JOURNAL OF POLYMER SCIENCE: Polymer Physics Edition VOL. 14, 1553-1560 (1976)

An X-Ray and Conformational Study of Kapton H

G. CONTE, L. D'ILARIO, and N. V. PAVEL, Laboratori Ricerche di Base, SNAMPROGETTI S.p. A., Monterotondo, Rome, Italy; and E. GIGLIO,

Istituto di Chimica-fisica, Universitd di Roma, Rome, Italy

Synopsis

Potential energy calculations were performed on the isolated chain of Kapton H (KH) in order to obtain information on the most probable conformations. A model with a C-0-C angle of 130° and with the segments joining the ether oxygens forming a planar zigzag chain satisfactorily fits the x-ray experimental data. Although the equatorial reflections are very few and diffuse, it seems likely that the space group is P21 and the unit cell parameters are a = 4.66 A, b = 32.9 A, c = 5.96 A, p = 100". The observed density is in agreement with two monomers in the unit cell. An intrachain vector distribution map was computed by rotating (angle 0) the pyromellitic acid diimide (DIPA) unit about the N-C bond in the KH monomer. This map was compared with a one-dimensional Patterson synthesis calculated along b, the fiber axis, to establish the value of 0. Van der Waals' energy cal- culations were subsequently accomplished in the crystal as a function of 0 and of the rotation around the helical axis of the KH chains. The results allowed us to choose a reasonable macromolecular conformation and packing in the crystal. KH is mainly ordered along the fiber axis but shows little order in the packing of adjacent helixes. The macromolecules are held together by Van der Waals' and, probably, by charge-transfer forces.

INTRODUCTION

The aromatic polyimide, available under the registered tradename Kapton H (Du Pont) is synthesized from pyromellitic dianhydride and diaminodiphenyl ether.' The chemical structure of KH is:

This polymer shows excellent chemical, mechanical, and electrical performance over a wide temperature range, so that it is interesting to elucidate its confor-

' mation and molecular packing in the solid state to establish relationships between the KH physical properties and structure. No organic solvent is known for this polymer, which is, moreover, infusible and flame resistant.

We proposed to verify the capability of potential energy calculations to solve the problem of the conformation and crystal packing of polymers, in which serious difficulties are often encountered, owing to the paucity of experimental data.

As the first stage of this investigation, the conformational analysis of the monomer was undertaken in order to establish an appropriate model for the x-ray study.

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0 1976 by John Wiley & Sons, Inc.

1664 CONTE ET AL.

CONFORMATIONAL ANALYSIS OF THE KH MONOMER

Van der Waals' energy calculations were carried out separately with the two subunits of the KH monomer shown in Figure 1, since other interactions are very weak. The bond distances (A) and angles (") employed are those of Figure 1, so that the length of the monomer is 18.15 A. The parameters of the five- membered rings were derived by averaging the values observed in the crystal structure of phthalimide.2 The hydrogen atoms lie on the bisectors of the C-C-C angles and a C-H bond length of 1.08 8, was assumed.

The Van der Wads' energy between nonbonded atoms was qualitatively computed by using semiempirical potential functions previously tested in known and unknown crystal structuress-7 and in conformational analysis of synthetic polymers and biopolymers.sl0 The coefficients for the potentials are listed in Table I and refer to the generalized form:

V ( r ) = a exp (-br)/rd - cr-6

where V(r ) represents the repulsive and dispersion energies and r the interatomic distance.

The conjugation energy VJ8) of the DIPA phenyl derivative was evaluated by means of the re1ation:ll

v,(e) = v: C O S ~ 8

where 8 the rotation angle around the N-C bond, is equal to 0' when the A unit of the KH monomer is planar (see Fig. 1). The total energy E depends on 8 and V,", the unknown conjugation energy at Oo, and is plotted in Figure 2. Increments of 5' and 5 kcal were used for 8 and V:, respectively. By varying V: over the rather wide range -25 to -10 kcal the minimum in E shifts slightly, from 8 = 30' to 8 = 40°, and only for < = -5 kcal does the minimum become flat and move to 8 = 50'. Thus if V: is about -10 kcal or less (algebraically) a twist angle of 35' can be reasonably assumed, but if Vi is greater than -10 kcal nothing can be said about the value of 8.

The analysis of the diphenylether was performed by considering two dihedral angles $1 and $2 with the same sense of rotation around the 0-C bonds, $1 =

A

B

Fig. 1. The two subunits of the KH monomer taken into account in the conformational analy- sis.

KAPTON H 1555

TABLE I Coefficients of Potentials Used for Calculating the Van der Waals' Energya

Interaction a x 10-3 b c d

H-H 6.6 4.08 49.2 0 H - C 44.8 2.04 125.0 6 H-N 52.1 2.04 132.0 6 H-0 42.0 2.04 132.7 6 C - C 301.2 0 327.2 12 C-N 34 0.0 0 340.0 12 C - 0 278.7 0 342.3 12 N-N 387.0 0 354.0 12 N--O 316.2 0 356.0 12 0-0 259.0 0 358.0 12

aThe energy is in kcal/atom pair and the interatomic distance is in A.

$2 = 0' corresponding to a planar molecule. The Van der Waals' energy map, calculated by fixing the C-0-C angle (hereafter indicated as q ) at 116' and by taking angular increments of lo', is shown in Figure 3. The minimum is a t $1 = $2 = 90' and lies in a shallow region, mainly elongated along the diagonal $ 1 = $2, so that it is difficult to determine the mutual arrangement of the two phenyl rings. Moreover, by opening 7 the minimum zone becomes flatter and the rotational freedom of the two phenyl rings increases. Similar results were obtained by other a ~ t h o r s . ~ ~ ~ ~ ~

Consequently, the conformational analysis, without other sources of infor- mation, does not permit a clear prediction of the KH monomer conformation in the isolated chain.

15 r-----7

0 20 40 60 00 0 ("1 -

Fig. 2. Total energy E of the KH fragment A as a function of 0. T h e curves refer to different values of V:, from -25 kcal (lowest curve) to -5 kcal (upper curve).

1556 CONTE ET AL.

I150 - - e $120 -

90 -

60 -

30 -

" 0 30 60 90 120 150 180

v,(")- Fig. 3. Van der Waals' energy of the fragment B as a function of J.1 and J.2. The contour lines

are drawn at 7.0,7.5, and 9.0 kcal.

X-RAY EXPERIMENTAL DATA

The polymer was studied at room temperature employing an Elliott GX6 ro- tating anode to obtain Cu K , radiation (A = 1.54 A) and a Stoe-Buerger pre- cession goniometer with flat films. X-ray diffraction patterns were obtained from amber colored KH films, 125 p thick, supplied by E. I. du Pont de Nemours & Co., with specimen to film distances in the range 50-125 mm. The KH film was stretched to increase the extent of order. Figure 4 shows a typical photo- graph, the incident collimated x-ray beam being perpendicular to the fiber. Large meridional arcs and very diffuse equatorial spots are visible. The low- est-angle (and most intense) meridional arc was considered to be second-order on the basis of geometrical and conformational criteria. The periodicity along b (fiber axis), observed by averaging the most reliable meridional spacings (see Table 11), was 32.9 8, and corresponded to the projection of two monomers on the b axis by assuming Q = 130" and that the segments joining the ether oxygens lie in the same plane. Thus the monomers give rise in the crystals and in non- crystalline domains to zigzag or fully extended chains, which are responsible for the stiffness of the polymer. Only arcs with k even, from 2 to 14, appeared, so that the presence of a twofold screw axis, parallel to b , was highly probable, and further because two consecutive KH monomers, in the fully extended confor- mation, may be related by a 21 axis passing through the centers of all the DIPA's. The spread of the equatorial spots does not allow an accurate measurement of their interplanar spacings, which are given in Table 11, and suggests that little order exists in the packing of adjacent helixes. To test the determination of the unit cell parameters the density of KH was measured by flotation in a pyri- dine-carbon tetrachloride density gradient. The value found was 1.42 g ~ m - ~ . If a and c are 4.66 and 5.96 A, respectively (see Table 11), the KH crystal is monoclinic, space group P21, with /3 = 100" and two monomers in the unit cell. The calculated density is 1.41 g in good agreement with the experimental

KAPTON H 1557

Fig. 4. X-ray photograph of a stretched KH film. The film-specimen distance is 50 mm and the fiber axis is horizontal.

value. The other two observed equatorial reflections of Table I1 have spacings nearly equal to those of the 101 and 101 planes of the above defined unit cell (3.34 and 3.96 A).

The intensities of the meridional reflections were measured by means of a Syntex AD-1 Autodensitometer, using a 109 X 54 p light spot and a scanning grid of 50 and 100 p along the meridional and equatorial direction, respectively. The integrated intensities are reported in Table 11.

TABLE I1 Spacings in X-Ray Diffraction Photographs of KH Stretched Films and Intensities of

the Meridional Reflections (on an Arbitrary Scale)

Meridional reflections Equatorial reflections

hkl d, A Intensity hkl d , A

020 16.3, 74 001 5.9,

0 60 5.5, 25 1 oi 3.9, 080 4.1, 19 101 3.3,

0120 2.7, 37 0140 2.3, 21

040 8.2, 28 100 4.6,

0100 3.3, 29

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OBSERVED AND CALCULATED ONE-DIMENSIONAL PATTERSON SYNTHESIS

Since the potential energy calculations on the KH monomer did not give an unambiguous answer, it was decided to compare the one-dimensional Patterson synthesis, calculated along b, with the intrachain atomic vector distribution of the KH zigzag conformations obtained by fixing 9 at 130° and by varying B by increments of 5O. In the starting position, 0 = Oo, the atoms of each monomer are coplanar and the planes of two consecutive monomers form a dihedral angle of 130O. A simple model, with the conformation of the fragment B corresponding to the energy minimum, was assumed and only DIPA was rotated in such a way to retain the twofold screw axis between two adjacent monomers. The Patterson synthesis was computed with increments of 0.1 8, by means of the meridional reflections of Table II and was sharpened by using a 2B value of 8 A2 (see full line curve of Figure 5). Of course, termination effects influence P ( y ) since the re- flection with the highest k index corresponds to a spacing of only 2.35 A. The independent part of P(y) showed three peaks at about 2.7,5.1, and 7.1 A besides the trivial one at the origin. By inspection of the vector distribution map along b it is clear that the best agreement with P(y) is achieved at B = 15O and 0 = 45O. The vertical bars of Figure 5 represent on an arbitrary scale, for B = 15O, the most probable interatomic vectors, within a range of 0.3 A, between two consecutive monomers, with the exception of the bar at the origin which covers a range of 0.15 A. The vectors between atoms of different chains also recur in the isolated chain too. In addition, if the first reflections not recorded (0 16 0 etc.) were included in the Patterson synthesis, the differences in values among the above-mentioned three peaks would tend to decrease, improving agreement with the calculated Patterson function.

At this stage we accomplished Van der Waals' energy calculations in the crystal

/300

h

Y %

a 200

100

0

~

1 I I I

2 4 6 .-. 8 y w ----D

Fig. 5. Calculated Patterson synthesis P(y) along the b axis (full line curve) and most frequent interatomic vectors of two monomers with t) = 130° and B = 15O (vertical bars).

KAPTON H 1559

to test both the correctness of the unit cell dimensions and the space-group as- signment and to decide about the actual value of 8.

VAN DER WAALS' ENERGY CALCULATIONS IN THE KH CRYSTAL

The Van der Waals' energy was computed by using the coefficients of the potentials listed in Table I and by changing the variables 6 and cp, which repre- sents the rotation of a chain around the twofold screw axis. The cp rotation occurs in a righthanded orthogonal framework O X Y Z oriented with respect to the monoclinic crystallographic system Oabc in such a way that OY and OZ coincide with Ob and Oc and the positive OX and Oa semi-axes lie on the same side of the Y Z plane. Then cp stands for a clockwise rotation about OY and 6 for a counterclockwise one about the axis passing through the nitrogen atoms, provided that O X Y Z is fixed and the atoms involved are moved. In the starting position, cp = 0", the central carbon and hydrogen atoms of DIPA, belonging to the mo- nomer a t (x,y,z), were situated on the X axis. Angular increments as small as 2" were given in the minimum zones and the lowest minimum found in the energy search is marked by a cross in Figure 6. All the interactions of the two asym- metric units a t (x,y,z) and (%,$ + y,Z) with the equivalent ones a t (1 + x), (1 + z ) , (1 + x, 1 + z ) and (1 + x , -1 + z ) were taken into account assuming a cut-off distance of 21 A.. The three most promising minima were located at cp and 6 values of (12Oo,O0), (35",25"), and (65O,-3Oo). An analysis of the interchain contacts indicated that the last two minima must be discarded, owing to some short distances in the corresponding crystal structures. Furthermore, the in- spection of the energy map allows us to reject 6 values near f45" since the energy is extremely high. Thus the region around (120" ,Oo) was explored and a very satisfactory packing, indicated by a black spot in Figure 6, was found at

Q 30

0

-60 t 1 -90 4

0 30 60 90 -90 -60 -30

y ("1 - Fig. 6. Van der Waals' packing energy in the KH crystal as a function of p and 0. The contour

lines are drawn at -50, -40, -20, and 0 kcal. The cross indicates the calculated deepest minimum and the black spot corresponds to the proposed packing.

1560 CONTE ET AL.

(108O,13O). This point is characterized by an energy nearly equal to that of the absolute minimum as well as by some C . . . . . C distances of 3.3 A. This supports the presence of charge-transfer forces, suggested by the amber color of the KH film and by the experimental density. Moreover, a B = 13’ agrees well with the 15’ value derived from the intrachain vector map.

Another point of interest concerns the shape of the deepest minimum mainly elongated along the B direction. The thermal resistance of KH could be ex- plained both by strong interchains forces (charge-transfer, T-T and Van der Waals’ forces) and by the conformational flexibility of the DIPA’s and phenyl groups, which can oscillate in phase with the equivalent groups of the surrounding chains without a sensible loss of packing energy.

In conclusion, by coupling the x-ray data with the potential energy results it is possible to present a reasonable simple model for the conformation and the crystal packing of KH. However, more complicated models, generated by con- sidering the abilityof the phenyl groups to rotate about the 0-C bonds, cannot be excluded.

References

1. H. Lee, D. Stoffey, and K. Neville, in New Linear Polymers, McGraw Hill, New York, 1967,

2. E. Matzat, Acta Cryst., B28,415 (1972). 3. E. Giglio, 2. Kristallogr., 131,385 (1970). 4. V. M. Coiro, E. Giglio, A. Lucano, and R. Puliti, Acta Cryst., B29,1404 (1973). 5. E. Gavuzzo, F. Mazza, an&E. Giglio, Acta Cryst., B30,1351 (1974). 6. V. M. Coiro, E. Giglio, and C. Quagliata, Acta Cryst., B28,3601 (1972). 7 . C. Dosi, E. Giglio, N. V. Pavel, and C. Quagliata, Acta Cryst., A29,644 (1973). 8. L. D’Ilario and E. Giglio, Acta Cryst., B30,372 (1974). 9. M. Cesari, L. D’Ilario, E. Giglio, and G. Perego, Acta Cryst., B31,49 (1975).

pp. 183,224.

10. E. Cernia, G. Conte, L. DIlario, N. V. Pavel, and E. Giglio, J. Polym. Sci., Polym. Chem. Ed.,

11. I. Fischer-Hjalmars, Tetrahedron, 19.1805 (1963). 12. A. Tonelli, Macromolecules, 6,503 (1073). 13. V. A. Zubkov, T. M. Birshtein, and I. S. Milevskaya, J. Mol. Struct., 27.139 (1975).

13,125 (1975).

Received December 8,1975