Synthesis and Self-Assembly of a Series of nPOSS-b-PEO Block Copolymers with Varying Shape Anisotropy

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.