Polymerization in liquid crystals—IX

European Poll.lu Journal Vol. 17. pp. 865 to 870. 1981

0014-3057,/81/080865-06502.00/0 Copyright © 1981 Pergamon Press Lid

Printed in Great Britain. All rights reserved

POLYMERIZATION IN LIQUID CRYSTALS--IX* STRUCTURAL INVESTIGATIONS OF COPOLYMERS CONTAINING p-METHYL-p'-ACRYLOYLOXY AZOXYBENZENE AND CHOLESTERYLVINYL SUCCINATE

F. CSER, K. NYITRAI,I. KocsIs and G. HARDY Research Institute for Plastics, H-1950 Budapest, Hungary (Received 2 December 1980) Abstract Structure of copolymers of p-methyl-p'-acryloyloxy azoxybenzene (MAAB) with cholesterylvinyl succinate (CVS) was studied by small- and wide-angle X-ray diffractometry, DSC, polarizing microscopy, and thermomechanical methods. The copolymers were found to be anisotropic non-crystalline substances in which the radial distribution of the radius of gyration of inhomogeneities corresponds to that of the diameter of the molecules determined by GPC. Both phase transition temperatures and clearing points of copolymers show a minimum as functions of the composition. On both sides of this minimum, the copolymers have different structures. At higher MAAB contents, the copolymer structure is similar to that of polyMAAB; at lower MAAB concentrations, alternating copolymer is formed similar in structure to polyCVS. For identification of the structures, copolymer/comonomer phase diagrams were also investigated.

In a previous paper on the copolymerization of p-methyl-p'-acryloyloxy azoxybenzene (MAAB) with cholesterylvinyl succinate (CVS) [1], the composition of the copolymer was found to be the same for reactions in the isotropic liquid and in the mesomorphic state. It was shown for the homopolymerization of MAAB [2] that the polymer was formed simultaneously in two nematic phases [3], viz. the nematic monomer and the nematic plasticized polymer. In the isotropic liquid state, the polymerization proceeds in the nematic plasticized polymer phase throughout the reaction except in the initial period. On the contrary, the polymer formed in the homopolymerization of CVS [4] brings the reaction about at first in the isotropic and then the smectic [1] plasticized polymer solution. Structural characteristics and molecular mass distribution of both homopolymers of MAAB and CVS are almost the same in the various stages of polymerization [2, 4]. In the present paper, results of structural investigations of their copolymers are reported.

X-ray scatterings were studied in transmission mode. Wide-angle diffractograms were recorded by a Philips powder diffractograph while the small-angle scattering was measured by a Rigaku Denki diffractometer. Ni-filtered CuKc~ radiation was used. The transmission technique permitted a diminution in the lower limit of the measurement to 20 = 1.9°; after background correction; useful inlbrmation could be obtained up to 20 = 1.3°. On the basis of the small-angle records, periods of electron density inhomogeneities were calculated by the Kratky-Porod method using the supposition of spherical, cylindrical, and planar inhomogeneities [6, 7]. Size distribution functions of the electron density inhomogeneities were also computed [8]. Polymer~monomer phase diagrams The copolymer/comonomer phase diagrams were recorded by polarization microscopy, thermomechanical investigations, and DSC measurements as described previously [2,9]. The starting monomer mixture of the copolymerization reaction was designated as monomer and the resulting copolymer as polymer. RESULTS

EXPERIMENTAL

Copolymerization studies Copolymers were prepared as described previously [1]. Three monomer compositions were used according to the phase diagram of the comonomers, one at about the eutectic composition and two at the midpoints between the eutectic composition and the pure monomers i.e. at 70, 40 and 20 mol?/,, of MAAB. The copolymers contained 85, 70.2, and 62.2 mol% of MAAB respectively. The reactions were conducted up to 20-30~o conversion. Copolymers were isolated and purified as reported previously [1]. X-ray d(ffracliometric studies Copolymers were pressed into tablets at 200 MPa. Their * Part VIII: G. Hardy, F. Cser and K. Nyitrai, Isr. J. Chem. 18, 233 (1979).

Wide-angle X-ray scatterings of homo- and copolymers are presented in Fig. 1. Two diffuse peaks can be recognized in the diffractograms at 11-23 and at 0-5 °. Maximum locations and relative intensities of these diffuse peaks are shown in Fig. 2. The size of the period of the wide-angle scattering maximum, characteristic of the packing of side chains [10], increases linearly with CVS content. The increase is greater than expected on the basis of additivity. The period value of the peak in the small-angle range increases also at a greater extent with the CVS content than calculated by the additivity rule. The copolymers essentially show the structural characteristics of a polyMAAB. Figure 3 demonstrates the Guinier [6] and the 865

866

F. CSERet al.

¢ (J I/l

I

o

t_

>hz i,i

z l

0

I

I

I

I

5

I

I

I

I

i

I0

I

i

I

I

I

I

I

15

I

I

I

I

I

I

I

20

I

I

i

i

l

25

I

i

30

20* Fig. 1. Wide-angle X-ray scattering of homo- and copolymers. Figures by the letters are proportional to the MAAB content of the copolymer. Kratky-Porod [7-1 representation of X-ray scattering of the three copolymers around 20 = 0 °. Relative errors of fitting the straight lines to both representations are identical. Gyration lengths calculated from the observed slopes correspond to nearly identical inhomogeneity periods. Thus, the formation of inhomogeneities (spherical, cylindrical or planar) cannot be ascertained on the basis of these representations. Sizes of inhomogeneities of samples with the highest CVS content fall into two distinct components viz. 4.4 and 10.4nm. The geometrical mean of these two components corresponds to the values observed for the other two samples. The size distribution function of inhomogeneities is illustrated in Fig. 4. The size of the maximum probability decreases along with the reduction of the asymmetry of the distribution function at greater sizes as CVS content is increased. Molecular size distributions of the copolymers and both homopolymers are shown in Fig. 5 as calculated from their GPC records. Average sizes (RN, RM) and polydispersities obtained from the size distribution functions and average electron density inhomogeneity values (calculated from the small-angle X-ray diffractograms) are collected in Table 1. No great changes are observed between the molecular mass distribution of CVS samples formed in the liquid and the cholesteric states I-4]. Polymers prepared in the isotropic liquid state have however smaller average size than those in the mesomorphic state. The two molecular mass distribution curves of polyMAAB in Fig. 5 refer to samples prepared in the nematic and isotropic liquid phases. Average molecular masses of polymers from the nematic state are higher than those produced in the isotropic liquid. Both homopolymers

have bimodal distributions; for the polymer formed in the nematic state, probability ratio of the peak of higher molecular mass is greater while, for that obtained in the liquid state, the ratio is reversed. Size of the molecule measured in THF solution decreases with increasing CVS content, accompanied by a reduction in the polydispersity. Polydispersity decreases with increasing CVS content. Polydispersity of the sample marked 4:6 is enhanced to a great extent by a low fraction while the distribution of the main fraction is not different from that of the other copolymers. 4.6

I 4.0i

E c d

~.o

!

/

24

°5"f / 0.50

0

/

0.2

0.4

0.6

0.8

ID

M

Fig. 2. Bragg periods from peak maxima in X-ray diffractograms of the copolymers plotted against composition (MAAB content).

Polymerization in liquid crystals--IX n

867

xxx

I

ooo

~,

oll

............ + ¢,%

@

~'~t"++~V.,I.~'X'X-X-X-x-X.I(_ - . , + + . + . + . ~ . ~ x x x------_~. =mr, -r "("4- ÷ ÷ ,vJ~,

+÷

• .

..

,

o

..~.~o.e_o_o..O_o~ ~

eoo x + ÷

3-

o-o-O-o-ro-o"o-o-

-e-e • •

_J

,

,

4

,

,

,

8

,

,

16

fO" 8

Fig. 3. Guinier

(+)

/\

,

,

,

20

,

,

,

24

=

and Kratky-Porod ( ×, ©) processing small-angle X-ray scattering of MAAB/CVS copolymers.

For further study of structures of the copolymers, phase diagrams of copolymer/comonomer systems were recorded. Phase diagrams should indicate the transitions observed in phase diagrams of the monomers (such as eutectic points, melting points, clearing points), the transition points of the copolymers, and the transitions brought about by the interactions. Considerable thermal effects are associated with the melting and dissolving of crystals. Figures 6, 7 and 8 illustrate the phase diagrams of monomer blends of MAAB/CVS at 0.2:0.8, 0.4:0.6, and 0.7:0.3 ratios and copolymers therefrom; symbols [~, V, O denote T~, fusing point, and clearing point respectively. Intensity of the depolarized light is constant up to the temperature indicated by • then begins to increase. At the temperature indicated by V,

O. 8

,

12

the rate of change is enhanced by the phase transition. At the temperature shown by ×, the light intensity decreases while at + , this decrease is accompanied by the appearance of an isotropic phase; finally at O, the system is homogeneous and isotropic. Each of the three phase diagrams exhibits a composition above MC MC Me MC Me

14

:.-..

12

t ft..... ~

0 4 7 IOL IOLC

-.... .... ....... ....

1.0

i,},

0.8

" I

0_6 0.4 ~x

O2

0,6

x 0.4

-6.5

~.~

',..5,.',. -6.0

-5.5

-50

-4.5

LG(R),cm _

°2

~,, o -

xx~ x~...IVlC2

Table 1. Average sizes (nm) of homo- and copolymers of MAAB and CVS derived either from the molecular mass distribution determined by GPC or from the size distribution of inhomogeneities measured by the small-angle X-ray diffractometry

0.4

/ o

O _615

Fig. 5. Distribution of gyration radii of molecular size of homo- and copolymers, ~om GPC measurements.

, \ % . _ MC4

/ n -6.0

~ -5.5

MCj7

LG ( R ] tom Fig. 4. Distribution function of gyration radii of inhomo-

geneities in the copolymers from consideration of intensities of small-angle X-ray scattering in the vicinity of origin.

Sample MAAB LC MAAB I 7:3 4:6 2:8 CVS

RN (nm)

RM (nm)

(R2) 1/2 (nm)

X-ray (nm)

7.3 6.8 7.7 2.9 -5.6

26.2 18.2 20.8 10.9 -7.6

9.29 7.85 8.73 3.41 -5.93

-15.0 12.5 8.5

RM/RN 3.6 2.7 2.7 3.8 -1.4

868

F. CSER et al, 180 160 140

A 120

40 20

i 0

, , i i jl 0.2 0.4 0.6

i

i 0.8

i 1.0

Xp

Fig. 6. Copolymer/comonomer phase diagram of the MAAB/CVS system. Initial molar ratio of MAAB to CVS is 0.2:0.8. [] = T~; V = TI from thermomechanical measurements; O = clearing point; 0, V, x, + = changes in the texture observed by polarization microscopy i.e. in the intensity of depolarization light. A = isotropic liquid; B2 = mesomorphic 2; B4 = mesomorphic 1; C = highelastic B4; D = glassy B4; B1 = A + B2; B3 = B2 + B4; E = B1 + cholesteric monomer mixture; F = B2 + cholesteric monomer mixture; G = B4 + B2 + cholesteric monomer mixture; H = B,, + cholesteric monomer mixture + crystalline monomer (CVS in this case); I = C + crystalline monomer + eutectic monomer mixture; J = D + crystalline monomer + eutectic monomer mixture. A peculiarity of the present phase diagram is the split of Region C into two parts. Between C1 and B4, a change in texture is observed. which (i.e. at higher copolymer contents) no crystalline phase can be detected. Region A of the phase diagrams denotes isotropic copolymer solutions. In Regions E-J, plasticized copolymers from equilibrium phases with monomers. Regions I and J are separated by the Tg of plasticized

polymers which coincides with the polymorphic transition point of MAAB [1]. Regions I and H are divided by the melting point of the eutectic mixture of the monomers corresponding to the large endothermic peaks in DSC records (Fig. 9). The area under the peak decreases to a greater extent than the area referring to the dissolution of crystals as functions of the increasing copolymer content. The over-all melting heat decre~tses proportionally with the copolymer content intersecting the zero level at Wet composition (Fig. 10). In Region H, therefore, the eutectic monomer mixture of cholesteric state forms a thermodynamic system with the excess of CVS and with the crystalline phase of MAAB as well as with the plasticized liquid copolymer phase. As the temperature is increased, the solid crystalline phase dissolves then disappears on reaching Region F. Temperature points on the border between Regions G and F are associated with the inherent phase transition of the copolymer involving no detectable thermal effect. No great thermal effect is found in the region of mesomorphic to isomorphic transition since the small enthalpy change accompanied by a big temperature change vanishes into the base line. In Regions D and C on the copolymer side, homogeneous glassy and homogeneous highly elastic copolymer phases exist respectively. In Region B4, the copolymer is in the liquid state with anisotropy showing increasing birefringence with temperature. Since Region B4 widens with decreasing temperature then narrowing in contact with Region H, it is supposed that the copolymer dissolves monomers in Region H. The extent of dissolution increases with CVS content. In Region Ba, the intensity of the depolarized light increases rapidly with temperature. The same change can be observed in Region F, thus the boundary between Regions H and G shows the structural changes in the copolymer. Entering Region B2, depolarized light intensity decreases abruptly with increasing temperature. This transition is ac-

,,°f

180 160

140

P. ,oo

40-[]

+./

-

B B ~~/~/3? ~ !

~ In

~

o

,,!E~/

D

J 0

0.2

0.4

0.6

0.8

1.0

Xp

Fig. 7. Copolymer/comonomer phase diagram of the MAAB/CVS system, lnitial molar ratio of MAAB to CVS is 0.4:0.6. Designations as in Fig. 6.

0

0.2

0.4

0.6

0.8

1.0

Xp

Fig. 8. Copolymer/comonomer phase diagram of the MAAB/CVS system. Initial molar ratio MAAB to CVS is 0.7:0.3. Designations as in Fig. 6.

Polymerization in liquid crystals--IX

869

Copolymer

:.-."/q /~/~

. . . . . .

310

350

- --\" - - X "

400

,

-- I r - - - ~

440

~

/~ 0.05

0/.9 T, K

Fig. 9. DSC records of blends of a monomer mixture at a molar ratio of MAAB to CVS of 0.7:0.3 with various amounts of copolymer. Figures by the curves are proportional to the mass fraction of the copolymer. companied by a weak unquantifiable endotherm while the system remains anisotropic and homogeneous. The transition coincides with the disappearance of the crystalline monomer phase in Region G - F . Reaching the Region Bt or E, the isotropic phase appears. It can be established from the phase diagrams that the copolymerization [1-1 started in the cholesteric state (at 83°), soon becomes heterogeneous due to the appearance of the isotropic phase where the enhanced reactivity results in copolymers with properties (such as composition, molecular mass) essentially indicating reaction in the isotropic state. Transition temperatures of homo- and copolymers are shown in Fig. 11. Tg follows the additivity rule while fusing point, transition points and clearing

point have minimum values. The copolymers show structural features of polyMAAB undergoing two textural and thermal phase transitions on cooling. These changes are not detectable in the X-ray diffractograms. It is remarkable that this system at the minimum of clearing point corresponds in composition to that containing the smallest possible MAAB blocks estimated by the reactivity ratios of the copolymerization.

280 260 240 220

6O

50

200 e ~

180 160

30

140

zo

~

,o

120

~

I00 80

02

o.,

0.6

oe

,.o

Xp Fig. 10. Integrated melting heats of blends of a monomer mixture at a molar ratio of MAAB to CVS of 0.7:0.3 with various amounts of copolymer plotted against the mass fraction of the copolymer. • = Over-all melting heat; ~7 = melting heat of the crystalline component; x = melting heat of the eutectic mixture.

60

i 0

J i i l I I I I 0.2 0.4 0.6 o.e 1.0

CVS

Fig. 11. Transition temperatures of MAAB/CVS copolymers plotted against the mole fraction of CVS. [] = Tg; ~7 = Ts, x and (:3 = mesomorphic to mesomorphic transition; + = clearing point.

870

F. CSERet al. DISCUSSION

CVS/MAAB copolymers cannot be considered as random copolymers. The ratio of propagation rate constants for CVS is very low (rcvs = 0.02) so that the copolymers can be regarded as polyMAAB blocks separated by single CVS units. Lengths of polyMAAB blocks are inversely proportional to the CVS content of the copolymer: 1 nMAAB - XCvs"

Extrapolated values of Tg and fusing point of copolymers intersect at 1:1 monomer ratio (alternating copolymer) around 80° which is the nematic melting point of MAAB. The minimum value of clearing points is extremely high (170°) suggesting that the formation of polymer chain results in a structural arrangement different from that of the monomer in its nematic state. The size of copolymer molecules is reduced by increasing CVS content in good agreement with kinetic data for CVS, which indicate that the molecular mass is determined by chain transfer with monomer. The concentration of terminal CVS units, consequently probability of the chain transfer, is enhanced by increasing the CVS content. The size of the polymer from a system with monomer composition of 7 to 3 is identical to that of the homopolymer formed in the isotropic liquid of MAAB. Figure 8 shows that the polymerizing system soon enters Region E of the phase diagram which is the region of the isotropic liquid. Since the samples used for structural studies were obtained at conversions of 30~, effects of the isotropic state suppressed those of the mesomorphic state. Homopolymer of MAAB was formerly considered nematic [2]. Later it was demonstrated [10, 11] that the related mesomorphous polymers have aperiodic helical structure when the polymeric molecule becomes rigid due to the radial arrangement of the

side-chains around the main polymeric chain. The polymer molecule itself is amorphous and disordered. Characteristics of aperiodic helix can be found also in copolymers. The size of distribution of molecules measured by GPC in solution is very close to that for inhomogeneity determined by small-angle X-ray diffractometry. Consequently the inhomogeneity surfaces detected by the small-angle X-ray scattering represent the macromolecules themselves, supporting our previous assumptions [10, 11] about their aperiodic helical character. Acknowledgements--The authors are indebted to Professor G. Bodor for the small-angle X-ray diffractometric measurements and to Dr G. Samay for the gel permeation chromatograms.

REFERENCES

1. K. Nyitrai, F. Cser, 1~. Seyfried, M. Lengyel and G. Hardy, Eur. Polym. J. 13, 673 (1977). 2. F. Cser, K. Nyitrai, 1~. Seyfried and G. Hardy, Eur Polym. J. 13, 678 (1977). 3. F. Cser, K. Nyitrai and G. Hardy, Acta Chim. Acad. Sci. Hung. 100, 464 (1979). 4. K. Nyitrai, F. Cser, G. Csermely, Bui Doc Ngoc, L. Fiizes, G. Samay and G. Hardy, Eur. Polym. J. 14, 467 (1978). 5. F. Cser, K. Nyitrai and G. Hardy, Mesomorphic Order in Polymers and Polymerization in Liquid Crystalline Media (Edited by A. Blumstein),p. 95 ACS Symp. Ser. No. 74, Washington (1978). 6. O. Krakty, Prog. Biophys. 13, 107 (1963). 7. A. Guinier, C.r. Hebd. Acad. Sci. 204, 1115 (1937). 8. P. W. Schmidt, Lecture in the Summer School Diffraction Studies on Non-Crystalline Substances, p. 51, P6cs, Coll. Abstr. (1978). 9. G. Hardy, F. Cser, G. Kowlcs, J. Szatmhri and G. Samay, Acta Chim. Acad. Sci. Hung. 79, 143 (1973). 10. F. Cser, J. Phys. 40, C3-459 (1979). 11. G. Hardy, F. Cser, K. Nyitrai, A. Kall6 and G. Samay, J. Cryst. Growth 48, 191 (1980).