Synthesis, Structure and Properties of Liquid Crystalline Polymers* Helmut Ringsdorf and Arnold Schneller

The following is an extended abstract of a paper presented at the conference on Liquid Crystalline Polymers in Leeds, 1980, summarising the principles of the snythesis of thermotropic liquid crystalline polymers, their structure dependant phase behaviour and their orientation in electric and magnetic fields.

1. HOW TO TAILOR THERMOTROPIC LIQUID CRYSTALLINE POLYMERS?

The term liquid crystal represents a number of different states of matter, whose degree of order lies between the almost perfect long-range order usually found in crystals and the statistical disorder found in ordinary liquids or gases. This extraordinary behaviour led to the well-known application of low molecular weight liquid crystals in optoelectronics. ,2 Liquid crystalline polymers, however, have only very recently become the subject of scientific research and application. In principle, there are two possible ways of obtaining thermotropic polymers with liquid crystalline properties:

(i) Attaching the mesogenic groups to a polymer back- bone leads to side chain liquid crystalline polymers.

(ii) Introduction of the mesogenic groups into a polymer backbone leads to main chain liquid crystalline polymers.

U

..------ ---m--- 7 1 : mesogenic group

: flexible spacer

Both methods have now been shown to give thermotropic liquid crystalline polymers. In practice this was achieved by decoupling - at least partially - the motions of the mesogenic units from the motions of the polymer segments by introducing flexible spacers. 314 Thus - mainly depend- ing on the nature of the mesogenic unit - all types of liquid crystalline phases (smectic, nematic, cholesteric) have now been obtained with polymeric systems. This concept, originally applied to the side chain polymers, has recently been used for the synthesis of main chain polymer^.^ 16

Institute fur Organische Chernie, Universitlft Chernie, Universitut Mainz, 6500 Mainz, West Germany. (Manuscript received 24 April 1981)

*Presented at the symposium on Liquid Crystal Polymers, Leeds, 16-1 7 July I980

2. VARIATION OF PHASE TRANSITIONS OF LIQUID CRYSTALLINE SIDE CHAIN POLYMERS

The validity of the speculative spacer concept - spacing the rigid mesogenic group from the backbone via flexible linkages - was first demonstrated using polymethacrylic and polyacrylic systems. Table 1 summarises the structure and the corresponding phase transitions of such polymers.

Table 1 and the polymer backbone in polyacrylics '3

Influence on the phase behaviour of the spacer

R' n R phase transitions ("C)

-CH3 2 -OCH3 g 101 n 121 i

-CH3 6 -OCH3 g 95n 105i

-CH3 6 -OC6H13 g 60s 115i

-H 2 -OCH3 g 6 0 n 1 1 5 i

-H 6 -OCH3 g 60s 98n 125i

-H 2 -CN g 6 3 n 9 3 i

g - glassy; s - smectic; n - nematic; i - isotropic.

By variation of spacer lengths n and mesogenic groups nematic as well as smectic phases are accessible. Polymers forming cholesteric phases could be realised by copolymerising a mesongic monomer, which exhibits a nematic phase as homopolymer, with a chiral c o r n p ~ n e n t . ~ These cholesteric polymers show the same optical properties as the low molecular weight cholesterics. In addition the cholesteric structure is preserved in the glassy state when cooling the polymer below the glass transition temperature. Replacement of the methacrylic by the acrylic backbone results in a decrease in a, of about 40°C (see Table 1) and is obviously due to the higher flexibility of the acrylic chain. Consequently, further lowering of the glass transition temperature should be possible by attaching the mesogenic groups to a highly flexible backbone. This consideration, however, does not hold true as it was proved by polymers with a polysiloxane backbone.10111 The replacerrlent of an acrylic by a siloxane backbone does not lower the glass transition temperature as far as would be expected from the Tg of polydimethylsiloxane (Tg = - 127°C)

THE BRITISH POLYMER JOURNAL, VOLUME 13, JUNE 1981 43

Table 2 crystalline polysiloxanes

Structure and phase transitions of liquid

n R phase transitions f C)

3 -OCH3 g 15n 61 i

4 -OCH3 ' g 1 5 n 95i

6 -OCH3 g 5s 4 6 n 1 1 2 i

11 -OCH3 k 5 3 s 133i

k -crystalline

This points to the fact that the model for the decoupling of the motions of the main chain and the mesogenic side chain seems to describe systems with T,s around or above room temperature. Below room temperature - due to lower thermal energy of the whole system - the motions of the polymer backbone are more strongly influenced by the interactions of the mesogenic side groups causing greater restriction of chain mobility. Thus one concept to overcome the problem of high glass transition temperatures is to reduce the interaction between the mesogenic side groups. This can be accomplished by increasing the distance between them, e.g. by introducing an additional spacer into the main chain (Spacer 2 Art). This is outlined in Table 3 using dimethylsiloxane units as flexible main chain spacers.

Table 3 Copolysiloxanes with low phase transitions

X n R phase transitions "C

3 5 -OCH3 g - 3 0 s 2 3 i

3 5 -CN g - 3 0 s 4 2 i

5 11 -OCH3 g - 465 11 Lcc 64 i

10 11 -OCH3 gl - 114 g2 - 57 s - 15 i+ LC = structure of the phase not determined:

+the polymer shows two distinct glass transitions

These copolymers still show liquid crystalline behaviour. The dominating siloxane character is preserved and leads to systems with the expected properties, e.g. high flexibility as indicated by low Tgs and, in contrast to the homo- polymers, liquid like behaviour at room temperature.

3. INVESTIGATIONS OF LIQUID CRYSTALLINE POLYMERS IN THE ELECTRIC AND MAGNETIC FIELD

Low molecular weight liquid crystals are widely used in display devices because they become oriented in electric and magnetic fields.l2-I6 The liquid crystalline polymers

of the side chain type behave in a similar fashion: they can be oriented under the influences of an electric field.' 7-19 Fig. 5 shbW the response and relaxation time of a liquid crystalline polymer when switching the molecules from a homogeneous orientation (molecules parallel to the glass plates) to a homeotropic orientation (molecules perpen- dicular to the boundaries) (Freedericksz transition).

relative intensity f (brightness)

,ton I 1 toff - I I

I 1 0 1 2 tlsec

-+CH,-CH ++CH2- CH+

c=o 0-R'

I I

O==C

0-R'

Fig.! Field orientation of a liquid cryrtallirfe copolymer at 186 C., 10 V and 20 V, cell thickness 20 Mm. * t l = response time, t2 = relaxation t h e .

The response and relaxation times are in the range of 200 ms and are thus comparable with times found for low molecular weight liquid crystals. The order within the liquid crystalline phases of a poly- meric system can be determined by spin probe techniques in a magnetic field.20 They reveal the glass transition temprature and the nematic-isotropic transition. Calcu- lation of the dependence on the temperature of the order parameter F2 lsee Fig. 2) indicates a random orientation above the clearing temperature ( P 2 5 ) and a high orienta- tion in the liquid crystalline phase (Pz =0,7) which can be frozen in below the glass transition.

1.01 I I 1 1

1 t I I I

'nko21L--LJ O P -20 2 0 60 100 140

T(OC) 3

~ i g . 2 liquid crystalline polymer.

Order parameter pZ as a function of temperature for a

44 THE BRITISH POLYMER JOURNAL, VOLUME 13, JUNE 1981

These measurements are confirmed by investigations of partially deuterated liquid crystalline polymers with 2H-n.m.r. 2 1

These extremly rigid systems show no liquid crystalline properties in bulk because they degrade before melting. In order to prepare thermotropic liquid crystalline polymers it is essential to insert flexible segments between the mesogenic elements along the main chain.

4. LIQUID CRYSTALLINE MAIN CHAIN POLYMERS WITH LOW PHASE TRANSITIONS

Studies concerning liquid crystalline main chain polymers were originated by Onsager 22 and Ishihara 2 3 when treat- ing theoretically the packing of rigid rod like molecules. In 1956 Flory 24 extended this theory to concentrated solutions. He proposed that above a critical concentration the macromolecules form a lyotropic liquid crystalline phase. These considerations were verified by the develop-

Depending on the mesogenic groups and the length of the spacer all three main types of liquid crystalline phase can be obtained - in analogy with the side chain polymers described above. Some examples are summarised in Table 4. Within homologous series of such liquid crystalline polymers of different spacer length the clearing temper- ature as well as the glass transition temperature is lowered with increasing spacer length. This regularity is consistent with the theory predicting a decrease in the transition

ment of a new type of fibre, the high tensile-strength fibre.25-27 These fibres became possible through the preparation and the subsequent spinning of anisotropic solutions of polyesters or polyamides.

Table 4 liquid crystalline main chain polymers

temperatures with increasing flexibility of the polymer chain. The phase transitions of main chain polymers with alkyl spacers and alkyloxy spacers cannot be lowered to values below room temperatures.

Structure and phase transitions of thermotropic

Structure Phase transitions (' C) Lit.

CH. 0

k 180 n 295 i 29

k 216 n 265 i 30

0 CH, 0 0

k 162 ch 278.5 i 30

ch: cholesteric

Table 5 with low glass transitions 1°,3'

Structure and phase transitions of polysiloxanes

K 35 Lc 130 i g 5 Lc 1 14 i g -13 Lc 94 i

2 3 5

2 g -14Lc 166 i g 4 7 Lc 127 i g -57 Lc 121 i g - s l 5 1 i

4 5 3

THE BRITISH POLYMER JOURNAL, VOLUME 13, JUNE 1981 45

In agrcement with results obtained with liquid crystalline side chain polysiloxancs main chain polymers with siloxane spacers were prepared. It was thus possible to lower the phase transitions remarkably (see Table 5 ) . In contrast to hydrocarbon spacer systems the introduction of a short siloxane unit (n = 2) results in a considerable decrease in both the glass and clearing temperatures compared to the polymers of Table 4.

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2

3

4

5

6

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