Side-chain liquid-crystal polymers: Gel-like behavior below their gelation points

We present microrheology experiments on a series of polysiloxane-type liquid-crystal polymers with mesogen-graft amounts of 25%, 70% and 100%. The experiments, conducted as a function of sample thickness, shaw that the low-frequency response of all of these polymers shifts progressively from a liquid-type behavior for thick samples (>100 mu m) to a purely elastic behavior for thin samples (<20 mu m). This surprising change in behavior, which occurs in all phases including the isotropic phase, points to the fact that these polymers do not behave like melts but like gels below their gelation point. Comparison of the results obtained in the nematic and isotropic phases shows that the clusters are not sensitive to the orientational, order. The absence of any rubbery plateau fin the case of thick samples) and the simultaneous presence of a low-frequency elastic plateau fin the case of thin samples) are indicative of the existence of temporary knots of a new type, associated with interaction between the mesogens. We also present a confined-geometry percolation model and a model consisting of an association in parallel of the elastic and viscous regions which make up the sample. This latter model offers an explanation of the whole set of results obtained, unlike the percolation model in confined geometry.