Smectic-A liquid single crystal elastomers - strain induced break-down of smectic layers

The mechanical response of a smectic-A liquid-crystalline side-chain polymer network with macroscopically ordered layers (smectic-A liquid single crystal elastomer) to extensive stress applied parallel to the layer normal is described. By stress-strain measurements a characteristic threshold strain of about 3% is observed, at which the elastic modulus changes dramatically. In the regime of small deformation the elastomer remains optically transparent possessing a large Young modulus. The macroscopically ordered smectic-A structure is conserved, probably showing some layer undulation. Above the threshold strain, however, the monodomain structure breaks down and the elastomer becomes completely opaque showing a much smaller elastic modulus. By strain dependent X-ray scattering measurements the structure of the polydomain is analysed. A splitting of the small angle reflection into four maxima is observed, indicating a layer rotation. The intensities of these reflections, however, decrease drastically with strain suggesting a partial distortion of the smectic layers toward a nematic like structure.