Ionic group-dependent structure of complex coacervate hydrogels formed by ABA triblock copolymers.

Abstract

This study investigates the nanostructure of complex coacervate core hydrogels (C3Gs) with varying compositions of cationic charged groups (<i>i.e.</i>, ammonium and guanidinium) using small-angle X-ray/neutron scattering (SAX/NS). C3Gs were prepared by stoichiometric mixing of two oppositely charged ABA triblock copolymers in aqueous solvents, in which A end-blocks were functionalized with either sulfonate groups or a mixture of ammonium and guanidinium groups. Comprehensive small-angle X-ray/neutron scattering (SAX/NS) analysis elucidated the dependence of C3Gs structures on the fraction of guanidinium groups in the cationic end-block (<i>x</i>) and salt concentration (<i>c</i>_s). As <i>x</i> increases, the polymer volume fraction in the cores, and interfacial tension (<i>γ</i>_core) and salt resistance (<i>c</i>*) of the coacervate cores increase, which is attributed to the greater hydrophobicity and non-electrostatic association. Furthermore, we observed that the salt dependence of the interfacial tension follows <i>γ</i>_core ∼ (1 - <i>c</i>_s/<i>c</i>*)^3/2 in all series of <i>x</i>. The results show that the variation of the ionic group provides a powerful method to control the salt-responsiveness of C3Gs as stimuli-responsive materials.