Unusual inward growth of hexagonal nanosheets by the self- assembly of a homopolymer Yirong Fan 2 and Hui Sun 1 State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, No. 489 West Helan Road, Yinchuan, 750021, China. Unrevealing the nucleation and growth of nanocrystals is very important for understanding the relationship between structure and properties. In particular, the crystallization behavior of polymers is complicated and has attracted extensive attention of scientists.Generally speaking, the crystallization starts from the nucleus and continues to epitaxial growth. However, in this study we discover an interesting phenomenon of inward growth of a crystalline homopolymer to form hexagonal nanosheets. The amphiphilic homopolymer poly(3-(4-(phenyldiazenyl) phenoxy)propyl methacrylate) (PAzoPMA) with azobenzene pendants is synthesized via reversible addition- fragmentation chain transfer (RAFT) polymerization, which can self-assemble into various morphologies including nanowires, hexagonal nanosheets, giant compound micelles in different alcohols (ethanol, isopropanol and n -butanol) by solvothermal method. Notably, we find that the hexagonal nanosheets are crystalline other than the amorphous state. Therefore, we monitor the growth process of the hexagonal nanosheets at different annealing time, and find that the hexagonal nanorings with hollow structure are formed at the very beginning of annealing. With the increase of annealing time, the homopolymer attaches at the internal surface of the hexagonal nanorings and the inner wall of the nanorings begins to grow inward gradually to form a thin film that fully covers the bottom of the hexagonal nanorings. As the annealing time continues to extend, the thin layer at the bottom further grows along the side of the hexagonal nanorings to form hexagonal nanosheets with a uniform thickness, eventually. Overall, we find an unusual crystal growth mode from the outside to inside of polymers for the facile preparation of hexagonal nanosheets, which may bring new insight for understanding the crystallization behavior of polymers.
Figure 1 TEM and AFM images of hexagonal nanosheets formed by the inward growth of PAzoPMA.
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