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than do smaller crystalline solutes. A given polymer may have no saturation solubility; it usually either dissolves completely or is only swollen by a given liquid. If the polymer is crosslinked, solution cannot occur and the polymer will only swell by imbibition of liquid to form a gel. Swelling decreases as the degree of crosslinking increases. Swelling is also a function of the solubility parameter of the liquid phase, and if the polymer is ionic, swelling will be dependent on the ionic strength of the solution as shown in Fig. 8.10 for crosslinked hyaluronic acid gels. Increasing ionic strength decreases the repulsion between the chains and allows the polymer to shrink.

Highly polar polymers like poly(vinyl chloride) and some cellulose derivatives require polar liquids as solvents, in which dipole interactions or hydrogen bonding between polymer and solvent molecules occur. However, sol-vation does not necessarily lead to solution because the liquid, if it is to act as a solvent, must dissolve the solvated polymer. This process may be very slow because of the high viscosity of the partially solvated system.

Swelling of hydrogels and drug release

The relative mobility of a drug diffusing in the swelling hydrogel is given by the swelling interface number, Sw defined as y ■ ô{t)

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