U.S. patent application number 09/987050 was filed with the patent office on 2002-05-30 for method of removing low molecular weight substance from polymide precursor or polymide containing low molecular weight substance.
This patent application is currently assigned to NITTO DENKO CORPORATION. Invention is credited to Fukuoka, Takahiro, Kanada, Mitsuhiro, Mochizuki, Amane, Yamamoto, Takayuki.
Application Number | 20020065390 09/987050 |
Document ID | / |
Family ID | 18835983 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020065390 |
Kind Code |
A1 |
Kanada, Mitsuhiro ; et
al. |
May 30, 2002 |
Method of removing low molecular weight substance from polymide
precursor or polymide containing low molecular weight substance
Abstract
A method of efficiently removing a low molecular weight
substance from a polyimide precursor or polyimide in which the low
molecular weight substance is dispersed as micro-domains, without
using a large amount of an organic solvent. The method of removing
a low molecular weight substance comprises subjecting either a
polymer composition having a micro-domain structure made up of a
continuous phase comprising a polyimide precursor and, dispersed
therein, a discontinuous phase comprising a low molecular weight
substance or a polyimide composition obtained from the polymer
composition by converting the polyimide precursor into a polyimide
to extraction with a combination of supercritical carbon dioxide
and a co-solvent to thereby remove the low molecular weight
substance. The co-solvent is preferably an aprotic polar solvent,
more preferably a nitrogen compound solvent or a sulfur compound
solvent.
Inventors: |
Kanada, Mitsuhiro; (Osaka,
JP) ; Yamamoto, Takayuki; (Osaka, JP) ;
Mochizuki, Amane; (Osaka, JP) ; Fukuoka,
Takahiro; (Osaka, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037
US
|
Assignee: |
NITTO DENKO CORPORATION
|
Family ID: |
18835983 |
Appl. No.: |
09/987050 |
Filed: |
November 13, 2001 |
Current U.S.
Class: |
528/481 ;
528/491 |
Current CPC
Class: |
Y02P 20/54 20151101;
H05K 1/0346 20130101; C08J 9/26 20130101; C08J 2203/08 20130101;
C08J 2379/08 20130101; C08J 2201/042 20130101; C08J 2201/032
20130101 |
Class at
Publication: |
528/481 ;
528/491 |
International
Class: |
C08G 002/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2000 |
JP |
P. 2000-365158 |
Claims
What is claimed is:
1. A method of removing a low molecular weight substance which
comprises subjecting either a polymer composition having a
micro-domain structure made up of a continuous phase comprising a
polyimide precursor and, dispersed therein, a discontinuous phase
comprising a low molecular weight substance or a polyimide
composition obtained from the polymer composition by converting the
polyimide precursor into a polyimide to extraction with a
combination of supercritical carbon dioxide and a co-solvent to
thereby remove the low molecular weight substance.
2. The method of removing a low molecular weight substance of claim
1, wherein said co-solvent is an aprotic polar solvent.
3. The method of removing a low molecular weight substance of claim
1, wherein said co-solvent is a nitrogen compound solvent or a
sulfur compound solvent.
4. The method of removing a low molecular weight substance of claim
1, wherein said co-solvent is N-methyl-2-pyrrolidone or
N,N-dimethylacetamide.
5. The method of removing a low molecular weight substance of claim
1, wherein said low molecular weight substance is a monomer or an
oligomer each having a molecular weight of 10,000 or lower.
6. A process for producing a porous polyimide which comprises
subjecting a polymer composition having a micro-domain structure
made up of a continuous phase comprising a polyimide precursor and,
dispersed therein, a discontinuous phase comprising a low molecular
weight substance to extraction with a combination of supercritical
carbon dioxide and a co-solvent to thereby remove the low molecular
weight substance, and then converting the polyimide precursor into
a polyimide.
7. A process for producing a porous polyimide which comprises
subjecting a polyimide composition obtained from a polymer
composition having a micro-domain structure made up of a continuous
phase comprising a polyimide precursor and, dispersed therein a
discontinuous phase comprising a low molecular weight substance by
converting the polyimide precursor into a polyimide to extraction
with a combination of supercritical carbon dioxide and a co-solvent
to thereby remove the low molecular weight substance.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method of removing a low
molecular weight substance by extraction from a polyimide precursor
or polyimide in which the low molecular weight substance is
dispersed as micro-domains and to processes for producing a porous
polyimide by utilizing the method. This porous polymer is extremely
useful as, for example, circuit substrates for electronic
appliances, etc.
DESCRIPTION OF THE RELATED ART
[0002] Polyimide resins have conventionally been used widely as
parts or members required to have reliability, such as circuit
substrates for electronic/electrical appliances and other
electronic parts, because of their features such as high insulating
properties, dimensional stability, moldability, and lightweight
properties. Especially in recent years, there is a desire for
higher-speed information transmission with the trend toward
performance and function advancement in electrical/electronic
appliances, and members for use in these appliances also are
required to be compatible with the desired higher-speed information
transmission. The polyimide resins for use in such applications are
also required to have a lower dielectric constant as an electrical
property necessary for the use of higher frequencies.
[0003] In general, the dielectric constant of a plastic material is
determined by the molecular skeleton thereof. This means that a
technique which may be effective in reducing dielectric constant is
to modify a molecular skeleton. However, in view of the fact that
the dielectric constants of polyethylene and
polytetrafluoroethylene, which are regarded as low dielectric
constant polymers, are about 2.3 and about 2.1, respectively, there
are limitations in the technique of controlling dielectric constant
based on skeleton modifications. In addition, the above technique
poses problems, for example, that a skeleton modification results
in changes in properties such as film strength and coefficient of
linear expansion.
[0004] As other attempts to obtain a lower dielectric constant,
various techniques have been proposed in which a plastic material
is made porous so as to utilize air, which has a dielectric
constant of 1, and to reduce and control the dielectric constant of
the plastic material based on the porosity.
[0005] Conventional techniques for obtaining general porous
polymers include dry processes and wet processes. Conventional dry
processes include a physical foaming method and a chemical foaming
method. In the physical foaming method, a low boiling solvent is
dispersed as a blowing agent into a polymer and this polymer is
then heated to volatilize the blowing agent, whereby cells are
formed to obtain a porous object. In the chemical foaming method, a
blowing agent is added to a polymer and then pyrolyzed to generate
a gas, whereby cells are formed to obtain a porous object. However,
these techniques have problems, for example, that a sufficiently
small cell size cannot be obtained and there are limitations on the
formation of finer patterns in circuit formation.
[0006] The present inventors have proposed a novel technique for
porosity impartation. This technique comprises preparing a solution
of a polyimide precursor in a solvent, adding thereto a dispersible
low molecular weight substance having an average molecular weight
of, e.g., 10,000 or lower, subsequently drying the resulting
mixture to remove the solvent and thereby cause a phase separation
between the polyimide precursor and the low molecular weight
substance, and then conducting a heat treatment to convert the
polyimide precursor into a polyimide and thereby obtain a porous
polyimide.
[0007] However, not only the low molecular weight substance added
for forming the two-phase structure but also the residual solvent
are present in the dried polyimide precursor and in the polyimide
to which the polyimide precursor has been converted. It is
therefore necessary to remove these substances from the polyimide
precursor and polyimide.
[0008] For removing the low molecular weight substance or residual
solvent from the polyimide precursor or polyimide, there may be
used a method in which the precursor or polyimide is dried at high
temperature for a prolonged time period to volatilize the low
molecular weight substance, a method in which the precursor or
polyimide is heated at high temperature for a prolonged time period
to pyrolyze the low molecular weight substance, or a method in
which the precursor or polyimide is sufficiently washed with a low
boiling solvent, e.g., THF (tetrahydrofuran), and then vacuum-dried
for a prolonged time period. However, these methods have a drawback
from the standpoint of production process because of the necessity
of a prolonged time period and further pose an environmental
problem because of the use of a large amount of an organic solvent.
With respect to the removal of oligomers having a relatively high
molecular weight of 1,000 or above, there has been no technique
proved to be effective in this purpose.
SUMMARY OF THE INVENTION
[0009] One object of the invention is to provide a method of
efficiently removing a low molecular weight substance from a
polyimide precursor or polyimide in which the low molecular weight
substance is dispersed as micro-domains, without using a large
amount of an organic solvent.
[0010] Another object of the invention is to provide processes by
which a porous polyimide having a small cell size and a low
dielectric constant can be efficiently produced.
[0011] The present inventors made investigations in order to
overcome the problems described above. As a result, they have found
that when a dispersible low molecular weight substance for forming
a discontinuous phase is added to a polyimide precursor serving as
a continuous phase to form a specific micro-domain structure in the
polymer and is subsequently removed therefrom by extraction with a
combination of supercritical carbon dioxide and a co-solvent, then
a porous object having extremely fine cells and a low dielectric
constant can be obtained. The invention is based on this
finding.
[0012] The invention provides a method of removing a low molecular
weight substance which comprises subjecting either a polymer
composition having a micro-domain structure made up of a continuous
phase comprising a polyimide precursor and, dispersed therein, a
discontinuous phase comprising a low molecular weight substance or
a polyimide composition obtained from the polymer composition by
converting the polyimide precursor into a polyimide to extraction
with a combination of supercritical carbon dioxide and a co-solvent
to thereby remove the low molecular weight substance. The
co-solvent is preferably an aprotic polar solvent, and is more
preferably a nitrogen compound solvent such as, e.g.,
N-methyl-2-pyrrolidone or N,N-dimethylacetamide or a sulfur
compound solvent. Examples of the low molecular weight substance
include monomers or oligomers each having a molecular weight of
10,000 or lower.
[0013] The invention further provides a process for producing a
porous polyimide which comprises subjecting a polymer composition
having a micro-domain structure made up of a continuous phase
comprising a polyimide precursor and, dispersed therein, a
discontinuous phase comprising a low molecular weight substance to
extraction with a combination of supercritical carbon dioxide and a
co-solvent to thereby remove the low molecular weight substance,
and then converting the polyimide precursor into a polyimide.
[0014] The invention furthermore provides a process for producing a
porous polyimide which comprises subjecting a polyimide composition
obtained from a polymer composition having a micro-domain structure
made up of a continuous phase comprising a polyimide precursor and,
dispersed therein, a discontinuous phase comprising a low molecular
weight substance by converting the polyimide precursor into a
polyimide to extraction with a combination of supercritical carbon
dioxide and a co-solvent to thereby remove the low molecular weight
substance.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The polyimide precursor to be used in the invention is not
particularly limited as long as it is an intermediate convertible
into a polyimide. It can be obtained by conventional methods. For
example, the polyimide precursor can be obtained by reacting an
organic tetracarboxylic dianhydride with a diamino compound
(diamine).
[0016] Examples of the organic tetracarboxylic dianhydride include
pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic
dianhydride,
2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane
dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride,
2,2-bis(3,4-dicarboxyphenyl) ether dianhydride, and bis
(3,4-dicarboxyphenyl) sulfone dianhydride. These organic
tetracarboxylic dianhydrides may be used alone or in combination of
two or more thereof.
[0017] Examples of the diamino compound include m-phenylenediamine,
p-phenylenediamine, 3,4'-diaminodiphenyl ether,
4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfone,
3,3'-diaminodiphenyl sulfone, 2,2-bis(4-aminophenoxyphenyl)propane,
2,2-bis(4-aminophenoxyphenyl)hexafl- uoropropane,
1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzen- e,
2,4-diaminotoluene, 2,6-diaminotoluene, diaminodiphenylmethane,
4,4'-diamino-2,2-dimethylbiphenyl, and
2,2-bis(trifluoromethyl)-4,4'-diam- inobiphenyl. These diamino
compounds may be used alone or in combination of two or more
thereof.
[0018] The polyimide precursor can be obtained by reacting an
organic tetracarboxylic dianhydride with a diamino compound
(diamine) usually in an organic solvent at from 0 to 90.degree. C.
for from 1 to 24 hours. Examples of the organic solvent include
polar solvents such as N-methyl-2-pyrrolidone,
N,N-dimethylacetamide, N,N-dimethylformamide, and dimethyl
sulfoxide. Also usable as the polyimide precursor is a poly(amic
acid) silyl ester obtained by reacting an organic tetracarboxylic
dianhydride with an N-silylated diamine.
[0019] The low molecular weight substance to be used in the
invention for constituting the discontinuous phase in the
micro-domain structure is an ingredient which is dispersible upon
mixing with the polyimide precursor. More specifically, the low
molecular weight substance is a compound which is capable of
separating as fine particles from the polyimide precursor to form a
sea-island micro-domain structure.
[0020] Examples of the low molecular weight substance include
monomers and oligomers having a relatively low degree of
polymerization formed by polymerizing one monomer or two or more
different monomers. Hereinafter, monomers and oligomers are often
referred to inclusively as "oligomer compounds". Specific examples
thereof include polyacrylate oligomer compounds, polyether oligomer
compounds, polyester oligomer compounds, and polyurethane oligomer
compounds.
[0021] Examples of the polyacrylate oligomer compounds include
hexanediol di (meth) acrylate, neopentyl glycol di (meth) acrylate,
trimethylolpropane tri(meth)acrylate, dipentaerythritol
hexa(meth)acrylate, epoxy (meth)acrylates, and oligoester
(meth)acrylates.
[0022] Examples of the polyether oligomer compounds include
polyalkylene glycols such as polyethylene glycol, polypropylene
glycol, and polybutylene glycol and such polyalkylene glycols
terminated at one or both ends by an alkyl group such as methyl, an
alkenyl group such as allyl, an aryl group such as phenyl, or an
acyl group such as (meth)acryloyl or by a combination of these.
[0023] Examples of the polyester oligomer compounds include
.epsilon.-caprolactone, polycaprolactone oligomers, and such
oligomers terminated at one or both ends by an alkyl group such as
methyl, an alkenyl group such as allyl, an aryl group such as
phenyl, or an acyl group such as (meth)acryloyl or by a combination
of these.
[0024] Examples of the polyurethane oligomer compounds include
urethane polyols which are products of the reaction of a high
molecular weight polyol such as a polyether polyol, polyester
polyol, polycarbonate polyol, or polybutadiene polyol with a
polyisocyanate monomer; and urethane acrylates which are products
of the reaction of a hydroxy (meth)acrylate monomer such as
hydroxyethyl (meth)acrylate, phenyl glycidyl ether acrylate,
pentaerythritol triacrylate, or glycerol dimethacrylate with a
polyisocyanate monomer such as methylene diisocyanate or with any
of the urethane polyols shown above.
[0025] Those low molecular weight substances may be used alone or
in combination of two or more thereof. Such low molecular weight
substances have a molecular weight (weight average molecular
weight) of preferably 10,000 or lower, e.g., from 100 to 10,000,
more preferably about from 200 to 3,000. If a low molecular weight
substance having a weight average molecular weight lower than 100
is used, there are cases where the low molecular weight substance
is completely compatibilized with the polyimide precursor, making
it impossible to obtain a porous object. On the other hand, if a
low molecular weight substance having a weight average molecular
weight exceeding 10,000 is used, there are cases where the removal
thereof in a later step is difficult.
[0026] The polymer composition having a micro-domain structure in
the invention can be formed by the conventional technique. For
example, the polyimide precursor and the low molecular weight
substance in a given proportion are dissolved in a solvent
(usually, an organic solvent) and this polyimide precursor solution
is formed into a desired shape, e.g., a film, by application to a
substrate. Thereafter, the solvent is removed by drying to thereby
insolubilize the low molecular weight substance in the polyimide
precursor. Thus, a polymer composition can be obtained which has a
micro-domain structure comprising a continuous phase made of the
polyimide precursor and a discontinuous phase dispersed
therein.
[0027] The amount of the low molecular weight substance to be added
can be suitably selected according to the combination of the low
molecular weight substance and the polymer. It is generally 200
parts by weight or smaller (e.g., from 5 to 200 parts by weight),
preferably 150 parts by weight or smaller (e.g., from 10 to 150
parts by weight), per 100 parts by weight of the polyimide
precursor.
[0028] The polyimide precursor is formed into, for example, a film
by application to a substrate. For applying the polyimide
precursor, a known means for coating, such as a spin coater, bar
coater, gravure coater or comma coater, may be used according to
the shape and thickness of the substrate. It is preferred to
conduct the application in such an amount that the resulting
polyimide precursor film after drying has a thickness of from 0.1
to 50 .mu.m, preferably from 1 to 25 .mu.m. Improved adhesion can
be obtained by priming the surface of the substrate with a silane
coupling agent or titanate coupling agent prior to the
application.
[0029] In drying the coating for removing the solvent, a
temperature of generally from 40 to 150.degree. C., preferably from
60 to 100.degree. C., is used according to the kind of the solvent
used. If temperature higher than 150.degree. C. is used, there are
cases where imidization of the polyimide precursor begins. Upon
removal of the solvent by this drying, the low molecular weight
substance is insolubilized in the polyimide precursor to thereby
form a micro-domain structure. This drying may result in porosity.
Even if the polymer constituting the continuous phase and the low
molecular weight substance constituting the discontinuous phase
react with each other to form bonds during the drying, this poses
no problem as long as the discontinuous phase can be removed later.
In the polymer composition thus obtained, the low molecular weight
substance constituting the discontinuous phase is present. There
are cases where part of the solvent also remains therein.
[0030] The polyimide composition in the invention can be obtained
by converting the polyimide precursor into a polyimide by, for
example, subjecting the polymer composition having a micro-domain
structure to a dehydrating cyclization reaction. The dehydrating
cyclization reaction of the polyimide precursor may be conducted,
for example, by heating the precursor to about 300 to 400.degree.
C. or by causing a dehydrating-cyclization agent, such as a mixture
of acetic anhydride and pyridine, to act on the precursor. In the
polyimide composition thus obtained, all or part of the low
molecular weight substance remains.
[0031] The method for removing the low molecular weight substance
constituting the discontinuous phase (or remaining) from the
polymer composition containing the polyimide precursor as the
continuous phase or from the polyimide composition containing a
polyimide as the continuous phase will be explained below.
[0032] In the invention, the low molecular weight substance is
removed by extraction with a combination of supercritical carbon
dioxide and a co-solvent. For this extraction, any temperature not
lower than the critical point for supercritical carbon dioxide may
be used. However, in the removal of the low molecular weight
substance from the polymer composition, wherein the polyimide
precursor constitutes the continuous phase, it is preferred to
conduct the extraction at a temperature in the range in which
imidization of the polyimide precursor does not proceed
excessively. As the temperature rises, the solubility of the low
molecular weight substance in supercritical carbon dioxide
decreases. Consequently, the temperature (extraction temperature)
at which the low molecular weight substance is removed with
supercritical carbon dioxide is preferably from 32 to 230.degree.
C., more preferably from 40 to 200.degree. C.
[0033] The pressure of the supercritical carbon dioxide is not
particularly limited as long as it is not lower than the critical
point for supercritical carbon dioxide. However, it is preferably
from 7.3 to 100 MPa, more preferably from 10 to 50 MPa.
[0034] The co-solvent is preferably an aprotic polar solvent.
Preferred examples of the aprotic polar solvent include nitrogen
compound solvents and sulfur compound solvents.
[0035] Examples of the nitrogen compound solvents include
N-methyl-2-pyrrolidone, 2-pyrrolidone, N,N-dimethylacetamide,
N-methylacetamide, acetamide, N,N-dimethylformamide,
N-methylformamide, formamide, and N-methylpropionamide.
[0036] Examples of the sulfur compound solvents include dimethyl
sulfoxide and sulfolane.
[0037] Especially preferred of such aprotic polar solvents are
N-methyl-2-pyrrolidone and N,N-dimethylacetamide. The amount of
those aprotic polar solvents to be added is, for example,
preferably from 1 to 30% by volume based on the supercritical
carbon dioxide.
[0038] The co-solvent may be supplied by the following methods. In
one method, the co-solvent and carbon dioxide are metered in terms
of volume, pressurized, and then continuously supplied with a
constant delivery pump to a pressure vessel containing the polymer
composition or polyimide composition having a micro-domain
structure. In another method, a given amount of the co-solvent is
introduced into the pressure vessel before pressurized
supercritical carbon dioxide is introduced thereinto.
[0039] The time necessary for extraction varies depending on
extraction temperature, extraction pressure, amount of the low
molecular weight substance added to the polyimide precursor, and
coating thickness. However, the extraction time may be about from 1
to 10 hours.
[0040] The extraction may be conducted while keeping the extraction
vessel closed (in a state in which the supercritical carbon dioxide
and co-solvent introduced and the ingredient extracted are
prevented from moving out of the vessel) or while continuously
supplying supercritical carbon dioxide and the co-solvent to the
extraction vessel with a constant delivery pump or the like.
[0041] According to the above-described method of removing a low
molecular weight substance, since an extraction operation is
conducted with a combination of supercritical carbon dioxide and a
co-solvent, the low molecular weight substance constituting the
discontinuous phase as well as the residual solvent and the like
can be removed in a relatively short period of time. Furthermore,
since there is no need of using a large amount of an organic
solvent, the method is preferred from the standpoint of
environmental preservation. In addition, the method is effective in
easily removing oligomers having a relatively high molecular weight
and in greatly improving the efficiency of removal of low molecular
weight substances.
[0042] By the method described above, the low molecular weight
substance is removed from the polymer composition containing a
polyimide precursor as a continuous phase by extraction with a
combination of supercritical carbon dioxide and a co-solvent.
Thereafter, the polyimide precursor is converted to a polyimide by
the method described above. Thus, a porous polyimide can be
obtained. Furthermore, a porous polyimide can be obtained also by
removing the low molecular weight substance from the polyimide
composition containing a polyimide as a continuous phase by
extraction with a combination of supercritical carbon dioxide and a
co-solvent by the method described above. The porous polyimides
thus obtained not only have a small cell size and hence an
exceedingly reduced dielectric constant but have high heat
resistance. The porous objects having these properties are
extremely advantageously utilizable as, e.g., an internal
insulator, buffering material, or circuit substrate in electronic
appliances, etc., while taking advantage of excellent properties
possessed by polyimide resins, such as heat resistance and
mechanical properties.
[0043] As described above, according to the method of the invention
for removing a low molecular weight substance, the low molecular
weight substance present as a discontinuous phase in a polyimide
precursor or polyimide can be efficiently removed because
extraction is conducted with a combination of supercritical carbon
dioxide and a co-solvent. Even oligomers having a relatively high
molecular weight can be removed. This method is advantageous also
from an environmental standpoint.
[0044] According to the porous polyimide production processes of
the invention, a porous polyimide having a small cell diameter and
a low dielectric constant can be efficiently obtained.
[0045] The invention will be explained below in more detail by
reference to Examples and Comparative Example, but the invention
should not be construed as being limited by these Examples and
Comparative Example in any way.
[0046] Method for Determination of Dielectric Constant:
[0047] Dielectric constant was determined with HP 4284A Precision
LCR Meter, manufactured by Yokogawa-Hewlett-Packard, Ltd.
SYNTHESIS EXAMPLE
[0048] Synthesis of Polyimide Precursor
[0049] Into a four-necked flask having a capacity of 500 mL
equipped with a stirrer and a thermometer were introduced 16.2 g
(0.15 mol) of p-phenylenediamine (PDA) and 227 g of
N-methyl-2-pyrrolidone (NMP). The contents were stirred at room
temperature to obtain a solution. Subsequently, 39.7 g (0.135 mol)
of 3,3',4,4'-biphenyltetracarboxylic dianhydride (diphthalic
dianhydride; BPDA) and 3.0 g (0.015 mol) of
2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)
were added to the solution. The resulting mixture was stirred at
room temperature for 2 hours to obtain a polyimide precursor
solution.
EXAMPLE 1
[0050] A polyethylene glycol monomethyl ether oligomer having a
weight average molecular weight of 600 was added to the polyimide
precursor solution obtained in the Synthesis Example, in an amount
of 66 parts by weight per 100 parts by weight of the polyimide
precursor. The resulting mixture was stirred to obtain a
transparent homogeneous solution. This solution was applied to a 25
.mu.m-thick stainless-steel foil (SUS304) with a comma coater in
such an amount as to result in a polyimide precursor film having a
thickness of 15 .mu.m on a dry basis. The coating was dried at
95.degree. C. for 5 minutes to remove the solvent and then further
heated at 180.degree. C. for 10 minutes to obtain a polyimide
precursor film which had a micro-domain structure containing the
polyethylene glycol monomethyl ether oligomer.
[0051] This polyimide precursor film was cut into a sheet having
dimensions of 50 mm by 100 mm. This sheet was placed in a 500 ml
pressure vessel, and 50 ml of N,N-dimethylacetamide was introduced
thereinto. The pressure vessel was closed, and supercritical carbon
dioxide pressurized to 25 MPa was introduced thereinto in a
100.degree. C. atmosphere. Thereafter, the pressure and temperature
were maintained for 2 hours to conduct supercritical extraction.
Throughout this operation, the pressure vessel was kept closed and
supercritical carbon dioxide was not passed therethrough.
[0052] Subsequently, the pressure inside the pressure vessel was
lowered and the polyimide precursor film was taken out. The amount
of the low molecular weight substance removed was calculated from
the weight change through the extraction. As a result, 100% by
weight of the low molecular weight substance was found to have been
removed. The matter extracted was analyzed by FT-IR spectroscopy.
As a result, the matter removed was found to be the polyethylene
glycol monomethyl ether oligomer added to the polyimide
precursor.
[0053] The polyimide precursor film thus treated was heated to
380.degree. C. at a reduced pressure of 1.33 Pa to obtain a porous
polyimide film having a thickness of 10 .mu.m. The porous polyimide
film obtained had a dielectric constant .epsilon. of 2.47
(measuring frequency, 1 MHz).
EXAMPLE 2
[0054] A polyimide precursor film was produced in the same manner
as in Example 1 above.
[0055] This polyimide precursor film was cut into a sheet having
dimensions of 50 mm by 100 mm. This sheet was placed in a 500 ml
pressure vessel, and 50 ml of N-methyl-2-pyrrolidone was introduced
thereinto. The pressure vessel was closed, and supercritical carbon
dioxide pressurized to 25 MPa was introduced thereinto in a
100.degree. C. atmosphere. Thereafter, the pressure and temperature
were maintained for 2 hours to conduct supercritical extraction.
Throughout this operation, the pressure vessel was kept closed and
supercritical carbon dioxide was not passed therethrough.
[0056] Subsequently, the pressure inside the pressure vessel was
lowered and the polyimide precursor film was taken out. The amount
of the low molecular weight substance removed was calculated from
the weight change through the extraction. As a result, 100% by
weight of the low molecular weight substance was found to have been
removed. However, a slight decrease in film thickness was
observed.
[0057] The polyimide precursor film thus treated was heated to
380.degree. C. at a reduced pressure of 1.33 Pa to obtain a porous
polyimide film having a thickness of 10 .mu.m. This porous
polyimide film had a dielectric constant .epsilon. of 2.80 (1
MHz).
COMPARATIVE EXAMPLE
[0058] A polyimide precursor film was produced in the same manner
as in Example 1 above.
[0059] This polyimide precursor film was cut into a sheet having
dimensions of 50 mm by 100 mm. This sheet was placed in a 500 ml
pressure vessel. The pressure vessel was closed without introducing
a co-solvent thereinto, and supercritical carbon dioxide
pressurized to 25 MPa was introduced thereinto in a 100.degree. C.
atmosphere. Thereafter, the pressure and temperature were
maintained for 2 hours to conduct supercritical extraction.
Throughout this operation, the pressure vessel was kept closed and
supercritical carbon dioxide was not passed therethrough.
[0060] Subsequently, the pressure inside the pressure vessel was
lowered and the polyimide precursor film was taken out. The amount
of the low molecular weight substance removed was calculated from
the weight change through the extraction. As a result, only 5.6% by
weight of the low molecular weight substance was found to have been
removed.
[0061] The polyimide precursor film thus treated was heated to
380.degree. C. at a reduced pressure of 1.33 Pa to obtain a porous
polyimide film having a thickness of 10 .mu.m. The porous polyimide
film obtained had a dielectric constant .epsilon. of 3.00 (1
MHz).
* * * * *