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(CH 3 )-bridges with a small proportion of B-N bonds, both connecting borazine rings, and -N(H)CH 3 groups. Both the ratio between flexible -N(CH 3 )-bridges and rigid B-N bonds and the relative amounts of plasticizing -N(H)CH 3 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 [B 3.0 N 4.4(0.1 C 2.0(0.1 H 9.3(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.
The present paper describes an access to polycrystalline boron nitride fibers from poly [B-(methylamino)borazine]. Solid-state NMR and IR spectroscopies, thermo-analytical experiments, SEM and XRD investigations were applied to provide a comprehensive mechanistic study of the fiber transformation and understand the role played by ammonia during the polymer-to-ceramic conversion. It was shown that a typical melt-spinnable poly[B-(methylamino)borazine] (T synthesis = 180 uC) is composed of borazine rings connected via a majority of NCH 3 bridges and a small proportion of NB 3 -containing motifs forming a cross-linked network. In addition, a low proportion of peripheral N(H)CH 3 groups, which are present in the starting molecular precursor, B-tri(methylamino)borazine, is identified. The polymer is capable of melting without decomposition in flowing nitrogen to produce high quality green fibers at moderate temperature. A curing process of green fibers in flowing ammonia at 400 uC through transamination and condensation forming cross-linked NB 3 motifs in the polymer network is seen as the most appropriate way to retain the fiber integrity during the polymer-to-ceramic conversion. The use of ammonia during the subsequent pyrolysis from 400 to 1000 uC allows the basal unit of the ''naphthalenic-type structure'' of boron nitride to be established at 1000 uC through important structural rearrangements and the crystallization tendency to be improved during further heating from 1000 to 1800 uC. Finally, incorporation of nitrogen using ammonia allows the production of polycristalline fibers in which the stoichiometry approaches that of BN.
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