Design of a Series of Preceramic B-Tri(methylamino)borazine-Based Polymers as Fiber Precursors:  Architecture, Thermal Behavior, and Melt-Spinnability

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(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.

Thermal behaviour of a series of poly[B-(methylamino)borazine] for the preparation of boron nitride fibers

“…Keeping in mind that molecular weights are unchanged going from one polymer to the other (see experimental section and Ref. [12]), we suggest that thermolysis occurs via intramolecular condensation decreasing the free volume in the polymer network, thereby reducing the softening capability of polymers.…”

“…12 B-tri(methylamino)borazine is thermolyzed at moderate temperatures to yield preceramic networks, namely poly[B-(methylamino)borazine], with different chemical and structural properties. 12 This thermolysis step is carried out in an argon atmosphere (3 L/h) at P Ar = 1 bar up to the desired temperatures (T thermolysis ) ranging from 140 to 200 • C. These polymers are delivered as air-and moisture-sensitive glassy solids at RT. Seven polymers are studied in the present paper.…”

“…All poly[B-(methylamino)borazine] samples display the same IR bands and positions as well as NMR signals in FT-IR and NMR spectra. 12 content is found to be lower than 2 at.% and was therefore omitted.…”

“…However, it is important to mention that 15 N solid-state NMR shows that this major mechanism occurs in complement with minor mechanisms through establishment of inter-ring B-N bonds and the occurrence of ring-opening process. 12 Experimental and simulated 13 C CP MAS NMR spectra (Bruker Avance 300 spectrometer at frequencies of 75.47 MHz) of three polymers prepared at 140, 160 and 180 • C (Fig. 4) confirm the formation of bridging -N(CH 3 )-units (δ iso = 30.5 ppm) and the presence of -N(H)CH 3 groups (δ iso = 28 ppm) in poly[B-(methylamino)borazine].…”

“…3 In our group, we showed that preceramic polymers of the type poly[B-(methylamino)borazine] are well suited to fill the requirements for the preparation of boron nitride fibers. [7][8][9][10][11][12][13][14] However, a lack of reproducibility in the mechanical performances of these fibers can be encountered if the viscoelastic and thermal properties of polymers are not defined.…”