Abstract
Polyelectrolyte gels are known to undergo significant conformational changes in response to external stimuli such as pH, temperature, or the dielectric constant. Specifically, an increase of the degree of ionization associated with an increasing number of counterions leads to swelling of the network. For a macroscopically large gel, which is electrostatically neutral in its interior, swelling is no longer governed by electrostatic interactions, but rather by the osmotic pressure of counterions. However, this electrostatic neutrality is typically violated for nanogels, because counterions are free to leave a gel particle. Although nanogel-swelling exhibits similar features as swelling of micro- and macrogels, another mechanism has to be relevant. Here, we use molecular dynamics simulations and scaling theory to unravel the structural properties of nanogels upon changing the electrostatic interactions. We demonstrate that the swelling of nanogels is governed by screened electrostatic interactions without a relevant contribution by the counterion osmotic pressure.
Highlights
Nano- and microgels are nanometer to micrometer size crosslinked polymer networks often comprised of polyelectrolytes
In a bulk gel, which can be considered as electrostatically neutral, there is a broad range of electrostatic interactions, where gel swelling is attributed to the osmotic pressure of the counterions[25,26,27]
The question arises on the mechanisms which govern gel swelling in such systems, on the range of interactions, where macrogels swell by counterion osmotic pressure
Summary
Nano- and microgels are nanometer to micrometer size crosslinked polymer networks often comprised of polyelectrolytes. In a bulk gel, which can be considered as electrostatically neutral, there is a broad range of electrostatic interactions, where gel swelling is attributed to the osmotic pressure of the counterions[25,26,27]. Such a mechanism does not apply for nanogels, since counterions are able to leave the gel particle. Differences, such as the charge distribution of star polymers, which increases toward the star center and gives rise to particular conformational properties[28]
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