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Manipulation of shape anisotropy of building blocks is creating diverse and complex assemblies, in particular in the field of colloid science. Self-assembly of block copolymers has created a great number of nano-objects and/or nanostructures. Herein we report the design and synthesis of a series of block copolymers with varying shape anisotropy using polyhedral oligomeric silsesquioxane (POSS) cluster and linear poly(ethylene oxide)s (PEO) with different molecular weights. The POSS-b-PEO block copolymers were prepared by means of first gathering one to four POSS clusters using short linear or dendritic linkers and then covalently linking them with linear PEO blocks. In this way, the shape anisotropy of the POSS-b-PEO block copolymers is varied widely. We found that the PEO block crystallization in a selected solvent drives the block copolymers to self-assemble into large nanothick sheets. We characterized the sheets using TEM and XRD and found a structure of one PEO crystalline layer sandwiched by two POSS layers. Then, we measured the melting, recrystallization, and remelting processes of the sheets using DSC and observed an effect of varying shape anisotropy on the melting and recrystallization temperatures of the PEO and POSS layer and the crystallinity of the crystalline PEO layer. We proposed ideal molecular structures to depict PEO crystalline structure and POSS packing in the sheets and then semiquantitatively discussed the effect of the varying shape anisotropy on the sheet formation from solution and on PEO melting and crystallization in the nanoconfined space. [ABSTRACT FROM AUTHOR]

Clusters are an important class of nanoscale molecules or superatoms that exhibit an amazing diversity in structure, chemical composition, shape, and functionality. Assembling two types of clusters is creating emerging cluster-assembled materials (CAMs). In this paper, we report an effective approach to produce quasi two-dimensional (2D) CAMs of two types of spherelike clusters, polyhedral oligomeric silsesquioxanes (POSS), and polyoxometalates (POM). To avoid macrophase separation between the two clusters, they are covalently linked to form a POM-POSS cocluster with Janus characteristics and a dumbbell shape. This Janus characteristics enables the cocluster to self-assemble into diverse nanoaggregates, as conventional amphiphilic molecules and macromolecules do, in selective solvents. In our study, we obtained micelles, vesicles, nanosheets, and nanoribbons by tuning the n-hexane content in mixed solvents of acetone and n-hexane. Ordered packing of clusters in the nanosheets and nanoribbons were directly visualized using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique. We infer that the increase of packing order results in the vesicle-to-sheet transition and the change in packing mode causes the sheet-to-ribbon transitions. Our findings have verified the effectivity of creating quasi 2D cluster-assembled materials though the cocluster self-assembly as a new approach to produce novel CAMs. [ABSTRACT FROM AUTHOR]