Influence of Water Content on the β-Sheet Formation, Thermal Stability, Water Removal, and Mechanical Properties of Silk Materials

Abstract

Silk, which has excellent mechanical toughness and is lightweight, is used as a structural material in nature, for example, in silkworm cocoons and spider draglines. However, the industrial use of silk as a structural material has garnered little attention. For silk to be used as a structural material, its thermal processability and associated properties must be well understood. Although water molecules influence the glass transition of silk, the effects of water content on the other thermal properties of silks are not well understood. In this study, we prepared Bombyx mori cocoon raw fibers, degummed fibers, and films with different water contents and then investigated the effects of water content on crystallization, degradation, and water removal during thermal processing. Thermal gravimetric analyses of the silk materials showed that water content did not affect the thermal degradation temperature but did influence the water removal behavior. By increasing the water content of silk, the water molecules were removed at lower temperatures, indicating that the amount of free water in silk materials increased; additionally, the glass transition temperature decreased with increasing water plasticization. Differential scanning calorimetry and wide-angle X-ray scattering of the silk films also suggested that the water molecules in the amorphous regions of the silk films acted as a plasticizer and induced β-sheet crystallization. The plasticizing effect of water was not detected in silk fibers, owing to their lower amorphous content and mobility. The structural and mechanical characterizations of the silk films demonstrated the silk film prepared at RH 97% realized both crystallinity and ductility simultaneously. Thus, the thermal stability, mechanical, and other properties of silk materials are regulated by their water content and crystallinity.

Silk: Optical Properties over 12.6 Octaves THz-IR-Visible-UV Range

“…Therefore, it is important to ascertain whether the observed peaks are related to water content. As silk is heated, water removal by evaporation efficiently proceeds in the range from 70 ${}^{\circ }$C to ∼200 ${}^{\circ }$C [36]. Therefore, if the peaks at around 240 cm${}^{-1}$ were related to water, they would be expected to gradually disappear in the thermally treated samples, which was not observed in experiment (Figure 3).…”

Relationships between physical properties and sequence in silkworm silks

“…Although silk fibres from the two main lineages investigated typically follow different deformation paths that can be related to the differences in modular organization of their repetitive sequences, individual samples exhibited a wide degree of variability in their measured tensile parameters. Indeed, although the amino acid sequence of the constituent fibroin undoubtedly plays an important role in shaping material properties, other factors such as the spinning process2526, reeling rate60, fibre morphology3661, flaw distribution30, temperature62, humidity6364 and degree of sericin binding65 have also been shown to modulate the mechanical properties of silk fibres. There is doubtless a subtle and complex interplay between these different intrinsic and extrinsic factors.…”

Experimental Methods for Characterizing the Secondary Structure and Thermal Properties of Silk Proteins

“…[46,47,51–54] Water in the form of freezing bound water, nonfreezing bound water, and freezing free water has a significant influence on the secondary structure, thermal properties, and mechanical properties of silks. [55,56] However, these interactions can be challenging to study because water overlaps with the amide I band in infrared analysis, and thus DD-MAS NMR is a powerful tool for studying this fundamental relationship. In one example, Ala, Ser, and Tyr residues in silk fibroin were isotopically labeled ( 13 C) and studied by DD-MAS NMR in both the dry and hydrated state, revealing site specific effects of water on silk (Figure 2).…”