Order-Order Morphological Transitions for Dual Stimulus Responsive Diblock Copolymer Vesicles

Area of Science:

• Polymer Science
• Materials Chemistry
• Nanotechnology

Background:

• Polymer vesicles are self-assembled nanostructures with potential applications in drug delivery and nanotechnology.
• The morphology of diblock copolymer vesicles can be tuned by altering block lengths and solution conditions.
• Previous work demonstrated pH-responsive morphological transitions in analogous PGMA-PHPMA copolymer worms.

Purpose of the Study:

• To synthesize PGMA-PHPMA diblock copolymer vesicles with varying poly(2-hydroxypropyl methacrylate) (PHPMA) block lengths.
• To investigate the pH-induced morphological transitions of these vesicles.
• To determine the influence of temperature, block length, and electrolyte concentration on vesicle morphology.

Main Methods:

• Reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization was used to synthesize PGMA-PHPMA diblock copolymers.
• Vesicle formation was induced at low pH.
• Morphological transitions were studied using turbidimetry, transmission electron microscopy (TEM), and dynamic light scattering (DLS) across a range of pH, temperature, and salt concentrations.

Main Results:

• Irreversible vesicle-to-sphere and vesicle-to-worm transitions were observed upon pH increase for specific PHPMA block lengths (175 and 200, respectively).
• These pH-induced transitions were found to be slow at 20 °C, requiring hours to complete.
• Longer membrane-forming blocks (PHPMA 225-250) did not undergo order-order transitions with pH alone, but a dual stimulus (pH switch followed by cooling) induced vesicle-to-sphere transition. Electrolyte addition suppressed pH-responsive behavior due to charge screening.

Conclusions:

• PGMA-PHPMA diblock copolymer vesicles exhibit pH-responsive morphological transitions, but the kinetics are slow and dependent on block length.
• A combination of pH and temperature stimuli can induce transitions in systems where pH alone is insufficient.
• Electrolytes can effectively suppress pH-induced morphological changes by screening electrostatic interactions.