Birefringence behaviour of annealed silk fibres

Silk fibres were annealed at 140 ° C for different annealing times ranging from 2 to 10h. Refractive indices and birefringence of annealed fibres were measured by two different interferometric methods. An interference polarizing microscope and multiple-beam Fizeau fringes in transmission techniques were used. The scanning electron microscope was used to estimate the geometrical parameters of silk fibres. The behaviour of birefringence for annealed silk fibres showed that the birefringence reach a maximum value when silk was annealed for 5 to 6 h at 140 ° C. The isotropic refractive index of annealed silk is also slightly increased with increasing annealing time. The results obtained clarify the effect of annealing time on the optical behaviour of natural silk fibres. Microinterferograms are given for illustration.