Polymer network 618 INDEX

Swelling Index Swelling is defined as the increase in volume of a gel or solid associated with the uptake of a liquid or gas [7]. In fact, swelling represents the first stage in every polymer solution process, in which the solvent molecules diffuse through the polymer matrix to form a swollen, solvated mass, known as polymer gel. However, the gel formed after swelling polymer networks does not break up and the polymer molecules do not diffuse out of the swollen mass, and therefore, no real solution can be formed. 

Post-polymerization functionalization has also been applied to the synthesis of terpyridine-modified polymers [ 126]. In a recent approach, Schubert and colleagues employed this method to prepare poly(pentafluorostyrene) with terpyridines in the side chains [127]. First, poly(pentafluorostyrene) with a narrow polydispersity index of just 1.08 was synthesized by nitroxide-mediated polymerization. In a second step, this polymer was converted with amine-functionalized terpyridine under microwave heating, selectively substituting the para-fluorines. Addition of iron(II) sulfate to a solution of the terpyridine-functionalized polymer in a mixture of chloroform and methanol leads to gelation at a polymer concentration of 33 g In another work, Schubert and coworkers prepared metal-cross-Iinked polymer networks from linear and tri-arm PEG precursors, both functionalized with terpyridine at their OH-termini [128]. Quantitative functionalization of these precursors was achieved by conversion of the hydroxy-functionalized PEG derivatives with 4-chloro-2,2 6, 2"-terpyridine under basic conditions. However, quantitative cross-linking with iron(II) chloride was not observed in methanol solutions, neither at room temperature nor at elevated temperature, but only a small quantity of cross-linked material precipitated from the solution. This observation was attributed to a strong tendency of the tri-arm PEG to form intramolecular complexes, acting as a chain stopper rather than as a cross-linker. 

Thus, we again find for inhomogeneously cross-linked gels a stretched exponential decay of the relaxation modulus G(t) on scales larger than the average size of the structmal inhomogeneities (cross-link agglomerations). Remarkably, the stretched exponential index here, 1 /3, coincides with that foimd by Sommer for polydisperse polymer networks [142], see Sect. 7.4. This is an expected result, since a ladder model is equivalent to a linear Rouse chain, see Sect. 6.2. 

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