Morphology of Polymers and Ions on the Atomic-Scale Revealed by Cryogenic Transmission Electron Microscopy*

The atomic-scale structures of proteins have been revealed by cryogenic transmission electron microscopy (cryo-TEM).  We leverage this development to image the molecular arrangement and crystal motifs in synthetic polymers.  We compare images of self-assembled nanosheets and nanofibers with simulation results, thereby validating the atomic-scale interactions assumed in the simulation with experimental data on the same scale.  We use atomically-defined amphiphilic polypeptoid block copolymers synthesized by solid-phase synthesis, and focus on halogen atoms that are either covalently or ionically bonded to the monomers of polypeptoids.  The covalently bonded case enables a study of the role of halogen-bonding in lattice structures in polymer self-assembly. The ionically bonded case, the polypeptoid which is positively charged, contains chloride counterions.   Cryo-TEM enables imaging of the counterions, providing direct images that we used to test theories of Stern layers and counterion condensation.