Silk proteins are of great interest to the scientific community owing to their unique mechanical properties and interesting biological functionality. In addition, the silk proteins are not burned out following heating, rather they are transformed into a carbonaceous solid, pyroprotein; several studies have identified potential carbon precursors for state-of-the-art technologies. However, no mechanism for the carbonization of proteins has yet been reported. Here we examine the structural and chemical changes of silk proteins systematically at temperatures above the onset of thermal degradation. We find that the β-sheet structure is transformed into an sp2-hybridized carbon hexagonal structure by simple heating to 350 °C. The pseudographitic crystalline layers grew to form highly ordered graphitic structures following further heating to 2,800 °C. Our results provide a mechanism for the thermal transition of the protein and demonstrate a potential strategy for designing pyroproteins using a clean system with a catalyst-free aqueous wet process for in vivo applications.

In this study, for the first time, thermal oxidation, which has only been considered as a degradation pathway for plastics, served as a simple and effective pretreatment protocol to modulate the chemical structure of linear low density polyethylene (LLDPE) for successful conversion of "noncarbonizable" LLDPE into an ordered carbon. More importantly, LLDPE based carbon could be graphitized into a highly ordered graphitic carbon with exceptional electrical performance exceeding that of Super-P, a pricey reference conductive agent for lithium ion battery fabrication. Upon thermal oxidative pretreatment, inherently noncarbonizable LLDPE was successfully transformed into an ordered carbon through heat treatment with a high conversion yield reaching 50%, a yield comparable to that obtained from polyacrylonitrile (PAN), a reference polymeric precursor. Systematic interrogation of the chemical structural evolution using X-ray diffraction, Raman, and dynamic scanning calorimetry (DSC) analysis, confirmed that an oxidation reaction occurred around 330 °C, which initiated transformation of aliphatic chains into cyclized ladder structures that allowed successful carbonization of LLDPE with high carbon yield. The thermally oxidized LLDPE evolved into a highly graphitic carbon that exhibited superior degree of ordering and electrical performance over a graphitized PAN counterpart. Finally, LLDPE waste, such as cling wrap and poly gloves was also successfully converted into an ordered carbon comparable to that obtained from the as-produced LLDPE precursor, suggesting opportunities associated with "upcylcing" of waste products. Thus, the proposed protocol represents an effective, potentially low-cost, and sustainable pathway providing for an exceptionally high quality conductive agent applicable in energy storage and flexible, printed electronics.

Silks are protein-based natural structured materials with an unusual combination of high strength and elongation. Their unique microstructural features composed of hard β-sheet crystals aligned within a soft amorphous region lead to the robust properties of silks. Herein we report a large enhancement in the intrinsic properties of silk through the transformation of the basic building blocks into a poly-hexagonal carbon structure by a simple heat treatment with axial stretching. The carbon clusters originating from the β-sheet retain the preferred orientation along the fibre axis, resulting in a long-range-ordered graphitic structure by increasing heat-treatment temperatures and leading improvements in mechanical properties with a maximum strength and modulus up to ∼2.6 and ∼470 GPa, respectively, almost four and thirty times surpassing those of raw silk. Moreover, the formation of sp 2 carbon configurations induce a significant change in the electrical properties (e.g. an electrical conductivity up to 4.37 × 103 S cm−1).