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The mechanical behavior of human hair fibers is determined by the interactions between keratin proteins structured into microfibrils (hard alpha-keratin intermediate filaments), a protein sulfur-rich matrix (intermediate filaments associated proteins), and water molecules. The structure of the microfibril-matrix assembly has already been fully characterized using electron microscopy and small-angle x-ray scattering on unstressed fibers. However, these results give only a static image of this assembly. To observe and characterize the deformation of the microfibrils and of the matrix, we have carried out time-resolved small-angle x-ray microdiffraction experiments on human hair fibers stretched at 45% relative humidity and in water. Three structural parameters were monitored and quantified: the 6.7-nm meridian arc, which is related to an axial separation between groups of molecules along the microfibrils, the microfibril's radius, and the packing distance between microfibrils. Using a surface lattice model of the microfibril, we have described its deformation as a combination of a sliding process and a molecular stretching process. The radial contraction of the matrix is also emphasized, reinforcing the hydrophilic gel nature hypothesis.

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Repetitive hair-relaxing treatments often applied to African-American hair weaken the hair structure. Therefore hair breakage is a common feature of African-American hair and an important cause of hair loss. Recently, by analysing the lipids extracted from human hair, a fraction of free-ceramide was isolated in which sphinganine was predominant. This study shows that this sphinganine-derived ceramide (i.e. C18-dhCer) binds to African-American hair and protects it from weakening caused by chemicals. To show this binding, we used two methods: radioactivity detection with a microimager and secondary ion mass spectrometry. We evaluated the benefits of C18-dhCer on African-American hair fibre, relaxed by guanidine hydroxide, using a new method called the Break'in Brush Technique (BBT). This method determines the hair breakage resistance during a brushing. Using this technique, we have shown less breakage when applying a shampoo with ceramide. The present study opens new prospects for the development of products able to increase the protection, provide better care and meet the needs of African-American hair thanks to the effect of ceramide binding.

The influence of the temperature of the melt T, on the kinetics and the morphology of a semicrystalline polymer (poly(oxymethy1ene)) was investigated using thermal analysis and optical microscopy. The thermodynamic melting point @ and the enthalpy of melting at thermodynamical equilibrium AI$ were determined by extrapolation of the graphs Tf = f(T3 and AHf = f(T3; (@ = 198"C, AI$ = 251 J/g). For different temperatures of the melt (T, = 185"C, 195 "C, 205 "C), isothermal and non-isothermal crystallizations were analysed using the Avrami and Ozawa equations. Nucleation and spherulitic growth in this polymer were studied by using optical microscopy at elevated temperatures. Using different analyses, we observed initial nucleation followed by spherulite growth with the following influence of the temperature of the melt on the distribution and the number of spherulites: Ti < Tf produces many small spherulites; T, > T! gives rise to few large spherulites. entre 1 et 2OoC/min.

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