Chemical cross-links, modulus contributions

Networks with tri- and tetra-functional cross-links produced by end-linking of short strands give moduli which are more in accord with the new theory if quantitative reaction can be assumed (3...13) However, the data on polydimethylsiloxane networks, may equally well be analyzed in terms of modulus contributions from chemical cross-links and chain entangling, both, if imperfect reaction is taken into account (J 4). Absence of a modulus contribution from chain entangling has therefore not been demonstrated by end-linked networks. 

Figure 3. Modulus contributions from chemical cross-links (Cx, filled triangles) and from chain entangling (Gx, unfilled symbols) plotted against the extension ratio during cross-linking, A0, for 1,2-polybutadiene. Key O, GN, equibiaxial extension , G.v, pure shear A, Gx, simple extension Gx°, pseudo-equilibrium rubber plateau modulus for a polybutadiene with a similar microstructure. See Ref. 10.

The two-network method has been carefully examined. All the previous two-network results were obtained in simple extension for which the Gaussian composite network theory was found to be inadequate. Results obtained on composite networks of 1,2-polybutadiene for three different types of strain, namely equibiaxial extension, pure shear, and simple extension, are discussed in the present paper. The Gaussian composite network elastic free energy relation is found to be adequate in equibiaxial extension and possibly pure shear. Extrapolation to zero strain gives the same result for all three types of strain The contribution from chain entangling at elastic equilibrium is found to be approximately equal to the pseudo-equilibrium rubber plateau modulus and about three times larger than the contribution from chemical cross-links. 

As a result, the concentration of the elastically effective subchains (i.e., subchains contributing to the elasticity modulus) differs from the concentration of subchains Vch calculated from the amount of cross-linker used in the gel synthesis under the assumption of the formation of an ideal network, where all the cross-linker molecules are incorporated in such a way that they connect elastically effective network subchains. To describe this difference, the elasticity modulus G is represented as a sum of two components Gch and Gg associated with chemical cross-links and entanglements, respectively. This approach was called a two-network model. " °... 

A conclusion can be drawn that to determine the effective cross-linking density, the methods based on the modulus measurements should be preferred. It seems possible in this case to distinguish between two contributions to the Veff, if Ve is determined chemically. An attempt to establish the interconnection between effective cross-hnking density in semi-IPNs and kinetic conditions of reaction has been made for IPNs based on cross-linked PU and hnear or partially cross-hnked PBMA. 

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