The dynamic mechanical properties of keratin fibres

Abstract

The complex modulus of single keratin fibres has been studied at various extensions or times as well as at different relative humidities, temperatures and frequencies . Two parameters of the complex modulus were measured, namely the dynamic modulus and the loss angle. To carry out measurements for the above studies a dynamic mechanical tester was designed and constructed. By using a piezoelectric element the apparatus allows for measurements to be taken in the frequency range 6 Hz to 1500Hz, while with an environment conditioning chamber the full range of relative humidities and the range of -100°C to + 50°C for temperatures can be covered. Fine fibre samples can be extended in the apparatus and tested at each extension. Considerable precautions were taken in the apparatus to reduce noise because of the small values of the signals detected. By using the above equipment it was found that the modulus of wool fibres decreases with strain up to intermediate extensions of about 20% and then increases with higher extensions. The loss angle variation with extension is inverse to the modulus changes. The complex modulus was also measured while fibres were extension cycled or relaxed at fixed strains. More measurements were taken under other specific conditions of strain. All of these results, it was shown, could be explained by the application of the two-phase structure model of keratin: one phase C being relatively impenetrable to water and possessing elastic properties at all extensions, and the other phase M being water penetrable and acting mechanically as a viscoelastic solid. Measurements on fibres were carried out during abrupt relative humidity changes at a constant frequency and temperature. For an abrupt relative humidity increase it was found that the loss angle vs time exhibits an overshoot at the time when the absorption is nearly completed, while the modulus curve is changing markedly at the same time. This result was compatible with the suggestion that the structural mobility of the keratin fibre reaches a maximum at the time when absorption is almost complete. The complex modulus of wet keratin fibres was measured in the frequency range of 6 - 1500 Hz at different temperatures between 0.2 and 45°C. Some measurements were taken at different relative humidities. These results together with results of other workers indicated the presence of a characteristic transition process in keratin dependent strongly on the water content. This process was attributed to the main chain motion in the M phase.