Unusual Structures of Interpolyelectrolyte Complexes: Vesicles and Perforated Vesicles

Area of Science:

• Polymer Science
• Materials Science
• Physical Chemistry

Background:

• Interpolyelectrolyte complexes (IPECs) are formed by the association of oppositely charged polymers.
• The morphology of IPECs in solution is influenced by various factors including polymer architecture, charge density, and environmental conditions.
• Understanding IPEC self-assembly is crucial for developing novel materials with controlled structures.

Purpose of the Study:

• To investigate the spontaneous formation of hollow spherical particles (vesicles) and perforated vesicles from interpolyelectrolyte complexes in dilute solution.
• To elucidate the role of polyion-solvent affinity and electrostatic interactions in determining IPEC morphology.
• To theoretically determine the conditions favoring core-shell and hollow vesicular morphologies.

Main Methods:

• Computer simulations were employed to model the self-assembly of interpolyelectrolyte complexes.
• Analytical theory was used to derive conditions for different morphologies.
• Free energy calculations were performed to determine the stability of core-shell and hollow vesicular structures.
• An order parameter based on spherical harmonics expansion was introduced to analyze pore distribution in perforated vesicles.

Main Results:

• IPECs can spontaneously form hollow spherical particles (vesicles) or perforated vesicles depending on the polyion-solvent affinity.
• When electrostatic interactions are weak, hydrophobic attraction leads to "dense core-loose shell" structures in poor solvent.
• Strong electrostatic interactions favor layered, hollow, and filled morphologies with segregated macroions.
• Perforated vesicles represent an intermediate morphology between vesicular and structured solid states.
• The solvent quality influences the pore distribution in perforated vesicles.

Conclusions:

• The differential affinity of polyions for the solvent is a key factor in the spontaneous formation of vesicular and perforated vesicular IPEC structures.
• Both electrostatic and hydrophobic interactions play critical roles in dictating the final morphology of IPECs.
• Theoretical calculations of free energy successfully predict the formation conditions for core-shell and hollow vesicular morphologies.
• The study provides a theoretical framework for understanding and controlling the self-assembly of IPECs into complex nanostructures.