Self-organization of sulfonated polystyrene ionomers in low-polarity solvents

Abstract

The self-organization of ionomers based on sulfonated polystyrene containing sodium sulfonate groups SO₃Na in chloroform, deuterated toluene, and deuterated benzene is studied by the method of small-angle cold neutron scattering. It is shown that in chloroform, at a content of ionogenic groups of 1.35 mol %, chains undergo coil-to-globule conformation transformations owing to their electrostatic interactions. An increase in the amount of SO₃Na groups to 2.6 mol % leads to the assembly of ionomer chains into a hollow spherical structure with the solvent inside it (vesicle). Ionomer chains in chloroform join and form stable pairs. The structural organization of ionomers containing different amounts of SO₃Na groups (0.5–5.8 mol %) in deuterated toluene manifests itself as the formation of effective chains comprising up to seven macromolecules and functioning as the main structural elements of the system. Chains preserve the conformation of the Gaussian coil even at a degree of sulfonation of 5.8 mol %, when the distance between ionogenic groups along the chain is comparable with the length of the Kuhn segment. The effective chains of ionomers with a high content of ionogenic groups are characterized by sizes smaller than PS coils; they are composed of one to two macromolecules, and their density is increased. The self-organization of such species in deuterated toluene facilitates formation of secondary structures, namely, clusters consisting of four to five effective chains. In deuterated benzene, ionomer chains form supramolecular structures (associates) that behave as Gaussian chains and include 13–18 macromolecules at moderate and high degrees of sulfonation of PS. When the centers of gravity approach each other at a distance of the diameter (40–70 nm), associates come into contact and give rise to clusters. It is found that, at the scale of segments within the chain segment, the rigidity of chains is increased due to formation of intermolecular physical crosslinks between chains arising from electrostatic dipolar interactions of ionogenic groups.