Thermal transport in single silkworm silks and the behavior under stretching

Abstract

This work reports on the first time study of thermal transport in the axial direction of single silkworm silks. The measured thermal diffusivity of relaxed silkworm silk and thermal conductivity are 0.39 × 10⁻⁶ to 2.03 × 10⁻⁶ m² s⁻¹ and 0.54–6.53 W m⁻¹ K⁻¹, respectively. The thermal diffusivity of silkworm silk increases up to 263% upon elongation up to 63.8%. For one of the samples studied (sample 5), the thermal conductivity goes up to 13.1 W m⁻¹ K⁻¹ after elongation of 68.3%, surpassing many other polymers. Three factors combine together to give rise to the remarkable thermal diffusivity increase: alignment improvement of β-sheet blocks, straightening of random coils under stretching, and structural transformation from random coil to β-sheet crystal by elongation (confirmed by our Raman spectroscopy study). Thermal path breakdown is observed when elongation is beyond 63.8%, suggesting that the length of the random coils under relaxed condition is about 61.1% of their real molecular link length. Our Raman spectroscopy study confirms this speculation: after 60% elongation, the Raman frequency started to increase, indicating that the internal strain/stress has been released due to internal structure breakdown.