The phase behaviour and the ferroelectric properties of liquid-crystalline side group polyacrylates containing an (R)-4''(1-methylheptyloxy)-4'-biphenylyloxycarbonyl-4-phenoxy moiety are compared with the properties of their monomers. A S*C phase polymorphism including monolayer and bilayer structures was found for the polymers. Some of the S*C subphases, consisting of smectic mono- or bi-layers, do not exhibit ferroelectric properties. These unusual properties do not exist for the corresponding monomeric acrylates. Therefore, one has to take into account that the polymer chain exerts a strong influence on the phase behaviour and the ferroelectric properties

Gießelmann, Frank

Kühnpast, Kurt

Scherowsky, Günter

Springer, Jürgen

Zugenmaier, Peter

Online Publikationen der Universität Stuttgart

LIQUID CRYSTALS, 1993, VOL. 14, No.3, 861-869 
Ferroelectric Iiquid-crystalline side group polymers 
Spacer length variation and comparison with the monomers 
by K. KOHNPASTt, J. SPRINGER"t, G . SCHEROWSKVt, 
F. GIEBELMANN§ and P. ZUGENMAIER§ 
t Institut fUr Technische Chemic, Fachgebiet Makromolekulare Chemic, 
Technische Universitat Berlin, D 1000 Berlin 12, Germany 
t Institut fur Organische Chemie, Technische Universitiit Berlin, 
01000 Berlin 12, Germany 
§ lnstitut fUr Physikalische Chemic., Technische Universitat Clausthal, 
D 3392 Clausthal-Zellerfeld, Germany 
The phase behaviour and the ferroelectric properties of liquid-crystalline side 
group polyacrylates containing an (R)-4"-{1-methylheptyloxy)-4'-
biphenylyloxycarbonyl-4-phenoxy moiety are compared with the properties of 
their monomers. A p~ phase polymorphism including monolayer and bilayer 
structures was found for the polymers. Some of the p~ subphases, consisting of 
smectic mono- or bi-Iayers, do not exhibit ferroelectric properties. These unusual 
properties do not exist for the corresponding monomeric acrylates. Therefore, one 
has to take into account that the polymer chain exerts a strong influence on the 
phase behaviour and the ferroelectric properties. 
1. Introduction 
Recently the influence of the spacer length (n = 2, 6, 8 and II methylene groups) and 
molecular weight on the polymorphism and the ferroelectric properties of liquid-
crystalline side group polyacrylates (Pin) has been described [1]. All polymers showed 
St: phases, and in one case a p~ phase was in evidence. A St phase polymorphism was 
found for most of the polymers. In some of these St: subphases, no ferroelectric 
properties could be detected. This unusual behaviour is in contrast to low molecular 
mass ferroelectric liquid crystals. Recently similar observations have been made for 
another liquid-crystalline side group polymer [2]. 
+ CH,- CH+ .rr==:\\.rr==:\\ ~lIDWDECeIF nJlJ©JCM2~ o~ 
Pin 
.=2,6, 8, 11 
The question arises as to what extent the polymer chain is responsible for the 
extraordinary properties of these polymers. A first approach to this question is to 
compare them with the corresponding monomers. In this paper the phase behaviour 
and results of electro-optic measurements for the monomers MIn are presented and 
compared with the properties of their polymers PIn. 
'" Author for correspondence. 
0267- 8292/93 s\O{)() <0 1993 Taylor & Francis Ltd. 
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Ferroelectric side chain polymers 863 
2. Experimental 
The synthesis of the monomers and polymers was described in a separate paper [1]. 
The phase behaviour was detennined by polarizing microscopy (Olympus, BHN-POL) 
and differential scanning calorimetry (DSC) (Perkin Elmer, DSC-7). The smectic layer 
thicknesses d were determined by X-ray measurements. The sample was aligned by 
slowly cooling from the isotropic phase to the liquid crystal phase in a magnetic field 
(0-7 T) and the diffraction patterns were recorded on a flat film [3]. 
Electro-optical properties were investigated using commercial 4 ~mJcells (E.H.C., 
Japan) with a rubbed polyimide layer on each of the glass substrates inducing planar 
orientation. The cells were filled using the temperature gradient method. The 
spontaneous polarization Ps was measured by the triangular wave method [4]. The 
response time 110/90 was determined from the change in the transmitted light intensity 
(10--90 per cent) on applying a rectangular wave voltage. The set-up for the 
measurement of the director reorientation during the switching process has been 
described elsewhere [5]. 
3. Results and discussion 
The transition temperatures and the liquid crystal phases of the monomers and the 
polymers are summarized in the table. 
The monomers only show smectic phases with a monolayer structure (subscript I). 
This was deduced from the smectic periodicit~ obtained from the X-ray measurements. 
For instance, the smectic periodicity of 44·2 A for the SBI phase (see figure 1) of MItt 
is in good agreement with the side group length of 44 A estimated using Stuart 
and Briegleb stereomodeLs with an extended conformation. In the p~f phase the smectic 
periodicity decreases to 40 A due to the tilt of the molecules in the layers and increases 
again to 41 ·3 A in the SAl phase. [n the SA l phase, where the orientation of the molecules 
0« 
-----"0 
44 
43 
42 
41 -
so 
. -• .j-
J~ . J
-
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' . . , 
, 
, 
, 
-i 
I ; • ___ . • 
: I 
60 
t 
I 
I 
; 
I "r --r-- ' 
70 80 90 100 
T I °c 
Figure J. Smectic periodicity d of monomer M I ll as a function of temperature (the broken 
lines denote the transition temperatures measured by DSC). 
864 K. Kiihnpast et .1. 
is on average parallel to the layer normal, the smectic periodicity is nearly 6 per cent less 
than the extended length ofthe molecules. This can be explained by a random tilt of the 
molecules in the layers and/or shortening by gauche conformations of terminal alkyl 
chains. Some theories of the SA phase propose that the molecules are randomly tilted 
within the layers, giving a tilt angle from 10-25' [6]. 
The smectic phases of the polymer samples are mainly bilayer structures (subscript 
2). Only if a long spacer is introduced (polymer PilI) two modifications of a mono-
layered p~f phase appear Ep~h and p~fFDf see figure 2). Below these phases, a Se phase was 
observed. This phase is presumed to have a SC2 type structure, with bilayer regions of 
alternating anti-parallel polarity. It is very similar to the SA phase [7], but the 
molecules are tilted with respect to the layer normals. The resu1ts of the X-ray 
investigations on thc polymers are described in detail in a separate paper [Il 
The appearance of bilayer structures in low molecular mass liquid crystals is 
restricted to molecules with strong longitudinal dipoles [8]. The monomers Min do not 
contain strong dipoles derived from groups such as nitro or cyano. Hence only smectic 
monolayers are observed. Nevertheless, the polymers PIn exhibit bilayer structures. In 
fact, Mauzac et al. have shown that liquid crystal side group polymers do not need 
strong dipoles to form bilayer structures [9]. They claim that the coupling of the 
mesogens to the polymer chain leads to a high anisotropy and is decisive for such 
behaviour. 
The table further shows that some of the polymers exhibit p~ subphascs, which do 
not show ferroelectric switching properties. This puts the 'polymeric' p~ phases in 
contrast to the 'monomeric' ones. The occurrence ofa ferroelectric switching behaviour 
in our polymers does not depend on whether they have mono- or bi-layer structures 
(compare P/6 with Pi lI in the table). In polymer PI li, there are two p~ phases with an 
identical X-ray pattern showing different electro-optical behaviour [1, 10]. Those 
70 
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., 
: I 
140 160 
T I ·c 
Figure 2. Smectic periodicity d of polymer PIt t as a function of temperature (the broken lines 
denote the transition temperatures measured by DSC). 
C\J 
'E 
u 
u 
c 
---0..'" 
Ferroelectric side chain polymers 865 
T 1°C 
Figure 3. Spontaneous polarization p. of the polymers P/6, PIS and PIt I as a function of 
temperature(the arrows indicate the temperatures at which the polymers begin to clear); in 
the case of polymer PIS, both p~I phases (x and y) exhibit ferroelectric properties. 
C\J 
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10 I ~ 
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T 1°c 
Figure 4. Spontaneous polarization PI of the monomers M IS (0 ) and M Ill ( _ ) as a function of 
temperature. 
866 K. Kiihnpast et af. 
phases are marked by the subscript x, for the low temperature phase, and y for the high 
temperature phase. In the high temperature p~f " phase of polymer PI lI , ferroelectric 
switching can be observed and a spontaneous polarization can be measured by 
polarization reversal (see figure 3). The low temperature p~yo phase shows no 
ferroelectric switching at the maximum field strength applied. Therefore. no sponta-
neous polarization could he detennined by polarization reversal measurements. This 
does not mean, that the p~" phase exhibits no spontaneous polarization at all; instead a 
spontaneous polarization may exist, which may be detectable by pyroelectric 
measurements [11]. 
The lack offerroeleetne switching in the p~. phases in the polymers P/6 and Pill 
could be due to a threshold behaviour. Ferroelectric switching in the Sc. phase of 
polymer PIS could only be observed at electric fields higher than 25 V J1m -' , In the case 
of polymer P/6, stronger electric fields could be applied, but no switching was observed. 
The threshold seems to be higher due to the higher elastic constants caused by the 
shorter spacer. On applying such strong fields to polymer PIll, the cells suffer short 
circuiting. 
In cont rast to the polymers, neither a St polymorphism nor non-switch able p~ 
phases were observed fo r the respective monomers. Figure 4 shows the spontaneous 
polarization of M IS and M I ll. The values of the spontaneous polarization for the 
polymers tend to be higher compared with the corresponding monomers. This may be 
due to a slight increase in the rotational hindrance of the mesogens around their 
molecular long axes as a result of the link to the polymer chain. The high viscosity of the 
polymers is responsible for the longer switching times in comparison with those 
observed for the monomers (for instance see figures 5 and 6). It should be mentioned 
that there are characteristic viscosities related to the different dynamic processes [12], 
which will not be discussed in detail. 
Vl 
'" 
-0 
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100 
90 
80 
70 
60 e-
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10 
M ~~JJD~~~DMJJ~~T~MJJDiJ~8~DMJJ~J=9~DMJJ~~N~MM 
, , 
, 
, .. . ..... . :. .. . 
T j"C 
Figure S. Response time y 10190 of monomer MIll as a function of temperature-measured by 
applying an electric field of 2·5 V I-'m - I (the broken lines denote the transition tempera-
tures measured by DSC). 
Ferroelectric side chain polymers 867 
20 
III 
E 
-0 
~ 15 , 
0 
~ 
10 
J J~JJtJJJjJJJtJgJ - +---+----i---. rJ JJiJ·iJJ~JJJJiJJHJ.tJ JJJ~. J
.. L L • ... 1.. .. 1 .... i ._-1--1.--- I 
I ' 1'. I i I I I 
1-- -,---- i --+--'-i " 1-·-l-·-t---l---j-·-· 
I ", I ~ I 
L ___ .I.. .J __ L... i._ · 1 I ____ J_ .. _ _._l......... 
, - . , . i -+--::--t I . 
; i :! ... J 
5 JJ.JJ.yJ~J DJJJJJHJ JJrJJ JJJ ~ f ·· !...fJJ " . 
l iJ_iJD ~D ~~iD ~~f=_Ji~~Ji~~~I~ 
140 145 150 155 160 
T I·e 
Figure 6. Response time f 10/90 of polymer PIll as a function oftemperature-rneasured in the 
ferroelectric p~I. phase by applying an electric field of 2·5 V J..tm - 1. 
To obtain more information about the nature of the p~" and Sty phases we have 
detennined respectively the director reorientation during the switching processes at 
13ye and 146°C for polymer Pill (see figure 7). The in·plane angle describes the 
orientation of the director in a...plane parallel to the glass substrates of the cell and is 
often denoted as the apparent tilt angle (see figure 8). The inclination of the director 
with respect to the substrate plane will be described by the off·plane angle. At I 46°e in 
the p~fv phase the director describes a strongly distorted semicircular trajectory 
between the two optically uniform ferroelectric switching states A and B. A 
semicircular trajectory corresponds to the well known reorientation of the director on a 
cone during the ferroelectric switching process [13]. The offset value off the off-plane 
angle (about 10°) in the ferroelectric switching states could be due either to a layer tilt or 
an inaccurate estimation of the refractive indices (necessary for an evaluation of the 
trajectory). At 13ye, for the p~f" phase, only electroclinic switching is-observed, giving 
low contrast. The director distribution of the liquid crystal sample remains optically 
uniform. since the cell can be set to a dark position between crossed polarizers. The 
director oscillates on the apex of the trajectory (in-plane angle of 0°). The trajectory 
determined at 135°e agrees excellently with theoretically predicted director trajectories 
[13] of electroclinic reorientations. The average position is located accurately between 
the two ferroelectric states and represents a third state. This state appears very similar 
to the third switching state which recently has been detected in a liquid crystal side 
group polymer with a 1,3-dioxolane-4-carboxylic acid as chiral moiety [15. 16]. In the 
latter polymer, contrary to P/11, the three switching states occur in the same liquid 
crystal phase. This behaviour has been explained by assuming a conformational 
interaction between the side groups and the polymer chain [16]. Since the tilt cone 
symmetry is broken by the polymer chain in the polymeric p~ phases, the director 
orientations along the tilt cone possess different potential energies. This leads to two 
868 K. Kiihnpast et al. 
25 
135 ~C 
• 
20 
-
• O! 15 
c 
« 
• c 
0 
1L 
, 
-
-a 
Figure 7. 
10 
A B 
5 
l~~~~~~~~~~~~~~~ryT~ 
-25 - 20 -15 -10 - 5 o 5 10 15 20 25 
In - Plane Angle I 0 
Director reorientation of polymer PIt I at 135°C and 146°C; 4 pm-cell. ± 30 V, SO Hz, 
triangular waveform. 
off -plol'lO angle direc tor n 
-4.",," __ gloss substrate 
cane QKis 
e 
Figure 8. Schematic representation of the director reorientation on a cone and definitions of in-
plane and off-plane angles. 
states where the projection of the director on to the smectic layers is in the eclipsed 
conformation with respect to the polymer chain. These unfavourable states. consider-
ing the repulsive [orces between polymer chain and side grouP. are identical to the 
ferroelectric switching states and appear at the high electric fields applied. If the 
external field is switched off or low electric fields are applied, a state with minimized 
repulsive conformational interactions between the polymer chain and the side groups is 
preferred. This is the case if the projection of the director is directed perpendicular to 
the polymer chain, i.e. the director lies on the apex of the trajectory. This state was 
observed as n third optically uniform switching state. The proposed explanation 
assumes that a certain long range orientational correlation exists between different 
polymer chains, which are placed preferably between the smectic layers. 
Similar interactions between polymer chain and side groups may be responsible for 
the unusual properties observed in polymer PIll . Contrary to the behaviour described 
above, the p~l . phase of PIll could not he forced into the ferroelectric switching states 
A, B. Only the intermediate (third) state is observed. This may be due to a stronger 
F erroeleclric side chain polymers 869 
interaction between the polymer chain and the side groups in polymer PI li. More 
experimental results are necessary to verify this assumption. However, the comparison 
of the polymers with the corresponding monomers shows that an influence of the 
polymer chain must be taken into account. 
This work was supported by the Deutsche Forschungsgemeinschaft (SFB 335 and 
Zu 27/14). 
References 
[1] KOHNPAST, K., SPRINGER, J., DAV(oSON, P., and SCHEROWSKY, G., 1992, Makromolek. 
Chem., 193, 3097. 
[2] BOMELBURG, J., HEPPKE, G., and HOLLlDT, J., 1991, Makromolek. Chern. rap. Cornmlm., 12, 
483. 
[3] PFEIFFER, S., 1989, Dissertation, Technical University Berlin. 
[4] MIYASATO, K., ABE, S., TAKEZOE, H., FUKUDA, A., and KUZE, E., 1983, Jap. ). appl. Phys., 22, 
L661. 
[5] GIEBELMANN, F., and ZUGENMAIER, P., 1990, Liq. Crystals, 8, 361. 
[6J GRAY, G. W.o and GOODBY, 1. W., 1984, Smectic Uquid Crystals (Leonard Hill), p. 3, and 
references therein. 
[7] HARDOUIN, F., SIGAUD, G., TINH, N. H., and ACHARD, M. F., 1981 , ) . Phys. Lett., Paris, 42, 
63. 
[8J HARDOUIN, F., LHVELUT, A. M.,AcHARD, M. F.,and SIGAUD,G., 1983, ). Chirn.phy.'i., SO, 53. 
[9] MAuZAc, M., HARDOUIN, F., RICHARD, H ., ACHARD. M. F., SIGAUD. G., and GASPAROUX. 
H., 1986, Eur. Polym. )., 22,137. 
[toJ SCHEROWSKY, G., SCHLlWA, A., SPRINGER, J., KUHNPAST. K., and TRAPP, W., 1989, Liq. 
Crystals, S, 1281. 
[l1J BLiNOV, L. M., BERESNEV, L. A., SHYTKOV, N. M., and ELASHVtLI, Z. M., 1979, J. Phys., 
Paris, Coli., 40, C3-269. 
[12J SCHEROWSKY, G., 1992, Polym. Adv. Tech., 3, 219. 
[13J LAGflRWALl., S. T., OTIERHOLM, B., and SKARP, K., 1987, Molec. Crystals liq. Crystals, 152, 
SOl 
[14] ANDERSSON, G., DAHL, I., KUCZYNSKI, W., LAGERWALL, S. T., SKARP, K., and STF.RLER, B., 
1988, Ferroefectrics, 84, 285. 
[15] SCHEROWSKY, G., KUHNPAST, K., and SPRIl\:GER, J., 1991 , Makromolek. Chern. rap. 
Commun., 12, 381. 
[16J dfb~biMA.kkI F., ZUGENMAIER, P., SCHEROWSKY, G., KUHNPAST, K., and SPRINGER,J., 1992, 
Makromolek. Chern. rap. Commun., 13, 489. 


Ferroelectric liquid-crystalline side group polymers - spacer length variation and comparison with the monomers

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Ferroelectric liquid-crystalline side group polymers - spacer length variation and comparison with the monomers

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