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In-plane orientation and graphitizability of polyimide films: II. Film thickness dependence
Carbon. Printed
Vol. 31, No. 8, pp, in Great Britain.
1307-1312,
1993 Copyright
0008-6223193 0 1993 Pergamon
$6.00 + .oO Press Ltd.
IN-PLANE ORIENTATION AND GRAPHITIZABILITY OF POLYIMIDE FILMS: II. FILM THICKNESS DEPENDENCE H. HATORI, Y. YAMADA, and M. SHIRAISHI National Institute for Resources and Environment, Onogawa, Tsukuba 305, Japan (Received,
30 November
1992; accepted
in revised form 12 April 1993)
AbstractsThe relation between the in-plane orientation of polyimide film and graphitizability was investigated. The degree of in-plane orientation was estimated by means of optical birefringence and ESR technique. The polyimide film was found to have non-uniform orientation in the thickness direction because the thinner the film was, the greater the orientation. The inhomogeneity of orientation caused multiphase graphitization in a film with a composite profile of the X-ray diffraction peak.
Key WordsPolyimide structure.
film, in-plane orientation, graphitizability,
1. INTRODUCTION Aromatic polyimides in film have unique graphitization characteristics. Some of these give graphitizable carbon through the solid state of the carbonization step[ l-31. The commercially available polyimide film “Kapton” is typical, being easily transformed into highly oriented graphite film by high-temperature heat treatment. Graphitizability of polyimide films varies with molecular orientation as well as type of molecular species. In the previous paper[4], the graphitizability of Kapton-type polyimide (PI) films prepared by various methods was found to depend largely on in-plane orientation-namely, alignment of molecular chains and/ or aromatic segments parallel to the film plane. Inplane oriented films showed graphitizability comparable to Kapton film with the same thickness, whereas isotropic ones were nongraphitizable. As an extension of the characteristic properties, we produced, from a polyimide-coated carbon fiber, a composite fiber possessing graphite layers well arranged along the fiber surface[5]. It is interesting that the graphitizability of a given polymer can be controlled by molecular orientation. We have proposed a reaction mechanism for the first step of carbonization of PI, such that benzene rings in molecules crosslink with each other by radical coupling reactions induced by elimination of carbonyl groups and the following hydrogen transfer[6]. The dependence of graphitizability on in-plane orientation implies that aromatic sheets develop with the orientation of aromatic segments being maintained parallel. The graphitization mechanism, however, is not fully clarified from the structural relationship between the polyimide and carbon films. The diversity of molecular packing, orientation, and conformation in moldings has been pointed out for PI[7-151. The density and crystallinity of film
multiphase graphitization,
skin-core
change with imidization temperature[7]. The inplane orientation of PI film is easily generated, and varies with tension during solvent removal and imidization[l2-141. The degree of the in-plane orientation depends on film thickness[ 121. Miwa et al. recently reported on thickness dependence of molecular conformation in PI film[ 151. Carbon film structure is also not very simple, even for film treated at high temperature. A thin layer of graphitic carbon was observed on nongraphitic film in our previous work[41. In the present work, we prepared PI films differing in thickness, and studied the correlation between the degree of in-plane orientation and graphitizability, with special attention to nonuniform structure in PI film and heat-treated film. An ESR study, together with a method using birefringence, was tried to assess in-plane orientation.
2. EXPERIMENTAL Synthesis of DATPP-VO 5,15-bis(4-aminophenyl)-l0,20-diphenyl-21H,23Hporphine (DATPP) was synthesized according to the method of Nishikata et af.[16]. The resulting product showed the analytical data as follows: IR absorption (KBr) 3460, 3365, and 3320 cm-‘; 300 MHz ‘H-NMR (CDCI,) 6 4.04 (broad s, 4H), 5.30 (s, 2H), 7.07 (d, 4H, J = 8.2 Hz), 7.7-7.8 (m, 6H), 8.00 (d, 4H, J = 8.2 Hz), 8.22 (dd, 4H, J = 7.5 Hz and 2.1 Hz), 8.82 (d, 4H, J = 4.7 Hz, P-H) and 8.93 (d, 4H, J = 4.7 Hz, P-H). A solution of 200 mg of DATPP and 300 mg of vanadyl sulfate in a mixture of glacial acetic acid (13.5 ml) and pyridine (6.5 ml) was refluxed for 3 h. A saturated aqueous solution of sodium hydroxide was added until the mixture became alkaline, and then products were extracted with chloroform. After evaporation of the extract in uacuo, a 1 : 1 2.1
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mixture of concentrated HCl and 95% ethanol by volume was added to dissolve the residue, and the solution was refluxed for deacetylation on amino groups. Vanadyl porphyrin chelate (DATPP-VO) was purified by silica gel column chromatography using a mixture of chloroform and ethyl acetate (10 : 1 by volume). The yield of DATPP-VO was essentially quantitative. FD mass spectrum of DATPP-VO, C,,HsON,VO, showed a mass peak at m/e 709. The IR spectrum (KBr) exhibited absorption peaks at 3465 and 3370 cm-‘. 2.2 Preparation of sample films A solution of poly (4,4’-oxydiphenylene pyromellitamic acid) (PAA) in N,N-dimethylacetamide (ca. 13 wt%) was prepared according to the method previously reported[l7]. The solution was cast onto a glass substrate. After the solvent was excluded at 60°C in a vacuum oven, cast films of different thickness gave the corresponding PAA films named A, B, C, and D in the thickness order. The films were imidized thermally at 200°C in uacuo and gave two types of PI films by the conditions; one was fixed on the glass substrate to cause in-plane orientation (ON), and the other was permitted an unhindered contraction (OFF) during imidization[U. Polyamic acid, the precursor of vanadyl porphyrin doped polyimide (PIVO), was obtained by polymerization of pyromellitic dianhydride with a mixture of diaminodiphenyl ether and 0.1 mol% DATPP-VO. PIVO films used for ESR measurements were also prepared by casting and imidization in the same manner as PI films. PI films were heated up to 800°C at a rate of YC/ min in flowing argon, and further heat-treated at 2800°C for 1 h without any compression (PIG). Thicknesses of PI samples used for heat treatment are shown in Table 1. Kapton film (25 pm, TorayDu Pont Ltd.) was heat treated in the same manner, as a reference. PI, PIVO, and PIG films are indicated by imidization conditions and thickness of the polyamic acid film (e.g., PI-ON-A, PIG-OFF-B, etc.). 2.3 Characterization Refractive indices of PI films were determined by use of Abbe refractometer (ATAGO 4T) with a polarizer at the wavelength of sodium D lines.
ESR experiments were performed at room temperature with a JEOL JES-FXlX spectrometer working at X-band. PIVO films of the same thickness were fixed in a quartz tube, and spectra were recorded with rotating the tube in a magnetic field. Spectral analysis was essentially the same as previously reported[ 181. The intensity’ ratio of parallel to perpendicular components (4,/Z,) at a vanadium nuclear spin quantum number, m, = -512, was calculated from the anisotropic spectrum of vanadyl chelate doped in PIVO film. X-ray diffraction profiles in reflection were measured for films and powdered samples of PIG with standard silicon, using a Regaku Ru-300 apparatus with CuKa radiation. 3. RESULTS AND DISCUSSION
3.1 Conformation of PI and birefringence X-ray profiles of the present PI films appear amorphous. The conformation of a PI molecule (Fig. 1) changes by the rotation of three bonds: a C-N bond connecting a benzene ring with an imide ring and two C-O bonds of the diphenyl ether moiety. The benzene ring connected by the C-N bond can be located in a skewed position at some angle between 0” and 90” with respect to the imide plane because of two opposing forces, that is, conjugation at the C-N bond and interaction of oxygen lonepair orbitals with benzene ring orbitals, which stabilize the ring at the position parallel and perpendicular to the imide plane, respectively[l9,20]. Positional correlation between two benzene rings, linked by ether bonds with a bond angle of ca. 120”, was indicated by theoretical approaches to conformational energy for the ground state of diphenyl ether[21]. Each position of benzene rings is restricted by repulsion of ortho hydrogen atoms, and the rings cannot be in a plane. Excepting the restriction, various conformations are possible and coexist in a PI film. An example of conformational structure is illustrated in Fig. lb. Conformation may be alterable by force applied during preparation.
(9
--N
Table 1. Thickness of the PI films used for heat treatment
al.
0
1; -(y&O kc4 n 0
Thickness (pm)
Samples PIG-ON-A -B -C -D PIG-OFF-A -A 1;
10 * 27 f 50 * 75 f 1.5 2 40’5 80 ”
0
0
I’
2 3 5 10 2 10
Fig. 1. (a) Structure of Kapton-type polyimide, (b) a conformation viewed from a direction parallel to the plane of the imide ring.
In-plane
orientation
The in-plane orientation of PI film is estimated by means of birefringence (A) determined by measuring refractive indices of two polarizations-parallel (nil) and perpendicular (nJ to the film plane. In-plane oriented film is isotropic in the film plane, but shows optical anisotropy in out-of-plane. The optical anisotropy is dominated by anisotropic polarizability of the aromatic segment. The Lorentz-Lorentz
equation relates birefringence to net polarizability per unit volume parallel (P,J and perpendicular (P,) to the film surface by
and graphitizability
0.10 0.08
8 5 0.06 2 .C ti ._2 0.04 m
0
(I) where ii is the average refractive index[l2]. Net polarizabilities for light polarized parallel and perpendicular to the plane of the film are determined by populations of each orientation, N,, and N,, and the polarizabilities of the aromatic segments, (~~1 and al, as schematically shown in Fig. 2[22]. Since Pi - P, = (Ni - N112) (ai, - (Ye) in eqn (l), and the anisotropy of the segment (~11- oyI is positive, a larger A indicates higher orientation of aromatic segments parallel to the film surface. Segmental alignment necessarily causes a chain orientation of the PI molecule having aromatic segments in the main chain[22]. Accordingly, separation of these contributing factors to birefringence is not possible. Birefringence is thus related to the orientation of molecular chains and conformation. The average refractive indices, K = (2ni1 + n,)l 3, of PI films were 1.691 -+ 0.002. Optical anisotropy in the film plane was checked by measuring ni both for casting and vertical directions. All PI samples are isotropic in the film plane in spite of anisotropy in the out-of-plane. A indicated tension during imidization to bring about high orientation of PI-ON films and thinner film to possess larger in-plane orientation for PI-ON and PI-OFF (Fig. 3). Thickness dependence has been reported for PI[12] and other polymers[22, 231; films were shown to have nonuniform orientation of molecules in the thickness
N_L@
NlOL
PII =
NII ali
+
2
+
P1 =
N,, al
t
NJalI 2
t -
-
2
N-l
ai1
2
Fig. 2. Schematic calculation of the polarizabilities of aromatic segment for light introduced into the edge of a PI film[22].
1309
20
40
60
go
I( D
Film Thickness @urn) Fig. 3. Relation between birefringence and film thickness (.A, AB, WC, VD).
direction. Such skin-core structure is due to casting force; that is, the greater force exerted on the film surface makes higher orientation of PAA molecules and, consequently, higher in-plane oriented skin of PI film. The degree of in-plane orientation in PAA film cannot be compared with that in PI by A, due to different anisotropy of segments resulting from the change in molecular structure[ 121. Nevertheless, it is apparent from the positive values of birefringence that PAA films have a certain degree of orientation (Fig. 3). It should be noted that in-plane orientation in PI films is alterable by conditions of casting and solvent removal, as well as imidization conditions.
3.2 Estimation of orientation ESR technique
by
Vanadyl porphyrin chelates doped in the PIVO polymer are chemically bonded to two imide rings by substitution of a part of the diphenyl ether moiety. Chelate concentration is so low that there may be little influence on orientation of polyimide molecules due to doping. The orientation of the porphyrin in a planar structure should reflect the in-plane orientation of aromatic segments in polyimide molecules. ESR spin probe technique gives information on the direction of porphyrin rings. The anisotropy of PIVO film was examined for variation in 1,/Zi when the angle between the film plane and magnetic field, 0, changed from 0” to 90”. The difference in relative intensity between 0” and 90”, (I,,/l,),=9~r - (I / ZL)e=oe,reflects the degree of orientation of the porphyrin rings in the film. This difference was plotted for PIVO-ON and -OFF samples against film thickness, as shown in Fig. 4. Thickness dependence and influence of tension during imidization are exhibited as well as the results of birefringence. The ESR method is thus applicable to the estimation of inplane orientation, and one should show ( 1Jthe correlation in orientation between different kinds of polymers and (2) changes in orientation during heattreatment.
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H. HATORI et al.
PI-OFF-A
Film Thickness (pm)
23
Fig. 4. Relation between orientation of the porphyrin chelate and film thickness (OA, AB, WC, VD).
24
25 26 28
21
28 23
24
25 26 28
27
28
Fig. 6. 002 X-ray diffraction profiles of PIG-OFF (a) films and (b) powdered samples.
3.3 Graphitizability Separation of two peaks by CuKa, and CUKCY~ lines is observed in 004 X-ray diffraction profiles of PIG-ON-A and -B (Fig. 5). Interlayer spacings determined from the profiles, c,,/2 (004), are 0.3357-0.3358 nm for films and powdered samples of PIG-ON-A and -B, this range being comparable to values of Kapton film (0.3357 nm) heat-treated under the same condition. Sharp peaks by CuKa, line in the profiles of PIG-ON-C and -D film samples (Fig. 5a) also give the same spacings, but the profiles have unsymmetrical shoulders, besides the peak by the CuKa, line, at a lower angle (Fig. 5). The relative intensity of the broad shoulder peak for each powdered sample is larger than that for the corresponding film. These results show that the thicker film has some regions differing in the degree of graphitization in a film; one is a graphitized region with high crystallinity and high orientation parallel to the
53
54
55
28
56 53
54
5.5
56
28
Fig. 5. 004 X-ray diffraction profiles of PIG (a) films and (b) powdered samples.
film plane, and the others have comparatively lower crystallinity. The regions could not be distinguished by observing cross sections of PIG-ON tilms, which exhibited the stacking structure ofgraphite layers[4] through a scanning electron microscope @EM). However, the brittle feature of the section in Fig. 7a may reflect lower crystallinity in PIG-ON-D film. PIG-OFF samples show more remarkable dependence on film thickness. The interlayer spacing c,/ 2 (004) determined from a film sample of PIG-OFF-A is 0.3361 nm, and the section has a stacking structure like PIG-ON. A graphitic region possessing the same crystallinity is present on PIG-OFF-B and -C, as indicated by the profiles of film samples (Fig. 6a). Raman spectra indicate this region to be located on the surface[4], because a band at 1355 cm-’ ascribed to the disordered structure of carbon is very weak on the surface of PIG-OFF films as well as PIG-ON films (Fig. 8). An SEM micrograph of the section of PIG-ON-D (Fig. 7b) shows an isotropic feature[4]; therefore the graphitic layers appear to be very thin. The section of PIG-OFF-B is substantially isotropic, though a stacking-like structure is observed near the surface in some sections. Interlayer spacings determined from diffraction angles where peak tops of 002 profiles are situated, co/2 (002), are 0.338 and 0.341 nm for powdered samples of PIG-OFF-B and -C, respectively (Fig. 6b). Moreover, these profiles are unsymmetric and broad at lower angles, even after correction for Lorentz-polarization, absorption, and atomic scattering factors. The major components of these films may thus be considered nongraphitic. Judging from X-ray profiles, graphitization of thinner films such as PIG-ON-A, -B, and PIG-OFFA is uniform to some extent, and hence graphitizability can be compared by lattice parameters. On the other hand, the degree of graphitization of PIGON-D is apparently heterogeneous in the film, and the graphitizability cannot be easily estimated by the lattice parameters. For example, it is difficult
In-plane orientation and graphitizability
Fig. 7. SEM micrographs of the cross section of (a) PIG-ON-D and (b) PIG-OFF-C
to compare the graphitizability of PIG-ON-D with that of PIG-OFF-A, since PIG-ON-D film has both regions of higher and lower crystallinity than PIGOFF-A. So-called multiphase graphitization that gives a composite profile such as those of PIG films has
PIG-ON-D
been observed on hard carbons heat-treated at high temperature[24,25]. This has also been reported for graphitization by catalysis[26] and under high pressure[27,28]. It should be emphasized that the graphitization of PI films was achieved under no compression and, of course, without metals. The formation of a wide variety of carbons, such as graphitic carbon with high crystallinity, nongraphitic carbon with the interlayer spacing over 0.34 nm, and the intermediates, at the same heat-treatment temperature is attributable to differences in the degree of in-plane orientation. Multiphase graphitization has often been estimated by peak separation of composite profiles into components of graphite and turbostratic structures. However, such separation was not made in the case of PIG films because graphitic and nongraphitic carbons having various crystallinities are capable of coexisting in a film. 4. CONCLUSION
PIG-OFF-C
1000
1200
1400 cm
1600 -1
1800 2000
Fig. 8. Raman spectra of the film surfaces of PIG-ON-D and PIG-OFF-C.
The in-plane orientation of PI film was estimated by birefringence and ESR technique. The orientation varies with film thickness and tension during imidization. The nonuniformity of molecular orientation in the thickness direction was indicated from the thickness dependence of in-plane orientation. As a result of high in-plane orientation, PI-ON films give graphitic carbons with high crystallinity and high orientation parallel to the film plane, though the thicker films include some regions of lower crystallinity. PIG-OFF-A film is classified as graphitic carbon, originated from the relatively high in-plane orientation of PI film. The degree of graphitization is apparently lower than that of PIG-ON-A or -B. As the degree of in-plane orientation becomes low, the
1312
H. HATORI ef a/.
graphitizability of PIG-OFF films decreases steeply. PIG-OFF-B and -C have thin layers of graphitic carbon on the surface, but the films are essentially nongraphitic. Accordingly, graphitizability of PI film is correlated to some extent with the degree of in-plane orientation. Multiphase graphitization can be explained in terms of non-uniform orientation of PI molecules in a film. However, other factors that inhibit graphitization in thick film should be considered, since the orientations of PI-ON-C and -D are comparable, but PIG-ON-D has components of lower crystallinity.
Acknowledgements-The
authors are indebted to Drs. Y. Sato, of NIRE, and H. Aoyama, University of Tsukuba, for helpful advice concerning characterization of the porphyrin samples. Thanks are also due to M. Shimizu, NIRE, for mass spectroscopic analysis.
REFERENCES 1. A. Burger, E. Fitzer, M. Heym, and B. Terwiesch, Carbon 13, 149 (1975). 2. Y. Hishiyama, S. Yasuda, A. Yoshida, and M. Inagaki, J. Muter. Sci. 23, 3272 (1988). 3. M. Inagaki and Y. Hishiyama, Carbon 29,1239 (1991). 4. H. Hatori, Y. Yamada, and M. Shiraishi, Carbon 30, 763 (1992).
5. H. Hatori, Y. Yamada, M. Shiraishi, and Y. Takahashi, Carbon 29, 679 (1991). 6. H. Hatori, Y. Yamada, and M. Shiraishi, Extended Abstracts of the International
Tsukuba, Japan, November
Symposiam on Carbon,
1990, p. 70.
7. S. Isoda, H. Shimada, M. Kochi, and H. Kambe, J. Polym. Sci. Polym. Phys. 19, 1293 (1981). 8. S. Isoda, M. Kochi, and H. Kambe, J. Polym. Sci. Polym. Phys. 20, 837 (1982). 9. M. Kochi, H. Shimada, and H. Kambe, J. Polym. Sci. Polym. Phys. 22, 1979 (1984).
10. L. G. Kazaryan, D. Ya. Tsvankin, B. M. Ginzburg, Sh. Tuichiev. L. N. Korzhavin. and S. Ya. Frenkel. Vysokomol. Soed. A14, 1199 (1972). 11. T. P. Russell, J. Polym. Sci. Polym. Phys. 22, 1105 (1984). 12. T. P. Russell, H. Gugger, and J. D. Swalen, J. Polym. Sci. Polym. Phys. 21, 1745 (1983). 13. K. Nakagawa, J. Appl. Polym. Sci. 41, 2049 (1990). 14. N. Takahashi. D. Y. Yoon. and W. Parrish. Macromolecules 17,‘2583 (1984). 15. T. Miwa, J. M. Sally, and C. W. Frank, Polymer Prep., Jpn. 40, 3733 (1991). 16. Y. Nishikata, A. Morikawa, M. Kakimoto, Y. Imai, and K. Nishiyama, Polymer J. 22, 593 (1990). 17. R. A. Dine-Hart and W. W. Wright, J. Appl. Polym. Sci. 11, 609 (1967). 18. Y. Yamada. K. Ouchi. Y. Sanada. and J. Sohma. Fuel 57, 79 (1978). 19. H. Ishida, S. T. Wellinghoff, E. Baer, and J. L. Koenig, Macromolecules 13, 826 (1980). 20. T. Matsuo, Bull. Chem. Sot. Jpn. 38, 557 (1965). 21. I. Baraldi, E. Gallinella, and F. Momicchioli, J. Chim. Phys. 83, 653 (1986). 22. W. M. Prest. Jr.. and D. J. Luca. J. Aod. “
23. 24. 25. 26. 27.
28.
Phvs. 50. ,
I
6067 (1979). W. M. Prest, Jr., and D. J. Luca, J. Appl. Phys. 51, 5170 (1980). R. E. Franklin, Proc. Roy. Sot. A209, 196 (1951). K. Kobayashi, S. Sugawara, S. Toyoda, and H. Honda, Carbon 6, 359 (1963). A. Oya and H. Marsh, J. Muter. Sci. 17, 309 (1982). T. Noda and H. Kato. Carbon 3. 289 (1965). T. Noda, K. Kamiya, and M. Inagaki, Bail. Chem. Sot. Jpn. 41, 485 (1968).
Vol. 31, No. 8, pp, in Great Britain.
1307-1312,
1993 Copyright
0008-6223193 0 1993 Pergamon
$6.00 + .oO Press Ltd.
IN-PLANE ORIENTATION AND GRAPHITIZABILITY OF POLYIMIDE FILMS: II. FILM THICKNESS DEPENDENCE H. HATORI, Y. YAMADA, and M. SHIRAISHI National Institute for Resources and Environment, Onogawa, Tsukuba 305, Japan (Received,
30 November
1992; accepted
in revised form 12 April 1993)
AbstractsThe relation between the in-plane orientation of polyimide film and graphitizability was investigated. The degree of in-plane orientation was estimated by means of optical birefringence and ESR technique. The polyimide film was found to have non-uniform orientation in the thickness direction because the thinner the film was, the greater the orientation. The inhomogeneity of orientation caused multiphase graphitization in a film with a composite profile of the X-ray diffraction peak.
Key WordsPolyimide structure.
film, in-plane orientation, graphitizability,
1. INTRODUCTION Aromatic polyimides in film have unique graphitization characteristics. Some of these give graphitizable carbon through the solid state of the carbonization step[ l-31. The commercially available polyimide film “Kapton” is typical, being easily transformed into highly oriented graphite film by high-temperature heat treatment. Graphitizability of polyimide films varies with molecular orientation as well as type of molecular species. In the previous paper[4], the graphitizability of Kapton-type polyimide (PI) films prepared by various methods was found to depend largely on in-plane orientation-namely, alignment of molecular chains and/ or aromatic segments parallel to the film plane. Inplane oriented films showed graphitizability comparable to Kapton film with the same thickness, whereas isotropic ones were nongraphitizable. As an extension of the characteristic properties, we produced, from a polyimide-coated carbon fiber, a composite fiber possessing graphite layers well arranged along the fiber surface[5]. It is interesting that the graphitizability of a given polymer can be controlled by molecular orientation. We have proposed a reaction mechanism for the first step of carbonization of PI, such that benzene rings in molecules crosslink with each other by radical coupling reactions induced by elimination of carbonyl groups and the following hydrogen transfer[6]. The dependence of graphitizability on in-plane orientation implies that aromatic sheets develop with the orientation of aromatic segments being maintained parallel. The graphitization mechanism, however, is not fully clarified from the structural relationship between the polyimide and carbon films. The diversity of molecular packing, orientation, and conformation in moldings has been pointed out for PI[7-151. The density and crystallinity of film
multiphase graphitization,
skin-core
change with imidization temperature[7]. The inplane orientation of PI film is easily generated, and varies with tension during solvent removal and imidization[l2-141. The degree of the in-plane orientation depends on film thickness[ 121. Miwa et al. recently reported on thickness dependence of molecular conformation in PI film[ 151. Carbon film structure is also not very simple, even for film treated at high temperature. A thin layer of graphitic carbon was observed on nongraphitic film in our previous work[41. In the present work, we prepared PI films differing in thickness, and studied the correlation between the degree of in-plane orientation and graphitizability, with special attention to nonuniform structure in PI film and heat-treated film. An ESR study, together with a method using birefringence, was tried to assess in-plane orientation.
2. EXPERIMENTAL Synthesis of DATPP-VO 5,15-bis(4-aminophenyl)-l0,20-diphenyl-21H,23Hporphine (DATPP) was synthesized according to the method of Nishikata et af.[16]. The resulting product showed the analytical data as follows: IR absorption (KBr) 3460, 3365, and 3320 cm-‘; 300 MHz ‘H-NMR (CDCI,) 6 4.04 (broad s, 4H), 5.30 (s, 2H), 7.07 (d, 4H, J = 8.2 Hz), 7.7-7.8 (m, 6H), 8.00 (d, 4H, J = 8.2 Hz), 8.22 (dd, 4H, J = 7.5 Hz and 2.1 Hz), 8.82 (d, 4H, J = 4.7 Hz, P-H) and 8.93 (d, 4H, J = 4.7 Hz, P-H). A solution of 200 mg of DATPP and 300 mg of vanadyl sulfate in a mixture of glacial acetic acid (13.5 ml) and pyridine (6.5 ml) was refluxed for 3 h. A saturated aqueous solution of sodium hydroxide was added until the mixture became alkaline, and then products were extracted with chloroform. After evaporation of the extract in uacuo, a 1 : 1 2.1
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H. HATORIet
1308
mixture of concentrated HCl and 95% ethanol by volume was added to dissolve the residue, and the solution was refluxed for deacetylation on amino groups. Vanadyl porphyrin chelate (DATPP-VO) was purified by silica gel column chromatography using a mixture of chloroform and ethyl acetate (10 : 1 by volume). The yield of DATPP-VO was essentially quantitative. FD mass spectrum of DATPP-VO, C,,HsON,VO, showed a mass peak at m/e 709. The IR spectrum (KBr) exhibited absorption peaks at 3465 and 3370 cm-‘. 2.2 Preparation of sample films A solution of poly (4,4’-oxydiphenylene pyromellitamic acid) (PAA) in N,N-dimethylacetamide (ca. 13 wt%) was prepared according to the method previously reported[l7]. The solution was cast onto a glass substrate. After the solvent was excluded at 60°C in a vacuum oven, cast films of different thickness gave the corresponding PAA films named A, B, C, and D in the thickness order. The films were imidized thermally at 200°C in uacuo and gave two types of PI films by the conditions; one was fixed on the glass substrate to cause in-plane orientation (ON), and the other was permitted an unhindered contraction (OFF) during imidization[U. Polyamic acid, the precursor of vanadyl porphyrin doped polyimide (PIVO), was obtained by polymerization of pyromellitic dianhydride with a mixture of diaminodiphenyl ether and 0.1 mol% DATPP-VO. PIVO films used for ESR measurements were also prepared by casting and imidization in the same manner as PI films. PI films were heated up to 800°C at a rate of YC/ min in flowing argon, and further heat-treated at 2800°C for 1 h without any compression (PIG). Thicknesses of PI samples used for heat treatment are shown in Table 1. Kapton film (25 pm, TorayDu Pont Ltd.) was heat treated in the same manner, as a reference. PI, PIVO, and PIG films are indicated by imidization conditions and thickness of the polyamic acid film (e.g., PI-ON-A, PIG-OFF-B, etc.). 2.3 Characterization Refractive indices of PI films were determined by use of Abbe refractometer (ATAGO 4T) with a polarizer at the wavelength of sodium D lines.
ESR experiments were performed at room temperature with a JEOL JES-FXlX spectrometer working at X-band. PIVO films of the same thickness were fixed in a quartz tube, and spectra were recorded with rotating the tube in a magnetic field. Spectral analysis was essentially the same as previously reported[ 181. The intensity’ ratio of parallel to perpendicular components (4,/Z,) at a vanadium nuclear spin quantum number, m, = -512, was calculated from the anisotropic spectrum of vanadyl chelate doped in PIVO film. X-ray diffraction profiles in reflection were measured for films and powdered samples of PIG with standard silicon, using a Regaku Ru-300 apparatus with CuKa radiation. 3. RESULTS AND DISCUSSION
3.1 Conformation of PI and birefringence X-ray profiles of the present PI films appear amorphous. The conformation of a PI molecule (Fig. 1) changes by the rotation of three bonds: a C-N bond connecting a benzene ring with an imide ring and two C-O bonds of the diphenyl ether moiety. The benzene ring connected by the C-N bond can be located in a skewed position at some angle between 0” and 90” with respect to the imide plane because of two opposing forces, that is, conjugation at the C-N bond and interaction of oxygen lonepair orbitals with benzene ring orbitals, which stabilize the ring at the position parallel and perpendicular to the imide plane, respectively[l9,20]. Positional correlation between two benzene rings, linked by ether bonds with a bond angle of ca. 120”, was indicated by theoretical approaches to conformational energy for the ground state of diphenyl ether[21]. Each position of benzene rings is restricted by repulsion of ortho hydrogen atoms, and the rings cannot be in a plane. Excepting the restriction, various conformations are possible and coexist in a PI film. An example of conformational structure is illustrated in Fig. lb. Conformation may be alterable by force applied during preparation.
(9
--N
Table 1. Thickness of the PI films used for heat treatment
al.
0
1; -(y&O kc4 n 0
Thickness (pm)
Samples PIG-ON-A -B -C -D PIG-OFF-A -A 1;
10 * 27 f 50 * 75 f 1.5 2 40’5 80 ”
0
0
I’
2 3 5 10 2 10
Fig. 1. (a) Structure of Kapton-type polyimide, (b) a conformation viewed from a direction parallel to the plane of the imide ring.
In-plane
orientation
The in-plane orientation of PI film is estimated by means of birefringence (A) determined by measuring refractive indices of two polarizations-parallel (nil) and perpendicular (nJ to the film plane. In-plane oriented film is isotropic in the film plane, but shows optical anisotropy in out-of-plane. The optical anisotropy is dominated by anisotropic polarizability of the aromatic segment. The Lorentz-Lorentz
equation relates birefringence to net polarizability per unit volume parallel (P,J and perpendicular (P,) to the film surface by
and graphitizability
0.10 0.08
8 5 0.06 2 .C ti ._2 0.04 m
0
(I) where ii is the average refractive index[l2]. Net polarizabilities for light polarized parallel and perpendicular to the plane of the film are determined by populations of each orientation, N,, and N,, and the polarizabilities of the aromatic segments, (~~1 and al, as schematically shown in Fig. 2[22]. Since Pi - P, = (Ni - N112) (ai, - (Ye) in eqn (l), and the anisotropy of the segment (~11- oyI is positive, a larger A indicates higher orientation of aromatic segments parallel to the film surface. Segmental alignment necessarily causes a chain orientation of the PI molecule having aromatic segments in the main chain[22]. Accordingly, separation of these contributing factors to birefringence is not possible. Birefringence is thus related to the orientation of molecular chains and conformation. The average refractive indices, K = (2ni1 + n,)l 3, of PI films were 1.691 -+ 0.002. Optical anisotropy in the film plane was checked by measuring ni both for casting and vertical directions. All PI samples are isotropic in the film plane in spite of anisotropy in the out-of-plane. A indicated tension during imidization to bring about high orientation of PI-ON films and thinner film to possess larger in-plane orientation for PI-ON and PI-OFF (Fig. 3). Thickness dependence has been reported for PI[12] and other polymers[22, 231; films were shown to have nonuniform orientation of molecules in the thickness
N_L@
NlOL
PII =
NII ali
+
2
+
P1 =
N,, al
t
NJalI 2
t -
-
2
N-l
ai1
2
Fig. 2. Schematic calculation of the polarizabilities of aromatic segment for light introduced into the edge of a PI film[22].
1309
20
40
60
go
I( D
Film Thickness @urn) Fig. 3. Relation between birefringence and film thickness (.A, AB, WC, VD).
direction. Such skin-core structure is due to casting force; that is, the greater force exerted on the film surface makes higher orientation of PAA molecules and, consequently, higher in-plane oriented skin of PI film. The degree of in-plane orientation in PAA film cannot be compared with that in PI by A, due to different anisotropy of segments resulting from the change in molecular structure[ 121. Nevertheless, it is apparent from the positive values of birefringence that PAA films have a certain degree of orientation (Fig. 3). It should be noted that in-plane orientation in PI films is alterable by conditions of casting and solvent removal, as well as imidization conditions.
3.2 Estimation of orientation ESR technique
by
Vanadyl porphyrin chelates doped in the PIVO polymer are chemically bonded to two imide rings by substitution of a part of the diphenyl ether moiety. Chelate concentration is so low that there may be little influence on orientation of polyimide molecules due to doping. The orientation of the porphyrin in a planar structure should reflect the in-plane orientation of aromatic segments in polyimide molecules. ESR spin probe technique gives information on the direction of porphyrin rings. The anisotropy of PIVO film was examined for variation in 1,/Zi when the angle between the film plane and magnetic field, 0, changed from 0” to 90”. The difference in relative intensity between 0” and 90”, (I,,/l,),=9~r - (I / ZL)e=oe,reflects the degree of orientation of the porphyrin rings in the film. This difference was plotted for PIVO-ON and -OFF samples against film thickness, as shown in Fig. 4. Thickness dependence and influence of tension during imidization are exhibited as well as the results of birefringence. The ESR method is thus applicable to the estimation of inplane orientation, and one should show ( 1Jthe correlation in orientation between different kinds of polymers and (2) changes in orientation during heattreatment.
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H. HATORI et al.
PI-OFF-A
Film Thickness (pm)
23
Fig. 4. Relation between orientation of the porphyrin chelate and film thickness (OA, AB, WC, VD).
24
25 26 28
21
28 23
24
25 26 28
27
28
Fig. 6. 002 X-ray diffraction profiles of PIG-OFF (a) films and (b) powdered samples.
3.3 Graphitizability Separation of two peaks by CuKa, and CUKCY~ lines is observed in 004 X-ray diffraction profiles of PIG-ON-A and -B (Fig. 5). Interlayer spacings determined from the profiles, c,,/2 (004), are 0.3357-0.3358 nm for films and powdered samples of PIG-ON-A and -B, this range being comparable to values of Kapton film (0.3357 nm) heat-treated under the same condition. Sharp peaks by CuKa, line in the profiles of PIG-ON-C and -D film samples (Fig. 5a) also give the same spacings, but the profiles have unsymmetrical shoulders, besides the peak by the CuKa, line, at a lower angle (Fig. 5). The relative intensity of the broad shoulder peak for each powdered sample is larger than that for the corresponding film. These results show that the thicker film has some regions differing in the degree of graphitization in a film; one is a graphitized region with high crystallinity and high orientation parallel to the
53
54
55
28
56 53
54
5.5
56
28
Fig. 5. 004 X-ray diffraction profiles of PIG (a) films and (b) powdered samples.
film plane, and the others have comparatively lower crystallinity. The regions could not be distinguished by observing cross sections of PIG-ON tilms, which exhibited the stacking structure ofgraphite layers[4] through a scanning electron microscope @EM). However, the brittle feature of the section in Fig. 7a may reflect lower crystallinity in PIG-ON-D film. PIG-OFF samples show more remarkable dependence on film thickness. The interlayer spacing c,/ 2 (004) determined from a film sample of PIG-OFF-A is 0.3361 nm, and the section has a stacking structure like PIG-ON. A graphitic region possessing the same crystallinity is present on PIG-OFF-B and -C, as indicated by the profiles of film samples (Fig. 6a). Raman spectra indicate this region to be located on the surface[4], because a band at 1355 cm-’ ascribed to the disordered structure of carbon is very weak on the surface of PIG-OFF films as well as PIG-ON films (Fig. 8). An SEM micrograph of the section of PIG-ON-D (Fig. 7b) shows an isotropic feature[4]; therefore the graphitic layers appear to be very thin. The section of PIG-OFF-B is substantially isotropic, though a stacking-like structure is observed near the surface in some sections. Interlayer spacings determined from diffraction angles where peak tops of 002 profiles are situated, co/2 (002), are 0.338 and 0.341 nm for powdered samples of PIG-OFF-B and -C, respectively (Fig. 6b). Moreover, these profiles are unsymmetric and broad at lower angles, even after correction for Lorentz-polarization, absorption, and atomic scattering factors. The major components of these films may thus be considered nongraphitic. Judging from X-ray profiles, graphitization of thinner films such as PIG-ON-A, -B, and PIG-OFFA is uniform to some extent, and hence graphitizability can be compared by lattice parameters. On the other hand, the degree of graphitization of PIGON-D is apparently heterogeneous in the film, and the graphitizability cannot be easily estimated by the lattice parameters. For example, it is difficult
In-plane orientation and graphitizability
Fig. 7. SEM micrographs of the cross section of (a) PIG-ON-D and (b) PIG-OFF-C
to compare the graphitizability of PIG-ON-D with that of PIG-OFF-A, since PIG-ON-D film has both regions of higher and lower crystallinity than PIGOFF-A. So-called multiphase graphitization that gives a composite profile such as those of PIG films has
PIG-ON-D
been observed on hard carbons heat-treated at high temperature[24,25]. This has also been reported for graphitization by catalysis[26] and under high pressure[27,28]. It should be emphasized that the graphitization of PI films was achieved under no compression and, of course, without metals. The formation of a wide variety of carbons, such as graphitic carbon with high crystallinity, nongraphitic carbon with the interlayer spacing over 0.34 nm, and the intermediates, at the same heat-treatment temperature is attributable to differences in the degree of in-plane orientation. Multiphase graphitization has often been estimated by peak separation of composite profiles into components of graphite and turbostratic structures. However, such separation was not made in the case of PIG films because graphitic and nongraphitic carbons having various crystallinities are capable of coexisting in a film. 4. CONCLUSION
PIG-OFF-C
1000
1200
1400 cm
1600 -1
1800 2000
Fig. 8. Raman spectra of the film surfaces of PIG-ON-D and PIG-OFF-C.
The in-plane orientation of PI film was estimated by birefringence and ESR technique. The orientation varies with film thickness and tension during imidization. The nonuniformity of molecular orientation in the thickness direction was indicated from the thickness dependence of in-plane orientation. As a result of high in-plane orientation, PI-ON films give graphitic carbons with high crystallinity and high orientation parallel to the film plane, though the thicker films include some regions of lower crystallinity. PIG-OFF-A film is classified as graphitic carbon, originated from the relatively high in-plane orientation of PI film. The degree of graphitization is apparently lower than that of PIG-ON-A or -B. As the degree of in-plane orientation becomes low, the
1312
H. HATORI ef a/.
graphitizability of PIG-OFF films decreases steeply. PIG-OFF-B and -C have thin layers of graphitic carbon on the surface, but the films are essentially nongraphitic. Accordingly, graphitizability of PI film is correlated to some extent with the degree of in-plane orientation. Multiphase graphitization can be explained in terms of non-uniform orientation of PI molecules in a film. However, other factors that inhibit graphitization in thick film should be considered, since the orientations of PI-ON-C and -D are comparable, but PIG-ON-D has components of lower crystallinity.
Acknowledgements-The
authors are indebted to Drs. Y. Sato, of NIRE, and H. Aoyama, University of Tsukuba, for helpful advice concerning characterization of the porphyrin samples. Thanks are also due to M. Shimizu, NIRE, for mass spectroscopic analysis.
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