Supramolecular networks that make use of specific non-covalent interactions furnish elastomer materials with superior processing and self-healing properties. However, they typically lack the hierarchical structure formation on different length scales observed in biomaterials that could be employed to tailor their mechanical properties. We prepared novel supramolecular materials based on oligopeptide-modified polymers that gave rise to “interpenetrating supramolecular networks”, that is, topologically independent networks with non-covalent interactions as network nodes. These materials displayed excellent energy dissipation. )
State of the Research
Structural biomaterials are built from a limited selection of chemical components but exhibit extraordinary properties specifically designed for their applications. This versatility is achieved by hierarchical structure formation on different length scales (Ritchie, Nature Mater. 2014, ASAP, doi:10.1038/nmat4089), through which identical supramolecular motifs are employed for different purposes. Supramolecular polymers and networks that make use of specific non-covalent interactions give rise to interesting novel elastomer materials (Seiffert, Chem. Soc. Rev. 2012, 41, 909); these undergo sharp thermal transitions to low viscosity melts that allow for convenient processing, and dynamic network reorganization processes may give rise to self-healing properties (Leibler Nature 2008, 451, 977). The reported examples, however, generally lack any hierarchical structure formation that would be required for a more sophisticated tailoring of the mechanical properties. Oligopeptide-modified polymers furnish well-defined nanostructures (Börner, J. Am. Chem. Soc. 2006, 128, 124142), but their role in bulk properties has not been addressed in detail.
We prepared novel supramolecular materials based on oligopeptide-modified polymers that, similar to what we had observed for organic nanowires, formed well-defined nanostructures. While polymers with very short oligopeptide end groups gave rise to small hydrogen-bonded aggregates that resulted in a supramolecular network, longer ones formed mixtures of single β-sheet tapes and stacked β-sheet nanofibrils that provided a secondary network and served as reinforcement. This process was “self-sorting” in the sense that the different types of nanostructures coexisted in mixtures of polymers with differently long oligopeptide end groups (Nature Commun. 2014). This control over the nanostructure and resulting network formation enabled us to prepare supramolecular elastomers with very similar molecular compositions but very different mechanical properties. While some of our materials were elastomers reinforced with oligopeptide nanofibrils and exhibited high moduli and elasticity, others exhibited excellent energy dissipation and were found to be high-performance mechanical-vibration damping materials (Figure 6).
The materials were the first example that featured “interpenetrating supramolecular networks”, that is, topologically independent networks with non-covalent interactions as network nodes. The excellent energy dissipation under maintained structural integrity is not displayed by either of the two networks alone.
Conclusions and Outlook
Our discovery of materials comprising interpenetrating supramolecular networks with tailored elasticity and energy dissipation is a unique technology platform of our laboratory that will be a corner stone of our future investigations across new classes of polymer materials and composites that are relevant for various applications in the automotive and aerospace industries.