Effect of Counterions on the Interaction among Concentrated Spherical Polyelectrolyte Brushes.

Yunwei Wang,Fan Yang,Li Li,Liang Sun,Xuhong Guo,Zhishuang Ye,Qingsong Yang,Yiming Wang

doi:10.3390/polym13121911

Abstract

The effect of counterions on interactions among spherical polyelectrolyte brushes (SPBs) was systematically investigated by rheology, small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The SPB particles consist of a solid polystyrene (PS) core with a diameter of ca.100 nm and a chemically grafted poly-(acrylic acid) (PAA) brush layer. Metal ions of different valences (Na+, Mg2+ and Al3+) were used as counterions to study the interactions among concentrated SPBs. The so-called “structure factor peak” in SAXS, the “local ordered structure peak” in WAXS and rheological properties indicated the interactions among concentrated SPBs. Combining SAXS, WAXS and rheology, the formation mechanism of the local ordered structure among PAA chains in the overlapped area of adjacent SPB, which was generated due to the bridge function of counterions, was confirmed. In contrast, excessive counterions shielded the electrostatic interaction among PAA chains and destroyed the local ordered structure. This work enriches our understanding of the polyelectrolyte assembly in concentrated SPBs under the effect of counterions and lays the foundations for SPB applications.

Highlights

Spherical polyelectrolyte brushes (SPB) are nanoparticles that consist of a spherical hydrophobic polymer core and are grafted with a linear hydrophilic polyelectrolyte shell on the surfaces of spherical core substrates [1,2,3,4]
This can be attributed to the enhancement of counterions, which leads to more ionized carboxyl groups in poly(acrylic acid) (PAA) chains via the increase in Na+ concentration from 0.01 to 5 mM
The phenomena observed by small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and the rheological methods confirm that the association among SPBs reflected by the structure factor peak in SAXS and local ordered structure peak in WAXS spectra should be driven by the interactions among PAA chains, which can be affected by the counterions

Summary

Introduction

Spherical polyelectrolyte brushes (SPB) are nanoparticles that consist of a spherical hydrophobic polymer core and are grafted with a linear hydrophilic polyelectrolyte shell on the surfaces of spherical core substrates [1,2,3,4]. Further increase in the concentration of counterions results in the disappearance of such local ordered structures, mainly because the counterions shield the electrostatic interaction among polyelectrolyte chains. To analyze this local ordered structure, concentrated SPB aqueous dispersions were characterized by SAXS, which can reveal the strength of interaction by structure factor in the SAXS curve. The rheological properties of SPB aqueous dispersions with different metal ions were studied to further certify the local ordered structures among SPBs. Combining rheology, WAXS and SAXS, the effect of different valence metal ions on the interactions among concentrated SPBs was systematically studied. The used water was obtained and purified by reverse osmosis of Millipore Milli-Q

Preparation of SPBs

Characterization

SAXS Theory

Results and Discussion

Conclusions