Reversible restructuring of aqueous block copolymer assemblies through stimulus-induced changes in amphiphilicity

Area of Science:

• Polymer Science
• Materials Science
• Physical Chemistry

Background:

• ABC triblock copolymers with specific block sequences exhibit unique self-assembly behaviors in aqueous solutions.
• The thermomorphic block's lower critical solution temperature (LCST) influences the copolymer's solubility and aggregation state.
• Understanding these temperature-dependent structural transitions is crucial for designing advanced materials.

Purpose of the Study:

• To investigate the self-assembly of hydrophilic-thermomorphic-hydrophobic ABC triblock copolymers in water.
• To characterize the temperature-induced structural changes from micelles to vesicles.
• To determine the reversibility and kinetics of the observed phase transition.

Main Methods:

• Synthesis and characterization of a representative poly(ethylene oxide)-block-poly(N-isopropylacrylamide)-block-polyisoprene copolymer.
• Dynamic and static light scattering (DLS/SLS) to analyze micelle and vesicle size and distribution.
• Transmission electron microscopy (TEM) for direct visualization of the assembled nanostructures.

Main Results:

• Copolymers self-assembled into distinct nanostructures depending on temperature.
• Small spherical micelles were observed in aqueous solutions below the thermomorphic block's LCST.
• Large vesicles were favored at temperatures above the LCST, indicating a significant structural change.
• The transition from micelles to vesicles was found to be reversible but slow, taking several weeks.

Conclusions:

• Hydrophilic-thermomorphic-hydrophobic ABC triblock copolymers can reversibly form micelles and vesicles in water.
• Temperature control above and below the thermomorphic block's LCST provides a mechanism for tuning nanostructure morphology.
• The slow kinetics of the reversible transformation suggest potential applications where controlled, gradual structural changes are desired.