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

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

P. Kahl, P. Baroni, and L. Noirez

Phys. Rev. E 88, 050501(R) – Published 6 November 2013

Abstract

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.

Received 19 July 2013

DOI:https://doi.org/10.1103/PhysRevE.88.050501

Authors & Affiliations

P. Kahl, P. Baroni, and L. Noirez^*

Laboratoire Léon Brillouin (CEA-CNRS), CE-Saclay, 91191 Gif-sur-Yvette, France

^*Corresponding author: laurence.noirez@cea.fr

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Issue

Vol. 88, Iss. 5 — November 2013

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Images

Figure 1
Scheme of an oscillatory shear (a) and of typical dynamic relaxation spectra. (b) Flow behavior ( $G^{'}$ < $G^{''}$ , with $G^{'}$ and $G^{''}$ vanishing with the frequency). (c) Solidlike behavior ( $G^{'}$ > $G^{''}$ , with $G^{'}$ and $G^{''}$ weakly dependent on the frequency).Reuse & Permissions
Figure 2
Input shear strain sine wave (open black squares, □) and output shear stress wave (red circles, $•$ ) of 8CB in the isotropic phase at 0.050 mm gap thickness for low [γ $_{0}$ = 6 $%$ , (a)], intermediate [γ $_{0}$ = 100 $%$ , (b)], and high [γ $_{0}$ = 8000 $%$ , (c)] strain amplitudes. The black and red lines correspond to a sine modeling of the input (– –) and the output waves (—) of the data points. Insets: Data resulting from the subtraction of the model sine function from the data points ( $σ_{experimental} - σ_{0}$ = Δσ, green squares, ■). Dynamic relaxation spectra ( $G^{'}$ : red circles, $•$ ; $G^{''}$ open blue squares, □) are shown for (a) and (c).Reuse & Permissions
Figure 3
Strain dependence of the viscoelastic moduli ( $G^{'}$ : red circles, $•$ ; $G^{''}$ open blue squares, □) of 8CB in the isotropic phase ( $T = 42.5$ °C) at 0.050 mm gap thickness at a frequency of ω = 5 rad s $^{- 1}$ ( $γ_{0}$ evolves from 2.8 up to 10 $^{4}$ $%$ ). The vertical dashed bars delimit the zone where the output signal exhibits periodic distortions. Insets: Details of the frequency dependence at weak (1) and at large (2) strain amplitudes ( $γ_{0}$ = 6 $%$ and at $γ_{0}$ = 8000 $%$ , respectively). The straight blue line in (2) shows the ω scale of $G^{''}$ , indicating a dynamic viscosity, in agreement with conventional experiments [12].Reuse & Permissions
Figure 4
(a) Shear modulus $G^{'}$ (logarithmic scale) of 8CB versus temperature at 0.090-mm gap thickness. The vertical lines indicate the transition temperatures of the smectic (Sm $A$ )-nematic transition ( $N$ ) and the nematic-isotropic ( $I$ ) transition. (b) Shear modulus $G^{'}$ (logarithmic scale) of 8CB versus gap thickness at $T$ (42.5 °C) > $T_{N I}$ . Above 70 μm, the values lower rapidly following an exponential decrease (small dashed points). The inset displays the shear modulus from 1 up to 40 rad s $^{- 1}$ at different gap thicknesses.Reuse & Permissions
Figure 5
Comparison of the shear moduli $G^{'}$ measured in the liquid phase classified versus degree of polymerization (repetitive unit): 8CB, a polybutylacrylate chain (PBuA), a liquid crystal polyacrylate with a phenyl benzoate moiety (SCLCP $^{1}$ ), and another liquid crystal polymer with a cyanobiphenyl moiety (SCLCP $^{2}$ ). The three polymers are described in Ref. [7] and Eur. Phys. J. E 19, 87 (2006).Reuse & Permissions

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