A new strategy to achieve easily scalable triple stimuli-responsive elastomeric opal films for applications as stretch-tunable photonic band gap materials is reported. Novel monodisperse highly functional core-interlayer-shell beads are obtained by semicontinuous emulsion polymerization featuring a temperature-sensitive fluorescent rhodamine dye either locally restricted in the core or the shell of prepared beads. After extrusion and compression molding, homogeneous elastomeric opal films with fascinating stretch-tunable and temperature-dependent fluorescent properties can be obtained. Applying strains of only a few percent lead to significant blue shift of the reflected colors making these films excellent candidates for applications as deformation sensors. Higher strains up to 90% lead to a tremendous Bragg reflection color change caused by transition from the (111) to the (200) lattice plane. The well-ordered opaline structure with its stop band at the emission frequency of the incorporated fluorescent dye shows remarkable angle-dependent fluorescence suppression. Herein described elastomeric opal films can be valuable in a wide range of applications such as rewritable 3D optical data storage, tunable laser action, and sensing materials.

Synthetic opals result from the crystallization of monodisperse silica or polymer beads of submicroscopic size. The beads self-organize to the face-centered cubic (fcc) lattice from which light is reflected wavelength selectively. At diameters of 0.15-0.3 µm, colors are singled out of white light by diffraction from the 111 plane of the lattice, the reflected color depending on the spacing a 111 . With elastic opal films of core-shell (CS) beads, this spacing and, thereby, the color can be changed by deformation. This mechanochromic effect has so far been studied only on opals made of beads that were not chemically interconnected so the deformation was partly irreversible. In this study, opal films of polymeric coreshell beads were prepared by a melt-flow technique developed earlier in this institute. Afterward, the films were photo-cross-linked. They deformed indeed reversibly, however, with mechanical hysteresis effects. The strained fcc lattice causes a blue shift of the reflected color, which is indicative of a hardsphere deformation mechanism. The shift is strong enough to switch monochromatic light on and off by only a few percent strain.

High-impact-modified polystyrene (HIPS) is made by thermal or radical polymerization of styrene containing dissolved polybutadiene (PB). The reaction leads, intermediately, to blends of yet ungrafted PB, of the homopolymer PS, and of graft copolymers PBgS varying in the number of PS grafts per PB chain. The polymerization induces a phase separation and a phase inversion which results in the well-known “salami” morphology of HIPS. To elucidate the mechanism producing this morphology, the polymerization of styrene in PB/styrene mixtures was studied kinetically and morphologically, in toluene solution and in the bulk. Besides the conventional techniques of polymerization kinetics, electron microscopy was employed to examine the PS/PB/PBgS blends that are formed during the polymerization. The electron micrographs reflect sensitively the composition of these blends, the architecture of the PBgS graft copolymers, and the miscibility of PBgS with PS and PB. The blends were isolated from the polymerizing system by two methods, i.e. (i) evaporation of styrene and (ii) dissolution and film casting. Method i preserves the in-situ morphology of the polymerizing system, while method ii leads to a thermodynamically controlled morphology. Pairwise comparison of these two types of morphology reveals that HIPS owes its salami structures to the fact that PBgS chains with two grafts or more can solubilize the homopolymer PS while PBgS chains with only one graft cannot.

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Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles.