Tensile properties of silkworm silk obtained by forced silking

Bave was acquired by the forced silking of three Bombyx mori silkworms, and its tensile properties were characterized. The material collected from any given silkworm yielded reproducible force-displacement plots, which were qualitatively similar to the plots obtained from silk collected from the other silkworms. This uniformity contrasts with the highly variable properties exhibited by silk which had been reeled from degummed cocoons. Scanning electron microscopy images were used to obtain information about the sample cross-sectional area, so that force-displacement plots could be resealed as stress-strain curves. Surprisingly, the scatter in the tensile properties increases after such rescaling. This finding can be explained in terms of the sericin coating of the bave (which contributes to the cross-sectional area but not significantly to the load-bearing capacity) having a variable thickness. When the sericin coating was eliminated by a degumming treatment, it was found that the fibers showed more consistent cross-sectional areas. Therefore, stress-strain curves of forced B. mori silk are reproducible, provided that force-displacement data are resealed by the correct cross-sectional area. Finally, the Weibull parameters of the forced silk were determined. The Weibull modulus, m, has a value of 13.0 +/- 0.3, which is more than double the value obtained previously from silk reeled from a cocoon, demonstrating that the process of degumming cocoons has a detrimental effect on the distribution of defects in the silk microstructure. (C) 2001 John Wiley & Sons, Inc.