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Preface to Forum on Block Copolymers for Nanotechnology Applications his issue of ACS Applied Materials & Interfaces features a Forum on Block Copolymers for Nanotechnology Applications. The Forum is linked with the EUPOC 2016 conference, which focused on block copolymers self-assembling to fabricate functional nanostructured materials. From 1998 onward, the European Polymer Federation decided to organize a series of Europolymer Conferences (EUPOCs) on topics of recent scientific and industrial interest. These annual conferences took place at the Palazzo Feltrinelli in Gargnano at Lake Garda, Italy. The scientific program of the EUPOC 2016 conference consisted of invited lectures, oral communications and poster presentations. Ample time was given to free discussions, encouraged by the residential style of the conference. Block copolymers can hierarchically self-assemble into chemically distinct domains with size and periodicity on the order of 10 nm or below. The final structural characteristics of these materials are dictated by the elementary block properties, covering the chain length, volume fraction and degree of block incompatibility. A wide variety of distinct morphologies, from spherical, cylindrical, and lamellar to more complex gyroid, Fddd, and hexagonally perforated layer structures can be obtained. In addition, the self-assembly is a highly parallel process, occurring spontaneously over the entire surface. The combination of these particular characteristics makes block copolymers extremely appealing for both fundamental studies and technological applications in several different fields that include nanostructured networks, nanoporous membranes, drug delivery vehicles, nanoparticle templates, organic photovoltaics, and next-generation lithography. Modern synthetic chemistry offers the possibility of designing these macromolecules with very specific length scales and geometries. Understanding the kinetics and thermodynamics of the block copolymer self-assembly process, in the bulk phase as well as in thin films, is a fundamental prerequisite necessary for exploiting these materials. The capability of precisely organizing these nano-objects on appropriate substrates represents a valuable tool to exploit novel technological developments. In this respect, block copolymer theory and simulation can provide new insights into the ordering mechanism and final structure. In coming years this area of research, at the intersection between fundamental science and technology, is expected to define new and innovative applications for these materials. For example, block copolymers are emerging as a costeffective nanofabrication tool to complement conventional optical lithography because they self-assemble in highly ordered polymeric templates with well-defined sub-20 nm periodic features. Related to this application, in “Effect of Entrapped Solvent on the Evolution of Lateral Order in Self-Assembled P(S-r-MMA)/PS-b-PMMA Systems with Different Thicknesses”, Giammaria et al.) focused on cylinder-forming polystyrene-block-poly(methyl methacrylate) block copolymers on a poly(styrene-random-methyl methacrylate) random
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copolymer brush layer grafted to the substrate prior to block copolymer deposition. The self-assembly process was promoted by a rapid thermal processing machine operating at 250 °C for 300 s. Related to such approach, in “Perpendicular Orientation Control without Interfacial Treatment of RAFT-Synthesized High-χ Block Copolymer Thin Films with Sub-10 nm Features Prepared via Thermal Annealing”, Nakatani et al. studied a series of perpendicular lamellae-forming poly(polyhedral oligomeric silsesquioxane methacrylate-block-2,2,2-trifluoroethyl methacrylate)s. The use of silicon-containing hybrid high-χ block copolymers provided easy access to sub-10 nm feature sizes. Directed self-assembly of the block copolymer on a topographically prepatterned substrate using the grapho-epitaxy method was used to successfully obtain perpendicularly oriented lamellae with a half pitch size of ca. 8 nm. To apply well-defined block copolymer nanopatterns to nextgeneration lithography or high-density storage devices, a small line edge roughness of the nanopatterns is essential. A “Reduction of Line Edge Roughness of Polystyrene-blockPoly(methyl methacrylate) Copolymer Nanopatterns By Introducing Hydrogen Bonding at the Junction Point of Two Block Chains” was reported by Lee et al.). Polystyrene-blockpoly(methyl methacrylate) copolymer (PS-b-PMMA) was synthesized with urea (U) and N-(4-aminomethyl-benzyl)-4hydroxymethyl-benzamide (BA) moieties at the junction of PS and PMMA chains (PS-U-BA-PMMA) to improve the line edge roughness. A reduction by ∼25% compared with that obtained from neat PS-b-PMMA without BA and U moieties was achieved. The contribution by Jiang et al.), “Ultrafast Self-Assembly of Sub-10 nm Block Copolymer Nanostructures by Solvent-Free High-Temperature Laser Annealing”, reports on laser spike annealing applied to PS-b-PDMS diblock copolymers to induce short-time (millisecond time scale), high-temperature (300 to 700 °C) microphase segregation and directed self-assembly of sub-10 nm features. A good long-range order for sub-10 nm cylinder morphology was achieved using grapho-epitaxy coupled with a 20 ms dwell laser spike anneal above 440 °C. To have the directed self-assembly of block copolymers implemented as a potential solution for patterning critical features for integrated circuits at future technology nodes requires a better understanding of how the template guides the assembly and induces subsurface changes in the lamellar structure as addressed by Sunday et al.) in “Characterizing Patterned Block Copolymer Thin Films with Soft X-rays”. Using a rotational transmission X-ray scattering measurement coupled with soft X-rays to improve contrast between polymer components, the impact of the ratio of the guiding stripe width to the block copolymer pitch was investigated. Special Issue: Block Copolymers for Nanotechnology Applications Published: September 20, 2017 31213
DOI: 10.1021/acsami.7b11706 ACS Appl. Mater. Interfaces 2017, 9, 31213−31214
ACS Applied Materials & Interfaces
Editorial
De Rosa et al. presented in “Controlling Size and Orientation of Lamellar Microdomains in Crystalline Block Copolymers” an alternative approach to obtain highly ordered lamellar nanostructures. The studied crystalline diblock copolymers constituted by crystalline polyethylene linked to an amorphous block of a propene-ethene random copolymer. Epitaxy was coupled with the technique of gold decoration so that the ordered nanostructures produced by epitaxy acted as template for the formation of long, straight, and parallel rows of gold nanoparticles. In “Vertical vs Lateral Macrophase Separation in Thin Films of Block Copolymer Mixtures: Computer Simulations and GISAXS Experiments” Berezkin et al. reported that mixtures of two diblock copolymers of very different lengths may feature both macro- and microphase separation. For thin films they observed that the film thickness and selective adsorption of different blocks to the substrate control the distribution of macrophases within the film as well as the orientation of the lamellae therein. The lamellar orientation in the layers rich in short copolymers was dictated by the surface selectivity, and this orientation only weakly affects the vertical orientation of lamellae in the film core. In “Amphiphilic Arborescent Copolymers and Microgels: From Unimolecular Micelles in a Selective Solvent to the Stable Monolayers of Variable Density and Nanostructure at a Liquid Interface” Gumerov et al. presented dissipative particle dynamics simulations. Self-assembly of amphiphilic arborescent block copolymers of two generations, and microgels in the selective solvent is compared with equivalent linear diblock copolymers. The pretty fast adsorption kinetics of second generation amphiphilic arborescent block copolymers make them efficient stabilizers of emulsions. Original porous catalytic supports can be engineered via an effective and straightforward synthetic route to polystyreneblock-poly(D,L-lactide) diblock copolymer precursors displaying an acid-cleavable acetal junction between both blocks as shown by Poupart et al. in “Porous Gold Nanoparticle-Decorated Nanoreactors Prepared from Smartly Designed Functional Polystyrene-block-Poly(D,L-Lactide) Diblock Copolymers: Toward Efficient Systems for Catalytic Cascade Reaction Processes”. In situ generated gold nanoparticles could then be immobilized within such functionalized porous nanoreactors, and these hybrid materials could find interesting applications in heterogeneous supported catalysis. In this regard, model catalytic reactions, including C−C homocoupling of benzeneboronic acid derivatives, hydride-mediated reduction of nitroaromatic compounds, and especially unprecedented “one-pot” cascade reactions consisting of the latter consecutive reactions from 3-nitrobenzeneboronic acid, were successfully monitored by different chromatographic and spectroscopic techniques. “Block Copolymer-Templated Hierarchical Porous Carbon Nanostructures with Nitrogen-Rich Functional Groups for Molecular Sensing” were reported by Sun et al. The abundant nitrogen atoms terminating on the surface of hierarchical porous carbon nanostructures played a critical role in promoting a large Raman enhancement generated via a chemical mechanism. Thus, the unique architecture of hierarchical porous carbon nanostructures based on the construction of a building block of a well-defined network of core−shell nanospheres provided a new design strategy for fabricating surface-enhanced Raman spectroscopy substrates. Hulkkonen et al. reported about “Block Copolymer Patterning for Creating Porous Silicon Thin Films with
Tunable Refractive Indices”. Porous silicon was prepared by etching a hexagonally ordered pore pattern onto the surface of silicon wafers using poly(styrene-2-vinylpyridine) to prepare the etch mask. A maximum decrease of 30% in the refractive index was observed at 34% porosity compared to bulk silicon. Nanoporous membranes with tailored size pores and multifunctionality derived from self-assembled block copolymers attract growing interest in ultrafiltration. The “Influence of Solvent on the Structure of an Amphiphilic Block Copolymer in Solution and in Formation of an Integral Asymmetric Membrane” is reported by Abetz et al.. The influence of the solvent system on the ordering and uniformity of the pores and pore characteristics is demonstrated for the separation layer as well as porous substructure of the final membranes. As a whole, the articles in this Forum highlight the significant progress that continues to be made in the development and application of block copolymer nanostructures, covering a broad range of realized and potential future applications. I acknowledge the contributions of our dedicated authors and reviewers, who through their efforts in this important research area ensure its progress and maintain its vigor.
Peter Mul̈ ler-Buschbaum, Associate Editor
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Technische Universität München
AUTHOR INFORMATION
ORCID
Peter Müller-Buschbaum: 0000-0002-9566-6088 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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DOI: 10.1021/acsami.7b11706 ACS Appl. Mater. Interfaces 2017, 9, 31213−31214
