Gels Cross-Linked in Solution

Although the term gel has been used with varied and imprecise implications in the literature, we shall adopt the definition that it is a substantially diluted system which exhibits no steady-state flow. A polymer gel is then a cross-linked solution, whether linked by chemical bonds or crystallites or some other kind of junction. 

Ratio G luP- plotted logarithmically against radian frequency for a 5% solution of ethylene-propylene copolymer (47h, = 6.6 X 10, broad molecular weight distribution) in Decalinat27 C superimposed rates of shear in parallel direction, from top to bottom 0,0.2,1, 3.2,10, and 25 sec. (Booij. ) 

A distinction must be drawn between gels which are diluted first and then cross-linked and those which are cross-linked first and then swollen (Section F below). In the former, the network strands have their average random configurations in a more or less unstrained state (except perhaps for gels linked at quite high dilution, where the polymer coils overlap each others domains only to a limited extent). In the latter, however, the strands are all extended beyond their normal root-mean-square lengths, in proportion to the cube root of the swelling factor p/c = uj.  

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The most recent experimental work to settle the controversy regarding the factor B is the swelling pressure work of Van de Kraats (105,106). This author used poly( >-nitrophenyl methacrylate) gels, cross-linked in solution by aminolysis with hexamethylene diamine. A typical example of his results is shown in Fig. 14, where it is seen that % is a constant over a range in polymer concentration from 5 to 20%. 

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