0.80 



0.70 



\j}' r^ "" empty space 

 ^^o' A^^ Water 



114 FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY Z 



Upon drying. This is more likely to occur in molecular frameworks. 

 Here the empty spaces shrink and form interstitial spaces which no. 

 longer possess the character of submicroscopic pores. It is therefore 

 easy to see why, in the poorly ^^^^ 

 crystallized rayon fibres, por- | 

 osity will disappear to a § 

 great extent in the drying | 

 process ; all the same, even c^o.90 

 these fibres contain about 5 % 

 of empty space (Fig. 78). In 

 native fibres whose incrusta- 

 tions have been removed, a 

 complete closing of the struc- 

 ture would hardly be poss- 

 ible. Otherwise it would be 

 hard to explain how the dens- 

 ity of ramie determined in 

 toluene could amount to i . 5 5 , 

 whereas the result of accurate 

 measurements of mass and 

 volume gives only 1.39. Fur- 

 thermore, an inner reserve of 

 space is necessary to explain 

 the great flexibility and capa- 

 city to twist; otherwise these 

 fibres would be as brittle and 

 elastic as glass fibres. 



Do^ible refraction of swollen gels. In the swelling process, isotropic 

 imbibition liquid penetrates between the anisotropic micellar strands. 

 In this way the rodlet birefringence of gels is enhanced, for it follows 

 from the formula given on p. 84 that if the other conditions remain 

 constant, this birefringence acquires its maximum value when the 

 relative volumes of rodlets and imbibition medium are equal {b^ = ^3)- 

 The intrinsic double refraction, however, is inversely proportional to 

 the volume so long as it is permissible to assume that no change in 

 micellar orientation occurs as a result of swelling. If the intrinsic 

 double refraction of the dry gel is called i-Do and the degree of swell- 

 ing is q (volume of swollen gel/volume of dry gel), then, according; 



%r 

 0.60 



' ' "" Cellulose 

 _i • -• " ■ '- 



0.!0 



0.20 0.30 



Regain g H2O per g cellulose 



Fig. 78. Increase in volume of swelling isotropic 

 (regenerated) cellulose threads (from Hermans^ 

 1946). Abscissa: absorption of water. Ordinate: 

 specific volume (i/density), q>o specific volume 

 of dried threads 0.66, ^^cr specific volume of crys- 

 tallized cellulose 0.63. The water absorption^ 

 increases linearly, but the volume does not. 



