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Nano Research

Article Title

Nanophase separation and structural evolution of block copolymer films: A “green” and “clean” supercritical fluid approach

Authors

Tandra Ghoshal, Materials Chemistry and Analysis Group and Materials Research Group, Department of Chemistry and Tyndall National Institute, University College Cork, Cork, Ireland Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN/AMBER), Trinity College Dublin, Dublin, Ireland
Subhajit Biswas, Materials Chemistry and Analysis Group and Materials Research Group, Department of Chemistry and Tyndall National Institute, University College Cork, Cork, Ireland Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN/AMBER), Trinity College Dublin, Dublin, Ireland
Colm O’Regan, Materials Chemistry and Analysis Group and Materials Research Group, Department of Chemistry and Tyndall National Institute, University College Cork, Cork, Ireland Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN/AMBER), Trinity College Dublin, Dublin, Ireland
Justin D. Holmes, Materials Chemistry and Analysis Group and Materials Research Group, Department of Chemistry and Tyndall National Institute, University College Cork, Cork, Ireland Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN/AMBER), Trinity College Dublin, Dublin, Ireland
Michael A. Morris, Materials Chemistry and Analysis Group and Materials Research Group, Department of Chemistry and Tyndall National Institute, University College Cork, Cork, Ireland Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN/AMBER), Trinity College Dublin, Dublin, Ireland

Keywords

block copolymer, supercritical CO2, self-assembly, swelling, nanopores

Abstract

Thin films of block copolymers (BCPs) are widely accepted as potentially important materials in a host of technological applications including nano- lithography. In order to induce domain separation and form well-defined structural arrangements, many of these are solvent-annealed (i.e. solvent swollen) at moderate temperatures. The use of solvents can be challenging in industry from an environmental point of view as well as having practical/cost issues. However, a simple and environmentally friendly alternative to solvo-thermal annealing for the periodically ordered nanoscale phase separated structures is described herein. Various asymmetric polystyrene-b-poly(ethylene oxide) (PS-b-PEO) thin films were annealed in a compressible fluid, supercritical carbon dioxide (scCO2), to control nanodomain orientation and surface morphologies. For the first time, periodic well defined, hexagonally ordered films with sub-25 nm pitch size were demonstrated using a supercritical fluid (SCF) process at low temperatures and pressures. Predominant swelling of PEO domains in scCO2 induces nanophase separation. scCO2 serves as green alternative to the conventional organic solvents for the phase segregation of BCPs with complete elimination of any residual solvent in the patterned film. The depressurization rate of scCO2 following annealing was found to affect the morphology of the films. The supercritical annealing conditions could be used to define nanoporous analogues of the microphase separated films without additional processing, providing a one-step route to membrane like structures without affecting the ordered surface phase segregated structure. An understanding of the BCP self- assembly mechanism can be realized in terms of the deviation in glass transition temperature, melting point, viscosity, interaction parameter and volume fraction of the constituent blocks in the scCO2 environment.

Graphical Abstract

Publisher

Tsinghua University Press

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