Orthogonally Spin-Coated Bilayer Films for Photochemical Immobilization and Patterning of Sub-10-Nanometer Polymer Monolayers

Versatile and spatiotemporally controlled methods for decorating surfaces with monolayers of attached polymers are broadly impactful to many technological applications. However, current materials are usually designed for very specific polymer/surface chemistries and, as a consequence, are not very broadly applicable and/or do not rapidly respond to high-resolution stimuli such as light. We describe here the use of a polymeric adhesion layer, poly­(styrene sulfonyl azide-<i>alt</i>-maleic anhydride) (PSSMA), which is capable of immobilizing a 1–7 nm thick monolayer of preformed, inert polymers via photochemical grafting reactions. Solubility of PSSMA in very polar solvents enables processing alongside hydrophobic polymers or solutions and by extension orthogonal spin-coating deposition strategies. Therefore, these materials and processes are fully compatible with photolithographic tools and can take advantage of the immense manufacturing scalability they afford. For example, the thicknesses of covalently grafted poly­(styrene) obtained after seconds of exposure are quantitatively equivalent to those obtained by physical adsorption after hours of thermal equilibration. Sequential polymer grafting steps using photomasks were used to pattern different regions of surface energy on the same substrate. These patterns spatially controlled the self-assembled domain orientation of a block copolymer possessing 21 nm half-periodicity, demonstrating hierarchical synergy with leading-edge nanopatterning approaches

Bidirectional Control of Flow in Thin Polymer Films by Photochemically Manipulating Surface Tension

The Marangoni effect causes liquids to flow toward localized regions of higher surface tension. In a thin film, such flow results in smooth thickness variations and may represent a practically useful route to manufacture topographically patterned surfaces. An especially versatile material for this application should be able to be spatially programmed to possess regions of higher or lower relative surface tension so that the direction of flow into or out of those areas could be directed with precision. To this end, we describe here a photopolymer whose melt-state surface tension can be selectively raised or lowered in the light exposed regions depending on the wavelength and dose of applied light. The direction of Marangoni flow into or out of the irradiated areas agreed with expected surface tension changes for photochemical transformations characterized by a variety of spectroscopic techniques and chromatographic experiments. The maximum film thickness variations achieved in this work are over 200 nm, which developed after only 5 min of thermal annealing. Both types of flow patterns can even be programmed sequentially into the same film and developed in a single thermal annealing step, which to our knowledge represents the first example of harnessing photochemical stimuli to bidirectionally control flow

Designing Intrablock Attractions To Increase the χ Parameter of a Symmetric Diblock Copolymer

Block copolymer (BCP) lithography is capable of forming features on the order of tens of nanometers, and this size is desirable for numerous applications, including data storage devices, microprocessors, and membranes. BCPs must be oriented to form device-relevant structures, and poly­(styrene-<i>block</i>-methyl methacrylate) (PS–PMMA) is the most widely studied BCP due to its ability to form perpendicularly oriented features when simply heated on an energetically nonpreferential substrate. However, the smallest practical feature sizes attainable by PS–PMMA are about 11 nm. In this work, we incorporate a self-interacting monomer, vinyl­naphthalene, into the styrenic block of PS–PMMA to effectively increase its Flory–Huggins interaction parameter. Introducing 35 mol % of vinyl­naphthalene into the BCP more than doubled its χ parameter, resulting in a BCP structure that is capable of forming features as small as 6.3 nm. We also demonstrate that like PS–PMMA, this new poly­((styrene-<i>random</i>-vinyl­naphthalene)-<i>block</i>-methyl methacrylate) (PSVN–PMMA) BCP can be oriented vertically with only thermal annealing

Ultrasmooth Polydopamine Modified Surfaces for Block Copolymer Nanopatterning on Flexible Substrates

Nature has engineered universal, catechol-containing adhesives which can be synthetically mimicked in the form of polydopamine (PDA). In this study, PDA was exploited to enable the formation of block copolymer (BCP) nanopatterns on a variety of soft material surfaces. While conventional PDA coating times (1 h) produce a layer too rough for most applications of BCP nanopatterning, we found that these substrates could be polished by bath sonication in a weakly basic solution to form a conformal, smooth (root-mean-square roughness ∼0.4 nm), and thin (3 nm) layer free of large prominent granules. This chemically functionalized, biomimetic layer served as a reactive platform for subsequently grafting a surface neutral layer of poly­(styrene-<i>random-</i>methyl methacrylate-<i>random-</i>glycidyl methacrylate) to perpendicularly orient lamellae-forming poly­(styrene-<i>block-</i>methyl methacrylate) BCP. Moreover, scanning electron microscopy observations confirmed that a BCP nanopattern on a poly­(ethylene terephthalate) substrate was not affected by bending with a radius of ∼0.5 cm. This procedure enables nondestructive, plasma-free surface modification of chemically inert, low-surface energy soft materials, thus overcoming many current chemical and physical limitations that may impede high-throughput, roll-to-roll nanomanufacturing

Interfacial Engineering for the Synergistic Enhancement of Thermal Conductivity of Discotic Liquid Crystal Composites

To develop an advanced heat transfer composite, a deeper understanding of the interfacial correlation between matrix and filler is of paramount importance. To verify the effect of interfacial correlations on the thermal conductivity, the conductive fillers such as expanded graphite (EG) and boron nitride (BN) are introduced in the discotic liquid crystal (DLC)-based polymeric matrix. The DLC matrix exhibits better interfacial affinity with EG compared to BN because of the strong π–π interactions between EG and DLC. Thanks to its excellent interfacial affinity, the EG-DLC composites show a synergistic increment in thermal conducting performance

A Photochemical Approach to Directing Flow and Stabilizing Topography in Polymer Films

Coatings and substrates with topographically patterned features will play an important role in efficient technologies for harvesting and transmitting light energy. In order to address these applications, a methodology for prescribing height profiles in polymer films is presented here. This is accomplished by photochemcially patterning a solid-state, sensitized polymer film. After heating the film above its glass transition temperature, melt-state flow is triggered and directed by the chemical pattern. A second light exposure was applied to fully activate a heat-stable photo-crosslinking additive. The features formed here are thermochemically stable and can act as an underlayer in a multilayered film. To exemplify this capability, these films were also used to direct the macroscopic film morphology of a block copolymer overlayer

Generating Large Thermally Stable Marangoni-Driven Topography in Polymer Films by Stabilizing the Surface Energy Gradient

Marangoni forces drive a fluid to flow in response to positional differences in surface energy. In thin polymer films, a difference in surface energy between two coincident liquid polymers could offer a useful route to manufacture topographically patterned surfaces via the Marangoni effect. Previously, we have demonstrated a photochemical method using the Marangoni effect for patterning thin polystyrene films. To generalize the approach, a theoretical model that gives the underlying physics of this process was also developed, which further revealed that low viscosities, low diffusivities, and large surface energy gradients favor rapid evolution of large film thickness variations. However, as described by the Stokes−Einstein equation or the Rouse model, low viscosity is generally correlated with high diffusivity in a single-component system. Herein, we report a strategy to decouple film viscosity and diffusivity by co-casting a high molecular weight surface energy gradient creating copolymer (low diffusivity) with a low molecular weight majority homopolymer (high diffusivity and low viscosity), which are miscible with each other. Patterned light exposure through a photomask imposes a patterned surface energy gradient between light-exposed and unexposed regions due to photochemical reactions involving only the low diffusivity component. Upon heating the film to the liquid state, the film materials (primarily the low viscosity homopolymer component) flow from the low to high surface energy regions. This strategy either eliminates or greatly slows dissipation of the prepatterned surface energy gradient while maintaining rapid feature formation, resulting in formation of ca. 500 nm high features within only 30 min of thermal annealing. Furthermore, the formed features are stable upon extended thermal annealing for up to one month. It is found that a ratio of Marangoni forces to capillary forces can provide a predictive metric that distinguishes which scenarios produce features that dissipate or persist

Marangoni Instability Driven Surface Relief Grating in an Azobenzene-Containing Polymer Film

The Marangoni effect describes fluid flow near an interface in response to a surface tension gradient. Here, we demonstrate that the Marangoni effect is the underlying mechanism for flow driven feature formation in an azobenzene-containing polymer film; features formed in azobenzene-containing polymers are often referred to as surface relief gratings or SRGs. An amorphous poly­(4-(acryloyl­oxyhexyl­oxy)-4′-pentyl­azobenzene) was synthesized and studied as a model polymer. To isolate the surface tension driven flow from the surface tension pattern inscription step, the surface tension gradient was preprogrammed via photoisomerization of azobenzene in a glassy polymer film without forming topographical features. Subsequently, the latent image was developed in the absence of light by annealing above the glass transition temperature where the polymer is a liquid. The polymer flow direction was controlled with precision by inducing different surface tension changes in the exposed regions, in accordance with expectation based on the Marangoni effect. Finally, the height of the formed features decreased upon extensive thermal annealing due to capillary leveling with two distinct rates. A scaling analysis revealed that those rates originated from dissimilar capillary velocities associated with different azobenzene isomers