The Frozen State in the Liquid Phase of Side-Chain Liquid-Crystal Polymers

International audienceQuenched isotropic melts of side-chain liquid-crystal polymers reveal surprisingly an anisotropic polymer conformation. This small-angle neutron-scattering (SANS) result is consistent with the identification of a macroscopic, solidlike response in the isotropic phase. Both experiments (rheology and SANS) indicate that the polymer system appears frozen on millimeter length scales and at the time scales of the observation. This result implies that the flow behavior is not the terminal behavior and that cross-links or entanglements are not a necessary condition to provide elasticity in melts. Liquids are known as weakly correlated systems. On cooling, glass-forming liquids transit to a strongly correlated glassy state. Nevertheless, the frontier between the liquid and solid states remains obscure; our experiments show that so far unidentified gigantic solid correlations exist far away from glass and phase transitions, in the ''molten'' state of side-chain liquid-crystal polymers (LC polymers). We thus show that polymers are solidlike far away from any phase or glass transition. This surprising finding is, however, consistent with the mode coupling developments (MCT) [1], the Fischer's clusters approach [2], or extended NMR experiments [3]. Such insights also provide a better understanding of unexpected flow behaviors in complex fluids [4] as the spectacular shear-induced phase transition in LC polymers [5]. Indeed, these shear-induced effects observed far away from any phase transition can be explained using neither the conventional visco-elastic concept (Rouse, reptation time) nor a conventional pretransitional coupling [6]. Time scales longer [7] than the individual polymer dynamics are involved and must be identified. We present here two original experiments that allow one to identify these slow relaxation modes and lead us to the fundamental result: the LC polymer is solidlike far from any phase or glass transition. Because of the coupling between the main chain and the mesogenic side group, LC polymers display various aniso-tropic main-chain conformations in the liquid-crystal phases [8]. Oblate or prolate chain anisotropies have been found depending on the symmetry of the phase and on the order parameter [9]. In the isotropic phase, the long-range liquid crystalline order is lost, giving rise to a macro-scopically averaged isotropic main-chain conformation. We demonstrate here that it is possible to identify a chain anisotropy within the isotropic phase. This is our first result, obtained by decoupling the fast phase dynamics from the slow melt time scales. It reveals the persistence of long nonrelaxed melt time scales. The second fundamental result is the unambiguous observation of a solidlike dynamic behavior within the isotropic phase. This contrasts with the expected flow behavior [10] and reveals that without cross-links or entanglements, LC polymers display elasticity. Both observations indicate millimeter-scale sol-idlike behavior. This so far unidentified macroscopic huge melt cohesion in a supposed ''liquid'' state has multiple consequences both from a theoretical and an experimental point of view. For example, the previously reported shear-induced phenomena [5] must be considered as the result of the solicitation of a gel and the conventional description (pretransitional orientational-order fluctuations, viscoelas-tic relaxation time) should be revisited. Finally, our observations together with the ''Fischer's clusters'' light scattering approach [2], Martinoty's piezorheometer measurements [11], and the MCT that predicts a crossover from liquid to glass behavior at about 50 to 80 K above the calorimetric glass transition temperature T g [1,2] provide firm indications that long-range solid correlations persist even far away from T g. The strategy adopted is the following. In the first part of the Letter, we examine the effect of nonequilibrium thermal treatments on the polymer conformation. We identify a persistence of the chain anisotropy in the quenched iso-tropic phase, which reveals nonrelaxed long time scales. In the second part of the Letter, we examine the viscoelastic behavior. We identify a solidlike behavior and analyze this elastic response versus temperature. The LC polymer used to illustrate our purpose is a methoxy-phenyl benzoate substituted polyacrylate [12] synthesized at the Laboratoire