Structure of rigid chain polyimides based on the dianhydride of pyromellitic acid

Structure of rigid chain polyimides

1417

was chemically-pure grade toluene. After extraction, the samples were dried to constant weight at 150° and weighed. The quantity of gel fraction (yield of polymer) was found as the difference between the weight of the sample initially and the quantity of material extracted (passed into solution).

Translated by E. O. PHILLIPS REFERENCES 1. S. R. NANUSH'YAN, V. V. SEVERNYI, M. B. FROMBERG, A. S. CHERNICHKINA, T. S.

BEBCHUK and K. A. ANDRIANOV, Izv. Akad. Nauk SSSR, ser. khim., 2244, 1970 2. S. R. NANUSH'YAN, G. I. PASHINTSEVA and V. V. SEVERNYI, Vysokomol. soyed. A18: 519, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 3, 593, 1976 3. E. G. NOVITSKII, Dissertation, 1966 4. A. A. BERLIN, T. Ya. KEFELI and G. V. KOROLEV, Poliefirakrilaty (Polyesteraerylates). Izd. "Khimlya", 1970

STRUCTURE OF RIGID CHAIN POLYIMIDES BASED ON THE DIANHYDRIDE OF PYROMELLITIC ACID* Yu. G. BAKLAGINA, I. S. M/LEVSKAYA, N. V. YEFANOVA,

A. V. SIDOROVlCH and V. A. ZUBKOV Polymer Institute, U.S.S.R. Academy of Sciences

(Received 2 April 1975) X-ray structure analysis and the calculation of intermolecular interaction energies with an atom-atom approximation have been used as methods to study the crystal structure of three polyimidcs based on the dianhydride of pyromellitic acid and differing with respect to the number of phenylene rings in the monomeric link. The mutual packing of the molecules in the ordered regions is put forward, corresponding to the minimum lattice free energy and to the principle of dense packing of the pyromellitimide fragments. The principal intermolecular forces determining this packing are the forces of van der Waals interaction in the densely packed layers of pyromellitimide fragments.

IN INVESTIGATIONSof polymers of the polyimide class (PI), considerable attention. has been given to establishing the c o n f o r m a t i o n a l s t r u c t u r e of isolated m a c r o molecules [1-3]. H o w e v e r , in order to u n d e r s t a n d the set of desirable p r o p e r t i e s t h a t these h e a t resistant p o l y m e r s have, it is essential to establish t h e t y p e o f m u t u a l p a c k i n g of t h e molecular chains in the crystal lattice, the c h a r a c t e r i s t i c s * Vysokomol. soyed. AI8: No. 6, 1235-1243, 1976.

Ytr. G. BAXI~GrNA et

1418

al.

~ f the forces acting between the chains and the parameters t h a t control the :structure. The present paper presents the results of an investigation of the structure o f three PI, capable of crystallization, obtained from the dianhydride of pyromellitic acids and the following different diamines: p-phenylenediamine (PMPPh), benzidine (PMB) and 4,4'-diamine-p-terphenyl (PMTPh). CO

CO

The structure of these maeromoleeules is such t h a t their conformations differ ~nly in the mutual rotation of the flat rings relative to one another. In connection with this, the enhanced chain rigidity makes it possible to use these P I to calculate of structural characteristics and to establish the structure of the pyromellitimide fragment and the mutual packing of these fragments in the ordered regions. Thus the mutual positioning of the macromoleeules in the erystM cell was sought by selecting the most probable structural variant from the X-ray d a t a and calculating the minimum lattice free energy by use of the atom-atom potential method. PMPPh, PMB, PMTPh fibres were obtained by Korzhavin and Prokopchuk [4] under identical conditions from solutions of the corresponding polyamido acids (PAA) in DMF by the wet spinning method with subsequent imidization of the PAA fibres in an inert medium at 370-450°C. PI films, 30-60/1 thick, have been obtained by the chemical and thermal imidization of the corresponding PAA [3]; heating was carried out in vacuum and in air thermostats in the temperature range 20-450°C. X-ray diffraction photographs of the fibres, films and powders were obtained with nickel-filtered CuK~radiation with URS-55, ~RS-60 and DI~ON-1 equipment; l~KV-86 and RKU-114 cameras were used. The intensities of the reflections were assessed by microphotometry of the powder diffraction patterns of the PI. The X-ray diffraction patterns of the initial and the chemically imidized films of PMPPh, PMB and PMTPh are characterized by the presence of two amorphous haloes (d-~12-16 and 3-5 A), the sharpness of which is markedly increased by heating the specimens to 170-200°C. Further heat treatment of the films (200-250°C) leads to the appearance of diffraction reflections on the X-ray patterns; at 300-380°C, X-ray diffraction patterns are obtained t h a t give evidence of a high degree of ordering of the macromolecules in the films. B y comparing t h e diffraction patterns (Fig. la, b) obtained from unorientated films with the plane of the specimen positioned either perper/dicular or parallel to the incident X-ray beam, a characteristic feature of the crystallization m a y be noted, namely, She formation of an axial-planar texture. Thus the axes of the maeromolecules

Structure of rigid chain polyimides

1419

(c-axis) are positioned isotropicly in the plane of the film and, at the same time, the crystal planes with d = 4 . 2 6 A are oriented in the plane of the film. An axial texture is created in fibres and oriented films of all three specimens upon heating, the axis of the texture coinciding with the direction of the axis of extention (Fig. lc, d). The reflections observed on the X-ray diffraction pattern may be indexed if it is assumed that the equatorial lattice of all three polymers is rec-

•

..a

tie

f

_

( fl L ~t

\

/f/

_

/ I ,c,,

"1/

d

,/I,/ /

I

J/

FIG. 1. Diagram showing the positions of the reflections on the X-ray diffraction patterns from: a a n d b unoriented P M P P h (X-ray beam perpendicular to and parallel to the plane of the specimen respectively in (a) and (b)); c - - P M P P h fibre; d - - P M B fibre. DE stands for the direction of the axis of extension.

tangular with parameters a--~8.584-0.05, b-----5.48±0.05 • and z = 2 . The period along the axis, calculated from the meridional reflections 001, was found to be equal to 12.3 A for PMPPh, 16.6/~ for PMB and 20.9 A for PMTPh. Thus the c period which determines the length of the monomeric link increases on going from P M P P h to P M B and from PMB to P M T P h b y 4.3/~, which is equal to the lengthening of the diamine component resulting from the coupling of a phenylene nucleus [5]. The densities of the crystalline regions, Por, calculated for two mohomeric units in the cell, and the experimental values of density, Pexp, are as follows: Per= 1.66, 1.56 and 1.49 g/cma; Pexp= 1.56, 1.48 and 1.45 g/cm a for PMPPh, PMB and P M T P h respectively. The fact that the ab section of the unit cell remains unchanged is evidence that the geometrically similar molecules of the series investigated b y us form exclusively a similar packing pattern. Precision measurements of the parameter c for the polymer PMPPh, PMB and P),]TPh enabled the dimensions of the pyromellitimide fragment to be determined and, b y taking into account the known data concerning the structure of the phthali-

Y u . G. BAI~T.AOINA et al.

1420

mide ring [6], a suggestion for its configuration in the polymer to be put forward: ,?,~o

o

o

o

The small number of reflections on the X-ray diffraction patterns and their overlapping does not make it possible to determine the space group unambiguously. The odd orders were absent from the reflections h00 and 0b0. I f these absences are not due to chance, the group P21212 m a y be proposed. However,

1

_

_

~

t_

t_ _ _ _ j

FIG. 2. Three possibilities for the positioning of the pyromcllitimide fragments in the ab section. only the reflections of even orders of h and k (021, 203, 404, 204) were found on the X-ray diffraction patterns in the small number of 0kl and 0k0 reflections, the structure was also analysed for the spece group P b a 2 . Having assessed the since

TABLE 1. POSITIONS OF THE STERIC-INTERACTION MINIIVLk FOR DIFFERENT TYPES OF PACKING EI,

~, deg

kcal/molo

45 50 55 58 60 62 65 68

--83.5 --89.9 --94.7 --95-3 --93.9

~, deg 20

--88.9

--77.2 --51-1

15

10

~II, keal/mole

--5

--10 --15 --20

--80.6 --75.9 --70.3

0

kcal/mole

50 45 40 35 30 25

165.7 --37.0 --76.75 --43-10 115.55 705.00

-- 70.3 --75.9 - - 80.6 --83.5 --84.5 --83.5

5

EIII,

~, deg

special features of the diffraction patterns obtained from fibre, oriented and unoriented film and from powder, we were led to the conclusion t h a t we could a t t e m p t to establish the structure by a purely geometrical method by using the

Structure of rigid chain polyimides

1421

principle of dense packing. B y assuming that the pyromellitimide fragment has a planar structure and is the initiator of the intermolecular packing of the P I chains [4], we attempted to realize, in the first instance, the principle of dense packing of these fragments. Figure 2 (I) shows the most probable version oi the mutual positioning of the pyromellitimide fragments in the ab section, which was obtained as a result of calculating the structural amplitudes as functions of the angle ~ which characterizes the position of the plane of this fragment relative to the crystallographic plane 010. At the same time, the intermolecular interaction energy of the pyromellitimide fragment was assessed using the atoma t o m approximation from the equation [7]

E ~ - 5 . 2 0 3 x 10er~12--2"248 X 10ar~ 6, where r e ~ ' - - r ~ - - r l - l - r o (r~ and rj are the van der Waals radii of the atoms i and j; r0+3-8824 A, a constant; and r is the distance between the atoms i and j). The interaction between one molecule A and the six nearest neighbours was considered in the calculation (Fig. 2). The best agreement between the diffraction pattern theoretically calculated for P M P P h and the experimental pattern was iound for the version I with values of q = 5 0 - 6 0 °. Calculation of the intermolecular interaction energy in this case also gave evidence of the existence of a minimum energy (Table 1) (Emin=--95 keal/mole, ~-~58 °) having quite a slope (80-95 kcal/mole) in the range of angles 45°< ~ < 6 5 °. When the angle ~ is varied within wide limits, a relative minimum in energy is observed at 9 = 0 ° ( E ~ - - 8 4 kcal/mole). B y varying the mutual positioning of the pyromellitimide fragments in the unit cell, we found yet another energy minimum corresponding to the case I I I in Fig. 2. In this structure III, the potential trough ( E = - - 7 7 kcal/mole, q = 4 0 °) is very narrow and deviation from the minimum b y ± 1 0 ° gives large positive values of energy. It follows from this that this structure should be unstable with respect to thermal vibrations of the lattice.

30

.~g

f5"13 ~°

Fro. 3. Dependence of the conformational energy on the angle of internal rotation. The contribution of electrostatic interaction to the intermolecular interaction energy was also calculated; the atom charges used were obtained b y quantumchemical calculation with the CNDO/2 method [8]. We were thus satisfied that,

1422

Yu. G. BAKLAGINAet al.

in all the cases considered, it did not play a substantial role since the contribution of the electrostatic energy to the total interaction energy did not exceed 0.5%. Thus the version of the positioning of the macro chains in the cell (the herringbone) [9], which was proposed on the basis of analysis of the X-ray data, corresponds to the lowest minimum of the intermolecular interaction energy. Having assessed the features of the diffraction pattern and taken into account the fact t h a t the molecules of all three P I always retain their own symmetry, 2, in the ordered retions, t h a t is, a second order axis along the polymer chains, we calculated t h a t the most probable space group is Pba2. The intensities of the reflections, I (hk,l), were calculated for various conformations differing in the angle of rotation, ~, of the pyromellitimide fragment relative to the diamine part of the molecule, with the assumption t h a t the phenylene rings of PMPPh, PMB and PMTPh lie in a single plane. In order to determine the angle ~ in an isolated molecule, the eonformational energy of the fragment was calculated quantumchemically by the EHT* method [10]. I t m a y be seen from Fig. 3 t h a t the energy profile has two minima ( ~ = 5 0 and 130 °) and a small maximum of approximately 1-4 kcal/mole at ~g--~90 °. TABLE

2. E X P E R I M E N T A L

AND

CALCULATED INTENSITIES

FOR

PMPPh

AS F U N C T I O N S

OF

THE ANOLES ~ AND Arcalc

hkl 110 220 310/ 1~o) 212 oo2

Iexp 100 57 19 30 3O

~ 56° _-50oj

100 25 ~'5 30

=60o]

(o= 58° =90 o

_

50o1 ,= 00o1 ,= 90o

q)= 60°

50o1 ,= 60o1 ,= 90o

100 24

100 36

100 35

100 32

100 46

100 47

100 44

100 58

7

13

8

8

15

9

9

19

12 30

24 30

9

30

12 30

25 30

8 30

11 30

30 30

Table 2 shows, in relative units, values of the intensity of the reference reflections obtained by photometry of P M P P h powder photographs (Fig. 4), as well as the intensities calculated for angles ~ = 5 0 , 60 and 60 °. The best agreement between the experimental and calculated intensities (with the temperature factor B----4), obtained for ~ = 9 0 and 60 °, makes it possible to propose a structure for an ordered region of P M P P h (Fig. 5). Tiffs does not contradict the conclusion concerning the value of the angle ~ for the isolated molecule, since the slight loss in conformational energy is masked b y the intermolecular interaction in the crystal. The interplanar spacings and indices of the observed reflections and also of the reflections which are absent because of low values of intensity (up to 0.1) a r e shown in Table 3. Table 4 gives the atomic coordinates for the independent * EHT--extended H~ckel theory.

Structure of rigid chain polyimides

1423

flO A 2OO /~j~

O02

i "~

I

14,

I

18 I

I

22 1

/ / f ~ ~

i

25 I

/

30 I

~

3428,c[.eq38 t

I

-7--

Fro. 4. Intensity curve obtained by p h o t o m e t r y of P M P P h diffraction photographs. The. numbers on the curve are the indices of the reference reflections hkl.

b

a

FIG. 5. a - - C r y s t a l structure of P M P P h ; b--projection of the structure on to the plane ac; b - - p r o je c t io n on to ab.

1424

Yu. G. BAKLAGINAet at.

s e c t i o n o f a P M P P h molecule. I t m a y be seen f r o m Fig. 5 t h a t densely p a c k e d layers of the large p y r o m e l l i t i m i d e f r a g m e n t s (A) a l t e r n a t e with less dense layers o f p h e n y l e n e rings (B). As a result o f this, a z i m u t h a l disorder m a y be e x p e c t e d t o be f o r m e d in layer B in the angular interval 24~, which does n o t lead to strong ~rABLE 3. EXPERIMENTALAND CALCULATEDVALUESOF THE INTERPLANARDISTAI~CES,d, AND II~TENSITIES FOR PMPPh ((p=60°; ~ = 9 0 °) hkl

dexp

dcalc

110 200 020 210 ~10~

4"64 4-26

4"62 4"29 2"74 3"37 2"53 2-61 2-31 2-00 1.97 2"15 1"95 1"69 1"64 1-67

120j 220 410] 4111 400[ 32P 4201

3"37 2"55

1-97

510 /

231:. 232 131J 520 021

1"69

Iexp -~calc

V.S. S

av av

W

v.w.

1.62 1"43

2"60

1.77 1.46 2.67

100 57

v.w. v.w.

19

100 57-6 0.2 28-0 14-4 4.7 0.7 1.9 2.3 1-2 4.4 0.9 0.3 0.5 2.1 0.8 2.7 4.6

hkl

dexp

d~al~

211 112 212 312 / 122/ 113 203 214 124 /

3"25 3"68 2"93

2.28

3"26 3"70 2"96 2"34 2"40 3"07 2"96 2"27

2"03

1"99 1"76

404j 114 /

2"30

2o4j

2"49

001 002 003 OO4 005 006

12"37 6"13 4"11 3.08 2.46 2.05

~'ote:a-visual assessmentof the intensity of the reflection; av--average;w--weak;v.w.--veryweak.

2"56 2"50 12"30 6"15 4"10 3"08 2"46 2"05

Iexp

Iealc

av

w" ~v

30

~v

W V.~W.

V.'~V. V.W. ~V

3O

V.W. aV V.W. W

b--intensity of the reference reflections;

32"0 8"7 29"7 13"0 1"6 0"1 0"1 9"0 2"8 3"0 4"6 0"3 4"2 30-5 1.4 11.5 0.8 5.8 v.s.-very

strong; s-strong;

.shear disturbances in the p o l y m e r i c n e t w o r k [9]. This t y p e o f d i s t u r b a n c e is e q u i v a l e n t to t h e r m a l t o r t i o n a l v i b r a t i o n s of t h e p h e n y l e n e rings a r o u n d t h e a x i s o f t h e molecule a n d leads t o considerable weaking o f the equatorial reflect i o n s even in regions w i t h i n t e r p l a n a r spacings o f 2.5-3 A whereas t h e diffraction field of the meridional reflections e x t e n d s t o d ~ 1-5-2 A. T h e t r a n s i t i o n f r o m P M P P h t o P M B a n d P M T P h , t h a t is, a n increase in the n u m b e r o f p h e n y l e n e rings in the m o n o m e r link, m u s t lead to the p r o p o r t i o n o f t h e specific v o l u m e •occupied b y less dense layers B being increased, evidence o f this coming f r o m the c h a n g e in the e x p e r i m e n t a l a n d also theoretical values of density [4]. I n t h e less d e n s e l y p a c k e d layers o f P M B a n d P M T P h , it is n o t necessary for d i p h e n y l a n d t e r p h e n y l to t a k e u p a flat c o n f o r m a t i o n [11], but, f r o m t h e diffraction patt e r n obtained, t h e r e is no possibility of more a c c u r a t e l y defining the s t r u c t u r e o f P M B a n d P M T P h or o f establishing the relative a z i m u t h a l position of the p h e n y l rings in t h e chain. T h u s the m u t u a l packing t h a t we h a v e proposed for the molecules in o r d e r e d

Structure of rigid chain polyimides

1425

regions o f all three p o l y m e r s corresponds to the m i n i m u m on the e n e r g y surface calculated b y means of a t o m - a t o m potentials and the principle of dense p a c k i n g o f the p y r o m e l l i t i m i d e fragments. The principal intermolecular forces deterTABLE 4. A~rOM COORDI:NATES

Atom C1

C2 Cs N, O1 CA C5

X ~-0.081 ~-0-039 Jr 0.069 0 ~-0.133 0 -~0.122

Y

Z

Atom

X

Y

Z

--0.220 --0.107 --0.187 0 --0-361 0 -[-0.111

0 0-097 0.210 0.273 0.248 0.386 0.443

C6

30.122 0 ~-0-133 0 +0.069 ~-0.039

-~0.111 0 --0.361 0 --0.187 --0-107

0.557 0"614 0"752 0"727 0'790 0"903

C7 O~ N~ Cs C9

mining this p a c k i n g are forces o f v a n der Waals interaction in densely p a c k e d layers of p y r o m e l l i t i m i d e fragments. The conclusion concerning the decisive role p l a y e d b y p y r o m e l l i t i m i d e f r a g m e n t s in the intermolecular p a c k i n g o f P M P P h , P M B a n d P M T P h molecules agrees with o t h e r results [12] t h a t showed t h a t the relaxation b e h a v i o u r of polyimides w i t h different s t r u c t u r e s d e p e n d s m a i n l y on t h e s t r u c t u r e of the d i a n h y d r i d e p a r t of the chain's repeating unit. Translated by G. F. MODLEN REFERENCES

1. L. G. KAZARYAN, D. Ya. TSVANKIN, B. M. GINZBURG, Sh. TUICHIYEV, L. N. KORZHAVIN and S. Ya. FRENKEL', Vysokomol. soyed. A14: 1199, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 5, 1344, 1972) 2. Yu. G. BAKLAGINA, N. V. MIKHAILOVA, V. N. NLKITIN, A. V. SIDOROVICH and L. N. KORZHAVIN, Vysokomol. soyed. A15: 2738, 1973 (Translated in Polymer Sei. U.S.S.R. 15: 12, 3109, 1973) 3. N. A. ADROVA, A. I. ARTYUKHOV, Yu. G. BAKLAGINA, T. I. BORISOVA, M. M. KOTON, N. V. MIKHAILOVA, V. N. NIKITIN arid A. V. SIDOROVICH, Vysokomol.

soyed. AI6: 1658, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 7, 1921, 1974) 4. Yu. G. BAKLAGINA, N. V. YEFANOVA, N. R. PROKOPCHUK, L. N. KORZHAVIN,

5. 6. 7. 8. 9. 10. 11. 12.

A. V. SIDOROVICH, F. S. FLORINSKII and M. M. KOTON, Dokl. AN SSSR 221: 609, 1975 A. I. KITAIGORODSKH, Molekulyarnyye Kristally (Molecular Crystals). Izd. "Nauka", 1971 C. S. PETERSEN, Acta Chem. Scand. 23: 2389, 1969 T. M. BIRSHTEIN, A. N. GORYUNOV and M. Ya. TURBOVICH, Molek. biol. 7: 683, 1973 J. A. POPLE and G. A. SEGAL, J. Chem. Phys. 44: 3289, 1966 B. K. VAINSHTEIN, Difraktsiya rentgenovykh luchei na tsepnykh molekulakh (Diffraction of X-rays by Chain Molecules). Izd. AN SSSR, 1963 R. HOFFMAN, J. Chem. Phys. 39: 1397, 1963 A. HARGREAVES and S. ttASANKIZVI, Acta Crystallogr. 15: 365, 1962 T. I. BORISOVA, M. I. BESSONOV arid A. P. RUDAKOV, Sb. Sintcz, struktura i svoistva polimcrov (Collection: Synthesis, Structure and Properties of Polymers). Izd. "Nauka", p. 88, 1970