Compositions – Liquid crystal compositions
Reexamination Certificate
1998-06-03
2001-05-08
Wu, Shean C. (Department: 1756)
Compositions
Liquid crystal compositions
C428S394000, C428S395000, C264S005000, C264S013000
Reexamination Certificate
active
06228285
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of making shaped articles from rigid-chain polymers, and more particularly, it relates to a method which utilizes a phase transformation of the rigid-chain polymer in a concentrated solution at elevated temperatures to form a wide variety of shaped articles including bulk components.
The use of organic polymers in the formation of structural materials has been desirable as such polymers can be melt processed at low temperatures, are environmentally stable, and are light in weight. However, such polymers are not suitable for high temperature use applications such as reinforcing fibers.
In recent years, high temperature, high-performance rigid-chain polymers have been developed by incorporating aromatic and heteroaromatic structures into the polymer backbone. Such rigid-chain, or rigid-rod, polymers include poly(p-phenylene benzobisthiazole) (PBT), poly(p-phenylene benzobisoxazole) (PBO) and poly(p-phenylene benzobisimidazole) (PBI), ladder polymers such as poly(imidazoisoquinolines) (BBL), extended-chain polymers such as poly(p-phenylene terephthalamide) (PPTA), poly(2,5(6)benzothiazole) (ABPBT) and poly(2,5(6) benzimidazole) (ABPBI), as well as their functionalized derivatives such as benzocyclobutene-functionalized PBT and PPTA. These polymers have a thermal and thermooxidative stability up to 500° C. When subjected to elevated temperatures, they normally decompose but do not melt.
However, these thermally intractable rigid-chain polymers are soluble in strong acids, such as polyphosphoric acid (PPA), methanesulfonic acid (MSA) and sulfuric acid, as well as organic solvents with Lewis acids. It has been found that when these polymers are dissolved in such acids, the resulting solutions may be extruded and coagulated to form uniaxially oriented fibers and biaxially oriented films. However, while such fibers have excellent tensile properties, they have poor compressive strength due to the fibrillar structure of the fibers, which is weak in lateral interactions and tends to buckle under a compressive load. This poor axial compressive strength has prevented the rigid-chain polymer fibers from being used as reinforcing fibers in more advanced structural composites.
Further, it is difficult to process the thermally intractable rigid-chain polymers into other structural materials such as bulk structural components because the extraction of solvent from such large components is time consuming and often results in shrinkage and deformation of the components.
Accordingly, there is still a need in the art for an improved method of processing thermally intractable rigid-chain polymers into structural components such as fibers and bulk components which exhibit good compressive strength and which do not undergo shrinkage or deformation during formation.
SUMMARY OF THE INVENTION
The present invention meets that need by providing a method which utilizes a phase transformation process to enable thermally intractable rigid-chain polymers to be fabricated into a wide variety of shaped articles including fibers, films, monolithic structural components and fiber-reinforced composites for structural applications. The resulting articles exhibit good mechanical properties and undergo little or no shrinkage during formation.
In accordance with one aspect of the present invention, a method is provided for forming a shaped article in which a rigid-chain polymer is dissolved in a strong acid, preferably sulfuric acid (H
2
SO
4
), and heated to a temperature sufficient to form a nematic liquid crystalline solution. The solution is then formed into the desired shape and cooled to form a solid. The method also includes the removal of solvent from the solid which can be extracted by conventional coagulation and drying methods.
The rigid-chain polymer for use in the present invention is selected from the group consisting of poly(p-phenylene benzobisthiazole), poly(p-phenylene pyridobisimidazole), poly(p-phenylene terephthalamide), and derivatives thereof. Preferably, the rigid-chain polymer comprises poly(p-phenylene benzobisthiazole) (PBT). The polymer is preferably present in the solution at a concentration of at least about 7% by weight, and more preferably, at least about 20% by weight, that is, preferably within the range of about 7% to about 40%, and more preferably, from about 20% to about 40%.
The heating temperature varies depending on the polymer concentration and generally increases with an increase in concentration. For example, for a solution having a concentration of about 7% by weight, a temperature of from about 122-127° C. is preferred, while for a solution having a concentration of 20% by weight, a temperature of from about 180-200° C. is preferred.
We have found that with the method of the present invention, the rigid-chain polymer/H
2
SO
4
solution undergoes a phase transformation from a liquid crystalline phase upon heating to a solid phase upon cooling which contains crystal solvates. Because of this phase transformation to a solid, the shaped article undergoes little or no shrinkage upon formation because the removal of solvent occurs when the article is in solid form.
In another embodiment of the invention, a method is provided for forming a fiber composite from a rigid-chain polymer in which a rigid-chain polymer is combined with fibers and dissolved in a strong acid (preferably H
2
SO
4
) and heated to form a nematic liquid crystalline solution. The solution is then formed into a composite and cooled to form a solid. Excess solvent may be extracted by conventional coagulation and drying methods as described above.
The fibers are preferably selected from the group consisting of glass, carbon, ceramic and heated-treated PBT fibers and may be provided in continuous or chopped form.
Preferably, the rigid-chain polymer is present in the solution in a concentration as described above.
The structural components formed by the process of the present invention have low temperature solution processability, good mechanical properties, high use temperature, and are lightweight. The structural components may be used in sporting goods applications, electronics assemblies, commercial and military aircraft and aerospace vehicles, automobiles, and construction materials.
Accordingly, it is an object of the present invention to provide a method of forming a wide variety of shaped articles from rigid-chain polymers. This, and other features and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.
REFERENCES:
patent: 4900805 (1990-02-01), Arnold et al.
patent: 4900806 (1990-02-01), Arnold et al.
patent: 5095075 (1992-03-01), Arnold et al.
patent: 5492666 (1996-02-01), Wang et al.
CAPLUS 1990: 149452, 1990.*
Jenekhe et al., “Solubilization, Solution, and Processing of Aromatic Heterocyclic Rigid Rod Polymers in Aprotic Organic Solvents: Poly(p-phenylene-2,6-benzobisthiazoldiyl)(PBT)”, Marcomoleclues 1989, 22, 3216-3222.
Lee Jar-Wha
Wang Chyi-Shan
Killworth, Gottman Hagan & Schaeff, L.L.P.
The University of Dayton
Wu Shean C.
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