Smectic surface structures in the isotropic phase of liquid-crystalline polymers studied by x-ray reflectivity

Elben, H.; Strobl, G.

doi:10.1021/ma00057a021

Cited by 34 publications

(20 citation statements)

References 2 publications

(3 reference statements)

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“…In 8 0 C B a significant smectic ordering has been detected at the free surface of a bulk nematic far above the S,/nematic transition temperature 1201. Similar results have also been obtained for other liquid crystals [21] as well as for a liquid crystalline side chain polymer; in the latter, boundary layers with a smectic structure were found even in the isotropic phase [22]. Liquid crystals with a tendency to form smectic phases, like the CCHs, CBs, CCNs and NCBs, will strongly exhibit the mentioned smectic-like order at solid surfaces.…”

Section: Discussionsupporting

confidence: 82%

On the alignment of thermotropic nematic and smectic liquid crystals on lecithin coated surfaces

Hiltrop

Hasse

Stegemeyer

1994

Ber Bunsenges Phys Chem

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The orientation of thermotropic nematic and smectic liquid crystals at monomolecular films of lecithin on glass as well as germanium substrates has been studied by polarizing microscopy and FTIR spectroscopy. Nematic liquid crystals with only a weak tendency to form smectic phases exhibit a dependence of their orientation on the temperature and the packing density of the monolayer which can be explained by a sterical model. Liquid crystals in the smectic A phase as well as nematics with a strong tendency to build up smectic layers, however, align homeotropically, independent on the packing density of the lecithin film and the temperature. This can be understood as a screening of the lecithin film properties by the smectic-like boundary layers of the nematic liquid crystal.Cr 43 N 45 Is0

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Section: Discussionsupporting

confidence: 82%

On the alignment of thermotropic nematic and smectic liquid crystals on lecithin coated surfaces

Hiltrop

Hasse

Stegemeyer

1994

Ber Bunsenges Phys Chem

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“…Thus, above a nematicsmectic [5][6][7][8][9] or isotropic-smectic transition [10][11][12][13] usually the formation of smectic layers at the free surface is observed. In particular, above the isotropicsmectic-A ͑Sm-A͒ transition of the compound 12CB [14] a sequence of five successive layering transitions was observed [10,13]; since the number of surface-induced smectic layers was finite, this behavior corresponds to partial wetting of the isotropic liquid-vapor interface by the smectic phase.…”

mentioning

confidence: 99%

Wetting Phenomena at the Free Surface of the Isotropic Phase of a Smectic Liquid Crystal

Lucht

Bahr

1997

Phys. Rev. Lett.

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We report an ellipsometric study of the free surface of a smectic liquid crystal possessing a smectic-A-to-isotropic phase transition. Approaching the transition from above, an ordered surface layer appears at a discontinuous transition one degree above the bulk transition temperature. With decreasing temperature the thickness of the surface layer increases continuously, indicating a possible logarithmic divergence at the bulk transition. The behavior may be described as complete wetting accompanied by a prewetting transition. [S0031-9007(97)03114-1] PACS numbers: 64.70.Md, 68.45.Gd The free surface of thermotropic liquid crystals exerts a pronounced ordering effect to the molecules close to the surface which becomes apparent even above the transition to the isotropic liquid phase: Beaglehole [1] has reported an ellipsometric study of the free surface of the liquid crystal 5CB [2] above its isotropic-nematic transition. The results showed the presence of a nematic layer interposing the isotropic liquid-vapor interface; the nematic coverage was found to diverge logarithmically at the bulk nematic-isotropic transition temperature thereby indicating complete wetting. Since then, a number of nematic compounds were found to show essentially the same behavior [3,4].Whereas in the nematic phase only a long-range orientational order of the rodlike molecules exists, smectic phases show in addition a quasi-long-range positional order leading to a layered structure. Thus, above a nematicsmectic [5][6][7][8][9] or isotropic-smectic transition [10][11][12][13] usually the formation of smectic layers at the free surface is observed. In particular, above the isotropicsmectic-A ͑Sm-A͒ transition of the compound 12CB [14] a sequence of five successive layering transitions was observed [10,13]; since the number of surface-induced smectic layers was finite, this behavior corresponds to partial wetting of the isotropic liquid-vapor interface by the smectic phase.In this Letter, we report a new wetting behavior above an isotropic-Sm-A phase transition. Compared to the behavior of 12CB there are two main differences: first, the thickness of the ordered surface domain grows continuously with decreasing temperature and possibly diverges at the bulk transition; second, the first appearance of an ordered surface layer on the isotropic liquid is characterized by a sharp, discontinuous transition. The corresponding layer thickness just below this transition is not that of a single smectic layer but about twice as large. Thus, our results may be described by complete wetting accompanied by a prewetting transition.We have studied the 4-hexyloxyphenylester of 4dodecyloxybenzoic acid, labeled in the following as 12.O.6 (cf. Fig. 1), the Sm-A-isotropic transition temperature T AI of the bulk sample is 89.1 ± C. The compound was purified by chromatography and several recrystallizations. The temperature width of the two-phase region at the Sm-A-isotropic transition, observed in a polarization micoscope, was found to be smaller than the resoluti...

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“…[40 -42] A respective large temperature range can be observed only on polymer LCs. [43] Yet, in our case, the surface excess order can be induced at higher gas pressures, corresponding to relatively high concentrations of sorbed gas. Such a surface order is quite strong and obvious, and can be stable even over a wide temperature range.…”

Section: Temperature-dependence Of Surface Tension Under Various Gas mentioning

confidence: 56%

“…Such pretransitional surface order has been observed by different authors also. [40][41][42][43] Under normal conditions (1 bar air or N 2 ), in the case of low-molecular-weight LCs some specific surface structures of molecular ordering at surface boundaries can be observed within a narrow temperature range above the i-n phase transition (T -T in = several K). [40 -42] A respective large temperature range can be observed only on polymer LCs.…”

Section: Temperature-dependence Of Surface Tension Under Various Gas mentioning

confidence: 99%

Gas-influenced surface and bulk properties of a liquid crystalline side-group polymer

Chen

Springer²

2000

Macromol. Chem. Phys.

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Smectic surface structures in the isotropic phase of liquid-crystalline polymers studied by x-ray reflectivity

Cited by 34 publications

References 2 publications

On the alignment of thermotropic nematic and smectic liquid crystals on lecithin coated surfaces

On the alignment of thermotropic nematic and smectic liquid crystals on lecithin coated surfaces

Wetting Phenomena at the Free Surface of the Isotropic Phase of a Smectic Liquid Crystal

Gas-influenced surface and bulk properties of a liquid crystalline side-group polymer

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