Changing the Shape of Hair 168 hair reduction.29 Evans et al. also used this terminology,44 but they applied intermittent strain at 2% rather than holding the hair at 1.5% strain. This change of protocol was executed to minimize the effect of strain on reduction rate.29,45 Chemical stress relaxation curves for the reduction of different sections of the same hair in 0.3 sodium thioglycolate at pH 9.0 at three different temperatures are shown in Figure 9A. The effect of pH on different sections of the same hair is shown in Figure 9B, while Figure 9C shows SFTK data in the pH range of 10.5–13.0. The curves from pH 9.0 at 39oC and pH 10.0 or 10.5 at 25oC illustrate the two shapes of SFTK curves typically observed. At pH 9.0 the reaction with NaTG on this hair appears to follow the pseudo first order kinetic model first published by Reese and Erying.43 To derive the pseudo first order model, it is first assumed that the concentration of the reducing agent, C 0 , is in large excess compared to the concentration of disulfide bonds, S-S, and that the reaction is slow compared to diffusion of the reducing agent. If we further assume that all S-S bonds are equally reactive, the rate of change in S-S is given by Eq.5: Eq. 5 Where C 0 = the concentration of reducing agent and k is the reaction rate constant. If each disulfide is assumed to support an equal amount of stress, then the force, F(t) at any time, t, is given by:29,43 Eq. 6 Plots of –ln(F(t)/F(0) versus t will be linear with a slope of kC 0 . Figure 10 shows plots of –ln(F(t)/F(0) versus t for different sections of the same hair treated with two concentrations of NaTG at pH 9.0 and 25oC. The a in Figure 10 show excellent fit to the pseudo first order model (r20.99 p 0.01). This author hypothesized that this might be due to diffusion of the non-reactive, unionized form of the