Effect of shape of mesogenic group on the formation of liquid crystalline phase in polymers

Eur. Polvm. J. Vol. 20, No. 11, pp. 1127-1130, Printed in Great Britain. All rights reserved

1984

0014-3057/84 $3.004 0.00 Copyright ' ! 1984 Pergamon Pres:~ Ltd

EFFECT OF SHAPE OF MESOGENIC GROUP ON THE FORMATION OF LIQUID CRYSTALLINE PHASE IN POLYMERS I. I. KONSTANTINOV, Y. B. AMERIK and A. A. SITNOV Institute of Petrochemical Synthesis, Academy of Sciences of the U.S.S.R., Leninsky Prospekt 29, 117912 GSP-1 Moscow V-71, U.S.S.R.

(Received 3 January 1984; in revised form 27 March 1984) Abstraet--Mesomorphic behaviour of four polymethacrylates with mesogenic side groups has been studied. In contrast to monomers which are nematogenic, mesogenic groups in the polymers form layer structures of the smectic type. A thermodynamic stable mesophase is supposed to be realized in polymers which consist of macromolecules of secondary structure, considering a macromolecule as the smallest structural element of the polymer system. INTRODUCTION

Polymers with mesogenic side groups, connected by flexible spacer to the main chain, form a liquid crystalline phase which has properties similar to those of the mesophase of low molecular weight compounds [1-3]. The role of the main chain in the formation of liquid crystalline structure [4] and mesophase behaviour [5] increases with decrease of the length of the flexible spacer. The extreme case where the mesogenic groups are connected directly to the main chain is of special interest. In such systems, thermotropic mesomorphism [6], having a number of unusual properties, was observed for the first time. The most remarkable properties are as follows. Firstly, macromolecules in a dilute solution are able to form an intramolecular mesomorphic structure [7,8]. Secondly, liquid crystalline ordering of the polymers in bulk can be reversibly improved on heating even above Tg [4,9]. Thirdly, small change of the mesogenic group constitution, e.g. change of length of an aliphatic para-substituent, causes change in the type of crystalline order [10]. From general consideration, however, mutual arrangement of mesogenic groups, as the first step of the formation of the thermodynamic stable mesophase, will take place only for a definite agreement of the sizes of the main chain elements with the mesogenic group shape. This paper deals with the relation between chemical constitution of mesogenic side groups and mesomorphous behaviour of polymers.

lowed by recrystallization from a suitable solvent. Constitutions of the monomers and some of their properties are give in Table 1. The polymers were prepared by free radical polymerization of the monomers ([M] = 0.2 m o l l ~) in dioxane solution at 60 ° using benzoyl peroxide as initiator ([I] = 0.5 x 10 -3 mol 1 ~). The polymers were isolated by precipitation in methanol or ether: the precipitates were collected on a glass filter and dried in vacuum to constant weight. Polymer yields were 8070. Optical investigations were carried out by means of polarizing microscope equipped with a hot-stage. Thermal analysis was performed by means of differential scanning microcalorimeter DSM-2M at heating and cooling rates of 2-4°/rain. Error in phase transition enthalpies (AHm) was +_ 6Oo. Glass transition temperatures (Tg) of the polymers were estimated from a deformation test performed with Kargin's balance at constant loading of 7 x 104 Pa and heating rate of 1.5"/min. X-ray diffraction data were obtained with a URS 55 instrument with Ni filtered CuK~ radiation at room temperature. Oriented samples of the polymers were prepared by stretching above Tg. Densities of polymers (p) were measured by flotation in aqueous CaC12 solution. Packing coefficients (K) of the polymers were estimated by the procedure described [11]: K = ( N A" V" p )/M where N a is Avogadro number, V and M are the molecular volume and mass of monomer unit, respectively.

MONOMERS EXPERIMENTAL

Synthesis of the monomers was performed by interaction of the chloride of a para-substituted acid with para-substituted phenol in solution in dry benzene under N2 flow at 80°. Reaction products were purified chromatographically through AI203, fol*C = crystal; I = isotropic; N = nematic phase; temperature in C; in parentheses are given phase transition cnthalpies in kJ mol -t. 1127

The phenyl benzoates of the acrylic series are known to form enantiotropic nematic phases, whereas the methacrylate derivatives tend to rnonotropic mesomorphism [12]. Thus, p-n-hexyloxyphenyl ester of p-methacryloyloxy benzoic acid (M06) has the following scheme of phase transitions:* C ._775 13y5) ~:~ I

52 (33.3)~N~665

' (0.71)

I. I. KONSTANTINOVet al.

1128

Table 1. Some characteristicsfor the compounds CH2~C(CH3)--CO0~/~

Tr (°C)

AHr kJ/mol

Mesomorphism

382.46

C23H260~

Methanol

77.5

33.5

Monotropic, nematic

382.46

C23H2605

Acetone

94.7

31.7

Nonmesomorphic

C24H2805

Methanol

75.0

41.8

Nonmesomorphic

C24H3404

Heptane

60.0

31.1

Enantiotropic nematic

M-06 --CO0-----~k--/~/------ O \L__J MB-6 - - O O C ~

MH-7 - - O O C - - - - ~ - - / k ~ - CH2 380.49 MC-7

~

OO C

~'/H'~ k_____/

CFI2 386.54

As suggested earlier [12] the formation of monotropic nematic phase occurs because of favourable location of molecules in the isotropic melt and to a suitable distribution of the interacting groups within the molecule. From this point of view, the case of M-06 provides perfect conditions for the formation of a nematic phase from the isotropic melt. In the structural isomer of M-06, p methacryloyloxy phenyl ester ofp-n-hexyloxybenzoic acid (MB-6), there is a change in the distribution of interacting grgups involving a change in polarity and polarizability. As a result, the isotropic melt of MB-6 passes into a metastable solid modification: C II - - ~947~(317)~ ~ I

t [ 75.4:

C6 H13

Formula

X

O

X --

Solvent for crystal

Molecular mass

Code

--

r C I ~---77"60~ I (29.8)

Increase in the geometric anisotropy of the molecule, e.g. lengthening of the alkoxy-substituent on passing to higher homologues such as MB-9 or MB-12, favours formation of the mesophase rather than the metastable solid phase. A situation analogous to that of MB-6 is observed with the p-methacryloyloxy phenyl ester of p - n heptylbenzoic acid (MH-7) which forms a less stable solid modification from the isotropic melt as compared to the original modification obtained by crystallization from solution: C II - - 75:~18j_ C I ~ - 60.(22.4j...~ I 52° (22.4) An enantiotropic nematic phase is realized in the p-methacryloyloxy phenyl ester of p-n-heptyl-transcyclohexane carboxylic acid (MC-7): C 6oL(~_.1..~N 7z~(°43-~I Replacement of the benzene ring by the cyclohexane ring essentially changes the molecular shape which permits their close packing with the displacement as *The structure of the polymers will be discussed in detail in a separate publication.

shown in Fig. 1. The packing in the solid state is known to be a precursor of molecular arrangement in the nematic phase. The shape of molecules of M-06, MB-6 and MH-7 (Fig. 1) suggests a variety of packing as indicated by polymorphism in the solid state; the preferred packing is mainly determined by energetic factors whereas in the case of MC-7 geometry of shape predominates. POLYMERS

In contrast to monomers, inclined to molecular organization of nematic type, mesogenic groups in the polymers form layer structures of smectic type (with liquid-statistical arrangement in layers). X-ray patterns of the polymers have some small angle reflections and a diffuse reflection within the range of wide angles. The diffuse reflection from oriented samples is located on the meridian (coincides with direction of stretching); there is a considerable difference within the range of small angles due to the effect of chemical constitution of mesogenic groups on layer packing type* (Fig. 2). The formation of the layer structures is due to the connection of mesogenic groups with the main chain and their suitable packing. The polymers with phenyl benzoate side groups (PM-06, PMB-6 and PMH-7) are characterized by a thermodynamic stable mesophase which passes into an isotropic melt accompanied by heat transition, AHf. The difference in AHf values seems to represent the intensity and extensivity of the interaction of mesogenic groups, the shape of which does not hinder their approach to the equilibrium space and the formation of thermodynamic stable ordering of mesomorphic type. This is proved by the equal packing coefficients (K) of the polymers which are similar to those of close packed ellipsoids [13]. Another situation is realized in PMC-7, in the polymer with cyclohexane rings in the side groups. Though the main chain makes the side groups form a layer, they are unable to approach each other to the equilibrium space owing to steric hindrance from the cyclohexane groups (Fig. 1). The value of K is rather low and equal to that of most poly(alkyl meth-

Effect of shape of mesogenic group

1129

MH-7

B

Fig. 1. Model of mesogenic molecules (A) and proposed packing elements in monomeric (B) and polymeric form (C).

acrylates) at temperatures nearly 100 '~ above Tg [11]. The low value of K is caused by the fact that the "forced" layer arrangement of the side groups falls out the close packing principle which is necessary when forming a thermodynamic stable structure. This appears to account for the absence of a liquid crystalline phase in PMC-7. Even an oriented sample does not exhibit birefringence. The dimensions of ordered regions seem to be much less than the wave length of visible light and are similar to the wave lengths of X-rays. Another feature of the mesomorphic polymers in question distinguishing them from semicrystalline polymers should be noted. It is known [14] that, for semicrystalline polymers, increase in the degree of crystallinity is accompanied by increase in Tg owing to the fact that crystallites hinder the mobility of main chains in the amorphous regions. The reverse correlation is observed in mesomorphic polymers. As follows from Table 2, increase in the entropy of mesophase-isotropic melt transition (ASr) as the criterion of ordering liquid crystalline structure, is attended by decrease in T~ of the poly-

(-)

(/OJ)

+'

,

oo

o o

PMB - 6

mers in the following sequence P M B - 6 > PMH7 > PM-06. Also it is known [15] that polyacrylates are characterized by improved mesomorphic structure as compared with polymethacrylates in spite of the former having lower Tg. This correlation between mesomorphic ordering and Tg seems to be caused by the peculiarities of polymer formation from solutions. As shown previously [8], the formation of liquid crystalline polymer systems on cooling of dilute solutions is due to two consecutive processes. For one thing, there is formation of a secondary structure by an isolated macromolecule as a result of a convolution of the main chain up to a compact conformation at the expense of side group interaction. Also there is the aggregation of macromolecules with the formation of macroscopic associates. Equilibrium liquid crystalline structures, however. are formed not only on cooling of dilute solutions of the polymers but also on precipitating the polymers from the solutions. There are at least two reasons which favour kinetically the formation of mesomorphic polymer systems, and primarily the for-

PM - 0 6

PMH - 7

Fig. 2. Scheme of X-ray diagrams of the polymers.

leo

PMC-7

1130

I.I. KONSTANTINOVet al. Table 2. Thermodynamic and structural parameters of the polymers

Polymer PM-06 PMB-6 PMH-7 PMC-7

Density [kg m-3]10-3 1.174 1.174 1.130 1.037

Packing coefficient 0.684 0.684 0.686 0.635

dt 35.8 37.0 30.0 33.9

d-Spacings (A) d2 d3 17.5 -23.0 11.6 -----

mation of secondary structure (intramolecular ordering). Firstly, it is high equilibrium flexibility o f the main chain [7] and secondly the fact that the formation of the secondary structure is completed before the solution reaches its 0-point [16,17]. The polymer systems formed in this manner are characterized by weak tendency to orientation by stretching (PM-06 is not able to orientate at all) and by high brittleness of films cast from solution. Such behaviour is characteristic of polymers of globular structure [18] in which intramolecular binding occurs mainly by Van der Vaals forces and interdiffusion of segments is practically absent. Hence, in order to explain the features of the liquid crystalline polymers, one can assume the smallest structural element of the polymer system to be a macromolecule with the secondary structure. In this case, the main relaxation transition (glass transition) in the polymers may be caused by local motions of the main chain fragments located at intermolecular boundaries; the mesophase-isotropic melt transition occurs because of destruction of intramolecular ordering. REFERENCES

1. H. Fikelmann, H. Ringsdorf, W. Siol and J. M. Wendorff, Am. chem. Soc. Syrup. Ser. 74, 22 (1978). 2. V. P. Shibaev, S. G. Kostromin, R. V. Tal'rose and N. A. Plat6, Dokl. Akad. Nauk SSSR 295, 1147 (1981). 3. V. S. Grebneva, V. L. Khodzhaeva, M. V. Shishkina, A. A. Sitnov, Y. B. Amerik and I. I. Konstantinov, Advances in Liquid Crystal Research and Applications

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

d4 4.80 4.70 4.80 4.90

Tg (°C) 115 150 135 90

TI (°C) 241 257 208 --

AHt (kj mol-t ) 9.20 3.65 6.47

ASr (J mol-lK -1 ) 18.0 7.2 13.5

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