Block copolymer strands with internal microphase separation structure via self-assembly at the air-water interface

Area of Science:

• Materials Science
• Polymer Chemistry
• Surface Science

Background:

• Block copolymers exhibit microphase separation in bulk.
• Amphiphilic block copolymers self-assemble at interfaces.
• Langmuir-Blodgett (LB) techniques enable thin film formation.

Purpose of the Study:

• To investigate the formation of hierarchical structures through coupled microphase separation and interfacial self-assembly.
• To explore how varying block copolymer compositions influence nanoscale and mesoscale organization.
• To understand the role of amphiphilic block copolymers in directing hierarchical self-assembly.

Main Methods:

• Preparation of blends of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and polystyrene-b-poly(butadiene) (PS-b-PB).
• Deposition of block copolymer blends at the air-water interface.
• Characterization of Langmuir-Blodgett films using Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM).
• Analysis of interfacial behavior using Langmuir compression isotherms.

Main Results:

• A blend with a high PS-b-PB fraction (f=0.75) spontaneously formed hierarchical aggregates with nanoscale cylinders in mesoscale strands.
• Lower PS-b-PB fractions (f=0.50, f=0.25) resulted in mesoscale structures (continents, strands, holes) but lacked internal nanoscale organization.
• Pure PS-b-PB self-assembly yielded less ordered, taller aggregates compared to blends containing PS-b-PEO.
• The presence of amphiphilic PS-b-PEO was crucial for regulating mesoscale organization.

Conclusions:

• Coupling bulk microphase separation with air-water interface self-assembly is an effective strategy for creating hierarchical block copolymer structures.
• The composition of the block copolymer blend significantly dictates the resulting hierarchical morphology.
• Amphiphilic block copolymers play a key role in controlling the mesoscale organization within these hierarchically structured materials.