Humidity‐dependent bending recovery and relaxation of human hair

Abstract

The time-dependent bending recovery of human hair fibers was investigated for a variety of relative humidities and aging times. The data were analyzed on the basis of a viscoelastic filament/matrix model and the Denby-equation, containing the parameter K as the ratio of the elastic bending rigidities of the matrix and the filaments and the Kohlrausch-Williams-Watts (KWW) function as relaxation function. The first stage of the analysis ascertained that the recovery curves shift with aging time on the time scale with the expected aging rate of l % 1. The second stage showed that the shape factor of the KWW function exhibits a mean value across the aging and humidity range of m ¼ 0.28, which is close to the "universal" value of 1/3. On this basis, it was found that virtually no change occurs for the modulus ratio for low water contents up to about 10%, being constant at K 0 ¼ 6 .1, while linearly decreasing beyond this threshold. The reduced, characteristic relaxation time drops on the log-time scale from logs r (0) ¼ 0.47 for the dry fiber linearly with water content, covering about two thirds of a decade for 0-20% water content. With the pronounced humidity dependence of the parameters, hair shows what is termed hydro-rheologically complex (HRC) in analogy to thermo-rheologically complex behavior. Using the HRC approach, the dynamical mechanical performance of hair (1 Hz) was calculated for a range of water contents and aging times and found to be in good general agreement with experimental data.

Effect of water desorption on the rheology and dynamic response of human hair to a non-contact impact

“…The change in the relaxation time t r is in accordance with the results found by Wortmann et al (2009) who studied hair bending properties. The lower the water content is, the slower the relaxation is.…”

Thermal denaturation and structural changes of α-helical proteins in keratins

“…Mode The theory was originally introduced as a hypothesis by Feughelman (1959) and confirmed in a variety of other studies including the mechanical, viscoelastic properties of keratin (Feughelman and Robinson, 1971;Wortmann and DeJong, 1985;Wortmann, 2007;Wortmann et al, 2009a). Cao and Leroy's (2005) observation that T D decreased significantly with water content, is in line with the well established view that water is an active plasticizer of the matrix as well as with the theory that the position of the helix-denaturation peak on the temperature scale is kinetically controlled by the matrix (Wortmann and Deutz, 1993;Istrate et al, 2009).…”

“…This stabilization of the helix at high temperatures is attributed to the highly viscous matrix, which kinetically impedes the unfolding of the helix and thus significantly enhances its temperature tolerance. Since water acts an effective plasticizer in the matrix (Wortmann et al, 2006b(Wortmann et al, , 2009a, the denaturation temperature drops accordingly with increasing water content (Haly and Snaith, 1967).…”

The effects of esterification on the humidity-dependent glass transition of human hair

“…As with all amorphous polymers, the thermal and physical history of the hair fibers affects the glass transition [2,19] as well as the related viscoelastic properties [8]. In light of this, the experimental procedure for the determination of T g was designed to control physical aging [2].…”

“…To describe the thermal [2,3] and some of the mechanical [4] properties of the hair fiber, it is well established that, the structure of the hair fiber may be simplified to a two-phase model [5], containing axially oriented, filamentous IFs embedded in an amorphous matrix. The glass transition, a property of the amorphous matrix, has been shown to be lowered by water as a plasticizer in keratin fibers [2,[6][7][8]. This is a common phenomenon for proteins [9,10].…”