Polymer Science U.S.S.R. Vol. 20, pp. 2884-2399. (~) PergamonPress Ltd. 1979. Printed in Poland
COMB-LIKE LIQUID CRYSTALLINE P O L ~ WITH SIDE GROUPS MODELLING THE SMECTIC TYPE OF LIQUID CRYSTAL* V. P.
N. Yu. LUKI~, N. A.
Z. A. I~OGANOVA, A. L. SMOLYANSKIZ and N. A. PI~T~
M. V. Lomonosov State University,Moscow Vologda Dairy Institute
(Received 27 February 1978) A description is given of the synthesis of several new comb-like polymers containing mesogenic groups in the side branches, simulating the structure of low molecular weight smectie type liquid crystals. The polymers obtained are able to form enautiotropic liquid crystalline phases, the type of which according to the terminology used for low molecular weight liquid crystalline compounds, may be defined as smectic. Thermodynamic boundaries of the liquid crystalline state were determined in the polymers synthesized, which enabled the liquid crystalline state in these polymers to be determined as a thermodynamically stable phase with spontaneous anisotropy of properties (particularly optical anisotropy). A typical feature of the structure of the polymers studied is the existence of layer ordering of side groups, which together with the packing of mesogenie groups ensures liquid crystalline properties. Mesogenic groups alone participate in the formation of crystalline packing in these polymers, whereas methylene chains, together with the main polymer chain, are in the amorphous phase.
W~, proposed an approach [1-3] to forming liquid crystalline polymers of thermotropic type involving the synthesis of comb-like polymers of poly-N-methacryloyl-co-aminocarboxylic acids containing mesogenic groups at the ends of side branches for which cholesterol esters were used simulating the cholesterol type liquid crystal __CtI2__C/ C H 3 ) CJ 0
(CH2)3--CH CH°(SL--TCH-\ ),, ) I
where n----2-11. * Vysokomol. soyed. A20: No. 9, 2122-2134, 1978. 2384
Comb-like l i q u i d crystalline polymers
The advantages of the approach proposed were tested by examining the structure and physical and chemical behaviour of polymer homologues of polycholesteryl esters of N-methaeryloyl-o~-aminocarboxylic acids (PCMMA-n), which enabled several relations to be established to determine the liquid crystalline state of polymers containing cholesterol [4, 5] as a thermodynamically stable phase condition with spontaneously arising anisotropy of properties. In this study we used this approach to synthesize another type of liquid crystalline comb-like polymer containing mesogenic groups that can form nematic and/ /or smectic type liquid crystal. As mesogenie groups we used ester molecules of hydroquinone and n-hexyloxybenzoic acid which, as in the case of polymers of a number of PCMMA-n were linked to the main chain of macromolecules with a sequence of methylene units of different length.
N H - - ( C I -CI , ) )-n --- ( }Q- - 0.- -_~___ ~~- - 0) - - - __ _
where n = 5 (13-5) and T = l l (13-11)*. Furthermore, to carry out a comparative structural study, M-11 copolymers were synthesized with butylacrylate (A-4) of different composition and poly-p-nhexyloxybenzoyl-p-oxyphenylmethacrylate (13-0) prepared
n C,;H I>
the macromolecules of which contained the same mesogenic groups as P-5 and 13-11, but were combined with an "ester bond" directly to the main chain of macromolecules. Results are presented of an investigation of the molecular and supermolecular structure of all polymers indicated (P-0, 13-11 and 13-5), their monomers and eopoiymers, in order to establish the relations which define the ability of the compounds indicated to from a liquid crystalline structure and find a correlation between structure and properties of thermotropic liquid crystalline polymers. * Monomers of polymers P-5 a n d P-11 were s u b s e q u e n t l y denoted as M-5 a n d M-11 respectively.
Y . P . SHXBA~-~V et a/. Monomers were synthesized according to the 'following conditions t I O - -~~ _- -_~J- - O t I q- I I 0 0 ~ ; - - ~__ - - O - - n - C ~ I I ~ 3 -+
C1t2 = C
/ -r (1) ~ [ ,
CII., = C
COOH ,~- (I) m CI--C--O--CH3
where n = 8 (MAC-5), n--- 11 (MAC-11) CII a //
0 0 " ~-~ 1! C _ ~ _ O N'C0 -- N I I - - ( C I I , _ . ) , - - ( - O - - ~ / - - O - - C - = where n = 8 (M-g), n = l l
p-n-Hexylooajbenzoyl-p.oxyphenol (I) was obtained b y direct estorification of p-n-hoxyloxybonzoic acid using a 4-8 fold hydroquinone excess b y methods previously described . Methacryloyl-p-oxyphenyl ester of p-n-hexyloxybenzoic acid (M-O) was obtained b y modified methods  b y the interaction of methacrylic acid chloride a n d I taken in a n equivalent ratio, in the presence of triethylamine in the solution of reagents indicated in pure benzene at 40 °. After the reaction benzene was distilled off, the residue dissolved in chloroform and the solution successively washed with 0.1 • H2S04 solution, water, 0.1 ~r soda solution, then again with water until neutral. Chloroform was evaporated to dryness, the residue twice recrystallized from acetone at 0°; yield 56%; Tmelt=88-90°; found, %: C 72.4; H 7.07; calculated, % C 72.5; t t 6.81. 2g-Methavryloyl-e-aminocaproic acid (MAC-5) and N-methaeryloyl-m-aminolaurie acid (MAC-11) were Obtained by methods previously described . p-n-Hexyloxybenzoyl.p-oxyphenyl ester-N.methacryloyl-aminocaproic acid (M.5) and the p-n-hexylobenzoyl-p-oxyphenyl ester of N-methacryloyl-eo-aminolauric acid (M-11) were obtained by acylation of phenol I with a corresponding acid MAC-5 and MAC-11 using the methyl ester of ehloroearbonie acid. 0.16 M / n e t h y l ester of chloroearbonie acid was added in pure chloroform to a solution of 0.16 ~ MAC-5 or MAC-11 a n d 0.16 M triothylamine. After 10 m i n a solution of 0-16 ~ phenol 1 and 0.16 M triethylamino in pure chloroform was added. The reaction was carried out at 0% After the reaction the reaction mixture was left overnight. The compounds obtained were separated a n d purified the same way as M-0. M-5: yield ~ 3 1 % ; Tmelt=100°; found, % C 70.62; H 7-33; N 3-20; calculated, %: C 70-31; t t 7.48; N 2.83. 3g.11: yield 43%; Tmelt=92°; found, % C 72.37; H 8.49; N 3.19; calculated, % C 72.50; H 8.45; N 2.42. All monomers were also identified according to I R spectra b y assigning the absorption bands of synthetic compounds observed to standard absorption bands. : Polymers P-O, P-5 and P-11 wore obtained b y radical polymerization of corresponding monomers in toluene solution at 75 ° for 48 hr in argon in the presence of a-g-'azoisobutyro.
Comb-like liquid crystalline polymers
dinitrile. Polymers were repeatedly reprecipitated with hot methanol from solutions irt toluene. Copolymers M-11 with A-4 were obtained in the same way as P-11. The compositiort of copolymers was calculated from the intensity ratio of absorption bands at 1660 cm-~ (C~O vibrations in the amide group) and 1735 cm -1 (C~O vibrations in ester groups). Copolymers containing about 75, 50 and 25 mole% M-11 were obtained. Thermographic investigations of monomers were carried out in a derivatograph (Hungary) at a rate of heating of 1-2 deg/min and using a DSM-2 differential scanning micro. calorimeter at a rate of heating of 12.5 deg/min. To study thermal properties of polymers, a DSC 'Rigaku Dcnki' differential scanning microcalorimeter was used (rate of heating 10 deg/min).* Transition temperatures were the temperatures corresponding to the maximum of endothermic and exothermie peaks. Transfer heats were evaluated by comparing peak areas of the compolmd studied and the standard. Stearie acid and indium metal were used as standard. X-ray studies were carried out in a URS-55 X-ray apparatus using CuK a radiation Faltered through a nickel filter. The apparatus is provided with a thermostatic cell enabling X-ray studies to be carried out in the temperature range of 20-250 °. Methods of spectroscopic investigations are described in a separate study . Polymer films were prepared from solutions in chloroform by evaporation of solvent on a KBr and NaC1 substratum, dried in vacuum at 40-50 °. During the I R spectroscopic measurements the film between two openings was isolated by an annular layer and clamped iu a heated holder. The radial temperature gradient did not exceed 3°. Optical studies in crossed polaroids were carried out in a MIN-8 polarization microscope equipped with a heating table. Temperature was regulated by an EPV-2-11 potentiometer. Before directly dealing the with structure and properties of polymers, let us examine results of investigating the physical and chemical behaviour of monomers.
Monomers. A s t u d y of t h e s t r u c t u r e a n d p h a s e condition of s y n t h e t i c m o n o m e r s w h e n c h a n g i n g t e m p e r a t u r e r e v e a l e d considerable difference. I t follows f r o m a n analysis of t h e r m o g r a m s s h o w n in Fig. 1 t h a t m o n o m e r s M-0 a n d M-11 a r e c h a r a c t e r i z e d b y t h e existence of several e n d o t h e r m i c transitions, w h e r e a s ol~ t h e r m o g r a m M-5 t h e r e is o n l y one e n d o t h e r m i c p e a k . M-5 m o n o m e r b e h a v e s in t h e " s i m p l e s t " w a y during h e a t i n g w h i c h a t 106 ° c h a n g e s into a n isotropic m e l t w i t h a m e l t i n g e n t h a l p y AHCrlt e q u a l to 22 callg. Melting of M-0 a n d M-11 t a k e s place differently, a t t e m p e r a t u r e s of 87 ° (AHCmrelt = 12"1 cal/g) a n d 100 ° ( A H c r e l t : l l ' 0 eal/g) t h e y f o r m anisotropic liquids s u b j e c t t o s t r o n g birefringence a n d changing into a n isotropic m e l t a t 110 a n d 115 °. L o w e n t h a l p i e s of t h e s e t r a n s i t i o n s w h i c h f o r m 0-95 cal/g for M-0 a n d 0.45 cal/g for M-11, are close ix) e n t h a l p y values of t r a n s i t i o n c h a r a c t e r i z i n g m e l t i n g of t h e liquid-crystalline p h a s e of low m o l e c u l a r weight c o m p o u n d s ; this t o g e t h e r w i t h results of optical a n d microscopic i n v e s t i g a t i o n s suggests a liquid crystalline s t r u c t u r e of t h e m o n o m e r indicated. I t is significant t h a t liquid crystalline p h a s e ~[-0 a n d M-11 is f o r m e d b o t h d u r i n g h e a t i n g a n d cooling of m o n o m e r s , w h i c h is t y p i c a l o f e n a n t i o t r o p i e liquid crystals. As far as a n o t h e r e n d o t h e r m i e p e a k is c o n c e r n e d in t h e region of 91-92 ° ( A H m e i t ~ l l cal/g), o b s e r v e d on t h e t h e r m o g r a m of M-11 * The authors are very gratoftd to I. V. Sochav (Physics Institute of the Leningrad State University) for enabling the studies to be carried out in the microcalorimotera.
V. P. SHIBAY~.Vet ed.
(:Fig. 1, curves 3 and 4), this is apparently due to melting a crystalline modification I, which is different in structure from modification II, which melts at 100 °. Unfortunately neither the liquid crystalline phase, nor crystalline modification 87
FIG. 1. Thermograms of monomers M-0 (1), M-5 (2) and M - l t (3, g) obtained a t a rate of heating of 12.5 (1-3) a n d 1.2 d e g / m i n (4).
I I can be recorded by X-ray in view of the very rapid polymerization of monomers at temperatures exceeding the temperature of the first endothermie transition. As shown by thermogram 4 (Fig. 1) obtained at a relatively low rate of increasing temperature, an exothermie peak is observed after the endothermie peak of melting, which corresponds to heat liberation during polymerization of
FIo. 2. Optical microphotography of M-11 showing transition of the spherulitie structure of the monomer to the liquid crystalline structure of the polymer formed during heating the monomer a t 110 °. .(The direction of the front of polymerization is shown by arrows).
Comb-like liquid crystalline polymers
the monomer. Optical investigations show that polymerization takes place at t h e moment of melting a spherulitic monomer structure and is accompanied by birefringence dependent on the formation of a liquid crystalline polymer structure (Fig. 2). The study of monomer structure therefore showed that some of them, namely M-0 and M-11 are able to form an enantiotropic liquid crystalline phase, the structure of which cannot be studied in practice in view of the high activity of monomers in polymerization. 2~7
/~ A__ Tme~ IM
A__ a 132
6 Tmelt i
Fro. 3. Thermograms of polymers P-0 (1), P-Sa (2), P-5b (3), P-5c (4), P-11b, c (5) and P-11a (6).
Polymers. To study the structure of synthetic polymers and find optimum conditions of crystallization, the following types of treatment of samples were used: a) precipitation of polymers with methanol from solution in toluene; b) slow cooling of polymer melts to room temperature; c) annealing of polymers for 6 h r at a temperature of 100°. * Results of investigating thermal properties of polymers in the form of DTA curves are shown in Fig. 3 and heats of phase transitions and X-ray results are given in Tables 1 and 2. An analysis of these results and results of optical and microscopic investigations show that P-0 polymer films at temperatures ranging from room temperature to 247 ° are characterized by considerable birefringence with all methods o f * Subsequently, polymer samples subjected to this type of treatment were denoted:~ by indices a, b and e.
V . P . SHIBAYEV et al.
treatment o f p o l y m e r s a m p l e s . A t 247 ° t h e p o l y m e r i n a n a r r o w t e m p e r a t u r e r a n g e ( 3 - 4 °) (Fig. 3, c u r v e 1) u n d e r g o e s a s u d d e n t r a n s i t i o n f r o m t h e o p t i c a l l y a n i s o t r o p i c t o t h e o p t i c a l l y i s o t r o p i c s t a t e ( s u b s e q u e n t l y , t h e t e m p e r a t u r e cor-
T E M P E R A T U R E S AND H E A T S OF PHASES T R A N S I T I O N S OF L I Q U I D C R Y S T A L L I N E POLYMERS
Method of treatment Traelt
Types of transition crystal -*liquid liquid crystalline crystalline state state -~ isotropic melt AHmelt, cal/g dHa-~l, cal/g Ta-~, °(3 (-4-1°) ° C ( ~ 1 °) (+0.02) (±0.02)
a, b, c,
132 57 73 51 51
0.17 1.15 1.25 0.90 0.90
b e a
r e s p o n d i n g t o t h i s t r a n s i t i o n , will be d e n o t e d as T a x i ) . Since t h e X - r a y s t u d y o f P - 0 d i d n o t r e v e a l t h e e x i s t e n c e o f c r y s t a l l i n e reflexes (Table 2), a n d t h e X - r a y p h o t o g r a p h o f t h e p o l y m e r is c h a r a c t e r i z e d b y t h e e x i s t e n c e o f a m o r p h o u s s c a t -
I N T E R P L A N A R DISTAI~CES OF L I Q U I D CRYSTALLINE POLYMERS EXA.iM:IArED AT D I F FERElqT TEMPERATURES
Method of treatment
a, b, c
Temp. of plotting, °C 20-150 20 100 150 20 150 20 55 60 65 70, 100, } 120, 140
Interplanar distances, A dl (:L1) 29 s 36v. w 43 v. w. 41 v. s. 40 s. 41 s. 38 s. 43 reed 43 reed 64 w. 4 6 v . w.
• d, (~1)
18 v. s. 25 rood 23 v. w. 19s. 22 s. 23 s. 22 s. 23 s .
d8 ( + 0.02) 4.60 4-41 4.41 4.70 4.70 4.70 4.41 4.41 4.41 4.41 4.60
cliff reed reed diff cliff diff rood reed mod mod diff
~ot6. ~otations of reflex intensity: v.s.--very strong; s.--strong; meal--medium; w.--weak; v . w . - - v e r y weak;
Comb-like liquid crystalline polymers
feting in the 4.6 A region and a sudden layer reflex, d1-~29 A., it may be assumed that liquid crystalline order can be formed in this polymer and a meso-pha~e formed which at Ta~t~247 ° melts with a value of AHa~t~2"6 cal/g. This heal of fusion is comparable with a similar value, AHa~t-----2-4 eal/g previously derived  for polymethacryloyl oxyphenyl ester of nonyloxybenzoic acid. The existence of a mesogenic group directly linked with the main chain of the maeromQlecule in non-stereospecific P-0 determines its particular behaviour, which involves the possible formation of liquid crystalline order. Up to the softening temperature which is ~ 180 °, the po)ymer is in the glassy state, so that the liquid crystalline structure is fixed in a rigid glassy matrix and it is only above this temperature that the polymer becomes flexible to some extent by the action of a mechanical field; however, a truly plastic state is only formed at a temperature higher than Ta~l, when the polymer changes into an isotropic melt. There is another type of structural ordering in polymers P-5 and P-11, where the same mesogenic group is "removed" from the main chain by a methylene group sequence of different length. In contrast to P-0, polymers P-5 and P-11 show several endothermic transitions on thermograms. X-ray and optical investigations of films of these polymers at different temperatures (Table 2) made it possible to interpret the type of phase transition observed. At temperatures lower than the temperature of first transition (Tmelt)both P-5 and P-11 are crystalline, which is confirmed by the existence of a sudden and intensive reflex in the 4.41 A region (Table 2). Polymer P-11 crystallizes readily and independently of conditions of treatment, whereas P-5 gives a "crystalline" X-ray photograph only after processing according to type a and c. However, although films of polymer 5b do not crystallize (Table 2), nevertheless they are optically anisotropic up to Taxi, like P-0 films (compare curves 1 and 2, Fig. 3). In other words, under given conditions of treatment full independence in the behaviour of.mesogenic groups cannot be achieved for P-5b samples, allowing possible independent crystallization, as observed ia P-11; a liquid crystalline structure similar to that of P-0 structure is formed in this case. Let us examine in more detail features of thermograms and results of thermal effects of phase transitions in the polymers studied. The first endothermic peak on thermograms of P-5 and P-11 (Fig. 3, curves 2-5) corresponds to melting of the crystalline phase of the polymer; it follows from the Figure and Table 1 that Tmelt and values of AHmelt for the same polymer treated by different methods somewhat differ. As far as the second peak is concerned in the 73° region on the DTA curve for P-11a samples, its formation is, apparently, due to the existence of another crystalline modification and will be examined later. The absolute heat effect of melting (Table 1) is very low and reflects the low level crystalline structure of polymers. Exceeding Tmeltchanges P-5 and P-11 into the liquid crystalline state and ia the range of Tmelt-Ta-*l these polymers are in the liquid crystalline state, charac-
V. P. S~BA~'EV et aL
terized by optical anisotropy with dimensions of birefringence regions of the order of 15-10 grn (Fig. 4). Since flow temperatures of P-5 and P-11 are the same, ~ 150 ° and ~ 125 °, respectively (according to results obtained by the thermomeehanieal method), i.e. are below Ta~, both polymers above T~ change into the plastic liquid crystalline state, characterized by a displacement of birefringence regions during the superimposition of a mechanical field, as observed with typicaI low hiolecular weight liquid crystals. Transition into isotropie melt when Ta~L is accompanied for both polymers by the disappearance of small angle reflections dl and d2 characterizing the layer packing of side branches. A comparison of heat values corresponding to transition from the liquid crystalline phase to an isotropie melt, showed the following features for the polymers studied.
FIG. 4. Optical microphotographs of the liquid-crystalline phase of P-5 (a) a n d P-11 (b) a t 150 ° .
Firstly, the value of AHa~i for each of the polymers studied is practically independent of conditions of treatment since, during plotting the cm~ve of DTA, polymer samples are subjected to specific annealing above Tmelt, which offsets the difference in initial structure. It is precisely for this reason that the segmental mobility of these polymers in melt ensures a fairly high level of liquid crystalline structure, which is confirmed both by results of X-ray analysis (sharp and intensive reflexes dl and d~) and the exceptionally narrow range of melting of the liquid crystalline phase of the polymers studied (3-4 °) (Fig. 3). Secondly, a comparison of AHa-.i in the P-0, P-5 and P-11 series indicates that the minimum value of AHa-.i corresponds to P-5, which is evidence of the highest degree of structural disorder of the liquid crystalline phase, since the presence of hydrogen bonds and the short length of the methylene bridge create considerable steric hindrance in packing of mesogenic groups. An increase in the length of the lateral methylene chain, in the same way as the "replacement" of the amide bond by an ester bond on transition from P-5 and F-11 and P-0, respectively, contributes to a more satisfactory structural organization of mesogenie groups, producing a very high value of A H a - , l for these polymers, compared with P-5. Therefore, to show the liquid crystalline structure in the polymer a certain mobility of mesogenic groups sufficient for ordering is essential .This mobility may
Comb-like liquid crystalline polymers
be achieved first of all by removing the mesogenic group from the main polymer chain by flexible (e.g. aliphatic) branches and secondly, by using a "flexible" bond which connects the main chain with side branches (e.g. replacement of the amide unit which tends to form hydrogen bonds by an ester group).
33 35 p • I0-~ cm-I
FIG. 5. I B spectra of P-11 (1, 1'), P-5 (2, 2'), eopolymer M-11-A-4 (75 : 25) (3, 3') films
and solutions of P-1I in CHCIa (1") and benzene (1'") at temperatures of 25 (1", 1"', 2, 3) and 190° (1', 3'). We examined features of the phase behaviour of polymers synthesized and established temperature boundaries of existence of the crystalline and liquid crystalline phase. However, up to the present, we had not dealt with the problem of ascertaining which of the polymer chain elements determine the possibility of crystallization of polymers P-5 and P-11, that are synthesized under conditions of radical polymerization and are not therefore stereo-specific. As indicated , polymers without mesogenic groups, namely poly-N-methacryloyl-co-aminocarboI xylic acids --CH2--C(CHs)--CO--NH--(CH2)n--COOH(PMAA-n), their methyl and benzyl esters with the same values of n-----5 and 11 are amorphous polymers with no liquid crystalline properties. It could, therefore, be assumed that the ability of P-5 and P-11 to crystallize is due to the interaction of mesogenie groups joined to side chains of PMAA-n. In order to establish the nature of interaction of side groups in various phase conditions of P-5 and P-11 samples, we undertook a detailed IB spectroscopic study of these polymers and M-1 l-A-4 copolymer (75 : 25) at different temperatures. Figure 5 shows IB spectra of these polymers and copolymers in solid phase at different temperatures and in solns using different solvents. It should be noted that the fairly complex structure of these compounds hinders the accurate assignment of all absorption bands observed in Fig. 5 as a con-
V. P. SHIBAY~EVet al.
sequence of the overlapping and displacement of vibrations. Therefore, we only selected as analytical bands those which are of interest for solving the problem described (Table 3). P-11
T A B L E 3. A S S I G N I N G SOME A B S O R P T I O N B A N D S I N T H E I I ~ SPECTRU1VI OF
Assignment Out-of-plane vibrations of C--H bonds and torsional vibrations of C--C bonds of the benzene ring [11-12] Pendulum vibrations of CH, groups of methylene chains [11-12] Out-of-plane deformation vibrations of C--H bonds and torsional vibrations of C--C bonds of the benzene rings [11-12] Skeletal vibrations with the participation of C--O bonds of parts
(~ ~ /
[13-15] Fan-type vibrations of CH~ groups of methylene chains  Bond stretching vibrations of C--C bonds, deformation vibrations of CCH and CCC angles of a benzene ring [11, 12] Skeletal vibrations with a C-- O bond of the part O
'According to a former paper , the stabilization of the intramolecular structure of comb-like poly-N-alkylmethacrylamides which in some sense are simpler analogues of P-5 and P-11 polymers, is considerably influenced b y hydrogen bonds. Figure 5 shows that both for P-5 and P-11 homopolymers and a copolymer in t h e frequency range of N H the band at 3370 cm -1 has the highest intensity. This band I
corresponds to vibrations of amide groups associated as dimers H N - - C = O . . . H N I
--C----O. The intensity of the band at 3468 cm -1 of free N H groups in spectra o f P-11 is low and in spectra of the M-11-A-4 and P-5 copolymer at room temperature it is generally negligible. At the same time absorption bands are observed in spectra of these two latter polymers at 3420 (copolymer) and 3440 cm -1. The b a n d . a t 3420 cm -1 corresponds to N H groups participating in the hydrogen bond I
with the oxygen atom of the ester group v N H . . . O = C - - O , while absorption at 3440 cm -1 is also considerably influenced b y vibrations of v NH... n-electrons o f benzene rings. A P-5 film contains N H groups in the proportion of 40%, according to approximate evaluation . On heating polymers and during solution in CI-IC13 band intensity at 3370 cm-1 decreases, which points to the decomposition o f dimers formed b y amide groups. The intensity of bands at 3420 and 3440 e m - 1
Comb-like liquid crystalline polymers
remains high, particularly in spectrum P-5. I t m a y be assumed that hydrogen I
bonds N H . . . O : C - - O and NH... ~-electrons fix the intramolecular packing of side branches. However, in spite of the fact that hydrogen bonds have a significant effect on the structure of the comb-like macromolecules studied, nevertheless, it wilI be shown that the existence of hydrogen bonds with the participation of amide groups is not a deciding factor in forming a long range order in the polymers studied in the crystalline and liquid crystalline states. It has been shown in m a n y I R spectroscopic investigations [17, 18] t h a t a sudden variation of parameters of individual bands m a y promote crystal-,liquid crystal phase transition. In fact I R spectroscopic investigations show that while the frequency and half-width of bands changes little with variation of temperature, the intensity of some of them in the 25-190 ° region varies by 20-50%. Figure 6 shows temperature relations of the optical density ratio in the maxim u m of bands at an experimental temperature Dt to its value at room temperature for two polymers and a copolymer. The Figure shows that these relations differ considerably from each other and are fairly complex. Without going into a detailed study of causes of increasing or reducing the D d D o ratio on changing temperature, which is a very complex problem  and requires special examination, we only note those changes, which may help us in understanding the nature of interactions studied. Figure 6 indicates that the intensity of bands vl, v2, and v~ corresponding to vibrations of atoms of benzene rings, changes suddenly in the temperature r a n g e of crystal-.liquid crystal and liquid crystal~isotropic melt phase transition. These temperature regions in Fig. 6 are denoted by vertical lines. Some deviations in temperatures corresponding to phase transitions derived by DTA and I R spectroscopy, are due to conditions of preparing the films, which were obtained by evaporation of solvents from polymer solutions with CHC13 and as indicated, conditions of treatment of polymers have a marked effect on temperatures and heats of phase conversion (Table 1). For bands of v2 and v3 (Table 3) corresponding to vibrations of methylene groups of the hydrocarbon part of long side branches, the temperature dependence of intensity is uniform (Fig. 6, curves 2 and 5). For comparison, a study was made of the temperature dependence of band i11tensity v2 and v5 in spectra of the cetyl ester of poly-N-methacryloyl-co-aminolauric acid (PCMAA-11). It is known t h a t this polymer has a crystalline structure as a result of the crystallization of methylene side branches . It appeared that the dependence of the intensity of bands ~'2 and v3 on temperature has a break when melting the polymer. As an example Fig. 6 shows the dependence of v2 on temperature (curve 2). In other words, bands v, and 1,~ are sensitive to phase transition in those cases when it is due to disruption of the order of the arrangement of methylene chains. For polymers 1)-5 and I)-11 and copolymers M-11 with A-4 there is no such marked change in v, and vs.
V.P. S m B A Y E V e~ aL
"8'~"4" I a
"~l2 180 T,°C
FIG, 6. Relation between DdDo of absorption bands of P- 11 fllm~ (a), copolymer M- 1l - A - 4 (75 : 25) (b) and F-5 (e) a n d temperature; numbers at the curves correspond to notations of frequencies in Table 3; 1', 6'--inverse course of temperature; curve 2" corresponds to PCMAA- 11. FIG. 7. Relation between log [D(vT)/D(vs)] a n d 1/T of P-11 films (1) a n d a 0.4% solution .of P-11 in mesitylene (2). (Values of v, a n d v, can be seen in Table 3).
Comb-like liquid crystalline polymers
This means that the crystalline and liquid crystalline state in P-5 and P-11 po]ymers and copolymers is due to the orientation and interaction of benzene rings of macromolecular side branches. Furthermore, using IR spectroscopy we were able to show initial stages of ordering of side branches even in solutions of comb-like polymers using P-11. This is confirmed by comparison of the temperature course of the dependence of tho intensity of bands v~ and r s in IR spectra of P-11 films and a 0.4% solution of P-11 in mesitylene (Fig. 7). Transition temperatures shown on curve 1 of Fig. 7 in P-11 films show satisfactory correlation with temperatures of phase transition obtained by differential scanning calorimetry (compare Fig. 3, curve g). However the sudden temperature dependence of log [D (vT)/D (vs)] for a P-11 solution in mesitylene is of most interest. The existence of two inflexions on curve 2 in tho temperature range close to corresponding phase transitions in P-11 films suggests that they are related to intra or intermolecular regrouping of comb-like macromolecules, these processes preceding phase transformation in the solid. We noted previously for polymers of PCMAA series containing cholesterol t h a t intramolecular order has a significant role in forming the liquid crystalline state . Even in this case, elements of liquid crystalline structure are apparently, arranged in the solution determining later the type of structural organization of polymers in the solid phase. Based on IR spectroscopic results and results of X-ray analysis (Table 2) the type of macromolecular packing of polymers studied may be presented as follows. As shown by Table 2, independent of the method of treatment and phase condition, P-0, P-5 and P-11 polymers are characterized by the existence on X-ray curves of small angle reflections dl, which at room temperature are close to tho calculated length of side branches and arc 23, 33 and 39 A, respectively, approximating to a maximum extent to experimental values on increasing n. These results prove the layer packing of side chains, including mesogenic groups, as shown for comb-like poly-n-alkylacrylates and poly-n-alkylamethacrylates . At the same time considering IR spectroscopic results concerning the interaction of amide groups with x-electrons of benzene rings in P-5 samples it should be assumed that side chains of adjacent macromolecules of this polymer arc somewhat displaced in relation to each other so that the indicated type of interaction could be brought into practice, thus accounting for some difference in experimental and calculated values of dl. In the liquid crystalline state of polymers studied which is higher than the melting point for samples P-5a and P-1 la diffraction lines show a marked widening in the range of 4.41 A, some increase in d 1 values and a marked redistribution in the intensity of small angle reflections dl and d2, compared with the crystallino state. Without considering the causes of these changes (this requires a special study) we only note that a comparison of d 1 values calculated and experimentally derived, apparently, suggests a smectic type of packing of side chains in polymers P-5 and P-11.
V. P. SmeArEr etal.
CoTolymers M-11 with butylacrylate. We examined the structure and properties of polymers containing mesogenie groups in each unit. I t was interesting to examine the structure and properties of polymers where mesogenic group content could be varied. Copolymers of M-11 with butylacrylate (A-4) obtained by radical copolymerization (Table 4) were used. TABLE 4. TEMPERATURES TZ AND Ts-~l OF M-11 COPOLYMERS WITH BUTYLACRYLATEDETERMINEDOPTICALLY Content of M-11, molo %
Tz, °C (±5)
100 75 50 25
125 100 70 48
Ta--, i, °C
160 131 107 75
35 31 37 27
Optical investigations show t h a t films of all copolymers synthesized have a noticeable birefringence. Even on adding up to 75% "diluent" units copolymers retain their liquid crystalline properties. As shown by Table 4, the existence o f units A-4 reduced values of both T~ and Taoi without changing in practice the width of the temperature interval of the liquid crystalline state (ATI.c). B y a suitable selection of components for copolymerization the temperature range of the liquid crystalline state can be varied and polymers obtained which form a mesophase in the requisite range of temperature. Results prove t h a t the addition of mesogenie groups to polymer macromolecules by methylene groups enables the range of the fluid liquid crystalline state to be widened and completely independent behaviour of mesogenic groups achieved. This ensures the possibility of crystallization of liquid crystalline nonstereospecifie polymers and copolymers with properties inherent in enantiotropio low molecular weight liquid crystals.
Translated by E. SEVERE REFERENCES
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Comb-like liquid crystalline polymers
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