Liquid crystals containing a suspension of silica particles of nanometric size (aerosil) present fascinating fundamental and applicative aspects. From the fundamental point of view, the interest in these “filled nematic” systems stems from the intricacies of the effect of a random perturbation on ordered phases. According to the “Imry-Ma argument” an arbitrarily low amount of static disorder should suppress long range order in a large enough continuous symmetry system in three dimensions [1]. These systems also present technological interest in view of scattering displays exhibiting the so-called “memory effect” [2]. We have studied a system composed of either the hydrophobic (R812) [3] or the hydrophilic (A380) aerosils (Degussa) in the liquid crystal pentylcyanobiphenyl (5CB) using the ESR spin probe technique. The order parameter and the rotational diffusion coefficients of a cholestane spin probe have been determined as a function of temperature for different aerosil concentrations, near and above the gelation threshold. We find that an increasing concentration of aerosil destroys the local order instead of stabilising it, as it was suggested [4], and that the effect is stronger for the hydrophilic aerosil. The local dynamics has a smaller and more subtle dependence from the aerosil-type and concentration. In the isotropic phase, it is of Arrhenius type and essentially concentration and aerosil-type independent. This is quite surprising, in particular at 10 wt % of aerosil, when the sample is clearly a gel and does not flow. In the nematic phase, only the reorientation of the main molecular axis (tumbling rate) depends from the aerosil-type and concentration while the reorientation around this axis is unaffected. Moreover, deviations of the tumbling rate from the Arrhenius behaviour are clearly observed in the nematic phase even at 1 wt % of aerosil and are well represented by a Vogel-Fulcher-Tammann type law, suggesting that a glass-like state could be induced by an arbitrarily weak quenched disorder. The permanence of local fluidity can help rationalizing the fast switching response time of a filled nematic. On the other hand, the slowing of the dynamics, employed in the memory effect, appears to be essentially macroscopic. Information on the origin of the macroscopic viscosity can be obtained by studying the dependence of the degree of alignment of the local nematic domains from the annealing conditions of the sample (aerosil type, sonication, thermal history, field intensity, aging…). We observe that 1 wt % of hydrophilic aerosil is capable to prevent almost completely the reconstitution of a monodomain in a zero field cooled sample, whereas a monodomain can be easily formed even at 3 wt % of hydrophobic aerosil. This result suggests that the reconstitution of a monodomain requires the reorganization of an H-bond based aerosil network, which appears to be more impaired in the hydrophilic case. [1] Imry, Y.; Ma, S.K., Phys. Rev. Lett. 1975, 35, 199. [2] Kreuzer, M.; Eidenschink, R., in Liquid Crystals in Complex Geometries Formed by Polymer and Porous Networks, edited by Crawford, G.P.; Zumer, S., chap.3, 307-324, Taylor & Francis, London, 1996. [3] Arcioni, A.; Bacchiocchi, C.; Grossi, L.; Nicolini, A.; Zannoni, C., J. Phys. Chem. B 2002, 106, 9245. [4] Glushchenko, A.; Kresse, H.; Reshetnyak, V.; Reznikov, Y.; Yaroshchuk, O., Liq. Cryst. 1997, 23, 241.

Effects of Dispersed Hydrophobic or Hydrophilic Aerosil Nanoparticles on the Order and Dynamics of the 5CB Liquid Crystal

ARCIONI, ALBERTO;BACCHIOCCHI, CORRADO;VECCHI, ILARIA;ZANNONI, CLAUDIO
2004

Abstract

Liquid crystals containing a suspension of silica particles of nanometric size (aerosil) present fascinating fundamental and applicative aspects. From the fundamental point of view, the interest in these “filled nematic” systems stems from the intricacies of the effect of a random perturbation on ordered phases. According to the “Imry-Ma argument” an arbitrarily low amount of static disorder should suppress long range order in a large enough continuous symmetry system in three dimensions [1]. These systems also present technological interest in view of scattering displays exhibiting the so-called “memory effect” [2]. We have studied a system composed of either the hydrophobic (R812) [3] or the hydrophilic (A380) aerosils (Degussa) in the liquid crystal pentylcyanobiphenyl (5CB) using the ESR spin probe technique. The order parameter and the rotational diffusion coefficients of a cholestane spin probe have been determined as a function of temperature for different aerosil concentrations, near and above the gelation threshold. We find that an increasing concentration of aerosil destroys the local order instead of stabilising it, as it was suggested [4], and that the effect is stronger for the hydrophilic aerosil. The local dynamics has a smaller and more subtle dependence from the aerosil-type and concentration. In the isotropic phase, it is of Arrhenius type and essentially concentration and aerosil-type independent. This is quite surprising, in particular at 10 wt % of aerosil, when the sample is clearly a gel and does not flow. In the nematic phase, only the reorientation of the main molecular axis (tumbling rate) depends from the aerosil-type and concentration while the reorientation around this axis is unaffected. Moreover, deviations of the tumbling rate from the Arrhenius behaviour are clearly observed in the nematic phase even at 1 wt % of aerosil and are well represented by a Vogel-Fulcher-Tammann type law, suggesting that a glass-like state could be induced by an arbitrarily weak quenched disorder. The permanence of local fluidity can help rationalizing the fast switching response time of a filled nematic. On the other hand, the slowing of the dynamics, employed in the memory effect, appears to be essentially macroscopic. Information on the origin of the macroscopic viscosity can be obtained by studying the dependence of the degree of alignment of the local nematic domains from the annealing conditions of the sample (aerosil type, sonication, thermal history, field intensity, aging…). We observe that 1 wt % of hydrophilic aerosil is capable to prevent almost completely the reconstitution of a monodomain in a zero field cooled sample, whereas a monodomain can be easily formed even at 3 wt % of hydrophobic aerosil. This result suggests that the reconstitution of a monodomain requires the reorganization of an H-bond based aerosil network, which appears to be more impaired in the hydrophilic case. [1] Imry, Y.; Ma, S.K., Phys. Rev. Lett. 1975, 35, 199. [2] Kreuzer, M.; Eidenschink, R., in Liquid Crystals in Complex Geometries Formed by Polymer and Porous Networks, edited by Crawford, G.P.; Zumer, S., chap.3, 307-324, Taylor & Francis, London, 1996. [3] Arcioni, A.; Bacchiocchi, C.; Grossi, L.; Nicolini, A.; Zannoni, C., J. Phys. Chem. B 2002, 106, 9245. [4] Glushchenko, A.; Kresse, H.; Reshetnyak, V.; Reznikov, Y.; Yaroshchuk, O., Liq. Cryst. 1997, 23, 241.
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A. Arcioni; C. Bacchiocchi; I. Vecchi; C. Zannoni
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11585/60183
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