In Situ Photocontrol of Block Copolymer Morphology During Dip-Coating of Thin Films

Abstract

We demonstrate a unique combination of simultaneous top-down and bottom-up control of the morphology of block copolymer films by application of in situ optical irradiation during dip-coating. A light-addressable and blockselective small molecule, 4-butyl-4'-hydroxyazobenzene (BHAB), is introduced into a diblock copolymer of polystyrene and poly(4-vinylpyridine) (PS-P4VP) of 28.4 wt% P4VP via supramolecular chemistry, notably by hydrogen bonding to P4VP. We show that the spherical morphology of thin films dip-coated from a THF solution at slow withdrawal rates in the dark convert to cylindrical morphology when dip-coated under illumination. This is attributed to volume expansion of the P4VP/BHAB phase due to trans-cis photoisomerization combined with a light-induced increase in BHAB uptake in the film. The demonstrated photocontrol highlights the potential of dip-coating as a scalable film preparation method that can be easily coupled with external stimuli to direct nanostructured self-assembly in the films as solvent evaporates.Controlling the properties of soft materials at the nanoscale is paramount for high-technology applications in diverse domains. Many of these applications require nanostructured materials to be in the form of thin films of good surface quality, 1 for example when used as nanolithographic templates for the electronics industry. 2 Block copolymers (BCPs) are well suited for this purpose, given their propensity to self-assemble into well-defined morphologies, with typical periodicities in the 10-100 nm range, due to block immiscibility. 3 In thin films, this leads to various surface patterns that are controllable at will by a number of parameters, both material (such as molecular weight, block volume fractions, interfacial interaction parameters, and addition of block selective components) and experimental (such as film preparation method, choice of solvent, and thermal/solvent annealing). 1

Hydrogen Bond Induces Hierarchical Self‐Assembly in Liquid‐Crystalline Block Copolymers

“…[6][7][8][9][10][11][12][13] Supramolecular cooperative motion (SMCM) plays an important role in the self-assembly process of LCBCs, driven by the interplay functions between the MPS and the elastic deformation of liquid crystal (LC) ordering. [14][15][16][17][18][19][20] Recent researches have realized the fabrication of sub-10 nm microstructures by supramolecular interactions www.advancedsciencenews.com www.mrc-journal.de…”

“…With a relatively high Flory–Huggins parameter (χ) and controllable responsiveness upon microphase separation (MPS), liquid‐crystalline block copolymers (LCBCs) have been utilized to fabricate multimorphologic nanostructures, such as sphere, cylinder, double gyroid, and lamella, depending on the volume ratio of each block . Supramolecular cooperative motion (SMCM) plays an important role in the self‐assembly process of LCBCs, driven by the interplay functions between the MPS and the elastic deformation of liquid crystal (LC) ordering …”

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

“…relative to the pyridine repeat units. 156 Normally, dip-coating of such supramolecular solutions without illumination at slow dip-coating rates gives rise to films whose small-molecule content is much lower than in solution but increases with increasing dip-coating rate, as observed also with BHAB. This led to a change in film morphology from spherical to cylindrical at a dip-coating rate of around 1.0 mm min À1 for the system studied.…”

“…69 Recently, Vapaavuori et al demonstrated that the BCP morphology of a thin film containing a block-selective hydrogenbonding azobenzene can be manipulated with light. 156 As explanatory background, some of the authors had previously shown that when the films are prepared using the dip-coating technique (i.e. where a substrate is pulled out of the polymer solution at a given rate), the hydrogen-bonding small molecule content in the film is controlled by solution azo/polymer repeat unit ratio, dip-coating withdrawal speed, and the solvent used.…”

Photocontrol of Supramolecular Azo-Containing Block Copolymer Thin Films during Dip-Coating: Toward Nanoscale Patterned Coatings

“…In a recent communication, we demonstrated that the thin film surface nanopattern, SM content, and thickness can be modified by visible light illumination of a film during slow dip-coating from a supramolecular BCP solution containing a photoactive azo-based guest SM. 28 Since light provides very precise spatial and temporal control, this in-situ photocontrol points to pathways for producing complex spatially varying surface patterns.…”