When black and white make green: the surprising interplay of structure and pigments

The natural world is teeming with color, which originates either from the wavelength- dependent absorp- tion of light by pigments or from scattering from nanoscale structures, or both. While the latter ' structural color ' has been a topic of intense study in recent years, the most vibrant colors in nature involve contributions from both structure and pigment. The study of structure–pigment interactions in biological systems is currently in its infancy and could inspire more technological applications, such as sustainable, toxin-free pigments and more efficient light harvesting

Structural control in polymerization-induced phase separation in the solid-state

The micro- and nanostructure of materials determines their macroscopic properties. Currently, most synthetic methods used to control structure at these length scales consist of controlling the size and morphology of structural building blocks. Typically, these fabrication processes involves two distinct steps: the synthesis of the blocks, followed by their assembly. Instead, living systems generate structures across a broad range of length scales in one step directly from macromolecules, whose dimensions are much smaller than the characteristic scale of the final structure. Inspired by these examples, we wanted to learn how to introduce and control structure in materials at the nano- and microscale without having to pre-engineer the building blocks. Our strategies leveraged polymerization-induced phase separation in the solid state. First, we demonstrated how polymerization has the unique capability of triggering, controlling, and arresting phase separation. Afterwards, we described how we exploited this ability to make durable, structurally correlated nanocomposites. Finally, we explain how we tuned the process parameters to control the length scale of the obtained structure. In some cases, the synthesized materials were able to display structural color

ATRP Enhances Structural Correlations In Polymerization-Induced Phase Separation**

Synthetic methods to control the structure of materials at sub-micron scales are typically based on the self-assembly of structural building blocks with precise size and morphology. On the other hand, many living systems can generate structure across a broad range of length scales in one step directly from macromolecules, using phase separation. Here, we introduce and control structure at the nano- and microscales through polymerization in the solid state, which has the unusual capability of both triggering and arresting phase separation. In particular, we show that atom transfer radical polymerization (ATRP) enables control of nucleation, growth, and stabilization of phase-separated poly-methylmethacrylate (PMMA) domains in a solid polystyrene (PS) matrix. ATRP yields durable nanostructures with low size dispersity and high degrees of structural correlations. Furthermore, we demonstrate that the length scale of these materials is controlled by the synthesis parameters.ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

Putting the Squeeze on Phase Separation

Phase separation is a ubiquitous process and finds applications in a variety of biological, organic, and inorganic systems. Nature has evolved the ability to control phase separation to both regulate cellular processes and make composite materials with outstanding mechanical and optical properties. Striking examples of the latter are the vibrant blue and green feathers of many bird species, which are thought to result from an exquisite control of the size and spatial correlations of their phase-separated microstructures. By contrast, it is much harder for material scientists to arrest and control phase separation in synthetic materials with such a high level of precision at these length scales. In this Perspective, we briefly review some established methods to control liquid-liquid phase separation processes and then highlight the emergence of a promising arrest method based on phase separation in an elastic polymer network. Finally, we discuss upcoming challenges and opportunities for fabricating microstructured materials via mechanically controlled phase separation.ISSN:2691-370

Metasurfaces Leveraging Solar Energy for Icephobicity

ISSN:1936-0851ISSN:1936-086

Enhancing the Refractive Index of Polymers with a Plant-Based Pigment

Polymers are essential components of many nanostructured materials. However, the refractive indices of common polymers fall in a relatively narrow range between 1.4 and 1.6. Here, it is demonstrated that loading commercially-available polymers with large concentrations of a plant-based pigment can effectively enhance their refractive index. For polystyrene (PS) loaded with 67 w/w% beta-carotene (BC), a peak value of 2.2 near the absorption edge at 531 nm is achieved, while maintaining values above 1.75 across longer wavelengths of the visible spectrum. Despite high pigment loadings, this blend maintains the thermoforming ability of PS, and BC remains molecularly dispersed. Similar results are demonstrated for the plant-derived polymer ethyl cellulose (EC). Since the refractive index enhancement is intimately connected to the introduction of strong absorption, it is best suited to applications where light travels short distances through the material, such as reflectors and nanophotonic systems. Enhanced reflectance from films is experimentally demonstrated, as large as sevenfold for EC at selected wavelengths. Theoretical calculations highlight that this simple strategy can significantly increase light scattering by nanoparticles and enhance the performance of Bragg reflectors.ISSN:1613-6810ISSN:1613-682

Optical Reflectance of Composites with Aligned Engineered Microplatelets

The reflection of light from distributed microplatelets is an effective approach to creating color and controlling the optical properties in paints, security features, and optical filters. However, predictive tools for the design and manufacturing of such composite materials are limited due to the complex light-matter interactions that determine their optical response. Here, the optical reflectance of individual reflective microplatelets and of polymer-based composites containing these engineered platelets as an aligned, dispersed phase are experimentally studied and analytically calculated. Transfer-matrix calculations are used to interpret the effect of the platelet architecture, the number of platelets, and their size distribution on the experimentally measured reflectance of composites prepared using a previously established magnetic alignment technique. It is demonstrated that the reflectance of the composites can be understood as the averaged response of an array of Fabry-Perot resonators, in which the microplatelets act as semi-transparent flat reflectors and the polymer as cavity medium. By using an analytical model and computer simulations to describe the interaction of light with platelets embedded in a polymer matrix, this work provides useful tools for the design and fabrication of composites with tailored optical reflectance.ISSN:2195-107

Metasurfaces Leveraging Solar Energy for Icephobicity

Inhibiting ice accumulation on surfaces is an energy-intensive task and is of significant importance in nature and technology where it has found applications in windshields, automobiles, aviation, renewable energy generation, and infrastructure. Existing methods rely on on-site electrical heat generation, chemicals, or mechanical removal, with drawbacks ranging from financial costs to disruptive technical interventions and environmental incompatibility. Here we focus on applications where surface transparency is desirable and propose metasurfaces with embedded plasmonically enhanced light absorption heating, using ultrathin hybrid metal–dielectric coatings, as a passive, viable approach for de-icing and anti-icing, in which the sole heat source is renewable solar energy. The balancing of transparency and absorption is achieved with rationally nanoengineered coatings consisting of gold nanoparticle inclusions in a dielectric (titanium dioxide), concentrating broadband absorbed solar energy into a small volume. This causes a > 10 °C temperature increase with respect to ambient at the air–solid interface, where ice is most likely to form, delaying freezing, reducing ice adhesion, when it occurs, to negligible levels (de-icing) and inhibiting frost formation (anti-icing). Our results illustrate an effective unexplored pathway toward environmentally compatible, solar-energy-driven icephobicity, enabled by respectively tailored plasmonic metasurfaces, with the ability to design the balance of transparency and light absorption