One of the most interesting characteristics of crystals is their profound morphology. Single crystal level crystalline structure can be divided into shape-translational symmetry commensurate crystals and shape-translational symmetry incommensurate crystals (SSICs). The former case is best represented by conventional polymer lamellae while SSICs typically show non-flat shapes, which are incommensurate with translational symmetry in Euclidean space. Helicoidal, tubular, and scrolled crystals are classical examples of SSICs.
This work reports that single crystals of a molecular bottlebrush (mBB) with crystalline poly(ethylene oxide) (PEO) side chains can be SSIC (https://www.nature.com/articles/s41467-020-15477-5). The PEO mBBs are synthesized by grafting alkyne end-functionalized PEO onto an azide-bearing backbone polymer using the highly efficient copper(I)-catalyzed azide-alkyne cycloaddition click reaction. Self-seeding solution crystallization leads to spherical hollow mBB crystals (Figure 1), which mimic classical polymer assembly structure such as vesicles and recently discovered crystalsomes obtained using a miniemulsion template. This unique crystalline structure is referred to as molecular bottle brush crystalsomes (mBBCs). The spontaneous translational symmetry breaking in mBBCs can be attributed to the competition between side-chain crystallization and local chain overcrowding which is intrinsic in mBBs with a high grafting density, as evidenced by fluorescence resonance energy transfer experiments (Figure 1). Consequently, the radius of curvature of the hollow crystalline spheres can be further tuned by side-chain grafting densities as well as crystallization conditions.
Figure 1. SEM image of the spherical molecular bottlebrush crystalsomes formed by solution self-seeding crystallization. Inserted schematics show the molecular bottlebrush chain (left) and the asymmetric packing of the crystal (right).
Detailed molecular mechanisms have been proposed for various types of polymer SSICs, including chiral packing, asymmetric folding, defects, self-induced compositional and mechanical field, etc. Our study unravels a new principle of spontaneous translational symmetry breaking. When both crystallizable and noncrystallizable components coexist in one chain, competing packing dictates the final assembled structure. As intriguing new properties have been observed in other crystalsome systems, this work, therefore, provides a facile route to fabricate such functional crystals and the general principle can be utilized towards designing versatile nanostructures.
Written by Hao Qi, Bin Zhao, and Christopher Li
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