Network Structure of Hydrogels

The structure of hydrogel networks is usually characterized by the following microscopic parameters (Fig. 4.14)  

The polymer volume in the swollen state, U2,. is a measure of the amount of fluid imbibed and retained by the hydrogel. The molecular weight between two consecutive crosslinks, which can be either of chemical 

The structure of hydrogels that do not contain ionic moieties can be analyzed by the Flory Rehner theory (Flory and Rehner 1943a). This combination of thermodynamic and elasticity theories states that a cross-Knked polymer gel which is immersed in a fluid and allowed to reach equiUbrium with its surroundings is subject only to two opposing forces, the thermodynamic force of mixing and the retractive force of the polymer chains. At equihbrium, these two forces are equal. Equation (1) describes the physical situation in terms of the Gibbs free energy. 

AGeiastic is the contribution due to the elastic retractive forces developed inside the gel and AGmixing is the result of the spontaneous mixing of the fluid molecules with the polymer chains. The term AGmixing is a measure of the compatibility of the polymer with the molecules of the surrounding fluid. This compatibility is usually expressed by the polymer-solvent interaction parameter, x (Flory, 1953). 

Differentiation of Eq. (1) with respect to the number of solvent molecules while keeping temperature and pressure constant, results in Eq. (2)  

mi is the chemical potential of the solvent in the polymer gel and /xj o is the chemical potential of the pure solvent. At equilibrium, the difference between the chemical potentials of the solvent outside and inside the gel must be zero. Therefore, changes of the chemical potential due to mixing and elastic forces must balance each other. The change of chemical potential due to mixing can be expressed using heat and entropy of mixing. 

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There are also some direct experimental techniques to study the molecular structure of hydrogel networks. For instance, the uptake of fluorescence-labeled tracer molecules (solutes) of different size into the networks can be investigated by confocal laser scanning microscopy (CLSM). Theoretically,... 

By radical copolymerization of poly(A/-isopropylacrylamide-co-N,iV-dime-thylaminoethyl methacrylate) [poly(NIPAM-co-DMAEMA)] macromonomer with the monomers NIPAM and DMAEMA, comb-type cationic hydrogels with poly(NIPAM-co-DMAEMA) backbone networks and grafted poly(NIPAM-co-DMAEMA) side chains can be successfully prepared. Within the comb-type hydrogels the grafted chains have freely mobile ends, which are distinct from typical network structures of normal-type crosslinked hydrogels, as shown in Figure 5.7. The obtained hydrogels show both temperature and pH sensitivity. They all deswell with an increase of temperature and/or pH, and exhibit a lower critical solution temperature (LCST) at about 34 °C and a p a value at about pH 7.3. 

Cationic polymers are defined as polyelectrolytes cariying positive charges, and they are either derived from natural sources such as chitosan, or chemically synthesized where the charges have been incorporated on their backbone and/or side chains. They may also exist as block copolymers, where one of the blocks is decorated with positive charges. When these block copolymers consist of a hydrophobic block, they readily undergo self-assembly in aqueous solutions and form micellar structures with a positively charged surface. Similarly, if the block copolymer consists of two hydrophobic blocks, one at each end, the system may self-assemble into network structures called hydrogels, which are water-rich 3D interconnected networks. 

Cruise, G.M., Scharp, D.S., Jeffrey, A. Hubbell, J.A. (1998) Characterization of permeability and network structure of interfacially photopolymerized poly(ethylene glycol) diacrylate hydrogels. Biomaterials, 19 (14), 1287-1294. 

Figure 21.2 Estimated polymer network structure of hybrid hydrogels.

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. 

Knowledge of the network parameters is important for understanding gelation processes, and relationships between the molecular structure and hydrogel synthesis conditions. The principles for the optimization of SAH characteristics for various application purposes can also be based on these parameters. 

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