Wood fibers, diffraction pattern

The extensive investigations by Astbury and Woods (1933) of the X-ray diffraction patterns of animal fibers before and after stretcihing, setting, or supercontraction (Section VI,5,2) have had a major influence on the models proposed to account for the load-extension properties. Astbury and Woods showed that when animal fibers are stretched, the characteristic a-pattern decreases in intensity with the simultaneous appearance of a d-pattern similar to that obtained from silk. The sharpness and intensity of the /3-pattern increases with sti ain, with temperature, and with time under strain. In the Hookean region the 5.1 A meridional spacing increases by up to 2 % (Astbury and Haggith, 1953). Astbury and Woods (1933) stated that virtually no /3-pattern appears until the fiber has been stret( hed by at least 20 %, and for 25 years this observation dominated the models proposed to explain the elastic properties of wool (Alexander and Hudson, 1954). 

A quantitative investigation of the X-ray diffraction pattern of a-keratin by Bendit (1957, 1960c) showed that the a-pattern begins to disappear and the /3-spacings begin to appear when the fiber has been extended by as little as 5 %. There is then a smooth continuous decrease in the intensity of the a-pattern and an increase in the intensity of the S-pattern until the fiber breaks. This observation and the confirmatory studies by Skertchly and Woods (1960) impose an important new requirement on mechanical models for the wool fiber. 

Sikorski (1960) and Woods (1960) have criticized the concept of a non-contractile matrix on the basis of the studies of Jeffrey ei al. (1955). Electron micrographs of preparations from the interiors of cortical cells derived from supercontracted fibers showed no apparent twisting of the fibrillar material, yet X-ray diffraction patterns showed marked disorientation of the a- or j3-structures in the fibers. Jeffrey el al. (1955) attributed this result to contraction of the matrix. 

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