Properties of polyelectrolyte gels in equilibrium with a reservoir of salt solution described within the primitive model have been investigated using Monte Carlo simulations. The polyelectrolyte gel was modeled as a charged defect-free three-dimensional network of diamondlike topology with explicit simple ions. The deswelling upon addition of salt was investigated for gels with different charge density, cross-linking density, chain flexibility, and counterion valence. All polyelectrolyte gels underwent a deswelling upon addition of salt. In most systems, however, even at high salt concentration, the equilibrium volumes remained significantly larger than those of the corresponding uncharged gels, and the salt concentration in the gel remained smaller than that in the reservoir. The salt content in the gel was larger as compared to that predicted by an ideal Donnan equilibrium due to a more negative excess chemical potential of the salt in the gel as compared to that in the reservoir. The predictions of the Flory-Rehner-Donnan theory were qualitatively in agreement with our results, but they displayed significant quantitative disagreement, mainly due to an overestimated contribution from the counterions to the osmotic pressure.
Volumes and other structural properties of polyelectrolyte gels in equilibrium with pure water have been determined by Monte Carlo simulations. The role of chain length polydispersity and topological network defects of four different networks with varying cross-linking density, monomer charge, and chain stiffness have been investigated. Generally, a chain length polydispersity reduced the gel volume, whereas the presence of chains with one end detached from the cross-linker (severed chains) led to an increased gel volume. Polyelectrolyte networks displayed the largest and uncharged polymer networks the smallest dependence on chain length polydispersity. The effect of severed chains was strongest for flexible polyelectrolyte gels and weakest for uncharged networks and stiff polyelectrolyte gels. Mechanical properties of uncharged and charged polymer gels were also investigated through uniaxially stretching and compared with theory.
The volume and structural changes upon replacement of oppositely charged network counterions for oppositely charged macroions in cross-linked polyelectrolyte gels have been investigated by Monte Carlo simulations using a coarse-grained model. Initially, the gel deswells, but after an approximately equivalent amount of macroions, the gel starts to swell again. The deswelling effect is greatest for small and highly charged macroions. The role of different network properties on the deswelling has also been examined. The initial deswelling is understood in terms of a replacement of confined counterions with macroions, thereby reducing the osmotic pressure originating from the counterions. At these conditions, macroions are located near network nodes with various degrees of network chains wrapping them. At charge equivalence, a profound change in the network structure has appeared. At these conditions, the cohesive electrostatic interaction and the excluded volume effect of the macroions strongly influence the equilibrium volume of the gel. Our model system reproduces many characteristic experimental observations of polyelectrolyte gels containing oppositely charged surfactants.
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