摘要:

Solidp-C6H4(CH3)2andp-C6H4(CD3)2have been studied over a temperature range from 10 K up to the melting point of ∼ 287 K by Raman- and Incoherent Inelastic Neutron-Scattering(IINS). The methyl torsion frequencies deduced from the Raman- and IINS-spectra are different because of the strong coupling of these modes with phonons and their dispersion in the Brillouin zone. A decrease in frequencies and a strong broadening, especially of the high frequency phonons where methyl torsions are predominant, have been observed in Raman- and IINS-spectra with increasing temperature. A lattice dynamics model based on the atom-atom potential set IVa of parameters given by Williams can, after some small modifications, satisfactorily explain the dynamics of ap-xylene crystal. Modified potential parameters better reproduce the experimental data and allow one to study the coupling of the internal methyl rotation with the lattice modes in more detail.

ISSN:1058-725X    

DOI:10.1080/10587259508030989

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The variation of the tilt angle with temperature in the smectic C phase has generally been shown to be non-existent or very slow for compounds or mixtures with the nematic-smectic C transition, while in the case of systems with the smectic A- smectic C transition, a relation between the steepness of this variation, near the transition, and the width of the smectic A domain has been observed. In this work, the variation of tilt angle in the smectic C phase is described forp-alkoxyphenyl-p-alkoxybenzoate homologous series, for which the evolution of polymorphism can be controlled systematically, by varying stepwise the length of the aliphatic chains, and for which large domains can be obtained for each type of phase sequence, nematic-, smectic A- and isotropic- smectic C. After completing the discussion made previously on the incidence of chain length on polymorphism, we confirm that the variation of tilt angle with temperature is slowest for compounds with intermediate chain lengths corresponding to the largest smectic A temperature range; this variation becomes continuously steeper when the smectic A domain becomes narrow. In addition, we show that the same description can be extended to the other types of phase sequences, by using the hypothesis of a virtual smectic A-smectic C transition above the observed nematic- or isotropic-smectic C transition. In fact, short chain lengths for homologues with a nematic/smectic C transition, or long chain lengths for homologues with an isotropic/smectic C transition, lead to an increase of the tilt angle at the phase transition and to a decrease of the amplitude of its variation with temperature; in our description, this behaviour corresponds to an increase of the temperature range between the real and virtual transitions. As a consequence, the homologues with very short and very long chain lengths show a quasi temperature-independent tilt angle, while the other homologues present a tilt angle variation similar to that observed for compounds exhibiting a smectic C/smectic A transition. This feature indicates that there is no need to distinguish between different types of smectic C phase.

ISSN:1058-725X    

DOI:10.1080/10587259508030990

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

4-Cyano-4′-n-undecylbiphenyl, C24N1H31Mr' 333.52, triclinic, P1,a' 9.755(2),b' 11.322(1),c' 19.911(2)Å, α '96.86(1), β ' 94.41(2), γ ' 109.16(1)˚,V' 2046.46 Å,Z' 4,Dx' 1.082 g/cc, μ ' 0.574 cm−1,F(000) ' 728.0, λ(MoKα) ' 0.71073 Å,T' 326 K, finalRandwR are 0.0476 and 0.0625 respectively for 7225 reflections included in least squares calculations.

ISSN:1058-725X    

DOI:10.1080/10587259508030991

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Static dielectric constant studies of three nematogenic compounds, viz., 4′-(hexyloxy)-4 biphenyl carbonitrile, 4′-(heptyloxy)-4 biphenyl carbonitrile and 4-isothiocyanatophenyl 4-pentylbicyclo (2,2,2) octane I-carboxylate, have been carried out and reported in this paper. The behaviour of the dielectric constants with temperature in these compounds have been explained in terms of the dipole moment.

ISSN:1058-725X    

DOI:10.1080/10587259508030992

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

1, 4-bis(1-naphthyl)-buta-1,3-diyne, C24H14,Mr, ' 302.38, monoclinic,P21/a,a'7.534(1),b' 6.012(1),c' 17.969(3)Å, β' 101.1(1)˚,V' 798.67 Å,Dx' 1.257g/cc, F(000) ' 316, λ.(Mok)α ' 0.71073 ÅT'300 K, final R and wR are 0.0389 and 0.0457 respectively using 1281 reflections taken in least squares calculations.

ISSN:1058-725X    

DOI:10.1080/10587259508030993

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In order to apply polyimide Langmuir Blodgett films to nematic liquid crystal (NLC) alignment, the optimum deposition conditions have been studied by measuring the azimuthal anchoring energy of 5CB aligned on the film. Special attention is paid to obtain good NLC alignment on the 2-type PI-LB flims, and we firstly prove that, by choosing the proper deposition conditions, the Z-type films show better performances of NLC alignment than Y-type films. The obtained azimuthal anchoring energy of the Z-type film exceeds 10−5J/m2, which is the strongest azimuthal anchoring energy of PI-LB films reported so far.

ISSN:1058-725X    

DOI:10.1080/10587259508030994

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Rotational and translational diffusion of a hydrophilic spin probe tanol have been studied for the hexagonal, lamellar and isotropic phases of a lyotropic system containing water and a nonionic surfactant decyldimethylphosphine-oxide. Rotation of the probe in the aqueous region of the lyomesophases and of the micellar solutions is only to a small extent hindered in comparison with rotation of tanol in pure water. The observed high values of translational diffusion coefficient of tanol provide complementary evidence of the high mobility of water molecules in lyomesophases. The hexagonal/liquid phase transition causes only subtle changes in the ESR spectra parameters whereas changes at the lamellar/liquid phase transition are more pronounced. Transition enthalpies and changes in diffusion parameters have been compared for the phase transitions in the system studied and in phospholipid-water systems. Existence of direct specific interaction of tanol molecules with the polar head groups on the surface of micelles is suggested in order to explain the obtained values of hyperfine splitting constant and of translational diffusion coefficients.

ISSN:1058-725X    

DOI:10.1080/10587259508030995

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

4-Cyan0-4′-n-decylbiphenyl, C23NH29,Mr' 319.49, monoclinic, P21/n, a ' 15.813(1)Å b ' 5.479(1)Å, c ' 23.179(2)Å, β ' 102.2(1)˚, V ' 1963.26Å3, Z '4. Dx' 1.067 gm/cc, μ ' 0.569 cm−1, F(000) ' 680, (MoKα) ' 0.71073Å, final R and wR are 0.054 and 0.062 respectively using 1281 reflections taken in least squares calculations.

ISSN:1058-725X    

DOI:10.1080/10587259508030996

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The synthesis and mesomorphic properties of three homologous series of β-diketones as well as one series each of copper(II) and palladium(II) complexes are reported. The β-diketones exhibit a smectic A phase while both the copper(II) and palladium(II) complexes show only a metastable nematic phase. Optical microscopy and differential scanning calorimetry have been used to characterize the mesophases.

ISSN:1058-725X    

DOI:10.1080/10587259508030997

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

1,4-bis(3-thianaphthyl)-buta-1,3-diyne, C20S2H10,Mr' 314.43, m.p ' 438 K, monoclinic,P21/c,a' 13.276(1),b' 3.958(1),c' 14.120(3)Å. β ' 92.11(1)Å,V' 741.45 Å3,Z' 2,Dx' 1.408 gm. cm−3, μ ' 3.36cm−1,F(000) ' 324.0, λ(MoK) ' 0.71073 Å,T' 300K, finalRand wRare 0.0303 and 0.0433 respectively using 1022 observations used in structure factor calculation. The monomer structure (withD' 3.664 Å,S1' 3.502 Å and the angle τ1' 62.25˚) does not satisfy the requirements for topochemical polymerisation (Wegner, 1977).

ISSN:1058-725X    

DOI:10.1080/10587259508030998

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor