Network formation involving polyelectrolytes in solution: the role of counterions

The network-forming ability of a small fraction of nanosized trianions of the triarylmethyl class (TAM) with poly(diallyldimethylammonium chloride) (PDADMAC) polyelectrolyte is studied by high-field/high-frequency (94 GHz) electron paramagnetic resonance spectroscopy. These tristar-shaped organic ions are expected to undergo both electrostatic and hydrophobic interactions with PDADMAC chains. The dependence of electron spin echo (ESE)-detected spectra of the TAM spin probe on PDADMAC concentration reveals a heterogeneous distribution of the spin-carrying counterions. One fraction of these ions forms densely packed clusters while another one is highly diluted. On varying the concentration ratio of TAM spin probe/PDADMAC, the mean distance between closest neighbors within clusters of approximately 1.5 nm does not change significantly, while their fraction increases with decreasing polyelectrolyte content. These findings indicate that the nanosized organic TAM trianions induce network formation in solutions of PDADMAC polyelectrolyte. The data are consistent with a zip-like cooperative binding effect of TAM ions, making this spin probe an interesting building block for electrostatic self-assembly.Dedicated to Professor E.W. Fischer on the occasion of his 75th birthday.