Unusual nanostructured morphologies enabled by interpolyelectrolyte complexation of polyions bearing incompatible nonionic segments.

Abstract

The nanostructures of polyelectrolyte complexes (PECs) fabricated from a series of rationally designed oppositely charged amphiphilic random copolymers with variable comonomer compositions were studied experimentally in 50:50 (v/v) methanol–H2O mixtures through morphological investigations. Both polycationic [poly(alanine methacryloyloxyethyl ester-co-styrene), P1–P4] and polyanionic [poly(acrylic acid-co-methyl acrylate), P1′–P4′] electrolytes were synthesized via reversible addition–fragmentation chain-transfer (RAFT) copolymerization while maintaining the mole fractions of charge-bearing groups within the range of 50–90%. Unlike polyanions, polycations showed well pronounced pH-responsive behavior owing to pH-induced protonation/deprotonation phenomena of pendant quaternary ammonium groups. On account of this fact, PEC formation with concomitant microphase separation was realized at pH 5.0 when all obtained compositions of both polycations and polyanions were totally soluble. Fine tuning of alanine-derived cationic and anionic acrylic acid compositions confers uncommon morphologies revealed by electron microscopic observations. Intriguingly, a “string-of-beads” like morphology was observed for PECs prepared with 50 and 60% charged ratios, whereas spherical micelles and vesicles were formed in the case of 70 and 90% ratios, respectively, attributed to the strength of electrostatic interaction and incompatibility of nonionic segments. These fascinating morphologies were further corroborated by computer simulations of two oppositely charged chains within a coarse-grained approach. A diagram of PEC morphologies was constructed depending on the hydrophobic/hydrophilic composition of polyions, fraction of ion-containing groups, and the strength of electrostatic interactions. Observation of these unique morphologies from a single pair of a polyelectrolyte system is scarcely reported in the literature.