Abstract
Nanopatterns can readily be formed by annealing block copolymers (BCPs) in organic solvents at moderate or high temperatures. However, this approach can be challenging from an environmental and industrial point of view. Herein, we describe a simple and environmentally friendly alternative to achieve periodically ordered nanoscale phase separated BCP structures. Asymmetric polystyrene-b-poly(ethylene oxide) (PS-b-PEO) thin film patterns of different molecular weight were achieved by annealing in supercritical carbon dioxide (sc-CO2). Microphase separation of PS-b-PEO (16,000–5000) film patterns were achieved by annealing in scCO2 at a relatively low temperature was previously reported by our group. The effects of annealing temperature, time and depressurisation rates for the polymer system were also discussed. In this article, we have expanded this study to create new knowledge on the structural and dimensional evolution of nanohole and line/space surface periodicity of four other different molecular weights PS-b-PEO systems. Periodic, well defined, hexagonally ordered films of line and hole patterns were obtained at low CO2 temperatures (35–40 °C) and pressures (1200–1300 psi). Further, the changes in morphology, ordering and feature sizes for a new PS-b-PEO system (42,000–11,500) are discussed in detail upon changing the scCO2 annealing parameters (temperature, film thickness, depressurization rates, etc.). In relation to our previous reports, the broad annealing temperature and depressurisation rate were explored together for different film thicknesses. In addition, the effects of SCF annealing for three other BCP systems (PEO-b-PS, PS-b-PDMS, PS-b-PLA) is also investigated with similar processing conditions. The patterns were also generated on a graphoepitaxial substrate for device application.
Highlights
Block copolymers (BCPs) are quintessential nanostructure forming materials due to their self-organizing capabilities [1]. This provides a large variety of potential technological applications such as nanolithography and “bottom up” microelectronic device fabrication depending on their different ordered morphologies produced after microphase separation [2,3]
In thin film form, typically obtained by spin-casting or dip-coating procedures, interfacial energies between air and a solid substrate as well as the film thickness relative to the bulk periodicity are additional factors which determine the morphology of a BCP [3,4,5,6]
Depressurisation was undertaken at rates of 20, 30, 60 and 120 psi/min to understand its effects on BCP morphologies and the microphase separation process
Summary
Block copolymers (BCPs) are quintessential nanostructure forming materials due to their self-organizing capabilities [1]. This provides a large variety of potential technological applications such as nanolithography and “bottom up” microelectronic device fabrication depending on their different ordered morphologies produced after microphase separation [2,3]. Solvent annealing is utilised to obtain ordered phase-separated, nanostructured thin films at low temperatures and in relatively short time periods through swelling the polymer blocks by a solvent, providing the necessary chain mobility and free volume to the Nanomaterials 2021, 11, 669. Supercritical carbon dioxide (scCO2 ) is an attractive solvent (or diluent) for the phase separation of BCPs as the fluid increases the free volume and chain mobility of dense polymers, thereby reducing the melt viscosity (Tg ) and melting point (Tm ) and facilitating the formation of self-assembled nanostructures [14,15,16]
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