ESR STUDIES OF SMECTIC PHASES

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ESR STUDIES OF SMECTIC PHASES

F. Poldy, M. Dvolaitzky, C. Taupin

To cite this version:

F. Poldy, M. Dvolaitzky, C. Taupin. ESR STUDIES OF SMECTIC PHASES. Journal de Physique Colloques, 1976, 37 (C3), pp.C3-77-C3-78. �10.1051/jphyscol:1976312�. �jpa-00216495�

JOURNAL DE PHYSIQUE Colloque C3, suppliment au no 6, Tome 37, Juin 1976, page C3-77

ESR STUDIES OF SMECTIC PHASES

F. POLDY, M. DVOLAITZKY and C . TAUPIN

Laboratoires de Physique de la Mati6re CondensCe (*) et de Chimie Organique des Hormones (**) Collkge de France, 11, place Marcelin-Berthelot, 7523 1 Paris Cedex 05, France

R6sum&. - L'organisation des chaines dans les diverses phases du 4,4'-dioctad~cyloxyazoxy- benzkne a et6 CtudiQ par RPE. Les marqueurs de spin sont des azobenzknes de structure trks voisine, marques en diverses positions de la chaine. Des corps mesomorphogi?nes paramagnetiques ont CtB synthetises. Leurs mesophases ont Bt6 identifiees.

Abstract. - ESR studies of chain organization in 4,4'-dioctadecyloxyazoxybenzene are reported.

The spin labels, closely resembling the host molecule, were marked at different positions along the chain. Various phase transitions were studied.

Some paramagnetic liquid crystals were synthetized. Their mesophases have been identified.

Spin labelled molecules can : 1. Spin label studies. - We have studied a number of features of the smectic polymorphism of 4,4'-octade- be as labels at low 'OnCentra- cyloxyazoxybenzene 1 (ODOAB) by means of a series (IoT3

''-3

and carrying-the paramagnetic labelling group at 2. give rise to mesomorphic phases. different points on the end chain :

1.1 SOLID o SB TRANSITION. - The observation of a large discontinuous decrease of the end chain order parameter at the solid + S, transition can be inter- preted as a melting of the chains and tends to support of model of this S, phase as a series of decoupled two- dimensional solids El].

A consideration of the calorimetric data (entropy change) at the solid ⁴ S, transition for a number of similar S, materials with end chains of different length suggests that chain melting may often be the dominant feature of this transition. On this view the S, phase would be characterized by an ordered two dimensional lattice within the smectic planes which are separated

(*) Equipe de Recherche Associ6e au C. N. R. S. (no 542).

(**) Groupe ^de Recherche du C. N. R. S.

by the dynamically disordered (liquid) end chains.

Short end chains would permit a relatively strong coupling between the smectic layers leading to local three dimensions order, while long chains would lead to a more or less complete decoupling of the layers.

Thus the S, phase would bridge the gap between plastic and liquid crystals [2].

1.2 TILT ^{ANGLE IN} SB AND SC. - The angle of tilt defined as the angle between the normal to the smectic layers and the apparent symmetry axis of spectral motioned averaging was measured in oriented samples, in both smectic phases. The observed values, 12O in S, and 230 in S, phases, are rather independant on the number of the labelled carbon. Moreover they are independant on temperature on each phase [3].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976312

C3-78 F. POLDY, M. DVOLAITZKY AND C. TAUPIN 1.3 THE SB o SC TRANSITION. - We can relate a

number of measurements at the S, o Sc transition.

Our ESR order parameter data show a slow decrease with temperature throughout the smectic range but with no discontinuity at the S, o Sc transition, which would thus seem to exclude a discontinuous change in mean interchain spacing. On the other hand Demus et al. have measured a density change of 1.25 % at this transition. This may be related to the change of tilt angle by a simple geometrical model [3].

1.4 ODD EVEN EFFECTS AND RESIDUAL ORDER IN THE ISOTROPIC PHASE. - Both order parameter and tilt angle measurements exhibit a pronounced odd even alternation. An examination of a molecular model of the spin label shows that the molecule as a whole is not linear, in fact, the passage from an odd to an even numbered label results in the oxazolidine ring moving from one side of the chain (interior with respect to the central kink for the odd carbon) to the other.

These physically non equivalent labels may have different steric interactions with the neighbouring molecules, the series of the alternation effect being

dependant of the type of environment (smectic or isotropic).

Odd-even alternation was also found on correlation time measurements in the isotropic phase of the liquid crystal.

This effect is much smaller when the labelled mole- cule is dissolved in chloroform but remains important when the solvent is a long linear alkane (octacosan).

These results suggest the existence of a residual mole- cular ordering in some isotropic phases. In the case of liquid alkanes X-ray studies have previously shown a local tendancy for the molecules to be parallel to each other [4] in agreement with our results. The type of the residual order (nematic or smectic) is under study.

2. Paramagnetic liquid crystals. - It was previously shown [5] that labelled derivatives of ODOAB present liquid crystalline (S,) phases. As the materials were inseparable mixtures of the two azoxy isomers, a different series of labelled molecules, analogs of p-biphenyl-p'-quinoxaline 3, was synthetized which present S,, S, and S, phases :

The phase diagrams of the various compounds were the exchange interaction. oriented homeotropic established by combination of polarizing microscope samples of these paramagnetic liquid crystals give observations and ESR detection of phase transitions ESR spectra characteristic of the exchange interaction (M. Dvolaitzky, J. Billard and F. Poldy, manuscript in two dimensions 161. This system offers a fine test of under press). These compounds, which carry one free theories of this type of exchange interaction.

spin on every molecule give ESR spectra dominated by

References

[I] DVOLA~TZKY, M., POLDY, F. and TAUPIN, C., Phys. Lett. 45A [4] STEWART, G. W., Phys. Rev. 31 (1928) 174.

(1973) 454. [ 5 ] DVOLA~TZKY, M., BILLARD, J. and POLDY, F., C. R. Hebd.

[2] LEVELUT, A. M., DOUCET, J. and LAMBERT, M., J. Physique 35 Skan. Acad. Sci. 279C (1974) 533.

(1974) 773. [6] RICHARDS, P. M. and SALAMON, M. B., Phys. Rev. B 9 (1974)

131 POLDY, F., DVOLAITZKY, M. and TAUPIN, C., J. Physique

Colloq. 36 (1975) C 1-27. 32.