Rheology of liquid crystalline elastomers in their isotropic and smectic Astate

The dynamics of two different side-chain liquid crystalline elastomers (SCL CE) exhibiting a smectic A phase is investigated by low-frequency shear and compression experiments. We find that both the isotropic and the smectic A phase have their own characteristic viscoelastic behavior,:which seems to be independent of the compound under investigation and thus universal. The relaxation in-the isotropic phase shows a distribution of relaxation times, which gives rise to a scaling law of the elastic moduli with an exponent o f 0.5. The viscoelastic response in the smectic A phase,however, exhibits a much broader spectrum of relaxation times containing very long-lived modes . It appears that there exists a low-frequency scaling law with an exponent of 0.3 characterizing the smectic A phase. We propose that this result ste ms from a transient smectic network with a very long lifetime that was set up by the smectic domains. At the phase transition, this transient smectic network disappears, which leads to a sharp decrease of the dynamic shear as well as the compression modulus.