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A STRUCTURAL STUDY OF THE SMECTIC MESOPHASES OF TWO BIPHENYL COMPOUNDS
AND AN X-RAY INVESTIGATION OF THE MISCIBILITY CRITERION
J. Lydon, C. Coakley
To cite this version:
J. Lydon, C. Coakley. A STRUCTURAL STUDY OF THE SMECTIC MESOPHASES OF TWO BIPHENYL COMPOUNDS AND AN X-RAY INVESTIGATION OF THE MISCIBILITY CRITE- RION. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-45-C1-48. �10.1051/jphyscol:1975106�.
�jpa-00215883�
JOURNAL DE PHYSIQUE Colloque C1, supplbment au no 3, Tome 36, Mars 1975, page C1-45
Classification Physics Abstracts
7.130
A S T R U C T U W STUDY OF THE SMECTIC MESOPHASES OF TWO BIPHENYL COMPOUNDS AND AN X-RAY INVESTIGATION
OF THE MISCIBILITY CRITERION
J. E. LYDON and C. J. COAKLEY
The Astbury Department of Biophysics, The University, Leeds LS2 9JT, England
RBsum6. - Les composks 4'-n-octyl-4-cyanobiphbyle et 4'-n-octyloxy-4-cyanobiphenyle for- ment des phases smectiques d'un type nouveau. Les photographies de diffraction des rayons X ne ressemblent B aucune autre. Elles posskdent un anneau intkrieur bien defini (qui correspond k l'kpaisseur des couches) et deux anneaux exterieurs moins dkfinis. L'kpaisseur des couches indique une structure avec des co-uches doubles dans lesquelles les mol~cules se recouvrent dans le centre.
Plusieurs possibilitks sont proposkes pour les positions exactes des molkcules.
Cette phase smectique se melange avec la phase habituelle qui existe dans le cas du 2-(4'-n-octade- cyloxybenzylid&neamino) fluorhe. L'observation au moyen des rayons X indique que l'kpaisseur des couches varie continfiment avec la composition. Bien qu'il y ait des diffkrences entre les lon- gueurs des mol6cules et les structures des deux phases smectiques, on obtient une phase homogkne en mklangeant ces deux types de molkcules.
Abstract.
-
The biphenyl compounds 4'-n-octyl-4-cyanobiphenyl and 4'-n-octyloxy-4-cyano- biphenyl form smectic phases of a novel type. The X-ray diffraction patterns are unlike any other previously reported ; they contain a sharp inner ring (corresponding to the layer thickness) and two diffuse outer rings. The layer thickness indicates some form of bilayer structure with molecules partially overlapping in the centre of the layer. A number of alternative models for the precise molecular arrangement are suggested.This smectic phase is miscible with the conventional SA phase formed by 2-(4'-n-octadecyloxy- benzylideneamino) fluorene. X-ray investigations indicated that the layer thickness changes smoothly across the composition range. In spite of the difference in molecular length and the diffe- rence between the structures of the two pure smectic phases, it appears that in mixtures there is a complete and intimate mixing of the molecules.
1. Introduction. - The 4'-n-alkyl and 4'-n-alkoxy- terns are similar and are of a novel type as shown in 4-n-cyanobiphenyl derivates prepared by Gray and figure 1 [4]. There are two diffuse reflections of similar co-workers [I] are of considerable importance because intensity corresponding to repeat distances of 8.6
a
they form colourless, stable mesophases. The octyl derivative is of especial interest because it is in the smectic phase a t room temperature. Preliminary investigations by Gray et al. [2] suggested that there might be something exceptional about this mesophase because they could not find any categorised type of smectic mesophase with which it was miscible. Subse- quently, however, these workers have found that it is completely miscible with the S, phase of some 2-(4'-n- alkoxybenzylideneamino) fluorenes [3].
We have examined both 4'-n-octyl-4-cyanobiphenyl (8 CB) and the corresponding ether, 4'-n-octyloxy-4- cyanobiphenyl (8 OCB). Their X-ray diffraction pat-
FIG. 1. -The X-ray diffraction pattern of 8 CB (taken with CuKa radiation). The inner sharp reflection corresponds to a
' 8 0 ~ ~ '
repeat distance of 29 A and the two outer diffuse reflections correspond to repeat distances of 8.6 A and 4.3 A.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975106
C1-46 J. E. LYDON AND C. J. COAKLEY and 4.3
A,
in contrast to the single diffuse reflection(ca 4
a)
found for conventional S, phases. The inner sharp reflection (the Bragg reflection relating to the smectic layer thickness) corresponds to a repeat dis- tance of 29.2 f 0.3PI
for 8 CB and 29.7 f 0.3PI
for 8 OCB. As indicated in figure 2 these dimensions areN K N N + * + y
(((( q\,yjjj
lib
CPI CP< h C CNk(
C t t ' + $ t i '+ail
N N N N + * a + .
v
1111
CN CN CN CN\k
c t t c . + + $ >FIG. 3. -The structure of the smectic phase of 8 CB and 8 OCB ; a schematic representation of five alternative models for the molecular arrangement. Model I corresponds to figure 2.
For details see text.
FIG. 2. - A drawing of two space-filling molecules of 8 CB in the arrangement of a possible bilayer structure for the smectic phase. Note that some degree of interlayer overlap of molecules
has been incorporated in this model.
considerably in excess of the molecular length (of approximately 23
A
for 8 CB) and some form of bilayer structure is implied, in which the molecules are partially overlapped. We have incorporated some interlayer penetration of molecules into this structure. This makes the layer thickness appear more compatible with the molecular dimensions and would seem to be comple- tely justified in light of previous investigations [4, 51.Other types of bilayer structure are possible and five alternative models for the molecular arrangement are shown schematically in figure 3. In the first of these (which represents the arrangement shown in Fig. 2b) the biphenyl groups lie normally to the layers and the alkyl chains are elongated and interdigitated. In this model the cyano groups lie parallel and side by side.
The dipole-dipole repulsive forces in such an arrange- ment must be appreciable and this perhaps argues
in favour of tilting of the biphenyl groups as in the second model.
we'
have no evidence, at this stage, about the state of the alkyl chains and it is possible that they are disordered as indicated in models I11 and IV rather than fully extended. If the chains are highly fluid this would make possible the further alternative shown in model V where the cyano groups are inter- digitated and held by electrostatic forces in the centre of the layer.This mesophase appears to be optically uniaxial.
For 111 and V this carries no additional implications, because the individual layers are uniaxial. In all of the other models the individual layers are biaxial and there must also be an overall rotational averaging of the orientations of the layers about an axis perpendicular to them.
It is tempting to speculate on the origin of the inner diffuse ring and the way in which it is a consequence of the bilayer structure. However we feel that this would be premature at this stage and we await the results of X-ray studies of oriented specimens (which we have in progress) and of spectroscopic investigations which should indicate which of the alternative structures is the correct one.
Whichever of the above models for the smectic phases of the biphenyl derivatives most closely repre- sents the real situation, this mesophase clearly repre- sents a new modification of the smectic A type and we
A STRUCTURAL STUDY O F THE SMECTIC MESOPHASES O F TWO BIPHENYL COMPOUNDS C1-47
considered that it would be interesting and possible l N T E N S I T Y informative to examine its miscibility with a conven-
(RELATIVE UNITS)
tional S, phase using X-ray diffraction.
2. X-ray investigation of the miscibility of 8 OCB
1
and ODOBAF.
-
The accredited SA phases with which the smectic phase of 8 CB and 8 OCB were found to be miscible were those of the 2-(4'-n-alkoxybenzylidene- amino) fluorenes [3]. The latter compounds apparently form a perfectly ordinary S, phase : the X-ray diffrac- tion patterns show only one diffuse ring and the sharp inner reflections indicate smectic layer thicknesses comparable with the lengths of the molecules. The particular compound selected for study was the octa- decyl derivative (ODOBAF) because its smectic layer thickness of 36A
was clearly distinguishable by X-ray diffraction from that of 8 OCB.ODOBAF
I
2 2 5 ' i . 5 2 75O 3 o 0
two peaks, has a similar profile, and does not have any
FIG. 6. -The variation of the position of the sharp inner subsidiary maxima. Note that with the diffractometer reflections with composition, for OCB,ODOBAF mixtures.
used, it is not possible to ensure quantitative agree- The temperatures at which these observations were made are ment of peak height between one sample and the next. indicated in figure 4.
CH 2 8
C 1 8 H J 7 0 ~ C H = N &
BRAGG ANGLE
FIG. 5.
-
The profiles of the sharp low angle reflections in the X-ray diffraction patterns of the smectic phases of 8 OCB,ODOBAF
ODOBAF and of a 50150 mixture (by wt). A Philips PW 1050125 powder diffractometer (with CuKa radiation) was used. See thenote in the text concerning the peak heights.
The temperature/composition diagram obtained by hot-stage microscopy, illustrating the miscibility of
8 OCB and ODOBAF is shown in figure 4. There did not appear to be any great variation in the peak heights across the composition range, but the precise peak heights in figure 5 are of no present signi-
TEMPERATURE ficance. The way in which the position of the peak
O c varied with composition is shown in figure 6. The
shapes of the tops of the peaks were often somewhat
150 irregular in both pure samples and mixtures and the
actual parameter plotted was the centre of the peak at half height.
BRAGG ANGLE
*.
50-
are shown in figure 5. It can be seen that the peak 0 10 20 30 40 50 60 70 8 0 90 100% O D O B A F
corresponding to the mixture lies between the other 100% ~ O C B
"-.
.
,',2 8
, ,' X
0 10 20 30 40 50 60 70 80 90 100%
loo% 8 0 C B
COMPOSITION, by wt. ODOBAF
,,
g - 3.0FIG. 4.
-
The temperature/composition diagram for 8 OCB 2'9 and ODOBAF. The row of points indicates the series of tempe-ratures and compositions used for the X-ray study of this system. 32 The corresponding X-ray data are shown in figure 6.
3 4 8 -
The profiles of the sharp low-angle reflections in the
+ i t
X-ray diffraction patterns of the smectic phases of 3 6 g - 2 ' 5
8 OCB, ODOBAF and of a 50150 mixture (by weight)
+ t
C1-48 J. E. LYDON AND C. J. COAKLEY
3. Discussion. - Various models would appear to be possible for the molecular arrangement in a mixed smectic phase. Two extreme models are illustrated in figure 7. In the first model the molecules are segregated into domains several layers thick. In the second model the molecules are intimately mixed within each layer.
The X-ray patterns of these two models should serve to distinguish between them.
FIG. 7. - TWO extreme models for the molecular arrangement in a mixed smectic phase. In model I the molecules are segregated into domains, each of which extends over several smectic layers.
In model I1 there is complete molecular mixing. Note that a range of intermediate possibilities exists, including the situation where the domains have an appreciable area but are only one layer thick.
There is a range of intermediate possibilities and X-ray diffraction may not be able to distinguish so readily between these. For example, a likely situation would be one where the molecules are segregated so that, over a limited spatial range within a layer, there are molecules of one type only
-
but where the layers are intimately mixed. Such a hypothetical structure has been suggested by De Vries [7] and would beexpected to give a diffraction pattern in which the reflection related to the layer thickness is noticeably broadened. As shown in figure 5 no signs of such broadening are apparent. We estimate that the angular resolution of the instrument used was about 0.050 and any gross broadening of the peaks should have been detectable. This, coupled with the smooth varia- tion of the Bragg angle (and hence layer thickness) with composition, indicates that the second extreme model suggested in figure 7 is essentially correct and that, in this system at least, miscibility occurs at a molecular rather than a domain level.
The self-consistency of the miscibility test strongly suggests that there is some underlying basic physical property of the molecular interaction which is common to two pure mesophases of the same miscibility group and to mixtures of them. In the light of the increasing amount of evidence that there are distinct structural sub-groups within each miscibility category [8], it would appear that this underlying factor can not be purely structural, i. e. a function of the molecular arran- gement. We tentatively suggest that the factor which causes two smectic phases of the same group to be completely miscible and which makes them distinct from those of all other groups is also related to the molecular motion
-
perhaps the precise parameter of importance is a function of the form and symmetry of the time-average potential field around each molecule.Acknowledgments.
-
We wish to thank Dr. G. W. Gray (University of Hull) for samples of all the mesogens used for this study and for much helpful discussion. One of us (C. J. C.) wishes to thank the S. R.for
a postdoctoral research fellowship.References
[I] GRAY, G. W., HARRISON, K. J. and NASH, J. A., Electron. [3] GRAY, G. W., (Plenary lecture, Session B of this confe-
Lett. 9 (1973) 130. rence).
121 GRAY, G. W., HARRISON, K. J., NASH, J. A., CONSTANT, J., [4] GRAY, G. W. and LYDON, J. E., Nature (1974) in press.
HULME, D. S., KIRTON, J. and RAYNES, E. P., Paper [5] DE VRIES, A., MoI. Cryst. and Liqu. Cryst. 11 (1970) 361.
presented at the ACS Conference on Liquid Crystals, [6] DE VRIES, A., Mol. Cryst. and Liqu. Cryst. 20 (1973) 119.
Chicago 1973. To be published in Ordered Fluids [7] DE VRIES, A., J. Physique Lett. 35 (1974) L-139.
and Liquid Crystals )> 1974. [8] DE VRIES, A., Paper presented in Session F of this conference.
