Self-assembly of soft materials is broadly considered an attractive means of generating nanoscale structures and patterns over large areas. However, the spontaneous formation of equilibrium nanostructures in response to temperature and concentration changes, for example, must be guided to yield the long-range order and orientation required for utility in a given scenario. In this review we examine directed self-assembly (DSA) of block copolymers (BCPs) as canonical examples of nanostructured soft matter systems which are additionally compelling for creating functional materials and devices. We survey well established and newly emerging DSA methods from a tutorial perspective. Special emphasis is given to exploring underlying physical phenomena, identifying prototypical BCPs that are compatible with different DSA techniques, describing experimental methods and highlighting the attractive functional properties of block copolymers overall. Finally we offer a brief perspective on some unresolved issues and future opportunities in this field.

Deposition of block copolymer thin films is most often accomplished in a serial process where material is spin coated onto a substrate and subsequently annealed, either thermally or by solvent vapor, to produce a well-ordered morphology. Here we show that under appropriate conditions, well-ordered block copolymer films may be continuously grown under substrate equilibrated conditions by slow deposition of discrete subattoliter quantities of material using electrospray. We conduct time-resolved observations and investigate the effects of process parameters that underpin film morphology including solvent selectivity, substrate temperature, block-substrate selectivity, and flow rate of the feed solution. For a PEO cylinder-forming poly(styrene-b-ethylene oxide) block copolymer, we uncover a wide temperature window from 90 to 150 °C and an ideal flow rate of 2 μL/min for ordered film deposition from dilute acetone solutions. PEO cylinders aligned with their long axes perpendicular to the film-air interface at optimal spray conditions. Using poly(styrene-b-methyl methacrylate) deposited onto neutrally selective surfaces, we show that the substrate-equilibrated process results in vertically oriented microdomains throughout the film, indicating a preservation of the initial substrate-dictated morphology during the film deposition. Electrospray offers a new and potentially exciting route for controlled, continuous growth of block copolymer thin films and manipulation of their microstructure.

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Electrospray has been recently advanced as a novel approach for the continuous deposition of self-assembled block copolymer thin films. It represents an analogue of physical vapor deposition in which the development of well-ordered microstructures is predicated on relatively rapid relaxation of the polymer compared to its rate of deposition. Here we describe the morphology development of a lamellae-forming poly(styrene-b-4-vinylpyridine) deposited by electrospray. Morphology was considered in the context of relative changes of the deposition and relaxation rates, with the latter significantly affected in some cases by the presence of residual solvent. We observe that the presence of residual solvent in deposited material accelerates the equilibration kinetics such that well-ordered alternating lamellar morphologies could be produced at deposition rates as high as 55 nm/min under “wet” spray conditions, whereas hexagonally packed micelles were produced when the polymer was deposited free of solvent, denoted as the “dry” spray limit. Molecular weight (MW) plays an important role in equilibration kinetics in the “dry” limit with a transition from poorly ordered to well-ordered lamellae produced by reducing MW. Film morphology was largely insensitive to temperature and flow rate over a broad range from 150 to 210 °C and from 3 to 18 μL/min respectively, although the orientation of the lamellae switched from parallel to perpendicular at elevated flow rates, potentially due to the influence of rapid solvent evaporation.

The delivery of sub-micron droplets of dilute polymer solutions to a heated substrate by electrospray atomization enabled precisely controlled and continuous deposition, or growth, of block copolymer thin films. It also provided, in principle, the ability to fabricate heterolattice materials using sequential depositions. This possibility was explored and the morphology of resulting composite films produced by such sequential electrospray deposition (ESD) of lamellar diblock copolymers of poly(styrene-b-4-vinylpyridine) with differing molecular weights was examined. The structure of the heterolattice interface was a strong function of temperature. Sharp interfaces with abrupt changes in the lamellar period were observed at lower deposition temperatures, while higher temperatures produced a smooth variation in the lamellar period from one molecular weight to the next. The ordering kinetics of a secondary high molecular weight layer could be substantially enhanced depending on the molecular weight of the polymer present in the underlying primary layer. These findings were discussed in the context of temperature and molecular weight dependent diffusion dynamics of the polymers in the melt which control the inter-mixing of the layers and therefore the structure of the heterolattice interface.