Response of a polyelectrolyte under oscillatory shear of low frequency

It is still a challenging task to gain a comprehensive understanding of the physical mechanism of rheological properties of polyelectrolyte solutions. For the purpose of investigating the molecular picture, a study is conducted to examine the counterion distribution of the charged main chain under oscillatory shear of low frequency, using single molecule fluorescence spectroscopy. Adopting sodium polystyrene sulfonate (NaPSS) as the model polyelectrolyte, the local concentration of hydronium ions around the PSS^- chain is measured <i>in situ</i> as a function of shear strain and shear frequency, by monitoring the fluorescence intensity of a pH-responsive fluorophore attached to the PSS^- chain end, by single molecule confocal fluorescence spectroscopy. The results not only show that the counterions depart from the charged main chain when shear is applied but also that the response increases nonlinearly with shear strain and shear frequency. Systematic investigations are performed with molecular weight, salt concentration and polyelectrolyte concentration. The discoveries indicate that the shear-induced counterion departure depends on the strength of the inter-chain electrostatic interaction, which determines the rigidity or resilience of the "cage-like" structure formed by multiple neighboring charged chains.