Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-11-13
2002-09-17
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
Reexamination Certificate
active
06451965
ABSTRACT:
FIELD OF THE INVENTION
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
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 light weight 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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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 in to a polyimide.
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
The polyimide precursor to be used in the invention is not particularly limited as long as it is a
Fukuoka Takahiro
Kanada Mitsuhiro
Mochizuki Amane
Yamamoto Takayuki
Boykin Terressa M.
Nitto Denko Corporation
Sughrue & Mion, PLLC
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