4.1 Crystalline Colloidal Arrays Embedded in Swollen Gel Networks

Monodisperse and highly charged colloidal particles can selfassemble in liquid media of very low ionic strength and form periodic structures at which Bragg reflection occurs. The driving force for the structure formation of the colloidal particles is their electrostatic repulsion.

A colour change with temperature caused by the formation of a crystalline colloidal array in a swollen gel network exhibiting a temperature-induced volume phase transition was reported in [1]. The system presented was obtained by the following two steps. First, highly charged monodisperse polystyrene spheres of 99 nm diameter were dispersed in N-isopropylacrylamide solution whereby a self-assembling of the polystyrene colloidal particles into a body-centred cubic structure occurred. Second, a photochemically initiated polymerisation of N-isopropylacrylamide was carried out to form a network which permanently locks in the ordering of the crystalline colloidal array. Temperature-induced swelling or shrinking of the polymer matrix results in changes of the lattice spacing of the crystalline colloidal array and thus of the Bragg reflection wavelength. To characterise the thermochromic properties of the novel material, thin films, 125 m thick at roomtemperature, were prepared and their Bragg reflection wavelengthdetermined as a function of temperature. Within the temperature range 11.7 34.9 C a continuous change of the diffracted wavelength from 704 to 460 nm was measured, which corresponds to nearly the entire visible range. It should be mentioned that the change of the Bragg reflection wavelength is accompanied by a proportional change of the layer thickness. To...