Chain-length dependence of polyion complex architecture bearing phosphobetaine block explored using SAXS and FFF-MALS

Synchrotron small-angle X-ray scattering (SAXS) was measured from aqueous solutions of polyion complexes (PICs), which were prepared by mixing oppositely charged diblock copolymers MAPTAC-b-PMPC and AMPS-b-PMPC. MAPTAC represents polycationic (3-(methacryloylamino) propyl) trimethylammonium chloride, AMPS represents polyanionic sodium 2-(acrylamido)-2-methylpropanesulfonate and PMPC represents hydrophilic poly-zwitterion (2-(methacryloyloxy) ethyl phosphorylcholine). The degrees of polymerization of MAPTAC and AMPS were selected to be the same for each complex, and the ratio (R-p) of the polymerization degree for the cationic to neutral chains was changed from 0.27 to 9.5. The aggregation number (N-agg) of PIC was determined with light scattering combined with field-flow fractionation (FFF). The inner structures of PIC were determined by analyzing SAXS with the core-corona model. When the MPC chain was much longer than the charged ones (Rp 0.27), a star-like spherical conformation was formed. With an increase of Rp, Nagg increased, while the spherical form was maintained up to R-p - 1.0. When R-p - 4.7 and 9.5, the PIC became a worm-like cylinder and vesicle, respectively. Based on the determined structural parameters and aggregation numbers, the index to describe how the tethered PMPC chains were crowded on the shell was determined. It was observed that the crowding parameter was much smaller than that of poly(ethylene glycol in spherical core-shell polymeric micelles.