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Frustrated phase condition:A way to room temperatureflc polymers with lowswitching voltage?Mikhail Kozlovsky a & Evgeni Pozhidaev ba Institut für Physikalische Chemie, TechnischeUniversität Darmstadt , Petersenstr. 20, 64287,Darmstadt, Germanyb P. N. Lebedev Physical Institute , Leninskii pr.53, Moscow, 117 924, RussiaPublished online: 09 Mar 2011.

To cite this article: Mikhail Kozlovsky & Evgeni Pozhidaev (2000) Frustrated phasecondition: A way to room temperature flc polymers with low switching voltage?,Ferroelectrics, 243:1, 145-158, DOI: 10.1080/00150190008008016

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Frustrated Phase Condition: A Way to Room Temperature FLC Polymers with Low Switching

Voltage ?

MIKHAIL KOZLOVSKYa and EVGENI POZHIDAEV'

alnstitutfir Phvsikalische Chemie, Technische Universitat Darmstadt, Petersensts 20, 64287 Darmstadt, Germany and 'PN.Lebedev Physical Institute, Leninskii p s

53, Moscow, 117 924, Russia

(Received August 30, 1999)

A variety of chiral LC polymers have been synthesized with the mesogenic side chains derived from asymmetrically substituted esters of terephthalic acid and hydroquinone. It has been shown that the fine ajustment of the chemical structure in those systems leads to poly- mer FLCk switchable to 28°C and by driving voltages to 0.2 V / p .

Keywords: ferroelectric liquid crystals; liquid crystalline polymers

1. INTRODUCTION

The FLC polymers are promising for applications in data storage and

display technologies due to easy processing and because of their

capabilities, to form fibres and films and to conserve polar structure in

glass after switching off the external field. As compared with low molar

mass FLC's, they have however such a disadvantage as much higher

viscosity. That results in the temperature range of the mesophase

145

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146 MIKHAIL KOZLOVSKY and EVGENI POZHIDAEV

shifted towards higher temperatures, in 10- 1000 times slower response

times, and in lower Ps values as well [I , 21. Hence, the molecular

design and synthesis of low-temperature FLC polymers switchable by

low voltages is of great interest.

Among the ferroelectric liquid crystals, both low molar mass

and polymer ones, there are numerous compounds with the phenyl

benzoate mesogenic core, -X,-C6H4-COO-C6H4-X2- [ 1-31. The phase

behaviour and important physical properties of those substances (such

as rotational viscosity or spontaneous polarization) are known to

depend strongly on the nature of the link groups XI and X2. Well

known are FLC side chain polymers with the mesogenic groups derived

from the hydroxybenzoic acid, i.e. with the link group XI =-COO-

andlor X2 = -COO-. They form FLC phases (Sm C* and others) in the

temperature range from ambient temperature (in a glass) to 120°C and

show spontaneous polarization of 1 - 100 nC/cm2 depending on the

spacer length, type of the polymer main chain, and chiral fragment

used.

In contrast, there are only few publications to date on the

polymers with chiral pendant groups derived from the terephthalic acid,

i.e. with the inversed ester link configuration, XI = -0OC- [4-71. Many

of them demonstrate however quite peculiar phase behaviour. Thus, the

polymethacrylate, P8*M, Dow

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FRUSTRATED PHASE CONDITION 147

forms an unusual phase state possessing a layered (smectic) structure

and a helical superstructure but being visually non-birefringent [6]. The

vanishing low birefringence of that "isotropic smectic phase", IsoSm*,

has been recently estimated as An 5 0.0012 for a relating copolymer [8].

We should notice here that the IsoSm* phase reveals neither

pyroelectric response nor current switching. A TGB-like structure with

an extra short helical pitch of 250-300 nrn has been suggested for the

IsoSm* phase [9 ] .

The peculiar mesomorphism of the chiral LC polymers with side

chains derived from the asymmetric esters of terephthalic acid and

hydroquinone can be related to the strictly alternating directions of the

three carboxylic groups in the mesogenic groups. To evaluate the

influence of the chemical structure on the phase behaviour in such

c h i d polymer systems, i.e. whether the ferroelectric Sm C* phase or

the "isotropic smectic" state, IsoSm*, would be formed, a variety of

side chain homopolymers were synthesized, and their structure and

ferroelectric properties have been comparatively studied.

2. EXPERIMENTAL

Synthesis of the monomers and Dolvmers

Synthesis of several polymers has been described earlier [5-71, and

seven more polymers were synthesized first in this work (Table 1).

Monomers for the polymers P8*ST, P6*ST, and P6*M have

been synthesized an polymerized as published for PS*M and P8*S.

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148 MIKHAIL KOZLOVSKY and EVGENI FQZHIDAEV

TABLE 1. Chiral side chain polymers with mesogenic groups derived from terephthalic acid and hydroquinone

Poly- Main Phase transitionsb’ ”’ Ref. mer chaina) nC/crn2

_h A. Side chain structure: --(cH,),o+

0

P5*A Ac Sm F* 77 Sm C* 97 Is0 - 1.5 5

PS*M Ma gl40 Srn C* 74 Sm A 85 Is0 0.9 5

B. Side chain structure: --(CH,),o+

0

P4*A Ac 2 SmB50SrnC*88Iso 4.0 5

P4*M Ma 2 gl40 Sm C* 78 Is0 16.5 5

P6*M Ma 4 gl30 IsoSm* 53 Is0

P8*M Ma 6 gl30 IsoSm* 64 Is0 6

P6*ST Sx 4 SrnA33SmC*51 Iso‘) - 1 5 - P8*ST Sx 6 gl25 IsoSrn* 61 Is0

C. Side chain structure:

P8*S Sx 6 gl24 IsoSm* 47 1.~0~’; or gl 24 Sm C* 43 Sm A 47 Isoe’ 1 1.5 7,9

PL4*A Ac 4 gl 10 Is0

PL4*M Ma 4 gl 15 Is0

PL4*S sx 4 gl 10 Is0

PL6*S Sx 6 RI 15 Is0

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FRUSTRATED PHASE CONDITION 149

')Ac stands for the acrylic main chain, Ma for the methacrylic

b, In heating.

one, and Sx for the polymethylsiloxane chain.

For the sample annealed 24 h at 20°C.

sample.

applied, or after several heating-cooling cycles.

d ) M e t ~ t a b l e phase sequence, for the fast cooled (20°K) bulk

') Equilibrium phase sequence, in thin films, under voltage

The monomers for the polymers of the structure D in Table 1 were

synthesized in a following way. First, the 2R-(+)-(4-hydroxy-

phen0xy)propionic acid was esterified by corresponding alkohol in

benzene with the azeotropic removal of water, using ptoluenesulfonic

acid as a catalyst. Further, the obtained phenol was treated with

equimolar amount of terephtaloyl chloride, and then the excess of 10-

undecenoi-1 (for the polyhydrosililation) or 1 1 -bromo-undecan- 1-01 (for

the acrylic and methacrylic monomers) was added. The goal products

were separated from the reaction media by column chromatography

(silica, ethyl acetate - toluene 1:19) with the typical yield of 45-55 %.

The bromo-precursor was then converted to the (meth)acrylic monomers

using the standard reaction with sodium acrylate or potassium

methacrylate in 1.3-dimethylimidazolidin-2-0n as a solvent.

Purity of the monomers was confirmed by TLC. and their

structure by NMR data. The monomers were then converted to the

polymers using standard polyhydrosililation reaction for the

polysiloxanes, PL4*S and P6*LS, and by the radical polymerization for

the poly(meth)acrylates, PL4*A and PL4*M.

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150 MIKHAIL KOZLOVSKY and EVGENI POZHIDAEV

Methods DSC curves were taken with a Perkin-Elmer DSC-2C calorimeter at 10

Wmin. Microscopic textures were observed under a Leitz microscope

supplied with a Mettler FP-82 heating stage and a videorecording

system. X-ray scattering curves were m d fkom 0 2 mm capillary

samples by a modified STOE STADI 2 diffiactometer using CUK,

radiation and PSD linear position scam@ detector.

The ferroelectric switching and the spontaneous polarization in P6*ST were studied using the experimental wt-up which is shown

schematically in Fig. 1. The set-up combines a standard device for the

electro-optical measurements with the device for the measurements of the polarkation reversal current, and for its integration as well.

FIGURE 1: Schematic drawing of the experimental &-up: 1 - function generator, 2 - FLC cell, 3 - oscilloscope, 4 - light source, 5 - photomultiplier, P - polarizer, A - analyzer, R - resistor for the polarization r e v d current measurements, C* - capacitor for integration of the polarization reversal current.

The automatic integration of the polarization reversal current

allows very simple and accumte evaluation of the spontaneous

polarization [lo]. Just this method was used for the spontaneous

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FRUSTRATED PHASE CONDITION 151

polarization estimations.The ferroelectric switching was studied by

means of the electrooptical measurements using the same equipment,

and the birefringence was estimated also on the base of electrooptical

measurements, using ideas which were proposed in [ 1 I ] .

3. RESULTS AND DISCUSSION

Phase behaviour of the mlvmers

The chemical formulae and phase transitions of the polymers are shown

in Table I . As seen from the Table, only the mesogenic core with three

ester link groups (structure B) ensures the formation of the IsoSm*

phase, provided the terminal chiral radical is long enough. If the radical

is shorter, the polymers are fenoelectric forming the SmC* phase

(structures A, B in Table 1).

At lower temperatures, the Sm C* structure is frozen in glass for

the methacrylic polymers (P4*M, P5*M), while the polyacrylates form

ordered smectic phases (Sm B phase for P4*A but the pyroelectric

SmF* phase for P5*A). At the same time, the polysiloxane P6*ST

shows an unusual nonequilibniim phase sequence Sm C* - Sm A (in

cooling): as seen from the Fig. 2, the layer thickness, dl, grows with

decreasing temerature and achieves finally the calculated value of the

side chain length. I - 34.1 A. Moreover, the maximum tilt value close

to the isotropization point, as calculated from the X-ray data,

6 ~ . ~ ~ = arccos (dlll) - 21" at 48"C, is in a reasonable agreement with

the optical switching angle measured at the same temperature,

eopt = 24.0'. We should note that the similar "inversed" Sm C - Sm A

phase sequence (in cooling) has been recently reported by Pelzl et al.

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152 MIKHAIL KOZLOVSKY and EVGENI FQZHIDAEV

[12] for symmetric dimeric liquid crystals. However, for the case of

P6*ST the phase transition is kinetically determined and can occur at

different temperatures depending on thermal prehistory of the sample,

as discussed in detail below, while the switchable Sm C* phase can be

supercooled down to 27°C.

Temperature, "C

FIGURE 2. The interlayer distance, dl, for the Sm C* phase of P6*ST versus temperature.

Considering the difference in the phase behaviour of the polymers

P6*M and P6*ST having the same mesogenic side groups but different

main chains, we might conclude that the methacrylic polymer chain

favours the formation of the IsoSm* phase, while the

polymethylsiloxanes require a longer terminal radical, to form that

mesophase (cf. P6*ST and P8*ST). We should note however that the

IsoSm* phase of P8*ST shows a low but nonzero birefringence which

was estimated as An - 0.004k0.001. The birefringence cannot be

increased by application of an electric field up to 17 V/pm, and no

switching can be detected.

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FRUSTRATED PHASE CONDITION I53

On the other hand, the polymers with chiral groups derived from

lactic acid (structure D) form no mesophases, being amorphous within

the whole temperature range studied (to O O C ) . The glass transition of all

the four polymers is below the ambient temperature, that being

surprisingly low for acrylic and methacrylic side chan polymers. The

absence of any layered structure in the polymers of D series confirms

the suggestion that three ester links with alternating orientation are

necessary for the equilibrium formation of IsoSm* phase.

The most interesting is however the phase behaviour of the

siloxane polymer with the ether link group between the spacer and the

mesogenic core, P8*S. When cooled fast enough (20 Wmin), especially

in bulk samples, it forms the IsoSm* below 47OC. On the other hand,

thin samples of the polymer, particularly after repeated heating-cooling

runs or under voltage applied, form the conventional phase sequence

Is0 - Sm A - Sm C* - glass, as shown in Table 1. Thus, the phase

behaviour of P8*S is influenced considerably by thermal prehistory of

the sample. The next section considers kinetic aspects of phase

transitions in the studied chiral side chain polymers.

Kinetically determined phase transitions

The phase transitions between mesophases, including the clearing

transition, are usually considered as thermodynamically controlled, in

contrast to the kinetically controlled crystallization of both amorphous

and LC polymers [13]. However, the heating/cooling mode in many

cases affects the transitions in the polymers of B series. Thus, the ISO - IsoSm* transition in cooling for P8*M is shifted by 22 K towards

lower temperatures, as compared with the corresponding transition in

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154 MIKHAIL KOZLOVSKY and EVGENI POZHIDAEV

heating. Such an overcooling of the mesophase is quite different from

the 5-8 K overcooling range typical for LC polymers.

The Sm C* - Sm A transition in P6*ST (in cooling) appears to

be kinetically controlled. The growth of transition temperature, T,, and

of transition enthalpy, AH, with the sample annealing is presented in

Fig. 3a, while Fig. 3b shows the saturated TcQ) and AHa, values after

24 h annealing, versus the annealing temperature, T,. As seen from the

Fig., the TCm is linear upon T, similar to the crystallization of

amorphous and LC polymers [14,15], while the AHa, value achieve

saturation for the samples annealed below room temperature. Therefore,

the conventional appearence of the polymer (as kept for several days at

20°C) does not correspond to the maximum volume fraction of the

mesophase that can be achieved. The detailed study of the transition

kinetics including the Avrami treatment is to be published elswhere.

The most drastic qualitative change in the phase behaviour,

depending on the cooling mode, has been observed for the polysiloxane

P8*S with ether attachement of the spacer to the mesogenic group [9].

The two abovementioned possible phase sequences in the polymer are

illustrated by DSC data shown in Fig. 4. Since training of a polymer

sample by repeated heatingkooling cycles results in the phase sequence

Is0 - Sm A - Sm C*, we assume that phase sequence as the equilibrium

one, but the Is0 - IsoSm* sequence as a metastable one.

Ferroelectric oromrties of the polymers

The "frustrated" ferroelectric Sm C* phase of the polysiloxanes P6*ST

and P8*S is distinguished by several peculiarities, as compared e.g.

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FRUSTRATED PHASE CONDITION I55

0 4 U I2 16

Annealing time. h

40

30

I- 20

10 20 30 40

Annealing temperature. I , 'C b)

FIGURE 3. a) The temperature, T, ( 0 ) and the enthalpy, AH (0 ) of the Sm A -Sm C* transition for the samples of P6*ST annealed at 30°C, versus the annealing time; b) the saturated values of the transition temperature,TCm ( 0 ) and enthalpy, AHa (0 ) versus annealing temperature.

with P4*M and P5*M. First of all, optical switching in the ferroelectric

phase can be observed close to the room temperature (down to 27"C),

probably due to the low glass transition temperatures. We should note

here that the SmC* phase for P6*ST at such a temperature is

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156 MIKHAIL KOZLOVSKY and EVGENI POZHIDAEV

FIGURE 4. DSC scans from PS*S in cooling at 20 Wmin (1) and 2.5 Wmin (2)

. I 6 - A 2

A . A

A

8 9 , A A A A

v ) . 3 AL?8 8

.- , M E A ~ 4 m rn rn

0, . . . . , . . . . , . . , . . . . , . . . . , . 2s M 35 40 45 so

61 A . I A 2

Temperature. 'C

FIGURE 5 . Minimum voltage of visually detectable switching, Urnin. in P6*ST ( I ) and PS*S (2) at 2.5 Hz, versus temperature.

supercooled, but the switching can be observed for several hours. As

seen from Fig. 5 , the minimum switching voltages for P8*S close to the

Sm C* - Sm A phase transition are somewhat lower, as compared with

P6*ST, while the phase transition appears as the threshold of the curve

2 at 43°C. At the same time, the latter polymer shows less growth in the

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FRUSTRATED PHASE CONDITION 157

Umin values with decreasing temperature. The driving voltage for the

switching even at lowest temperatures is rather low, U,,, - 4 - 6 V/pm,

as compared with 20-35 V/pm at 30°C reported for standard FLC

polymers [16, 171. Moreover, at somewhat higher temperatures (35-

50°C) the minimal switching voltage achieves extremely low values of

0.2 - 0.5 V/pm.

As a conclusion, we should suggest that the high sensibility of

the FLC polysiloxanes, P6*ST and P8*S, to external fields relates to

their frustrated phase behaviour. In other words, in a certain

temperature range the polymers can form two different LC phases with

a very close free energies, so that the particular phase condition of a

sample can be “pushed’ to either side by a very small external action.

That might be an approach to polymeric FLC materials switchable at

ambient temperature by a low voltage.

Acknowledgments

We are greateful to Prof. W.Haase (Institute of Physical Chemistry, Darmstadt University of Technology, Darmstadt, Germany) for fruitful discussions, and to M.Darius (the same Institute) for the studies of ferroelectric switching in P8*S. M.Kozlovsky is greateful to Dr. B.Helgee (Department of Polymer Technology, Chalmers University, Gategorg, Sweden) for his assistance in the synthesis of the polymers of D series). The work has been supported by Volkswagen Foundation (project I 74 471) and by the Deutsche Forschungsgemeinschaft 436 RUS 113 / 401 / 0 (S).

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