Chemistry of inorganic compounds – Boron or compound thereof – Binary compound
Reexamination Certificate
1999-12-27
2001-08-21
Vollano, Jean F. (Department: 1621)
Chemistry of inorganic compounds
Boron or compound thereof
Binary compound
Reexamination Certificate
active
06277348
ABSTRACT:
This application is the national phase of PCT/FR98/01403, filed Jul. 1, 1998, now WO99/01389.
The present invention relates to:
a novel process for the preparation of mesophasic polyborazylene. Said process is of particular value in that it affords a quality product from an appropriate polyborazylene, rapidly (virtually instantaneously), reproducibly and with a good yield. Furthermore, said process is easy to carry out,
mesophasic polyborazylene which is novel in that it is in the presence of a particular solvent and/or by virtue of its quality,
the use of said polyborazylene of the invention, and/or prepared according to the invention, as a boron nitride precursor.
Since the midseventies, the appearance of new technological stakes in materials sciences has rekindled interest in organometallic polymers, and numerous developments have appeared. Thus there have been reports on the synthesis of siliconcontaining polymers, of the SiC and SiC/Si
3
N
4
type, capable of being shaped and of being used as precursors to ceramics. These discoveries, combined with increasing practical needs for molding compounds and for new materials for hightemperature applications, have encouraged new research into organometallic and inorganic polymers.
Boron nitride is the basis of commercial ceramics. It can easily be obtained in powder form by the pyrolysis of relatively inexpensive reactants. Unfortunately, it is very difficult to obtain said boron nitride in the form of fibers or films from powders. The use of polymers based on boron and nitrogen seemed to be a very attractive way of doing this. Thus polymers of this type, obtained by the polycondensation of borazine, have already been used. Borazine or “inorganic benzene”, which is easy to prepare, is in fact an appropriate precursor (being a reactive planar molecule capable of generating planar polycondensed molecules consisting of borazine rings). Said borazine has the following chemical formula:
and is characterized by a B/N atomic ratio of 1.
In a first method, said borazine was polymerized in bulk, with regular degassing (the dihydrogen is evacuated as it is formed), to generate a polymer, polyborazylene, which can be deposited especially in the form of a film. Said polyborazylene, its preparation by this method and its uses as a boron nitride precursor have been described in particular by:
FAZEN P. J., BECK J. S., LYNCH A. T., REMSEN E. E., SNEDDON L. G., in “Thermally Induced Borazine Dehydropolymerization Reactions, Synthesis and Ceramic Conversion Reactions of a New Highyield Polymeric Precursor to Boron Nitride”, Chem. Mater., 1990, 2, 96-97;
FAZEN P. J., REMSEN E. E., BECK J. S., CARROLL P. J., McGHIE A. R., SNEDDON L. G., in “Synthesis, Properties and Ceramic Conversion Reactions of Polyborazylene. A Highyield Polymeric Precursor to Boron Nitride”, Chem. Mater., 1995, 7, 1942-1956.
The specific use of said polyborazylene in the form of a film has been described in particular by:
CHAN V. Z-H., ROTHMAN J. B., PALLADINO P., SNEDDON L. G., COMPOSTO R. J., in “Characterization of Boron Nitride Thin Films Prepared from a Polymer Precursor”, J. Mater. Res., 1996, 11(2), 373-380;
SNEDDON L. G., BECK J. S., FAZEN P. J., in “Direct Thermal Synthesis in the Absence of Catalyst, and Ceramic Applications of Poly(borazylenes)”, patent U.S. Pat. No. 5,502,142.
In a second method, said borazine was polymerized in bulk with the pressure being allowed to increase in the reactor. Economy et al. showed that, under such conditions, it was possible to obtain oligomers with a very low “graphitization” temperature (1500° C. instead of 2500° C. for an ordinary boron nitride) (graphitization being a process of three-dimensional organization of the sheets of BN hexagons, analogous to the organization of the graphite crystal).
To obtain, under these conditions, a polymer capable of developing a mesophase, Economy et al. either stored the resulting oligomer for several weeks at a temperature of between 0 and 5° C., or baked said oligomer at 100° C. under a pressure of 20 MPa. (Observations by polarized light microscopy show that the application of a heat treatment up to 100° C. has the effect of disordering the material. However, after return to room temperature, Economy et al. observe a return to an anisotropic state, although this reversibility is not total. In fact, X-ray diffraction shows a peak corresponding to the (002) planes, broadened after treatment at 100° C.)
Both of the above methods, which therefore afforded mesophasic polyborazylene, have been described respectively by:
KIM D., ECONOMY J., in “Occurrence of Liquid Crystallinity in a Borazine Polymer”, Chem. Mater., 1994, 6, 395-400 (storage at low temperature);
and by:
COFER C.G., KIM D., ECONOMY J., in “Formation of an Ordered Boron Nitride Matrix for Fiber Reinforced Composites”, Ceram. Trans., 1994, 46, 189-197 (baking under pressure).
However, said methods are difficult to reproduce and relatively expensive to carry out and, in any case, generate polyborazine mesophases of mediocre quality with only a very low yield.
Now, those skilled in the art are not unaware of how valuable it would be to have easy and quantitative access to a sufficiently fluid mesophasic polymer which already contained broad anisotropic domains, foreshadowing the sheet structure of the desired ceramic, for use as a precursor to boron nitride, especially hexagonal boron nitride (BN-hex), with a view to applications as a matrix and interphase, or even fibers, etc.
Faced with this technical problem of providing such a boron nitride precursor—mesophasic polyborazylene—the Applicant has developed a novel process for the preparation of said precursor. Said process makes it possible to obtain said precursor under very advantageous conditions and in novel forms.
According to its first subject, the invention therefore relates to a novel method of preparing a mesophase which is a boron nitride precursor, said method affording the desired product (virtually) instantaneously and quantitatively and with a high yield. Said method consists essentially in adding a solvent to a polyborazylene. It has the advantage of adjusting the viscosity while developing the liquid crystal structure. This facilitates control over the rheology when shaping materials, such as an interfacial film for composite materials.
More precisely, the process proposed according to the invention is a process for the preparation of mesophasic polyborazylene which comprises two essential steps. In the first of said steps, polyborazylene is prepared by the polycondensation of borazine in a closed reactor. This synthesis of polyborazylene, which has been known per se since at least 1959 (D. T. Haworth, L. F. Hohnstedt, J.A.C.S., 1959, vol. 82, 3860), is carried out by the technique of the prior art referred to above. In the second of said steps, as a characteristic feature, the desired mesophase is obtained virtually instantaneously by the addition, to said polyborazylene obtained by polycondensation in a closed reactor, of an appropriate additive (called a solvent) selected from aromatic solvents, borazine solvents and mixtures thereof.
Totally surprisingly, the Applicant has found that a polyborazylene prepared in a closed reactor (i.e. in particular under the pressure of the dihydrogen released) leads instantaneously, under the effect of an appropriate solvent, to the formation of a mesophase, reproducibly and with high yields.
Within the framework of the process of the invention, the simple intervention of a suitable solvent is advantageously substituted for the techniques of the prior art involving slow ageing at low temperature or baking under pressure, which afford a mesophasic polyborazylene of low quality, nonreproducibly and with only a low yield. Such a substitution is advantageous by virtue of its ease of use, its high yield, its reproducibility and the quality of the product obtained.
The solvent used, which dissolves the polyborazylene or is itself dissolved in said polyborazylene, favors the organization of the polycondensed molecules. Its use, in larger
Birot Marc
Bourrat Xavier
Dunogues Jacques
Manfe Olivier
Naslain Roger
Societe Nationale d'Etude et de Construction de Moteurs d&a
Vollano Jean F.
Weingarten, Schurgin Gagnebin & Hayes LLP
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