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1. Introduction
Carbon materials are made of the elemental unit consisting of graphite-like
layers. The structural relationship between a precursor polymer and carbon
microtexture is important because the property of carbon materials depends
on the microtexture and pore structure derived from the arrangement of the
elemental units. Recently, it was reported that commercial available polyimide
film, "Kapton", and some other polymer films with heat-resistance
provide graphite film with high crystallinity by heat-treatment at high
temperatures. In this study, regulation of microtexture and pore structure
in carbons by structural control of Kapton-type polyimide film used as the
precursor was investigated.
Fig. Chemical Structure of Kapton-type Polyimide
2. Graphitization of Polyimide Films
The relation between the in-plane orientation of polyimide film and graphitizability
was obtained. Polyimide films with higher degree of in-plane orientation
give graphitic carbons, whereas isotropic ones provide non-graphitic films.
Further, the thicker the film is, the lower the degree of graphitization.
The thick film gave a composite profile of X-ray diffraction peaks because
of the multiphase graphitization in a film. Thin layers of graphitic carbon
are formed even on the surface of the film which is on the whole non-graphitic.
One of the reasons why multiphase graphitization occurs is that the precursor
film has a skin-core structure of the in-plane orientation.
Fig. Microtexture (TEM micrographs) of graphitized and non-graphitized films
prepared from in-plane oriented and non-oriented polyimide films.
[Publications]
(1) H. Hatori, Y. Yamada and M. Shiraishi: "In-plane orientation and
Graphitizability of Polyimide Films"; Carbon, Vol. 30, No. 5, pp. 763-766
(1992)
(2) H. Hatori, Y. Yamada and M. Shiraishi:"In-plane orientation and
Graphitizability of Polyimide Films: II. Film Thickness Dependence"
Carbon, Vol. 31, No.8, pp. 1307-1312(1993)
3. Preparation of Composite Carbon fibers by Coating
In-plane orientation of the polyimide film is easily brought about by the
imidization on a substrate and the film gives graphitizing carbon. Composite
carbon fibers with a graphitizing carbon layer were prepared by coating
of polyimide on core fibers. The polyimide layer coated on the PAN
based carbon fiber is well graphitized after heat-treatment up to 2800 C,
though the layers are bent along the grooves on the surface of the core
fiber. Polyimide coated Kevlar fiber also gives graphitic carbon. In this
case, however, the Kevlar fiber changes from non-graphitizable to graphitizable
due to polyimide coating.
Fig. Polyimide coated PAN fiber heat-treated at 2800 CûC
[Publication]
H. Hatori, Y. Yamada, M. Shiraishi and Y. Takahashi:"Carbonization
and Graphitization of Polyimide Coated on Carbon Fiber" Carbon, Vol.
29, No.4/5, pp. 679-680 (1991)
4. Regulation of the Micropore Structure in Carbon Films Prepared
from Polyimide
Carbon films prepared from Kapton-type polyimide have microporous structure
and show molecular sieving properties, that is, selectivity of molecules
in adsorption. Modification of micropore-size distribution was studied using
these Carbon-Molecular-Sieve (CMS) films. The micropore size varies with
heat-treatment temperature. As a result, polyimide film of 25 micrometer
in thickness gives carbon films having molecular sieving properties comparable
to zeolite 4A and 5A. The CMS prepared from a thick polyimide film (125
micrometer) has smaller size and volume of micropores than thin film treated
at the same temperature. On the other hand, a change in the degree of in-plane
orientation in pristine film, which influences the graphitizability remarkably,
is not effective for modifying the pore structure.
The CMS film also shows the permselectivity of gases. For example, the permeation
rate of CO2 is 50 times faster than that of N2. It is noteworthy that the
permeability of CO2 is higher than that of He which is smaller in molecular
weight, indicating little contribution from a direct mass transfer in the
gas phase. The high permeability of CO2 may be due to a rapid transfer of
molecules adsorbed in micropores, caused by the large difference in concentration
across the film. The results show that the CMS film has homogeneous fine
pores without cracks and large pores.
[Publications]
(1) H. Hatori, Y. Yamada, M. Shiraishi, H. Nakata and S. Yoshitomi:"Carbon
Molecular Sieve Films from Polyimide" Carbon, Vol. 30, No. 2, pp. 305-306(1992)
(2) H. Hatori, Y. Yamada, M. Shiraishi, H. Nakata and S. Yoshitomi, Masaki
Yoshihara, Takayasu Kimura: "Modification of Pore Structure in Carbon
Molecular Sieve Films Prepared from Polyimide"Tanso, Vol. 1995 (No.167)
pp. 94-100.
5. Regulation of the Macropore Structure by Phase Inversion Method
Preparation of porous carbons from phase inversion membranes was investigated
as a control method of the pore structure in carbon materials. When phase
inversion membranes of Kapton-type polyimide are carbonized, the macroporous
structure is maintained and carbon film with high porosity can be obtained.
However, the micro- and mesopore structure in the carbons are not changed
by phase inversion in the polymer stage, and thus, the macroporous carbon
has molecular sieving properties like carbons prepared from non-porous polyimide
film without phase inversion. Various types of phase inversion membranes
have the capability of being porous carbon precursors, if they satisfy
two requirements: the solid-state carbonization and a relatively high carbonyield.
Shaping in the precursor stage enables the porous carbon to be finely processed
and conforms to practical applications. Composite carbon film with a macroporous
and microporous layer combined with good adherence is easily prepared by
fabrication in the polymer stage.
Fig. Composite carbon film with a macroporous and microporous layer
[Publications]
(1) H. Hatori, Y. Yamada and M. Shiraishi:"Preparation of Macroporous
Carbon Filmsfrom Polyimide by Phase Inversion Method" Carbon, Vol.
30, No. 2, pp. 303-304 (1992)
(2)H. Hatori, Y. Yamada and M. Shiraishi:" Preparation of Macroporous
Carbons from Phase Inversion Membranes" J. Appl. Polym. Sci., Vol.
57, pp. 871-876 (1995)
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