In-plane orientation and graphitizability of polyimide films

Carbon. Vol. 30, No. 5. pp. 763-766,

1992

OCW6223/92 $5.00 + .OO Copyright 0 1992 Pergamon Press Ltd.

Printed m Great Britain.

IN-PLANE ORIENTATION AND GRAPHITIZABILITY POLYIMIDE FILMS

OF

H. HATORI, Y. YAMADA and M. SHIRAISHI National Research Institute for Pollution and Resources, Onogawa, Tsukuba 305, Japan (Received

14September I99 1;accepted in revised.form 5 November 1992)

Abstract-The relationship between in-plane orientation of polyimide film and the graphitizability was obtained. In-plane oriented films in which the molecular chains are aligned parallel to the film plane give graphitic carbons, whereas isotropic ones afford non-graphitic carbons. A thin region of graphitized carbon can be observed even on the non-graphitic film, the skin-core structure of which is derived from the non-uniform orientation of the polyimide film in the thickness direction. Key Words-Polyimide

film, graphitizability, in-plane orientation, skin-core structure.

1. INTRODUCIION Some highly heat-resistant polymers, such as polyimide[ 1,2], poly(p-phenylene oxadiazole)[3], and poly(p-phenylene vinylene)[4,5], are known to give, by heat treatment, graphitic carbons with high crystallinity. For example, the commercially available polyimide film “Kapton” yields by heat treatment up to 3050°C a graphite film in which crystallinity is as high as that of pyrolytic graphite annealed up to 32Oo”C[2]. The graphitization mechanism of these polymers is of interest because most polymers carbonized in the solid phase have been considered to give non-graphitizable carbons until recently. In-plane orientation-namely, the alignment of molecular chains parallel to the film plane-has been investigated for poly(4,4’-oxydiphenylene pyromellitimide) (PI)[6-91 and is quantitatively estimated by determining the birefringence An = ny-nl, where n,, and n, are the refractive indices parallel and perpendicular to the film plane, respectively. It is necessary to explore what kinds of structural correlation exist between the starting polymers and the resultant carbons. In this paper, we present the relationship between in-plane orientation and the graphitizability for the PI film.

of acetic anhydride and pyridine (Chem-OFF). The flawless films can be obtained except for the film chemically imidized on the substrate. Thickness of the PI films varies 20-50 pm, depending on the imidization conditions. Vacuum deposition polymerization (VDP) of PAA[ 1 I] and subsequent imidization on the substrate at 200°C also yielded an in-plane oriented film (VDP-ON) of about 5 pm in thickness. Kapton 1OOHfilm (Toray-Du Pont Ltd.), which is denoted Kapton hereafter, was also used as a reference. The PI films were heated to 800°C at a heating rate of YC/min in flowing argon, and further heat-treated at 2800°C for 1 h without any compression. These heat-treated samples are indicated by the final heattreatment temperature (e.g., Therm-ON-2800, Kapton-2800, etc.).

2. EXPERIMENTAL

2.1

Preparation

A solution of poly(4,4’-oxydiphenylene pyromellitamic acid) (PAA) in N,N-dimethylacetamide (ca. 1Owt%)[ lo] was cast onto a glass substrate and the solvent was excluded at 60°C in an air oven. An inplane oriented PI film and an isotropic one were prepared according to the method given by Russell et al.[6,7]; the former was produced by imidization of a PAA film fixed on the substrate and the latter was obtained by removing it from the substrate before imidization. They are termed ON and OFF, respectively. The PAA films were thermally imidized at 200°C in vucuo (Therm-ON, Therm-OFF), o. chemically imidized by immersing them in a 1: 1 mixture by volume

2.2 Characterization Refractive indices of the PI films were determined by use of Abbe refractometer (ATAGO, 4T) at the wavelength of sodium D lines. X-ray diffraction patterns were measured using a Regaku Ru-300 apparatus with CuKa radiation. The microscopic structure was observed with a scanning electron microscope (SEM) (JEOL, JXA-733) and a transmission electron microscope (TEM) (Philips, CM30). Raman spectra were recorded on a Spex Ramalog 1403 spectrophotometer using a 514.5 nm line of argon laser. 3. RESULTS AND DISCUSSION The observed values of birefiingence of the PI films are listed in Table I. The values for Therm-ON and VDP-ON are comparable to those reported as the in-plane oriented films[6-91, whereas the lower An values for Therm-OFF and Chem-OFF reflect the lower degree of the in-plane orientation. All of the PI samples gave black films after heattreatment at 8Oo”C, and there was no difference in appearance even on the SEM observation. The ther-

163

764

H.

HATORI

Table 1. Birefringence (An) of PI films Therm-ON 0.070

Therm-OFF

Chem-OFF

VDP-ON

0.01 I

-0

0.09 I

Table 2. Interlayer spacing, c0/2, and average crystallite size, L,, of PI films heat-treated at 2800°C

mogravimetric analysis (TGA) also showed the similar curve above 450°C at which pyrolysis and polycondensation of the film begin to take place (Fig. 1). The interlayer spacing and the crystallite size were determined from the x-ray diffraction profiles for the samples heat-treated at 28Oo’C (Table 2). The inplane oriented films (Therm-ON and VDFON), including Kapton[6], yield graphitic carbons after the heat treatment; their lattice parameters are close to those of graphite. On the other hand, the isotropic films (Therm-OFF and Chem-OFF), heat-treated at the same temperature, show the lattice parameters comparable to those of non-graphitic carbons. TEM observation also indicates the considerable difference in structure between them. Therm-ON2800 shows the typical graphitic features, whereas Chem-OFF-2800 gives non-graphitic ones (Fig. 2). SEM micrographs display the representative cross sections of the heat-treated films as shown in Fig. 3. The stacking structure ofgraphite layers[5] like Kapton-2800 is clearly observed on the sections of Therm-ON-2800 and VDP-ON-2800, whereas Therm-OFF-2800 and Chem-OFF-2800 have an isotropic texture. From the results described above, it is obvious that the graphitizability ofthe PI film depends on their inplane orientation, although the quantitative correlation is still not clarified. It has been reported that Kapton film has a regular structure in the intramolecular conformation and intermolecular aggregation, although the regularity is not so high as in the crystalline state[ 12,131. But the x-ray diffraction profiles of the present PI films we prepared show that these films are amorphous. Therefore, the similarity in

the

crystallinity

between

Kapton-2800

eta/.

Sample

co/2 (A)

Therm-ON-2800 Therm-OFF-2800 Chem-OFF-2800 VDP-ON-2800 Kapton-2800

3.359” 3.43b 3.46b 3.359a 3.358”

L (A) > IOOOB 71b 34b > IOOW >lOOO”

“determined from 004 diffraction peak. bdetermined from 002 diffraction peak.

Therm-ON-2800 suggests that the regular intramolecular conformation and intermolecular aggregation in Kapton film are not necessary conditions for the graphitization of the PI film. The Raman spectra obtained at the film surface are presented in Fig. 4, together with those obtained for pellets made of powdered samples of the films. An absorption band located at 1580 cm-’ is due to a graphitic structure, and that at 1355 cm-’ is ascribed to a disordered structure in carbon. The intensity ratios ofthe 1355 cm-’ band against the 1580 cm-’ one, denoted as I,355/11580, for the pellets show a qualitatively good correlation to the x-ray parameters indicating average graphitizability of the film. The intensity ratios for the film surface are, however, always smaller than those of the corresponding pellets, and there is not so much difference between the samples used.

and

10

I

- 20 ul ! 2 30 .z B 40

I 400

500

I

1

600

700

Temperature

800

I

I

900

1000

I

("Cl

Fig. 1. Typical TGA curve of PI film (Therm-ON).

Fig. 2. TEM micrographs and electron diffraction patterns (inset) of (a) Therm-ON-2800 and (b) Chem-OFF-2800.

765

Graphitizability of polyimide films

Fig. 3. SEM micrographs of cross section of (a) Therm-ON-2800 and (b) Therm-OFF-2800.

,a)

I

Therm-ON-2800

lb1

Therm-OFF-2800

(cl

Chem-OFF-2000

I(

A.... Il355'Ii580 = 0.7

1200

1400

Fiaman shift

1600

is00

(cm-')

C 200

j

1600

1400

Raman

shift

I

(cm-l1 (el

(d) VDP-ON-2800

moo

J

IO

,600

1400

Raman shift

(cm-l 1.

Kapton-ZBOO

I1355~~1580 =-0 I1355'~1580 = O.1

11355’1i580=

3

1400

Raman

shift

1600

(cm-l)

is00 1200

0.4

1400

Raman shift

1600

II

(cm-')

Fig. 4. Raman spectra of the film surface (upper) and the pellets (lower): (a) Therm-ON-2800, (b) ThermOFF-2800, (c) Chem-OFF-2800, (d) VDP-ON-2800, (e) Kapton-2800.

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H. HATORIet al.

4. CONCLUSION Katagiri et a/.[ 141 reported that the ratio of 1,355/1,580 depends both on degree of graphitization and orienThe graphitizability ofthe PI films largely depends tation ofgraphite planes. Accordingly, the higher valon their in-plane orientation. Even in a film, a local ues of the intensity ratios for the pellets can be exdifference of the degree of the orientation induces plained in terms of random orientation of graphite such non-uniform graphitization as the skin-core powders, or the graphite planes on the pellets. It is structure. worth noting that a band at 1355 cm-’ is very weak or missing even on Therm-OFF-2800 and ChemOFF-2800, suggesting the presence ofgraphitic layers Acknowledgement--The authors are grateful to Mr. Y. Taknear the film surface. It is considered that these films ahashi, ULVAC Jpn. Ltd., for providing the VDP samples. We also thank Dr. R. Yokota, Institute of Space and Astrohave skin-core structures, since they are non-granautical Science, for helpful advice concerning the prepaphitic on average, as mentioned above. It was reration of polyimide films. ported that the degree of molecular orientation of PI[6] or some other polymers[ 15,161 is dependent upon the film thickness, and hence the films possess REFERENCES a non-uniform orientation of the molecules in the thickness direction. We also have found that a thinI. A. Burger, E. Fitzer, M. Heym, and B. Terwiesch, Carner film shows larger birefringence both for Thermbon 13, 149 (1975). 2. Y. Hishiyama, S. Yasuda, A. Yoshida, and M. Inagaki, ON and Therm-OFF[ 171. Therefore, the graphitic J. Mater. Sci. 23,3272 (1988). layers may be formed from the in-plane oriented 3. M. Murakami and S. Yoshimura, Synth. Met. 18, 509 skin, and a local difference in the degree of the ori(1987). entation causes non-uniform graphitization even in 4. T. Ohnishi, I. Murase, T. Noguchi, and M. Hirooka, such a thin film. Many workers have so far studied the thermal degradation mechanism of the Kapton film[ 1,18-211, because of the interest in high-temperature heat-resistance or in good graphitizability. These investiga-

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