Linking of Long Linear Molecules

The above picture of the network structure of vulcanized rubber is supported by -the success of the kinetic theory of rubberlike elasticity (see part 4, page 14) calculations based on this model agree well with experimental measurements of stress-strain curves and other properties (James and Guth, 1943 Flory, 1944). Excellent evidence that the swollen gel contains the same network as the unswollen rubber has been presented by Flory (1944, 1946), based on studies of butyl rubber. Using the network model, the number of cross-links in the structure can be calculated in three ways (o) from measurements of the proportions of insoluble (network) and soluble (unattached) material in samples of different initial molecular lengths (b) from the elastic modulus of the unswollen rubber (c) from the maximum amount of liquid imbibed by the gel when swollen in equilibrium with pure solvent. The results of these three calculations for butyl rubber samples were in good agreement. 

A third method for obtaining the same geometrical arrangement of a three-dimensional network interspersed by liquid is to start, again, with 

The existence of a stable, dilute gel with a network of long molecules cross-linked by primary bonds demands that the cross-links be spaced far apart. In the butyl rubber samples described above, the average length of strands between cross-links was of the order of a thousand carbon atoms. If too many cross-links are introduced in the vulcanization of rubber, the strands will be tied together too closely and the network cannot imbibe enough solvent to form a dilute gel. If too many crosslinks are introduced in the treatment of dilute nitrocellulose with silicon tetrachloride, the progressively closer binding of the network strands results in expulsion of solvent and contraction of the gel to a more concentrated state—i.e., syneresis. 

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