[ACS Symposium Series] Inorganic and Organometallic Polymers II Volume 572 (Advanced Materials and Intermediates) || Synthesis and Preceramic Applications of Poly(aminoborazinyls)

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<ul><li><p>Chapter 28 </p><p>Synthesis and Preceramic Applications of Poly(aminoborazinyls) </p><p>Yoshiharu Kimura and Yoshiteru Kubo </p><p>Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan </p><p>Soluble and thermoplastic derivatives of poly(aminoborazinyls) were used as ceramic precursors for fabricating hexagonal boron nitride (h-BN) ceramics. They were readily prepared by solution condensation of B,B,B-tri(methylamino)-borazine (MAB) and bulk co-condensation of MAB and laurylamime (LA). The structure analysis revealed that both the solution and bulk condensates consist of several borazinyl rings which have two- and three-point linkages with other rings as well as a one point-linkage. The bulk co-condensate of MAB-LA was melt-spun and pyrolyzed to fabricate BN fiber, while the solution condensate was used as a ceramic binder in the powder consolidation molding of h-BN and as a matrix material in the fabrication of fiber-ceramic composites (FCC). These examples demonstrated that these BN precursors are useful for developing well-designed super heat-resistant materials that may be essential for future technological innovation. </p><p>Among the newly developed high-tech ceramics, hexagonal boron nitride (h-BN) has been occupying an unique position (1), because of its excellent heat resistance, low density, and structural similarities with graphite. In the utilization of h-BN, however, there have been intrinsic problems related with its processability and mechanical properties. Since the processing of h-BN cannot usually be performed by conventional thermal sintering, a high-energy process like hot-pressing has been utilized. Therefore, fabrication of fibers and molding with complicated, fine shape are difficult. Furthermore, mono-lithic h-BN is likely to crack from a surface micro flaw, and reinforcement with ceramic fiber is preferable (2). For these purposes the so-called precursor method using an appropriate inorganic polymer as the ceramic precursor should be effective (5), since die polymeric precursor should be easily processed to a desired shape and then pyrolyzed into the desired ceramics by the ordinary thermal sintering process without using sinter-aiding agent (4, 5). In this method the structure and properties of the precursor have a close relationship with the performance of the ceramics obtained, and poly(aminoborazinyls) consisting of the unit components of h-BN have received much attention for use as the h-BN precursor (Z). In particular, Paciorek (6), Paine (7), and Sneddon (8) designed various derivatives of poly(aminoborazinyls) which can be readily converted to h-BN. Recently, we demonstrated that poly(aminoborazinyls) prepared from ,,-</p><p>0097-6156/94/0572-0375508.00/0 1994 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h028</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>376 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>triaminoborazine (TAB) and B,B,B-tri(methylamino)borazine (MAB) are potentially useful for the fabrication of BN fibers and molds (9-11). The further applications of this technique, bench-scale preparation of BN fiber, powder consolidation molding of BN ceramics, and fabrication of fiber-ceramic composite (FCC) have been studied in detail. This paper deals with the concepts of these methods as well as a preliminary feasibility study utilizing poly[(methylamino)borazinyl], i.e., the MAB condensate, as the precursor for making BN ceramics. </p><p>Synthesis of MAB </p><p>MAB was prepared in high yield by the following reaction scheme (12). It was isolated as white waxy material. </p><p>CI NHCH3 I I 3 BCI3 n o . c H - N - B ^ N - H r.t. ^ ' + 1 I I </p><p>NH4CI " H C 1 c i ^ N ^ C I 3 N H 2 C H a N H ^ N ^ N H C H a I I H H </p><p>MAB: IR (nujol) 3460 (NH), 1410 cm'1 (B3N3 ring), etc.; *HNMR (C6U6) 1.4 ppm (broad, 3H, M6NH), 2.4 (d, 9H, CH3), 2.6 (s , 3H, Ni/), n B NMR (QD6) 26 ppm (the line width at half height = 284 Hz). </p><p>Solution polymerization of MAB </p><p>MAB polymerizes in the bulk state when heated above 130C to form a thermoplastic condensate. However, its thermoplasticity is likely to be lost by crosslinking during the bulk condensation (10). In order to control the degree of condensation, the thermal condensation of MAB was carried out in refluxing chlorobenzene solution. With extended reflux an insoluble crosslinked product precipitated. Therefore, the reaction was stopped upon observation of the first trace of the precipitate, and the MAB condensate was isolated from this solution as white solid material. </p><p>Table I shows a typical result of the solution condensation of MAB as compared with that of the bulk condensation. The yields of the condensate were almost identical, but the solution condensate was wholly soluble in organic solvents, while the bulk condensate contained some insoluble gel. The average molecular weight of the former was slightly lower than that of the soluble part of the bulk condensate. The softening point of the solution condensate was about 80 C, which was similar to that of the co-condensate of MAB and laurylamine (LA) (90/10 in wt; vide infra). </p><p>Table I, Thermal condensation of MAB Solvent Temp. </p><p>f Q Time (h) </p><p>Condensate (wt%) </p><p>Mn </p><p>chlorobenzene 130 150 </p><p>2 2 </p><p>85 87 </p><p>500 720 </p><p>The X H NMR spectrum of the condensate is shown in Figure 1(b). As compared with the spectrum of MAB (Figure 1(a)), the signals at 1.4 and 2.4 ppm were reasonably ascribed to the NH and methyl protons of the substituent </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h028</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>28. KIMURA &amp; KUBO Synthesis of Poly(aminohorazinyls) 377 </p><p>Figure 1. X H NMR spectra of (a) MAB, (b) the solution condensate of MAB, and (c) the bulk co-condensate of MAB-LA (at 200 MHz, in QDo). </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h028</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>378 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>methylamino groups, respectively. In contrast, the NH protons of the borazine ring appeared as three sets of signals around 2.6, 3.5, and 4.3 ppm. While the highest-field one is ascribed to the NH protons of the borazine ring that is bonded with only one borazinylamino group (one-point-linkage form). The middle- and the lowest-field signals are reasonably ascribed to those of the borazine rings that are bonded with two and three borazinylamino groups (two- and three-point-linkage forms), respectively. The splittings of the latter signals may be due to the asymmetry of the borazinyl substituents. With this signal assignment combined with the signal intensities and the molecular weight, the structure of the condensate could be represented by 1, for example, whose average degree of polymerization was five. These facts supported that the homo condensation of MAB is easily controlled in solution state. </p><p> CH3NHx xNHCH 3 </p><p> I I </p><p>HN^ ^NH CH </p><p>3 ^ N ^ NHCH3 NHCH3 </p><p> H N ' X N H H N ' X N H H N ' ^NH </p><p>I I I I I I </p><p> 3 " ' ^ ' " ^ ' ^ ' % 3 B H CH 3 CH 3 </p><p>HN" V NH </p><p>, B \ , B X CH3NH NHCH3 </p><p> 1 </p><p>Bulk Co-Condensation of MAB and Laurylamine </p><p>Another effective method for preparing thermoplastic poly(aminoborazinyl) is to make a co-condensate of MAB with such a higher alkylamine as LA. We have shown that the latter can be trapped in the condensate to decrease the crosslinks and work as plasticizer (9). </p><p>In the present study, a larger scale synthesis of the co-condensate was studied by heating about 20 g of MAB and 10 wt% of LA at 200-260C for 1 h in a nitrogen flow. Since in this larger scale reaction the elimination of methylamine was slowed, a relatively higher reaction temperature was necessary to effect the condensation. </p><p>Table . Thermal Co-Condensation8 of MAB and LA Run No. LA </p><p>(wt%) Temp. rc) </p><p>Time (h) </p><p>Weight Loss (%) </p><p>Mn D (dalton) </p><p>Spinnability </p><p>1 10 200 1 10.1 _ good 2 10 220 i 13.4 good 3 10 240 1 17.1 470 excellent 4 10 260 1 18.6 - poor </p><p>a20 g of MAB and 2.0 g of LA were reacted. b By GPC relative to PPG standards. </p><p>As summarized in Table , the degree of condensation in terms of weight loss wassuccessfully controlled by changing the reaction temperature, and colorless thermoplastic products were formed when the weight loss due to the elimination of </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h028</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>28. KIMURA &amp; KUBO Synthesis ofPoly(aminoborazinyls) 3 7 9 </p><p>MA was within 10-20 wt%. Mn of the soluble condensates were in the range of 400-800 as determined by GPC, which corresponds to that of pentamers - decamers of MAB. The typical NMR spectrum of the condensate is shown in Figure 1(c). The characteristic signals due to the methyl and methylene protons of lauryl group were noted at 0.9,1.28, and 2.2 ppm, respectively. The methyl signal of NHCH3 group at 2.4 ppm became broader, and the two signals at 1.3 and 1.8 ppm may be ascribed to the pendant NH of the methylamino and laurylamino groups, respectively. The signals of the NH protons of the borazine ring were detected at the region where those of the borazine rings with one- and two-point linkages should appear. Therefore, it appears that the number of three-point-linkage was decreased. The U B NMR signal of this product was detected as a single peak at the same chemical shift ( 26 ppm) as observed for MAB. Based on these spectroscopic data, the structure of the MAB-LA condensate is postulated as 2, in which LA units are incorporated as alkylimino and alkylamino units to interfere with the formation of crosslinks in the product. </p><p>R H N . ,NHCH 3 i I </p><p>HN^ ^NH R ^ N ^ B NHCH3 NHCH3 </p><p> HN^ X N H H N ' X N H HN^ ^NH </p><p>I I I I I I B R </p><p>RHN' - N " -NH - N ' N N ' % H R 6H3 A </p><p>2 (R=C12H25) </p><p>The DTA of these condensates exhibited a broad endothermic peak around 85C and an exothermic peak ranging from 200 to 430C which are ascribed to their melting and thermal decomposition, respectively. A large weight loss was observed at 200-500 C in TGA, and the final weight loss at 900C was 38 %. </p><p>Fabrication of BN Fiber from Bulk Co-Condensate of MAB-LA </p><p>As discussed in the Introduction, the poly (aminoborazinyls) can be pyrolyzed into h-BN ceramics and have various potential applications for use as preceramics. In this article, some examples based on the different processabilities of the above MAB condensates are shown. </p><p>In the early 1970s, Economy et al invented a chemical conversion method of fabricating BN fiber, in which boron oxide fiber was melt-spun and converted to BN by solid phase nitridation in an ammonia atmosphere (13). In this process, however, quality control of the precursor fiber and the process control of the nitridation were not easy. Therefore, an alternative preceramic process was developed (14, 15), although the spinnability of the polymer precursors utilized were too poor to fabricate precursor fibers with excellent quality. Recently, the author reported a new method of fabricating high-performance h-BN fiber based on a thermoplastic derivative of poly[(methylamino)borazinyl] as the precursor polymer (9, 11). In this process the precursor fiber is spun by melt-spinning techniques, pyrolyzed directly up to 1000eC in an ammonia gas flow, and further sintered up to 1800C in a nitrogen flow. The strength of the sintered BN fiber was found to reach 1 GPa. In the present study, the bench-scale preparation of BN fiber was investigated by a similar approach using the co-condensate of MAB-LA prepared above. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h028</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>380 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>Melt spinning. Using a micro screw extruder with a nozzle of 500 mm in diameter, the condensate was melt-spun at 160-170C. The extrudate from the spinneret was taken up by a winder to give a continuous filament of about 50 mm in diameter. The best melt-flow of the extrudate was obtained for the condensate obtained at 240C (Run No.3 in Table I). The good spinnability of the MAB-LA condensate in spite of its low molecular weight is attributed to the strong intermolecular aggregation between the planar borazine rings. This phenomenon is comparable to that of amorphous carbon pitch (16), which is utilized as the precursor to carbon fiber and has an excellent spinnability. The latter consists of aromatic rings in place of borazine networks with Mn of 800 - 1000. A typical SEM micrograph of the precursor fiber (see Figure 2) indicates that the cross-section of the fiber is circular and that the inner phase is homogeneous. </p><p>Pyrolysis of fiber. During the spinning, the fiber surface was slightly hydrolyzed, and the fiber became infusible. Therefore, it was directly pyrolyzed in an electric tube furnace without special thermosetting. The temperature of the furnace was raised to 600-1000C at a rate of 100C/h in a flow of ammonia gas (30 ml/min), held at the predetermined temperature for 1 h, and then cooled to room temperature. At this stage the sintered fiber was gray due to the carbonization of the organic residue. It was further sintered up to 1400 and 1800eC in a nitrogen flow. Then, its color turned white above 1400C. Figure 2 shows the typical SEM photographs of the fibers pyrolyzed at various temperatures. The inner phase of the fibers pyrolyzed at 1000 and 1400eC was homogeneous, while that of the fibers sintered above 1600eC exhibited a slight grain growth. </p><p>Mechanical properties. We reported in the former paper (9) that the modulus of the BN fiber prepared by the precursor method increased with increasing pyrolysis temperature, while its tensile strength decreased around 1400C and increased again at the higher temperature. The average tensile strength and modulus observed were 1 GPa and 78 GPa, respectively, which are lower than those of the common ceramics fibers reported thus far (3). Most of t...</p></li></ul>