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Liquid crystalline polymers

14. Synthesis and thermal behaviour of some polyethers

containing azo-mesogens

S. Alazaroaie a, V. Toader a, I. Carlescu b, K. Kazmierski c, D. Scutaru a,N. Hurduc a,*, C.I. Simionescu a

a Faculty of Chemical Engineering, Department of Macromolecules, Technical University of Iasi, Bd. Mangeron 71,

6600 Iasi, Romaniab

Faculty of Chemical Engineering, Department of Organic Chemistry, Technical University of Iasi, Bd. Mangeron 71, 6600 Iasi, Romaniac Center of Molecular and Macromolecular Studies of Polish Academy of Science, Bd. Sienkiewicza 112, 90-363 Lodz, Poland

Received 20 November 2002; received in revised form 20 November 2002; accepted 6 January 2003

Abstract

The paper presents a study on the relationship between the structure of macromolecular chain and its capacity to

generate a mesophase, when mesogens with an azobenzene structure are implied. The polymers have been synthesized

by phase transfer catalysis starting from 1,9-dichlorononane and different bisphenols: diphenyl-4,4 0-bis[(azo-4-)phenol],

4,40-dihydroxyazobenzene, 4,40-dihydroxydiphenyl, bisphenol A and 4,40-dihydroxybenzophenone. The polymers have

been characterized by 1H-NMR spectroscopy, DSC calorimetry, optical microscopy in polarized light and thermo-

gravimetrical analysis. Theoretical conformational studies, using molecular simulations have also been performed. Due

to their particular geometry, bis-(azobenzene) units are better mesogenic groups as compared with the azobenzene ones.

The highly aromatic structure makes impossible the samples isotropisation, as the degradation processes starting ad-

vance. For these polymers, under UV irradiation, due to the presence of two azo groups in each mesogen unit, strong

conformational modifications are expected. The replacement of the bis-(azobenzene) moieties with azobenzene ones

reduces the transition temperatures, making possible the samples isotropisation.

2003 Elsevier Science Ltd. All rights reserved.

Keywords: Phase transfer catalysis; Liquid crystals; Polyethers; Molecular modeling; Azo-polymers

1. Introduction

In the last few years, scientific research in the polymer

area has focused on the synthesis of new polymers

having special properties, i.e. liquid crystalline behavior.

A special attention was attributed to the polymers con-

taining azobenzenic groups due to their potential ap-

plication in holographic techniques, as optical memory

systems, dendritic systems, etc. [1–7]. The capacity of

azobenzene chromophore to undergo changes in itsconformation, from the more thermodynamically stable

trans form into the less favored cis one, under UV ra-

diation was demonstrated [8]. During this isomerisation,

significant modifications both in geometry and in dipole

of the chromophore take place, making the azobenzene

moiety a potentially useful unit for the photocontrol of

polymer structure either when it is connected in the side

chain or incorporated directly in the main chain [9].

Supramolecular assembly is a powerful tool for the

control of polymer structure. Modulation of noncova-

lent interactions through external stimuli is a solution

for regulating the structure of these systems.

* Corresponding author. Tel.: +40-232-2786802219; fax: +40-

232-271311.

E-mail address: [email protected] (N. Hurduc).

0014-3057/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0014-3057(03)00002-8

European Polymer Journal 39 (2003) 1333–1339

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8/10/2019 eur polym j 39, 2003, 1333-1339


The paper presents a study on the relationship be-

tween the structure of macromolecular chain and the

capacity to generate a mesophase, when mesogens hav-

ing an azobenzenic structure are implied. For a better

understanding of the correlation between the chain

geometry, flexibility, inter-chain interactions over LC

behavior, a very significant number of polyethers and

copolyethers (over 1500) are taken into consideration,

some of them have already been reported in previous

papers [6,10–14]. All the polymers have been synthesized

by phase transfer catalysis polycondensation starting

from 1,9-dichlorononane (DCN) and different bisphe-

nols: diphenyl-4,40-bis[(azo-4-)phenol] (BAB), 4,40-di-

hydroxyazobenzene (AB), 4,40-dihydroxydiphenyl (DHD),

bisphenol A (BPA) and 4,40-dihydroxybenzophenone

(BPC). The polymers have characterized by 1H-NMR

spectroscopy, DSC calorimetry, optical microscopy in

polarized light and thermogravimetrical analysis. The-

oretical conformational analyses using molecular simu-lations have also performed.

2. Experimental

The polymers were synthesized by phase transfer ca-

talysis in a liquid–liquid system. In a typical polycon-

densation reaction, 1.3 mmol mixture of bisphenols, 2.5

g NaOH and 5 ml H2O were vigorously stirred into a 50

ml flask (for 10 min); 1.3 mmol DCN dissolved in 5 ml

nitrobenzene were added and stirred for 5 min; 0.2 mmol

tetrabutylammonium bromide were added to the flask

and the temperature was raised to 85 C. The mixture

was maintained at this temperature for 5 h with stirring.

Then the organic layer was washed with water and the

polymer was precipitated in methanol. The precipitate

was filtered, washed with methanol and water, and dried

at 45 C under reduced pressure.

All the materials (except the BAB and AB) were

supplied by Aldrich and used without further purifica-

tion. AB was obtained according to literature data [15].

BAB was synthesized as follows: benzidine hydrochlo-

ride (5 g, 19.4 mmol) was suspended into 45 ml of cold

water and stirred until a fine suspension is obtained; thesuspension was cooled to 0–5 C and 6.42 g (5.6 ml, 51.5

mmol) HCl 30% were added; a solution of sodium nitrite

(2.81 g, 40.74 mmol) in 7 ml of water was added drop-

wise, under stirring, the temperature being kept between

0 and 5 C; the reaction mixture was stirred further for

30 min and then added to a cold solution of phenol (3.64

g, 38.8 mmol) and sodium acetate (6.36 g, 77.6 mmol) in

65 ml water; the reaction mixture was further reacted for

30 min at 8 C; finally, the suspension was filtered off,

washed several times with water and dried; the product

was purified by extraction with ethylacetate in a Soxhlet

extractor (Yield: 4.5 g (58.9%)).

The copolymerization ratios and the numerical mo-

lecular weights ( M n) were calculated using the end chain

signals of 1H-NMR spectra recorded on a BRUKER

AVENCE 300 MHz device (DMSO-d6 as a solvent).

DSC thermograms were recorded on a METTLER 12 E

device with a heating/cooling rate of 10 C/min. Optical

microscopy studies in polarized light were performed on

an OLYMPUS BH-2 device equipped with a LINKAM

TP 92 temperature controller. The thermogravimetrical

analyses were performed on a MOM-Budapest deriva-

tograph, in static air, with a 10 C/min heating rate. A

HYPERCHEM force-field program (version 4.5) was

used to perform the molecular simulations [16].

3. Results and discussion

The general reaction scheme for the synthesis of polymers is given in Scheme 1.

The proposed chemical structure of the polymers is

sustained by the 1H-NMR spectra. The signals corre-

sponding to the end-chains groups (ACH2Cl at d ¼ 3:6

ppm and ACH@CH2 at d ¼ 5:0–5.5 ppm) are taken into

consideration in order to calculate the numerical mo-

lecular weights. Only certain structures present a small

amount of vinyl end groups because of the dehydro-

chloruration reactions that take place at the interface of

the phases (favored by the high concentration of

NaOH 40%). For all the structures, the calculated

molecular weights have values ( M n ¼ 2500–4000) which

place them in the oligomeric domain. Two reasons are

important for this choice:

• due to the presence of the aromatic mesogenic groups

in the main chain, high transition temperatures are

expected; as a consequence, a lower molecular wei-

ghts will impose a decrease of the transition temper-

ature;

• in our opinion, the correlation between the chain

structure and its capacity to generate a mesophase

can be better observed in the oligomeric domain.

Because previous studies showed a relatively lowthermal stability for the polymers containing azo groups

[17–19], a thermal behavior investigation, using ther-

mogravimetry analyses is necessary. In the case of

bis(azobenzenic) products, the limit of the thermal sta-

bility is situated around 240 C. In these conditions, only

the crystalline/liquid crystalline transition has evidenced,

the isotropisation of the samples being impossible with-

out the decomposition occurred. For the azobenzenic

structures, a thermal stability limit situated around 250

C is observed. For these polymers, the isotropisation of

the system is possible, but is not good to maintain the

polymer in the isotropic state too much, taking into

1334 S. Alazaroaie et al. / European Polymer Journal 39 (2003) 1333–1339

8/10/2019 eur polym j 39, 2003, 1333-1339


consideration the fact that the isotropisation values are

situated relatively close, to the thermostability limit.

In Table 1 some characteristics of the homopolymerscorresponding to the investigated bisphenols, are pre-

sented.

Only the homopolymers containing BAB or AB as

mesogens present a LC behavior (Samples 1 and 2).

Nevertheless, the isotropisation is not possible for

Sample 1 due to the high value of the crystalline/LC

transition. For the same sample, the first heating DSC

curve (Fig. 1) presents two endotherms, the first one is

attributed to a solid/solid transition. At the second

heating, this first endotherm disappears.

A decrease in both polarity and length of the mesogen

induces the decrease of the transition temperatures

(Sample 2); therefore, the isotropisation of the polymer

becomes possible. Fig. 2 presents the mesophase corre-

sponding to this polymer at 215 C. One can underline

that the mesophase is not very large.

The other two homopolymers (Samples 3 and 4)

present semi-crystalline and amorphous structure, re-

spectively.

0 T, C˚

EXO

a

bc

50 100 150 200 250

Fig. 1. DSC thermogram corresponding to Sample 1: (a) firstheating; (b) second heating; (c) first cooling.

Cl (CH2)9 Cl + HO R OH R'HO

R

OH+ Bu4NBr

-HCl

(CH2)9 O O R'x

(CH2)9 O Oy

where R, R' are :

N N

BAB

DHD

C

CH3

CH3

BPA

N N N N

AB

C

BPC

O

Scheme 1.

Table 1

Characteristics of the homopolymers corresponding to investi-

gated bisphenols

Spl. no. Bisphe-

nol type

M n Thermal

behaviour

1 BAB 4100 K!215 C (LC) LC

2 AB 3000 K!209 C

(LC)!228 C (I)

LC

3 DHD 2350 K! 207 C (I) SC

4 BPA 3300 K! 70 C (I) A

K: crystalline; LC: liquid crystalline; I: isotrope; SC: semi-

crystalline; A: amorphous.

S. Alazaroaie et al. / European Polymer Journal 39 (2003) 1333–1339 1335

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In order to reduce the values of the transition tem-

perature, copolymerization reactions are performed.

Table 2 presents some characteristics of the obtained

copolymers containing BAB and AB as mesogenic

groups.

Excepting BAB/BPA copolymers, the others have the

copolymerisation ratio very close to the feed ratio. It is

worth noting that all the products containing BAB

present a LC behavior. Unfortunately, excepting sample

13, the polymers cannot be isotropised, due to the de-

composition reactions.

A typical DSC thermogram for the first group of

polymers (Samples 5–7) is shown in Fig. 3 (Sample 7).

Like in the case of BAB homopolymer, two endothermicsignals can be signaled for the first heating. The optical

micrograph of Sample 7 (Fig. 4) reflects the formation

of a nematic texture.

The presence of the second mesogen (DHD) does not

influence in a significant manner the crystalline/LC

transition temperatures.

Surprisingly, only an insignificant decrease of the

transition temperatures was observed after the replace-

ment of DHD with AB despite of the highest AB

Fig. 2. Optical micrograph in polarized light corresponding to

Sample 2 at 215 C (150).

Table 2Characteristics of the copolymers containing BAB mesogens groups

Spl. no. Bisphenol type Copolym. ratio M n Thermal behaviour

5 BABþDHD 2.7:1 3200 K!215 C (LC) LC

6 BABþDHD 1:1.1 3500 K!220 C (LC) LC

7 BABþDHD 1:2.9 2800 K!200 C (LC) LC

8 BABþAB 2.8:1 2800 K!190 C (LC) LC

9 BABþAB 1:1 3100 K!195 C (LC) LC

10 BABþAB 1:3 2900 K!190 C (LC) LC

11 BABþBPA 1:1.2 3000 K!220 C (LC) LC

12 BABþBPA 1:2.08 3500 K!210 C (LC) LC

13 BABþBPA 1:4.5 4500 K!130 C (LC)!200 C (I) LC

K: crystalline; LC: liquid crystalline; I: isotrope; SC: semi-crystalline; A: amorphous.

50 100 150 200 250 T, C˚

EXO

a

b

c

Fig. 3. DSC thermogram corresponding to Sample 7: (a) first

heating; (b) second heating; (c) first cooling.


Sample 7 at 220 C (150).


8/10/2019 eur polym j 39, 2003, 1333-1339


polarity in comparison with that of DHD. The DSC

thermograms corresponding to Sample 10 are presented

in Fig. 5. The first endotherm is very small for this group

of compounds, but it is still present. The formed meso-

phase is a nematic (Schlieren) one (Fig. 6).

Neither the presence of the BPA on the chain (Sam-

ples 11–13) does not induce spectacular changes in the

thermal behavior, except when a large excess of BPA

units counts for (Sample 13). Because the isotropisation

takes place at 200 C, a standard characterization is

possible for this polymer. Fig. 7 presents the DSC curves

corresponding to the heating/cooling processes. The

crystalline/LC transition occurs at 130 C and the mes-

ophase is very large comparatively with those recorded

for other similar structures investigated by us. Fig. 8

presents the nematic texture of the formed mesophase.

The thermal behavior of Sample 13 is somehow un-

expected, taking into consideration the large excess of

BPA units. Based on previous observations, it has es-tablished that a large excess of azobenzenic units besides

the flexible spacer favored a LC behavior. At this point

one exception should be mentioned: polymers based on

semi-flexible spacer as oxetane derivative [14,20]. For

these, the LC behavior is explained by the chains high

rigidity, consequently the ordering process takes place at

macromolecular level and not at the mesogenic level.

In the case of polymers with flexible spacers, the mi-

cro-phase separation processes (aliphatic/aromatic) play

an important role and the formed mesophases are sta-

bilized by a preferred linear conformation of the chains.Probably, due to the high value of the asymmetrical

ratio of BAB mesogen, a low content of these groups is

enough to induce a LC behavior. Moreover, the BAB

units due to their polarity and inter-chain interactions

can assure a certain degree of chain conformational

stability.

For the polymers containing BAB units, the theoret-

ical conformational analyses reveal two conformers with

both similar energies and asymmetrical ratio coefficients

(4.4 for Structure 1 and 4.0 for Structure 2, respectively)

(Fig. 9). Therefore, both isomers have a similar proba-

bility of formation. The BAB geometry has a very


Sample 10 at 210

C (150).

EXO

50 100 150 200 250 T, C˚

a

b

c



EXO

T, C˚

a

b

c

50 100 150 200 250




Sample 13 at 170 C (150).


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important role concerning the LC behavior, as none of

the similar polymers with azobenzenic mesogens does

present mesophases at a similar copolymerisation ratio(Table 3). Additionally, the BPA homopolymer has not

a LC behavior.

This behavior is probably due to the soft/hard ratio.

The flexible spacer has a length of 10.5 AA, whereas the

BAB, AB and BPA units are 22.6, 12.1 and 19.8 AA re-

spectively long. Thus, for Sample 13, the hard/soft ratio

is 1.2 while for the BPA homopolymer is only 0.97.

For the polymers containing AB and BPA units

(Samples 14–16, Table 3) the hard/soft ratio is situated

between 1.11 (Sample 14) and 1.0 (Sample 16). Taking

into consideration these values one can conclude that for

the polymers containing a flexible spacer with nine

methylene groups, a hard/soft ratio above 1.1 is neces-

sary to generate a mesophase.

The investigations continue with polymers containing

AB units as mesogen (Table 3). The samples with AB

and DHD units present LC behavior for all the copo-

lymerization ratios (Samples 14–16). The presence of AB

groups induces a decrease of the transition temperatures,

the isotropisation being possible for all the polymers.

These significant changes comparatively with the first

group of copolymers (Table 2) are attributed to the

conformational differences induced by AB particular

geometry. Isotropisations occur around 210 C, with 40

C below the thermal stability limit. Fig. 10 shows a

typical DSC thermogram corresponding to Sample 14.

The modification of the ratio between AB and DHD

does not present a significant influence upon the tran-

sition value, but affects the stability of the mesophase.

An excess of DHD units diminishes the stability with

20 C.

Among the polymers belong to the next group

(Samples 17–19), only that containing an excess of AB

units presents LC behavior (Sample 17). The other two

samples have only semi-crystalline structures.

In order to establish either the chain geometry or

polarity plays the major role in mesophase formation,

the BPA units were replaced with BPC ones (Sam-

ples 20–22). Even the BPC and BPA have very similar

geometries; a significant difference of the polarity is ex-

pected due to the presence of carbonyl group. Unfor-

tunately, none of them presents a LC behavior.

Fig. 9. Minimum energy geometries corresponding to BAB.

Table 3Characteristics of the copolymers containing AB mesogens groups

Spl. no. Bisphenol type Copolym. ratio M n Thermal behaviour

14 ABþDHD 2.6:1 2600 K!170 C (LC)!212 C (I) LC

15 ABþDHD 1:1 2500 K!168 C (LC)!210 C (I) LC

16 ABþDHD 1:2.4 2850 K!180 C (LC)!200 C (I) LC

17 ABþBPA 2.7:1 3100 K!130 C (LC)!180 C (I) LC

18 ABþBPA 1:1.4 2800 K!170 C (I) SC

19 ABþBPA 1:3.3 2800 K!90 C (I) SC

20 ABþBPC 2.9:1 2450 K!198 C (I) SC

21 ABþBPC 1.1:1 2300 K!170 C (I) SC

22 ABþBPC 1:2.7 2350 K!130 C (I) SC

K: crystalline; LC: liquid crystalline; I: isotrope; SC: semi-crystalline; A: amorphous.

100 200150 T, C˚

E

X

O

a

b

c

0 50




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4. Conclusions

New polymers containing azo-mesogens have syn-

thesized. The bis-(azobenzene) units are better mesogens

groups, due to their particular geometry. The high aro-

matic structure makes the isotropisation of these sam-

ples impossible, as the degradation processes start in

advance. Strong conformational modifications are ex-

pected for these polymers (under UV irradiation) due to

the presence of two azo groups in each mesogen unit.

Using different bisphenols as comonomers (DHD,

BPA) no significant differences concerning the LC be-

havior are signaled, all the samples being capable to

generate a mesophase. The hard/soft ratio is important

for the LC behavior. No mesophase have formed at

value lower then 1.1.

The replacing of BAB units with AB ones reduces the

transition temperatures, making the isotropisation of the

samples possible. Even so, only the AB/DHD and AB/BPA copolymers with a large excess of AB units have an

LC behavior.

Probably, the aromatic/aliphatic micro-phase separa-

tion has an important role in the stability of the meso-

phase, but no matter which group of polymers was taken

into consideration, only insignificant differences are

observed when bisphenols pairs have changed, or the

copolymerization ratio has been modified.

Acknowledgement

The authors want to thank to the Romanian Minister

of Education and Research for the financial support of

this work.

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