Mechanical properties and conformational transitions of natural polyamide fibres and comparison with synthetic polyamide fibres

Natural polyamide fibres have important application fields especially in textile industry, engineering and medicine. To produce fibrous materials with higher characteristics and improve their performances requires determining how the mechanical properties of these fibres change depending on usage conditions. Although there are a lot of researches to understand the mechanical characteristics of wool and silk fibres but still it is seen these fibres were not investigated completely. In this thesis, the mechanical characteristics of alpha-keratin (wool and hair) and silk fibres after different effects such as preliminary stretching, temperature, and water were investigated. It was shown that after the alpha-keratin fibres were pre-extended in the region of 16% to 24% they show similar stress-strain curves of silk fibres due to alpha-beta conformational transitions. Pre-extension more than 32% leads to similarity to the stressstrain curves of synthetic polyamide fibres. New transitions observed on these fibres were interpreted by comparing with stress-strain behaviour of naylon and kapron fibres. It was seen that preliminary stretching effect on mechanical properties of silk fibres depends on extension level either. It was observed higher pre-extension level leads to higher stress values and a second transition appears for pre-extension more than 8% on the stress-strain curve. After water effect, wool and silk fibres show lower breaking stress and higher elastic properties. Here, a new transition which has not been observed in the literature before appears on the stress-strain curves of alpha-keratin fibres. Important decrease on mechanical properties of these fibres due to thermal effects is seen after heating more than 150 °C. With increasing time of heating, the fibres show worse mechanical characteristics. The stress-relaxation and creep-recovery processes are investigated in detail. It was seen that wool fibres have nonlinear viscoelastic in the preliminary extension region of 0 to 10% and linear viscoelastic behaviour for extension more than 10%. Mathematical model for estimating of relaxation modulus and behaviour of wool fibres in the nonlinear region for long time was proposed. Keywords: alpha-keratin, wool, silk, conformational transition, microfibril, stress, strain, stress relaxation, creep.