Polypeptides have been widely studied and applied in biomedicine, energy storage, and sensing due to the precise ability to edit sequence, structure and function. The incorporation of a polypeptide backbone into ionic polymers allows us to understand the role of secondary structure (alpha helix, beta sheet) on properties such as the ionic conductivity, as most polymer electrolytes exist as random coils. Here three polypeptide polymerized ionic liquids (PILs) with different secondary structure (left-handed helix, right-handed helix and random coil conformations) were synthesized using the chirality of the amino acid. For ammonium bis(trifluoromethane sulfonimide) peptide PILs with a degree of polymerization (DP) of 50, We find that the polypeptide-based PIL with a random coil conformation exhibit the highest Tg-normalized ionic conductivity compared to the either left or right handed helix conformation (which are the same within error). Interestingly, high temperatures (200 °C) can lead to large changes in the Tg of the random coil while the helix is stable and unchanged, which we attribute to the disruption of intermolecular hydrogen bonding. To further probe the role of helix on conductivity, we also synthesized a series of polypeptide PILs with DP = 50-1000 to vary the length of helix along the backbone. Unlike most polymer electrolytes where Tg-normalized ionic conductivity decreases with increasing chain length, the ionic conductivity of helical polypeptide-based PILs increases with increasing helix polymer chain length, which we attribute to the persistent helical conformation of the polypeptide. This work provides a systematic understanding of how secondary structure affects the ionic conductivity, and presents a new platform for the development of bioinspired high-performance polypeptide electrolytes.

Understanding the role of secondary structure and molecular weight on polypeptide polymerized ionic liquids

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