We report effects of disorder due to random cross-linking on the nematic to smectic-A phase transition in smectic elastomers. Thermoelastic data, stress-strain relations and high-resolution x-ray scattering profiles have been analyzed for two related compounds with a small and a larger nematic range, respectively, each for 5% as well as 10% cross-links. At 5% cross-link density the algebraic decay of the positional correlations of the smectic layers survives in finite-size domains, providing a sharp smectic-nematic transition. At an increased cross-link concentration of 10% the smectic order disappears and gives way to extended short-range layer correlations. In this situation neither a smectic-nematic nor a nematic-isotropic transition is observed anymore. The occurrence of disorder at a relatively large cross-link concentration only, indicates that smectic elastomers are rather resistant to a random field. The temperature dependence of the correlation lengths and thermoelastic behavior suggest a shift to a "parasmectic" regime of a first-order smectic-isotropic transition.
©2011 American Physical Society