Structural changes of Bombyx mori silk fibroin in the course of drawing.

Abstract

Structural changes by drawing of Bombyx mori silk fibroin were investigated by means of X-ray diffraction, differential scanning calorimetry, and thermomechanical analysis (TMA). At the early stage of the drawing, the degree of orientation of silk fibroin molecules increased with increasing draw ratio and the content of the randomly coiled region decreased. This orientation may be attributed to the formation of hydrogen bonds between the C=O and N-H groups of adjacent molecules. Further drawing accelerated the formation of the β-sheet structure. When the draw ratio increased more than 6 times, the degree of orientation of silk fibroin molecules increased up to the value comparable to those of the silk fibroin fiber in which a β-pleated-sheet structure with anti-parallel chains was formed. The rate of the thermal contraction of the specimen observed on the TMA curves decreased with increasing draw ratio in the temperature range of 230-260°C and the length of the specimen remained constant due to the high thermal resistance associated with the intersheet and interchain hydrogen bonds, when the pleated sheet structure was formed. At this stage, the specimen exhibited the endothermic shoulder peak at about 310°C and an additional major endothermic peak at 282°C.