The importance of the σ-hole in the self-assembly of halogenated peptoid nanosheets

Halogens are gaining increasing importance in designing functional polymeric materials. The most prominent feature of halogens is their high electronegativity, which facilitates the exploration of halogen-containing self-assembled structures. However, when halogens are involved in a covalent bond, the halogen atom tends to polarize and forms a region of positive electrostatic potential on its surface. This region of positive potential is referred to as a σ-hole. In this study, a series of amphiphilic diblock copolypeptoids were synthesized with a standard phenyl or a halogenated phenyl in the side chain. These diblock copolypeptoids self-assemble into crystalline nanosheets in water, and high resolution cryo-TEM imaging of the nanosheets reveals a surprising change in the crystal motifs of the halogenated nanosheets. This change in the crystal motifs was only observed by doing direct, atomic-scale imaging on the radiation sensitive materials. Molecular dynamics simulations are used to understand the intermolecular forces that gave rise to the varying crystal motifs. This combination of atomic-scale imaging and atomic-scale simulations can be broadly applied to study the effect that halogens have on the self-assembly of polymeric materials.