Swelling Of Polyelectrolyte Gels Research Articles

Charge Regulation of Polyelectrolyte Gels: Swelling Transition

We study the effects of charge-regulated acid/base equilibrium on the swelling of polyelectrolyte gels by considering a combination of the Poisson–Boltzmann theory and a two-site charge-regulation model based on the Langmuir adsorption isotherm. By exploring the volume change as a function of salt concentration for both nanogels and microgels, we identify conditions where the gel volume exhibits a discontinuous swelling transition. This transition is driven exclusively by the charge-regulation mechanism and is characterized by a closed-loop phase diagram. Our predictions can be tested experimentally for polypeptide gels.

Effect of Mechanical and Chemical Constraints on Swelling of Polyelectrolyte Gels

Effect of Mechanical and Chemical Constraints on Swelling of Polyelectrolyte Gels

Collapse–Decollapse of Thermosensitive Polyelectrolyte Gels in Aqueous Media

The effect of temperature on water sorption by gels based on the copolymers of N-isopropylacrylamide with sodium p-styrene sulfonate containing 1, 3, and 5 mol % of charged monomer units is studied in aqueous media in the presence of salt (NaCl, KCl) or urea. It is shown that the incubation of gels in salt solutions leads to compression of the gels. With increasing temperature, the collapse of gels is observed in all studied media. The difference in the gel collapse temperature in water and salt solution increases with an increase in the fraction of charged units in the polymer gel network and in the salt concentration in the solution. With a return temperature decrease and gel decollapse, the mass of all gels in salt solutions (0.5–5.0 wt %) is completely restored. The incubation of gels in aqueous solutions of urea (0.5–5.0 wt %) has practically no effect on water sorption by gels before the collapse and the collapse temperature. However, the degree of swelling of polyelectrolyte gels during decollapse caused by a temperature decrease is much lower than the initial value, in contrast to the completely reversible swelling of the poly(N-isopropylacrylamide) gel under similar conditions.

Modeling the effects of pH and ionic strength on swelling of polyelectrolyte gels.

A model is developed for the elastic response of a polyelectrolyte gel under unconstrained and constrained swelling in a water bath with an arbitrary pH, where a monovalent salt is dissolved. A gel is treated as a three-phase medium consisting of an equivalent polymer network, solvent (water), and solute (mobile ions). Transport of solvent and solute is thought of as their diffusion through the network accelerated by an electric field formed by mobile and fixed ions and accompanied by chemical reactions (self-ionization of water molecules, dissociation of functional groups attached to polymer chains, and formation of ion pairs between bound charges and mobile counter-ions). Constitutive equations are derived by means of the free energy imbalance inequality for an arbitrary three-dimensional deformation with finite strains. Adjustable parameters in the governing relations are found by fitting equilibrium swelling diagrams on several hydrogels. The effects of pH, ionic strength of solution, and constraints on equilibrium water uptake are studied numerically.

Swelling of pH-sensitive hydrogels.

A model is derived for the elastic response of polyelectrolyte gels subjected to unconstrained and constrained swelling. A gel is treated as a three-phase medium consisting of a solid phase (polymer network), solvent (water), and solutes (mobile ions). Transport of solvent and solutes is modeled as their diffusion through the network accelerated by an electric field formed by ions and accompanied by chemical reactions (dissociation of functional groups attached to the chains). Constitutive equations (including the van't Hoff law for ionic pressure and the Henderson-Hasselbach equation for ionization of chains) are derived by means of the free energy imbalance inequality. Good agreement is demonstrated between equilibrium swelling diagrams on several pH-sensitive gels and results of simulation. It is revealed that swelling of polyelectrolyte gels is driven by electrostatic repulsion of bound charges, whereas the effect of ionic pressure is of secondary importance.

Effect of charge inhomogeneity of polyelectrolyte gels on their swelling behavior

The purpose of this paper is to discuss the role of ionic charges in the swelling of polyelectrolyte gels. According to the classical Flory's theory, osmotic pressure arising from mobile ions is regarded as a swelling force due to ionic charges; in other words, the gel was assumed to be surrounded by Donnan potential barriers. Although this assumption was criticized by Katchalsky et al., many studies have accepted it to account for the nature of the swelling or the phase transition in polyelectrolyte gels. Thus, we have tried to verify whether the ‘concept’ of osmotic pressure due to mobile ions is adequate or not. For this purpose, we have studied the swelling behavior of ionic gels with inhomogeneous charge distributions. A gel sample was obtained via inhomogeneous binding of anionic surfactants (e.g. sodium dodecylbenzene sulfate) to lightly crosslinked polymer of N-isopropylacrylamide (NIPA), while the other was based on the physical entrapment of poly(acrylic acid) within the NIPA chain network. Using both NIPA-based ionic gels, we observed a strong effect of the charge inhomogeneity on the degree of swelling. It is therefore indicated that our observations can no longer be explained in terms of the concept of osmotic pressure arising from mobile counterions within the gel phase.

Swelling Kinetics of a Polyelectrolyte Gel in Water and Salt Solutions. Coexistence of Swollen and Collapsed Phases

The swelling of polyelectrolyte gels in water and salt solutions is associated with the presence of up to three coexisting phases: a dry phase, a swollen phase, and a collapsed one due to the binding of metal ions to polyions. Swelling in pure water proceeds in two ways: a transition from the dry to swollen state, whose kinetics scales with the square of the initial gel size, and a further swelling that scales linearly with the initial gel size. We show that there exist various swelling paths depending on the gel interaction with metal ions and their concentration.