High-performance boron nitride fibers obtained from asymmetric alkylaminoborazine

Abstract

In order to prepare boron nitride fibers displaying improved mechanical properties, different polymers have been prepared from 2-[(CH₃)₂N]-4,6-(CH₃HN)₂B₃N₃H₃ (1) by varying thermolysis conditions. Analytical data showed that the resulting polyborazines 2, 3, 4 and 5 present polymerisation degrees included between 0.7 and 1.1, and confirmed that they are composed of borazinic rings connected mainly through direct B–N inter-ring bonds and to a lesser extent through –N(CH₃)– bridges. Spinning properties of these polymers and ceramisation conditions of the derived crude fibers have been studied. While the spinnability of the higher cross-linked polymer 5 was poor, all of the others displayed very good processing properties as well as no reactivity in spinning conditions allowing the fabrication of high performance boron nitride fibers. Their tensile strengths are over 0.80 GPa and often reach 1.10 GPa. The three main characteristics of the polymeric precursors of fibers, namely polymerisation degree, glass-transition and spinning temperature are shown to be closely related. We observed that when a polymer displays good processing properties, the higher the polycondensation degree is, and the higher the mechanical properties are. A final pyrolysis temperature of 1600 °C is sufficient to ensure the crystallization of the BN fibers obtained.