Relaxation of a Viscoelastic Gel Bar: I. Theory

When a rod of gel is deflected in 3-point bending, two types of relaxation process occur: hydrodynamic relaxation caused by flow of liquid within the gel network, and viscoelastic relaxation of the network itself. The kinetics of load relaxation have previously been analyzed for the case of hydrodynamic relaxation within a perfectly elastic network. That analysis describes, for example, the behavior of silica gel in a nonreactive solvent, such as acetone. When the liquid can attack the gel network, then true viscoelastic relaxation is superimposed on hydrodynamic relaxation, and that situation is examined in the present paper. To a very good degree of approximation, the total relaxation is equal to the product of the hydrodynamic and viscoelastic relaxation functions. In Part II, it will be shown that the present analysis describes the behavior of silica gel in an aqueous solvent and in an alcohol/amine solution.