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Chem. Mater. 1998, 10, 412-421
Amine-Modified Polyborazylenes: Second-Generation Precursors to Boron Nitride Thomas Wideman,1 Edward E. Remsen,*,2 Enriqueta Cortez,2 Vicki L. Chlanda,2 and Larry G. Sneddon*,1 Department of Chemistry and Laboratory for the Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, and Analytical Sciences Center, Monsanto Corporate Research, Monsanto Company, 800 North Lindbergh Blvd., St. Louis, Missouri 63167 Received August 18, 1997. Revised Manuscript Received October 14, 1997X
New second-generation polymeric precursors to BN ceramics have been synthesized in high yield by the reaction of polyborazylene (PB), [B3N3H∼3.5]x, with diethylamine (DEA), dipentylamine (DPA), and hexamethyldisilazane (HMD). Elemental analyses and the spectroscopic data indicate that the resulting DEA-PB, DPA-PB, and HMD-PB polymers contain boron-bonded amino groups attached to the polyborazylene backbone. Analysis of volatile byproducts of the reaction suggests modification of PB with DEA and DPA occurs primarily through dehydrocoupling reactions, while the reaction with HMD involves amine Si-N bond cleavage with elimination of trimethylsilane. Combined molecular weight/ infrared spectroscopy studies show the polymers are modified throughout the molecular weight distribution. Modification with HMD results in increased molecular weights due to cross-linking reactions involving the silazane. The DEA-PB and DPA-PB polymers have lower molecular weights than the starting PB, with the highest amine concentrations in the lower molecular weight fractions, suggesting some backbone scission occurs during polymer modification. The modified polymers show increased solubility in organic solvents compared to the parent PB polymer. Also unlike PB, the DPA-PB polymers become fluid, without weight loss, in the range 75-95 °C. The DPA-PB polymers were melt-spun using a crude ram extruder to yield continuous polymer fibers 30-40 µm in diameter. After a brief air-cure, pyrolysis of the polymer fibers under ammonia yielded ∼30 µm BN ceramic fibers of good quality, as determined by SEM, DRIFT, XRD, and RBS measurements, as well as oxidation and mechanical studies.
Introduction Boron nitride has a wide range of attractive properties, including high-temperature stability and strength, a low dielectric constant, large thermal conductivity, hardness, and corrosion and oxidation resistance, leading to a number of potential applications as a structural or electronic material.3 Boron nitride powders may be easily obtained,4 but it has proven more difficult to prepare BN in more complex forms, especially fibers and coatings. Thus, there has been intense interest by many research groups on the development of polymeric precursors to BN.3a Abstract published in Advance ACS Abstracts, December 15, 1997. (1) University of Pennsylvania. (2) Monsanto Company. (3) (a) Paine, R. T.; Narula, C. K. Chem. Rev. (Washington, D.C.) 1990, 90, 73-91 and references therein. (b) Meller, A. Gmelin Handbuch der Anorganische Chemie, Boron Compounds; Springer-Verlag: Berlin, 1983; 2nd Supplement, Vol. 1. (c) Meller, A. Gmelin Handbuch der Anorganische Chemie, Boron Compounds; Springer-Verlag: Berlin, 1988; 3rd Supplement, Vol. 3, and references therein. (d) Bracke, P.; Schurmans, H.; Verhoest, J. Inorganic Fibres and Composite Materials: A Survey of Recent Developments; Pergamon: New York, 1984; p 54 and references therein. (e) Pouch, J. J.; Alterovitz, S. A. Synthesis and Properties of Boron Nitride; Trans Techn: Brookfield, 1990. (4) Greenwood, N. N.; Earnshaw, A. In: Chemistry of the Elements; Pergamon Press: New York, 1994. X
We have previously shown that borazine can be readily dehydropolymerized to give the soluble preceramic polymer, polyborazylene (PB), in excellent yields.5
The combined analytical, spectroscopic, and molecular weight data indicate the polymer has a complex structure, related to those of the organic polyphenylenes, with linear, branched-chain and fused-cyclic borazine segments, and typical Mn ) 500-900 g/mol and Mw ) 3000-8000 g/mol. According to its powder X-ray diffraction spectra, the polymer also appears to have a layered structure in the solid state. Further studies have shown that polyborazylene is an important preceramic polymer that converts to boron nitride in (5) (a) Fazen, P. J.; Remsen, E. E.; Carroll, P. J.; Beck, J. S.; Sneddon, L. G. Chem. Mater. 1995, 7, 1942-1956. (b) Fazen, P. J.; Beck, J. S.; Lynch, A. T.; Remsen, E. E.; Sneddon, L. G. Chem. Mater. 1990, 2, 96-97.
S0897-4756(97)00572-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/19/1998
Amine-Modified Polyborazylenes
Figure 1. Proposed two-dimensional cross-linking process for the ceramic conversion reaction of polyborazylene.
excellent chemical (89-99%) and ceramic yields (8493%) by a process involving a two-dimensional crosslinking reaction similar to that shown in Figure 1. While polyborazylene has proven to be an excellent precursor for the production of boron nitride coatings, films, and shaped materials, the initial polymer-crosslinking reaction depicted in Figure 1 occurs at low temperature and has prevented the use of the polymer in applications requiring melt-processing. Thus, the key to the utilization of this polymer in other more demanding applications, such as the melt-spinning of fibers, is to control the dehydrocoupling reaction that leads to the formation of boron-nitrogen cross-links between the polyborazylene chains. One strategy by which this could be accomplished is to reduce the number of reactive B-H or N-H sites by functionalizing the polymer with suitable substituents. In this way it should be possible to improve both the thermal stability and lower the glass transition temperature (Tg) of the polymer. Indeed, we have previously prepared polyorganoborazylenes,6 including both B-ethyl- and B-propylsubstituted polyborazylenes, and found that these polymers have increased thermal stabilities compared to the parent polyborazylene. However, it was not possible to lower the Tg of the polyorganoborazylenes below the onset of cross-linking, and thus, these polymers are not meltable. The polyorganoborazylenes also have the added disadvantage that because of the inefficient elimination of the alkyl groups during the ceramic conversion process, carbon is retained in the BN ceramics derived from these polymers. Given these problems, investigations of alternative approaches for the formation of functionalized polyborazylenes with enhanced properties were initiated.7 Herein, we now report the high-yield syntheses and properties of new types of amine-substituted polyborazylene polymers that have proven to be excellent BN precursors for applications that require stable melts.
Chem. Mater., Vol. 10, No. 1, 1998 413 Materials. Borazine was prepared by the reaction of NaBH4 and (NH4)2SO4,9 and purified through a -63, -78, and -196 °C trap series, with only the material in the -78 °C trap used in the experiments. Dipentylamine (DPA), diethylamine (DEA), and hexamethyldisilazane (HMD) were distilled from CaH2 before use. Glyme was freshly distilled from sodium benzophenone ketyl. Diethylborazine was prepared as recently reported.6 Physical Measurements and Instrumentation. Diffusereflectance IR spectra (DRIFT) were recorded on a PerkinElmer 1760 Fourier transform spectrophotometer equipped with a diffuse-reflectance attachment. 11B NMR spectra were obtained at 64.2 MHz, and 1H NMR were obtained at 200.1 MHz on a Bruker AF-200 spectrometer equipped with the appropriate decoupling accessories. All 11B chemical shifts are referenced to BF3‚O(C2H5)2 (0.0 ppm) with a negative sign indicating an upfield shift. All 1H chemical shifts were measured relative to internal residual protons from the lock solvents and are referenced to Me4Si (0.0 ppm). Highresolution mass spectra were obtained on a VG Micromass 7070H mass spectrometer. GC/MS analyses were performed on a Hewlett-Packard 5890 gas chromatograph equipped with a 5970 Series mass-selective detector. The composition of each component was established both by its observed m/e cutoff and by comparison of the calculated and observed isotope patterns in the parent and/or fragment envelopes. Thermogravimetric analyses were obtained on a PerkinElmer TGA 7 thermogravimetric analyzer using an argon gas or breathing air purge. TGA/MS studies were performed on a Seiko Instruments Model 320 TG/DTA with a Fison Thermalab mass spectral analyzer. X-ray powder diffraction spectra were obtained on a Rigaku Geigerflex X-ray powder diffractometer. X-ray diffraction studies on single BN ceramic fibers were carried out on a MSC/R-AXIS IIc area detector employing graphite-monochromated Mo KR radiation. Rutherford backscattering spectroscopy (RBS) measurements were obtained on a NEC Model 5SDH Pelletron tandem accelerator operating at 5.1 MeV. Mechanical studies of BN fibers were performed at Dow Corning Corp. on an Instrom 1122 Mechanical Analyzer. Scanning electron microscopy was performed on a JEOL 6300 electron microscope. Elemental analyses were performed at the Nesmeyanov Institute of Organoelement Compounds (INEOS), Moscow, Russia. Analyses were performed in duplicate and percentages are reported as the average of the two assays. Densities were measured by floatation in halogenated hydrocarbons. Molecular Weight Analysis. Molecular weight distribution averages and weight-average intrinsic viscosity were determined by size-exclusion chromatography employing inline viscometric detection (SEC/VISC). Chromatograms were obtained with a Model 150-CV SEC/VISC system (Waters Chromatography Inc.) operated at 35 °C. The resulting Mark-Houwink relationship for polystyrene in THF at 35 °C was
[η] ) 1.2175 × 10-4M0.712
(2)
All synthetic manipulations were carried out using standard high-vacuum or inert-atmosphere techniques as described by Shriver.8
where M is the peak molecular weight of the calibrant. A third-order polynomial was least-squares fitted to the log hydrodynamic volume versus retention volume data. From this universal calibration curve, hydrodynamic volume at each chromatographic data point, φi, was determined. Intrinsic viscosity at the corresponding data point, [ηi], was calculated from the combined outputs of the VISC and DRI detectors following previously described10 methodology. Prior to these calculations, DRI and VISC chromatograms were corrected to account for the retention volume delay between the two detectors and band broadening due to axial dispersion. The delay volume and band broadening parameters were deter-
(6) Fazen, P. J.; Sneddon, L. G. Organometallics 1994, 13, 28672877. (7) Wideman, T.; Sneddon, L. G. Chem. Mater. 1996, 8, 3-5. (8) Shriver, D. F.; Drezdzon, M. A. The Manipulation of AirSensitive Compounds, 2nd ed.; Wiley: New York, 1986.
(9) (a) Wideman, T.; Sneddon, L. G. Inorg. Chem. 1995, 34, 10023. (b) Wideman, T.; Fazen, P. J.; Lynch, A. T.; Su, K.; Remsen, E. E.; Sneddon, L. G. Inorg. Synth., in press. (10) Kuo, C.-Y.; Provder, T.; Koehler, M. E. J. Liq. Chromatogr. 1990, 13, 3177-3199.
Experimental Section
414 Chem. Mater., Vol. 10, No. 1, 1998
Wideman et al.
Table 1. Synthesis and Polymer Composition polymer
PBZ/amine/glyme (g/mL/mL)
time (h) at 75 °C
yield (g, %)
compositiona
PB PB-∆ DEA-PB-1 DPA-PB-1 DPA-PB-2 DPA-PB-3 DPA-PB-4 DPA-PB-5 HMD-PB-1
3.98/0/32 4.98/40/0 5.02/20/20 4.97/20/20 5.01/20/20 5.03/20/20 2.55/25/25 4.98/10/30
120 60 1 15 60 120 192 120
3.96, 100 7.05, 98 5.88, 85 6.82, 98 7.16, 94 7.94, 98 4.58, 97 6.23, 100
(B3.0N3.2H∼3.5-4) (B3.0N3.3H∼3.5-4) (B3.0N3.2H∼3.5-4)[NEt2]0.50 (B3.0N3.6H∼3.5-4)[NPn2]0.16 (B3.0N3.0H∼3.5-4)[NPn2]0.23 (B3.0N3.3H∼3.5-4)[NPn2]0.26 (B3.0N3.0H∼3.5-4)[NPn2]0.33 (B3.0N3.1H∼3.5-4)[NPn2]0.47 (B3.0N3.0H∼3.5-4)[N(SiMe3)0.88]0.29
a
See Experimental Section for elemental analyses.
mined by the program TRISEC (Viscotek Inc.) using chromatograms for the polystyrene calibrants. Concentrations at each chromatographic data point, ci, were obtained from the DRI peak height, hi, and the mass of polymer injected, m:
∑h
ci ) m(hi)/(vi)
i
(3)
where, vi is the incremental volume corresponding to data point i. Molecular weight at each chromatographic point, Mi, was calculated from φi and [ηi]:
Mi ) φi/[ηi]
(4)
Molecular weight distribution averages, Mn, Mw, Mz, and the weight-average intrinsic viscosity, [η]w, were calculated by using the appropriate summations of Mi, [η]i, and ci across a chromatogram. Reported molecular weight and intrinsic viscosity averages are mean values of two determinations. Data acquisition and reduction were provided by either a micro pdp 11/23+ computer (Digital Equipment Co.) or a 486 desktop computer. Data acquisition performed with the 11/ 23+ computer employed a modified version of program MOLWT3 (Thermo Separations Inc.). Data acquisition performed by the 486 desktop computer employed the program TRISEC (Viscotek Corp.). Universal calibration and molecular weight calculations made with the MOLWT3-acquired data employed customized software. The same calculations performed with TRISEC-acquired data employed calculation modules in the TRISEC software package. Compositional Heterogeneity Analysis. The variation of composition across a polymer molecular weight distribution was determined by infrared spectroscopic analysis of collected SEC fractions. The approach employed generally followed methods previously described11,12 for the characterization of copolymer compositional heterogeneity. The use of this technique is, to the best of our knowledge, the first reported application to the analysis of compositional heterogeneity in inorganic polymers. Specific conditions used in the present study were as follows: a Model LC-Transform (Lab Connections Inc.) was placed in-line with the SEC/VISC system and used to deposit eluting polymer fractions onto a Ge disk rotating at a constant angular speed of 10°/min. The deposition was performed by splitting off 15% of the eluting solution stream and devolatilizing the stream with a 20 psi flow of He heated to 51 °C. The fractions were deposited around the circumference of the Ge disk as a continuous polymer film. The chromatographic conditions used with the LC-transform were identical with those used for SEC/VISC molecular weight analysis described above. The offset between VISC, IR, and DRI chromatograms was determined with a monodisperse polystyrene standard by overlaying the polymer’s Gramm-Schmidt reconstructed IR chromatogram and the corresponding VISC and DRI chromatograms. (11) Wheeler, L. M.; Willis, J. N. Appl. Spectrosc. 1993, 47, 11281130. (12) Willis, J. N.; Wheeler, L. M. Adv. Chem. Ser. 1995, 247, 253263.
Composition analysis was provided by a Model 800 FT-IR spectrophotometer (Nicolet Instrument Co.) which was used to measure the relative change in functional group content along the polymer film. Spectra were collected with a resolution of 4 cm-1. The sample disk was rotated clockwise at 10°/ min as dictated by the deposition conditions described above. Preparation of Polyborazylene (PB). Polyborazylene was prepared by the literature method.5,9b In a typical preparation of the polymer used in this study, borazine (56.57 g, 0.703 mol) was vacuum transferred into a 500 mL stainless steel reaction vessel, equipped with a stirbar and high-pressure stainless steel valve. The vessel was then submerged in a 70 °C oil bath for 48 h. The vessel was then cooled to -196 °C and attached to a vacuum line, and the evolved hydrogen removed. Any unreacted borazine was removed by vacuum evaporation at room temperature for 36 h. The polyborazylene was then removed from the reactor under an inert atmosphere as a white solid (49.06 g). Anal. Found for PB: B, 40.24%; N, 55.20%; H, 4.44%. The polymer was soluble in THF and glyme and exhibited spectroscopic properties and molecular weights similar to those previously reported.5,9 Reaction of Polyborazylene (PB) with Diethylamine (DEA). In a typical reaction, a 4.98 g sample of PB was charged under an argon atmosphere into a 100 mL one-piece glass vessel equipped with a high-vacuum stopcock and stirbar. The addition of 40 mL of DEA via syringe completely dissolved the polymer. The vessel was then evacuated and sealed. The mixture was heated in an oil bath as summarized in Table 1. The vessel was then cooled to -196 °C, attached to a vacuum line, and the evolved hydrogen removed. Any volatile materials were then vacuum distilled from the flask at 25 °C for 18 h. Vacuum-line fractionation of these volatile products showed only unreacted DEA (-78 °C trap). The DEA-PB-1 polymer was removed from the vessel under an inert atmosphere and isolated as a moisture sensitive, white solid (7.05 g, 98% yield). Anal. Found for DEA-PB-1: B, 26.65%; N, 42.72%; C, 20.07%; H, 7.33%. NMR data for DEA-PB-1: 1H NMR (δ, 200 MHz, THF-d8) 6.0-3.6 (br, B-H and N-H), 3.0-1.9 (CH2), 1.4-0.6 (CH3); 11B NMR (δ, 64.2 MHz, THF-d8) ∼32, ∼28 ppm (br). In some samples, a quartet resonance at -13.5 ppm was also observed suggesting the formation of BH3‚NHEt2. IR data for DEA-PB-1: 3440 (s) (N-H), 3240 (w), 2970 (s) (C-H), 2940 (s) (C-H), 2920 (s) (C-H), 2500 (s) (B-H), 2370 (m), 2330 (sh, m), 1640 (sh, m), 1450 (vs, br) (B-N), 1100 (m), 1060 (sh, m), 1010 (sh, w), 910 (s) (B-N), 760 (sh, m), 680 (s) cm-1. The polymer was soluble in THF but only sparingly soluble in benzene. Upon heating, the polymer initially softens, but within minutes it begins to decompose with loss of hydrogen gas to produce a gel. Additional polymer properties are listed in Tables 1 and 2. Reaction of Polyborazylene (PB) with Dipentylamine (DPA). A series of reactions were carried out in which 2.55.0 g samples of PB were charged under an argon atmosphere into a 100 mL one-piece glass vessel with a high vacuum stopcock and stirbar. The polymers were dissolved in a 1:1 v:v mixture (8 mL/g of PB) of glyme and DPA. The vessels were then evacuated and sealed. The mixtures were heated in an oil bath as summarized in Table 1. The vessels were then cooled to -196 °C, and attached to a vacuum line, and the evolved hydrogen was removed. Any volatile products were then vacuum distilled from the flasks at 50 °C for 22 h
Amine-Modified Polyborazylenes
Chem. Mater., Vol. 10, No. 1, 1998 415 Table 2. Polymer Molecular Weights
a
polymer
Mz (g/mol)
Mw (g/mol)
Mn (g/mol)
Mw/Mn
[η]w (dL/g)
PB PB-∆ DEA-PB-1 DPA-PB-1 DPA-PB-2 DPA-PB-3 DPA-PB-4 HMD-PB-1
18 750 a 4 850 18 800 32 750 14 300 4 310 109 350
2700 a 920 2355 2600 1700 930 6670
725 a 430 833 836 756 444 1509
3.72 a 2.14 2.83 3.11 2.25 2.09 4.42
0.031 a 0.004 0.016 0.009 0.008 0.008 0.024
Gel observed.
followed by 70 °C for 2 h. NMR and GC/MS analyses of the volatile products showed only glyme and unreacted DPA. The DPA-PB polymers were removed from the vessels under an inert atmosphere and isolated as moisture-sensitive clear solids. Anal. Found for DPA-PB-1: B, 28.68%; N, 46.56%; C, 17.27%; H, 7.36%. DPA-PB-2: B, 27.98%; N, 39.33%; C, 24.19%; H, 7.52%. DPA-PB-3: B, 26.40%; N, 40.41%; C, 25.61%; H, 7.84%. DPA-PB-4: B, 24.20%; N, 35.13%; C, 29.67%; H, 7.97%. DPA-PB-5: B, 20.73%; N, 32.20%; C, 36.13%; H, 9.22%. NMR data for DPA-PB-5: 1H NMR (δ, 200 MHz, THF-d8) 6.2-3.6 (br, B-H and N-H), 2.90 (br, CH2), 1.44 (br, CH2), 1.31 (br, CH2), 0.91 (br, CH3); 11B NMR (δ, 64.2 MHz, THF-d8) ∼29 ppm (br). IR data for DPA-PB-5: 3460 (s) (N-H), 2975 (s) (C-H), 2935 (s) (C-H), 2870 (s) (C-H), 2520 (s) (B-H), 2375 (m), 2330 (sh, m), 2280 (sh, w), 1430 (vs, br) (B-N), 1375 (s), 1264 (m), 1200 (m), 1140 (m), 915 (s), 770 (sh, m), 690 (s) cm-1. Each of the polymers had similar NMR and IR spectra with the intensities of the peaks varying according to the amine content. Increasing the DPA content also led to improved solubility and meltability. For example, while PB is insoluble in benzene, DPA-PB-4 was partially soluble and DPA-PB-5 was very soluble. DPA-PB-4 and DPAPB-5 could be melted without visible decomposition. Additional properties of the modified polymers are listed in Tables 1 and 2. Reaction of Polyborazylene (PB) with Hexamethyldisilazane (HMD). In a typical reaction, a 4.98 g sample of PB was charged under an argon atmosphere into a 100 mL one-piece glass vessel with a high-vacuum stopcock and stirbar. The polymer was dissolved in 40 mL of a 3:1 v:v mixture of glyme and HMD. The vessel was then evacuated and sealed. The mixture was heated in an oil bath as summarized in Table 1. The vessel was then cooled to -196 °C and attached to a vacuum line, and the evolved hydrogen was removed. Any volatile materials were then vacuum distilled from the flask at 50 °C for 22 h followed by 70 °C for 2 h. Vacuum line fractionation and NMR and GC/MS analysis of these volatile materials showed glyme and unreacted HMD (-78 °C trap) and trimethylsilane (1.22 g, 16.5 mmol; -196 °C trap). The HMD-PB polymer was removed from the vessel under an inert atmosphere and isolated as a moisture sensitive white solid (6.23 g). Anal. Found for HMD-PB-1: B, 31.30%; N, 45.04%; C, 10.18%; H, 6.03%. NMR data for HMD-PB-1: 1H NMR (δ, 200 MHz, THF-d ) 6.0-3.5 (br, B-H and N-H), 8 0.09 (br, Si-CH3); 11B NMR (δ, 64.2 MHz, THF-d8) 30.9 ppm (br). IR data for HMD-PB-1: 3440 (s) (N-H), 2970 (s) (C-H), 2930 (m) (C-H), 2510 (s) (B-H), 2400 (sh, m), 1420 (vs, br) (B-N), 1280 (s), 1050 (w), 1010 (sh, w), 910 (s) (B-N), 860 (s), 780 (s, sh), 680 (s) cm-1. The polymer was soluble in THF but only sparingly soluble in benzene. Upon heating HMDPB-1 did not soften before decomposition. Polymer molecular weights are given in Table 2. Control Experiments. A control experiment to determine any thermally induced molecular weight changes in the starting PB was carried out by charging a 3.98 g sample of PB under an argon atmosphere into a 100 mL one-piece glass vessel with a high-vacuum stopcock and stirbar. The flask was evacuated, and 32 mL of glyme was vacuum transferred into the flask. The vessel was then sealed and heated in an oil bath as described in Table 1. The vessel was then cooled to -196 °C and attached to a vacuum line, and the evolved
hydrogen was removed. Any volatile materials were vacuum distilled from the flask at 25 °C for 6 h, followed by 50 °C for 2 h. The white solid material (3.96 g) was then removed from the vessel under an inert atmosphere. Anal. Found for PB-∆: B, 35.54%; N, 50.84%; C, 2.83%, H, 4.64%. The material showed signs of gelation and could not be fully dissolved in THF. Therefore, molecular weights could not be determined. Reaction of Diethylborazine (DEB) with Dipentylamine (DPA). DEB (50 mg, 0.04 mmol) and DPA (500 mg, 3.2 mmol) were charged under an argon atmosphere into a NMR tube equipped with a vacuum stopcock (Chemglass part UP 9703-044). The tube was placed in a 75 °C oil bath for 190 h. Analysis of the reaction mixture by 11B NMR and GC/ MS showed mostly unreacted DEB and DPA, as well as 2,4diethyl-6-(dipentylamino)borazine. DEB-DPA: 11B NMR (64.2 MHz), 35.8 (s), 25.9 (s); cutoff m/e: 292 (2%), 235 (100%, P-C4H9). Smaller amounts (
