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Figure 8.10 Relative swelling of hyaluronic acid hydrogels in different concentrations of NaCl and Na2SO3 solutions.

Reproduced from R. Tomer et al., J. Control. Release, 33, 405 (1995).

we so that substituting in equation (8.20) obtain the relationship between A G1 and temperature

Swollen state (low temperature)

Shrunken state (high temperature)

where is the volume fraction of polymer, and k1 and are heat and entropy parameters, respectively.

It is sometimes convenient to define the temperature at which a polymer of infinite molecular weight just becomes insoluble in a given solvent; this temperature is the Flory temperature or theta temperature, 0, which may also be defined by

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