"Raft" formation by two-dimensional self-assembly of block copolymer rod micelles in aqueous solution

Area of Science:

• Polymer Science
• Materials Science
• Supramolecular Chemistry

Background:

• Block copolymers self-assemble into diverse structures in solution.
• Forming free-standing planar polymer assemblies like sheets is challenging due to the tendency for closed structures (e.g., vesicles).
• Existing polymer crystallization methods typically involve individual chain uptake, not aggregate assembly.

Purpose of the Study:

• To report a novel mechanism for forming planar lamellar structures from block copolymers in solution.
• To investigate the hierarchical assembly process from spherical micelles to planar arrays.
• To explore the influence of co-assembly with homopolymers on morphology control.

Main Methods:

• Electron microscopy was used to visualize the lamella formation mechanism.
• Light scattering techniques were employed to study the kinetics of the assembly process.
• System studied: poly(ethylene oxide)-block-polycaprolactone (PEO-PCL) in water, with co-assembly using homopolycaprolactone (HPCL).

Main Results:

• A novel lamella formation mechanism was identified: core-crystalline spherical micelles link to form rods, which then associate into planar arrays.
• Co-assembly with HPCL induced morphological changes, leading to either rod or lamellar structures.
• The hierarchical growth involves 1D rod subunits formed from spherical assemblies, leading to 2D lamellae.

Conclusions:

• This study presents a new, controllable method for hierarchical lamellar formation in block copolymer solutions.
• The process involves a novel pathway from spherical micelles to rods, and subsequently to planar arrays.
• The size and aspect ratio of the resulting lamellae can be controlled through this assembly process.