MECHANICAL HYSTERESIS OF CHEMICALLY MODIFIED HAIR 401 0.7- 0.6- •, 0.2- 0 I/Tt 0 io 45 7'5 9'o Temperature øC Figure 1. Illustration of graphical technique for determination of transition temperature the control. Temperatures were held to +2 øC. In the case of the extensions at 0øC, ice was deposited in the experimental chamber by immersing it in a dry ice/acetone bath until a substantial layer of ice had formed on the bottom and sides of the chamber. Temperatures of 65 ø and 80 øC were maintained with a circulating hot water bath the circulating pump was turned off during loading and unloading to avoid turbulence. RESULTS AND DISCUSSION Most of the experimental results obtained during the course of this investigation are summarized in Table I. The work necessary to extend fibers by 20% (W20) is given in arbitrary readings from the integrator without conversion into conventional units of energy. The 20% hys- teresis ratio is defined as: Work regained in unloading from 20% extension ti/20 • Work required to extend by 20% The apparent second order phase transition* temperature (Tt) is obtained by plotting the hysteresis ratio vs. the temperature and inter- polating to the point at which the slope changes (Fig. 1). * "A second order phase transition is characterized by a sudden change in slope of the pri- mary thermodynamic variable" (16.) The change in heat capacity of a solid going to a liquid is a first order phase transition, i.e., a discontinuity exists at the melting point the softening point of a wax is a second order phase transition, i.e., the heat capacity vs. temperature curve changes slope.

402 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Strain Strain C Strain D Strain A:0 øc, B =25øc, C:65øC, D:80 øc I.-Phenyl Is0cyanate, 2-Ninhydrin, 3.-C0ntr01 Figure 2. Typical stress-strain curves Alexander and co-workers (27) have reviewed the interpretation of load-extension curves, but relatively little work has been devoted to an explanation of unloading curves (16, 28). The present study shows (Fig. 2) that the work of unloading and especially the shape of the un- loading curve vary appreciably with temperature and chemical modifica- tion. By contrast, the loading curves show only minor changes in shape with temperature. It may be noted that W20 at 80 øC is essentially the same for most of the treatments performed since the average devia- tion is of the order of + 15%-30% the larger deviations are germrally found whenever the chemical change of the hair is more drastic, such as in the case of treatment with phenyl isocyanate. In contrast to the work required for extension, hysteresis ratio values are more significant the average deviations for this parameter are small, approximately + 1-3%. The 20% hysteresis ratio, rather than a 30% ratio, was chosen to avoid any added complexity from unfolding of the a-helix structure of the fiber which starts at 20% extension (27).* Admittedly, some of the data at higher temperatures suggest that the a-helix is involved in the hysteresis mechanism. The loading and unloading curves of the control fibers yield an in- creasing hysteresis ratio with increasing temperature. In other words, at higher temperatures the work regained during unloading increases * It is to be understood that some of the chemical treatments described below may have a m,?re or less permanent effect on the a-helix structure.