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.