Molecular weight dependence of chain conformation of polyelectrolytes

Molecular conformation of permanently charged polyelectrolytes is investigated at single molecular level by fluorescence correlation spectroscopy (FCS). By taking sodium polystyrene sulfonate (NaPSS) and quarternized poly 4-vinylpyridine (QP4VP) as model systems, the hydrodynamic radius (R_h) was measured for a series of samples covering molecular weight about an order of magnitude. The results show that the scaling power index depends on the ionic strength in the aqueous solution, and especially under moderate ionic strength, the scaling power index differs depending on the molecular weight of the polyelectrolyte, indicating the dependence of molecular conformation on the molecular weight, a process of conformation change from rod-like to random coil. The physical origin is found to be the enhancement of counterion adsorption (binding) to the charged chain.