J. Phys. II France
Volume 3, Numéro 12, December 1993
Page(s) 1759 - 1778
DOI: 10.1051/jp2:1993102
J. Phys. II France 3 (1993) 1759-1778

Conformational and orientational characteristics of chain molecules placed in a nematic field : n-decane and 1,6-dimethoxyhexane dissolved in 4 '-methoxybenzylidene-4-n-butylaniline (MBBA)

Yuji Sasanuma

Department of Polymer Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo 152, Japan

(Received 18 March 1993, revised 3 August 1993, accepted 1 September 1993)

Conformational and orientational characteristics of n-decane (n-C 10) and 1,6-dimethoxyhexane (1,6-DMH) chains dissolved in a liquid crystal 4 '-methoxybenzylidene-4-n-butylaniline (MBBA) have been investigated. The two chain molecules, although having the same number of skeletal bonds, are known to take quite different configurations in the isotropic state. The phase behavior of the MBBA + 1,6-DMH system was observed ; the initial slopes $\beta_{\rm N}^{\infty}$ and $\beta_{\rm I}^{\infty}$ of the phase boundaries, representing the ability of the solute to disturb the nematic order, were determined, and compared with those of the MBBA + n-C 10 system. The thermodynamic data show that, in the nematic environment, the n-C 10 molecules is more rigid and extended than 1,6-DMH. Deuterium NMR spectra of partially deuterated MBBA incorporated in the solutions were measured, and the orientational order parameters of the solvent were evaluated. Using the Photinos-Samulski-Toriumi (P-S-T) model and the single-ordering-matrix (SOM) model, 2H-NMR quadrupolar splittings and D-D dipolar couplings observed for the perdeuterated solutes were analyzed. The results obtained by the P-S-T model indicate that, even in the nematic phase, the n-C 10 molecule is very flexible as in the isotropic phase. On the other hand, the SOM analysis gave a consequence that the n-C 10 chain possesses considerable rigidity. Thus, it can be concluded that the SOM model rather than the P-S-T model afforded results consistent with the thermodynamic data. For 1,6-DMH, the P-S-T simulation did not satisfactorily reproduce the D-D dipolar couplings observed, while the SOM scheme achieved the good agreement between the calculations and observations in all the examples. According to the SOM analysis, the 1,6-DMH molecule, keeping its inherent conformational preference, conforms itself to the nematic field by taking anisotropic configurations such as kink ( $g^{\pm}~tg^{\mp}$), crankshaft ( $g^{\pm}~tg^{\pm}$) and jog ( $g^{\pm}~tttg^{\mp}$ arrangements.

33.25F - 36.20C - 61.30G

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