Liquid crystalline polymers

LIQUID CRYSTALLINE POLYMERS* V. P. SHIBAYEV and N. A. PLATE M. V. Lomonosov State University, Moscow (Received 29 October 1976)

Methods for production of high molecular compounds in the liquid crystalline (LC) state are discussed, including crystalline polymer melts, solutions of rigid chain polymers, block copolymer gels and polymers with side groups simulating the structure of liquid crystalline molecules. Examples of the realization of LC orientational order in melts and solutions of certain polymers are given, which clearly demonstrate the role of preliminary ordering of the maeromoleeules for the purpose of improving the physicomechanical properties of plastics. The results of work on preparation of polymers with mesogenic side groups are systematized and critically discussed. Methods are discussed for formation of thermotropic, LC polymers, which must be regarded as a special type of highly ordered, polymeric systems with spontaneously arising anisotropy of properties, occupying an intermediate position between amorphous and crystalline polymers.

IN RECENT years the attention of research workers in the field of the physical chemistry of high molecular compounds has been drawn to the problem of creation of polymeric, liquid crystalline (LC) systems [1-13]. The great interest in such systems is obviously brought about by advances in the study of the structure, properties and practical application of low molecular liquid crystals in various branches of technology and medicine [14-17], as well as of the nature and specific features of t h e liquid crystalline state as a special state of macromolecular substances. Progress in this field of investigation is inevitably associated with the development of ways of producing polymeric LC systems and of controlling structure formation processes in polymers for the purpose of producing plastics with a desired combination of physicomechanical properties. Unfortunately, not only is no systemization of experimental data on polymeric liquid crystals available in the literature, not even criteria indicating the very fact of the existence of the LC state in polymers, have been worked out. In the present paper an a t t e m p t is made t o fill this gap. The authors have not tried to generalize all the material in the literature on polymeric LC systems. * Vysokomol. soyed. AI9: No. 5, 923-972, 1977. 1065

1066

V. P . SHIBAYEV a n d N. A. PLATE

I n this review only some, and what we feel to be the most fundamental, of the problems relating to the possibility of attaining the LC state in polymers are discussed, and some rational approaches to production of polymeric LC systems are mentioned. SOME GENERAL INFORMATION ON THE CHEMICAL AND PHYSICAL STRUCTURE OF LOW MOLECULAR LIQUID CRYSTALS

Liquid crystals were discovered about ninety years ago. They were first detected by the Austrian botanist Reinitzer [18], by his observation t h a t eholesteryl benzoate has two melting points, in the regions of 145 ° and 179 °. At 145 ° a

Z

II

~

..

FIc. 1. Three types of thermotropic liquid crystals: a--smeetie with ordered ( I ) - m i d disordered (II) arrangement of the molecules in layers; b - - n e m a t i e and c--eholesterie.

the Solid, crystalline substance changed to a turbid, anisotropic liquid, which at 179 ° became transparent and isotropic. The state of cholesteryl benzoate in the temperature region of 145-179 ° was called liquid crystalline, and sub-

Liquid crystalline polymers

1067

TABLE l. SOME LOW MOLECULAR LIQUID CRYSTALLINE COMPOUNDS [19--23] S u b s t a n c e a n d its s t r u c t u r a l f o r m u l a

Phase transition temperatures*

Nonan-2,4-dienic acid 23

C

H,Ca--CH2--CH= CH--CH~ CH--C00H

53"5

+N

_>I

n - O c t y l o x y b e n z o i e acid n.CsH~,--0--~C00H

C

100.7 ÷ S - - - 107.5 -~N

4,4-Azodiphenetol

C

160"2 _>I

\

\

/

- - _147.3 __

+I

/

\~/156-1 N

p-Azoxyanisole

} ~ C - - O ~ N ~ ~--~z~/O--C~ 0 n-Heptyloxyazoxybenzene

n-C,H,,--O--<~/__~-N= N--C~--O--C,H,, 0

c

116

~N

C

73"1

÷ S

~136

95'1

+I

~N

123.9

~I

4 - C e t o x y - 3 - n i t r o - 4 ' - d i p h e n y l e a r b o x y l i e acid n C1.H88 O~Z/~

C / ~~

COOK

C--126"8 S: 1 7 1 ' 0 S, 197"2~.S, 201"9_> I

NO~ Di-p-methoxyphenyl terephthalate

t

c

205

c

115

C

102

C

70

~N

277" + I

o o 2-p-n-Decyloxybenzylideneaminofluorene ->S

170.5 ~N

181

+ I

85

-~I

CH~ Cholesteryl propionate H.C / \ 1

CH,

CH8 I CH--(CHD,--CH

~,c_cR._c_o_~/@ / v o~,

CH

116

c~8

Cholestcryl myristate

o

/\

If

!

/ II

CH~

CH8

AV L\) I / CH,

CH.

i

I

~S

80 ---2 CH

~ , c - ( c H , ) , , - c - o - % / %2

*C--represents the crystalline state.N,S, and CH the nematie, smectic and cholesteric types of m e s o p h a s e respectively, I an isotropic melt and S~, S~, S. . . . . . various modifications of the smectic phase. stances terval) form

capable

above

of simultaneously droplets)

and

the

melting

point

combining

the

of crystalline

bodies

Tm (over properties

(namely

a certain of

temperature

liquids (flow, ability

anisotropy),

into

were called liquid

1068

V . P . SHIBAYEV and 1~. A. PLATE

crystals. Despite the unusual combination of these two words this designation has acquired the right of existence alongside such terms as "mesophase", "mesomorphic substance" and "mesomorphic state". Some tens of thousands of organic substances that can exist in the LC state are known and the number of such compounds is continuously increasing. The main structural characteristic of LC compounds is the anisodiametrie, rod-like or plate-like form of the molecules. 17

~trt~rtr~rtm~trrt~t~f~!t¢ ~,~

b

U

,~

"~\~

FIG. 2. Some types ~of lyotropic LG structures: a - - l a m e l l a r : b--cylindrical a n d c--spherical.

Table 1 lists some of the most characteristic representatives of organic compounds capable of forming, within a certain range of temperatures, an LC phase. These are mainly aromatic organic compounds with one or more benzene rings containing a number of substituents. This elongated shape of the molecules.is the factor that, giving rise to anisotropic polarizability, establishes a tendency to arrangement of the molecules predominantly parallel to one another, which is perhaps the specific characteristic of low molecular liquid crystals. According to the classification proposed by Friedel [24] three main types of LC compounds can be distinguished, depending on the arrangement of the molecules, namely smectic, nematie and eholesterie (Pig. 1). Smectic liquid crystals are closest to truly crystalline bodies. The molecules are arranged in an ordered (Fig. la, I) or disordered (Fig. la, II) manner in layers and their centres of gravity can move in two dimensions (on the smectic plane). The long axes of the molecules in each layer can be arranged both perpendicularly to the plane of the layer and at some angle. This provides the possibility of the existence of various polymorphic modifications within the limits of the smectic type of mesophase. The nematic form is characterized by orientation of the long axes of the molecules along a certain direction, with a disordered arrangement of their centres of gravity (Fig. lb). The cholesteric type of ordering of the molecules is the most complex and is

Liquid crystalline polymers

1069

produced as a rule in derivatives of cholesterol. The molecules are assembled in layers in which their arrangement is like the arrangement in a nematic phase, but each layer is turned relatively to the previous one through a certain angle, so t h a t over all there is a screw-like twist of the molecules, forming a helix with a pitch Z (Fig. lc). These types of mesophase are classed as the so-called thermotropic liquid crystals, which are formed only by thermal action on the substance (heating or cooling). Another type of mesophase is that of lyotropic liquid crystals, which is formed when some compounds are dissolved in certain solvents. The structure of these liquid crystals is more complex than that of the thermotropic type. In these the structural units are not molecules by molecular complexes, which are arranged in the solvent medium and can have, for example, a lamellar, cylindrical or spherical shape (Fig. 2). I t must be pointed out t h a t the above forms of both thermotropic and lyotropic liquid crystals are only boundary cases of ordered molecules. At the present time within the limits of these forms a large number of polymorphic modifications have been found and some of these can be identified fairly precisely [25]. Despite the fact that the above structural features of liquid crystals, bringing about a combination of orientational order with high lability of the molecules, were known several decades ago, and that by the 1940-1950' s much experimental evidence had accumulated, most researches saw nothing more t h a n theoretical interest in study of the LC state. The position changed sharply at the beginning of the 1960' s, when the unique properties of low molecular LC compounds (mainly thermotropic), due to the high anisotropy and lability of the molecular structure, began to find extensive areas of practical application. The ability of LC compounds to "respond" rapidly to change in temperature, mechanical stress, electromagnetic radiation and even chemical environment, is now widely used for construction of heat detectors and special devices for picking up information in electronics and electro-optics. Use of liquid crystals has begun in medicine, as heat indicators, for diagnosis of a number of vascular diseases and of various tumours and surface neoplasms. Detailed examination of the possibility of practical use of low molecular, liquid crystals can be found in references [14-16]. It can now be said without exaggeration that a number of branches of technical physics would be unthinkable without the use of liquid crystals. The practical uses liquid crystals already found, in turn are stimulating a vigorous surge of scientific investigations. The rate of increase in the number of these investigations is indicated by the figures presented below on the number of scientific papers and patent specifications published in this field (provided by t h e English information service "Locus").

1070

V.P. SHIBAYEVand N. A. PLATE

Year

Number of publications

1968

1969

1970

1971

1972

1973

180

300

330

500

600

700

From 1970-1972 investigations of liquid crystals began to embrace the field high molecular compounds. Papers were published in which various approaches to production of thermotropic, polymeric LC systems are developed, and the terms "liquid crystalline" and "mesomorphic" are entering generally into the lexicon of polymer chemists.* Far from always however are the polymer systems studied called liquid crystalline b y the authors, or realization of the special, liquid crystalline state in them established with sufficient basis. In the present paper we shall use the term liquid crystalline to describe a thermodynamically stable phase state of polymers or polymeric systems, characterized b y spontaneous appearance, regardless of the state of aggregation, of anisotropy of properties (especially optical anisotropy), without the existence of a three-dimensional crystal lattice. At the same time it must be pointed out that in addition to this definition, which describes the LC state as a phase state, the term "LC structure" is often used to indicate only that orientational order exists in the system. Despite the less precise meaning of the latter definition, the concept of LC structure (or order) is widely used in the field of research into polymer structure. We shall a t t e m p t in the rest of this review to indicate wherein ¢he differences and similarities between these terms lie. At the present time it is evident that four types of polymer systems can be distinguished, about which one can speak of attainment of the LC or mesomorphie state, where occurrence LC orientational order is possible. These are, melts of crystalline polymers and amorphous polymers, lyotropic LC systems, mesomorphic structures of block copolymers in gels and polymers with anisodiametrie side groups of structure similar to that of the molecules of low molecular liquid crystals. In this paper the last of these types of LC polymers will be discussed in greatest detail. With regard to the other three systems, in a brief examination of these greatest attention will be given to showing the possibility of their practical use. ORIENTATIONAL LC ORDER IN AMORPHOUS POLYMERS AND MELTS OF CRYSTALLINE POLYMERS

The display of anisotropie properties b y low molecular LC compounds is a result of a definite mutual ocientation of anisodiametric molecules, which, re* We must emphasize once again that here we are speaking mainly of research in field of thermotropic liquid crystals, because lyotropic LC systems, based for example on natural and synthetic polypeptides, have now been studied for a fairly long time, beginning with papers by Robinson and his collaborators [26, 27]. Extensive work on lyotropic LG, polymeric systems based on aromatic polyamides, was published recently by Papkov [13].

Liquid crystalline polymers

1071

gardless of the type of mesophase, are predominantly arranged parallel to one another in individual regions, called domains or realms, containing, according to different authors, from 10~ to 106 molecules. It would seem that the presence of long chains of highly asymmetric molecules should predetermine the possibility of realization of the LC state in any polymeric substance. In practice, however, this is far from being the ease. That the macromoleeules should be anisodiametric is only a necessary, but not sufficient condition for ensuring formation of an LC phase. Investigation of the structure of amorphous polymers in many instances gives a definite indication of the presence of fairly extensive regions with parallel aggregation of the macromolecules, whether or not a bundle [28, 29] or domain [30-32] structure is attained. The spontaneous arising of orientational order, which is preserved even under conditions where segmental motion is possible (in the high elastic and viscous flow states for example), still does not give grounds for stating that the LC state exists in the polymer. The size of the regions of spontaneous ordering of segments of the polymer molecules in amorphous polymers is too small for the polymer to display anisotropic properties (in the absence of an orientating field), which is a necessary condition for them to be classed as an LC phase. Nevertheless the authors of a number of papers [33-35], on the basis for example of electron diffraction investigation of melts of a number of crystalline polymers (polyethylene, polytrifluorochloroethylene, poly(ethylene sebacate)), having detected the presence of aggregates (regions) of macromolecules with parallel packing of polymer chain segments, write of an analogy between such a structure and nematic liquid crystals. Within these regions (of dimensions up to 25 A) long-range order in the orientation of the molecules is preserved, whereas there is no long range order in the arrangement of their eentres of gravity in projection on a plane perpendicular to the axes of the molecules. In the opinion of these authors the great length of the chains and the spread of molecular weights here exclude the possibility of formation of a smectic modification. Liquid crystalline order in a melt can be preserved in the solid polymer by rapid cooling (chilling) of the melt, as was found in reference [36] in chilling of isotactic poylpropylene. Here the authors observed formation of a partially ordered PP structure, where the polymer molecules, having a 31 helical conformation, are arranged parallel to one another, with azimuthal disorder in the plane perpendicular to the axes of the macromoleeules. This type of structure, called smectic by these authors, is obviously an example of the packing characteristic of the gas crystalline state [37, 38], which is a modification of the LC state. The authors of the above papers, however, in drawing an analogy between polymer melts and low molecular liquid crystals, fixed their attention only on the similarity of the structural, orientational ordering of these compounds, without touching upon the question of their phase state. However, this structural evidence obtained confirmation in the work of Smit [39], who, while studying the temperature dependence of the specific heat capacity C v of polyethylene

1072

V. P. SHIBAYEV and N. A. P ~ A ~

and isotactie polypropylene, discovered some reversible changes in the molten polymers, at temperatures some 50-100 ° above the melting point. From a comparison of the value of ACp with some corresponding values for LC compounds of the type of higher aliphatic acids, Smit made the direct statement that several types of smectic modifications can occur in both PE and PP. The value of AC~between a supercooled, isotropic PP melt and one of its smectic modifications is 0.12 cal/g, which is in fact close to A C ~ 0 . 1 1 cal./g for the typical LC compound p-azoxyphenetole on passing from its smectic modification to an isotropic liquid. Comparison of the transition temperatures of PE and PP, found in this work, with corresponding known transition temperatures for salts of fatty acids, made it possible to determine the number of monomer units included in the ordered regions of the two polymers. In Smit's opinion these regions consist of lamellae of thickness 20-50 A and contain six and nine monomer units of P P and PE respectively. It is interesting to note that IR spectroscopy has revealed the existence in molten PP of molecular segments in a helical conformation, containing at least five monomer units [40]. The formation of the so-called high pressure modification of PE during its crystallization or annealing under a pressure above 2 tonnes/cm2 [41], and existing in the temperature range of 260-290 ° (260° being the temperature of the phase transition of PE under pressure from the rhombic to the hexagonal form, and 290 ° the melting point of the hexagonal form), can obviously also be regarded as attainment of LC order. In a review devoted to a detailed examination of the results of investigation of the structure of PE with straightened out chains, Bassett also mentioned the possibility of drawing an analogy between the hexagonal form of PE and the structure of liquid crystals [42]. The suggestion that an LC phase can exist in molten polydiethylsiloxane (PDES) was made recently in reference [43]. From the results of X-radiographic, optical, thermographic and NMR studies, the authors reached the conclusion that there is a "partially ordered phase" in PDES melts at temperatures from --5 ° to ~-16 °, which is similar to the LC phase of low molecular compounds. In this temperature region the polymer possesses optical anisotropy and its diffractogram contains a single, sharp maximum, corresponding to the distance between the PDES chains (8.7 A). The transition to an isotropic melt produces a small endothermie peak in the thermogram and a sharp fall in the intensity of X-ray scattering (Fig. 3). The existence of a mesomorphic structure in two, isomeric, hetero-organic polymers of closely similar structure, namely poly-m- and p-(bis-chlorophenoxy) phosphazenes [(//~/~--O)2PN]n and[(Cl--~/~--O)2PN]n was detected in reference [44]. C1 Thus examination of the above papers indicates fairly clearly that it is possible for both an LC structure and the LC state to exist in molten polymers, despite the fact that the authors often differ with respect to identification of the type of LC modification present.

Liquid crystalline polymers

1073~

The papers m e n t i o n e d dealt with polymers with comparatively flexible chains, such as PE, P P and polydiethylsiloxane. With regard to rigid chain polymers, which have considerably higher melting points, no evidence is yet available to indicate the possibility o f formation of an LC structure in these. Nevertheless, it can obviously be asserted (and we shall prove this later) t h a t in polymers in which there is a combination of strongly inter£cting polar groups and long hydrocarbon segments, the tendency to form an LC structure and to display LC properties should be exhibited to the highest degree. Only high melting points t h a t are close to the decomposition temperatures restrict or completely suppress the interval of existence of a mesophase in molten, rigid chain polymers. This probably reduces considerably the range of polymers potentially capable of attaining the LC state. I

A

J

la

~

l

2a

I

30 Z~ °

FIG. 3. Diffractograms of partially ordered (LC) (1) and iso~ropie phase (2) (,f polyethylsiloxane at 15° and 30° respectively [43]. The possibility of existence and detection of LC order in polymers in the solid state is demonstrated by the results of determination of the so-called p~cking coefficient K, defined as the ratio of the intrinsic volume of all the atoms in a structural unit to the complete or true volume of the unit. The values of K actually found for synthetic, crystalline polymers lie within the range of 0.69-0.75. Polyethylene and isotactic P P after conversion to the amorphous state have K~0.62. For mesomorphic structu:'~s with hexagonal packing of the macromolecules (PVC in the gas crystalline state, smectic P P and 7-irradiated P E for example) K has values within the range of 0.65-0.67 [45]. It is quite possible that the observed values of K are indications of LC orientational order, existing in these polymers in the molten state and then fixed in the solid phase. The possibility of producing LC orientational order in a melt, under the action of a mechanical field for example (which is in practice equivalent to formation of a nematic phase in the melt) can be used for production of polymer fibres::

4074

V. P. SHIBAYEVand N. A. PLATE TABLE 2. PHYSICOMECHANICAL PROPERTIES OF P E SAMPLES [48]

Property

Standard PE sample 130

~/Tm, ° C

Breaking strength, kg/mm~ Elasticity modulus,

3

60

PE obtained by orienrational crystallization 142 30.5 417

kg/mm ~

Extension at break, ~o

1000

a n d films of high strength. This was shown most clearly in references [47] and [48], by the example of formation of films from previously orientated, molten PE. The drawing process stimulates formation of a nematic phase in the melt, from which orientational crystallization of the PE film is accomplished. As is seen from Table 2 the properties of the film differ substantially from the properties of standard PE. These results provide a clear example ~of the effect of previous ordering of macromolecules, produced in molten polymers by formation of LC orientational order, on the physicomechanical properties of polymers. Thus a future subject for research consists above all in finding conditions to promote LC order in already known polymers, for the purpose of improving their mechanical characteristics. In fact, because of the chain structure of macromolecules, a l m o s t any crystallizable polymer above its melting point should be capable of forming LC orientational order to some degree of perfection or another. The effect of previous ordering of polymer molecules in highly anisotropic melts, in producing improved physicomechamcal characteristics of polymers was demonstrated quite recently in the case of copolyesters obtained by acidolysis of poly(ethylene terephthalate) by p-acetoxybenzoie acid [49]. Unfortunately however, though the authors called these polymers liquid crystalline, they did not forward any evidence to confirm their LC nature, promising to do this in subsequent publications. Location of the temperature interval of existence of a mesophase in molten polymers, creation of conditions promoting maximal emergence of LC order in micro-regions of large dimensions (by application of mechanical, electrical or other fields) --all this is a way of producing special, structural modification of polymers, aimed at increasing their strength. LYOTROPIC LIQUID-CRYSTALLINE SYSTEMS

The examples dicussed above of organization of macromolecules in an LC ~system in the melt, concerned flexible chain polymers, mainly polyolefins. With regard to rigid chain polymers the nearness of their Tm to the temperature of chemical decomposition prevents attainment of the LC state in bulk. However, in view of the high tendency of these polymers to undergo spontaneous organization, they display LC properties when dissolved in certain organic solvents,

Liquid crystalline polymers

1076

giving lyotropic liquid crystals. Tlle formation of a mesophase in polymer solutions was discovered in 1950 by Elliot and Ambrose [50] and was then studied in detail by Robinson [26, 27, 51, 52] and by Miller and his collaborators [53]. Formation of lyotropic LC systems is characteristic of natural (proteins, cell membranes) and synthetic (polypeptides, aromatic polyamides) polymers, where in a limited range of temperatures and concentrations an LC phase arises spontaneously, as a rule with parallel orientation of the rod-like molecules. The physical meaning and definition of the critical concentration ccr, corresponding to separation of an isotropie polymer solution into isotropie and anisotropic phases, are given in references [54] and [55]. Presumably this "breakdown" is a result of the impossibility of packing macromolecules of low flexibility in a disordered fashion in a restricted space. The critical concentration % is related to the degree of anisodiametry of the macromolecules P in'the following way: Ccr= 8/P ( 1-- 2P). Since P is the ratio of the length of the molecule to its diameter and is proportional to the degree of polymerization, when P is large % is inversely proportional to the molecular weight. When P=200, for example, Cc~ is 5.7% by weight. Experimental studies of the structure and phase separation of polypeptide solutions [26, 27] have confirmed the correctness of this equation and enabled a detailed picture to be put forward of the stages of formation and breakdown of LC structure in solutions of poly-(?-benzyl glutamate) [56-59]. The following is the most important example of the practical use of lyotropie systems. By making use of a previously created nematic structure in solutions of rigid chain polyamides (poly-p-benzamide (PBA) in DMAA and poly-p-phenyleneter(Ththalamide in DMAA), fibres were formed from anisotropic solutions in reference [60]. The strength of the fibre formed from an anisotropic solution of I)BA (120 gs/tex) is four times that of fibre formed from an isotropic solution (30 gs/tex). This is an exceptionally clear and vivid example of the use of the special properties of the LC structure of a two-component system realized in practice. Thus the basis for production of highly orientated polymeric structures, both from a nematic melt (in the case of PE) and an anisotropic solution (as of rigid chain polyamides), is the use of LC orientational order, which under suitable conditions is characteristic to some degree or another of any high molecular compound. It is therefore profitable to consider the results of investigation of the structure of molten polymers and of polymer solutions from the point of view of the possibility of formation of LC order with the aim of making practical use of the special properties of the LC structural organization of this phase. MESOiMORPHIC STRUCTURES OF COPOLYMERS IN GELS

Block copolymers consisting of blocks with markedly different properties, such as polystyrene-poly(ethylene oxide) [61], polystyrene-polyisoprene [62], ~polyisoprene-poly-4-vinylpyridine [63, 64], polybutadiene-polystyrene-pol~bu-

1076

V. P. S m B A Y E V

a n d N. A. P L A T E

¢adiene [62, 65] and others [66, 67], form a special type of LC system. Systems obtained by anionic polymerization ~aave been studied in detail mainly by French authors, who have developed methods of high precision for control of the coma 0 "/o

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position, isolation and identification of block copolymers. By selective precipitation or crystallization from solution of one type of block of the block copolymer molecule and keeping the second type of block in solution, so-called mesomorphie gels are obtained. Electron microscopical and low-angle X-ray scattering studies show that these gels have a regular, periodic structure with a cubic, hexagonal or lamellar type of packing of the different structural elements, consisting of aggregates of blocks of the same chemical nature. Figure 4 shows electron photomicrographs (a, c) and schematic diagrams (b, d) of mesomorphie gels of polybutadiene-polystyrene-polybutadiene (PB-PS-PB) triple block copolymers, swollen in partially polymerizcd methyl methacrylate. These diagrams show the

Liquid crystalline polymers

1077

periodic structure, consisting of layers (Fig. 4a) and cylinders (Fig. 4b) of aggregated PB blocks, uniformly separated from one another by "swollen blocks" of PS [65]. Such gels possess birefringence that is easily observed under a polarizing microscope, and X-ray analysis shows that they display low angle periodicity. Block copolymer gels are essentially micro-heterogeneous systems, Where polymeric segments of the same chemical nature are "sorted out" in micro-regions measuring some hundreds of angstrSms. Their classification as lyotropic liquid crystals is explained mainly by the similarity of the structural formations in the block copolymers to low molecular liquid crystals. The structure of block copolymers, the macromolecules of which include in their composition chain segments of different structure and flexibility, undoubtedly warrants attention, because it enables planned "design" of polymer molecules bearing the fragments necessary for display of LC properties, to be achieved. Moreover by carrying out within a mesomorphic gel, polymerization of any monomer (styrene or methyl methacrylate for example), it is possible to fix the original LC structure of the original block copolymer in "a solid body. In the opinion of Sadron, one of the authors of papers dealing with this type of work, it is thereby possible to obtain polymeric systems with entirely new optical, electrical and other properties [68]. Unfortunately there is as yet no information on this in the literature and one can only hope that future development of the work indicated in this section will produce more significant, practical results. POLYMERS WITH ANISODIAMETRIC SIDE GROUPS, OF STRUCTURE SIMILAR TO THAT OF THE MOLECUI.ES OF LOW-MOLECULAR LIQUID CRYSTALS

The examples discussed above demonstrate how, by making use of LC orienrational order previously produced in melts or solutions of linear polymers, it is possible to exert a controlled effect on some physicomechanical characteristics of polymeric materials. Evidently however this is only one of the possible manifestations of the LC state in linear polymers. In addition to this one can imagine high molecular compounds where the macromolecules are chemically bound to low molecular substances capable of existing in the LC state. It would seem that in such systems it is possible to combine the "polymeric qualities" of high molecular compounds (with their ability to form films and fibres), with the properties of low molecular LC compounds, which could lead to creation of qualitatively new polymeric materials. The principle on which preparation of this type of polymer is based is either that of synthesis of monomers bearing "LC groups", with subsequent polymerization of these monomers, or of addition of molecules of low molecular LC compounds to a previously formed polymer by chemical reaction on the polymer. Among the fairly large number of papers on this subject only the first of these methods is of any importance at present.

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Table 3 lists the main types of monomers synthesized up to the present time, capable either of themselves existing in the LC state, or bearing "liquid crystalline" (mesogenic) groups. It should be mentioned that most of the papers concerned deal with methods of preparation of the polymers and the kinetics of polymerization. In the present paper we shall not undertake a discussion of specific details of the po]ymerization of these monomers, which is itself of independent interest and has been analysed in detail in papers b y Krentsel, Amerik and their collaborators [3, 69]. In what follows below we shall p a y most attention to the structure and some properties of the polymers, which are called liquid crystalline, not always with good foundation. Generally speaking this hampers demonstration of the specific features and the role of the LC state in the polymeric compounds. It is because of this that Table 3, in addition to LC monomers, contains monomers that do not form a mesophase, b u t from which polymers stated to be liquid crystalline have been obtair~ed. Even though many of these polymers have not been investigated, we have included the corresponding monomers in the Table, mainly for the purpose of showing that the range of mesogenie monomers is considerably wider than the number of such polymers already obtained and investigated. I t is possible that this fact will a t t r a c t more steadfast attention o f research workers to study of the properties of such polymers. The principle on which preparation of the LC polymers listed in the Table is based, includes synthesis of LC monomeric compounds capable of existing in one or more LC modifications. Then after polymerization of the liquid crystal in a suitable form or forms (in bulk or in solution) the authors of most of the papers compare the properties of the polymers obtained, thus bringing out the effect of the nature of the preliminary ordering of the monomer on the eonfigurational structure and conformation of the maeromolecules, as well as on the supermolecular structure of the polymer. We shall first examine the results of study of the structure of polymers formed b y polymerization of monomers capable of existing :in nematic or smectie phases:

Polyraers with side groups simulating the nematic or smectic types of liquid crystals. !]?he first LC monomer to be studied was vinyl oleate (VO), which forms a nematic mesophase in the temperature range of --32 ° to --18 ° [70, 71]. When VO is po]ymerized in bulk a transparent polymer is produced, in the form of a viscous liquid, whereas the authors of the above papers concluded that the polymer obtained by polymerization in the LC phase or in the solid state, is crystalline and in external appearance is reminiscent of a fat, melting in the range of 34-38 °. Despite the fact that in the papers cited above [70, 71] no experimental evidence was p u t forward to indicate the LC state of poly(vinyl oleate), in later papers of the review type [3, 69] the same authors call this polymer liquid crystalline. A t t e m p t s to obtain LC poly(cetyl vinyl ether) also proved unsuccessful and in spite of the ability of VE-16 (Table 3, monomer 2) to exist in a smedtie form,

1092

V.P. SHIBAYEV and N. A. PLATE

the comb-like PVE-16 crystallized on account of packing of the side chains, b u t became converted to the isotropic state above its melting point [72]. In a series of papers b y Krentsel', Amerik, Constantinov and their collaborators [3, 69, 73, 74], devoted mainly to study of the kinetics of polymerization of a number of methacrylic derivatives, especially methacrylyloxyphenyl esters I q

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Fro. 5. Wide angle (a) and small angle (b) diffractograms for poly-p-acryloxybenzylidenep-ethoxyaniline (Table 3, polymer 8.2): /--polymer obtained by anionic polymerization in solution at 20°; 2--the same but at --78°; 3--the same at --78 ° and after annealing at 180°; 4--polymer obtained by polymerization of monomer in nematic phase; 5--nematic phase of monomer [86] ofp-n-alkoxybenzoic acid (MPEBs) (Table 3, monomers 3), and also in papers by Svetkov and his collaborators in collaboration with the above-mentioned authors [75-77], on the hydrodynamic, dynamo-optical and electro-optical properties o f these polymers, the role of the length and type of interaction of the side branches in attainment of LC orientational order of the polymer molecules in solution is discussed in detail. This t y p e of polymer, the melecules of which have been called crystal-like [78], are shown b y study of their solutions to have anomalously high segmental anisotropy and dipole moments, which gave the authors grounds for speaking about attainment of orientational order of the mesomorphic type, the side chains of the molecules forming a labile LC structure of the nematic type [75-78]. The conformational state and properties of solutions of polymers of the M P E B series (Table 3) have been worked out in a series of experimental and review papers b y Tsvetkov and his collaborators [75-78], b u t there would be undoubted interest in establishment of certain correlations between the unique properties of these polymers in solution and their structure in the solid phase. With regard to the structure of this series of polymers in bulk, despite their amorphous nature low angle X-ray analysis shows that they have single layer packing of the side branches, indicating rather that there is some analogy between their structure and the structure of low molecular liquid crystals of the smeetie t y p e [79]. Differential thermal analysis of the polymethacrylyloxyphenyl ester~ of nonyl- and cetyloxybenzoie acids (Table 3, polymers 3.2 and 3.3) shows thab

Liquid crystalline polymers

1093

t h e y give sharp melting peaks in the thermograms at 218 ° and 230 ° respectively [180] (in spite of their amorphous structure). An approximate estimation of the heats of melting of these polymers gave the value 2-4 cal/g (1.0-1.5 kcal/mole) [80], which however is higher than the usual values of the heat of transition from a mesophase to all isotropic melt [93]. Unfortunately, apart from a statement that one of them, the polymethacryloxyphenyl ester of cetyloxybenzoic acid (Table 3, polymer 3.3), is capable of displaying birefringenee in the viscous flow state, in the temperature range of 160-225 ° [81], no other information about the structure of this polymer is available. A wide range of monomers capable of forming mesophases has been synthesized by introducing groupings of the Schiff's base type into side chains of vinyl and acrylic compounds (Table 3, monomers 4-12). The first communication of Paleos and his collaborators [82] was devoted mainly to description of the preparation of such monomers and to discussion of the kinetics of their polymerization (Table 3, monomers 4.1-4.3). In later investigations of German [7, 86, 87], French [4, 84, 85, 88-90] and American [6, 12, 91, 92] authors, main attention has been turned to study of the structure of the corresponding polymers. Ringsdorf and his collaborators [7, 86, 87], after synthesizing a large number of LC monomers based on aeryloxy- and methacryloxybenzylidene-p-alkyl(and p-alkoxy) aniline, made a fairly detailed study of the structure of the two polymers from acryloxy- and methacryloxybenzylidene-p-ethoxyaniline (Table 3, polymers 8.2 and ll.2) [86]. By polymerizing monomers 8-2 and l l.2 in solution and as isotropic and anisotropic melts, the authors discovered a substantial difference in the structure of the polymers formed, as indicated by low angle, X-ray scattering measurements. Despite the occurrence of a fairly broad amorphous halo in the largeangle region (regardless of the polymerization method) (Fig. 5a), the polymers prepared by polymerization in the molten state give a sharp, intense, low angle reflection, as is seen from Fig. 5b) (curve 4), whereas the nematie phase of the monomer and the polymers obtained by polymerization in solution give almost no low angle reflections (Fig. 5b), curves 1, 2 and 5). Since monomer 8.2 (Table 3) gives no low angle reflection, in the opinion of the authors polymerization in the melt results in folmation of a smectie phase of the polymers, with formation of the layer structure sho~n in Fig. 6, which is reminiscent of the type of packing of the macromolecules proposed by Plate and Shibayev Ibr crystallizable polymers of comb-like structure [94]. The polymers of acryloxy- and methacryloxybenzylidene-p-ethoxyaniline obtained by polymerization in solution do not give low angle reflections (Fig. 5b, curves I an([ 2), but after they have been annealed at 180 ° (the softening point of the polymers) a sharp reflection appears in the low angle scattering region (Fig. 5b, curve 3), and both the acrylic and methacrylic polymers become optically anisotropic. When the methacrylic polymer is heated further it enters the viscous flow state at 230 ° and at 308-310 ° the anisotropy disappears completely. The authors state that this polymer exhibits the

V. P . SItIBAYEV a n d N. A. PLATE

1094

properties of an. enantiotropic liquid crystal, maintaining its LC structure and anisotropic properties when cooled (i.e. undergoes transition to the glassy state). Although the authors report the fact the polymers obtained by polymerization in the nematic phase have the most highly ordered structure, there is no information about the microstructure of these polymers, in comparison with their optical characteristics. Obviously the most important result of this work is establishment of the relationship between the emergence of layer order, indicated by a low angle reflection and the ability of the polymer to display optical anisotropy. Also in the opinion o f the authors it is possible to obtain only polymers of the smectic type, either from an isotropic melt, or from the nematic phase. :.

]

I

UlIIIIINIINIfltllJ ]~IG. 6. Diagram of layer (smectic) packing of molecules of poly-p-aeryloxybenzylidenep-ethoxya~iline (Table 3. polymer 8.2) [86].

Monomers of the Schiff's base type, similar to those used by t~ingsdorf and his collaborators, as well as some other monomers (Table 3, monomers 7.3-7.10, 8-3-8.9, 9.1-9-6, 10.4-10.10, 11.4-11-9) and diacrylic compounds (Table 3, monomers 12-16) were synthesized and investigated by Strzelecki and Liebert in references [4, 84, 85, 88-90]. Since there is a definite type of arrangement of the molecules in each of the three types of thermotropic liquid crystal, in the opinion of these authors, by carrying out polymerization in each of the mesomorphie phases it is possible to fix the original mesophase in the polymeric compound also. Assuming, a priori, that in the case of linear polymers it is not possible to preserve an "organized" structure above the melting point, in the molten polymer, Strzeleski and Liebert propose making use of an idea of De Gennes [95], of fixing the LC structure by polymerization with crosslinking in anisotropic media. By polymerizing the monoacrylic compounds thermally or by copolymerizing them with diacrylates (Table 3, monomers 12-15), from observation of the polymerization process directly on the stage of a polaxing microscope, or from study of optical photomicrographs of sections of block specimens, the authors came to the conclusion that is possible to fix a nematie, smectic or even cholesteric structure in the polymer. Apart from description of the morphological pattern observed and of the optical photbmicrographs, which are fundamental for assigning the polymer to

1095.

Liquid crysSalline polymers

one or other of the types of m~sophase, the authors did not, however, p u t forward any structural evidence from their investigation of the polymers and copolymers. All these polymers are brittle, insoluble and infusible, and are in the form of brown glasses.

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FIo. 7. Arrangement of side chains of macromolecules in crosslinked polymers obtained by copolymerization of mono- and diaerylic monomers in smectic (a) nematic (b) and eholestrie (c) mesophases [84]. With regard to schematic diagrams in these papers representing the arrangement of the polymer molecules in the nematic, smectic and cholesterie modifications (Fig. 7), no arguments of any moment are put forward in support of these structures. Moreover the authors do not provide any information a b o u t the degree of conversion of the monomers or on monitoring the purity of the polymers. I t is therefore not impossible that the optical photographs correspond to mixtures of the polymers and LC monomers, so that the question of the LC' state and the actual molecular and supermolecular structure of the polymers themselves remains open. Even though it. is possible to obtain anisotropic copolymers in the nematic phase b y copolymerizing D I - A B D A B with ABCA (Table 3, monomers 12 and 9.1) in a magnetic field (field strength 5000 C) [89], tile properties of such systems with a fixed mesophase structure, mentioned above, make them little suitable for practical use and inconvenient for structural investigation. Nevertheless the method developed b y these authors, of using crosslinking agents for fixing an LC structure, is certainly of interest and was used later b y Blumstein and his collaborators, who, choosing the same and other monomers, investigated the structure of the polymers in greater detail. In contrast to Strzeleski a n d Liebert, Blumstein and his collaborators [12, 91] were able to show that the polymers obtained b y polymerization of diacrylate LC monomers do not necessarily preserve the type of mesophase characteristic of the original monomer. I t is seen from Table 3 (monomer 12) that polymerization of Di-ABDAB at 250 ° (i.e. from the nematic state) produces a crosslinke4

1096

V. P. SHIBAYEV and N. A. PLATE

"polymer with a smeetic type of structure, whereas according to references [84] and [88] a polymer of nematic structure is formed under these conditions. Similar results were obtained by Ringsdorf and his collaborators, in polymerization of the monofunctional monomers 8.2 and 11.2 (Table 3), which, when polymerized from the nematic phase or an isotropic melt produce a polymer with a smectic structure.

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F r o . 8. Arrangement of side chains of molecules of poly-di-N-p-aeryloxybenzylidenep-diaminobenzene (Table 3, polymer 12, Di-ABDAB). The m a i n chains lie in parallel planes. T h e directions of the a-parallel (~[I) a n d perpendicular (~±) smectie layers are shown b y arrows [91].

Hence polymerization of LC monomers by no means always results in fixation of the original molecular organization existing at the start of polymerization, but in some instances can lead to formation of polymers of more highly organized structure. This is largely dependent on the ability of side groups to undergo regular packing of the layer type, without significant disturbance of the order of arrangement of the main chains, as occurs for example in Di-ABDAB polymer molecules, which are arranged in parallel planes, separated by a distance equal to the length of the side branches (Fig. 8). The formation of a network structure of this type results in the appearance in the X-radiograms of Di-ABDAB polymers (Table 3) of two sharp, low angle reflections at 22.2 A and 43-5 A [91], the ratio of the intensities of which is dependent to a considerable extent on the degree of conversion. According to reference ~91], in the initial stages polymer formation occurs by opening of only one double bond in the bifunctional monomer molecules. This results in predominant formation of the structure characteristic of two layer packing of the side chains of the macromolecules, which explains the 43.5 A reflection, which corresponds to twice the length of the side chain. Increase in the degree of conversion brings about opening of the 'second double bond, which upsets the two layer type of packing and results in a sharp increase in the intensity of the reflection in the 22-2A region, which corresponds to single-layer packing of the side chains. The same type of packing is evidently produced in the eopolymer obtained by polymerization of Di-ABDAB and •-p-acryloxybenzylidene-p-n-butylaniline (Table 3, monomer 7.3) in bulk in the ratio 1 : 1.

Liquid crystalline polymers

1097

Determination of the coefficient of linear expansion ~ of the Di-ABDAB polymer, from X-radiographic measurements showed that this charaeteristio displays considerable anisotropy. Whereas the value of ~ tl in the direction perpendicular to the side chains (i.e. parallel to the smcctic layers) is 1.6× 10 -4 deg -1, ~± in the direction perpendicular to the smectic layers is 0 (see Fig. 8). Comparison of the values of ~lr and ~z for this polymer with those for low-molecular LC compounds, formed the basis on which the authors described the structure of Di-ABDAB polymer as having the smectic type of packing of the macromolecules. Comparison of references [84], [88] and [91] shows however, as is seen from Table 3 (polymers 12), that the authors differ to some extent in their interpretation of the results of investigation of the structure of the crosslinked polymers based on Di-ABDAB. It must be pointed out that the difficulty of working with crosslinked polymers complicates investigation of their structure and of the more important molecular characteristics, to a considerable extent. Nevertheless to obtain even insoluble polymers with optical anisotropy can be of considerable interest in some instances, if it it possible to obtain highly orientated polymeric compounds directly during the polymerization process itself. This was recently discovered in a paper by Lorkowski and Reuther [96], by the example of anisotropic polymerization of 4-p-hexyloxyphenyl ester of 4'-acryloxybenzoic acid (Table 3, monomer 17). By making use of the inherent, oricntational order of the LC nematic phase of the monomer, as well as a magnetic field (field strength 5-5 kG), the authors obtained polymers with a high degree of molecular orientation. Structural investigation showed that these polymers are uniaxial]y orientated and optically positive (the birefringencc An-= 0.070± 0.005), and the side branches form double layers (d~42/~), as shown in Fig. 6. Since when polymerization is carried out from an isotropic melt optically isotropic polymers are formed, the authors consider that polymerization from an LC phase (both with and without application of a magnetic field) can be regarded as a new, convenient method of production of "uniformly orientated polymers". With regard to the insolubility of these polymers, although the authors explain this by strong intra- and intcrmolecular interaction of the side groups, attention must be paid to the possibility of occurrence of transfer reactions in polymerization of this acrylic monomer, as was observed in reference [4], in study of the polymerization of another LC acrylic monomer (Table 3, polymer 18). Another way of fixing a monomeric LC structure is based on the ability of a number of organic compounds to form dimeric products, by hydrogen bonding for example. When the compound is polymerized these bonds function as crosslinkages, fixing the mesomorphic structure of the monomer in the final polymer. Polymers of this type have been obtained from acrylic and methacrylic monomers containing carboxyl groups at the ends of side chains (Table 3, monomers 18-21). For example Strzeleski and Licbert [4] have described the preparation of poly-(p-acryloxybenzylidene-~0-aminobenzo;.cacid) (PABAA) (Table 3, polymer

1098

V . P . SHIBAYEV a n d N . A. PLATE

/

18), in which, in the authors' opinion, the polymer molecules are packed in t h e following way.

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The authors consider that this special "crosslinking" of the polymer b y hydrogen bonds fixes the mesomorphic (smectic) phase of the monomer, though it is still not clear how the polymer can also take up a nematic structure, as the evidence of reference [4] shows (Table 3, polymer 18). Moreover these polymers, b o t h the nematic and smectic types, are insoluble, evidently as a result of chain transfer because of the presence of an ~-hydrogen atom, causing crosslinking

Liquid crystalline polymers

1099

during polymerization. This is indirectly confirmed in reference [12], where Blumstein and his collaborators, b y polymerization of methacryloxybenzylidene-p-aminobenzoic acid (MABAA) in solution and in bulk, were able to obtain a soluble polymer, w i t h a nematic structure, which is preserved up to the decomposition temperature of the polymer. The considerable difference in glass temperature of the MABAA polymers (154 ° and 87 °) obtained b y polymerization in bulk and in solution (Table 3, polymer 19), is worthy of attention. This could be a result of a different stereochemical structure of the macromolecules, though this requires further confirmation. In describing MABAA polymers of nematic structure the authors mention a reflection in the region of 90 • in their X-radiograms, which it has not yet been possible ¢o explain. Thus the presence of strong polar interaction in the end groups of long side branches (in the form of hydrogen bonds for example) evidently promotes formarion of an LC structure, not only in polymerization of monomers in the solid or LC phase, but also in preparation of polymers from isotropic solutions. Moreover in later work Blumstein and his collaborators [92] were able to obtain not only LC, b u t also crystalline polymers b y polymerization of such monomers as ABA and MBA (Table 3, monomers 20 and 21), under conditions that excluded the possibility of formation of stereoregular polymers. Figure 9 shows diffractograms of two samples of polymers obtained b y radical polymerization of ABA in bulk (curve 1) and in an isotropic solution in heptyloxybenzoie acid (curve 2). It is seen that curve 1 has a number of sharp, diffraction maxima, from which the structure of this polymer can be indexed as crystallizing in a monoclinic cell with the parameters a = 6.68/~, b = 7.42 A, c-- 32.50 A and the angle fl-----124 °. X-ray analysis shows that the degree of crystallinity of this polymer is 40 ~o. It is seen from Table 3 that, depending on the conditions of polymerization in bulk and on the conditions of treatment, PABA (polymer 20) forms two crystalline modifications, whereas polymerization in solution produces an amorphous polymer. The effect of the conditions of treatment on the tendency of PABA to crystallize and to form LC and amorphous structures is shown schematically below [92] Precipitation Amorphous polymer

Annealing 215Q

] Form II Smecgie structure

Film obtained from solution ]~orm I of polymer in DMF . Crystalline structure

The continuous lines on this scheme correspond to the observed transitions between the amorphous and crystalline structures, while the dotted lines show the directions of possible transitions, which however have not been brought about experimentally.

V. P. SHIBAYEV a n d N . A. P I ~

II00

Formation of a crystalline polymer b y radical polymerization of 4-(2-vinyloxyethoxy)benzoic acid CHz=CH

0

O--(CHzh--O--~,~._.f--C--OH in an LC phase and as an isotropie melt has also been reported in reference [97], though the authors did not suggest any explanation or any ideas about the packing of the polymer molecules, which carry carboxyl groups on side chains. I

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Fro. 9. Diffractograms of samples of polyacryloxybenzoic acid (Table 3, polymer 19), obtained by radical polymerization in bulk (1) and in solution in heptoxybenzoic acid (2) [92]. A similar method, taking into account the effect of terminal carboxyl groups, was used in papers by Amerik and Krentsel' [3, 69], and by Blumstein [6, 12], where an LC matrix was produced as a result of unusual dimerization of molecules of the monomer and of an LC solvent. We shall not stop to discuss a number of very interesting features of the kinetics of polymerization in such systems (which have been reported in detail, in reference [31] for example), but shall consider only results of investigation of the structure of the polymer. I t is seen from Table 3 (sample 21.1) t h a t in polymerization of MBA in admixture with ~-heptyloxybenzoic acid the polymer has an LC structure only when the mixture itself is an LC state. An amorphous polymer is obtained from an isotropic solution of MBA (Table 3, sample 21.2). I f however films are obtained from solutions of these amorphons polymers (samples 21-21.2) in DMF, regardless of the nature of the original structural organization of the mixture, PMBA of smeetic structure, with twolayer packing of the side chains, which are inclined to the plane of the layers at an angle of 45-47 °, is obtained. Thus previous structural organization of the medium in which polymerization is carried out is by no means always a necessary condition for formation of an LC structure in polymers. Transition from an amorphous structure of a polymer

Liquid crystalline polymers

l 101

bearing anisodiametric side groups, to its LC structure can in many instances be brought about by suitable choice of conditions of treatment of the polymer, namely temperature and the type of solvent, which obviously promote a definite type of packing of the side chains, establishing an LC structure of the polymer as a whole. The latter is very clearly seen from the work of Shibayev, Plate, Talroze and Karakhanowa [98, 99], who studied the structure of polymers from N~-acyl derivatives of N~-methacrylyl-L-lysine (PML-n) (Table 3, polymers 22-23), which are comb-like polymers containing carboxyl groups in each monomer unit in addition to long, aliphatic branches. Since chemical linking of the asymmetric side chains occurs only at the end groups, as is seen from Table 3 (samples 22-23), the side branches can be regarded as a kind of structurally organized assembly of long chain molecules, joined together by the main chain, which would be expected to display the combination of properties characteristic of compounds capable of attaining an LC state. Examination of films of polymers of the PML-n series in polarized light in fact showed the presence of optically anisotropic structures, over a wide range of temperatures, covering the regions of both the glassy and high elastic states. It is interesting to note that for this series of polymers the method of treatment has a decisive effect on the phase state. Precipitation of polymers of the PML-15, 17 and 21 series from solution in chloroform produces amorphous systems, whereas preparation of films of the polymers from solution by addition of polar solvents (precipitants) leads to crystallization of the polymers as a result of packing of the aliphatic side chains in, as before, a hexagonal cell, as for the previously studied, comb-like polymers of the acrylic and methacrylic series [9, 94, 100-102]. Here, however, the packing is more defective (Table 3, Tm). At the same time the well defined amphipathic nature of PML-n molecules is explained by the special place occupied by these systems among polymers of comb-like structure. The occurrence of two types of interaction, namely hydrogen bonding between functional groups and dispersion forces between the methylene side branches, brings about the micro-phase separation that occurs in PML-n homopolymers, especially under the influence of specifically acting solvents. As a result of this phase separation on the micro-scale, regions are formed that are capable of undergoing the molecular rearrangements and phase transitions typical of LC compounds. Formation of such regions occurs as a result of ordering of anisodiametric side groups having a tendency to form liquid crystals, and stabilization of the structure is brought about because of reinforcement of its matrix of"rigid" main chains. As a result, crystalline PML-15, 17 and 21 polymers above the melting point pass into an optically anisotropic state, thus displaying LC properties. Since the glass temperatures of these polymers are considerably above the melting point of regions formed by the main chains, the crystallization and melting processes take place only in micro-regions of the glassy matrix, which permits molecular rearrangement only within these regions. This type of structural organization of the macromolecules covers

1102

V, P., SHII]AYEV and N..A. PLATE

wide range of temperature,, lying between the limits of t h e melting point and decomposition temperature, of the polymer. However, by reducing intermolecular (and intramolecular) interaction in the PML-n series of polymers by replacing t h e carboxyl groups by ester groups, i.e. passing from polymers PML-17 and PML-21 to polymers PMML-17 and PMML-21 (Table 3, polymers 23.2 and 23.3), it is seen from Table 3 that it is possible to alter the glass temperature, hence, and this is very important, the temperature of transition from the optically anisotropic to the optically isotropic state. It thus follows from all that has been discussed above, that formation of an LC structure in polymeric compounds is mainly controlled by the nature of the packing of anisodiametric side groups. In turn the possibility of this packing place is largely dependent on the ability of the main chain of the polymer to take up the conformation that would promote an LC structure of the side groups. This conformation can be produced during the polymerization process itself and then the polymer has an LC structure (as occurs in the cases discussed, of polymerization in the melt or in an LC matrix, see for example, Table 3, polymers 8.2, 11.2, 12, 13 and 19). On the other hand, polymerization in solution as a rule produces polymers not having an LC structure, and only subsequent treatment of the polymers, involving conformational changes, can produce (and even then not always) the necessary level of structural organization of the side groups. Very recently, however, in papers published under the leadership of Magagnini [103-106] the preparation has been described of the aromatic polymers poly-(p-diphenyl acrylate) (PDPA) and poly-(p-cyclohexylphenyl acrylate) (PCPA), which, being prepared in the melt and in solution by radical polymerization (i.e. under conditions excluding the possibility of formation of stereoregular polymers, as was shown by examination of the NMI~ spectra), exhibited properties in many ways analagous to liquid-crystalline properties (Table 3, polymers 24-29). Without calling the polymers liquid crystalline, the authors nevertheless draw attention to their unusual structural properties. Despite the fact that these polymers are not stereoregular they have a well defined melting point (see Table 3) and X-ray analysis shows that they give a broad amorphous halo in the region of large-angle scattering and a narrow reflection corresponding to layer order of magnitude 23.2 A for PDPA and 19.2 _~ for PCPA [94]. In view of the fact that interaction of side chains is essential for attainment of a layer structure (as was observed in the case of non-stereoregular, comb-like polymers [94]), the authors suggest the type of packing shown in Fig. 10 [104]. The side groups form a layer, the width of which is approximately twice the length of the side branches, and the main chain is arranged perpendicularly to the plane of the diagram. The lack of truly crystalline order in this type of packing is explained by inadequat, e coplanarity in the arrangement of the molecules. The proposed system of packing corresponds essentially ~o a smeetic arrangement of molecules (side groups), arranged however, ac-

Liquid crystalline pblymers

1103

cording to an unknown principle, in double layers. Optical investigation shows t h a t both PDPA and PCPA display spontaneous optical anisotropy. In an attempt to discover the reasons for the unusual structure of these polymers, the authors prepared compounds of chemical constitution differing only slightly from that of PDPA and PCPA, as Table 3 shows (polymers 25-28). Detailed investigation of the structure and properties of these compounds showed that they all have an amorphous structure and do not display the LC properties of PDPA and PCPA. Particular attention must be paid to the properties of PDPA and PVPB, which differ in chemical structure only in the different directions of arrangement of the ester groups. In spite of the similar values of Tg of these polymers, PVPV is amorphous, does not form layer structures like PDPA and is not optically anisotropic. The difference in physical structure and properties of these two polymers (PDPA and PVPB) once" again supports the above suggestion that the presence of anisodiametric side groups is only a necessary, but far from sufficient, condition for attainment of an LC structure. The formation of a mesophase involves the setting up of a certain structural (layer) order in the arrangment of the side groups, the formation of which is dependent both on the possibility of occurrence of eonformational changes in the main chain, and on the mobility (or free rotation) of the side groups. It is evidently the latter that causes the difference in structure between PVPB and PDPA, as has been noted before in a study of the structure and thermodynamic properties of comb-like polymers and polymers of similar structure (poly(heptadecyl acrylate) and poly(vinyl stearate) [9]). The effect of steric hindrance imposed on the packing of the side groups, because of the presence of the main chain, is revealed most clearly in the results of attempts to prepare LC polymers containing cholesterol. Polymer with side groups simulating the cholesteric type of liquid crystal. Polymers containing cholesteryl esters as side groups are of considerable interest, because it is well known that low-molecular, cholesteric LC compounds have the widest range of practical application. At the present time about ten papers are known that deal with preparation of polymers containing cholesterol, either by polymerization of monomers containing cholesteryl or cholestanyl groups [107-115] (see Table 3, manomers 30-33) or by conversion of pre-formed polymers [116]. The authors of most of the papers pay greatest attention to the kinetics of polymerization of these LC monomers and to discovering the effect of the phase state of the monomers on the kinetics and on stereoregulation. It must be mentioned here that despite the limited range of monomers investigated, mainly the acrylic and methacrylic esters of cholesterol and cholestanol, the temperature limits of their phase trav~sitions, as given by the various authors, are by no always reproducible (as is seen from Table 3). Evidently in these instances, as is generally the case in study of phase transitions in low molecular liquid crystals, it is necessary not only to know the purity of the preparation,

V. P. SmBAY~cvand N. A. PLATJS

1104

but also the absolute, standard conditions of conducting the experiment (the

rate of change of temperature, the method of determining the transition temperatures etc., for example), which provide the possibility of obtaining consistent comparative characteristics, relating to the same substance. With regard to investigation of the structure of the polymers derived from the eholesteryl and cholestanyl acrylates and methacrylates, apart from mention of the fact that these polymers are rigid substances with high softening points, no information on their structure and LC properties is provided. i

T.... 1

Io

o

|

a

b

I~IG. lO. Schematic diagrams of poly-(p-diphenyl acrylate) molecules (a) and the packing of layers of the polymer molecules (b) [104]. Thus the attempts described in the literature to obtain polymeric liquid crystals by synthesis of polymers containing cho!esteryl or cholestanyl side groups directly linked to the main chain, have not been successful. In our opinion, for formation of an LC structure a certain lability of the side branches is required, which, in spite of the presence of the main chain, permits attainment of a definite order in the arrangement of the cholesteryl groups, as occurs in low molecular liquid crystals. In order to overcome steric hindrance to packing of the side branches it is necesary to remove the cholesteryl groups (or other groups modelling the structure of the molecules of low molecular liquid crystals) some distance away from the main chain. I n discussion of methods of production of thermotropic LC polymers containing cholesterol, Shibayev, Freidzon and Plate [9, 10, 117-120] started from ideas previously developed by themselves, about the relationship between the structure and properties of comb-like polymers bearing long, aliphatic side branches in each monomer unit [9, 94]. The autonomous behaviour of the side chains of comb-like polymers, exhibited by their ability to form layer structures and even to crystallize, regardless of the configuration of the main chain [9, 94, 100, 101], provides the possibility of production of LC polymers based on these. In fact, since chemical attachment of the asymmetric appendages in these polymers is only at the end groups of the side branches, the latter, as we have already mentioned, can be regarded as a kind of structurally organized assembly of long chain molecules linked to the main chain, which is probably one

Liquid crystalline polymers

1105

o f t h e n e c e s s a r y conditions for formation o f a n LC structure. Having synthesized cholesterol containing monomers and polymers (Table 3, polymers 35) based o n long chain monomerie compounds, and by making use on the one hand o f

the tendency of the side chains of such polymers to undergo spontaneous organization, and on the other hand of their high lability (which reduced steric hindrance to packing of the cholesteryl groups at the side of the main chain), the authors succeeded in 1973 [9, 10, 117-119] in obtaining a wide range of polymeric compounds, namely the ChMAA-n homopolymers and copolymers (Table 3, polymers 35 and T,%ble 5, copolymers), possessing LC properties in all three physical states of the polymer, i.e. the glassy, high elastic and viscous flow states. It should be noted that Kamogawa [116] also prepared the cholesteryl ester of poly-(N-acrylyl-co-aminolauric acid) by addition of cholesterol to the macromolecules of poly-(N-acrylyl-aminolauric acid) by the reaction

C~1-12

CH3

CH3

CIH--CO--NH--,CH2),,COOH + C H . ~ / \ I ~ H

I

--,

AI/(?\) HO

I

CH3 CH3 CH3 CH2

! -. CH--CO NH--(CH.,)~tCO0

/ \ CH3

Ctt3

As the author pointed out, however, the polymer obtained by him cont~ined about 10 mole % of carboxyl groups and did not display LC properties. This could be because Kamogawa studied films of the polymer, obtained by evaporation of the solvent from solutions of the polymer in toluene. As will be shown below, this method of preparation hinders formation of an L(! structure. An interesting example of synthesis of the comb-like polycho!esteryl-llmethacryloxyundecanoate (Table 3, polymer 34) is deserib,:d by Imoto and his collaborators [115]. Apart from investigation of the phase state of the monomer, nothing is mentioned, however, of an LC structure of the polymers. We shall now turn to a more detailed discussion of the structure and properties of the LC, cholesterol containing polymers prepared and studied by Shibayev, Freidzon and Plate. Figure 11 shows, as examples, optical photomicrographs of films of one of ~ e i r cholesterol containing polymers in the glassy and viscous flow states [9,

:Ill)~

V. P. SHIBAYEV and 'N. Ai PLATE

~10, t17]. I t is seen that' tl~e texture seen with ~crossed polaroids consists of a ~collection of birefringent regions of dimensions from.2# to 10p. This spontaneously arising pattern of birefringence is preserved, unchanged, in all three physical states of the PChMAA-n series of polymers up to a certain temperature Ta~l, above which, over a narrow temperature interval (2-3 °) tl/e polymer film changes from the optically anisotropic to the optically isotropic state. It must be specially emphasized t h a t at temperatures below Taoi polymers of the PChM_dA-n series (where n i>5) (Table 4) are in the flow state, in the form of a mobile, viscous, anisotropie liquid, flow of w h i c h causes movement of the birefringent regions, similar to the behaviour of low molecular liquid crystals [119].

FIG. 11. Optical photomicrographs of films of PChMAA-11 in the glassy (a) and viscous flow (b) states; crossed polaroids [117]. Thus the authors were first to bring about and observe formation of an LC mesophase in cholesterol containing polymers, by the example of the PChMAA-n series, in the high elastic and viscous flow states [9, 10, 117-119]. The temperature interval covering the appearance of segmental mobility combined with optical anisotropy i ~ dependent on the length of the methylene "bridge" joining t h e cholesterol group to tJ~e main polymer chain (Table 4). I t is seen f r o m T a b l e 4 t h a t PChMAA-2 has the highest T~, above which t h e r m a l decomposition of the polymer begins. As n is increased the Tf-Ta~i interval gradually increases and for PChMAA-11 this covers 55 °. I n this temperature interval the texture* of the polymer is practically the same as t h a t * Here and subsequently the term texture is used to indicate ~ern seen between crossed polaroids in an optical microscope.

the morphological pat-

Liquid Crystalline polyrhers

1107

* f the LC monomer, (cf. Figs. l l and 12 a)"displaying the Same optical propcities. The only difference is that in the case of the polymers the optical picture is preserved at temperatures corresponding to the glassy state, whereas in most instances when low molecular liquid crystals are cooled t h e y crystallize. This is illustrated in Fig. I2, which shows the gradual growth of "solid" spherulites from the LC phase of monomer ChMAA-11 (Fig. 12a) during cooling [10, 117].

Fxo. 12. Optical photomicrographs taken in polarized light, showing the growth of solid crystals (b, c, d) fl'om the liquid crystalline phase of ChMAA-11 monomer (a) [9, 10]. The external analogy seen between the textures of the LC monomers and polymers of the PChMAA-n series (cf. Figs. l l a and 12a) and also with the eonfocal texture of a number of low molecular, cholesteric liquid crystals [23], would seem to provide grounds for stating t h a t the cholesteric type of liquid crystal is present in the polymers. In contrast to the monomers and low molecular, cholesteric liquid crystals, however, which easily undergo transition to

1108

V . P . SmBAYEV and N. A. PLATE

a fiat texture under the action of a mechanical field [15, 24], the polymers do not form this t y p e of texture. X-ray analysis indicates rather t h a t the packing of the side chains of PChMAA-n polymers is of the smectic type. TABLE 4. TRANSITION TEMPERATURES IN THE PChMAA-n SERIES OF POLYMERS*

Polymer

Ts, °C

T~, °C

Ta_1, °C

PChMAA-2 PChMAA-5 PChMAA-6 PChMAA-8 PChMAA-10 PChMAA-11

185 deeomp. 130 130 130 125 120

200 190 180 150 135

220 215 200 185 180

* Tg--glass temperature, Tf--flow ~emperature, T a ~ l - temperature of transition from anisotropie to isotropie melt.

Figure 13 shows the dependence of the diffraction maxima on the number of carbon atoms in the methylene bridge of the side branch of the PChMAA-~ molecules [119].

qO-

..jd3 2O

..... {

Z

: ,I

:

-_ I

6

[

-d 1

: l

tO n

FIG. 13. Dependence of the dx-- d4 maxima on the number of carbon atoms in the methylene chain of the side branch of PChMAA-n macromolecules [117]. The size of the broad, diffuse halo in the region of 5.9 A is practically independent of the length of the side chain and is close to the distance between the parallel side substituents in orientated samples of the polymers. In t h e region of low scattering angles there are still another three diffraction maxima, the values of which, as is seen from Fig. 13, b~ginning with n = 5 increase linearly as n increases. This type of relationship indicates t h a t the structure of polymers of the PChMAA-n series when n~>5 differs from their structure when n ~ 5 . Comparative investigation of the structure of the PChMAA-n polymers a n d of a number of model compounds not containing cholesterol, such as poly-(l~-

Liquid crystalline polymers

1109

methacrylyl-o~-aminocarboxylie acid) (PMAA-n)

--[CH;.--C (CH3)]

l

HOOC--(CH2)2--HN--0C and its methyl ester (PMMAA-n) --[CH~--C (CH3)]--,

]

C0---NH--(CH~)n--COOCH 3 where n varies between 2 and 11, (Table 3, polymer 36), enabled the nature o f the above reflections to be explained [120]. Reflection d2 corresponds to the length of the side branch joining the cholesterol to the main chain and indicates the existence of layer order in the arrangement of the side groups. Reflection da, which does not appear in the X-radiograms of the model compounds not containing cholesterol, corresponds to the packing of the cholesteryl groups. Without giving a definite interpretation of the d4 reflection, the authors state that its value depends little on the length of the side branches, and t h a t the reflection itself is evidently caused by scattering from the main chain, possibly indicating some pcrcodicity resulting from its folded structure. On the basis of the X-radiographic evidence, of examination of molecular models and of evidence in the literature on the crystal structure and the structure of the mesophase of low-molecular esters of cholesterol [121, 122], the authors propose the packing of the macromolecules in orientated samples of PChMAA-n indicated in Fig. 14. In the PChMAA-n polymers the side groups, containing cholesterol, are arranged predominantly perpendicular to the axis of the main chain and at a slight angle to one another, so that they are almost parallel. In PChMAA-n polymers with n/> 5 the side branches form a layer in such a way that the cholesteryl groups of one polymer molecule are surrounded by the methylene chains of neighbouring macromolecules, i.e. antiparallel packing of the side chains occurs (Fig. 14). In PChMAA-n polymers with n < 5 the "rigid" cholesteryl groups are not able to arrange themselves between the methylene chains and in these another type of packing of the side branches obviously occurs. In view of the relatively "poor" X-ray scattering pattern of the PChMAA-n polymers and the diffuse nature of the reflections, the authors have so far refrained from giving a more detailed description of their structure [117, 119]. It must be pointed out that consideration of models of the structure of polymers containing cholesterol is hampered by the fact that at present there is no sufficiently well reasoned model of the packing of low molecular, cholesterie liquid crystals. In the opinion of the authors of reference [122] "the crystallography of cbolesteryl alkanoates is very complex, there being at least eight types of crystal structure and no clear correlation between the crystal structure and the mesomorphic properties". Nevertheless the results of investigation of the structure of PChMAA-n

1110

V. P. SHIBAYEVand N. A. PLATE

polymers leaves no doubt t h a t t h e display of LC properties b y these polymers is a result of a definite older in the arrangement of the cholesteryl groups. The lack of spontaneous optical anisotropy in the already mentioned PMAA-n a n d PMMAA-n polymers, and also in such polymers as the methyl, benzyl and cetyl esters of poly-(N-methaerylyl-co-aminolauricacid) (Table 3, polymers 36-38), indicates the decisive role of mesogenic groups in the production of LC properties.

FIG. 14. Model of the PChMAA-11 macromolecule (a) and a scheme of the packing of macromolecules PChMAA-n in orientated samples of the polymers when n~>5 (b, c): b -- the continuous lines denote molecules lying in the plane of the diagram and the dotted l~nes molecules lying in a plane parallel to the plane of the diagram: c -- projection along the main chain of the macromolecule. With regard to the type of LC phase formed in polymers containing cholesterol, the existence of layer order in the arrangement of the side branches would seem to indicate formation of a smectic type of structure. In polymers of t h e PChMAA-n series, however, the side branches are parallel only to a first approximation (as was mentioned above, they are actually inclined at a small ~ngle ~ one another), so t h a t one can say only t h a t the structure of these polymers is analogous to t h a t of the smeetic type of liquid crystals. On the other hand the optical birefringence pattern observed ill films of PChMAA-n polymers has a morphological resemblance to the confocal texture of cholesterie LC compounds. The flat texture characteristic of cholestcrie liquid crystals, representing essentially a single c r y s t a l of the cholesteric type, is not formed in cholesterol containing polymers. This is evidently associated with the presence of defects,

Liquid crystalline polymers

111 I

which the main chain brings in to the LC structure being formed, as in crystallization of comb-like polymers [94]. Hence the external analogy between t h e texture of low molecular liquid crystals and of polymers of the PChMAA-n series does not indicate packing of the structural elements of their macromolecules in a eholesteric phase. The presence of the main chain considerably hinders formation of the helical structure characteristic of low molecular liquid crystals of the cholesterie type. TABLE 5. TRANSITION TEMPERATURES OF COPOLYMERS OF ChMAA-n WITH ACRYI~kTES (A) AND AJ.KYL METHACRYLATES M A ) Copolymer (mole ChMAA-n) C h M A A - 11 w i t h A - 4 42 37 17 ChMAA-11 with MA-4 9O 67 4O Ctfi~IAA- 11 w i t h M A - 10 75 58 ChMAA-11 with A-16 45 C h M A A - 11 w i t h M A - 2 2 75 ChMAA-6 with A-4 45 30

Tg,°C

T f , °C

T a ~ ~, °C

65 60 < 20

ll5 100 60

160 140 100

115 105 85

140 140 135

180 170 160

90 70

140 120

180 170

45

70

100

7O 100 70

ll0 170 105

No anisotropy 180 115

* The figure following the code letters of the monomers indicates the nmnber of carbon a$oms in the alkyl radical.

The tendency of cholesterol groups to undergo spontaneous ordering is exhibited also b y a number of copolymers of some cholesterol containing monomers with n-alkyl acrylates and n-alkyl methacrylates (Table 5). It is seen from Table 5that introduction of the second component, acting as a "diluent" of the cholesterol containing units and lowering the glass temperature, in some instances increases the interval covering the high elastic and viscous flow LC states, and in others completely suppresses optical anisotropy. It is seen from Table 5 t h a t copo!ymers of ChMA_A-11 with butyl acrylate (A-4) and butyl methacrylate (1VIA-4) form an LC phase over a wide range of ratios of the components. For example a copolymer containing more t h a n 800/o of A-4 is still capable of existing in an LC state, whereas a copolymer of ChMAA-11 containing only 25% of MA-22 is amorphous and optically isotropic. I f t h e second component in a copolymer does not hinder packing of the cholesteryl

1112

V. P. SHIaAYEVand N. A. PLATE

groups, formation of a mesophase is possible. In copolymers with MA-22 the long side branches screen the cholesteryl groups and optical anisotropy does not appear in any of the three physical states of the polymers. Thus a necessary condition for appearance of LC properties in copolymers is that the length of the side substituent should not be greater than a certain limit, which is not more than the the length of the methylene "bridge" joining the cholesterol to the rhain polymer chain. The possibility of producing copolymers with an LC structure considerably widens the range of polymers possessing LC properties and shows that by suitable choice of monomers for copolymerization it is possible to obtain a broad selection of polymers forming a mesophase in different ranges of temperature. After having established the fundamental principles of the physicochemical behaviour in the soli2 phase, of polymers containing cholesterol, the authors [123, 124] were faced with the problem of finding the relationship between the conformationa! state and intramolecular mobility (IMM) of these polymer molecules in solution, and their structure in the solid phase. This problem is directly related to discovery of the role of intramolecular interaction in attainment of an LC state in polymeric systems. By using PChMAA-11 and a number of copolymers of ChMAA-11 with MA-4 as the main materials for investigation, the authors followed the conformational behaviour and IMM of these polymers in solution over a wide range of temperatures [123-:125], using for this purpose the methods of spectropolarimetry [123], polarized luminescence [124] and dielectric relaxation [125]. The results of investigation of the temperature dependence of the specific optical rotation [a] and the relaxation time ~, characterizing the conformational state and IMM of PChMAA macromolecules are presented in Figs. 15 and 16. Comparison of the [a]=f(T) functions for PChMAA'-ll in different solvents (Fig. 15) shows that change in temperature does not affect the value of [g] o f this polymer in toluene and chloroform, whereas in n-heptane'and n-dodecane [a] increases considerably as the temperature is reduced. Since the value of [a] of the monomer ChMAA-11 in solution in heptane remains constant over the entire range of temperatures and is a little less than its value for the polymer, the differences between the values of [a] for the polymer and monomer are produced only by the conformational state of the PChMAA-11 molecules. This is indicated also by the temperature dependence of ~ (Fig. 16, curve\ 2). A point worthy of attention is the fact that the curves of these relationship contain extrema, indicating an extraordinarily sharp reduction in the IMI~ under conditions close to phase separation of a solution of PChMAA- 11 in heptane (the temperature of phase separation is 23-28 ° at polymer concentrations above 0-1 ~/o). The maximum in the curve of the dependence of ~ on T (Fig. 16, curve 2) indicates transition of the macromolecules to a compact state. The decrease in v as the temperature is further reduced cannot be explained only in terms of the IMM and here account must be taken of the mobility of the macromolecule

1i~3

Liquid crystalline polymers

as a wholE. According t o views developed in reference [126], the maximum in the T=f(T) curve is due to formation of a compact intramoleeular structure, similar to that of globular proteins, referrred to in reference [127], for example. Comparison of curves 1 and 2 clearly shows that a conformational transition precedes formation of a compact structure. Examination in the electron microscope of the structures formed from solutions of PChMAA-11 in heptane confirm their globular nature [120]. [ ~ ] , deg.dl/dm.# Z#O

[ct], deg.d//drn. 9,

ns~ /

1000

.1

I

/~+~.~

oZ

/ ~°I'~-,

lJo

+J

/

I\o'~

IO

3g

Aq

-

-

gOD #0

---

:

.

.

.

.

.

2#g t

20

I

q#

I

I

#0

FIa. 15

L

_J

8O

T,%

5#

T, °g

Fm. 16

FIG. 15. Dependence of the specific optical rotation [~] on temperature for PChMAA-11 in heptane (1), dodecane {2), toluene (3) and chloroform (4), and for ChMAA-11 in heptane (5) (A=350 nm) [123]. FIG. 16. Temperaturo depondeneo of [~] (1) and z ( 2 - 6 ) for solutions of PChMAA-11 (1, 2) and of copolymers of ChMAA-11 with MA-4 containing 10 (3), 25 (4) and 60 mole ~o (5) of MA-4, and of PCMAA-11 (6) in heptane [123].

Study of the causes of formation of such intramolecular structures showed that a considerable part is played by the reinforcing effect of the cholesteryl groups when the temperature is reduced, which under conditions close to phase separation results in intramolecular aggregation, which brings about the observed conformational change. "Dilution" of the sequence of cholesterol con~ining monomer units with butyl methacrylate units (ChMALA-MA-4 copolymers) gradually causes "degeneration" of the conformational transition (Fig. 16, curves 3-5). The decisive part played by the cholesteryl groups in formation of intramolecular structures is confirmed by study of the dependence of v on T for solutions of PCMAA-11, a polymer in which cholesterol is replaced by cetyl groups (Table 3, polymer 38). It is seen from Fig. 16 (curve 6) that this polymer does not form a compact structure in solution. Hydrogen bonding plays an important part in stabilization of the structures formed. Addition to a solution of PChMAA- 11 in heptane of a powerful competitor for hydrogen bonds as trifluoroaeetie acid (TFAA) causes [a] to fall and when

1:114

V . P . SHIBAYEV a n d N . A . PLATE

the concentration of TFAA is more than 0.2% the conformational change does not take place (Fig. 17). The breaking of hydrogen bonds brings about a sharp increase in intramolecular mobility, as is shown by comparison of the values of v for PChMAA-11 in different solvents (Table 6). I t is seen from Table 6 t h a t when, for example,

[=_7,deg.d//dm.g 150- I

1~0 q -

70

~×-O-x--O-x-q-I

0

t

I

,~0

I

I

l

50

I

I

l

90

T,°C

FIG. 17. Temperature dependence of [~] for solutions of PChMAA-11 in heptane containing 0 (1)7 0.01 (2), 0.03 (3), 0.08 (4), 0.14 (5) and 0.20 vol. ~o TFAA (~=350 nm) [123]. 0.2% of TFAA is added to a solution of PChMAA-11 in heptane, r decreases by more t h a n an order of magnitude and the thermodynamic quality of the solvent has a decisive effect on the intramolecular structure and IMM. The value of r for PChi%IAA in non-polar solvents is higher t h a n in polar solvents. TABLE 6. ]:~ELAXATION TIME (r, m s e c ) OF P M M A - 1 1 DETERMINED BY THE POLARIZED LUMINESCENCE M E T H O D IN VARIOUS SOLVENTS AT 25 °

Solvent I-Ieptane Cyclohexane Toluene o-Dichlorobenzeno Dichloromethane Chloroform

T~ SeC

490 130 100 47 32 22

Solvent Heptane-k 0.2% TFAA* Cyclohexane ~-0.15~o TFAA Toluene-k 0"6~o TFAA o-Dichlorobenzene~-0.1 ~o TFAA Dichloromethane d- 0.1 °/o TFAA Chloroform-k0.1 ~o TFAA

T~ S e C

47 23 23 20 20 20

* TFAA-trifluoroacetic acid, vol. o~.

The ability of cholesterol containing polymers to form a compact intramolecular structure in a certain range of temperatures, and the coolaerative nature o f the conformational change, discovered in references [120], [123] and [124] and which is demonstrated most clearly by the T-~f (T) relationship, gives every reason for drawing a n analogy between t h e beh~viour of these synthetic, non:-

1115

Liquid crystalline polymers

stereoregular polymers and t h a t of natural and synthetic biopolymers in solu: tion. This analogy consists in the discovery b y the authors of references [123] and [124] of the ability of cholesterol containing polymers to form secondary (and even tertiary) intramolecular structures in dilute solution, b u t stabilized not b y interaction of the main polymer chains, b u t b y interaction only in th~ side chains. T A B L E 7. L A T T I C E SPACINGS OF P C h M A A - 1 1

Conditions of film preparation From solution in toluene or chloroform (isotropic film) From solution in heptane (anisotropic film)

dli0"05

SAMPLES

Lattice spacings, A d~±0"5 dail

5-30 diff

19"8 w

5-30 diff

19'8 s

30'5 w

d4i 1

52 m

* Intensity of reflections: diff--diffuse, w - w e a k , m - medium, s - s t r o n g .

The formation of intramolecular structures in solutions of cholesterol containing polymers has an important effect on formation of an LC structure in polymer films, as was clearly shown in references [129] and [123], b y comparison of the results of investigation of polymer films obtained b y evaporation of t h e solvent from solutions in which had been measured (Table 6).

FIG. 18. Electron photomicrograph of a PChMAA-11 film [123]. Formation of films from solutions of PChMAA-11 in toluene or chloroform produces optically isotropic specimens, whereas the films obtained from solution in heptane and dodecane are optically anisotropic (as was also found when PChMAA melts were cooled [117, 119, 120]. X-ray analysis of these films indi-

1116

V. P. SmBAVEvand N. A. PLATm.

cares that the degree of ordering of the side branches of PMA/k macromoleeules, Containing cholesterol, is higher in optically anisotropic, L C specimens than in isotropic films (Table 7). Examination in the electron microscope shows that these optically anisotropic, ILC polymer films have a peculiar, "mosaic" supermolecular structure (Fig. 18), whereas the optically isotropic films (obtained from solution in toluene or chloroform) appear in the electron microscope to be "structureless". In the light of this evidence it becomes possible to understand the results presented in the above papers, where it is stated that the possibility or impossibility of producing LC structures in polymers with side groups simulating the structure of low molecular liquid crystals is substantially dependent on the conditions under which they are produced and on the subsequent conditions of preparation of the films. Comparison of the results of investigation of the conformational state of the molecules of cholesterol containing polymers in solution with the structure and properties of films formed from these solutions has provided the possibility of discovering one of the main causes of formation of an LC structure. In our opinion this necessary condition for attainment of an LC state in comb-like polymers containing groups modelling the structure of low molecular liquid crystals is formation of a perfect, secondary intramolecular structure, which brings about the subsequent stages of formation of the more complex, supermolecular forms, responsible for the display of optical anisotropy. In summing up the results of research on the preparation and study of polymers containing cholesterol, it can be stated with confidence that the principle of creation of thermotropic, polymeric LC systems based on comb-like polymers is completely confirmed. Here we have demonstrated the possibility of obtaining only LC polymers containing cholesterol. There can be no doubt that in place of cholesterol other molecular groupings capable of forming both the smectic and nematic types of mesophase can be used. At the same time, the structural investigation of cholesterol containing polymers has clearly shown how carefully b o t h the problem of determining the type of mesophase and in general the classification of polymeric LC compounds must be approached. It is obvious that the classification of thermotropic liquid crystals proposed for low molecular compounds, is by no means applicable in its fullest extent to polymeric systems, the structural organization of which is different from that of low-molecular compounds, l Comb-like polymers are naturally not the only type of polymer from which thermotropic, polymeric LC systems can be obtained. The principle underlying ~he formation of polymeric LC systems based on comb-like polymers requires a combination of only two necessary factors, namely the presence of sufficiently flexible anisodiametric fragments of side chains and mesogenic groups covalently linked to them (in some instances these groups can be polar groups) [99]. On t h e o t h e r hand it is possible for these fragments to be joined to the system

Liquid crystalline polymers

1117

not as side branches, b u t as part of a linear macromolecule. Quite recently in fact, a group of Italian chemists [11] have succeeded in synthesizing some polyalkanoate polymers (PALK-n) from p,/-dihydroxy-a,a'-dimethylbenzalazine and dibasic acid dichlorides, which, as is seen below, combine in the composition of the macromolecules, flexible aliphatie chains and fairly rigid fragments

[

--

.... \ --O--\,. //--C /

l

,

C--\\

I N--]~ CHa

k

/~,

o]

It //--O--C--(CH.,)Tz--C--

I Cila

o

/ ._]

Above the melting point these crystalline polymers pass over to an LC state, the temperature range within which it exists being dependent on the conditions of heat treatment of the specimen (Table 8). Table 8 shows the melting points T A B L E 8. P H A S E

T R A N S I T I O N T E M P E R A T U R E S AND E N T H A L P I E S OF M E L T I N G OF

PALK-n

SAMPLES

n 8 10 12

.Experimental conditions Heating First heating First cooling Second heating Second cooling Third heating

T1, °C 238 203 210 180 203 210 181 206

T=, °C 295 256 241 216 239

AH1,

AH~ ,

kcal/mole 2"53 1"90 2"10 1"57 0'71 1'32 1"55 2-10

kcal/mole 2'68 2"33 1"90 "

1"80 1"82

of the crystalline (T1) and LC (T~) phases and also the enthalpies of melting corresponding to these phase transitions (zIH1 and AH~). A noteworthy point, not explained b y the authors, is the fact that the difference between AH1 and AH 2 is small, whereas in the case of low molecular liquid crystals the lmat of transition of an LC phase to an isotropic melt (AHs) is always considerably iess than AH 1. Optical measurements show that in the temperature interval corresponding to the LC phase the polymers are optically anisotropic. The authors do not reach a final conclusion about the type of mesophase obtained, ascribing to it a nematie or smectic type of structure. CONCLUSIONS

The results discussed in the present paper, of study of the physicochemical behaviour of liquid crystalline polymeric systems, must be regarded as a new and very promising trend in the physical chemistry of polymers. The examples presented of attainment of LC orientational order in polymer melts and solutiona

1118

V . P . SHIBAYEV and N. A. PLATE

clearly show t h e w a y t o w a r d m a k i n g practical use o f t h e special properties o f a n LC s t r u c t u r e , for t h e purpose o f i m p r o v i n g t h e p h y s i c o m e c h a n i c a l characteristics o f p o l y m e r i c materials. H e r e t h e p r o b l e m facing research workers is ~hat of finding m e t h o d s for creating conditions t o p r o m o t e p r o d u c t i o n of LC o r d e r e v e n in a l r e a d y k n o w n polymers. W i t h respect t o t h e p r e p a r a t i o n a n d s t u d y of t h e r m o t r o p i c LC p o l y m e r s t h a t include mesogenic groups, this m u s t be r e g a r d e d as one aspect of t h e general p r o b l e m o f p o l y m e r structure. T h e r m o t r o p i c LC p o l y m e r s f o r m a special t y p e o f highly o r d e r e d p o l y m e r i c s y s t e m w i t h s p o n t a n e o u s l y arising, anisotropic properties, occupying a position i n t e r m e d i a t e b e t w e e n a m o r p h o u s a n d crystalline polymers. W e can look f o r w a r d t o t h e f a c t t h a t success in investigation of systems o f this t y p e will lead to b r o a d e n i n g of our knowledge of the s t r u c t u r e of p o l y m e r s in general, a n d as a consequence t o p r o d u c t i o n of p o l y m e r i c materials o f a new quality. Translated by E. O. PHILLIPS REFERENCES 1. A. C. DE VISSER, K. DE GROOT, J. F E Y F ~ and A. B ~ T J E S , J. Polymer Sci. A-I, 9:

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103. 104. 105. 106. 107. 108. 109. 110. 111.

112~

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THE EFFECTS OF BUTYL-o-TITANATES ON THE RADICAL POLYMERIZATION OF BUTYL METHACRYLATE* A. V. Svxr~, M. A. BVLATOV, S. S. SPASSKIL A. L. SUVOROV and YE. A . KHRUSTALEVA Chemistry Institute, Ural Research Centre, U.S.S.R. A c a d e m y of Sciences

(Received 23 January 1976) n-Butyl- and tort. butyl-o-titanato (NBT and TBT) a d d e d to n - b u t y l m e t h a c r y l a t e (NBM) act as weak polymerization inhibitors a n d chain transfer agents. N B T and TBT produce a 1000 fold increase in the rate of the decomposition of ketone- and hydro-peroxides, while N B T produces a 50-100 fold increase in decomposition diacyl peroxides * Vysokomol. soyed. A19: No. 5, 973-976, 1977.