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In this study, the mechanical and structural characteristics of wool fibers were examined, and the contributions of the microfibrils and matrix to the deformation processes were analyzed. The experiments were carried out on single wool fibers. To understand the structural changes at different deformation levels, the wool fibers were treated with a complex mechanical regime. One of the most significant results from the mechanical tests was the estimation of the modulus of the matrix, which changed from 0.07 to 1.6 GPa during stretching. This result proves the proposal of the netlike structure of the matrix. On the basis of the obtained experimental data and the structural model, where the microfibrils and matrix play the role of a reinforcing phase and a filler, respectively, the rigidity of the microfibrils in the initial unstretched state was determined to be equal to about 20 GPa. The experimental data obtained from the Fourier transform infrared–attenuated total reflectance spectroscopy method proved the α–β conformational transition both in the yield and postyield regions and revealed the presence of the stable β form, which was not sensitive to stretching. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

The recovery process in stretched wool fibers at different strain levels ranging from 5% to 40% was investigated at room conditions for a long time, up to one year, and in water. The recovery process in stretched wool fibers is quite slow at room conditions; thus this slow recovery process causes quite high remaining deformation on the wool. The recovery process in the strain (ε) and logarithm time (log t) coordinates has a linear dependence in the wide time range that allows estimating the required time for a complete recovery. In contrast to the rather slow recovery process at room conditions, a complete recovery in water at room temperature was observed within approximately 30 s. Structural changes during the recovery processes at room conditions and in water were analyzed by an FTIR/ATR method. The influences of water content and new formations of hydrogen bonds in the recovery processes were examined. Slow recovery at room conditions was associated with the reorganization of the hydrogen bonds between microfibrills and matrix which results in formation of a new and rather stable structure. The absorption of water by the matrix phase causes the disruption of the strong hydrogen bonds holding the stretched form of the fiber and leads to a rapid recovery.

The mechanical properties of Bombyx mori silk yarns and baves were investigated with tensile testing method. After silk yarns were pre-extended at different strain levels and fixed for a while followed by recovery process, the tensile characteristics were examined in detail. It was commonly observed that low preliminary extensions up to 2-3% do not cause the changes of the mechanical properties and stress-strain curves because they result in small structural changes and distortions, which were recovered within relatively short time ($ 1 min) in recovery process. However, pre-extension values >3% strain lead to great changes of the mechanical properties and fibre structure, i.e., the changes of the shape of stressstrain curve where additional transition point was observed, increase in the rigidity and stress at rupture, but decrease in extensibility as a result of orientation and destruction of the fibre structure especially in the amorphous region. It was stated that silk fibre consists of two distinct deformation regions, namely first linear region extending up to 2-3% strain and the second region beyond 2-3% strain where the main reorganization processes of the fibre structure, that is, the straining of macromolecular chains especially in the amorphous regions, the orientation of structural units such as b-sheet microcrystals in stretching direction, and the destruction of macromolecules take place.

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