Gel-like elasticity in glass-forming side-chain liquid-crystal polymers

We study the complex shear modulus G of two side-chain liquid-crystal polymers (SCLCPs), a methoxyphenylbenzoate substituted polyacrylate (thereafter called PAOCH(3)), and a cyanobiphenyl substituted polyacrylate supplied by Merck (thereafter called LCP105) using a piezoelectric rheometer. Two methods of filling the cell are used: (a) a capillary method, which can be used only at high temperature because of the low value of the viscosity, and (b) the classical one, thereafter called compression method, which consists in placing the sample between the two slides of the cell and to bring them closer. By filling the cell at high temperature either with the compression or the capillary method, we show that the response of both compounds is liquidlike (G' similar to f(2) and G '' similar to f, where f is the frequency) for temperatures higher than a certain temperature T-0 and gel-like (G' similar to const, G '' similar to f) below T-0. This change in behavior from the conventional flow response to a gel-like response, when approaching the glass transition, is observed for nonsliding conditions and for very weak-imposed shear strains. It can be explained by a percolation-type mechanism of preglassy elastic clusters, which correspond to long-range and long-lived density fluctuations that are frozen at the time scale of the experiment. The sample response is therefore the sum of two contributions: one is due to the flow response of the polymer melt and the other to the elastic response of the network formed by the preglassy elastic clusters. By filling the cell below T-0 with the compression method, both compounds exhibit a gel-type behavior by gently bringing closer the slides of the cell and an anomalous low-frequency behavior characterized by G' = const and G '' = const by increasing the pressure used to bring closer the slides of the cell. A compression-assisted aggregation of the preglassy elastic clusters can explain both the increase in the low-frequency elastic plateau when the sample thickness is decreased and the anomalous low-frequency behavior. Further evidence for the existence of these elastic clusters is provided by the following results: (a) the nonlinear response of the samples as a function of the strain amplitude, which can be explained by the Payne effect, and (b) the aggregation effects, which can be mimicked by a polydimethylsiloxane melt filled with silica particles, the silica particles playing the role of the preglassy elastic clusters. All these observations show that PAOCH(3) is not a macroscopically solidlike material with an unconventional type of elasticity, as claimed by Mendil et al. [Phys. Rev. Lett. 96, 077801 (2006)]. The gel-type behavior observed here on two SCLCPs (PAOCH(3) and LCP105) and previously on some conventional flexible polymers (atactic polystyrene, poly-n-butylacrylate) seems to be a generic effect of the glass transition. The presence of the preglassy elastic clusters questions the widely accepted hypothesis of ergodicity in the supercooled state.