THE KERATINIZATION PROCESS 



255 



super contraction since it may follow also on the ordinary recovery of length 

 occurring when stretched fibres are released (Fig. 108). 



Supercontraction is a property that the keratin fibres share with other 

 systems containing long oriented molecular chains. The most probable 

 configuration of a long, free, flexible molecular chain in solution is a 

 random coil, for in this condition the entropy is a maximum. However, 

 less probable configurations may be assumed as a result of the molecule 

 interacting with other molecules or with other parts of itself. These inter- 



laxation time 



Fig. 108. Recovery of Cotswold wool in steam, after being held at 50% 



extension in steam or hot water for given times of relaxation. Times to be 



multiplied by 3 for the 40% (Astbury and Woods, 1933). 



actions provide the " forces of crystallization " and thermodynamically the 

 reduction in internal energy resulting from these interactions opposes the 

 randomizing effects of entropy. A system of long, flexible molecules with 

 very weak interaction between the chains may constitute a rubber. The 

 form of the molecules and of the bulk specimen in the rubbery state is 

 largely controlled by the entropy factor, i.e. the contractile force opposing 

 extension arises from the tendency of the extended molecules to return to 

 a shorter, more probable configuration. 



Among the protein fibres a large contribution to the internal energy 

 factor is provided by hydrogen bonding which can occur between the 

 numerous peptide groups (Fig. 109). In general terms, where steric 

 conditions are favourable and the close packing of portions of many chains 

 is possible, a crystallite stabilized by multiple H-bonds is formed. We may 

 say then that such systems are stabilized by " crystallite cross-links " to 

 distinguish the condition from the single-chain covalent linkages due to 

 cystine bridges. Some protein fibres, collagen and silk for example, owe 

 their insolubility and stability almost entirely to crystallite cross-linking. 



