Structural changes of silk fibers induced by heat treatment

The structural changes of silk fibers induced by heat treatment were investigated by thermomechanical analysis, dynamic mechanical measurements, X‐ray diffraction analysis, and refractive indices measurements. Bombyx mori silk fibers exhibited only a slight contraction, while tussah silk fibers showed a prominent two‐step contraction during the heating process. The dynamic storage modulus of B. mori fibers remained unchanged until the final decrease at about 170°C. The E' value of tussah fibers showed a slight increase ranging from 30 to 70°C and a final abrupt decrease at about 190°C. The broad peak of dynamic loss modulus showed a maximum at 220 and 230°C for B. mori and tussah silk fibers, respectively. The crystalline structure of silk fibers did not show any significant change regardless of heat treatment. The birefringence value of B. mori silk remained unchanged in the range from 25 to 240°C and then decreased, while that of tussah fibers decreased linearly. The isotropic refractive index increased for both silk fibers, even though with a different rate with increasing temperature. We proposed a schematic representation showing the structural changes of silk fibers induced by heat treatment, taking into account the changes in molecular orientation in the amorphous, laterally ordered and crystalline regions.