Photochemically Induced Marangoni Patterning of Polymer Bilayers.

Surface-tension gradients created along a polymer film by patterned photochemical reactions are a powerful tool for creating surface topography. Here, we use mathematical modeling to explore a strategy for patterning photochemically inactive polymers by coupling a light-sensitive and light-insensitive polymer to form a polymer bilayer. The light-sensitive polymer forms the top layer, and the most dominant surface-tension gradients are introduced at the interface between this layer and air. Lubrication theory is used to derive nonlinear partial differential equations describing the heights of each layer, and linear analysis and nonlinear simulations are performed to characterize interface dynamics. Patterns form at both the polymer-air and polymer-polymer interfaces at early thermal annealing times as a result of Marangoni stresses but decay on prolonged thermal annealing as a result of the dissipative mechanisms of capillary leveling and photoproduct diffusion, thus setting a limit to the maximum individual layer deformation. Simulations also show that the bottom-layer features can remain "trapped", i.e., exhibit no significant decay, even while the top layer topography has dissipated. We study the effects of two key parameters, the initial thickness ratio and the viscosity ratio of the two polymers, on the maximum deformation attained in the bottom layer and the time taken to attain this deformation. We identify regions of parameter space where the maximum bottom-layer deformation is enhanced and the attainment time is reduced. Overall, our study provides guidelines for designing processes to pattern photochemically inactive polymers and create interfacial topography in polymer bilayers.

Controlling Surface Deformation and Feature Aspect Ratio in Photochemically Induced Marangoni Patterning of Polymer Films.

Thin liquid polymer films can be topographically patterned when polymer/air interfaces are deformed by surface-tension gradients. Toward this end, a recently developed method first photochemically patterns surface-tension gradients along a solid, flat polymer film. On heating to the liquid state, the film initially develops topography reflecting the patterned surface-tension gradients. But capillary leveling and diffusion of the photoproduct oppose this causing the features to eventually decay back to a flat film upon extended thermal annealing. Intuitively, this interplay between competing mechanisms sets a limit on the maximum film deformation during the process. Prior studies show that the initial film thickness, photomask periodicity, and amount of photochemical conversion significantly affect the maximum film deformation. Here, we use a model based on lubrication theory to develop additional insights into this observation. We identify two regimes, capillary-leveling-dominated and photoproduct-diffusion-dominated, wherein the respective dominant mechanism determines the maximum film deformation that can be additionally related to various experimental parameters. Scaling laws for the variation of maximum film deformation and aspect ratio with film thickness and surface-tension pattern periodicity are also developed. Complementary experiments show good agreement with model predictions. Insights into the effect of surface-tension pattern asymmetry on the maximum film deformation are also provided. These findings reveal mechanistic detail and fundamental principles that are useful for controlling the process to form target patterns of interest.

Achieving Stable Patterns in Multicomponent Polymer Thin Films Using Marangoni and van der Waals Forces.

Liquid-air interfaces can be deformed by surface-tension gradients to create topography, a phenomenon useful for polymer film patterning. A recently developed method creates these gradients by photochemically patterning a solid polymer film. Heating the film to the liquid state leads to flow driven by the patterned surface-tension gradients, but capillary leveling and diffusion of surface-active species facilitate eventual dissipation of the topography. However, experiments demonstrate that using blends of high- and low-molar-mass polymers can considerably delay the decay in topography. To gain insight into this observation, we develop a model based on lubrication theory that yields coupled nonlinear partial differential equations describing how the film height and species concentrations evolve with time and space. Incorporation of a nonmonotonic disjoining pressure is found to significantly increase the lifetime of topographical features, making the model predictions qualitatively consistent with experiments. A parametric study reveals the key variables controlling the kinetics of film deformation and provides guidelines for photochemically induced Marangoni patterning of polymer films.

Connecting Microscopic Structures, Mesoscale Assemblies, and Macroscopic Architectures in 3D-Printed Hierarchical Porous Covalent Organic Framework Foams.

The induction of macro and mesopores into two-dimensional porous covalent organic frameworks (COFs) could enhance the exposure of the intrinsic micropores toward the pollutant environment, thereby, improving the performance. However, the challenge is to build a continuous hierarchically porous macro-architecture of crystalline organic materials in the bulk scale. In this regard, we have strategized a novel synthetic method to create hierarchically porous COF foams consisting of ordered micropores (2-2.2 nm) and disordered meso and macropores (50 nm to 200 µm) as well as ordered macropores (1.5 mm to 2 cm). Herein, graphene oxide was used for creating disordered macro and mesopores in COF-GO foams. Considering the rheological features of the precursor hydrogel, we could integrate crystalline and porous COF-GO foams into self-supported three-dimensional (3D)-printed objects with the desired shapes and sizes. Therefore, we have engineered the 3D macro-architecture of COF-GO foams into complex geometries keeping their structural order and continuous porosity intact over a range of more than a million (10-9 m to 10-3 m). The interconnected 3D openings in these COF-GO foams further enhance the rapid and efficient uptake of organic and inorganic pollutants from water (>95% removal within 30 s). The abundant distribution of interconnected macroporous volume (55%) throughout the COF-GO foam matrix enhances the flow of water (1.13 × 10-3 m·s-1) which results in efficient mass transport and adsorption.

Light-Triggered, Spatially Localized Chemistry by Photoinduced Electron Transfer.

It is of immense interest to exert spatial and temporal control of chemical reactions. It is now demonstrated that irradiation can trigger reactions specifically at the surface of a simple colloidal construct, obtained by adsorbing polyethyleneimine on fluorescent colloidal particles. Exciting the fluorescent dye in the colloid affords photoinduced electron transfer to spatially proximal amine groups on the adsorbed polymer to form free radical ions. It is demonstrated that these can be harnessed to polymerize acrylic acid monomer at the particle surface, or to break up colloidal assemblies by cleaving a cross-linked polymer mesh. Formation of free radical ions is not a function of the size of the colloid, neither is it restricted to a specific fluorophore. Fluorophores with redox potentials that allow photoinduced electron transfer with amine groups show formation of free radical ions.

Camphene as a Mild, Bio-Derived Porogen for Near-Ambient Processing and 3D Printing of Porous Thermoplastics.

Porous structures are ubiquitous in nature due to their advantageous mechanical and transport properties. These structures have inspired various synthetic porous polymer technologies, including lightweight structural materials and membranes. While many manufacturing processes have been developed to generate porous thermoplastics, these usually include hazardous processes, such as high pressures and temperatures, or chemical components. Furthermore, few are compatible with additive manufacturing methods, such as 3D printing. Herein, we introduce bio-derived terpene camphene as a solvent and porogen for the freeze-casting of thermoplastic parts under mild conditions. Enabled by a low melting point (50 °C), camphene is used as a solvent for melt processing camphene-polymer solutions at moderate temperatures that later undergo room-temperature crystallization to template polymer-rich domains. Due to its high vapor pressure, camphene can be sublimed directly from these biphasic structures, resulting in an interconnected microporous polymer structure. Various polymers are demonstrated to be soluble in camphene, including polystyrene, an olefinic elastomer, a polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene elastomer, a cyclic olefin copolymer, and poly(ethyl methacrylate). Porous samples of each polymer were generated from camphene mixtures via compression molding, cooling, and subsequent vacuum annealing at room temperature to remove camphene. The porosity and pore structures were dependent on solution composition, including both the polymer type and polymer loading. Across the compositions investigated, porosity decreased monotonically from 93 to 65% with increasing polymer content. In the case of polystyrene, samples with pore diameters varying from ∼20 to <5 µm were generated. Rheological measurements were conducted on a series of polystyrene-camphene solutions to understand and optimize the formulation and conditions for direct ink write 3D printing. Porous parts with complex structures were successfully printed under mild conditions. These results underscore the advantages of camphene as a sustainable, nontoxic porogen and will inform future development of porous polymer systems derived from these methods.

Correction: Synthesis of high surface area porous carbon from anaerobic digestate and it's electrochemical study as an electrode material for ultracapacitors.

[This corrects the article DOI: 10.1039/C9RA06603A.].