The influence of the coordination of crosslinks on the mechanical properties of polymers

Polymeric structures are ubiquitous in natural as well as technological systems. The mechanical properties of these systems are strongly dependent on the degree and kind of crosslinking of the different polymeric chains. The vast majority of theoretical concepts investigating the effect of crosslinking deal with two-fold coordinated crosslinks, i.e. crosslinks formed between two monomers only. Nevertheless, a variety of materials is known that crosslink via so called tris-complexes, i.e. one crosslink is formed between three monomers. One example for this kind of material is the iron-dopa complex that depending on pH can exist in the mono-, bis- or tris-state. It is the aim of this project to close the aforementioned gap in the theoretical description and to investigate the effect of three-fold coordinated crosslinks on the mechanical properties of polymeric structures. It is expected that this effect is highly non-trivial. First, the force flow through a three-fold coordinated structure is completely different than through a bis-complex. Second, rupture of a tris-complex is always a two stage process. When the first monomer detaches, the tris-complex is transformed into a bis-complex. Subsequently, this bis-complex may either completely dissociate through a rupture of the remaining bond, or the tris-complex may be restored through the attachment of a different monomer. In the framework of this project a simple generic potential shall be used to describe reversible crosslinks of different coordination. The static and dynamic mechanical properties of the systems will be studied using Monte Carlo and molecular dynamics techniques. Two different geometries will be investigated: aligned fiber bundles and random fiber networks. Special emphasis in the analysis will be put on understanding the relation between backbone bending elasticity, sticky site density and polymer density on the one hand and the mechanical behavior of the system on the other hand. Computational loading tests will be performed to assess key mechanical parameters like stiffness, toughness or strength of the systems. The energy dissipation of these structures will be estimated by doing cyclic loading tests. It is the ultimate goal of this project to reveal the influence of coordination of crosslinks on the mechanical properties of crosslinked polymeric systems. This will lead to a better understanding of the functionality of biological structures as well as to the possibility to specifically tailor the mechanical properties of artificial systems.

Biological materials are a great source of inspiration for material scientists in their quest to build new materials with specifically tailored properties. One especially fascinating material are mussel byssal threads that marine mussels use to adhere to rocky substrates. From a mechanical point of view the mussel byssus is stiff and tough, possesses a hard outer coating, shows a high extensibility and naturally adheres well in aqueous environment. Furthermore, mussel byssal threads show self healing behavior, although no living cells are present in these structures. Last but not least, these structures are clearly environmental friendly. All these properties make it desirable to transfer the design principles of the byssus into a technological solution. The main reason for the byssus' remarkable properties can be found in metal coordination bonds that cross-link the polymers making up the structure. A metal coordination bond consists of a metal ion (in the case of mussels mostly iron or zinc) that binds to several organic ligands (sidechains of the proteins making up the structure) cross-linking the proteins. The number of organic ligands defines the coordination of the cross-link. This coordination can be influenced by external parameters, like the pH value, giving the organism the possibility to tailor its mechanical properties. To transfer the advantages of biological materials into technological solutions it is inevitable to understand the design principles of the former on all length scales. In current microscopic models describing the mechanical properties of polymeric systems, a cross-link is often defined as an additional bond between two ligands. The important case of three ligands is mostly neglected. At this point the current FWF project sets in. In this project microscopic models of polymeric systems were developed allowing to include cross-links of different coordination. Numeric techniques were used to perform computational loading tests on the systems to assess their mechanical properties. One important result of the investigations is that a large number of cross-links is not always beneficial for the mechanical performance of the structures. The presence of many cross-links may even lead to a reduction of the strength of the system. This surprising result is due to the fact that a large number of cross-links may deform cooperatively leading to a combined strength of the cross-links that exceeds the strength of the covalent backbone of the structures leading to permanent damage of the material. This effect is largest for two-fold coordinated cross-links and reduces significantly when the coordination of cross-links increases. Another result is that a certain degree of disorder in the system is beneficial for the mechanical performance. It is expected that these discovered principles will help to synthesize novel materials that exhibit some of the remarkable properties of their biological counterparts.