Self-assembly of the dipole-driven physical polyzwitterions in solutions*

Dipolar polymers are capable of forming complex, self-regulating structures which could be employed in various fields from drug-delivery systems to the next-generation polymers. Here in a polycation-negatively charged organic salt complaxation system, we found that dipolar interaction will transform a polycation into a physical polyzwitterion. In dilute solutions we found isolated polycation chain shrinks upon a decrease in ionic strength, exhibiting a “globule-to-coil” conformational transition with increasing ionic strength. Such an “anti-polyelectrolyte effect” , which is a feature of traditional chemical polyzwitterion, is due to the intra-chain dipolar interactions. In concentrated solutions, by tuning the ionic strength, the system exhibits rich phase behavior, including phase separation at low ionic strength and a homogeneous solution at very high ionic strength, with a stable mesomorphic state of monodispersed spherical aggregates with the size around 100-200 nm as an interlude between the two limits. By using light scattering we found that the size of the aggregates depends on the polymer concentration Cp according to the scaling law R~Cp^1/6. Further increasing the polymer concentration will make the mesomorphic aggregates disassemble into single chains by a self-poisoning mechanism.

Our finding highlights the unacknowledged role that dipoles play in the self-assembly of charged macromolecules and biological materials. The discovered principle of dipole-directed assembly is essential to understand and control the structures and phase behaviors of charged macromolecular in biological and living systems, such as biomolecular condensates, membraneless organelles, and DNA/RNA complexation.