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A distinct smectic C2 liquid crystal observed in main-chain liquid crystalline polymer
Junji Watanabe, Satoshi Kinoshita
To cite this version:
Junji Watanabe, Satoshi Kinoshita. A distinct smectic C2 liquid crystal observed in main-chain liquid crystalline polymer. Journal de Physique II, EDP Sciences, 1992, 2 (5), pp.1237-1245.
�10.1051/jp2:1992196�. �jpa-00247710�
Classification Physics Abstracts
61.30
A distinct smectic C~ liquid crystal observed in main-chain
liquid crystalline polymer
Junji Watanabe and Satoshi Kinoshita
Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
(Received J6 September J99J, revised 9 December J99J, accepted 7 January J992)
Abstract. We present experimental evidence for the formation of a smectic C2(Sc~) phase in
main~chain liquid crystalline polymers with the general repeat unit,
~fj Q Q ~O~CH@~o~j~ The Sc~ phase was identified by definitive observation
I I
O O
using X~ray and optical methods and found to have a novel layered structure. The mesogenic groups in this phase are tilted with respect to the layer normal, but unlike the SC phase the tilt
direction is opposite in adjacent layers. This distinct structure is considered to arise from a
coupling of polymeric and mesogenic effects in which the spatial orientation of mesogenic groups
is controlled by the confined conformation of the polymer chain.
1. Introduction.
Mesogenic groups can be incorporated into a polymer as side chains or as part of the main chain [il. The former case gives rise to the so-called side-chain liquid crystalline (L.C.) polymers, in which the chain conformation of the backbone is not significantly altered by the formation of the mesophase. Although the radius of gyration of the backbone appears to be different between the isotropic phase and mesophase or between the different mesophases according to the neutron scattering analysis [2, 3], the polymeric and mesogenic effects are
relatively uncoupled and the mesophase behavior is often predictable from the behavior of the low-molecular-weight mesogen. On the other hand, if the mesogenic group forms part of
the main chain as in main~chain L-C- polymers, the polymer molecule must adopt a
conformation and packing that is compatible with the structure of the mesophase. As a result, the polymeric and mesogenic properties are closely coupled. Alteration of the repeat unit may influence the molecular packing and result in the properties of the mesophase departing from
those of the low-molecular-weight mesogen. Conversely, the formation of a mesophase can
affect the conformation of this type of polymer.
In this paper, we report that a new type of smectic liquid crystal, the smectic
C~(Sc~), can be formed in main-chain L-C- polymers. The definitive observation of the
1238 JOURNAL DE PHYSIQUE II N° 5
Sc~ phase was performed by optical and X-ray experiments, leading to the conclusion that in this phase the mesogens form a layer with their long axes tilted to the layer normal but the tilt direction is opposite between neighboring layers. We also argue that such a curious structure results from a coupling of polymeric and mesogenic effects.
2. Materials.
The novel Sc~ phase was initially found to occur in a homologous series of main-chain L-C-
polyesters (BB-n) [4], composed of a bibenzoate group as a mesogen and an alkylene group as
a flexible spacer :
fiC-fi-fi-C~©4<Hz-£,C-~,
I I
O O
BB-n (n
= 3-9 : the carbon number of alkylene group
These polymers were synthesized by melt transesterification of dimethyl p,p'-bibenzoate and the desired alkane diol. The degree of polymerization was controlled by reaction time to be in
a range of 20 to 40. All of the BB-n polyesters with n ranging from 3 to 9 exhibit a single
smectic phase, with the type of smectic phase depending on the odd-even parity of n. The
novel Sc~ phase was observed to occur for BB-n polymers in which n was an odd number
whereas the even-membered BB-n series exhibits a SA Phase. The corresponding thermody-
namic data are reported in reference [4].
3. X-ray studies.
Evidence from X-ray measurements was obtained for the smectic nature of the new
Sc~ phase. Figure i shows the typical X-ray pattem for a highly oriented Sc~ phase of BB-5. In this case, the oriented sample was prepared as a fiber by drawing from the isotropic melt and
Fig, I. X-ray diffraction pattem for an oriented smectic C2 Phase of the BB-5 fiber taken at 150 °C.
The fiber was prepared by drawing from the isotropic melt and the fiber axis is placed in a vertical direction.
the fiber axis corresponding to the polymer chain axis is placed in the vertical direction of
figure. The pattem includes two distinct reflections characterizing the Sc~ layered structure one is the sharp layer reflection and the other the broad reflection. The layer reflection
appears just on the meridional, indicating that the layers are oriented perpendicular to the
polymer chain axis. The Bragg spacing of this reflection was found to approximate the length
of the fully extended repeat unit [4], indicating that each repeat unit participates in an individual layer. On the other hand, the broad reflection with a spacing of around 4.5 I is
attributable to the disordered lateral packing of mesogenic groups within a layer and appears
split into two portions lying above and below the equatorial line. This indicates that the
mesogenic groups are arranged with their long axes tilted to the layer normal or to the
polymer chain axis. The structure is thus similar to that of the Sc Phase, but unlike the Sc phase the tilt angle as estimated from the splitting angle of the broad reflections is
independent of temperature, remaining constant around 25°.
4. Optical textures.
The Sc~ phase can be characterized by two distinct optical textures. One is a fan-shaped
texture (Fig. 2a) which can be observed for the untreated mesophase. Pressing and
succeedingly shearing the mesophase between clean glasses gives the other; a distinct
homeotropic texture (Figs. 2b and 2c). The homeotropic texture is highly birefringent and includes a schlieren texture with the singularities of s
= I (Fig. 2b) and s
= 1/2 (Fig. 2c). This observation dictates that the mesogenic groups are tilted with respect to the layer normal and therefore the c-director [5] exists for this phase. Both textures resemble those observed for the normal Sc phase. However, the appearance of the schlieren texture with s
= 1/2, indicates that the c-director in this phase is similar in nature to the n-director of nematic phase rather
than to the c-director of the Sc phase [6].
5. Chiral St~ phase and its reflection property of light.
Introduction of a chiral component converts the Sc~ phase to a chiral St~ phase with a
characteristic helical structure. A typical example of a polynJer that adopts a chiral
S(~ structure is the copolymer, BB-5,3*(CH~),
CH~
£§-fi-fi-C~-OiHz-~O@~ (C-fi-fi-C-/H~CHt~H~©~~
I I I I
O O O U
which is composed of BB-5 and the chiral BB-3*(CH~) units as comonomers. The helical
structure of the chiral S(~ in this copolynJer series can be confirmed by the microscopic
observation of dechiralization lines superimposed on the fan-shaped texture and also from the
ability of this phase to selectively reflect light. The helical pitch, P, evaluated by microscopy
and absorption spectroscopy was found to be independent of temperature, but decreases with
an increase in the molar fraction of the chiral component, y/(x+y), according to the
equation, IIF (nm~ )
=
l. 2 x 10~ ~y/ ix + y).
The homeotropic chiral S(~ phase results in the formation of a Grandjean planar texture, which was conflrrned by the observation that the rotation of the sample between crossed
polarizers produced no extinction. The helical structure is, thus, formed with its helical axis
perpendicular to the layers, in other words, the helical structure results from the helical
twistings of the c-directors as in the chiral St phase.
IOURNAL DE PHYSIQUE II T 2,N' 5, MAY 1992 45
1240 JOURNAL DE PHYSIQUE II N° 5
j ~C
~
~ j ~j~
~
Fig. 2. The Sc~ phase of BB-5 observed at 180 °C by polarizing microscopy (x 200). A sample with a thickness of 10 ~m was placed between the slide and a coverslip and observed through crossed polarizers
with an Olympus BH-2 microscope. a) The fan-shaped texture of the Sc~ phase was observed for the untreated specimen prepared by slow cooling from isotropic melt. b) and c) Shearing the specimen
between glasses gave a homeotropic texture. The homeotropic texture is birefringent and includes schlieren with point singularities of s
=
I (b) and s
= 1/2 (c).
The chiral S/j phase in a Grandjean alignment results in the selective reflection of the visible light when the chiral component of BB-3*(CH~) is in the range of 0.2 to 0.4. Figures 3a and 3b show typical reflection spectra that were observed for BB-5, 3*(CH~) with a chiral
component of 0.25 with incident light normal and oblique to the helical axis. In these figures,
we see the single normal reflection band arising from the periodicity of half the pitch (so- called half-pitch band) for both normally and obliquely incident light. This observation is very
significant and again reveals the essential difference between the structure of the Sc~ and Sc phases, since for a chiral St phase the obliquely incident light produces the extra
full-pitch band arising from the period equal to the full pitch [7, 8]. The reflection spectra for a chiral St phase are shown in figures 3c and 3d as measured for the following related chiral
copolyester, BB-6, 4*(CH3) 19]
CH~
-~§ Q Q -D~CHr~~O4~ ~C o j C~CHr-/H-CHr-CH~O~~
I I I I
O O O O
100
~
100
~
S 80
~
f 80
'
w
60 chiral Sc2 j
60 chiral SC
~
i~°~
~ d
$ 80
~
60
500 1000 15C0
Wave length In m Wavelength / nm
Fig. 3. The reflection spectra for chiral S$~ and chiral St phases with a homeotropic Grandjean alignment. The chiral S$~ and St phases were prepared for the copolymers BB-5, 3*(CH3) with the chiral content y/(x + y)
=
0.25 and BB-6, 4*(CH~) with y/(x + y)
=
0.70, respectively (see text). The
homeotropic specimens with a thickness of 10 ~m were prepared by shearing between two glass slides.
a) and c) The reflection spectra as measured by the incident light parallel to the helical axis (the incident
light normal to glass surface), indicate a half-pitch band at around 500nm for both the chiral S$~ (al and St (cl phases. b) and d) On incidence inclined by 60° to the helical axis, the chiral S$~ still indicates only a half-pitch band (b) while the chiral St reveals the extra full-pitch band in
addition to the half-pitch band (d).
6. Model of Sc structure.
The Sc~ phase has a similar structural properties to the Sc phase but in some aspects both are
essentially different. The qualitative model that emerges for this new phase is only one in which the mesogenic groups are tilted to the layer normal and their tilt direction of mesogenic
1242 JOURNAL DE PHYSIQUE II N° 5
axis is opposite between neighboring layers. In other words, the c-directors lie in the same direction in the every second layer but in the opposite direction between the neighboring layers. The layer structure is illustrated in figure 4a. Here, the polymer chain lies along the z- direction perpendicular to the layers, assuming the average conformation with the successive
mesogenic groups tilted around 50° with respect to each other. Crystallographically, this structure can be illustrated by a doubling of the period in the z-direction giving rise to a screw axis of second order along to z-axis [10]. Further, the structure is invariant under gliding
reflection in the xz-plane, I-e- reflection combined with a translation in the z-direction by a
half of the period. The space group is analogous to the crystallographic D~~ group, although
the local symmetry is C~~ as in the Sc phase. The structure is optically biaxial and its optical
index ellipsoid can be illustrated for each of the two neighboring layers in a period as shown in
figure 4b. The optic plane is located in the xz-plane and the optic axes Oa and Ob lie
symmetrically on both sides of Oz axis.
~l~Z
a) b)
Fig. 4. -Molecular ordering of the Sc~ phase and its optical index ellipsoid.
7. Discussion.
The layer structure proposed for the
SC~ phase is a reasonable model for the following
reasons. Firstly, the X-ray pattem predicted from this model is consistent with observation.
Secondly, this model explains the appearance of the schlieren texture with the singularities of
s = 1/2, since the head and tail of the c-directors are globally indistinguishable because of the D~~ symmetry of the layer structure resulting in the c-director being similar in nature to a
nematic director. In fact, we can build up a continuous stacking of layers around the
singularity of s
=
1/2 if a screw dislocation is coupled [11, 12], as illustrated in figure 5.
Thirdly, in this model a helical structure can be formed by the twisting of the c-directors and hence with its helical axis perpendicular to the layers as in a chiral St phase. However, one notices, by considering the helical arrangement of the index ellipsoid given for the basic lattice in figure 4b, that a remarkable difference exists between the optical properties of chiral
St~ and St phases. In the chiral St~ phase, the index ellipsoid has the optic axes which lie
symmetrically on both side of Oz-axis equivalent to the helical axis. This means that the
optical property has always the period equal to half the pitch in any view or on any
,
iii
~ Ill
II z
/
Fig. 5. The c~director orientation around the wedge dispiration with rotational component of w. The translational component gives rise to the step.
propagation of light to the helical axis. This situation is similar to that in cholesterics [13], but
completely different from that in the chiral St phase ; the chiral St phase has the periodicity
of the full pitch for light propagating in any direction except for the direction along the helical axis [7]. These results have been obtained through the reflection properties of light as shown in figure 3.
In most types of liquid crystals so far observed [14], uniaxial orientation has been achieved
for the long axes of the mesogens, as described by the n-director. In this sense, the
Sc~ phase is quite novel and interesting since uniaxial ordering can not be observed for the n- director but only for the c-director. The formation of such a curious
SC~ structure is believed to
result from a coupling of the polymeric and mesogenic effects in which the spatial
arrangement of mesogenic groups is strongly related to the conformation of the intervening alkylene spacers of the polymer chain. In order to account for these effects, Abe [15] has
performed conformational analysis, within a framework of the rotational isomeric state
model, and evaluated the angle o, defined by unit vectors attached to two successive
mesogenic groups, for all possible conformations of the alkylene spacer. The results indicate that the angular distribution for the even- and odd-membered BB-n polymers appears to be
different. When n is even, o are found to be distributed in the two ranges 0° to 30° (30-40 9b) and 85° to 130° (60-70 9b). For the n = odd system, the major portion of the angles is located in the region 50° to 90° and to some degree orientations are also permitted in the region above 150° [15]. In each system, the conformers with the smaller angular displacement of successive
mesogens are in the more extended form whereas the ones with the larger angular
displacement are in the folded form. According to this calculation, we can arrive at the
following interesting conclusions that can be closely related to the present observation.
(I) In the n
= even series, the parallel orientation of successive mesogenic groups is allowed, conforming more or less to the concept of an ordinary uniaxial ordering of the n-
director but in the n
= odd series uniaxial orientation of successive mesogenic groups is not
expected.
(2) The conformers with the smaller angular displacement in both odd and even systems
Correspond to the conformers participating in the observed layer structures.
These explain the odd-even appearance for the different types of smectic phases and at the
same time, account for the specific formation of a SC~ phase in the odd-membered BB-n series.
1244 JOURNAL DE PHYSIQUE II N° 5
At this stage this argument is somewhat qualitative but sufficient to conclude that the confined conformation of the alkylene spacer jointing the mesogens is likely responsible for the formation of the novel Sc~ phase. This conclusion can be supported by the observation
that dimeric model compounds with the following formula :
~~~~7~~~ ~~ ~j _~ _~~~ ~~~~)-n _Q~
~
~ ~~~~3
exhibit the same mesomorphic behavior with a variation of n as the BB-n polyesters. The Sc~ phase can be seen if the number of methylene unit in the flexible spacer is odd (n
= 5, 7
and 9) while the even-membered compounds with n = 4, 6 and 8 show the SA Phase.
Finally, we would like to comment on that the smectic phase with the similar structure has been reported by Galeme and Liebert [16]. It is called the smectic O phase [16, 17], the
structure of which has been clarified in I-(methyl)-heptyl-terephthalidene-bis-aminocinna-
mate (MHTAC) from the observations of the X-ray pattem and optical texture just similar to the ones described in this study. Also almost simultaneously, Fukuda et al. [18] have reported
that the SCA phase in chiral 4-(1-methylheptyloxycarbonyl) phenyl 4'-carboxylate (MHPOBC)
should assume the similar layer structure on the basis of the reflection property of light due to its helical structure. These phases have also been clarified to have the antiferroelectric
properties [19, 20], which can be expected from D2 symmetry of the packing structure [21].
We believe that the present Sc~ phase can be classified into the same type of phase as the smectic O and CA phases. If this is the case, however, another significant reason except for that proposed here, must be given for such a curios formation of smectic structure since there is no similarity in the chemical structure between MHTAC or MHPOBC and the present
materials. This interesting point warrants further investigation.
References
[1] DEER A., KRIGBAUM W. R. and MEYER R. B., Polymer Liquid Crystals (Academic Press, New York, 1982).
[2] KELLER P., CARVALHO B., COTTON J. P., LAMBERT M., MoussA F. and PEPY G., J. Phys. Lett.
46 (1985) L-1065.
[3] DAVIDSON P., NOIREZ L., COTTON J. P, and KELLER P., Liq. Cryst. IQ (1991) it1.
[4] WATANABE J. and HAYASHI M., Macromolecules 21 (1988) 278 22 (1989) 4083.
[5] The director obtained by the projection of the long axes of tilted mesogens onto the plane in which the layers lie.
[6] NEHRING J. and SAUPE A., J. Chem. Sac. Faraday Trans. II 68 (1972) 1. In the Sc Phase, the upper ends of the tilted molecules have to be distinguished from the lower ones. This means that the
c~director has a distinguishability of the head and tail and hence the singularities of
s = 1/2 are incompatible with the structure of Sc Phase.
[7] BERREMAN D. W., Mol. Cryst. Liq. Cryst. 22 (1973) 175.
[8] CHILAYA G. S., ARONISHIDzE S. N. and KUSHNIRENKO M. N., Mol. Cryst. Liq. Cryst. (Lett.) 82 (1982) 281.
[9] WATANABE J., Liq. Cryst., to be published.
[10] MICHELSON A., CABID D. and BENGUIGUI L., J. Phys. France 38 (1977) 961.
[I I] Such a kind of defect consisting of a wedge disclination and a screw dislocation, has been described
as a dispiration by Hams ; HARRIS W. F., Philos. Mag. 22 (1970) 949.
[12] TAKANISHI Y., TAKEzOE H., FUKUDA A., KOMURA H. and WATANABE J., J. Mater. Chem. 2
(1992) 71.
[13] BERREMAN D. W. and SCHEFFER T. J., Phys. Rev. Lett. 25 (1970) 577.
[14] GRAY G. W. and GOODBY J. W., Smectic Liquid Crystals (Leonald Hill, Glasgow and London, 1984).
[15] ABE A., Macromolecules 17 (1984) 2280.
[16] GALERNE Y, and LIEBERT L., Phys. Rev. Lett. 64 (1990) 906.
[17] LEVELUT A. M., GERMAIN C., KELLER P., LIEBERT L. and BILLARD J., J. Phys. France 44 (1983)
623.
[18] CHANDANI A. D. L., GORECKA E., OUCHI Y., TAKEzOE H. and FUKUDA A., Jpn J. Appl. Phys.
28 (1989) L-1265.
[19] GALERNE Y. and LIEBERT L., Phys. Rev. Lett. 66 (1991) 2891.
[20] CHANDANI A. D. L., HAGIIVARA T., SUZUKI Y., OUCHI Y., TAKEzOE H. and FUKUDA A., Jpn J.
Appl. Phys. 27 (1988) L729.
[21] Each layer in the chiral smectic O and CA phases has C2 symmetry and as a result may exhibit the spontaneous polarization as in the chiral Sc Phase. Globally, however, these phases have D2 symmetry which entails zero average polarization, leading to the antiferroelectric phase.
