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Macroscopic supramolecular assembly through adjusting the surface-flexibility of the building block

Macroscopic supramolecular assembly (MSA) is a recent progress in supramolecular chemistry to associate large building blocks with a size larger than 10 μm through non-covalent interactions, which provides new ideas and solutions to the fabrication of supramolecular bulk materials. However, with increasing the size of the building blocks, it is difficult to achieve MSA because of insufficient binding strength between macroscopic building blocks. According to previous reports, the key element to achieve MSA is the high surface-flexibility, which endows the supramolecular groups on the surface with high freedom to reach interactive distance and interacts with each other through multivalent effect, thus enhancing the apparent binding constant of the macroscopic building blocks. Herein, we synthesized the grafted supramolecular polyelectrolytes which contained the host and guest functional groups as PAA-CD and PAA-Azo, respectively. Through modifying the polydimethylsiloxane (PDMS) building blocks with different number of polyelectrolyte multilayers contained β-cyclodextrin (CD)/azobenzene (Azo) groups by layer-by-layer method, we adjusted the surface-flexibility of the building blocks and systematically investigated the assembly behaviors of building blocks with varied number of polyelectrolyte multilayers. The results represent the building blocks modified with CD/Azo groups respectively cannot achieve MSA even stacking in water for long time (600 min) when the number of multilayers on the surface is few (5 bilayers); with increasing number of polyelectrolyte multilayers, the time for realizing MSA exhibits a decreasing trend. In addition, the interactive forces between PDMS building blocks with different number of multilayers were in situ measured in water with a DCAT apparatus. The interactive force between building blocks modified with CD/Azo groups gradually increase with the increased interaction time, and the growing rate of the forces is larger as the number of multilayers increases. The in-situ force measurement result corresponds well with the above assembly behaviors. Moreover, the major contribution of the host-guest supramolecular interaction is confirmed by both qualitative controlled MSA experiments and quantitative results with an in-situ force measurement method. This work verified the design principle of a highly flexible surface being requisite to realize MSA and elaborated that the macroscopic assembly behavior can be regulated by adjusting the surface flexibility of the building blocks.