MOLECULAR AND MACROMOLECUL AR STRUCTURE 



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reason referred to as the Hooke's law region. Here the fibre is behaving 

 like a conventional elastic solid and the axial molecular-lattice spacings, 

 as found by X-ray photographs, show a reversible increase of the same 

 order of extension as the fibre. The value of Young's modulus and the 

 amount of the Hookian extension decrease as the humidity and tempera- 

 ture are increased. 



AB (~2-~20%): At the point A the fibre yields suddenly and 

 there is a rather rapid extension of the fibre to about 20%. During this 

 extension the X-ray photograph remains of the a-type but becomes less 



20 30 



% extension 



Fig. 70. Idealized stress-strain curve for a keratin fibre^(wool or hair) 

 stretched in water at room temperature : OA (0-2%) Hooke's law region ; 

 AB (2-20%) extension of amorphous phase (phase I) a-fades and /? 

 appears; BC (20^4-5%) extension of crystalline phase (phase II); CD 

 (45%) extension of phase III non-crystalline (adapted from Astbury and 

 Woods, 1933). 



perfect as extension increases. Dry fibres break usually at about 20% 

 extension. At B a shoulder develops showing that the fibre is becoming 

 more difficult to stretch; the a-pattern rapidly fades and ^-reflections 

 appear. This shows that in this region the a-crystalline phase is being 

 destroyed and that a new configuration is appearing. 



CD (> ~ 45%): The fibre again becomes more easy to stretch. The 

 transformation of the a-crystallites into the jS-modification seems complete. 



Extension rarely exceeds 60-70% in cold water, but is greater in hot 



