Control of Macroscopic Polymer Mechanics through Supramolecular Nanostructure

Nature utilizes supramolecular architectures to sustain life. One significant role thereof is to resist external stimuli including mechanical stress and damage. The programmed assembly of biopolymers into the supramolecular structures enables precise control of macroscopic properties and leads to superior functions including mechanical toughening and self-healing. Such supramolecular structures are constructed by the elaborate arrangement of chemically imprinted non-covalent bonds in biopolymers. In the case of synthetic polymeric materials, however, it is challenging to implement stable supramolecular structures in the condensed polymeric matrix where molecular motion is highly restricted. In this study, we have proposed a new strategy to incorporate well-defined supramolecular nanofibers into the polymer matrix and clearly demonstrated the effect of the fibers on the macroscopic mechanical and dynamic properties. We employed flexible PDMS (polydimethylsiloxane)-based polymers with periodically positioned urea functional groups and different structural flexibility around the urea motifs. With low flexibility, the consecutive hydrogen bonding between urea motifs is less feasible due to the limited chance of physical contacts. In contrast, strong consecutive hydrogen bonding can be achieved by high structural flexibility, which enables the growth of stable, robust, and well-defined supramolecular fibers even in highly entangled polymer matrix. The formation of supramolecular fibers in the polymer with high structural flexibility was confirmed by AFM imaging, X-ray scattering, and UV-vis spectroscopic studies. This polymer having rich supramolecular fibers shows higher elastic modulus, compressive strength, and more solid-like behavior compared to the polymer without supramolecular structure, despite the subtle differences in the chemical structure and molecular weight. More intriguingly, we found that selective and orthogonal self-healing takes place when the fibers are embedded in the polymer matrix, which means the fibers recognize each other. To the best of our knowledge, this is the first example of macroscopic self-sorting in self-healing process. We anticipate that our design strategy modulating the structural flexibility around dynamic bonding will provide new insights into the development of versatile, multi-functional supramolecular polymeric materials.