In general polymers are incompatible with one another as a result of low entropy of mixing and the positive energy of mixing between polymers. Exceptions to this rule are for example metastable, partly miscible systems which exhibit a Lower Critical Solution Temperature (LCST). At low temperatures the polymers interact via salt formation, hydrogen bonding, complex formation, п-electron interaction or dipolar interaction. The miscibility decreases with increasing temperature associated with the formation of domains. As a result the layer turns opaque. Thermotropic polymer blends are poured mostly as films from an organic solvent on a glass or a polymer substrate [2,3,20].

Typical polymer blends developed for overheating protection purposes are based on acrylate polymers mixed with either chlorinated rubber or polystyrene [23]. Other systems are styrene- hydroxyethylmethacrylate based with polypropyleneoxide as a second polymer [24,25].

In general thermotropic polymer blends are environmental-friendly and can be produced in a large area at low costs. As to their switching range thermotropic polymer blends are well suited for solar thermal applications. Thermotropic polymer blends undergo a transition from a highly transmitting state to a highly reflecting state (change in solar transmittance by 52%) at temperatures variable between 30 and 130°C [20,23,24]. However, these material types show a switching within a broad temperature range along with high reversibility within a broad time-frame (up to 15 hours) [26]. Furthermore the materials are susceptible to humidity and UV radiation and exhibit problems with long-term stability [27]. To apply thermotropic polymer blends as overheating protection devices of solar collectors further developments should focus on the adjustment of switching temperatures between 55°C and 80°C and on the improvement of the long term stability and switching performance.