Preparation and characterization of liquid crystalline poly(schiff base) polymers

Eur. Polym. J. Vol. 26, No. 9, pp. 947-950, 1990 Printed in Great Britain. All rights reserved

0014-3057/90 $3.00 + 0.00 Copyright © 1990 Pergamon Press plc

PREPARATION AND CHARACTERIZATION OF LIQUID CRYSTALLINE POLY(SCHIFF BASE) POLYMERS AMAR H. AL-DUJAILI Department of Chemistry, College of Education, University of Baghdad, P.O. Box 7358, Baghdad, Iraq

RIADTH J. A. SHALASHand THAIR S. YASAGH Petroleum Research Centre, Jadiriyah, P.O. Box 10039, Baghdad, Iraq (Received 8 September 1989; in revised form 25 January 1990) Abstract--Liquid crystalline poly(Schiff bases) have been synthesized by condensing a non-mesogenic aldehyde moiety with o- and m-phenylenediamine, 4,4'-diaminodiphenylmethane and 4,4'-diaminodiphenyl ether. The polymers were characterized by i.r. spectroscopy, viscosity measurements, differential scanning calorimetry and optical polarizing microscopy. Variation of the chemical structure was found to be an important factor for the occurrence of mesomorphic properties.

INTRODUCTION Thermotropic liquid crystal behaviour of polymers is of a considerable current interest for a variety of reasons including theoretical challenges presented by macromolecular fluid phases and the possibility of obtaining ultra-high strength fibres from such fluid phases or solution [1, 2]. It is well known that mesophasic properties are related to the stereochemical nature of both the rigid and flexible groups and to the way they are connected to form the macromolecular chain [3-5]. Recently, the synthesis and properties of polyazomethine (Schiff's bases) having the following structural formula have been described [6]:

m-Phenylenediamine (Aldrich) was purified by distillation, (b.p. 128°/2mmHg). 4,4'-Diaminodiphenylmethane, 4,4'-diaminodiphenyl ether and o-phenylenediamine were purified by recrystallization from benzene, tetrahydrofuran (THF) and water respectively; their melting points were 93 95, 191-194 and 102-103°C, respectively [7]. Polymerization All polymers were prepared by the same method, illustrated here by reference to o-phenylenediamine and di-(pformylphenyl) ct,~o-alkanedicarboxylate (I). In a flask equipped with mechanical stirrer, reflux condenser and gas-inlet tube, a mixture of o-phenylenediamine (0.54 g, 5 mmol), di-(formylphenyl)-alkanedicarboxylate (5 mmol), ethanol (15 cm 3) and ethanoic acid (1 drop) was

~ H C ~ - OCO(CH2)nC(O)O~-CH=N-Ar-N~= x where n = 3, 4, 7 or 8. These polymers were prepared by condensation of non-mesogenic aldehyde and amine moieties with the aim of elucidating the order introduced by the development of the macromolecular structure. The present investigation is concerned with the effect of varying the amine moieties on the macromolecular structure. In the reported first series [6] the length of the methylene units in the aldehyde moiety was varied in an odd/even series between n = 3 and n = 8. In the present series, different amine units are examined. EXPERIMENTAL PROCEDURES

Materials Di-(p-formylphenyl)~t,og-alkanedicarboxylates of structural formula I were synthesized by the method already described [6].

OHC~OCO(CH2)nC(O)O~CHO (I) where n = 3, 4, 7 or 8.

refluxed under N 2 for 6 hr. The pale yellow precipitate was removed by filtration, washed with methanol and propanone, and dried in vacuum at 60~'C.

Polymer characterization Inherent viscosities of the prepared polymers were measured with a suspended Ubbelohde-type viscometer in a bath controlled to T-0.01°C at 30°C. Solutions (0.5%) of the polymer in c o n e . H2SO4 were used; in view of possible protonation by the solvent, the results have only indicative value. The thermal properties of the polymers were examined by DSC using Mettler TA 3000 instrument. Samples (ca 10mg) were examined at a heating rate of 20K/min in dry N: flow. The maximum in the DSC endotherm peak was taken as the transition temperature. The textures of polymers were examined on an optical microscope (Leitz, Laborulex 2Pol) equipped with heating stage (Leitz 350) and a photographic camera (Vario Orthomate 2). i.r. Spectra were obtained on a PyeUnicam IR 7912 spectrophotometer. Elemental analysis was performed using Perkin-Elmer 240 elemental analyser. 947

948

AMAR

.f,.........

H. AL-DUJAILIet al.

Ill ', I

.,'

t¢n( l ,ll

8

""

t,

{,4/t7!,/

" I

I

I

I

3500

3000

2500

2000

1800

Wove

\

I

I

I

I

I

I

I

I

1600

4400

1200

1000

8o0

600

40o

2o0

number

(cm -~ )

Fig. 1. i.r. Spectra of (a) di-(p-formyl)~t,to-alkanedicarboxylate(I), (b) 4,4'-diaminodiphenyl ether and (c) polymer IVe. RESULTS AND DISCUSSION The chemical structures of the polymers were established from i.r. spectra and elemental analysis. The spectra of the polymers showed characteristic carbonyl absorption at (1600-1800cm -t) corresponding to the di-(p-formylphenyl)~t,~o-alkanedicarboxylate (I), In all the polymers, there was a conspicuous absence of absorption bands corresponding to NH groups (3200-3500 c m - ' ) of diamine monomers. Figure 1 shows the i.r. spectra of I, 4,4'-diaminodiphenylether and the resultant polymer. Typical results of the elemental analyses of the poly(Schiff bases) are listed in Table 1; there is good agreement between calculated and theoretical values. Results of DSC measurements on the polymers are tabulated in Table 2. Columns 3, 4 and 5 show the glass transition temperature (Ts), the crystalliquid crystal transition temperature (tic) and liquid crystal-isotropic transition temperature (tO, respec-

tively. It was found that the poly(Schiff bases) were insoluble in all common organic solvents but soluble in conc. H2SO4. The polymers were precipitated during the polycondensation before reaching high molecular weight, as indicated by their inherent viscosities (column 2 Table 2). Typical examples of DSC therrnograms of poly(Schiff) polymers (lib and lie) are shown in Fig. 2 in which no distinct Tg could be detected for some polymers. The polymers derived from 4,4'-diaminodiphenylmethane (Id-IVd) and 4,4'-diaminodiphenyl ether (le-IVe) moieties exhibt liquid crystalline properties. Figure 3 shows a polarized microscope photograph of polymer IVd in the liquid crystalline state. All other polymers containing 4,4'-diaminodiphenylmethane and 4,4'-diaminodiphenyl ether units showed a similar texture as observed on a polarizing microscope. However, the polymers having o r t h o - and m e t a phenylenediamine moieties do not show such thermotropic texture. In view of the high melting temper

Table 1. Elemental analysis of poly(Schiff bases)*

)0~-CH=N-Ar-I

E=CH-~-OCO (CH2)nC(O

Calculated n

Ar

C%

H%

Found N%

C%

H%

N%

la

3

~ -

72.82 4.85

6.80 70.41 4.89

6.31

lib

4

@)--

73.24 5.16

5.57 71.83 4.96

6.12

llle

7

77.42 6.09

5.02 73.00 5.32 6.09

lVd

8

_ ~ C H 2 . ~

77.62 6.29 4.90 77.96 6.32

5.21

le

3

-~

73.81 4.76

4.98

-G--

0

_

~

5.56 71.81 4.87

*Full information about the other polymersis available from the authors.

Liquid crystalline poty(Schiff base) polymers

~

Table 2. Characteristics of poly(Schiff bases) Y/inh

Tg

Iic

ti

Designation (dl/g) ('C) (' C) (C) la 0.11 288 lb 0.09 300 le** 0.23 65 170 303 id 0.25 290 330 le 0.28 323 400 lla 0.07 105 388 llb 0.13 90 300 1I¢ 0. I8 80 250 320 lid 0.35 74 350 420 lie 0. I0 70 368 440 Illa 0.06 75 222 lllb 0.20 98 273 ille 0.20 75 160 244 llld 0.18 280 380 llle 0.16 350 428 IVa 0.13 248 IVb 0.13 100 285 IVe 0.27 55 175 270 IVd 0.18 275 335 IVe 0.20 260 300 *Liquid crystal. **Data of (lc- le) from Ref. [6] for comparison.

Remarks

LC* LC LC

LC LC LC

949

IIe

W

LC LC LC

LC LC LC

ature o f some of the polymers viz. le, lid, l i e a n d IIle, the microscopic studies were c o n d u c t e d at r o o m temperature. These investigations were p e r f o r m e d on polymer film samples p r e p a r e d by q u e n c h i n g from the liquid crystalline melt. The molecular structural criteria for m e s o m o r p h i s m are rigidity, rod-shape a n d polarizability. A n y d e p a r t u r e from linearity [8] of the polymer chain m a y cause complete loss o f ability to form liquid crystalline state. The i n t r o d u c t i o n of ortho or meta substituted rings in the polymer backbone m a y cause significant deviation from linearity, leading to a decrease in m e s o p h a s e stability. A n o t h e r point o f interest is t h a t the transition temperatures (tic a n d ti) o f the polymers derived from 4,4'd i a m i n o d i p h e n y l ether a n d 4,4'-diaminodiphenylm e t h a n e units were higher t h a n those of phenylenedia m i n e units. The longer rigid groups in the polymer

I

I

100

200

I

I

300 400 Temp. (°C)

I

3

5~

Fig. 2. DSC thermograms o f polymers l l b and |Id.

must be responsible for e n h a n c e d thermal stability of the mesophases, thus increasing phase transition temperatures. The reported higher thermal stability [9] o f the ether linkage c o m p a r e d with methylene is in good agreement with the present results. According to the optical texture observed with the polarizing microscope, the mesophases of all these polymer a p p e a r to be nematic but no additional i n f o r m a t i o n is presently available on this subject. REFERENCES

1. T. S. Chung. Polym. Engng Sci. 26, 901 (1986). 2. A. Ciferri and I. M. Ward (Eds). Ultra-High Modulus Polymers. Applied Science, London (1979). 3. R. W. Lenz and J. Jin. Liquid Crystals and Order Fluids (edited by A. C. Griffin and J. F. Jhonoson). Plenum Press, New York (1984). 4. A. A. Wojtkowski. Macromolecules 20, 740 (1987).

Fig. 3. Nematic Schlieren texture of polymer IVd in liquid-crystalline state (between crossed polars).

950

AMAR H. AL-DUJAILI et al.

5. P. W. Morgan, S. L. Kwlek and T. C. Pletcher. Macromolecules 20, 729 (1987). 6. A. H. Al-Dujaili, A. D. Jenkins and D. R. M. Walton. Molec. Cryst. Liq. Cryst. 146, 76 (1988). 7. D. D. Perrin, W. L. F. Armarego and D. R. Perrion. Purification of Laboratory Chemicals, 2nd Edn. Pergamon Press, New York (1980).

8. G. W. Gray. Molecular Structure and the Properties of Liquid Crystals. Academic Press, New York (1962). 9. P. E. Cassidy. Thermally Stable Polymers, p. 2. Marcel Dekker, New York (1980).