Hidden solidlike properties in the isotropic phase of the 8CB liquid crystal

P Kahl (1) , P Baroni (1) , Laurence Noirez (1)

1 LLB - UMR 12 - Laboratoire Léon Brillouin

Novel dynamic experiments have enabled the identification of a macroscopic solidlike response in the isotropic phase of a low molecular weight liquid crystal, 4,4'-n-octylcyanobiphenyl (8CB). This unknown property indicates that the low frequency shear elasticity identified in the isotropic phase of liquid crystal polymers is not reminiscent from the glass transition but reveals likely a generic property of the liquid state. The comparison to high molecular weight liquid crystals indicates, however, that the shear modulus is much enhanced when the liquid crystal moieties are attached to a polymer chain. The macroscopic length scales probed (0.050–0.100 mm) exclude wall-induced effects. DOI: 10.1103/PhysRevE.88.050501 PACS number(s): 61.30.Hn, 68.08.−p, 87.15.hg, 83.85.Vb The knowledge of the timescales involved in liquid crystalline systems is of outmost importance to understand, control, and improve their characteristics. The submillimeter scales properties attract a tremendous research interest [1–4]. However, few studies concern the isotropic phase away from pretransitional effects. Assimilated to ordinary viscous liquids, the isotropic phase is not supposed to exhibit solidlike properties, or at very high frequency only (mega-or gigahertz) as ordinary liquids. For this reason, the low frequency behavior of the isotropic phase remains mistakenly unexplored. Experimentally, the viscous or solidlike nature of a material is deduced from its response to a low frequency mechanical solicitation. A couple of years ago, careful dynamic experiments carried out in the isotropic phase of high molecular weight liquid crystals [side-chain liquid crystalline polymers (SCLCPs)] have revealed an as-yet unknown property: the isotropic melt does not flow but exhibits a finite shear elasticity of about several thousand Pascals at low frequency (0.1–10 Hz) [5–9]. The identification of low frequency shear elasticity in the isotropic phase of SCLCPs away from the isotropic-nematic transition opens numerous questions on the origin of this new property. It neither seems to result from the contribution of the liquid crystal moieties nor from surface anchoring effects, but likely from a generic property of the liquid state. Measurable in SCLCPs at macroscopic length scales as far as 100 • away from the glass transition temperature [5,7,9], the shear elasticity of SCLCPs still raises the debated question of reminiscent glass transition effects. In this Rapid Communication, we probe the dynamic properties of the low molecular counterpart: the 4,4'-n-octylcyanobiphenyl (8CB). The widely studied molecule can be considered as a representative liquid crystal molecule. 8CB exhibits a crystalline phase at low temperatures that enables one to rule on the question of pretransitional glass transition effects. We reveal a low frequency, solidlike response at several tens of micrometers sample thickness in the isotropic phase of 8CB, meaning that long range correlations are preserved when the orientational order is lost. This shear elasticity is detectable if special attention is paid to boundary conditions between the substrate and the sample. Under these conditions, the shear stress is optimally transmitted between the sample

Domaines: Matière Molle [cond-mat.soft]

P Kahl, P Baroni, Laurence Noirez. Hidden solidlike properties in the isotropic phase of the 8CB liquid crystal. Physical Review E , 2013, ⟨10.1103/PhysRevE.88.050501⟩. ⟨hal-01361932⟩