Pure & App!. Chem., Vol. 57, No. 1 1, pp. 1715—1726, 1985.Printed in Great Britain.© 1985 IUPAC

Cholesteric and other phases in thermotropic liquidcrystalline polymers with side chain mesogenicgroups

N.A.Platé, Ya.S.Preidzon, V.P.Shibaev

Department of Chemistry, Moscow State University, 119899,Moscow, USSR

Abstract A new field of physical chemistry of macromolecu-lar compounds - physical chemistry of synthetic liquid-crys-.talline polymers, particularly, comb-.like polymers with meso-genic side..groups is considered. The classification of meso—phase types of such polymers is given and the dependence ofits structure on the chemical constitution of monomeric unitsis analized. The approaches to creation of liquid crystallinepolymers with definite interval of existance and of mesophasetype are demonstrated. Particular attention is paid to thestructure and the optical properties (selective reflection oflight) of cholesteric polymers. The formation of an intramo-.lecular liquid crystalline structure in solutions of polymerswith mesogenic side groups is discussed.

INTRODUCTION

Thermotropic liquid crystalline (LC) polymers belong to relatively novelclass of iquid crystalline compounds. Pirst attempts to create thermotropicpolymeric liquid crystals have been undertaken some fifteen years ao (ref s.1 8).The study of both thermotropic and lyotropic LO polymers is directly rela.-ted to the solution of practical problems connected with the creation ofpolymeric materials endowed with pre—assigned properties. Thus, for instan-ce, the use of the LO state anisotropy in the processing of polymeric mate-rials proves to be highly promising for the production of ultra-strong high-modulus fibers and films (ref s. 9 — 13).At present, at least three types of thermotropic LC polymers can be conside-red. These are (i) melts of some linear crystallizable polymers; (ii) thepolymers containing the mesogenic groups within the backbone chains, and(iii) the polymers with side chain mesogenic groups (ref. 14).The present paper deals with the LC polymers with mesogenic groups withinthe side branchings of macromolecules. We emphasize the most significantand principal questions relating to some aspects of the synthesis of nema—tic, smectic and cholesteric LC polymers and some features of their struc-ture.

SYNTHESIS OF LIQUID CRYSTALLINE POLYMERS WITH SIDECHAIN MESOGENIC GROUPS

The main pathway to obtain polymers with side chain mesogenic groups invol-ves the synthesis of monomers followed by either their homopolymerizationor copolymerization with other mesogenic or nonmesogenic compouns (Fig. 1(ref a. 14 — 18). An alternative method, that is the addition of moleculesof low molar mass liquid crystals to a polymeric chain by means of polymer—analogous reactions, is used so far at seldom, and quite a few LC polymerswere synthesized by this method (ref a. 19 — 21).The length of the side chain connecting the mesogenic group with the back-bone chain (the spacer) is of extremely high significance for the realiza-tion of the LO state. The point is that the attachment of the mesogenic

groups directly to the backbone chain (with no spacer) not necessarily leadsto the formation of the LC polymer. This is accounted for by the notablesteric hindrance imposed on the packing of mesogenic groups by the backbonechain, and, as a consequence, most polymers turned out to be amorphous. Theattachment of mesogenic groups to the side branching of the so—called comb—like polymers, i.e. the polymers having long aliphatic branchings, allows

1715

1716 N. A. PLATE, VA. S. FREIDZON AND V. P. SHIBAEV

to lower essentially the limitations imposed on the packing of niesogenicgroups by the backbone chain, as compared to the polymers with the mesogenicgroups directly attached to the backbone chain.

a

Pig. 1. Synthesis of LC poly.-÷ mers with mesogenic side

U V groups: (a) - homopolymeri-zation; (b) copolymeriza—

b tion of mesogenic and non—mesoenic monomers; (c) —copolymerization of mesoge—nic monomers; (d) — poly—

+ I mer—analogous reaction;1 — mesogenic groups;2 - main chain;C A,B — functional groups

There are several methods to build the monomer molecules with the mesogenicgroups parted from the reactive double bond by the flexible spacer.The first method is as follows: initially eminocarboxylic acid, which methy—lene chain will now serve as a spacer, is attached to the reactive acrylicor methacrylic group, and then the mesogenic groups (cholesterol fragmentsfor instance) are attached to the terminal carboxyl group (ref. 8):

1) 2_O + NH2—(0H2) —000H —OH2=CHCONH_(CH2)n_COOH

2) CH2=OH..CONH—(CH2) ...000H + Ohol—OHOH2=OH—CO1I}I—(OH2)—OOO.-Chol

OHOH 9H—(CH )

111_OH3 3

where Chol-OH is HO-

There is an alternative method, i.e. initially the spacer is attached to themesogenic group, and then follows the reaction with acrylate or methacrylateto give the monomer (ref. 22):

1) Br-(CH2)n_000H + Chol—OHBr—(CH2)n—000_Chol

2) 0H2=CH—000Na + Br—(CH2)—COO—Chol —CH2=CH—COO—(CH2)—OOO—Chol

In those cases when it is troublesome to carry out the reaction with alreadyavailable mesogenic group, as is, e.g., the case for Schiff bases, the mono-mer can be synthesized as follows. Pirst, the spacer is attached to thefragment of prospective mesogenic group, then to obtain thus far non—mesoge—nic monomer, and finally the required mesogenic monomer is prepared by thereaction with the other fragment of the mesogenic group (ref. 24):

1) HO-(0H2 ).Br + HO_O_OOH HO-(CH2)_O_Q_OOH

Cholesteric phases in thermotropic liquid crystalline polymers 1717

2) CH2=CH.-COC1 + HO—(CH2).O..COH CH2=CH—COO—(CH2)..O_COH

3) CH2=CH—COO—(CH2)—O__COH + NH2..Q-.R-.-..

CH2=CH-COO-(CH2

Up to date, some hundred new LC polymers with various side mesogenic groupshave been synthesized (refs. 14,17).

THE STRUCTURE OF SMECTIC POLYMERS

In the polymers with side rnesogenic groups the spacer provides the suffici—ent autonomy of the backbone chain and side mesogenic groups. If there wasthe total autonomy then the structure of polymer would be completely deter—mined by that of mesogenic groups, i.e. the polymers with the nematogenicor smectogenic groups wou]4produce the nematic or smectic mesophase, respec—tively, whereas the chiral mesogenic groups would lead to the LO polymers ofcholesteric or chiral smectic type. As a matter of fact this is not the case.The total autonomy cannot be achieved, and thus the structure of the polymeris determined not only by its mesogenic groups, but also by the chemicalstructure of the spacer and backbone.Fig. 1 shows that the backbone chain induces the layer ordering in the arra-ngement of mesogenic groups. That is why the most abundant mesophase typefor such polymers is the smectic one. Presently, more than a hundred ofcomb—like polymers are known to form the smectic mesophase (ref s. 14 — 17,23, 25 — 30). Now we consider several types of smectic mesophases which canoccur in polymers.The polymer (1) of the following structure

-CH -OH-2 ooo(CHoO_CHN4D_CIi (1)

forms below 90°C the smectic phase featuring a fan-shaped texture. An X—raypattern of the polymer shows an intensive narrow reflection at wide anglesand a series of low—angle reflections. The orientation results in the arc—shaped splitting of all reflections, with the low—angle reflections beingtransformed into the equatorial and the wide—angle ones into the meridionalarcs (Fig. 2a). Such a character of X-ray patterns indicates the formationof the smectic phase with the ordered layers, in which mesogenic groups arearranged normally to the layer plane (ref. 30). This ordering is typical ofthe smectic S phase for low molar mass liquid crystals (ref. 31). Withinthe temperatu'e range between 90 and 149°c the general character of the ref-lection arrangement is pertained but the wide—angle reflection becomes dif-fuse (Fig. 2b). This fact indicates that the order in layers is destroyed,and, hence, is typical of the smectic 5A phase. Thus, the smectic dimorphism

— S is found in the polymer (1). Th structure of these phases is shownitI Fig. 3.If the butyl group in the mesogenic group of the above polymer is substitu-ted by the nitrile one (the polymer (2)) (ref. 32), or the azomethyne meso—genie group is substituted by the cyanodiphenyl one, leaving unaffected thebackbone acrylate chain and the spacer of 11 methylene units (the polymer(3)) (ref. 28), one arrives at the other type of dimorphism.

_CH2_H_) i—0_O.CH=N_4D_cN (2)

-CII -CII-2aoo_CH211_o_O_O_CN

The high—temperature phase of these polymers is the same as in the polymer(1), while the low—temperature phase has a tilted disordered arrangement ofmesogenic groups within the layers, that is, represents the smectic S phase.The other type of smectic polymorphism is observed for the comb—like poly-mers with acrylic backbone chains and phenylbenzoate mesogenic group. AnX—ray pattern of the polymer (4) (ref. 29) at room temperature shows the

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1720 N. A. PLATE, YA. S. FREIDZON AND V. P. SHIBAEV

phase :Ls observed (ref. 29).

CH2.-gH_ (OH2 )5._COO..._COO_3._OC3H7(7)

...CH2—H-.. OH2 )5_COO_OOC...4..OC3H7(8)

S—-N*.-- -

-CH2—H(OH2) 5...Coo_.OOO..4...OCH3

(9)

i

The polymers with cyanodiphenyl and azomethyne mesogenic groups and the spa—cer of 11 methylene units form, as it was already shown, smectic phases. Ta-king a shorter spacer of 2-6 methylene units (and leaving unaffected themesogenic group and the acrylate backbone) gives rise to the nematic poly—mers (10) and (ii) (ref s. 24, 32).

(OH2)20OQ (10)

(11)

The nematic polymers show the textures which are typical of nematic liquidcrystals, i.e. marbled, schlieren, threaded. The X—ray patterns of nematicphases of polymers show the presence of only one diffuse maximum (Pig. 6a).And besides the usual nematic phase with just orientational order in the ar—rangement of mesogenic groups, we have found in polymers a new type of thenematic ordering (ref. 29), which was not observed in low molar mass liquidcrystals. An X—ray pattern for the polymer (12) of the following structure

_CH2_H_) 5_C00_(O...00O_43.0OH3 (12)

shows one narrow wide-angle reflection with no low—angle reflections (Pig.Gb). Such a character of the X-ray pattern indicates the ordered hexagonalpacking of the side. mesogenic groups with no translational order in the di—rection of their long axes (Pig. 7). This new type of the nematic structuremay be denoted as the nematic N phase, which in the considered polymer pre-cedes the usual nematic NA phas

NB NA- IOne of peculiar features of the smectic and nematic polymers is their beha-viour upon the uniaxial orientation (ref. 29). Under the action of a mecha-nical field on nematic polymers it is the mesogenic groups that are arrangedalong the orientation axis, not the backbone chains, i.e. there occurs aconcerted turning of the LO domains formed by the mesogenic groups (Pigs.Ga and b). In the case of the smectic polymers it is the smectic layers thatare arranged along the orientation axis, while the mesogenic groups are ar-ranged either normally to them (the smectics 5A' 5B' 5E (Pigs. 2a,b and 4))or at some angle (the smectics Si,, St. (Pig. 2e )). The polymers that formboth smectic and nematic mesophases tthe polymer (8)) are readily orientablein the nematic phase, with the mesogenic groups being oriented along the on—

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1722N. A. PLATE, YA. S. FREIDZON AND V. P. SHIBAEV

were synthesized in (refe. 18, 36, 37).All the cholesterol—containing polymers form mesophase within the broad tern-.perature range. X—ray patternsof these polymers in the wide-angle regionshow just a diffuse halo corresponding to a spacing of 0.58 -. 0.62 run. Theuniaxial orientation increases the intensity of the halo in the rneridionaldirection. Istthe low-.angle region a series of intensive reflections is obser-.ved which transform to equatorial arcs upon the uniaxial orientation. Such apattern of the reflections intensity is defined primarily by the normal ar-rangernent of the side groups with respect to the backbone chains. The positi—on of the low—angle reflections depends on the length of the rnethylene spa—cer connecting cholesterol to the backbone chain. This fact indicates theformation of the layered structures with the layer thickness defined by thelength of the side mesogenic groups. The structure of the polymers is shownschematically in Pig. 9.

aPig. 9. Schemes of macromole.-

cular packing of cholesterol-.containing polymers:a) one—layerb) two—layerc) intermediateShaded molecules are withinthe plane parallel to theFigure plane.

Depending on the length of the spacer there can be formed a one layer pack-ing with the mesogenic groups being antiparallelly arranged in such a waythat the cholesterol groups of one macrornolecule are surrounded by the methy-lene chains of the neighboring macromolecules. The short spacers give riseto the two-layer packing with mesogenic groups being arranged in parallels.An intermediate packing in which the aliphatic parts of the cholesterol gro-ups overlap is also possible.As is known a typical feature of the cholesteric liquid crystals is theirability to reflect light selectively and, as a consequence, to display cir-cular dichroism. In fact, for a series of cholesterol—containing polymers inthe TJl1-region there was observed the maximum of reflection and circular di-chroism (ref. 38).Thus, the cholesterol—containing hornopolymers of the acrylic and methacrylicseries form both smectic and cholesteric mesophases, the latter reflects se-lectively the light in the UV—region.In order to obtain the cholesteric polymers having the ability to reflectvisible light the property of the nematic liquid crystals to untwist the he-lix of the cholesteric mesophase is usually employed.Wide opportunities provide copolymerization of cholesteric monomers with ne—matogenic ones (refs. 39, 40). At present, a large number of copolymerswhich reflect selectively visible light has been synthesized on the basis ofnematogenic and cholesterol—containing monomers (ref s. 17, 40, 41).As an example let us consider the copolymers of the nematogenic monomer AM—5with various cholesterol—containing monomers

0H2=0H-.C0O_(CH2)5_C00_O_COO__OCH3AM—5

2=C(R)—C00—(0H2)—C00—0hol where R=H (ChA—n), R=CH3 (OhM—n)

b

Cholesteric phases in thermotropic liquid crystalline polymers 1723

Increasing the fraction of the nernatic monomeric units in copolymers leadsto the shift of the wavelength of the selective reflection to the long—waveregion, which means the untwisting of the cholesteric helix. The formationof the cholesteric mesophase in copolymers can be considered also as the in—dusing of the helical structure in the nematic polymer under the action ofchiral colesterol—containing units. A useful characteristic, which is widelyused in studying optical properties of the induced cholesteric mesophase innemato—cholesteric mixtures, is the helical twisting power (HTP) of a chiraladditive

HTP=xc-it

where X- molar fraction of chiral additive. Taking into account that ArtP(n is the average refractive index, P is the helix pitch) and assumingrteon}t the latter expression can be written as

J :LHTP=Vt "i

i.e. HTP is determined by the slope of ,g as a function of Xe4.Pig. 10 shows the dependence of on Xc4 for a series of cholesteric copo—lymers. As can be seen from this Figure, the copolymers of the same nemato—genic monomer and different cholesterol-containing monomers have the valueof the HTP which is close to that for the mixtures of low molar mass chole—sterics and nematics, namely, cholesteryl propionate and p—buthoxybenzylide-ne-p-buthylaniline (ChP-BBBA).

-Hi

ri—X0 Pig. 10. Dependence ofon the molar frac—

tion of cholestericS — AM-5+ChA-5 component for copoly-

1.5 - AM-5÷ChA-lo mers and for mixturesx' of low molar mass li—

1 00 — AM—5+ChM—1Q quid crystals.C — AM—5+cholesteryl

acrylate

0.5 x - ChP-BBBA///

0.1 0.2 0.3 0.4 0.5

Now let us consider the influence of the temperature on the helix pitch ofthe mesophase of cholesteric copolymers. Por low molar mass cholesterics inthe region far from phase transitions the helix pitch is,as a rule, slightlydependent on the temperature. A sharp dependence on temperature is observednear the point of transition tothe smectic mesophase, provided the latterexists. The copolymers of AM—S and ChA—5 give rise to the cholesteric meso—phase only, and the helix pitch for these copolymers is defined by their com-position and is slightly dependent on temperature within the whole range ofthe mesophase existence (Fig. ii). Weak dependence of the helix pitch on thetemperature is observed also for the copolymers AM—S with ChA—lO and AM—5with ChM—10 for low concentrations of cholesterol—containing units. Increa-sing the concentration of the latter enhances their tendency to form layerstructures, i.e. smectogenety, in consequence the helix pitch in the copoly—mers containing more than 30 mol.% of ChA—lO or CIiIVI-10 units sharply increa-ses with decreasing of temperature (Pig. 11).A significant feature of polymeric systems is their ability to pass to theglassy state. This property allows one to fix the cholesteric mesophase witha definite helix pitch within a glassy film. Since all comb—like cholestericpolymers form the left—hand helix, they reflect the left—hand circular com-ponent and transmit the right-hand one, and thus, they are transformers ofthe plane—polarized light into the circular— polarized one.

1724 N. A. PLATE, VA. S. FREIDZON AND V. P. SHIBAEV

XR, rjfl

700

I • S p s S I 2____________ I —• - 4

I

600 I

500

:z:::::::—:

400 ..s11

20 40 6o 80 100 120 T, OJ

Pig. 11. Temperature dependences of XR for copolymers of AM—5with 20 mol.% of ChA—5 (1) with 28 mol.% of OhA—lO (2)with 39 mci. % of ChA.-1 0 (3 5 , with 22 mci .% of ChM-'l 0 (45and with 33 moi.% of ChM-.10 (5)

THE FORMATION OF INTRAMOLECULAR STRUCTURE INCHOLESTEROL-CONTAINING POLYMERS

The chain structure of macromolecules of the LO polymers leads to the forma-.tion of a principally new type of mesophase -. the intramolecular mesophaseat the level of a single macromolecule. This effect is displayed most clear-.ly in the cholesterol—containing polymers.

—CH2—C(CH3)—

C00—(CH2)—C00—Chol ( PChM—n )

Pig. 12 shows the temperature dependence for the radius of gyration (p2)1/2,intrinsic viscosity ['q] , specific optical rotation [a] and relaxationtime (characterizing the intramolecular mobility) for one of cholesterol.-containing polymers (PChM—10) in heptane solution.It is easily seen from this Figure that sharp change of all the above para-meters takes place2w.in the temperature region 40 — 60°C.The decrease in ( ) and [] result from the formation of compact struc-ture. The latter is substantiated also by the decrease in the intramolecularmobility (the increase in the T ). The internal structure of the globulecan be inferred from the polarimetry data. It can be seen that the [od va-lues for temperatures below 40°C are essentially greater than those resul-ting from the intrinsic molecular activity of the cholesterol group (the da-ta for the monomer, which are unaffected within the whole temperature range,are also listed). High optical activity (roughly of the order as observedfor the polymer at 20°C) is characteristic for liquid. crystals of the choles—teric type. All the results show unambiguously that below 40°C the choleste—nc liquidcrystalline globules are formed.Analysing the relationships shown in Pig. 12 allows to distinguish two sta-ges in the observed transition. At the first stage (60—50°C) there occursce—rtain reduction in size of the molecule and decrease in the intramolecularmobility. The [a] values are almost unaffected. This is the transition "acoil — an isotropically liquid globule", with random arrangement of mesoge-nic groups in the latter. The second stage (below 50°C) is characterized bythe sharp drop of the intramolecular mobility and sharp increase in [a]This fact indicates the transition "isotnopically liquid globule — liquidcrystalline globule of the cholesteric type" with the mesogenic groups pac—ked,in a dense helical structure.The mechanism of formation of globule with the intramolecular cholestericstructure is clearly manifested in studying the fractions of the polymerwith different molar mass.

Cholesteric phases in thermotropic liquid crystalline polymers 1725

Fig. 12. Temperature dependences for (p2)1/2 (1, [] (2),[a] (3) and T (4) for PChM—10 in heptane solution.

Dotted line [a] for monomer ChM—10.

For the high—molecular sample (1 =6.6.106_ the molar mass is unaffected within the whole temperature range, !.e. the LC globules consist of a single ma-cromolecules (Pig. 13). The formaton of the LO globule for the macromolecu-les of molar mass less than 1.510 is accompanied by the effective growthof the molar mass, i.e. the association of macromolecules takes place. Theseresults show that the formation of the intramolecular mesophase requires adefinite critical content of mesogenic groups, which for the polymer consi-dered amounts to —3000.

M /M°w w

5

20 40 60 T,°C

Fig. 13. Temperature dependence forchanges in PChIVI-10 molar mass inheptane solution (M/M°) for frc-tion M°=0.351O (1); w O.73.lob (2)

1.510 (3); 6.6.106 (4)

Thus wide variety of types of structural arrangements in thermotropic LC po-lymers and possibility to change this structure give chance to regulate pro-perties of these very promissing and interesting polymeric materials.

3

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