Abstract
The following new results have been obtained by the differential thermal analysis of silk materials:
1. All the silk fibers show an initial endothermic reaction at about 100°C on account of the evaporation of absorbed water. The second endothermic peak occurs at 305°C in domestic silk and around 350°C in wild silks. Silk is known to consist of polypeptide chains in a predominantly extended configration with the intermolecular hydrogen bonding. The second endothermic peak may be attributed to the destruction of β-configuration.
2. For Bombyx fibroin, α-β transition during the wet thermal treatment begins to occur around 120°C, and the α-fibroin almost changes to the β-fibroin at 130°C. On the other hand, the α-fibroin begins to change to the β-form around 240°C during the dry thermal treatment and completely changes to the β-form at 260°C. But this α-β transition could not be detected by DTA.
3. The α-β transition for native Tussah silk fibroin in silkgland can be detected by DTA. Its endothermic peak occurs at 54°C. This behavior is in agreement with the result of X-ray diffraction. For the α-form of coagurated fibroin, an initial endothermic peak appears at about 100°C due to the loss of absorbed water. The α-helical structure of fibroin begins to be destroyed at about 160°C, and small endothermic peak by the α-β transition appears at about 200°C. The next major reaction is also endothermic and begins around 270°C. A single peak at 336°C is obserbed for coagurated fibroin. It is thought that these endothermic reactions may arise from the decomposition of the β-fibroin component.
4. The thermogram obtained from poly-L-alanine is very similar to these from wild silks. The endothermic peaks appear at 95°C and 353°C and a exothermic peak occurs at 342°C. The X-ray diffraction powder diagrams show that crystallites of poly-L-alanine begain to show the β-form at 280°C, and become perfectly β-form at 324°C.
5. According to the DTA thermograms obtained from Bombyx sericin, the sericin extracted with water at 120°C for 1 hr. shows endothermic reaction starting gradually around 160°C and with the peak at 200°C. On the other hand, the coagurated sericin in silkgland shows the peak at 223°C. These endotherms correspond to the temperature of disappearance of birefringence for the sericin. Therefore, these endothermic peaks show the decomposition point of the sericin crystallites. It is thought that the difference of endothermic peak depends on the size of crystallites of sericin.
6. When silk fibers are heated in water under pressure at the temperature higher than 100°C, the second endothermic peak showing the decomposition of β-structure is shifted to higher temperature with decreasing temperatures. This phenomena are explained by the crystallization.
