to be mainly responsible for the decrease
in free volume size and not other processes such as diffusion of the degraded low-molecular-weight protein fragments from the cuticle to the cortex as pointed out by Ruetsch et al. [7], because even these fragments are large in size for occupying the ang- strom-sized cavities. The most used experimental method to determine photo degra- dation of polymers is spectroscopy. However, the formation of new covalent bonds that lead to cross-linking in a polymer is not easily observable by optical spectroscopy [32], and more so in the case of a complex biopolymer like human hair due to its heterogeneous, multi-component composition. Because of the lack of direct char- acterization methods for assessing the cross-linking, the underly- ing mechanism is often not clear. However, few possibilities can be predicted for the photo-irradiated hair: The photochemical fis- sion of disulfide bonds in hair results in cystine S-sulfonate resi- dues as primary product, which, in turn, may cross-link. Similarly, the carbonyl groups and amide groups resulting from the photo-oxidation at the peptide backbone carbon may establish
Fig. 1. 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 virgin hair sample.