Double helical conformation and extreme rigidity in a rodlike polyelectrolyte

Abstract

The ubiquitous biomacromolecule DNA has an axial rigidity persistence length of ~50 nm, driven by its elegant double helical structure. While double and multiple helix structures appear widely in nature, only rarely are these found in synthetic non-chiral macromolecules. Here we report a double helical conformation in the densely charged aromatic polyamide poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) or PBDT. This double helix macromolecule represents one of the most rigid simple molecular structures known, exhibiting an extremely high axial persistence length (~1 micrometer). We present X-ray diffraction, NMR spectroscopy, and molecular dynamics (MD) simulations that reveal and confirm the double helical conformation. The discovery of this extreme rigidity in combination with high charge density gives insight into the self-assembly of molecular ionic composites with high mechanical modulus (~ 1 GPa) yet with liquid-like ion motions inside, and provides fodder for formation of other 1D-reinforced composites.

Induced Spirals in Polyethylene Terephthalate Films Irradiated with Ar Ions with an Energy of 70 MeV

“…The only difference between PBDT and PET is that the repeat unit of PET contains a single benzene ring and a single type of dipole unit. We consider the explanation given by the authors of [26] to be physically well-founded and confirmed experimentally. In our explanation of the spiral formation effect we found, we follow the physical representations described in [26].…”

“…This, together with the fact that the benzene-carboxyl subunits are already ordered, allows dipole-dipole interactions between the subunits of neighboring chain molecules, causing correlated rotations of these subunits. The asymmetric nature of the dipoles in the subunits, together with their rotations, is sufficient in this situation for the formation of a spiral conformation of the subunits in neighboring chain molecules, as shown in [26]. As with PBDT, repeat units of PET chain molecules contain an aromatic ring with an attached dipole -CO unit.…”

“…After aging the picture is more complicated but still contains some cross-like symmetries. Such X-ray diffraction patterns are typical of solid polymer materials consisting of chain molecules coiled in spirals, in which rings due to the polycrystalline character of the material are superimposed with arcs due to the spiral configuration of the chain molecules [22,25,26]. All of the diffraction rings described above have azimuthal distributions of intensities consisting of rings on which long or short arcs with variable intensity are superimposed.…”

“…For example, in [26], the formation of a double helix in a sample of synthetic sulfonated aramid polyanion, poly-2,2' -disulphonyl-4,4' -benzidine terephthalamide (PBDT), was shown to be due to rotation of benzene rings associated with dipole units such as -SO 3 − and -NHCO-. A repeat unit of PBDT polymer molecule contains three benzene rings separated from the axis of the molecule by σ-hinges, but only two of them contain attached asymmetric dipole groups, which allows them to rotate during intermolecular interactions.…”

Strong graphene oxide nanocomposites from aqueous hybrid liquid crystals

“…In the Onsager excluded-volume picture of lyotropic liquid crystal formation 10 , a low onset concentration suggests a high aspect ratio. This results because the rod-like PBDT double helix aggregates into prolate particles-with an axial persistence length of~260 nm (see Supplementary Note 1) and an aspect ratio of 330 [10][11][12] . Graphene oxide (GO), a precursor to "synthetic graphene" 13 , has a lower modulus (~250 GPa) 14 than G, but the heterogeneity of the GO primary structure enables formation of a stable aqueous colloidal nematic phase above a critical concentration 15 with its director n GO specifying the average direction of platelet normals; the largest refractive index is perpendicular to n GO , hence the negative sign in the phase designation, N -.…”