Liquid crystalline polymer networks - rubber elastic material with exceptional properties

Abstract

If low molar mass mesogenic molecules, which are able to form a liquid crystalline phase, are linked as side groups to a polymer main chain, a new class of polymeric substances with liquid crystalline behaviour is obtained. Nematic, smectic and cholesteric elastomers have been synthesized. The new material is marked by a new combination of liquid crystalline and elastomer specific properties. Above the glass temperature, Tg, the substances are rubber-elastic. Starting from the elastomer in the optical isotropic state, a first order phase transformation into the liquid crystalline state is obtained by decreasing the temperature. By further cooling the liquid crystalline elastomer is converted into an anisotropic liquid crystalline glass. The thermoelastic and optoelastic properties of the networks have been investigated by uniaxial deformation (compression, elongation) and birefringence measurements. Above the transformation point nematic-isotropic, Tn-i, the thermoelastic behaviour is the same as for usual elastomers. However, when the transformation temperature is reached, the nominal stress decreases rapidly. If one passes to temperatures below the transformation temperature by maintaining the deformation of the network, the liquid crystalline order is built up in a sample having a preferential direction. This is the reason for the unusual stress-temperature behaviour observed. Stress optical measurements above Tn-i show a considerable temperature dependence, which is untypical for common rubbers. The steep increase of the stress optical coefficient, some 20 k above Tn-i, indicates that there are marked pretransformational effects. The sign of the birefringence indicates the orientation of the mesogenic side groups with respect to the axis of deformation or to the direction of the polymer backbone, respectively. In the LC state the investigations of the LC elastomers by X-ray diffraction- and polarizing microscopic measurements lead to same results with respect to the orientation of the mesogenic side groups in the deformed networks. It is remarkable that the results depend on the length of the flexible spacer between the mesogenic side groups and the polymer backbone.