Shear modulus of polydomain, mono-domain and non-mesomorphic side-chain elastomers: Influence of the nematic order

We investigate the behavior of the complex shear modulus of a series of ela stomers including mono-domain and poly-domain liquid crystal samples, and a non-mesomorphic sample. We find that the dynamics of the glass transition are strongly modified by the nematic order. This result explains why the tr uly elastic response of liquid crystal elastomers can only be observed in t he isotropic phase at very high temperatures and at very low frequencies. B etween the elastic regime and the glassy state, the elastomers have a visco -elastic regime, which is characterized by a Rouse-like behavior for mono-d omain and poly-domain samples, and by a Zimm-like behavior for the non-meso morphic sample. We also show that the mono-domain sample exhibits marked an isotropy of the shear-modulus G'. This anisotropy, which is observed for th e first time. is a function of frequency and is inverted between low and hi gh frequencies, due to relaxation effects of the orientational order.