electro-optical effects of azo dye containing liquid crystalline copolymers

Makromol. Chem. 185,1327 - 1334 (1984) 1327

Electro-optical effects of azo dye containing liquid crystalline copolymers

Helmut Ringsdorp, Hans- Werner Schmidt

Institut fur Organische Chemie, Johannes Gutenberg-Universitat, Johann-Joachim-Becher- Weg 18 -20, D-6500 Mainz, FRG

(Date of receipt: January 19, 1984)

SUMMARY: The synthesis and phase behaviour of azo dye containing liquid crystalline side group copoly-

mers are described. These copolymers show the same elctro-optical effects as low molar mass guest-host systems. Their macroscopic oriented nematic structure can be frozen in below the glass transition temperature resulting in a polymer film with dichroic properties. The behaviour of the copolymers (phase behaviour, surface and electric field orientation) can be improved via mixtures with low molar mass liquid crystals.

Introduction

For liquid crystalline side group polymers all relevant types of mesophases such as nematic, cholesteric, smectic A or smectic C have been realized. These polymers com- bine well-known properties of low molar mass liquid crystals with those of polymers and have scientific and technical interest').

Liquid crystal displays using pleochroic dyes dissolved in low molar mass nematic liquid crystals were first described by Heilmeier et al.*). The electro-optical effect, a colour intensity change, in such guest-host displays is based on the cooperative alignment of the guest dye in the nematic host and the different absorption of the dye de- pending on its orientation. Mixtures of dyes in liquid crystalline side group polymers have been investigated and show similar orientation and electric field behaviour as low molar mass systems3s4).

Coloured polymeric liquid crystals with properties comparable to low molar mass guest-host systems can be obtained by copolymerization of mesogenic monomers and monomeric dyes4-@. They might open interesting application possibilities in the field of guest-host systems. The schematic structure of such dye containing liquid crystalline copolymers is shown in Fig. 1.

Fig. 1. Schematic structure of dye containing liquid crystalline side group copolymers MESOGENIC UNIT

0025-1 16)</84/$03.00

1328 H. Ringsdorf, H.-W. Schmidt

By covalent fixation of both the dye and the mesogenic group to the same polymer main chain it is possible to synthesize systems with high, temperature independent dye concentrations5). This is one of the main distinctions to low molar mass guest-host systems in which the solubility of the dye in the nematic host is frequently insufficient and temperature dependent '3 *). As examples of coloured liquid crystalline polymers illustrated in Fig. 1, copolymers with an azo dye content up to 40 weight-% were synthesized. In addition, the electro-optical effects of these copolymers and their mixtures with low molar mass liquid crystals were investigated.

Results and discussion

Synthesis and phase behauiour of the copolymers

The synthesis of the polymerizable azo dye 3 is summarized in the following reaction scheme.

1

1 + C1-(CH,)6-OH - HO-(CH,)6-0 e N = N o C N 1 \ - HCI

2

2 + H,@CH-COCI - H,C=CH-C00-(CI-1,)6-0 O N - N - Q - C N 1 \ - HCI

3

H2C=CH-COO-(CH,),-0

4

4-(4-Hydroxyphenylazo)benzonitrile (1) was synthesized by diazotation of 4-amino- benzonitrile and coupling with phenol. The spacer was introduced by etherification with 6-chloro-I-hexanol to give 4-[4-(6-hydroxyhexyloxy)phenylazo] benzonitrile (2). Esterification of 2 with acryloyl chloride leads to the azo dye monomer 6-[4-(4-cyano- phenylazo)phenoxyJhexyl acrylate (3). As comonomer the corresponding benzoate 4 was choseng). Neither monomer 3 nor monomer 4 form liquid crystalline phases. Monomers 3 and 4 were polymerized by free radical polymerizations in toluene solution.

Electro-optical effects of azo dye containing liquid crystalline copolymers 1329

n L 5

L -In 6

The liquid crystalline behaviour of the resulting polymers was studied by polarization microscopy and DSC measurements. All polymers show a nematic mesophase as detectable by characteristic droplets at the nematic-isotropic transition. The DSC curves in Fig. 2 demonstrate that the phase behaviour of the polymers is not in- fluenced by their composition.

Fig. 2. DSC-curves of polymer 6 and copolymer 5(e). Heating rate: 10 K/min; Tg: glass transition temp.; T,,: clearing point

0 LO 80 120 T P C

Dye content and the phase transition temperatures of the nematic copolymers 5(a) -5(e) are listed in Tab. 1 and compared with the homopolymer poly((6-14-(4- cyanophenoxycarbonyl)phenoxy]hexyloxycarbonyl)ethylene) (Q9) isolated from a free radical polymerization under the same conditions. It should be noted that the phase transitions of these nematic polymers are, within the limits of error, independent of the azo dye concentration. This can be explained by the similarity of the structure of the two monomers.


Tab. 1 . tion temperatures of copolymers 5 and polymer 6

Composition of copolymers 5 in weight-% of monomeric units of 3 and phase transi-

Sample Copolymer compositiona) Phase transitionsb) Weight-% of monomeric units of 3

temp. in "C

6 10 14 19 40 -

g 33 n 130 i g 34 n 129 i g 33 n 129 i g 34 n 130 i g 32 n 132 i g 33 n 133 i

a) Average of the values determined by UV measurements and by elemental analysis (see Exptl.

b, g: glassy; n: nematic; i: isotropic. Part).

Electro-optical effects of the copolymers

Dye molecules in liquid crystalline media are co-oriented by the ordered structure of the liquid crystalline host. They can be macroscopically oriented by surface forces, an electric or a magnetic field.

The nematic copolymers 5(a) - 5(e) can be aligned by surface effects between glass slides to give homogeneous orientation with the long axes of the mesogenic units parallel to the glass slides. In contrast to low molar mass guest-host systems, the orientation can be frozen in below the glass transition temperature resulting in a clear

- - Coloured Slightly coloured

- Coloured

Fig. 3. meier cell (A& positive). 0 : Dye molecule; -: mesogenic unit; - polarization plane

Schematic representation of the electro-optical effects of guest-host systems in a Heil-


polymer film exhibiting optical dichroism. If the polarization plane of polarized light is parallel to the homogeneous orientation, the film exhibits the maximal colour intensity and the minimal intensity if the polarization plane is perpendicular. The contrast ratio between both positions depends on the order parameter of the dye.

In the electric field liquid crystalline side group polymers can be oriented like low molar mass liquid crystals10-'2). If dichroic dyes are dissolved in liquid crystalline media an orientation change is connected with a colour change which is used in guest host displays. From the homogeneous alignment the copolymers 5(a) - 5(e) can be homeotropically oriented by an electric field due to their positive dielectric aniso- tropy. The behaviour of the copolymers is identical with the effects of low molar mass guest-host systems and is schematically represented in Fig. 3. The response times of copolymers 5 for a light intensity change of 90% are summarized in Tab. 2.

As expected, the rise times increase when the applied voltage is reduced, the time for the relaxation from homeotropic to homogeneous orientation takes longer and is voltage independent within the limits of experimental error.

Tab. 2. Rise and decay times (ton, foff) for the copolymer S(e) at different voltages. Condi- tions: Heilmeier cell (one polarizer); cell thickness: 12 pm; temperature: 115 "C; reduced temperature (T/Tc,): 0,87; frequency: 50 Hz

Potential difference Rise timea) Decay timea) in V ton/' *off/s

30 1 2 44 20 2 3 38 10 6 8 42 5 12,2 41

a) Response times measured for an intensity change of 90% (cf. Fig. 3).

Mixtures of the copolymers with low molar mass liquid crystals

The application of liquid crystalline polymers in displays is complicated by their relatively high viscosities which results in high application temperatures and relative long rise and decay times. To overcome this problem, mixtures of liquid crystalline side group polymers with low molar mass liquid crystals have been investigatedi3. 14). It could be shown that polymer 6 is miscible with the low molar mass 4-cyanophenyl Cheptylbenzoate in the nematic phase over the whole concentration range14).

Similarly the copolymers 5(a) -5(e) are homogeneously miscible with 4-cyanophenyl Cheptylbenzoate. To illustrate the lowered phase transition temperatures and the faster response time in the electric field, the results for one mixture of copolymer 5(e) with 4-cyanophenyl4-heptylbenzoate are given in Tabs. 3 and 4.

With the copolymers discussed above the guest-host effect of dye containing liquid crystalline copolymers and their mixtures with low molar mass liquid crystals was demonstrated. However, the incorporated azo dye ist not ideal for an application in


Tab. 3. Phase transitions of copolymer 5(e) and its mixtures with 4-cyanophenyl 4-heptylbenzoate (CHB)

Systema) Dye content Phase transitions weight-% of MU of 3b) T/"C

5(e) 40 Mixtures 5(e)/CHB 30

g 32x1 132i g -2 n 103 i

a) The mixtures contain 74 weight-% of 5(e) and 26 weight-% of CHB. b, MU: monomeric unit.

Tab. 4. Comparison of the rise times ( f0J for copolymer 5(e) and its mixtures with 4-cyanophenyl 4-heptylbenzoate (CHB) at different voltages. Conditions: Heilmeier cell (one polarizer); cell thickness: 12 pm; frequency: 50 Hz

System Potential T/"C Reduced Rise timea) difference temperature f 0 J s in V T/Td

5(e) 30 115 0,87 172 Mixture b, 30 81 0,79 03

5(e) 20 115 0,87 2,3 Mixtureb) 20 81 0,79 1 3

5(e) 10 115 0,87 6 8 Mixtureb) 10 81 0,79 2 8

Rise times measured up to an intensity difference of 90%. b, Composition see Tab. 3.

guest-host displays due to its low UV stability. Preliminary experiments have already shown that the concept can be transferred to anthraquinone dyes with extremely low solubility and high UV stability6).

Experimental part

Synthesis of the monomers

4-(4-Hydroxyphenylazo)benzonitrile (1): 0,5 mol(59 g) of 4-amino-benzonitrile was dissolved in 140 ml of conc. sulfuric acid and 140 ml of water. For the diazotation 0,5 mol (343 g) of sodium nitrite dissolved in 200 ml of water was slowly added at temperatures below 5 "C. To the cooled solution, 1 mol of 2 N sodium hydroxide solution with 0,5 mol(47,O g) of phenol were added and the reaction mixture was stirred for 1 h at room temperature. The precipitated azo dye was isolated by filtration and washed with water. The residue was dissolved in 1 I of ethanol/water (vol. ratio 1 : 1) and acidified with conc. hydrochloric acid. The precipitate was isolated by filtration and dried. Yield: 105 g (95%); m. p. 203 "C (Lit. 15): m. p. 203 "C).


C13qN30 (22392) Calc. C 69,96 H 4,04 N 18,83 Found. C 70,OS H 4,22 N 18,76

4-[4-(6-Hydroxyhexyloxy)phenyluzo] benzonitrile (2): 0,l mol(22,3 g) of 4-(4-hydroxyphenylazo)benzonitrile (1) and 14 g of dry potassium carbonate were dissolved in 100 ml of absolute acetone, a pinch of KI is added, and 0,11 mol (15,3 ml) of 6-chloro-1-hexanol is then added dropwise. The reaction mixture was refluxed for 6 days. The precipitated potassium chloride was filtered off under suction from the warm mixture and the filtrate was evaporated. Chloro- form (200 ml) was added to the crude product and the undissolved material was filtered off. The chloroform solution was extracted four times with water and evaporated. The product was recrystallized from 300 ml of acetone. Yield: 15,3 g (47070); m. p. 147 "C.

clgHZl (323,3) Calc. C 69,71 H 6,47 N 12,83 Found C 70,38 H 6,24 N 13,20

6-[4-(4-Cyunophenyluzo)phenoxy]hexyl ucrylute (3): 0,02 mol (6,5 g) of 4-[4-(6-hydroxyhexyloxy)phenylazo]benzonitrile (2) was dissolved in 30 ml of absolute tetrahydrofuran at 60°C. 0,022 mol (3,l ml) of triethylamin was added and a solution of 0,022 mol (1,8 ml) of acryloyl chloride in 5 ml of the tetrahydrofuran was then added dropwise at 60 T. The reaction mixture was cooled and stirred at room temperature for 20 h. After the addition of 150 ml of chloroform the solution was extracted by shaking with water. The chloroform solution was dried over N%,S04 and evaporated. The crude product was recrystallized from 350 ml of a mixture of ethanol/acetone (vol. ratio 6: 1). Yield: 4 3 g (60070); m. p. 93 "C.

UV (chloroform): A,, = 366 nm.

C2,H,,N,O, (377 74) Calc. C 70,Ol H 6,14 N 11,13 Found C 69,94 H 6,34 N 11,14

The synthesis of the phenylbenzoate monomer 4 is described elsewhere9).

Synthesis of the polymers

About 1 g of the monomer mixture (3 and 4) was dissolved in 8 ml of toluene and 1 mol-070 (based on the monomer) of AIBN was added. The monomer solution was degassed by passing nitrogen for 10 min and polymerized at 70°C for 42 h. The copolymers obtained were precipitated in cold ether, dissolved in methylene chloride and reprecipitated. This procedure was repeated until the monomer was no longer detectable by thin layer chromatography. The purified polymers were dried under reduced pressure. For elemental analysis see Tab. 5 .

Tab. 5 . Yield, elemental analysis, and composition of copolymers 5 ~~ ~ ~~ ~~

Polymer Yield Elemental analysis (EA)a) Copolymer composition weight-% of MU of 3b) in 070

C H N EA uv

5(a) 28 70,14 6,06 4,02 6 6 5(b) 24 70,28 6,13 4,37 11 8

5(d) 16 70,21 5,86 5,24 21 17 5(c) 20 70,92 6,24 4,69 15 12

5(e) 18 70,03 6,24 6,81 42 37

a) C,,H,,NO, (393,4) Calc. C 70,21 H 5,89 N 3,56. b, MU: monomeric unit.


The copolymer compositions were determined from the nitrogen values of the elemental analysis (EA) and by UV measurements made in CHC13. The absorption band at 366 nm was used to calculate the copolymer composition based on the absorption of monomer 3.

Preparation of the mixture

The mixture of the copolymers with low molecular weight liquid crystals were prepared in various weight ratios and dissolved in CH2C12. Solvent was evaporated and the mixtures were dried in vacuo.

Measurements

The thermal behaviour of the compounds was investigated by a DSC-2C differential scanning calorimeter (Perkin Elmer). The scan speed of the heating runs was 10 K/min. Investigations of the liquid crystalline textures of the polymers and the mixtures were carried out with the POL- BK I1 polarizing microscope (Leitz).

The displays to study the electro-optical effects were prepared in the following manner: Electrical conducting glass slides coated with Sn02, In203, and with a rubbed polyimide surface (Siemens AG, Miinchen) were used. A polycarbonate film of 12 pm thickness with a hole of about 0,25 cm2 at the center was used as distance holder. The polymers were melted on one glass slide and the display was put together to a sandwich cell. The display was then placed in a metal holder on a hot-stage. To achieve a homogeneous preorientation the display was annealed at 10°C below the nematic-isotropic transition for 1 h. The behaviour of the liquid crystalline systems in the electric field was investigated with the same measuring arrangement described in the literature”).

’) A. Ciferri, W. R. Krigbaum, R. B. Meyer, “Polymer Liquid Crystals”, Academic Press,

’) G. H. Heilmeier, J. A. Castellano, L. A. Zanoni, Mol. Cryst. Liq. Cryst. 8, 293 (1969) 3, R. V. Talroze, V. P. Shibaev, V. V. Sinitzyn, N. A. PlatC, M. V. Lomonosov, Polym.

Prepr., Am. Chem. SOC., Div. Polym. Chem. 24 (2), 309 (1983) 4, H. Finkelmann, H. Benthack, G. Rehage, J. Chim. Phys. Phys. Chim. Biol. 80, 163 (1983)

H. Ringsdorf, H.-W. Schmidt, A. Schneller, “12. Freiburger Arbeitstagung Fliissigkri- stalle”, Freiburg (1982) H. Ringsdorf, H.-W. Schmidt, G. Baur, R. Kiefer, Polym. Prepr., Am. Chem. SOC., Div. Polym. Chem. 24 (2), 306 (1983)

New York 1982

9 J. Cognard, T. Hieu Phan, Mol. Cryst. Liq. Cryst. 68, 207 (1981) *) S. Aftergut, H. S. Cole, Mol. Cryst. Liq. Cryst. 78, 271 (1981) 9, M. Portugall, H. Ringsdorf, R. Zentel, Makromol. Chem. 183, 2311 (1981)

lo) R. V. Talroze, S. G. Kostromin, V. P. Shibaev, N. A. PlatC, Makromol. Chem., Rapid Commun. 2, 305 (1981) H. Ringsdorf, R. Zentel, Makromol. Chem. 183, 1245 (1982) H. Finkelmann, U. Kiechle, G. Rehage, Mol. Cryst. Liq. Cryst. 94, 343 (1983)

13) H. Finkelmann, H.-J. Kock, G. Rehage, Mol. Cryst. Liq. Cryst. 89, 23 (1982) 14) H. Ringsdorf, H.-W. Schmidt, A. Schneller, Makromol. Chem., Rapid Commun. 3, 745

Is) I. Socha, J. HorskB, M. Vecera, Collect. Czech. Chem. Commun. 34, 2982 (1969) (1982)

electro-optical effects of azo dye containing liquid crystalline copolymers

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