Shear-Induced Counterion Release of a Polyelectrolyte
Kaikai Zheng, Kuo Chen, Weibin Ren, Jingfa Yang, Jiang Zhao
Abstract
For the purpose of understanding the molecular origins of the rheological properties of polyelectrolyte solutions, an in situ investigation into the distribution of counterions of a polyelectrolyte under shear is conducted. An apparatus named rheo-microspectrometer is constructed by integrating an advanced rheometer with multiple highly sensitive single-molecule fluorescence microscopic and spectroscopic techniques, including fluorescence correlation spectroscopy, photon counting histogram, single-molecule fluorescence imaging, and fluorescence emission spectroscopy. It has the capacity of measuring the directional and diffusive motions of single fluorescence-labeled polymer molecules as well as their fluorescence emission spectra when the samples are under shear. The shear-induced changes in counterion distribution of a model polyelectrolyte, sodium polystyrene sulfonate (NaPSS), is investigated at the single molecular level. Fluorescence resonance energy transfer between the fluorescence donor attached at the PSS– chain end and the cationic acceptor serving as the counterion probe in solution shows the increase of average counterion–chain distance. The emission spectra of the pH-responsive fluorophore labeled at the PSS– chain end show the dilution of hydronium ions near the chain by shear. The results indicate that the shear applied to the NaPSS solution releases the counterions from the PSS– chain. The mechanism of counterions’ redistribution is attributed to the shear-induced transient overlap of electric fields of multiple PSS– chains. The results indicate that the effective charge of a polyelectrolyte molecule increases with shear, in contrast to the conventional assumption of the invariable charges.