Defect-mediated creep of structured materials

Low-stress creep measurements on a nematic liquid crystal polymer indicate that it is a viscoelastic solid, with a modulus of 100 dynes/cm(2) and a yi eld stress of 50 dynes/cm(2). Both smectics and nematics are viscoelastic s olids at very low stress levels, with a modulus that is related to their de fect texture. At stress levels somewhat above the yield stress, there is a yielding regime where the deformation rate and defect spacing are power law s in the applied stress. We understand these power laws using the ideas dev eloped long ago by Orowan for the motion of line defects in crystalline sol ids. The exponents of these power laws are different for nematics and smect ics, but the nematic universality class also appears to apply to superplast ic metals and ceramics.