As a way to prepare special functional materials, controllable self-assembly on nano scale has been widely concerned and studied in the past decades. For example, in block copolymers, dendrimers and other organic based soft material systems, various nanostructures have been widely reported. Among them, the unconventional Frank Kasper phase (F-K phase for short) in soft matter has been found in the process of self-assembly system in recent years and gained more attention. At present, through the corresponding simulation and theoretical research, it is found that the f-k phase formed by self-assembly needs to have specific topological structure restrictions and secondary forces.
In recent years, the academician team of Akron University and South China University of technology, Stephen Z. D. Cheng, has been committed to studying the relationship between structural primitives and the formed supramolecular structure based on giant molecules. Giant molecule is a kind of macromolecule constructed by "molecular nano particles" with precise chemical structure, nano size and rigid three-dimensional skeleton. Due to the use of rigid structural elements, the giant molecular system is different from other flexible systems (such as block copolymers, etc.), which can more intuitively show the influence of the topological structure of structural elements on the assembly structure in the process of self-assembly.
Recently, academician Cheng zhengdi's team reported a series of giant molecules based on trisubstituted benzene and polyhedral silsesquioxane (POSS). Through the differential design of their chemical structure, the influence of chemical structure, especially steric hindrance, on the formation of supramolecular nanostructures during self-assembly was studied. This series of giant molecules consists of two different Planar cores and different numbers of POSS around them linked by covalent bonds (Fig. 1a, e and Fig. 2a, d). Among them, the competition between π - π stacking between Planar cores and steric hindrance repulsion between rigid poss can be controlled by the number of poss. A rare method of constructing spherical supramolecular F-K phase with planar giant molecules has been successfully realized.
Figure 1. Structural characterization of the columnar phase formed by self-assembly of teb-bposs3 and tab-bposs3 with three poss. Photo source: ACS appl. Nano mater
Figure 2. Structure characterization of F-K A15 phase formed by self-assembly of teb-bposs6 and tab-bposs6 with six poss. Photo source: ACS appl. Nano mater
After annealing at 190 ℃ - 230 ℃ and quenching, the supermolecular nanostructures formed by these giant molecules were characterized by SAXS and verified by TEM. When the number of POSS contained in the planar giant molecule is three, the two samples (teb-bposs3 and tab-bposs3) form a columnar phase structure (Figure 1); when the number of POSS increases to six, the two samples (teb-bposs6 and tab-bposs6) form a rare F-K structure A15 phase (Figure 2). It is worth mentioning that although the f-k phase has been reported in the giant molecular system, most of them rely on the hydrophilic / hydrophobic interaction as the driving force, using the π - π interaction of planar molecules to form the spherical phase and then form the f-k structure, which is rarely reported in the giant molecular system or other systems.
Figure 3. Molecular arrangement mechanism assumption: (a) conventional columnar structure and (b) unconventional F-K A15 structure. Photo source: ACS appl. Nano mater
In this paper, the formation mechanism of self-assembly structure is discussed. Because the shape and volume of POSS are relatively fixed, and the size of POSS (1.1-1.2 nm) is larger than the π - π stacking distance of plane core (0.3-0.4 nm), when the molecules are assembled close to each other due to the force of plane core, the spatial hindrance of POSS will produce repulsive force, and the two forces will compete with each other. The resulting supermolecular nanostructures will depend on the balance of these two forces. When the number of POSS contained in each molecule is three, there is enough space around the column stem formed by the plane core stack to accommodate all poss through rotation, and the column phase can be maintained (Fig. 3a). When the number of posss contained in each molecule becomes six, the space around the column stem is not enough to accommodate all posss. The steric resistance effect produced by poss conflicts with the maintenance of the column structure, resulting in the forced fracture of the column structure to release the steric resistance. The plane molecules bend into a bowl and form a spherical aggregate (Fig. 3b), and then pile up into a F-K structure.
This achievement was recently published in ACS applied nano materials (cover, figure 4). The first author is Dr. Jiang Jing of Akron University, and the corresponding authors are academician Cheng zhengdi and Dr. Zhang Wei.
Figure 4. Cover concept. Photo source: ACS appl. Nano mater
Original text (scan or long press QR code, and then go to the original page after identification): modular constructed polygonal oligomeric silsesquioxane based giant movements for unconventional nanostructure fabricingjiang, Yu Wang, Lu Jin, Chih Hao Hsu, shuailin Zhang, Jialin Mao, weinbin Yin, Tao Li, Bo Ni, Zebin Su, Jiahao Huang, Chris wesdemiotis, Kan Yue, Wei Zhang,* Stephen Z. D. Cheng*ACS Appl. Nano Mater., 2020, 3, 2952-2958. DOI: 10.1021/acsanm.0c00231
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