In thermotropic systems with fixed domains scattering particles, which exhibit a sudden change of refractive index with temperature, are statically embedded in a matrix material. At low temperatures the layer is translucent, as the refractive indices of matrix and domain are almost equal. The differing temperature dependence of the refractive index for the components above the switching threshold causes the thermotropic film to turn opaque [2].

One way to manufacture thermotropic systems with fixed domains is the incorporation of thermotropic core-shell polymer particles in a thermoplastic matrix. Such a material is commercially available from EMS Chemie AG (Switzerland) [28]. However, the gradual transition from clear to cloudy of this material is not optimal for solar collector applications.

Other thermotropic systems with fixed domains are based on the dispersion of paraffin in the matrix of a curable resin [6,29,30]. Such thermotropic systems possess a high potential for active solar thermal systems. The materials show a steep and rapid switching process within a small temperature range and an extraordinary high reversibility at low hysteresis [6,30]. The switching threshold is easily adjustable between 25 and 100°C [6,29,30]. A solar transmittance below 85% in clear state is reported. At present a moderate change in transmittance by 25% is achieved [31]. The material production is environmental friendly and cost-efficient. For thermotropic systems with fixed domains no comprehensive research on long-term stability and ageing was reported yet. However, as the additive is statically embedded in the matrix, the materials may posses a high durability. Further research should focus on the adjustment of appropriate switching temperatures, an improvement of the switching range and on the guarantee of sufficient long-term stability even under demanding conditions.