On the confinement of liquid crystals in molecular sieves: dielectric measurements

Frunzǎ, S.; Frunză, Ligia; Schoenhals, Andreas; Zubowa, Heide-Lore; Kosslick, H.; Carius, H.-E.; Fricke, R.

doi:10.1016/s0009-2614(99)00523-0

Cited by 49 publications

(27 citation statements)

References 30 publications

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“…The nematogen can be adsorbed in different kinds of macroporous membranes [2][3][4][5][6], glasses [7][8][9][10] and molecular sieves [11], dispersed with different types of aerosil particles [12][13][14][15][16][17][18][19] or even aerosil dispersions can be confined in Anopore membranes [20,21]. In particular, aerosil-dispersed organic composites are very promising from the technological as well as from the scientific point of view, and interest for these types of materials is growing considerably.…”

Section: Introductionmentioning

confidence: 99%

“…However, only a few papers deal with the problem of the influence of the different types of confinement on the relaxation rate of the librational process observed in the nematic phase [2,3,5,8,10,12]. Particularly extensive studies were devoted to the investigation of molecular dynamics of 4-n-pentyl-49-cyanobiphenyl (5CB) and 4-n-octyl-49-cyanobiphenyl (8CB) in different kinds of confinement such as organic Synpor membranes [2], inorganic Anopore membranes [3][4][5][6], porous silica glasses [7], controlled pore glasses (CPG) [8,9], molecular sieves [11] or aerosil dispersions [12][13][14][15][16][17][18][20][21][22]. In these mesogens the strong dipole moment is oriented nearly parallel to the molecular long axis, which greatly simplifies the interpretation of the dielectric spectra.…”

Section: Introductionmentioning

confidence: 99%

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Influence of hydrophilic and hydrophobic aerosil particles on the molecular modes in the liquid crystal 4‐n‐pentyl‐4′‐cyanobiphenyl

2006

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Dielectric spectroscopy in the frequency range 10 6 -10 9 Hz was applied to investigate the influence of hydrophilic and hydrophobic aerosil particles on molecular processes in the liquid crystal 4-n-pentyl-49-cyanobiphenyl. The dynamics of the molecular process in the isotropic phase is non-Arrhenius; it weakly depends on the aerosil density and shows critical temperature dependence of the activation energy near the isotropic-nematic phase transition. The relaxation rate of the process related to the hindered rotation of the molecule around its molecular short axis (slower process) follows an Arrhenius law over about thirty degrees in the nematic range (except close to the phase transition) with an activation energy comparable to the bulk and is almost independent of the aerosil density. The relaxation rate of the process originating from the fluctuation of the molecular long axis around the director (librational mode, faster process), however, follows the Vogel-Fulcher-Tammann law. With increasing disorder achieved by increasing the aerosil density, the relative dielectric strength of the librational mode increases in comparison with the bulk. The relaxation frequency of the slower process increases but that of the faster process decreases with increasing aerosil density. Both these effects are less pronounced for hydrophobic than for hydrophilic aerosils.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Influence of hydrophilic and hydrophobic aerosil particles on the molecular modes in the liquid crystal 4‐n‐pentyl‐4′‐cyanobiphenyl

2006

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“…I/V (current versus voltage) measurements were also carried out. Literature contains various publications exploiting the modification of these parameters indeed [16][17][18][19][20][21][22][23][24][25][26][27]. Wave-mixing experiments, extensive in the literature, including different kinds of materials and lasers, claims various grating formation mechanisms or combination of these mechanisms.…”

Section: Resultsmentioning

confidence: 99%

Dielectric properties of laser-induced conditions in a hybrid composite made by doping a side-chain liquid crystalline polymer

San

Okutan

Köysal

et al. 2005

Journal of Non-Crystalline Solids

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“…Thus, the strong anchoring of the CB molecules implies a surface-induced order of these molecules resulting in a local para-nematic state. This was indicated by investigations of LCs confined to pores [19][20][21][22][31][32][33] and for 8CB-aerosil composites in a large range of silica density [14]. The surface-induced order persists also in the temperature range corresponding to the isotropic bulk state, but decreases with increasing distance from the surface.…”

Section: 2mentioning

confidence: 93%

“…Comparison of the surface layer on an aerosil with that inside the pores Recently the molecular dynamics of 5CB and 8CB confined inside the pores of molecular sieves of AlMCM-41 type (2 nm pore diameter) [33] was also studied by dielectric spectroscopy. As for the aerosil composites, a low frequency relaxation process was observed which was assigned to the layer of CB molecules inside the pores.…”

Section: 4mentioning

confidence: 99%

Dynamics of the surface layer in cyanobiphenyl–aerosil nanocomposites with a high silica density

et al. 2004

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Composites were prepared from an aerosil and 4-n-alkyl-4'cyanobiphenyls with five to eight carbon atoms in the alkyl chain. Their high silica density of y7 g aerosil in 1 cm 3 of liquid crystal (LC) allows the observation of the behaviour of a thin cyanobiphenyl layer (having nearly a monolayer structure) on the silica particles. The systems are investigated by dielectric spectroscopy (10 22 -10 9 Hz) in a large temperature range (220-370 K). All the composites show a (main) relaxation process at frequencies much lower than the processes observed for the bulk LC that was assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its characteristic frequencies obeys the Vogel-FulcherTammann law, which is found to be typical for glass-forming liquids. The quasi twodimensional character of the glass transition in the surface layer is discussed for the first time. At the nematic-to-isotropic transition temperature of the bulk, the composites show a continuous decrease of the characteristic frequencies as a function of the alkyl chain length, while the bulk LCs show the well known odd-even behaviour. The magnitude and temperature dependence of the slow relaxation process in the composites (molecules on an outer surface) agree with those of the same molecules confined to the nanopores of molecular sieves (internal surface).

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On the confinement of liquid crystals in molecular sieves: dielectric measurements

Cited by 49 publications

References 30 publications

Influence of hydrophilic and hydrophobic aerosil particles on the molecular modes in the liquid crystal 4‐n‐pentyl‐4′‐cyanobiphenyl

Influence of hydrophilic and hydrophobic aerosil particles on the molecular modes in the liquid crystal 4‐n‐pentyl‐4′‐cyanobiphenyl

Dielectric properties of laser-induced conditions in a hybrid composite made by doping a side-chain liquid crystalline polymer

Dynamics of the surface layer in cyanobiphenyl–aerosil nanocomposites with a high silica density

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