4. Conclusions
The present PALS study shows that UV radiation induces micro- structural alterations in virgin/bleached hair, the primary effect being cross-linking of protein fragments within the amorphous do- mains. Bleached hair suffers accelerated photodamage, compared to virgin hair, due to the degradation of melanin pigments and also because of the more uniformly distributed hydrophilic centers. The changes in the PALS parameters upon bleaching are different from that of UV irradiation, signifying the differences in the underlying mechanisms of the two processes. The study points out that, even under ambient conditions the fiber can swell upon irradiation, pos- sibly because of the hydrophilic nature of the photoreaction prod- ucts. Another new observation is the limiting role of the cross-links (that result from UV irradiation) towards fiber swelling on humid- ification or bleaching. Upon irradiation and/or bleaching, the hair fiber progressively becomes brittle as evidenced by the increased denaturation temperature (TD). The decrease in average crystallite size and increase in lattice strain upon irradiation/bleaching, an
Fig. 6. DSC scan showing the denaturation temperature of virgin, UV treated (UV50h, etc.), and 1h post-bleached UV treated hair (UVB).
Fig. 7. DSC scan showing the denaturation temperature of virgin, 5h bleached (B5h), and ‘bleached (5h) + UV irradiated’ hair (BUV200h, etc.).
Fig. 8. Typical XRD scan of virgin hair sample.
Table 1 Average crystallite size and lattice strain calculated from W-H plot.
Sample Average crystallite size in Å Lattice strain
Virgin (Ref) 164 0.15 Ref + UV275h 119 0.21 Ref + UV600h 88 0.21 Ref + UV800h 50 0.23 Ref + UV800h + bleach 1h 126 0.19 Ref + bleach 5h 91 0.21 Ref + bleach 5h + UV500h 66 0.22
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