Abnormal viscoelastic behavior of side-chain liquid-crystal polymers

Abstract

%e show that, contrary to what is commonly believed, the isotropic phase of side-chain liquid-crystal polymers has viscoelastic properties which are totally different from those of ordinary flexible melt polymers. The results can be explained by the existence of a transient network created by the dynamic association of mesogenic groups belonging to different chains. The extremely high sensitivity of the compound to the state of the surfaces with which it is in contact offers us an unexpected method of studying surface states.PACS numbers: 83.10.Nn, 61.30.Eb, 83.70.Jr Side-chain liquid-crystal polymers (SCLCP) are composed of a polymer chain onto the sides of which mesogenic molecules have been grafted using a flexible chain called a spacer [1]. This physical link between the mesogenic groups and the polymer chain should in theory make the "polymer" characteristics (viscosity, elasticity) anisotropic, and modify the order of magnitude of the purely "liquid-crystal" characteristics (torsional viscosity yl, curvature elastic constants). The Fredericks transition experiments [2] and the transient electrical birefringence experiments [3,4] performed in the nematic and isotropic phases show that the static properties of these mesophases are close to those of conventional liquid crystals (no substantial modification of either the birefringence or the curvature elastic constants), while their dynamic properties are essentially governed by the chains themselves (increase of viscosities and relaxation times). A certain number of rheology experiments have also been carried out [5], and these confirm that the presence of the liquid-crystal order has no significant role to play in the dynamics of the nematic and isotropic phases, which consequently have viscoelastic properties which are qualitatively similar to those of ordinary flexible polymers.However, these experiments have not revealed the rubbery plateau which is characteristic of entanglement efl'ects, and which is replaced by an extended distribution of relaxation times the origin of which is unknown. The polymer's polydispersity or/and a relaxation of the mesogenic groups have been invoked to explain this distribution [5].In this Letter, we present new experiments realized with a rheometer, which enabled us to study orientated samples with very low shear strains. The results obtained reveal a viscoelastic behavior which is quite different from that of conventional polymers, and this suggests the existence in the melt of elastic clusters of varying sizes up to 100 pm. We will also show that the rheological behavior observed is extremely sensitive to the state of the surfaces in contact with the sample. The experimental setup was inspired by that previously developed by Cagnon and Durand in their study of the plastic behavior of certain lamellar liquid crystals [6].The sample, of thickness e between 13 and 120 pm, is placed between two glass or silica slides, adjusted for parallelism to within~5x10 rad. The sample can be observed de tisu through an aperture. A sine ...

Richness of Side-Chain Liquid-Crystal Polymers: From Isotropic Phase towards the Identification of Neglected Solid-Like Properties in Liquids

“…The measurement was performed at one frequency (73 Hz) using the vibration of a piezoelectric system (resonator). In liquid-crystal polymers, the earlier works are due to Martinoty and co-authors, in 1994 reporting on "abnormal" gel-like behavior in the isotropic phase [51,52] attributed to clusters assisted by liquid crystal properties. In 2003, a low frequency gel behavior was identified by the same group up to 50 µm thickness on a low molecular weight polystyrene melt attributed to pre-transitional glass transition clusters [53].…”

Dynamic soft elasticity in monodomain nematic elastomers

“…It is also interesting that many authors, e.g. [7,6], reported their success in time-temperature superposition across the nematic-isotropic phase transition, which of course is related to the absence of pronounced soft drops in G ′ values. We can only suppose that a different synthetic procedure of their materials led to the very non-soft (highly semisoft) nematic networks.…”

“…Gallani et al and Weilepp et al [7,9] were both pointing out that the nematic-isotropic transition is trivial for the dynamic mechanical measurements. The reason could be that polydomain nematic elastomers were used, although we have earlier seen the effect of dynamic soft elasticity in a variety of polydomain elastomers [26].…”

Influence of the nematic order on the rheology and conformation of stretched comb-like liquid crystalline polymers

“…The simplest idea was to compare with a linear isotropic polymer, such as polystyrene. But recent microrheology experiments [15] carried out on CLLC polymers have shown that these polymers do not behave like melts but like gels below their gelation point even in the isotropic phase. Moreover, our rheological study shows that the PMA-CH 3 chains studied here, are not entangled [5] and, oddly enough, no study of this kind is available with non-entangled polystyrene chains.…”