A generalized method for alignment of block copolymer films: solvent vapor annealing with soft shear

Abstract

One of the key issues associated with the utilization of block copolymer (BCP) thin films in nanoscience and nanotechnology is control of their alignment and orientation over macroscopic dimensions. We have recently reported a method, solvent vapor annealing with soft shear (SVA-SS), for fabricating unidirectional alignment of cylindrical nanostructures. This method is a simple extension of the common SVA process by adhering a flat, crosslinked poly(dimethylsiloxane) (PDMS) pad to the BCP thin film. The impact of processing parameters, including annealing time, solvent removal rate and the physical properties of the PDMS pad, on the quality of alignment quantified by the Herman's orientational factor (S) is systematically examined for a model system of polystyrene-block-polyisoprene-block-polystyrene (SIS). As annealing time increases, the SIS morphology transitions from isotropic rods to highly aligned cylinders. Decreasing the rate of solvent removal, which impacts the shear rate imposed by the contraction of the PDMS, improves the orientation factor of the cylindrical domains; this suggests the nanostructure alignment is primarily induced by contraction of PDMS during solvent removal. Moreover, the physical properties of the PDMS controlled by the crosslink density impact the orientation factor by tuning its swelling extent during SVA-SS and elastic modulus. Decreasing the PDMS crosslink density increases S; this effect appears to be primarily driven by the changes in the solubility of the SVA-SS solvent in the PDMS. With this understanding of the critical processing parameters, SVA-SS has been successfully applied to align a wide variety of BCPs including polystyrene-block-polybutadiene-block-polystyrene (SBS), polystyrene-block-poly(N,N-dimethyl-n-octadecylammonium p-styrenesulfonate) (PS-b-PSS-DMODA), polystyrene-block-polydimethylsiloxane (PS-b-PDMS) and polystyrene-block-poly(2-vinlypyridine) (PS-b-P2VP). These results suggest that SVA-SS is a generalizable method for the alignment of BCP thin films.

Effect of elongational flow on immiscible polymer blend/nanoparticle composites: a molecular dynamics study

“…25 Shear flow has also been shown to improve the alignment and uniformity of BCP thin films. 26,27 Molecular dynamics simulations of BCPs under shear flow have investigated the transition between the parallel and perpendicular morphologies 28,29 as well as the placement of NP within lamellar BCP domains. 16 Under extensional flow, Kim et al experimentally studied poly(styrene-co-acrylonitrile) blended with poly methyl methacrylate but was unable to reach steady state.…”

Photothermally Directed Assembly of Block Copolymers

“…Repeated SS‐LZA sweeps effectively impose an oscillatory shear history on the sample, where an internal stress field is repeatedly generated and relaxed. For typical SS‐LZA processing of PS‐b‐PMMA films, uniaxial alignment throughout the entire sample could be achieved in ≈0.3 s, with quality of ordering saturating within ≈20 s. Interestingly, while previous SS annealing studies suggested that excessive shear rates could decrease the quality of BCP alignment due to insufficient time for material response, SS‐LZA demonstrated that BCP films can respond to mechanical stresses even within millisecond timescales (18 ms) due to the large shear rates and high local temperatures.…”

Directional Self‐Assembly of Fluorinated Star Block Polymer Thin Films Using Mixed Solvent Vapor Annealing

“…Because the SVA process can readily control the orientation of self‐assembled BP domains through the appropriate choice of solvents, swollen film thickness, deswelling rate, and vapor pressure, parallel and perpendicular orientations of self‐assembled morphologies are achievable from a diverse array of BP systems. Additionally, SVA can effectively reduce T g and enhance the chain mobility of polymer blocks, while also mitigating the slow diffusion kinetics of higher‐ χ BPs during the self‐assembly process to a much greater extent than thermal annealing . Recently, Vogt and coworkers reported a modified SVA approach that showed the directional alignment of BP thin films through the application of shear during solvent annealing, termed solvent vapor annealing with soft shear (SVA‐SS) .…”

“…Additionally, SVA can effectively reduce T g and enhance the chain mobility of polymer blocks, while also mitigating the slow diffusion kinetics of higher‐ χ BPs during the self‐assembly process to a much greater extent than thermal annealing . Recently, Vogt and coworkers reported a modified SVA approach that showed the directional alignment of BP thin films through the application of shear during solvent annealing, termed solvent vapor annealing with soft shear (SVA‐SS) . The swelling and deswelling of a soft poly(dimethylsiloxane) (PDMS) pad generated a macroscopic shear stress that could directionally align BP thin film morphologies as the films were drying.…”