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Macromolecules 2007, 40, 1018-1027
Design of a Series of Preceramic B-Tri(methylamino)borazine-Based Polymers as Fiber Precursors: Architecture, Thermal Behavior, and Melt-Spinnability† Sylvain Duperrier,‡ Christel Gervais,§ Samuel Bernard,*,‡ David Cornu,‡ Florence Babonneau,§ Corneliu Balan,⊥ and Philippe Miele*,‡ Laboratoire des Multimate´ riaux et Interfaces, UMR CNRS 5615, UniVersite´ Claude Bernard Lyon 1, 43 Bd du 11 noVembre 1918, Baˆ timent Berthollet, 69622 Villeurbanne Cedex, France; Laboratoire de Chimie de la Matie` re Condense´ e, UMR CNRS 7574, UniVersite´ Paris 6, 4 Place Jussieu, tour 54, e´ tage 5, 75252 Paris Cedex, France; and REOROM Laboratory, Hydraulics Department, “Politehnica” UniVersity of Bucharest, Splaiul Independentei 313, 060032 Bucharest, Romania ReceiVed October 5, 2006; ReVised Manuscript ReceiVed NoVember 30, 2006
ABSTRACT: A series of poly[B-(methylamino)borazine] were synthesized by thermolysis of a monomeric B-tri(methylamino)borazine at various temperatures between 150 and 200 °C and then characterized for suitability as a fiber precursor. Polymerization mechanisms and polymer architectures are discussed. It was shown that poly[B-(methylamino)borazine] represents a network combining a majority of -N(CH3)- bridges with a small proportion of B-N bonds, both connecting borazine rings, and -N(H)CH3 groups. Both the ratio between flexible -N(CH3)- bridges and rigid B-N bonds and the relative amounts of plasticizing -N(H)CH3 groups cause different responses to thermal properties and spinnability (glass transition, spinning temperatures, melt throughput, and fiber drawing). Based on fiber shape visualization using CCD camera during extrusion, appreciable meltspinnable compounds are prepared between 160 and 185 °C. Such polymers display a chemical formula of [B3.0N4.4(0.1C2.0(0.1H9.3(0.2]n (n ∼ 7.5), a glass transition between 64 and 83 °C, tailored flexibility, and sufficient plasticity to successfully produce fine-diameter green fibers.
1. Introduction With the need for the development of non-oxide ceramics with high purity for thermostructural applications, the pyrolysis of inorganic precursors creates substantial interest, both scientifically and for practical purposes.1-4 Such compounds provide a means for controlling and adjusting composition and nano/ microstructure shaping of ceramic materials which allows the desired materials to be designed. This elegant chemical approach, the so-called polymer-derived ceramics (PDCs) route, has been introduced in the early 1960s by Poppers and Chantrell4 and is mainly applied to the preparation of non-oxide ceramic fibers. Then, precursor-derived ceramic fibers were historically proposed by Yajima5 and Verbeck6 in the 1970s to Si/C/N(O) fibrous systems, and different compositions have been provided since then, most of them including Si-based ceramic fibers such as Si/C/N and Si/B/C/N systems.7-9 In the case of fibers, the method consists of four major steps: (i) synthesis of a molecular precursor containing the constitutive elements of the desired ceramics in a homogeneous distribution, (ii) transformation of the precursor into a preceramic network with defined rheological behavior to provide proper processing capabilities, (iii) spinning of this preceramic polymer into a green fiber, and (iv) subsequent conversion of as-spun fibers to the desired ceramic fibers through appropriate thermal and/or chemical protocols under selected oxygen-free atmospheres. Twenty years ago, * Corresponding authors. E-mail:
[email protected],
[email protected]; Tel: +33 472 433 612; Fax: +33 472 440 618. † We wish to dedicate this paper to the memory of Dr. Jean-Marie Le ´ toffe´, who managed the center of thermal analyses in the Laboratoire des Multimate´riaux et Interfaces. ‡ Universite ´ Claude Bernard Lyon 1. § Universite ´ Paris 6. ⊥ “Politehnica” University of Bucharest.
Wynne and Rice10 rationalized this route, as they set a series of general empirical rules which are still valid for the design of suitable spinnable polymers. Among non-oxide advanced ceramics, hexagonal boron nitride (h-BN) is an advanced ceramic that could offer great potentialities as fibrous reinforcing agent in specific applications.11 h-BN12-15 represents a crystalline ceramic with a layered anisotropic structure, similarly to that of carbon graphite. It offers some attractive properties such as high stiffness and toughness along the basal layers, a nonwettability against many metallic and silicate melts, a good oxidative resistance up to T ∼ 1000 °C, and a low coefficient of thermal expansion in the direction of basal layers. In addition, this poorly dense ceramic (d ) 2.27) exhibits potentialities in infrared and microwavetransparent structures and excellent electrical insulation properties. The main idea behind the preparation of BN fibers is to combine in a same fiber the high strength of polyacrylonitrilederived carbon fibers with the specific properties of h-BN. Although controlling the various demands with respect to processing of ceramic fibers, i.e., fusibility and/or solubility, thermal stability at low temperature for melt-spinning, and high ceramic yield, and combining them in only one molecule remains an ambitious objective, preceramic polymers which use the borazine ring as a basal structural unit can be well suited for filling the requirements as BN fiber precursor.11-22 As an illustration, several attempts were successful in our lab for producing BN fibers with high mechanical performances from B-tri(methylamino)borazine-based polymers, namely poly[B(methylamino)borazine].20 However, the preparation of poly[B-(methylamino)borazine]derived BN fibers remains a complex and difficult task in terms of polymer synthesis and spinning, resulting in a lack of reproducibility in mechanical performance. Such variations in
10.1021/ma0623035 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
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the quality of the final fibers are the result of the nature and structure of these polymers and the thermal and rheological phenomena that occur during their spinning. It is therefore our intent to investigate the synthesis and spinning of a set of representative poly[B-(methylamino)borazine] and understand the role of their architecture on their thermal and rheological behavior upon melt-spinning through two papers . The first aim of the present paper is to ascertain the chemical steps and establish the structural changes which occur during the polymer preparation using a combination of 15N solid-state NMR, GC/ MS, elemental analyses, and density measurements. Second, we investigate the thermal properties and the spinning behavior of polymers using thermal analysis and CCD camera visualization of fiber geometry during extrusion and fiber drawing. We therefore discuss the effects of architecture on the thermal behavior and spinnability of poly[B-(methylamino)borazine], and we provide synthesis conditions that allow us to develop meltspinnable polymers. In the following paper, we will describe how polymer architecture affects shear rheology behavior of polymer melts and provides reliable melt-spinnability assumptions that help us to predict the melt-spinnability of poly[B(methylamino)borazine]. It should be mentioned that the literature dedicated to melt-spinning of preceramic polymers is rather scarce,7-9,23-27 and to our knowledge, there are no detailed reports focused on the effects of polymer architecture on thermal, spinning, and rheological behavior of preceramic polymers as fiber precursors. 2. Experimental Section 2.1. General Comment. Syntheses were carried out in an argon atmosphere, using argon/vacuum lines and Schlenk-type flasks. Argon (>99.995%) was purified by passing through successive columns of phosphorus pentoxide, siccapent, and BTS catalysts. Purified B-tri(chloro)borazine was purchased from Katchem Ltd. (Praha, Czech Republic). This molecule was analyzed by 1H and 11B NMR and IR spectroscopies. 11B NMR (96.29 MHz, C D , 6 6 ppm): 29.7 (br). 1H NMR (300 MHz, CDCl3, ppm): 5.29 br (N(H) borazine). IR data (KBr pellets, cm-1): 3450 (m); 1438 (s); 1031 (m), 743 (w), 704 (w). Methylamine (99+%) obtained from Sigma Aldrich was purified by passing through a column of potassium hydroxide. Toluene was dried and purified using standard glass manipulation and was freshly distilled under argon from sodium/benzophenone prior to use. Preparation of samples for characterization was performed inside an argon-filled glovebox (Jacomex BS521; Dagneux, France). 2.2. Synthesis of Molecular and Polymer Precursors. B-tri(methylamino)borazine was prepared in a three-necked 2 L Schlenk flask equipped with a methanol reflux condenser by dropping a solution of 77 g of B-tri(chloro)borazine (419 mmol) in toluene on a solution of 105 g (3.387 mol) of methylamine in toluene, at -50 °C with vigorous magnetic stirring, whereby methylamine hydrochloride precipitation was observed immediately. After the addition of B-tri(chloro)borazine was complete, the reaction mixture was allowed to warm to room temperature (RT). The precursor solution was then separated from the precipitated methylamine hydrochloride by filtration through a pad of Celite. The precipitate was thoroughly extracted three times with 50 mL of toluene and then disposed. The filtrate and the extract were combined, and toluene was partially removed at RT in high vacuum (10-1 mbar) from the remaining solution until the concentration of the B-tri(methylamino)borazine (1) in toluene reached 65 wt %. It should be mentioned that the B-tri(methylamino)borazine was partially dried to facilitate its introduction in the polymerization reactor for the subsequent thermolysis step. Synthesis of the poly[B-(methylamino)borazines] 2-6 proceeded as follows. A 250 mL three-necked polymerization reactor equipped with a mechanical stirrer was charged at RT with 70 g (419 mmol) of the
B-Tri(methylamino)borazine-Based Polymers
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solution of 1. The mixture was heated in vacuum to 75 °C with vigorous stirring to remove residual toluene, giving 45.5 g (267.2 mmol) of product. The reaction was continued in a flowing argon (3 L/h) at PAr ) 1 atm step by step to the desired final synthesis temperature (Tthermolysis) ranging from 150 °C (polymer 2) to 200 °C (polymer 6). After cooling to RT, poly[B-(methylamino)borazines] 2-6 were recovered as air- and moisture-sensitive solids. Samples were stored inside an argon-filled glovebox. Polymer 2: Tthermolysis ) 150 °C. IR data (KBr pellets, cm-1): 3434 (m); 2958 (w) 2928 (w), 2898 (w), 2820 (m); 1597 (s); 1515 (s); 1460 (s); 1411 (s); 1178 (s); 1095 (m), 707 (w). 11B NMR (96.29 MHz, C6D6, ppm): 25.7 (br). 1H NMR (300 MHz, CD2Cl2, ppm): 1.86 br (-N(H)CH3); 2.47 vbr (-N(H)CH3); 2.56 vbr (bridging -N(CH3)-); 2.70-4.10 br (N(H) borazine). 13C NMR (75 MHz, CD2Cl2, ppm): 27.6, 27.9 (-N(H)CH3); 31.2 (bridging -N(CH3)-). TGA (ammonia, 1000 °C (1 °C/min); 47.3% weight loss): 0-45 °C, ∆m/m0 ) 0%; 45-400 °C, ∆m/m0 ) 26.8%; 4001000 °C, ∆m/m0 ) 20.5%. Polymer 3: Tthermolysis ) 160 °C. IR data (KBr pellets, cm-1): 3430 (m); 2936 (w), 2894 (w), 2816 (m); 1597 (s); 1519 (s); 1460 (s); 1413 (s); 1182 (s); 1091 (m), 707 (w). 11B NMR (96.29 MHz, C6D6, ppm): 25.77 (br). 1H NMR (300 MHz, CD2Cl2, ppm): 1.86 br (-N(H)CH3); 2.47 vbr (-N(H)CH3); 2.55 vbr (-N(CH3)-); 2.64-3.37 br (N(H) borazine). 13C NMR (75 MHz, CD2Cl2, ppm): 27.6, 27.9 (-N(H)CH3); 31.2 (bridging -N(CH3)-). TGA (ammonia, 1000 °C (1 °C/min); 46.3% weight loss): 0-45 °C, ∆m/m0 ) 0%; 45-400 °C, ∆m/m0 ) 25.4%; 400-1000 °C, ∆m/ m0 ) 20.9%. Polymer 4: Tthermolysis ) 175 °C. IR data (KBr pellets, cm-1): 3434 (m); 2958 (w) 2928 (w), 2898 (w), 2820 (m); 1597 (s); 1515 (s); 1460 (s); 1411 (s); 182 (s); 1088 (m), 707 (w). 11B NMR (96.29 MHz, C6D6, ppm): 25.77 (br). 1H NMR (300 MHz, CD2Cl2, ppm): 1.86 br (-N(H)CH3); 2.47 vbr (-N(H)CH3); 2.55 vbr (-N(CH3)-); 2.64-3.37 br (N(H) borazine). 13C NMR (75 MHz, CD2Cl2, ppm) 27.6, 27.9 (-N(H)CH3); 31.2 (bridging -N(CH3)-). TGA (ammonia, 1000 °C (1 °C/min); 45.5% weight loss): 0-45 °C, ∆m/m0 ) 0%; 45-400 °C, ∆m/m0 ) 24.0%; 400-1000 °C, ∆m/m0 ) 21.5%. Polymer 5: Tthermolysis ) 185 °C. IR data (KBr pellets, cm-1): 3435 (m); 2958 (w), 2893 (w), 2817 (m); 1597 (s); 1515 (s); 1460 (s); 1418 (s); 1187 (s); 1087 (m), 710 (w). 11B NMR (96.29 MHz, C6D6, ppm): 25.77 (br). 1H NMR (300 MHz, CD2Cl2, ppm): 1.86 br (-N(H)CH3); 2.47 vbr (-N(H)CH3); 2.55 vbr (-N(CH3)-); 2.64-3.37 br (N(H) borazine). 13C NMR (75 MHz, CD2Cl2, ppm): 27.6, 27.9 (-N(H)CH3); 31.2 (bridging -N(CH3)-). TGA (ammonia, 1000 °C (1 °C/min); 42.5% weight loss): 0-45 °C, ∆m/m0 ) 0%; 45-400 °C, ∆m/m0 ) 21.6%; 400-1000 °C, ∆m/ m0 ) 20.9%. Polymer 6: Tthermolysis ) 200 °C. IR data (KBr pellets, cm-1): 3434 (m); 2936 (w), 2893 (w), 2816 (m); 1601 (s); 1528 (s); 1460 (s); 1433 (s); 1187 (s); 1087 (m), 711 (w). 11B NMR (96.29 MHz, C6D6, ppm): 25.77 (br). 1H NMR (300 MHz, CD2Cl2, ppm): 1.86 br (-N(H)CH3); 2.47 vbr (-N(H)CH3); 2.55 vbr (-N(CH3)-); 2.64-3.37 br (N(H) borazine). 13C NMR (75 MHz, CD2Cl2, ppm): 27.6, 27.9 (-N(H)CH3); 31.2 (bridging -N(CH3)-). TGA (ammonia, 1000 °C (1 °C/min); 41.6% weight loss): 0-45 °C, ∆m/m0 ) 0%; 45-400 °C, ∆m/m0 ) 19.2%; 400-1000 °C, ∆m/ m0 ) 22.4%. 2.3. Melt-Spinning. Polymers 2-6 were tested with regard to melt-spinning combining a lab-scale piston extrusion system (Mate´riau Inge´nie´rie-St-Christol les Ale`s, France) to melt the polymer and to supply definite throughputs and a wind-up device, i.e., spool, to supply the take-up velocity, both set up in a nitrogenfilled glovebox (Figure 1). The distance from the rotating spool to the spinneret was fixed at 18 cm. Extrusion and drawing units are designed for small-scale spinning and can support flow throughputs from 0.1 to 2 mm/min and take-up velocity from 9 to 330 m/min. The piston chamber can support an internal pressure of 600 N. Four grams of solid polymers was placed at RT into the piston chamber and heated (5 °C/min) without compressive load to a certain temperature, namely
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Figure 2. Schematic representation of the B-tri(methylamino)borazine.
Figure 1. Description of the melt-spinning process.
Tspinning. At Tspinning, the polymer melt flow was forced by pushing it with the piston along the extrusion line through a melt filtering and then a single-capillary spinneret of 200 µm in diameter akin to liquid being squirted out of the capillary at a controlled piston velocity. As-extruded molten filament fell with gravity at an ideal pressure of ∼350 N to be drawn during cooling by the take-up spool and continuously recovered onto the spool. It should be mentioned that the process of producing BN fibers continues through a curing step of green fibers in an ammonia atmosphere (RT to 400 °C) to retain fiber integrity, i.e., avoid melting of the polymer fibers, during the further pyrolysis. The latter is performed in an ammonia atmosphere (400-1000 °C) to remove carbon residues and then in a nitrogen atmosphere (1000-1800 °C) to achieve the complete ceramic transformation.28 2.4. Polymer and Green Fiber Characterization. 2.4.1. 15N Solid-State NMR. 15N solid-state NMR experiments were performed at RT on a Bruker Avance-300 spectrometer, at a frequency of 30.41 MHz using a Bruker magic angle spinning (MAS) probe. Solid samples were spun at 5 kHz, using 7 mm ZrO2 rotors filled up inside an argon gas glovebox. 15N MAS NMR spectra were recorded with a pulse angle of 90° and a recycle delay between pulses of 100 s. 15N chemical shifts were referenced to solid NH4NO3 (10% 15N-enriched sample, δiso(15NO3) ) -4.6 ppm compared to CH3NO2 (δiso(15NO2) ) 0 ppm)), and spectra were simulated with DMFIT.29-31 15N-enriched polymers 2-6 were obtained from a same batch of 15N-enriched 1. The latter was synthesized by aminolysis of a 15N-enriched B-tri(chloro)borazine using homemade methylamine 15N enriched at 10 at. %.28 2.4.2. Elemental Analyses. Elemental analyses were made at the Max-Planck Institute (Stuttgart, Germany) using various apparatus (ELEMENTAR, Vario EL CHN-Determinator; ELTRA, CS 800, C/S Determinator; LECO, TC-436, N/O Determinator, and atom emission spectrometry (ISA JOBIN YVON JY70 Plus). 2.4.3. GC/MS. GC/MS measurements were performed in a continuous thermolysis process using a Hewlett-Packard model Agilent micro-GC M200 equipment coupled with a quadripole mass spectrometer (Agilent 5973 network mass selective detection). Gaseous species were identified on the basis of their MS molecular ion peaks and by comparison of the GC retention times of their corresponding GC signals to those of known gas such as hydrogen, ammonia, methylamine, or argon. A quantitative GC analysis was carried out from the area of the signals. Signal areas were normalized to the same, overall, integrated area values. 2.4.4. Differential Scanning Calorimetry (DSC). DSC measurements were carried out on a TA8000 Mettler-Toledo apparatus, using alumina crucibles in a nitrogen atmosphere and the following temperature program: -30 to 200 °C (10 °C/min). 2.4.5. Picnometry. Density measurements were carried out at RT in a controlled inert environment with a density analyzer (AccuPyc 1330 Helium pycnometer from Micromeritics). 2.4.6. Size Exclusion Chromatography (SEC). Molecular weight distribution was determined by SEC. Analysis was performed using a Shimadzu SPD 6A UV detector and Waters s-Styragel columns in distilled tetrahydrofuran (THF; Sigma-Aldrich) as eluent with
N,N-dimethylacetamide (DMAC). A calibration curve was generated from the chromatograms of the B-tri(methylamino)borazine (M ) 167.6 g/mol). 2.4.7. Fiber Shape Visualization. Green fiber pictures were recorded during melt-spinning operation using a Sony DXC-9100P 3CCD camera equipped with a 40× zoom. The camera resolution was 782 × 582 pixels (800 vertical lines × 575 horizontal lines). Pictures were analyzed using the Analysis software, and precision of the diameter measurements was of 1 pixel (
