Creating coloured polymer films without the use of pigments might seem impossible. But using miniature polymer spheres, and a novel assembly process, this feat has been accomplished over large film areas.
Humans have been intensely interested in colour for tens of thousands of years. Until the advent of modern organic chemistry, the colours found in art, decorations and clothing were obtained from pigments, for example indigo, cochineal and ochre, which are derived from naturally occurring compounds found respectively in plants, insects and earth. Amazingly, until about 50 years ago we did not realize that the natural world also makes extensive use of structural colour — colour formed not by the interaction of light with a dye, but by the diffraction of light from a material containing a periodic or quasiperiodic structure on the length scale of the wavelength of visible light. Writing in Advanced Materials, Finlayson et al.1 describe a method for creating intensely coloured polymer films based on the principles of structural colour used by nature.
The iridescence of mother-of-pearl, the shimmer of an opal, the striking coloration of many beetles and the brilliant colours of peacock feathers (Fig. 1a) — these are all due to structural colour, not pigment molecules2. For example, the coloration of an opal comes exclusively from its microstructure, which consists of a periodic arrangement of silica spheres embedded in a silica-based matrix that has a slightly different refractive index from that of the spheres. As different wavelengths of light interact with this structure, they are diffracted in different directions, leading to the iridescence with which we are all familiar.
Over the past two decades, scientists have become exceptionally good at creating materials exhibiting strong colorations that are based on the principles of structural colour found in nature. These 'artificial opals', or more generally — when not made of colloidal building blocks — photonic crystals, have been proposed as potentially forming the basis of a wide range of technologies, ranging from chemical sensors to on-chip optical waveguides3. It has been extremely difficult, however, to form structurally coloured materials over the large areas required for applications such as decorative coatings and fabrics. Finlayson et al.1 now show that highly scalable processing techniques can be used to create coloured, large-area polymer films based on the principles of structural colour (Fig. 1b).
The coloured films reported by the authors derive their hue from the same principle as the coloration of an opal, and are in fact also based on spheres. In their case, the specially designed polymer-based spheres, each with a diameter of about 250 nanometres, form the films when merged together. Until recently, artificial opals tended to have rather muted colours, with flashes of iridescence at specific angles but overall a rather milky and unappealing appearance. But a few years ago, researchers discovered4 that the colour could be made much more intense by the addition of a fraction of a per cent of carbon black. At that time, however, it was not clear how these materials could be made into the large-area structures required for many applications.
Until Finlayson and colleagues' study, the artificial opals with the largest areas were created through the technique of spin coating5, in which a solution is placed on a rotating substrate to form a thin film. Spin coating works well for coating flat surfaces of up to about a metre across with an opalescent film, but becomes entirely impractical either for coating a curved surface or for making free-standing films. Opalescent films can also be formed by convective assembly processes, although these struggle to coat surfaces much larger than a glass slide6, and by other generally slow self-assembly processes based on particle interactions in solution. The authors now report1 their use of edge shearing, a potentially rapid and industrially scalable process in which colloidal particles are sandwiched as a thin film between two removable polymer sheets and drawn over a sharp edge. The edge-shearing process induces the particles to form a highly periodic structure, greatly strengthening their response to light and making their colour much more intense.
In addition to this novel colloidal assembly process, Finlayson et al. used colloidal particles that are different from normal. They consist of a hard, crosslinked polystyrene core surrounded by a soft polyethylacrylate shell. Between the core and shell is a thin polymer layer that binds the core and shell together. When heated and pressed, the particles' soft shells merge, forming a continuous structure, with the cores remaining intact and separated. Once the cores have been induced to assemble into a periodic structure, through the edge-shearing process, a strong iridescence results that is due to the difference in the refractive indices of the core and the shell.
The edge-shearing process as demonstrated by the authors is rather slow (about 1 centimetre per second), and there may be challenges to scaling up the synthesis of the core–shell colloidal particles. But through appropriate engineering these shortcomings are almost certainly resolvable. A more interesting question is whether the films can be made multispectral — that is, with more than one colour at once. The current films exhibit highly saturated colours that vary as a function of the viewing angle. Thus, they can provide a brilliant rainbow of colours, but they cannot present two colours concurrently from the same viewing angle.
Might it be possible to self-assemble structures that contain two characteristic length scales, and thus colours, at the same time? Of course, other colours could be added using dye molecules, but such an approach would not harness the power of the technology. Still, even with this limitation, an exciting range of possibilities emerges, both for consumers and for industry. Imagine a car whose colour changes as it drives by, or fabrics that shimmer in an unexpected way. Looking further, it is likely that the structures could be engineered to be responsive and change colour as a function of their environment, providing both aesthetically and technologically important capabilities.
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Urea-Functionalized Poly(ionic liquid) Photonic Spheres for Visual Identification of Explosives with a Smartphone
ACS Applied Materials & Interfaces (2019)