Opaline hydrogels:: Polycrystalline body-centered- ubic bulk material with an in situ variable lattice constant

Opaline hydrogels were produced as a polycrystalline bulk material with a body-centered-cubic (bcc) structure by immobilization of self-ordered charged colloidal particles crystallized under equilibrium conditions in a poly(acrylamide) matrix. The final size of a polycrystalline sample is about 7 cm 3; a single crystal is up to several millimeters in length. The crystal size is tunable by varying the amount of photoinitiator and the hydrogel volume change due to swelling. The resulting photonic crystals are of high quality, showing high-order reflections. These hydrogels show a reversible shift of the diffraction Bragg peak wavelength depending on external conditions due to swelling or shrinking and under applied mechanical stress. The wavelength of the photonic band gap can be shifted over the entire spectrum of visible light (Delta lambda approximate to 500 nm). Bulk material offers the possibility of shifting the position of the main Bragg reflection to smaller and larger wavelengths simultaneously: under compression, the wavelength of the (110) reflection parallel to the direction of compression decreases, while that perpendicular to the direction of compression increases. This observation can only be explained by a uniaxial transformation of the bcc lattice to a body-centered-orthorhombic lattice. Other uniaxial transformations to-body-centered-tetragonal, rhombohedral, or triclinic symmetry are possible depending on the direction of compression.