Atomic-scale imaging of the effect of side-chain chemistry on the crystal motifs in polypeptoid nanosheets

Polypeptoids are sequence-defined polymers that have received considerable attention as a platform to create biomimetic nanomaterials. When varying the side chain chemistry of an amphiphilic peptoid between hydrogen, fluorine, chlorine, bromine, and iodine, the arrangement of the polypeptoid molecules in a self-assembled crystalline nanosheet changes; this change cannot be revealed by conventional X-ray scattering or diffraction techniques due to the lack of phase information in position space. However, by combining low-dose cryo-TEM with crystallographic and single-particle image analysis techniques, we are able to reconstruct atomic-scale images of the crystal motifs of polypeptoid nanosheets. The different crystal motifs are interpreted in light of electronegativity, steric hindrance, and non-bonded internal energy obtained by molecular dynamic simulations. Precise engineering of polymer nanomaterials is enabled with this understanding of the competing factors that determine the polypeptoid crystal motifs at the atomic level.