2010 TRI/PRINCETON CONFERENCE 277 protein structure resulting in changes in hair water absorption and desorption profi les. In this work, water sorption/desorption and the kinetics of these processes on thermally treated hair were studied. The effect of polymer pretreatment on the water sorption/de- sorption performance of hair was evaluated. Figure 9a shows the water sorption and desorption isotherms of hair fi bers with and with- out thermal treatment and polymer protection. The thermally treated hair has a lower maximum water regain than the unheated hair in each sorption step. The maximum Figure 9. Water sorption and desorption isotherms and apparent diffusion coeffi cients of hair fi bers with and without thermal treatment and polymer protection. Dark brown European hair. Figure 8. SEM images of the hair fi ber surface with and without thermal treatment at 232°C and polymer protection. Dark brown European hair.

JOURNAL OF COSMETIC SCIENCE 278 water regain for the unheated hair at 90% RH is 21.95% while the heated hair is 17.27%. To avoid the variation among different hair tresses, the unheated and the heated hair are from the same hair tress split into two halves. One half is heated and the other is not heated. The hysteresis (the difference in net moisture changes between the desorption and sorption processes) is higher for heat-treated hair than unheated hair, indicating a lower water retention of the heated hair on drying. The less water regain and lower retention for thermally treated hair might be attributed to the helical pro- tein conformation change to the beta sheet or other uncoiled denatured cross-linking structure. The new protein conformation may have reduced water accessibility or bind- ing sites. Figure 9a shows that polyquaternium-55 pretreatment increases the water regain of heated hair compared with its untreated control possibly due to the protective effect of the polymer on thermally induced hair protein damage. As shown in the FTIR and DSC studies described previously, polymer pretreatment reduces protein degrada- tion and denaturation, thereby protecting the protein structure and native hydrogen bonding interactions. The data indicates that this has the effect of improving the water sorption of hair, compared with the unprotected thermally damaged hair. The mecha- nism of increased water restoration of hair via polymer protection of native protein structure is further supported by studying the water sorption and desorption of virgin hair without thermal treatment, both with and without 1% polyquaternium-55 treat- ment. This is illustrated in Figure 10a. Both isotherms are identical, indicating that the polymer treated and untreated unthermally-stressed hair fi bers have the same water sorption and desorption performance. This supports a mechanism in which thermal protection of the native protein structure is a major factor in moisture restoration and, thus, thermal protection. The apparent diffusion coeffi cients have been utilized to measure the kinetics of moisture uptake and loss in hair fi bers (12,13). Diffusion rates for moisture into and out of the fi ber at each relative humidity were calculated from the sorption and desorption data in each Figure 10. Water sorption and desorption isotherms and apparent diffusion coeffi cients of virgin hair fi bers with and without polymer treatment. Dark brown European hair.