In summary, the value of TD increases (from that of virgin hair)
with UV irradiation time, the increase being more prominent after a bleaching step. The increase in TD is indicative of some embrittlement of the fiber, possibly due to new molecular forces set up within it compensating or even over- riding the effects of the lost disulfide bonds. Evidence [52] shows that the value of TD is kinetically controlled by the cross-link den- sity of the matrix that surrounds the intermediate filaments. The new forces, which may be hydrogen or salt linkages, are suscepti- ble to moisture, but still capable of rendering thermal stability to the fiber, at least under the ambient conditions. In fact, Wortmann et al. [53] have reported that in the wet state the value of TD de- creases after bleaching. The observed pattern of changes in the va- lue of TD supports our claims made before about the UV induced cross-linking within the cortex region. The marked increase in TD after bleaching is attributable to the more even distribution of these new bonds within bleached hair than that in the UV exposed virgin hair.
3.3. X-ray diffraction results
XRD scans (e.g., Fig. 8) for the virgin, bleached, and UV irradi- ated hair show two amorphous halos around 2h 9.2 and 20.3, and a shoulder at about 24 for the second halo, which conforms to the fact that human hair has a overall crystallinity of about 20% [54]. The shape of these curves and the peak positions do not alter much between the samples, implying that the fibrillar
Fig. 4. Variation of (a) o-Ps lifetime s3 (free volume hole size Vf), (b) o-Ps intensity I3, and (c) fractional free volume Fv as a function of UV exposure time for bleached hair sample.
Fig. 5. Effect of humidification on o-Ps lifetime s3 and intensity I3 of ‘bleached + UV exposed’ hair sample. Here, BUV200h represents a bleached (5h) + 200h UV irradiated hair; BUV200hW is a bleached (5h) + 200h UV exposed + humidified hair.