Systematic Study on the Effect of Solvent Removal Rate on the Morphology of Solvent Vapor Annealed ABA Triblock Copolymer Thin Films

Abstract

Nanoscale self-assembly of block copolymer thin films has garnered significant research interest for nanotemplate design and membrane applications. To fulfill these roles, control of thin film morphology and orientation is critical. Solvent vapor annealing (SVA) treatments can be used to kinetically trap morphologies in thin films not achievable by traditional thermal treatments, but many variables affect the outcome of SVA, including solvent choice, total solvent concentration/swollen film thickness, and solvent removal rate. In this work, we systematically examined the effect of solvent removal rate on the final thin film morphology of a cylinder-forming ABA triblock copolymer. By kinetically trapping the film morphologies at key points during the solvent removal process and then using successive ultraviolet ozone (UVO) etching steps followed by atomic force microscopy (AFM) imaging to examine the through-film morphologies of the films, we determined that the mechanism for cylinder reorientation from substrate-parallel to substrate-perpendicular involved the propagation of changes at the free surface through the film toward the substrate as a front. The degree of reorientation increased with successively slower solvent removal rates. Furthermore, the AFM/UVO etching scheme permitted facile real-space analysis of the thin film internal structure in comparison to cross-sectional transmission electron microscopy.

Morphological Evolution of Gyroid-Forming Block Copolymer Thin Films with Varying Solvent Evaporation Rate

“…Moreover, the formation of perpendicular cylinder from the solvent evaporation is attributed to kinetic factor of the swelling process. 31 As the film thickness reduces to 520 nm, an interesting phase transition from hexagonally packed perpendicular cylinders to gyroid texture with the crystallographic plane of (111) G parallel to the air surface can be found. The coexisting planes of hexagonal array of perpendicular cylinder and gyroid (111) G suggest that the initiation of phase transition in the thin-film was originally from cylinder to gyroid (Figure 2c).…”

“…31 As a result, to avoid energy penalties and to correspond with the commensuration for the consideration of thermodynamic stability will lead to the formation of perpendicular cylinders rather than parallel cylinders. Also, the evaporation may induce the formation of perpendicular orientation of cylinder microdomains due to the permeation discrepancy between phase-separated microdomains.…”

“…Recently, Epps and co-workers reported a systematic studies on induced cylinder orientation in BCP thin films through the control of solvent evaporation rate for swollen thin films by solvent. 31 In our previous study, the morphological evolution for the gyroidforming block copolymer thin films was studied as a function of solvent annealing time by keeping the swollen thickness at two times of the initial thickness. When the solvent annealing of the gyroid-forming PS−PLLA thin film was keep at a certain swollen thickness for a sufficiently long time, interesting morphological evolution from gyroid to cylinder can be found.…”

Strategies for Inorganic Incorporation using Neat Block Copolymer Thin Films for Etch Mask Function and Nanotechnological Application

“…Judicious selection of particular reactive tri-BCP systems is essential to allow infi ltration and fulfi ll ones eventual desired nanofeatures. Other tri-BCPs of note include those studied by Epps and co-workers focusing on solvent vapor anneal effects in PdS-b -PI-b -PdS (d = deuterated) fi lms, [ 196 ] phase changes due to partial hydrogenation in a PI-b -PS-b -PMMA system, [ 197 ] and the precise placement of gold nanoparticle location in well-ordered PS-b -PI-b -PS patterns. [ 198 ] An endless range of architectures are possible beyond di-and tri-BCPs, and recently multiblocks have been suggested to open a "Pandora's box" of design space by virtue of synthetic polymer advances.…”

Nanopatterns Produced by Directed Self-Assembly in Block Copolymer Thin Films

“…4.5). This process is however still considered difficult to reproduce, mainly because reproducibility requires a precise mastering and controlling of many experimental [65], annealing time with characteristic times sometimes shorter than 1 min [63], vapor pressure and the rate of vapor removal [39,66], glass transition temperature. All these can affect, often in coupled ways, the resulting metastable morphologies.…”