2020同步年報

Soft Matter 037 Hierarchical Superhelices from Achiral Polymers Artificial periodic helical nanostructures on a sub-optical wavelength scale have been demon- strated to have prominent optical properties for application of metamaterials. A helix is a fascinating structure in nature; from DNA to cell, tissues are related to the helical structure, but helical structures exist typically in a chiral system. Herein, Wei-Tsung Chuang (NSRRC), Yeo-Wan Chiang (National Sun Yat-sen University) and I-Ming Lin (National Sun Yat-sen University) proposed a novel concept of focal asymmetry to fabricate hierarchically helical structures from nanome- ter to submicrometer scale in the self-assembly of achiral dendron-jacketed block copolymers (DJBCP). As shown in Fig. 1 , to create the focal asymmetry in dendrons, they syn- thesized two positional isomers of dendrons featuring two amphiphilic tails at the 3,4- and 3,5-positions of the benzoic acids, which behave as asymmetric and symmetric den- drons (denoted aD and sD ), respectively. The ortho-position poly(2-vinylpyridine) (P2VP) in polystyrene- block -poly(2- vinylpyridine) (PS- b -P2VP) and the para-position poly(4-vin- ylpyridine) (P4VP) in polystyrene- block -poly(4-vinylpyridine) (PS- b -P4VP) respectively represent asymmetric and sym- metric concepts in the supramolecular dendron-jacketed blocks. Through hydrogen-bonding interactions, aD and sD units can be grafted to the P4VP blocks and the P2VP blocks, resulting in DJBCP of four types with varied levels of asymmetry. Figure 2 presents transmission-electron-microscope (TEM) images of the self-assembled nanostructures obtained from the four types of DJBCP with a constant PS volume fraction, 0.23, so the PS domains (bright area in TEM images) are sur- Fig. 1 : Schematic representation of hierarchical helical assemblies of supramolecular DJBCP induced on controlling the asymmetric chemical structures of the dendrons and the interactive hydrogen-bonding (H-bonding) sites of the diblock copolymers. [Reproduced from Ref. 1] Fig. 2 : TEM micrographs of films as cast of (a) PS-b-P4VP(aD) 0.5 , (b) PS-b-P4VP(sD) 0.5 , (c) PS-b-P2VP(sD) 0.75 and (d) PS-b-P2VP(aD) 0.3 . After staining with I 2 , the dendron-jacketed blocks and PS blocks appear dark and bright, respectively. [Reproduced from Ref. 1] rounded by dendron-jacketed blocks (dark area in TEM images). The helical structures appeared in the three types of DJBCP with asymmetric compo- nents, which are PS- b -P4VP( aD ) 0.5 , PS- b -P2VP( aD ) 0.3 and PS- b -P2VP( sD ) 0.75 (here, 0.5, 0.3 and 0.75 represent the molar fraction of aD or sD to P4VP or P2VP unit); PS- b -P4VP( sD ) 0.5 with both symmetrical components formed a cylindrical structure. Combining the above result, as long as it contains an asymmetric element, the helical struc- ture can be produced. As shown in Fig. 3 , PS- b -P4VP( aD ) 0.5 was selected for small-angle X-ray scattering (SAXS) with oscillatory- shearing alignment in situ at TLS 23A1 . A highly ordered SAXS pattern reveals a highly oriented and ordered (a) (b) (c) (d)