Reversible deformation of opal elastomers

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 mu m, colors are singled out of white light by diffraction from the I I I plane of the lattice, the reflected color depending on the spacing a I I,. 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 core-shell 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.