No 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.

electronics and sensors mounted on drill bits to record geological data in the oil and gas industry need to be powered by batteries that operate in the temperature range from 60 to 120 °C and sometimes even up to 200 °C. [7] Additionally, the utilization of liquid electrolytes also makes it difficult to use lithium metal as an anode to further improve the energy density of lithium batteries, primarily due to lithium dendrite growth and active lithium consumption. [11,12] Because of these issues, alternative electrolyte materials are desired in order to meet the wide application demands of high temperature rechargeable lithium batteries. Solid polymer electrolytes are potential candidates for high temperature lithium battery electrolytes due to the absence of volatile liquid components. Poly(ethylene oxide) (PEO) is the most widely studied polymer electrolyte and has been shown anecdotally to work at up to 120 °C. [13] However, the cycling is not stable and the increased fluidity of PEO at such high temperature may lead to short circuit. [14,15] Other solid polymers have also been explored but none have shown stable cycling or adequate Coulombic efficiency above 100 °C.

Polymer electrolytes show promise as alternatives to conventional electrolytes in energy storage and conversion devices but have been limited due to their inverse correlation between ionic conductivity and modulus. In this study, we examine surface morphology, linear viscoelastic, dielectric and diffusive properties of molecular ionic composites (MICs), materials produced through the combination of a rigid and charged double helical polymer, poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), and ionic liquids (ILs). To probe temperature extremes, we incorporate a non-crystallizable IL to allow measurements from −90 to 200 °C. As we increase the PBDT weight percentage, shear moduli increase and do not decay up to 200 °C while maintaining room temperature ionic conductivity within a factor of 2 of the neat IL. We connect diffusion coefficients of IL ions with ionic conductivity through the Haven ratio across a wide temperature range and analyze trends in ion transport based on a relatively high and composition-dependent static dielectric constant. This behavior may result from collective rearrangement of IL ions in these networks. We propose that these properties are driven by a two-phase system in MICs corresponding to IL-rich "puddles" and PBDT-IL associated "bundles" where IL ions form alternating sheaths of cations and anions around each PBDT rod. These polymer-based MIC electrolytes show great promise for use in electrochemical devices that require fast ion transport, high modulus, and a broad thermal window.

Molecular ionic composites (MICs) are a new class of solid electrolytes that combine ionic liquids (ILs) and a rigid-rod double helical polyelectrolyte, poly(2,2′)-disulfonyl-4,4’benzidine terephthalamide (PBDT). In this study, we focus...