Thermotropic hydrogels are chemically or physically cross-linked polymer networks which are poured with appropriate water content. At low temperatures the aqueous solution is dissolved homogeneously on the molecular level, so that a clear state is achieved. Above the switching temperature scattering domains are formed by an aggregation of the polymers and/or by separation of free water from the polymer network [2,3,7,8].

Numerous thermosensitive hydrogels exhibiting a cloud point are discussed in the literature. Examples are aqueous solutions of polyvinylethers [9], polyvinylalcohol [10], poly(N-substituted acrylamide) [11], poly(N-vinyl alkylamide) [12], ethyleneoxide-polypropyleneoxide [13], copolymers of N-vinyl — 2-pyrrolidone and hydroxyethylmethacrylate [14], poly(methyl-2-acetamidoacrylate) [15],

poly(methyl-2-acetamidoacrylate-co-methyl acrylate) [16], polyvinylacetone [17], poly(N-oxazolines) [18] or cellulose derivatives [4].

As to performance properties thermotropic hydrogels possess a high potential for solar collector applications. The materials are characterized by a high transparency in the clear state (>82%) along with low haze and a change in solar transmittance by 77% at temperatures adjustable between 5 and 100°C. The materials exhibit an excellent switching performance with a steep switching gradient, a high reversibility, and low hysteresis [4,19,20,21]. But as the transition is based on physical interaction between the components the materials have several problems with long-term stability and ageing. Furthermore the materials have to be UV protected [4]. The water makes the thermotropic hydrogel susceptible to freezing and limits the operation temperature range [21]. Thermotropic hydrogels filled into the intervening space of a double glazing, place high demands on sealing of the glazing. If not sealed properly the layer will dry out [2]. If synthetic materials are used thermotropic hydrogels are quite expensive. If biopolymers are used costs decrease significantly. However, the use of biopolymers requires a thorough protection against microorganisms [22].

A thermotropic hydrogel glazing, a sandwich of two glass panes and the encapsulated hydrogel, is announced by Affinity Ltd. (Japan) [2]. However, no thermotropic hydrogel is described currently that exhibits the working temperature range and switching temperatures required to provide adequate overheating protection for solar collectors. Further developments should focus on the adjustment of adequate switching temperatures (55-80°C) and the improvement of the long-term stability.