Phase Behavior of Sugar-based Block Co-oligomer Modulated by Molecular Chirality

Abstract

Sugar-based block co-oligomer (BCO) consisting of saccharide block exhibits high segregation strength that overcomes the limitation of the traditional block copolymers for accessing the ordered nanostructures with ultrasmall feature size. This study explores the influence of molecular chirality on the self-assembly behavior of glucose-block-tocopherol (Glc-b-Toc) BCOs, wherein the Toc blocks were either pure in chirality or being a mixture of the stereoisomers. Both the chiral and racemic BCOs formed a hexagonal perforated layer (HPL) structure with an unusually small aspect ratio (c/a) of the unit cell, attributable to the propensity of the Glc block to enhance hydrogen bonding formation. As the temperature was increased, the dominance of hydrogen bonding interactions diminished, resulting in a gradual increase in the c/a ratio and eventually a transformation of the HPL structure to a double gyroid (DG) phase. The chirality of the Toc block enhanced the effective segregation strength of the BCO, leading to elevated order-order transition temperatures associated with the HPL-to-DG and DG-to-hexagonally packed cylinder (HEX) transitions. Within the HPL phase window, the chiral BCO exhibited significantly slower kinetics during thermally-induced structural reorganization in adjusting the c/a ratio, especially in the cooling process. Consequently, a pronounced hysteresis in the structural reorganization of the chiral BCO was observed. This phenomenon was ascribed to the chiral interaction between the Toc blocks, which limited the diffusion of the BCO molecules for the structural reorganization.