LAUSR

Nanoscale two-dimensional (2D) polymer-based platelet structures are of considerable interest for a range of applications [1]. In principle, solution self-assembly of block copolymers (BCPs) provides a convenient route to analogous planar nanostructures derived from soft matter [2]. However, the formation of 2D platelet micelles is generally uncommon relative to other morphologies. In literature, this type of materials is always prepared by crystallization-driven selfassembly or some other crystalline-related methods, while non-crystallization approach is quite rare [3]. Very recently, Prof. He from Southern University of Science and Technology and his coworkers [4] achieved highly controllable 2D squares in nano and millimeter scale by using π-π interaction as the main driving force, which is another non-crystallization approach. The building block they employed, is a series of rod-coil diblock copolymers with precise molecular weight prepared by the combination of Siegrist polycondensation and anionic polymerization. The copolymers contain the same rod block, which was poly (2,5-di(2′-ethylhexyloxy)-1,4-phenyle nevinylene) (PPV) with DP=12, while the coil block was made of poly(2-vinyl pyridine) (P2VP), the DPs varied from 12 to 71. Then the copolymers were dissolved in 2-propanol, which is a selective solvent to P2VP. The solution was heated at 85 °C for 45 min, followed by aging for 24 h at 25 °C (Figure 1). By using transmission electron microscopy (TEM), 2D square micelles were found from the solutions of block copolymers PPV12-b-P2VPn. The squares had uniform diameters, which could be tuned by the ratios of P2VP to PPV blocks as well as concentration of the solution. By varying the block ratio from 1 to 3, the square diagonal lengths decreased from about 3 μm to 240 nm and for 1:1 copolymer, diagonal lengths decreased from 2750 nm (0.01 mg/mL) to 1300 nm (0.0025 mg/mL). Interestingly, the concentration dependence disappeared completely when P2VP block was twice longer than PPV core, which correlates with the suppressed hydrophobic interaction due to a slowdown in dynamic exchange of unimers. From the height of these squares measured by atomic force microscope (AFM), one may find their dependency on P2VP block length, which supports nicely to the sandwich packing mode purposed by the authors. To further explore the formation mechanism of these squares, the group used UV-Vis spectroscopy and light scattering intensively to characterize the square formation process. From the linear dependency of Rg/Rh value (the morphology factor, ρ) vs. temperature, one may find that the growth of these squares was from worm-like micelles to squares during the cooling process. Moreover, no crystalline morphology was observed by both selected-area electron diffraction (SAED) and high resolution TEM (HRTEM) whereas grazing-incidence wide-angle X-ray scattering (GIWAXS) results suggested an edge-on configuration of the π-π stacking PPV backbones in the 2D squares. The contribution of branched alkyl side chains in herringbone molecular arrangement was also recognized. Meanwhile, the solution investigation by using light scattering also indicated that this non-crystallization-driven self-assembly process had a high yield of nano-objects, which might be very useful in further application studies. In short, this paper opens a new avenue to obtain 2D mate-