Equilibrium swelling, characterization

After all drug is dissolved, the osmotic pressure decays and the beads shrink to their equilibrium swelling value, as observed for beads with high water swelling (11,12) during this second release phase normal Fickian diffusion kinetics becomes more important, characterized by a very low rate which depends on the hydro-philicity of the polymer. 

It is of interest to note that experimental values of the parameter d characterizing the strain dependence of equilibrium swelling ratio for elastomers subjected to uniform biaxial extension closely approximate the theoretical values. It will be recalled that this parameter is specified by Eq. [6.1.21]. Moreover, the Flory theory defines it as d = 2/9 = 0.22(2),... 

J. L. Thiel and R. E. Cohen, Synthesis, Characterization, and Viscoelastic Behavior of Single-Phase Interpenetrating Styrene Networks, Polym. Eng. Sci. 19, 284 (1979). Polystyrene/polystyrene homo-IPNs. Swelling equation for single-phase IPNs. Equilibrium swelling studies as a function of crosslink level. 

In Chapter 3 of this book, L. Brannon-Peppas offered a thorough analysis of the preparation and structure of hydrogels. In that chapter. It was noted that the number average molecular weight between crosslinks. Me, and the volume equilibrium swelling ratio, Q, or its reciprocal parameter equilibrium polymer volume fraction, x>2,s. are important parameters for the characterization of the network. It is then necessary to review theories and equations used for calculation of such parameters. 

The immobilization of reagents onto sorbents often results in increase of their sensitivity and, in some cases, selectivity, allows to simplify the analysis and to avoid necessity of use of toxic organic solvents. At the same time silicas are characterized by absence of swelling, thenual and chemical stability, rapid achievement of heterogeneous equilibrium. 

The parameters which characterize the thermodynamic equilibrium of the gel, viz. the swelling degree, swelling pressure, as well as other characteristics of the gel like the elastic modulus, can be substantially changed due to changes in external conditions, i.e., temperature, composition of the solution, pressure and some other factors. The changes in the state of the gel which are visually observed as volume changes can be both continuous and discontinuous [96], In principle, the latter is a transition between the phases of different concentration of the network polymer one of which corresponds to the swollen gel and the other to the collapsed one. 

However, it yields dynamic modulus. Some other techniques were also used to characterize hydrogels, for example, viscoelastic measurements [28, 30, 31] and swelling equilibrium [20]. 

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