Strain Dependent Effects in Networks

Swollen networks usually show a negligible C2 term. If they do exhibit a C2 term, there is reason to believe that the network structure is inhomogeneous, or even heterogeneous due to microsyneresis (see Chapter II, Sections 3 and 4). These observations suggest that the explanation for C2 might be coupled with a certain structuring in the network beyond that implied by the ideal network picture. 

Such structuring is necessarily an intermolecular effect. The simplest type of an intermolecular effect, which should be treated first, is due to the crosslinks between the chains themselves. Dobson and Gordon (50) have remarked that most crosslinks are actually short chains of one or several links, which upon straining the network, become oriented but cannot be stretched. As a result an additional entropy force should arise, which has not yet been accounted for in the Gaussian theory. This force can be calculated on the basis of the Kuhn and Grun (114) chain vector orientation argument, which yields in extension 

Several authors have addressed themselves to the problem of excluded volume type obstruction in networks. The first such calculation stems from Khasanovich (102). He evaluates the excluded volume effect in swollen networks by considering the interactions between pairs of segments in three sets of mutually perpendicular planes. These inter- (and 

DiMarzio s subsequent calculation of the packing entropy-effect then yields rather small negative corrections to the Gaussian force, which are insufficient to explain the observed behaviour (see Fig. 27). 

Jackson, Shen and McQuarrie (95) use an alternative method for the calculation of the obstructional effect of the molecular volume. Their starting point is the assumption that the obstruction becomes anisotropic as soon as the network is stretched. Rather than using probabilities of 1/3 for the random walk steps in any of the three cartesian directions, they derive for unidirectional strain 

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