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 deformational rigidity increases due to relaxation of stresses in oriented PP fibres. Elongation of PP fibres is accompanied by mechanical failure in all stages, which is a distinctive feature of deformation processes in these materials. Ordering of the structure of amorphous regions, manifested by an increase in the deformational rigidity of these fibres, is the result of stress relaxation in oriented PP fibres.Isotactic polypropylene is widely used for manufacturing industrial fibres, and the high strength at relatively low density should be noted above all [1][2][3]. The manufacturing process for polypropylene (PP) fibres is a multistage process, and the fibre undergoes temperature and deformation treatment in different stages [4,5]. To improve product quality and optimize industrial orientation of PP fibre, it is necessary to understand the structural processes that take place in fixing the fibre in the stitched state, i.e., as a result of stress relaxation [6].The relaxation processes that take place in stitched (loaded) fibres and yarns in many cases significantly alter their performance properties. There is currently no complete representation of the structural processes that cause such changes in the deformation properties of fibres and yarns. The change in the deformation characteristics of fibres in the stretched state must undoubtedly be taken into account for reliable prediction of the deformation properties of synthetic fibres and the articles made from them, which is not always done in practice.We established the character of the change in the rigidity of stretched film PP fibres during stress relaxation and elucidated the structural nature of these changes. We investigated PP film fibre (manufactured by Tverkhimvolokno OJSC) with a linear density of 130 tex and draw ratio of λ = 6, tensile strength of σ t = 580 MPa, and elongation at break of ε b = 22%.The deformation-strength properties in the stress relaxation and active stretching regime and the structural rearrangements that take place in stitched fibers and yarns were investigated. The studies of the mechanical properties of the fibres in active stretching (obtaining the stress-strain diagrams) and stress relaxation regimes were conducted on an Instron-1122. The tests in conditions of stopping stretching and then stretching again were performed on the same setup. IR spectroscopy was used to reveal the moleculardecomposition processes that take place in stretched oriented samples. The valence bond breaks in the macromolecules were recorded with this method based on the newly formed stable terminal groups formed as a result of decomposition. The data were obtained by frustrated total internal reflection on a Spectrum One Fourier IR spectrometer. The absorption bands at 1742 and 1650 cm -1 were considered the molecular-decomposition bands in the IR spectrum of PP [7]. Low-frequency Raman spectroscopy, which is sensitive to the regular segments of macromolecules of any type regardless of their localization both in amorphous and in crystalline ...
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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