The thermal transition of a thermoresponsive microgel of poly‐N‐isopropylacrylamide (PNIPAM; transition temperature=34 °C) is shifted to higher temperatures when it is embedded in a shell of temperature‐sensitive poly‐N‐isopropylmethacrylamide (PNIPMAM; transition temperature=44 °C). The magnitude of the shift depends on the shell/core mass ratio. A thick shell induces a third transition arising from strong mechanical forces exerted on the core.
Staying in the tradition of Astbury's hypothesis about the role of the α ⇔ β transformation for the stress/ strain-curve of wool fibers and of Feughelman's X/Y-zones model, the interrelation between the morphological structure of keratin fibers and the shape of their stress/strain-curve in water is reevaluated. The yield and post-yield regions can be attributed to the opening up of two distinctly different and well defined portions of the monomer of the intermediate filament; the increased slope in the postyield region can be attributed to the influence of the sulfur bonds in one of the segments.The role of the sulfhydryl-disulfide interchange reaction for the appearance of the post-yield region is pointed out and the molecular mechanisms for achieving the maximum possible strain are discussed.J. B. Speakman was the first prominent protagonist of the theory that the mechanical properties of wool fibers have an important influence on their processing performance. In consequence, he placed a major emphasis on measuring stress/strain-curves and their changes with physical and chemical modifications [ 29 ] . The upper curve in Figure 1 shows somewhat schematically the stress/strain curve of a wool fiber in water. FIGURE 1. Stress/strain curve of a wool fiber in water (schematically ) and the components of the two-phase model. Stress and strain are not drawn to scale. _ On the molecular level, structural investigations using x-ray diffraction led to the discovery of axially oriented a-helices by Pauling and Corey and of the ahelix b 0-pleated sheet transformation with strain by Astbury and Woods. Studies by transmission electron microscopy revealed the existence of lightly stained cylindrical structures, traditionally referred to as &dquo;microfibrils,&dquo; embedded in a more darkly stained &dquo;matrix&dquo; phase [ 16 ] .On the basis of knowledge of the microscopic and molecular morphology of a-keratin fibers, major attempts have been made, especially by Hearie et al. [ 21,22 ] and Feughelman [ 15 ] , to consistently interpret the shape of the stress/strain-curve in relation to fiber structure. Both these approaches are based on the twophase model proposed by Feughelman [ 10 ] , where the filament phase dominates the stress/strain curve in the wet state, while the amorphous matrix phase either plays a minor role [ 15 or exhibits a significant contribution only at high strains [ 21,22 ] .Si nce these versions of the structure / property relationships [ 15,22 ] , more detailed information has become available on the structure of the &dquo;microfibrils,&dquo; in modern semantics referred to as &dquo;intermediate filaments.&dquo; This has enabled us, when expanding a previous account of our research [ 39 ]. to reevaluate the models on a molecular level. The model refinement, which stays in the tradition of Feughelman's X/Yzones model [ 1 l ,15 ] , achieves an improved consistency of the property / structure relationship. Structural Principles and Stress/Strain Properties of Fibrous Keratins a-Keratin f...
The glass transition of human hair and its dependence on water content were determined by means of differential scanning calorimetry (DSC). The relationship between the data is suitably described by the Fox equation, yielding for human hair a glass transition temperature of T(g) = 144 degrees C, which is substantially lower than that for wool (174 degrees C). This effect is attributed to a higher fraction of hydrophobic proteins in the matrix of human hair, which acts as an internal plasticizer. The applicability of the Fox equation for hair as well as for wool implies that water is homogeneously distributed in alpha-keratins, despite their complex morphological, semicrystalline structure. To investigate this aspect, hair was rendered amorphous by thermal denaturation. For the amorphous hair neither the water content nor T(g) were changed compared to the native state. These results provide strong support for the theory of a quasi-homogeneous distribution of water within alpha-keratins.
SYNOPSISApplying differential scanning calorimetry (DSC) for temperatures up to 18O"C, the denaturation transition of the a-helical material of various keratins in excess water was studied at conditions of equilibrium water vapor pressure (high-pressure DSC) . The results show a generally cystine content and material invariate denaturation range of 20-30°C with peak temperatures around 140°C. Though analysis of variance and multiple comparison tests show a pronounced inhomogeneity of the enthalpy data, the results generally support the material invariance of the denaturation enthalpy and hence of the helix content of the keratins. The denaturation enthalpy for the helical coiled-coil-structures in the intermediate filaments is determined as AH = 6.5-7.8 kJ/mol. A significant positive correlation was found between the denaturation temperatures and the cystine content. It is concluded, that the helix denaturation temperatures are kinetically controlled by the amount and/or the chemical composition of the surrounding nonhelical matrix, and that the double-peak endotherms observed for wool and other keratins originate from two cell types that are sufficiently different in sulfur content to allow endotherm separation. In the case of wool the cell types can be identified as "ortho-" and "paracortical" cells.
The relation between the glass transition temperature and the water content of wool can be described by the Fox equation. Data on transitions from a wide range of sources, including mechanical, dielectric, and calorimetric measurements, are well described by the equation. The utility of the relation for understanding and interpreting experiments on the wrinkle recovery of wool fabrics is shown.
The most prominent view of the diffusion mechanism of dyes into wool fibers assumes that the molecules primarily enter the fiber in a fast process along the cell membrane complex (CMC), that is, by an intercellular mechanism. From the CMC, they are subsequently distributed in a slower process throughout the other morphological components according to their respective diffusion coefficients and dye affinities. This view, referred to here as the CMC-diffusion model, is based on investigations of the diffusion performance of heavy metal complex and fluorescent dyes under anhydrous and aqueous conditions. An evaluation of various key aspects of the evidence for this model suggests that, due to differences in the glass transition and fluorescence quenching performance of the various morphological components in a wool fiber, there is, in fact, little evidence to support the CMC-diffusion model. Instead, the evidence supports the alternative, more general view that under normal dyeing conditions, diffusion proceeds primarily by means of all the nonkeratinous components of the wool fiber according to a restricted transcellular diffusion mechanism.
The final shape of a head hair is predetermined through a variety of factors during its formation in the follicle. These are genetic pathways, specific growth factors, cell differentiation and segregation, etc, with spatial as well as chronological dynamics. The cortex of hair consists of two major cell groups. These are characterized by parallel (para‐type) or roughly helical arrangements (ortho‐type) of the intermediate filaments (IF). There are also cell‐specific differences in the disulphide content, that is, of the cross‐link density of the IF‐associated matrix proteins. Given the current state of the academic discussion, we consider it as timely to support and broaden the view that, the structural differences of the cell types together with their lateral segregation are the main driving factor of curl formation. The mechanical effects, which derive thereof, are triggered in the transition zone of the follicle, that is, upon formation of the mature hair shaft. Furthermore, an irregular, “flat” cross section of the hair shaft is shown to be a synergistic but not determining factor of curl formation. The degree of cell type segregation along the mature hair shaft together with dynamic changes of the location of the plane of segregation, namely in a non‐circular cross section can account for very complex curl patterns. Against the background of these views, we argue that contributions to hair curl are implausible, if they relate to physical mechanisms which are active below the transition zone from the living to the mature (dead) hair.
Data on the linear viscoelastic relaxation of wool fibers for a range of relative hu midities below the gas transition ( Tg) are systematized by a humidity-time superposition method. The differences in the relaxation behavior in water where the fiber is above its Tg are pointed out. The analysis relates to the properties of the water sensitive, viscoelastic component of the wool fiber and consists of superimposing the relaxation curves for different humidities onto the curve for the dry state by multiplicative scaling and shifting on the logarithmic scale of the reduced time variable λ, that corrects for the influence of ageing. The change of the scaling and shift factor with the absorption mechanism of the water is discussed.
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