Effects of the Density of Chemical Cross-links and Physical Entanglements of Ultraviolet-Irradiated Polystyrene Chains on Domain Orientation and Spatial Order of Polystyrene-block-Poly(methyl methacrylate) Nano-Domains

Epitaxial Growth of Surface Perforations on Parallel Cylinders in Terraced Films of Block Copolymer/Homopolymer Blends

Due to incommensurability between initial thickness and interdomain distance, thermal annealing inevitably produces relief surface terraces (islands and holes) of various morphologies in thin films of block copolymers. We have demonstrated three kinds of surface terraces in blend films: polygrain terraces with diffuse edges, polygrain terraces with step edges, and pseudo-monograin terraces with island coarsening. The three morphologies were obtained by three different thermal histories, respectively. The thermal histories were imposed on blend films, which were prepared by mixing a homopolystyrene (hPS, 6.1 kg/mol) with a weakly segregated, symmetry polystyrene-block poly(methyl methacrylate) (PS-b-PMMA, 42 kg/mol) followed by spin coating. At a given weight-fraction ratio of PS-b-PMMA/hPS = 75/25, the interior of the blend films forms parallel cylinders. Nevertheless, the surface of the blend films is always dominated by a skin layer of perforations, which epitaxially grow on top of parallel cylinders. By oxygen plasma etching at various time intervals to probe interior nanodomains, the epitaxial relationship between surface perforations and parallel cylinders has been identified by a scanning electron microscope.

Interface Manipulations Using Cross-Linked Underlayers and Surface-Active Diblock Copolymers to Extend Morphological Diversity in High-χ Diblock Copolymer Thin Films

Top and bottom interfaces of high-χ cylinder-forming polystyrene-block-maltoheptaose (PS-b-MH) diblock copolymer (BCP) thin films are manipulated using cross-linked copolymer underlayers and a fluorinated phase-preferential surface-active polymer (SAP) additive to direct the self-assembly (both morphology and orientation) of BCP microdomains into sub-10 nm patterns. A series of four photo-cross-linkable statistical copolymers with various contents of styrene, a 4-vinylbenzyl azide cross-linker, and a carbohydrate-based acrylamide are processed into 15 nm-thick cross-linked passivation layers on silicon substrates. A partially fluorinated analogue of the PS-b-MH phase-preferential SAP additive is designed to tune the surface energy of the top interface. The self-assembly of PS-b-MH thin films on top of different cross-linked underlayers and including 0-20 wt % of SAP additive is investigated by atomic force microscopy and synchrotron grazing incidence small-angle X-ray scattering analysis. The precise manipulation of the interfaces of ca. 30 nm thick PS-b-MH films not only allows the control of the in-plane/out-of-plane orientation of hexagonally packed (HEX) cylinders but also promotes epitaxial order-order transitions from HEX cylinders to either face-centered orthorhombic or body-centered cubic spheres without modifying the volume fraction of both blocks. This general approach paves the way for the controlled self-assembly of other high-χ BCP systems.

Photoimageable Organic Coating Bearing Cyclic Dithiocarbonate for a Multifunctional Surface

We report the fabrication of photocross-linkable and surface-functionalizable polymeric thin films using reactive cyclic dithiocarbonate (DTC)-containing copolymers. The chemical functionalities of these material surfaces were precisely defined with light illumination. The DTC copolymers, namely, poly(dithiocarbonate methylene methacrylate-random-alkyl methacrylate)s, were synthesized via reversible addition-fragmentation chain transfer polymerization, and the reaction kinetics was thoroughly analyzed. The copolymers were cross-linked into a coating using a bifunctional urethane cross-linker that contains a photolabile o-nitrobenzyl group and releases aniline upon exposure to light. The nucleophilic attack of the aromatic amine opens the DTC group, forming a carbamothioate bond and generating a reactive thiol group in the process. The surface concentrations of the unreacted DTC and thiol were effectively controlled by varying the amounts of the copolymer and the cross-linker. The use of methacrylate comonomers led to additional reactive surface functionality such as carboxylic acid via acid hydrolysis. The successful transformations of the resulting DTC, thiol, and carboxylic acid groups to different functionalities via sequential nucleophilic ring opening, thiol-ene, and carbodiimide coupling reactions under ambient conditions were confirmed quantitatively using X-ray photoelectron spectroscopy. The presented chemistries were readily adapted to the immobilization of complex molecules such as a fluorophore and a protein in lithographically defined regions, highlighting their potential in creating organic coatings that can have multiple functional groups under ambient conditions.

Distributions of Deuterated Polystyrene Chains in Perforated Layers of Blend Films of a Symmetric Polystyrene-block-poly(methyl methacrylate)

We have examined the spatial distributions of polymer chains in blend films of weakly segregated polystyrene-block-poly(methyl methacrylate) [P(S-b-MMA)] and deuterated polystyrene (dPS). By fine-tuning the composition (ϕ_PS+dPS = 63.8 vol %) of the total PS/dPS component and annealing temperature (230 and 270 °C), P(S-b-MMA)/dPS blend films mainly form perforated layers with a parallel orientation (hereafter PLs^//). The distributions of dPS in PLs^// were probed by grazing-incidence small-angle neutron scattering (GISANS) and time-of-flight neutron reflectivity (ToF-NR). GISANS and ToF-NR results offer evidence that dPS chains preferentially locate at the free surface and within the PS layers for blend films that were annealed at 230 °C. Upon annealing at 270 °C, dPS chains distribute within PS layers and perforated PMMA layers. Nevertheless, dPS chains still retain a surface preference for thin films. In contrast, such surface segregation of dPS chains is prohibited for thick films when annealed at 270 °C.

Influence of Osmotic Pressure on Nanostructures in Thin Films of a Weakly-Segregated Block Copolymer and Its Blends with a Homopolymer

We studied the influence of osmotic pressure on nanostructures in thin films of a symmetric weakly-segregated polystyrene-block-poly (methyl methacrylate), P(S-b-MMA), block copolymer and its mixtures with a polystyrene (PS) homopolymer of various compositions. Thin films were deposited on substrates through surface neutralization. The surface neutralization results from the PS mats, which were oxidized and cross-linked by UV-light exposure. Thus, thermal annealing produced perpendicularly oriented lamellae and perforated layers, depending on the content of added PS chains. Nevertheless, a mixed orientation was obtained from cylinders in thin films, where a high content of PS was blended with the P(S-b-MMA). A combination of UV-light exposure and acetic acid rinsing was used to remove the PMMA block. Interestingly, the treatment of PMMA removal inevitably produced osmotic pressure and consequently resulted in surface wrinkling of perpendicular lamellae. As a result, a hierarchical structure with two periodicities was obtained for wrinkled films with perpendicular lamellae. The formation of surface wrinkling is due to the interplay between UV-light exposure and acetic acid rinsing. UV-light exposure resulted in different mechanical properties between the skin and the inner region of a film. Acetic acid rinsing produced osmotic pressure. It was found that surface wrinkling could be suppressed by reducing film thickness, increasing PS content and using high-molecular-weight P(S-b-MMA) BCPs.

Photo-directing chemoepitaxy: the versatility of poly(aryl methacrylate) films in tuning block copolymer wetting

UV irradiated poly(aryl methacrylate) films can induce a change in the orientation of the domains of an overlayer of PS-b-PMMA from parallel to perpendicular lamellar structures.

Universal Interfacial Control through Polymeric Nanomosaic Coating for Block Copolymer Nanopatterning

The employment of self-assembly of soft materials has been accepted as an inexpensive, robust, and reliable patterning method. As their self-assembly relies on the delicate molecular interactions near the substrate, a precise prediction/control of the interface structure and dynamics is critical to achieve desired nanostructures. Herein, a polymeric nanomosaic (PNM) pattern is created from the air/water interfacial self-assembly of a block copolymer (BCP) and introduced as an effective interfacial energy control for substrates. As a demonstration, the PNM coating is employed to control the BCP film structures. The perpendicular orientation of BCP self-assembly, which requires neutral wetting conditions for both blocks, is difficult to achieve but can readily be obtained with the PNM coating upon a fine resolution of the pattern quality. The universal applicability of the PNM coating as an interfacial control has been confirmed on curved, flexible, and three-dimensional substrates. In addition, the PNM is introduced as an etching-free and reusable topcoat imparting free surface neutralization even for the high-χ BCP nanopatterning.

Photoinduced Modulation of Polymeric Interfacial Behavior Controlling Thin-Film Block Copolymer Wetting

The tunable surface-wetting properties of photosensitive random copolymer mats were used to spatially control the orientations of thin-film block copolymer (BCP) structures. A photosensitive mat was produced via thermal treatment on spin-coated random copolymers of poly(styrene-ran-2-nitrobenzyl methacrylate-ran-glycidyl methacrylate), synthesized via reversible-deactivation radical polymerization. The degree of UV-induced deprotection of the nitrobenzyl esters in the mat was precisely controlled through the amount of UV-irradiation energy imparted to the mat. The resulting polarity switching of the constituents collectively altered the interfacial wetting properties of the mat, and the tunability allowed lamellar or cylinder-forming poly(styrene-b-methyl methacrylate) BCP thin films, applied over the mat, to change the domain orientation from perpendicular to parallel at proper UV exposures. UV irradiation passing through a photomask was capable of generating defined regions of BCP domains with targeted orientations.