Shear‐Induced Organization in Flexible Polymer Opals

Abstract

Synthesis of 3D opaline photonic crystals has developed into a standard procedure during the last decade. [1][2][3][4][5] However, the conventional methods suffer from multiple drawbacks, with cracking and polycrystallinity [6][7][8][9] leading to degradation of the optical properties of these photonic crystals through, e.g., scattering. These special difficulties in 3D photonic crystal fabrication have hindered the utilization of the technology in commercial applications, and such photonic information technology [10] is still in its infancy. Clearly there is a need for industrial-scale, high-yield methods for producing functional photonic crystals. A novel cost-effective large-scale technique to produce flexible opals through shear-ordering during compression, utilizing a core/shell approach based on polymers, has recently been developed [11,12] and further demonstrated to have possible applications, e.g., sensor, security, and structural color applications. [13] In this Communication we present a key analysis of the 3D rheologically derived properties of shear-ordered opaline thinfilm photonic crystals using optical tracking of the strain-induced anisotropy. Probing UV-surface diffraction combined with band-gap measurements reveals a complete picture of the unit cell changes under strain. The results demonstrate that our polymer opals consist of a coherently ordered ''super-domain'' characterized by a radial director vector and show anisotropic photonic behavior depending on the relative vectorial orientation of strain and director.Shear-ordering of colloidal suspensions has been studied extensively in recent years. [14][15][16][17] In these systems the crystal ordering is dependent on both the applied shear profile and the strength of shear, and with suitable conditions long-range ordering is achieved, possibly with some dislocations or stacking faults. Our approach relies on the compression-induced shear-flow ordering of core/shell polymer particles resulting in highly-ordered solid photonic crystals with spectacular structural color features (Fig. 1a). We start with precursor core/shell particles composed of a polystyrene-polymethylmethacrylate (PS-PMMA) core and polyethylacrylate (PEA) shell. The detailed precursor preparation is described elsewhere.[11] By uniaxially compressing the precursor powder between two heated plates (Fig. 1b), we create a viscous shear flow in the polymer melt forcing the spheres to assemble into an fcclattice. The resulting structure formed here is a circular thin (ca. 300 mm) film disk (diameter 15 cm) of low-refractiveindex-contrast fcc-crystal, with the PS-PMMA cores forming the lattice and the PEA filling the interstitial sites. In this photonic crystal film the (111) planes are oriented parallel to the compression plates. [11][12][13] The (111)-plane resonance wavelength can be tuned by varying the precursor PS-PMMA particle size, and the sample can be doped with nanoparticles leading to interesting photonic behavior. [18] In this paper we show results obtained from opals us...

A colloidoscope of colloid-based porous materials and their uses

“…(E, F) Melt extrusion of core-shell particles can fabricate elastomeric, mechanically stable opals over large areas. 135,136,160 Reproduced with permission from John Wiley and Sons, Elsevier, and the American Chemical Society. (G, H) Droplet-based microfluidics can be used to prepare spherical superparticles in a continuous-flow method 105 (reproduced with permission from the National Academy of Sciences), and (H) the throughput can be increased by parallelization while maintaining monodispersity 161 (reproduced with permission from the Royal Society of Chemistry).…”

“…The shear forces induce ordering of the particles and upon cooling, a large area colloidal crystal results ( Figure 5E). 135,136 The shell can further be crosslinked, producing elastomeric colloidal crystals with strain-responsive structural color ( Figure 5F). 160 Finally, superparticles can be prepared in a microfluidic setup, allowing a continuous flow production of self-assembled materials with high fidelity (Figure 5G-H).…”

High Frequency Acoustics in Colloid-Based Meso- and Nanostructures by Spontaneous Brillouin Light Scattering

“…The crystallographic orientation of the silica spheres in the pressed film is known to be an f cc lattice with orientation of lines of next neighbors in the radial direction from the center of the film. [68] Due to the melt compression, in the film the silica cores come nearer then in the initial opal, as is schematically shown in Fig. 5.17b.…”

“…It could be shown only recently that in such films the direction corresponding to the (111) plane of the f cc lattice is radial from the center of the round press cylinder, i.e., multi-domain ordering must be assumed, however, the crystallographic directions are well defined. [68] Fig. 3.3 shows a typical film prepared consisting of PS cores and PEA matrix.…”

“…From light diffraction experiments on similar samples, also synthesized by DKI, it is experimentally proven that stretching clearly changes the intersphere distances. [68] In the direction of the strain, the distance between the spheres increases, while perpendicular to that the spheres come nearer together. Accordingly, also the BG should be shifted to lower frequencies, which is not the case within the experimental error, or, with other words, the gap is robust against structural variation.…”