Boron Nitride Fibers Prepared from Symmetric and Asymmetric Alkylaminoborazines

Abstract

The thermolysis of 2,4,6‐[(CH3)2N]3B3N3H3 (1), 2,4‐[(CH3)2N]2‐6‐(CH3HN)B3N3H3 (2), and 2‐[(CH3)2N]‐4,6‐(CH3HN)2B3‐N3H3 (3) led to polyborazines 4, 5, and 6 respectively. The polymers display direct B–N bonds between borazinic B3N3 rings and, in addition, a proportion of –N(CH3)– bridges for 5 and 6, as clearly underlined by 13C NMR spectroscopy. Melt‐spinning of these three polymeric precursors exemplified that their ease of processing increases in the order 4 < 5 < 6. Nevertheless, polyborazine filaments could be prepared from each of them and a subsequent thermal treatment up to 1800 °C resulted in the formation of crystalline hexagonal boron nitride fibers, which were characterized by X‐ray diffraction analysis, Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. Scanning electron microscopy (SEM) images showed that the ceramic fibers are circular and dense without major defects. The mechanical properties for 4‐derived fibers could not be measured because of their brittleness, whereas measurements on 5‐ and 6‐derived fibers gave tensile strength σR = 0.51 GPa, Young’s modulus E = 67 GPa, and σR = 0.69 GPa, E = 170 GPa, respectively. The improvement in mechanical properties for ceramic fibers prepared respectively from 4, 5, and 6 could be explained to a large extent by the improvement of the processing properties of the preceramic polymers. This evolution could be related to the increased ratio of bridging –N(CH3)– groups between the B3N3 rings within the polymers 4, 5, and 6 and therefore to the functionalities of the starting monomers 1, 2, and 3.

Correlation between structural features and mechanical properties of boron nitride fibres derived from alkylaminoborazines

“…The preparation of polyborazines 4, 5 and 6 by thermal polycondensation of alkylaminoborazines 2,4,6-[(CH 3 ) 2 N] 3 B 3 N 3 H 3 (1), 2,4-[(CH 3 ) 2 N] 2 -6-(CH 3 HN)B 3 N 3 H 3 (2) and 2-[(CH 3 ) 2 N]-4,6-(CH 3 HN) 2 B 3 N 3 H 3 (3), respectively, and their subsequent conversion into boron nitride fibres 4.1, 5.1 and 6.1 was described in a first paper. 3 The improvement in the preparation process of a polyborazine 7 derived from 3 allows the fabrication of BN fibres 7.1 with enhanced mechanical properties as it was reported in a second paper in. 4 Table 1 summarized the …”

“…1 Thus, BN based composites are of particular interest for applications at high temperature even though the use of this ceramic could be extended regarding its insulating properties. The development of such composites is restricted by the fact that BN fibres are not commercially available which has motivated several studies devoted to their elaboration particularly in our group [2][3][4] …”

“…Three borazinic precursors 2,4,6-[(CH 3 ) 2 N] 3 B 3 N 3 H 3 (1), 2,4-[(CH 3 ) 2 N] 2 -6-(CH 3 HN)B 3 N 3 H 3 (2) and 2-[(CH 3 ) 2 N]-4,6-(CH 3 HN) 2 B 3 N 3 H 3 (3) as well as derived polyborazines have been studied within the goal of evidence relationships between the mechanical properties of the final ceramic fibres and the nature of the starting molecular precursors as well as the derived polymers. 3,4 Thus, the thermolysis of 1 and 2 led to polyborazines 4 and 5, respectively, whereas two different polymers 6 and 7 have been obtained from 3. The main conclusion of these works is that the melt-spinnability of the asprepared polyalkylaminoborazines is mostly improved by the presence of bridged inter-ring N(CH 3 ) groups to the detriment of direct B N intercyclic bonds.…”

Influence of Monomer Structures on the High Temperature Properties of Poly[(Alkylamino)Borazine)]s Derived BN

“…Alkylaminoborazines (AAB) derived from 2,4,6-trichloroborazine (TCB) have been deemed to be attractive molecular monomers for processable polymers to BN in a complex form [7][8][9]. Nonetheless, as far as we know, it was not easy to synthesize AAB monomers to processable polymers due to the measurement of gaseous alkylamines [10].…”

“…The reactivity of AAB monomers, which were in close connection with the processability of polymeric precursors, was strongly dependent on the substituent alkylamino groups linked with boron atoms [7,10,12]. However, the research that the structural influence of monomer on the poly[(alkylamino)borazine)]s derived BN's high temperature properties was limited, except that Toury et al compared the crystallinity of BN from different molecular precursors [13].…”