Can Oppositely Charged Polyelectrolyte Stars Form a Gel? A Simulational Study

We present a Langevin molecular dynamics study of an equimolar mixture of
monodispersed oppositely charged di-block four-armed polyelectrolyte stars. We use
an implicit solvent coarse-grained representation of the polyelectrolyte stars and varied
the length of the terminal charged blocks that reside on each arm. By varying the
polymer concentration we computed P-V diagrams and determined the free-swelling
equilibrium concentration with respect to a pure water reservoir as a function of the
charged block length. We investigate various structural properties of the resulting
equilibrium structures, like the number of ionic bonds, dangling arms, isolated stars,
and cluster sizes. The ionic bonds feature a broad distribution of the number of arms
involved and also display a distribution of net charges peaked around the neutral ionic
bond. Our main result is that for charged block length equal to 4 and 5 ionized beads
the resulting macro-aggregate spans the box and forms a network phase. Furthermore,
we investigated the dynamics of ionic bonds, and computed their lifetimes and turing dynamics. The bonds are weak enough to allow a network restructuring under
thermal fluctuations but are still strong enough to yield a stable gel phase.