Swelling Properties of Gels

A similar situation arises in the case of the swelling of rubber. Here the isoprene units of the rubber molecule, when not part of a rubber molecule, are soluble in aromatic hydrocarbons, but when incorporated in a polymer network they are constrained to remain as part of a gel. 

The swelling of clays in water represents an example in which the internal osmotic pressure is associated with double-layer repulsion between the clay platelets. 

The swelling of a gel can be prevented by subjecting it to a pressure applied by a piston permeable to the solvent. This is called the equilibrium swelling pressure, equilibrium being set up between external solvent at atmospheric pressure and solvent confined in the compressed gel. The swelling pressure depends on 

It often happens that the network structure initially formed in a gelling process is not the most stable. The relatively slow diffusion of portions of the polymer chains may lead to the formation of more stable and more compact structures which exert an increased swelling pressure on the imbibed licjuid, which then exudes from the gel. This slow expulsion of imbibed liquid is called syneresii. 

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