissipation mechanism, while the overall material integrity survives.3 Furthermore, these bonds are reversible and may reform when the load is released, allowing for molecular repair. SBs have been found in a large variety of biologial materials like wood, 4 bone 5-7 and in some softer biological fibres such as silk, 8 whelk egg capsule 9 and mussel byssus threads. 10-13In materials such as silk, SBs are often weak hydrogen bonds combined in large numbers in regular protein conformations in order to collectively produce high stiffness 14 ; however, in the case of the mussel byssus, much stronger interactions between metal ions and proteins are employed. In this regard, the mussel byssus is an especially fascinating material. The mussel secretes the collageneous byssal threads as a means of creating a secure attachment in wave-swept rocky seashore habitats. Among the impressive properties of the mussel byssus are its high extensibility of over 100%, high stiffness and toughness, 10 its hard and wearresistant outer coating [15][16][17][18] and, last but not least, its ability to create strong and long-lasting underwater adhesion to a variety of surfaces.17 A fundamental aspect shaping each of these prope...

Influence of reversible cross-link coordination on the mechanical behavior of a linear polymer chain

“…The left graph of figure 5 shows the work to straighten the molecule W. For all systems, W shows a linear dependence on the sticky site density (i.e., the number of cross-links). While W does not considerably change between the random and ordered arrangement (as was already observed in [63]), it is consistently higher for the system containing bis-complexes only compared to the system containing trimers. This is remarkable because the complex energies are the same for the dimer and trimer.…”

“…Previous investigations identified the number and the distribution of sticky sites as a major factor determining the mechanical behavior of a polymer cross-linked with dimers [62][63][64]. Therefore, the influence of the same parameters on the mechanical performance is also investigated in the current study and shown in figure 5.…”

Weak reversible cross links may decrease the strength of aligned fiber bundles

“…Previous results showed that the topology and arrangement of RCLs have a strong influence on the mechanical response of single polymer chains. 28,45 In the present paper the focus is put on the influence of the RCLs on the strength of the polymeric Fig. 1 Sketch of the used model.…”

“…The model presented here is a generalization of the single chain model described previously. 28,45 Each polymer in the system is described as a covalently linked chain of N hard spheres with diameter R oriented along the Z-axis (in the following R is set to the unit of length). Depending on the situation investigated the entire system is made of one, two or nine single chains.…”

Bioinspired composites – next generation of materials and devices

“…The topic self-healing concepts is addressed by Nabavi et al 3 by computational studies of a simple model for sacrificial bonding, a prominent mechanism for self-healing used in many natural systems such as in mussel byssus threads, for instance. It is demonstrated that the work to fracture in the bioinspired model systems is mainly determined by the number of sacrificial bonds, while stiffness and strength are rather dependent on their distribution.…”