Controlling the chemistry, morphology and structure of boron nitride-based ceramic fibers through a...

Controlling the chemistry, morphology and structure of boron nitride-basedceramic fibers through a comprehensive mechanistic study of the reactivityof spinnable polymers with ammonia

Sylvain Duperrier,a Christel Gervais,b Samuel Bernard,*a David Cornu,a Florence Babonneaub andPhilippe Mielea

Received 27th March 2006, Accepted 6th June 2006

First published as an Advance Article on the web 20th June 2006

DOI: 10.1039/b604482d

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]

(Tsynthesis = 180 uC) is composed of borazine rings connected via a majority of NCH3 bridges and a

small proportion of NB3-containing motifs forming a cross-linked network. In addition, a low

proportion of peripheral N(H)CH3 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 NB3 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.

Introduction

The thermal decomposition of non-oxide molecular and

polymeric precursors in oxygen-free atmosphere is an elegant

chemical approach for the preparation of so-called polymer-

derived ceramics (PDCs).1–6 This molecular route allows the

chemistry (elemental composition, compositional homogeneity

and atomic architecture) of starting precursors and the shaping

properties and reactivity of related polymers to be controlled

and designed in order to provide PDCs with desirable com-

position, structure and shape. As the preparation of ceramic

fibers is one of the most challenging prospects, this chemical

process receives increased attention to prepare fibers in various

compositions.7–9

In our group, B-tri(methylamino)borazine-based polymers,

i.e., poly[B-(methylamino)borazines], are appropriate candi-

dates for the successful preparation of boron nitride fibers

via a ceramic transformation process of green fibers derived

therefrom in selected ammonia and nitrogen atmospheres up

to 1800 uC.10–13 In these studies,11,12 it was shown that the

mechanical and structural performances of fibers are

intimately affected by the processing steps including synthesis,

spinning and ceramic transformation. A systematic mechani-

stic study of the chemical and thermal reactivities of polymers

with the selected oxygen-free atmospheres, i.e., ammonia and

nitrogen, during fiber preparation therefore appears vital to

provide desirable properties. According to our knowledge, no

comprehensive study has been undertaken on the reactivity of

borazine-based polymers with ammonia during the polymer-

to-ceramic conversion. Researchers briefly pointed out its

effect on the carbon composition in the final BN ceramics.14–19

Furthermore, only one group described the interaction of

ammonia with melt-spinnable polychloromethylsilanes for the

preparation of nitrogen-containing amorphous SiC fibers.8

This study was mainly devoted to the effect of ammonia on the

curing of green fibers.

Here, we are discussing the feasibility of accessing poly-

crystalline boron nitride fibers using the thermal stability and

melt-spinnability of poly[B-(methylamino)borazine] at moder-

ate temperature in a nitrogen atmosphere and the reactivity

of green fibers derived therefrom with ammonia during the

polymer-to-ceramic conversion. It is the purpose of this work

to investigate the individual chemical processing steps, and

then to identify the related architectural, structural, and

morphological changes during heat-treatment in ammonia

aLaboratoire des Multimateriaux et Interfaces (UMR CNRS 5615),Universite Claude Bernard, Lyon, 43 bd du 11 nov. 1918-69622,Villeurbanne Cedex, France. E-mail: [email protected];Fax: +33 472 440 618; Tel: +33 472433 612bLaboratoire de Chimie et de la Matiere Condensee de Paris (UMRCNRS 7574), Universite Pierre et Marie Curie, Paris, 6 – 4 placeJussieu, Tour 54, E. 5, Couloir 54-55-75252, Paris Cedex 5, France

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3126 | J. Mater. Chem., 2006, 16, 3126–3138 This journal is � The Royal Society of Chemistry 2006

and nitrogen atmospheres in order to provide a detailed

picture of the role of ammonia during fiber preparation. The

identification of the relevant chemical reactions was made

based on the chemistry of the polymer using a combination of

thermo-analytical experiments, solid-state NMR and FT-IR

spectroscopies and chemical analyses. SEM experiments

were used to investigate changes in the fiber integrity during

preparation, while X-ray diffraction allowed us to gain

information on the role of ammonia in the development of

the boron nitride phase.

Experimental

General comments

All synthesis reactions were carried out in a purified argon

atmosphere passing through successive columns of BTS-

catalyst and potassium hydroxide by means of standard

Schlenk manipulations and vacuum/argon-line techniques.

Manipulations of the polymer and fiber samples were made

inside an argon-filled glove box (Jacomex BS521) dried with

potassium hydroxide. Toluene was freshly distilled under

argon from sodium/benzophenone prior to use. Ammonia

and nitrogen were used as received (>99.999%).

Thermogravimetric analysis (TGA) of the polymer-to-ceramic

conversion was recorded on a Setaram TGA 92 16.18.

Experiments were performed in ammonia and nitrogen atmo-

spheres at 0.8 uC min21 from 25 to 1000 uC using silica

crucibles (sample weight of y40 mg) at ambient atmospheric

pressure. Experimental differential thermogravimetric (DTG)

data were generated from the TGA measurements, then

simulated using the multi-peak fitting Lorentz distribution.20

TGA-GC/MS measurements were performed in flowing

nitrogen in a continuous process using Hewlett-Packard model

Agilent micro-GC M200 equipment coupled with a quadripole

mass spectrometer (Agilent 5973 Network Mass Selective

Detection). Gaseous by-products were identified on the basis

of their MS molecular ion peaks and, by comparison of their

GC retention times to those of known gases (hydrogen,

ammonia, methylamine, dimethylamine, nitrogen, argon and

oxygen). A quantitative GC analysis has been carried out from

the area of the signals corresponding to the identified gaseous

species. Chemical analyses were made at the Service Central de

Microanalyse du CNRS (Vernaison, France). The polymer and

related fibers, which were isolated at different intermediate

temperatures during their transformation into ceramics using

either ammonia or nitrogen, were analyzed at room tempera-

ture with respect to their IR and/or 15N and 13C solid-state

NMR spectra. Fourier transform infrared spectra (FT-IR)

were obtained from a Nicolet Magna 550 Fourier transform-

infrared spectrometer in a KBr matrix (dried at 120 uC in air).

All experiments were performed under a slight argon flow.

The 15N-enriched polymer was obtained from 15N-enriched

B-tri(methylamino)borazine. The latter was synthesized by

aminolysis of 15N-enriched B-trichloroborazine with enriched

borazinic nitrogen atoms using home-made 10at%15N

enriched methylamine. The 15N-enriched methylamine was

prepared from a mixture of 10 g (187 mmol) of 100at%15N

enriched ammonium chloride and 90 g (1683 mmol) of non-

enriched ammonium chloride. 15N and 13C solid-state NMR

experiments were performed on a Bruker Avance 300 spectro-

meter, at frequencies of 30.41 and 75.47 MHz respectively.

Solid samples were spun at 5 kHz, using 7 mm ZrO2 rotors

filled up inside an argon-filled glove box. All 15N and 13C CP

MAS experiments were performed under the same Hartmann–

Hahn match condition: both RF channel levels, v1S/2p

and v1I/2p were set at about 42 kHz. Inversion recovery

cross-polarization (IRCP) measurements were performed

as described in refs 21,22. 15N MAS NMR spectra were

recorded with a pulse angle of 90u and a recycle delay between

pulses of 100 s. 15N and 13C chemical shifts were referenced

to solid NH4NO3 (diso (15NO3) = 24.6 ppm compared to

CH3NO2 (diso (15NO2) = 0 ppm)) and TMS via solid

adamantane respectively. Spectra were simulated with the

DMFIT program.23

Powder X-ray diffraction (XRD) studies of as-prepared

ceramic fibers were achieved using a Philips PW 3040/60

X’Pert PRO diffractometer operating at 30 mA and 40 kV.

Fibers were crushed prior characterization. Scanning electron

microscopy (SEM, Hitachi S800 operating at 150 kV) was used

to observe at room temperature the morphology and cross-

sectional microtexture of pyrolyzed fibers. Fibers were

mounted on stainless pads and, due to the insulating properties

of samples, the samples were sputtered with y10 A of a

Pd–Au mixture to prevent charging during observation.

Polymer synthesis

BN fibers were obtained by melt-spinning of a specially

designed poly[B-(methylamino)borazine] followed by curing

and pyrolysis of as-spun fibers.

Poly[B-(methylamino)borazine] was prepared by thermo-

lysis of B-tri(methylamino)borazine at Tsynthesis = 180 uC.

Synthesis of the polymer proceeded as follows.

A 250 mL three-necked reactor equipped with a mechanical

stirrer was charged with 95 g (565.5 mmol) of B-tri(methyl-

amino)borazine dissolved in 35wt% of toluene. The mixture

was heated in vacuum up to 75 uC under vigorous stirring to

remove the residual toluene and deliver 61.7 g (368.1 mmol) of

a dried B-tri(methylamino)borazine. The synthesis reaction

was continued step by step in flowing argon (3 L h21) at PAr =

1 atm up to 180 uC. 54 g of poly[B-(methylamino)borazine]

was recovered as an air- and moisture-sensitive glassy solid at

room temperature. The sample was stored inside an argon-

filled glove box.

Anal. found: C, 19.1%; H, 7.3%; N, 49.0%; B, 24.6%

[B3.0N4.6C2.1H9.5]n, calcd (according to Fig. 1): C, 17.7%;

H, 6.7%; N, 51.7%; B, 23.9% [B3.0N5.0C2.0H9.0]n. IR data

Fig. 1 Methylamine condensation of B-tri(melthylamino)borazine

yielding poly[B-(methylamino)borazine].

This journal is � The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 3126–3138 | 3127

(KBr pellets, cm21): n (N–H): 3442 (m); n (C–H): 2963 (w),

2937 (w), 2894 (w), 2812 (m); d (NHCH3): 1595 (s); d

(borazinic NH): 1511 (s); n (B–N): 1460 (s); d (CH3): 1416

(s); n (C–N): 1105 (m); d (BN out-of-ring): 707 (w). 11B NMR

(96.29 MHz, C6D6, ppm): d = 25.77 br; 1H NMR (300 MHz,

CD2Cl2, ppm): d = 1.86 br (N(H)CH3), 2.47 vbr (N(H)CH3),

2.55 vbr (bridging NCH3), 2.64–3.37 br (NH borazine). 13C

NMR (75 MHz, CD2Cl2, ppm) d = 27.6, 27.9 (N(H)CH3), 31.2

(bridging NCH3). Simulated 15N MAS NMR (30.41 MHz,

ppm): d = 2285 (NB3), 2307 (NH, borazine ring), 2320

(bridging NCH3), 2347 (N(H)CH3). Simulated 13C CP MAS

NMR (75.47 MHz, ppm): d = 28 (N(H)CH3), 30.5 (bridging

NCH3). TGA (nitrogen, 1000 uC (0.8 uC min21); 41% weight

loss): 0–180 uC, Dm/m0 = 0%; 180–450 uC, Dm/m0 = 15.5%;

450–1000 uC, Dm/m0 = 25.5%. TGA (ammonia, 1000 uC(0.8 uC min21); 44.5% weight loss): 0–50 uC, Dm/m0 = 0%; 50–

400 uC, Dm/m0 = 21.0%; 400–1000 uC, Dm/m0 = 23.5%.

Spinning and ceramic conversion procedures

Poly[B-(methylamino)borazine] was spun at 173 uC using a

lab-scale melt-spinning apparatus set up inside a nitrogen-

filled glove box purified with potassium hydroxide. Thus

as-spun fibers were heated in an ammonia atmosphere, and,

for comparison, in a nitrogen atmosphere, to the desired

temperature from 25 to 1000 uC (0.8 uC min21) with a dwell

time of 1 h at the final temperature. Ammonia- or nitrogen-

treated fibers were further pyrolyzed in a nitrogen atmosphere

up to 1800 uC (10 uC min21) and held for 1 h at 1800 uC.

Results and discussion

Structure of a spinnable poly[B-(methylamino)borazine]

B-tri(methylamino)borazine is composed of a borazine ring

capped by N(H)CH3 groups bound to boron atoms. The exact

structure of the derived polymer, i.e., poly[B-(methylamino)

borazine], is not known. Presuming quantitative methylamine

condensation of B-tri(methylamino)borazine, a polymer struc-

ture as presented in Fig. 1 is expected.

Fig. 1 shows that the chemical environment of nitrogen

atoms is modified during the polymerization, while boron

atoms remain in a BN3 environment. Previous NMR investiga-

tions of a borazine-type polymer, i.e., polyborazylene,21 have

shown that, due to a relatively small chemical shift range,

modification of the second neighbours of the boron atoms

cannot be identified through 11B NMR experiments. In

accordance with such findings, the present study is focused

on 13C and 15N solid-state MAS NMR spectroscopy to

identify the structural elements composing poly[B-(methyl-

amino)borazine] ([B3.0N4.6C2.1H9.5]n) obtained by thermolysis

at Tsynthesis = 180 uC. A poly[B-(methylamino)borazine] pre-

pared at Tsynthesis = 180 uC represents an appreciable melt-

spinnable polymer which allows us to produce green fibers at

Tspinning = 173 uC (Tspinning; spinning temperature) where a

suitable viscosity of the polymer melt is obtained.

The experimental 13C CP MAS NMR spectrum of this

typical melt-spinnable poly[B-(methylamino)borazine] (Fig. 2)

exhibits a single broad composite signal that can be simulated

with two components at 28 and 30.5 ppm assigned to methyl

groups in peripheral BN(H)CH3 and bridged B2NCH3

environments respectively according to our 13C liquid-state

NMR studies (see Experimental and ref. 13). Although cross-

polarization (CP) is not quantitative, the identification of a

relatively high intensity signal for BN(H)CH3 groups means

that N(H)CH3 groups are still present in the polymer, and

therefore that the polymerization via methylamine condensa-

tion remains incomplete at Tsynthesis = 180 uC.

The presence of B2NCH3 and peripheral BN(H)CH3 environ-

ments was confirmed by 15N solid-state NMR experiments.

The experimental 15N MAS NMR spectrum (Fig. 3a) of the15N-enriched polymer shows one broad and asymmetric signal

which probably corresponds to a superposition of peaks

ranging from 2270 to 2370 ppm. It is inherently difficult to

interpret. CP techniques were therefore used to differentiate

these individual signals (Fig. 3b,c).

The experimental 15N CP MAS spectra of the 15N-enriched

polymer (Fig. 3b) and its non-enriched analogue (Fig. 3c) are

very similar, and both allowed two signals to be distinguished.

In addition, the structure recorded for the former case (Fig. 3b)

with a much better signal-to-noise ratio allowed us to run a

series of inversion recovery cross polarisation (IRCP) spectra.

The N sites were distinguished from their proton environments

by taking advantage of the 1H–15N dipolar coupling.24 This

sequence has already successfully been used to identify 15NHx

sites (x = 0–1) in similar systems.25

IRCP MAS NMR spectra of the 15N-enriched poly[B-

(methylamino)borazine] were thus recorded for various inver-

sion times ti (Fig. 4) showing that the individual components

do not invert at the same rate.

Two signals at 2347 and 2307 ppm are inverted for ti =

750 ms, whereas a third signal at 2320 ppm inverts at a

much slower rate with a behaviour characteristic of a non-

protonated nitrogen site. Signals at 2347 and 2307 ppm can

be thus assigned to peripheral BN(H)CH3 groups and B2NH

units of borazine rings respectively based on both their IRCP

dynamics which is typical of NH sites and the chemical shifts

previously reported for B-tri(methylamino)borazine and other

derivatives.25 Regarding the idealized polymer structure

represented in Fig. 1, the non-protonated signal at 2320 ppm

may be ascribed to a B2NCH3 bridge connecting two borazine

Fig. 2 Experimental 13C solid-state CP MAS NMR of poly[B-

(methylamino)borazine] and simulation of C resonances in bridging

and terminating NCH3 groups.


rings. This assignment is confirmed by a previous study in

which a similar bridge has been observed at 2317 ppm in a

borylborazine molecule.25

15N MAS (Fig. 3a) and 15N CP MAS (Fig. 3b) NMR

spectra of the 15N-enriched poly[B-(methylamino)borazine]

have been simulated with the peaks extracted from IRCP

experiments. As expected, the signal at 2320 ppm, assigned to

a non-protonated site, is more intense in the MAS spectrum,

since it can be under-estimated by cross-polarisation at contact

times optimised for protonated N. It can be noticed that the

protonated sites at 2347 and 2307 ppm are broader on the

MAS spectrum, since no proton decoupling was applied.

It is interesting to observe that an additional resonance with

low intensity appears at 2285 ppm on the MAS spectrum

(Fig. 3a). Its absence in the CP MAS sequence suggests that no

protons are present in a rather large surrounding (Fig. 3b,c). A15N signal with a similar chemical shift (diso (15N) = 2286 ppm)

has been already observed in a preceramic polyborazylene.21 It

was assigned to an NB3 environment. This suggests in the

present system that some B3N3 rings are either connected

through B–N bonds (models I and II) or fused like

borazanaphthalene-type molecules (model III) as depicted

in Scheme 1.

Although the formation of fused borazine rings is expected

to proceed during fiber preparation thereby providing the

consolidation of the final boron nitride structure, it is difficult

to distinguish between the formation of inter-ring B–N bonds

and fused B3N3 rings only by the structural study of a

single type of poly[B-(methylamino)borazine]. The thermolysis

Fig. 4 Experimental and simulated 15N IRCP MAS NMR spectra

using the 15N-enriched poly[B-(methylamino)borazine] for three

inversion time values.

Fig. 3 Experimental and simulated 15N solid-state (a) MAS, and (b)

CP MAS NMR spectra of 15N-enriched poly[B-(methylamino)bor-

azine], and (c) CP MAS NMR spectrum of non 15N-enriched poly[B-

(methylamino)borazine]. The different nitrogen environments are also

presented.

Scheme 1 Schematic representation of different NB3 motifs.


process must be investigated in more detail through a

combination of GC/MS analyses and structural studies of

polymer intermediates at different levels of the thermolysis

process. This should allow us to characterize the chemical

reactions and gain information about the exact model which

composes the polymer network. Such studies are under

investigation.

As a consequence of the thermolysis process at 180 uC, it

turns out that poly[B-(methylamino)borazine] is composed of

at least four structural nitrogen sites. The relative amounts of

the various sites extracted from the simulation of the 15N MAS

NMR spectrum are summarised in Table 1 (the relaxation

time T1 measured for 15N in similar compounds22 was taken

into account).

Although the relative amount of the characteristic structural

elements is expected to vary with the thermolysis temperature,

it may be proposed that the building blocks of a typical melt-

spinnable poly[B-(methylamino)borazine] (Tsynthesis = 180 uC)

are composed of borazine rings, which are connected via a

majority of NCH3 bridges and a small amount of NB3-

containing motifs forming a cross-linked network. This

branched molecular network is capped by a low proportion

of peripheral N(H)CH3 groups.

Thermal decomposition under selected atmosphere

Weight losses and decomposition rates (differential thermo-

gravimetric (DTG) data) of poly[B-(methylamino)borazine]

have been monitored during its decomposition by TGA

experiments. DTG curves were simulated by fitting their

amplitude using the multi-peak Lorentz function.20

TGA measurements were conducted in a nitrogen atmo-

sphere then, for comparison, in an ammonia atmosphere. It

should be mentioned that TGA experiments were carried out

up to 1000 uC (0.8 uC min21), since the great majority of the

weight loss necessary for boron nitride production from

poly[B-(methylamino)borazines] occurs below 1000 uC.10,11

It is important to emphasize that the overall weight loss

measured at 1000 uC is closely related to the synthesis

procedure of the polymer.

Poly[B-(methylamino)borazine] (Tsynthesis = 180 uC) exhibits

an appreciable thermal stability up to y200 uC in flowing

nitrogen (Fig. 5a). This is confirmed by comparing the

empirical formulas of the polymer unit (B3.0N4.6C2.1H9.5) and

the material obtained therefrom at 200 uC (B3.0N4.5C2.0H8.9).

Such data suggest that almost no enhanced cross-linking nor

decomposition occur below 200 uC. This allowed spinning of

the polymer melt in a stable process and production of high

quality green fibers.

The decomposition under nitrogen atmosphere occurs

through a continuous weight loss from 200 uC up to 450 uC

Table 1 Chemical shifts and relative intensities for structural elements present in poly[B-(methylamino)borazine] and related pyrolyzed fibers

diso (¡2 ppm)

Relative intensity (%) (¡2)

AssignmentPolymer

Pyrolysis temperature

200 uC 400 uC 600 uC 800 uC

2285 6.5 30 47 81 100

2307 44.2 34 17 14 —

2320 35.4 36 36 5 —

2347 13.9 — — — —

Fig. 5 (a) TGA, (b) experimental and (c) simulated DTG curves for

poly[B-(methylamino)borazine] (Tsynthesis = 180 uC) in a nitrogen

atmosphere. Heating rate: 0.8 uC min21.


(Dm/m0 # 15.5 wt%) that accelerates in the range from 450 uCto 650 uC (Dm/m0 # 20.0 wt%). The as-produced residue is

obtained in ca. 60% yield at 1000 uC and its black colour

suggests that carbon is retained upon heating to 1000 uC.

It is interesting to see that, in contrast to predictions made

from weight loss measurements, the associated DTG curve

(Fig. 5b) distinguishes four regions during decomposition. This

finding is confirmed using the Lorentz simulation (Fig. 5c),

since a simulated four-peak curve really matched the weight

loss rate of the poly[B-(methylamino)borazine] as a function of

temperature. Each weight loss is associated with two steps

which seem to proceed simultaneously.

Using ammonia under identical conditions, the same

polymer also undergoes a continuous weight loss in two steps

which are identified in the temperature ranges from 50 to

400 uC (Dm/m0 # 21.0wt%) and 400 to 1000 uC (Dm/m0 #23.5wt%) with similar rates (Fig. 6a). TGA data reveal an

overall weight loss of y44.5%, thus a ceramic yield of ca.

55.5% at 1000 uC for the as-obtained white fine powder.

Fig. 6b reports the corresponding DTG curve as a function

of temperature. DTG data show that the plotted curve is

characterized by two Tmax temperatures, at which the decom-

position rate is maximum. Such data confirm the occurrence of

a two-stage thermal process, but the broadening and asym-

metry of the peaks suggest peak overlapping, and thus a more

complicated decomposition process. As an illustration, the

Lorentz simulation suggests that there are five regions for the

decomposition process in an ammonia atmosphere (Fig. 6c).

The first weight loss in the lowest temperature range (50–

200 uC) is due to an additional process which does not occur

under nitrogen atmosphere, whereas the second one proceeds

in a similar way through two steps. Decomposition steps

also seem to occur simultaneously for each weight loss range

suggesting concurrent or even interdependent mechanisms

during the polymer-to-ceramic conversion.

A comparison of the empirical formulas of the TGA

residues obtained at 1000 uC (Table 2) clearly confirms that

thermal decomposition proceeds differently using either

ammonia or nitrogen. As expected, the TGA residue released

after heating under nitrogen (B3.0N2.9C0.5H1.5) retains a higher

amount of carbon compared with that obtained at 1000 uCunder ammonia (B3.0N2.9C0.2H0.7).

In addition, the stoichiometry of the TGA residue (B : N

ratio) is approaching that of h-BN using either nitrogen or

ammonia atmosphere, whereas the carbon and hydrogen

content are significantly lower if using ammonia. This points

out the fact that there is either nitrogen incorporation in the

material or a carbothermal reaction according to eqn (1) using

ammonia as pyrolysis atmosphere.

3C + 4NH3 A 3CH4 + 2N2 (1)

Since the polymer-to-ceramic conversion of the poly[B-

(methylamino)borazine] into boron nitride represents a com-

plex series of concurrent or even dependent reactions, 15N

and 13C solid-state NMR and FT-IR spectroscopies as well as

GC/MS experiments were used to explore the structural

changes of the poly[B-(methylamino)borazine]-derived green

fibers occurring up to 1000 uC (0.8 uC min21) in ammonia

and nitrogen atmospheres.

Structural changes during the ceramic transformation

FT-IR and solid-state NMR spectroscopies were carried out at

room temperature.

Pyrolysis under nitrogen

Green fibers and those annealed in a nitrogen atmosphere

to temperatures ranging from 200 to 1000 uC have been

characterized by IR spectroscopy (Fig. 7).

The similarity of the IR spectra of green fibers and samples

heated at 200 uC excludes decomposition below 200 uC as

suggested through the TGA experiments. Upon heating to

450 uC, the most fundamental change is the decrease in

intensity of bands assigned to N(H)CH3 groups in the range

2800–2970 cm21 which indicated elimination of methylamine.

This is in good agreement with GC/MS data recorded during

TGA experiments (Fig. 8), indicating methylamine (m/z = 31)

elimination around 210 uC.

Fig. 6 (a) TGA, (b) experimental and (c) simulated DTG curves for

poly[B-(methylamino)borazine] (Tsynthesis = 180 uC) in an ammonia

atmosphere. Heating rate: 0.8 uC min21.

Table 2 Measured elemental composition (wt%) of a typical melt-spinnable poly[B-(methylamino)borazine] (Tsynthesis = 180 uC) and resulting TGresidue obtained after decomposition in ammonia and nitrogen atmospheres. Heating rate: 0.8 uC min21

Samples N : B ratio B (wt%) N (wt%) C (wt%) H (wt%)Empirical formulaper monomer unita Mass per unit

Polymer 1.53 24.6 49.0 19.1 7.3 B3.0N4.6C2.1H9.5 131.7TG residue (1000 uC, N2) 0.97 40.5 50.0 7.6 1.9 B3.0N2.9C0.5H1.5 80.6TG residue (1000 uC, NH3) 0.97 42.5 54.1 2.4 1.0 B3.0N2.9C0.2H0.7 76.1a Empirical formulas are standardized at B3.0. Oxygen values are ,2% and omitted.


Assuming that methylamine is mainly eliminated during the

first weight loss, it is reasonable to suggest that the first

decomposition step is initiated by condensation reactions of

N(H)CH3 yielding N(CH3) bridges (B2NCH3 sites) and

generating methylamine (eqn (2)).

ð2Þ

As the first weight loss is represented by a two-stage process

(see DTG curve in Fig. 5) and according to the higher stability

of the N–H bonds in borazine compared with those in

N(H)CH3 groups, we assume that the mechanism associated

with the second step represents a continuation of what started

during the first process (eqn (2)). Condensation reactions of

N(H)CH3 groups and borazinic N(H) units occur leading to

the formation of cross-linked NB3 sites (eqn (3)) thereby

releasing methylamine.

ð3Þ

Therefore, it is reasonable to suggest that the first weight

loss detected between 200 and 450 uC is mainly assigned to

condensation reactions that yield a higher cross-linked

polymer network at 450 uC (B3.0N3.9C1.5H7.0).

A striking fact observed during the polymer-to-ceramic

conversion of green fibers in a nitrogen atmosphere is the

identification of ammonia (m/z = 17) by GC/MS (Fig. 8).

Similarly to methylamine developed from N(H)CH3 groups

(eqn (2) and (3)), ammonia is clearly formed from NH2 groups

bound to the boron atoms in the material. These B–NH2 units

are not identified in the polymer. It is relevant to mention

here that GC/MS experiments show an increased loss of

ammonia during the second weight loss (Fig. 8), so associated

mechanisms have a minor role during the first weight loss. IR

spectroscopy reflected such findings.

Although spectra at 300 and 400 uC display new bands

between 1600–1650 cm21 and 3000–3300 cm21 that may be

assigned to NH/NH2 modes in substituted borazines,26 the

related bands are clearer in samples heated at temperatures of

600–800 uC (Fig. 7).

Inevitably, the presence of NH2 groups and/or bridging NH

sites in the material suggests that a certain amount of borazine

units is destroyed through complex ring-opening pathways.

Such mechanisms include (i) a series of promoted cleavages of

intra-ring boron–nitrogen bonds, (ii) the subsequent appear-

ance of NH2 groups and (iii) the final reformation of the

borazine ring newly generating NH units with subsequent loss

of ammonia. Such borazine ring-opening processes have

already been described for borazine derivatives to form a

fused-ring borazine network called ‘‘naphthalenic-type struc-

ture’’.27 As the latter represents the basal structural unit of

h-BN, it is reasonable to suggest that such a rearrangement

occurs during the second weight loss, since it is necessary to

achieve full consolidation of the BN planar structure at high

temperature. Besides, loss of methylamine, which is higher

than that in the first weight loss, continues during the second

weight loss (GC/MS; Fig. 8). We suggest that this large loss of

methylamine arises from the cleavage of the inter-ring B–N

bond in B2N(CH3) sites, generating N(H)CH3 groups which

are expulsed to further consolidate the BN planar structure.

Considering the highest bond energy of the intra-ring B–N

bond in borazine (DB–N = 501 kJ mol21),28 it is reasonable to

suggest that the second weight loss is initiated by the cleavage

of the inter-ring B–N bond in N(CH3) bridges (3rd step, loss of

methylamine), followed by a 4th step which includes ring-

opening pathways (loss of ammonia). This means that the

second decomposition is associated with chain depolymeriza-

tion, before rearrangements gradually lead back to a

consolidated network. According to the complexity of these

Fig. 7 IR spectra of (a) poly[B-(methylamino)borazine] (Tsynthesis =

180 uC) and related fibers annealed (0.8 uC min21) in a nitrogen

atmosphere at: (b) 200 uC, (c) 300 uC, (d) 450 uC, (e) 500 uC, (f) 600 uC,

(g) 700 uC, (h) 800 uC, (i) 900 uC and (j) 1000 uC.

Fig. 8 GC curves representing the continuous evolution of gaseous

by-products, (a) methylamine and (b) ammonia, during TGA

experiments carried out in a nitrogen atmosphere. Heating rate:

0.8 uC min21.


mechanisms, a simple model for the individual reactions

cannot be proposed.

By 1000 uC, only bands assigned to B–N valence vibration

(n(B–N) = 1385 cm21, d(B–N, out-of-ring) = 795 cm21) with

N–H units in different environments are found, despite

chemical analyses (B3.0N2.9C0.5H1.5) indicating the formation

of a solid that contains a slight deficit in nitrogen compared to

BN and a large excess of hydrogen and carbon.

Pyrolysis under ammonia

In comparison, the same batch of green fibers was annealed

under an ammonia atmosphere covering the temperature

range from 100 to 1000 uC (0.8 uC min21). Due to the

complexity in the determination of involved mechanisms, 13C

and 15N solid-state NMR analyses were investigated in

conjunction with FT-IR.

Experimental and simulated 15N MAS NMR spectra

recorded for the polymer (Fig. 9a) and its pyrolysis products

up to 800 uC (Fig. 9b–e) show that the signal of N(H)CH3 end

groups (diso (15N) = 2347 ppm) vanishes after heat treatment

to 200 uC (Fig. 9b), while the signal corresponding to an NB3

environment at 2285 ppm increases in intensity in the sample

heat-treated below 800 uC (Fig. 9c–e). The latter is the only

visible resonance in the sample heat-treated at 800 uC (Fig. 9e).

It should be mentioned that the spectra of the pyrolyzed

samples can be simulated with similar signals to those used in

the polymer spectrum with an increasing amount of the NB3

site. The proposed simulation was confirmed by using the

IRCP sequence to check the degree of protonation of the

various signals (Fig. 10), but it is interesting to observe that

heating at 200 uC and 400 uC results in the appearance of an

additional IRCP signal around 2298 ppm.

According to its rapid inversion behaviour, this signal

corresponds to a protonated site. It is not clearly observed in

the IRCP spectrum for fibers isolated at 600 uC, most probably

due to a loss in the signal-to-noise ratio.13C CP MAS NMR spectra of the heat-treated samples do

not show additional signals compared to the polymer spectrum

(Fig. 11), suggesting that the signal centred at 2298 ppm in the15N IRCP MAS spectra is attributed to a C-free protonated

nitrogen site, either a B2NH unit or a BNH2 group.

It should be noticed that, according to the 13C CP MAS

NMR experiments, the amount of peripheral BN(H)CH3 sites

remains important compared to the bridged B2NCH3 units at

high temperature, while the 15N MAS studies indicated that

BN(H)CH3 sites have almost disappeared at 200 uC. This can

be explained by an overestimation of the BN(H)CH3 environ-

ment using the CP method due to the high amount of protons

around this site.

The relative amounts of the various 15N sites (using a single

peak at 2307 ppm including the minor signal at 2298 ppm for

the BxNH32x units (x = 1,2)) with increasing temperature are

summarised in Table 1 along with the relative amount of the

same sites identified in the polymer.

Based on NMR data, we assume that the first weight loss

involves the successive disappearance of (i) N(H)CH3 end

groups, (ii) N(CH3) bridges and (iii) B2NH units and the

concomitant formation of NB3 sites. Therefore, it is reasonable

to suggest that the decomposition below 400 uC is most

probably due to a thermally-induced cross-linking process.

We have focused on clarifying the relevant chemical

reactions caused by cross-linking with FT-IR (Fig. 12).

IR spectra of fibers isolated below 300 uC appear signifi-

cantly changed after ammonia treatment compared to nitrogen

treatment. The reaction with ammonia generates two new

bands at y1620 and y3300 cm21 which may be consistent

with the presence of NHx (x = 1,2) groups.26 New bands

around 1000–1100 cm21 are also due to NH units.26 This

allowed us to assign the signal at 2298 ppm in the 15N IRCP

MAS NMR spectra at 200 and 400 uC (Fig. 10) to NH/NH2

environments. However, it remains difficult to distinguish

between NH and NH2 groups bound to boron atoms only by

their chemical shifts.

In addition, compared to the IR spectrum of the polymer,

the lower intensity of the bands ascribed to C–H bond

stretching vibrations in the range 2800–2970 cm21 indicates

that methylamine is removed in this temperature range.

Fig. 9 Experimental and simulated 15N MAS NMR spectra of:

(a) poly[B-(methylamino)borazine] and fibers derived therefrom and

heated at various stages of the transformation process at: (b) 200 uC,

(c) 400 uC, (d) 600 uC, and (e) 800 uC. Heating rate: 0.8 uC min21.


Therefore, by combining solid-state NMR and IR data, we

propose to describe the first weight loss through the series of

reactions illustrated in eqn (4).

ð4Þ

It is thus established that incorporation of ammonia into the

polymer occurs upon heating at low temperature whereby

reactive B–NH2 moieties form instead of B–N(H)CH3 motifs

according to an amine-exchange pathway (route 1 in eqn (4)).

Hence, we propose that the insertion of amino groups is

responsible for the formation of aminoborazine units during

the first step. This transamination mechanism initiates facile

cross-linking reactions in two ways:

– First, we assume that the presence of highly reactive NH2

groups offers great cross-linking capabilities forming a net-

work consisting of hydrogen-containing groups in bridging

positions, i.e., NH bridges, through condensation reactions

and loss of methylamine/ammonia (routes 2 and 3 in eqn (4))

in a second step.

– Second, we suggest that as-formed B2NH sites promote

condensation reactions in a third step to provide a highly cross-

linked network (B3.0N3.8C1.0H5.8) through the development of

tertiary functions, i.e., NB3 sites, generating methylamine

and/or ammonia (routes 4 and 5 in eqn (4)). Relevant

mechanisms formulated in eqn (4) are most probable. It is

important to indicate that the incorporation of ammonia

during conversion was predicted by chemical analyses of the

TGA residue at 1000 uC (see above, Table 2). It is also pointed

out that the found B3.0N3.8C1.0H5.8 composition for the solid

Fig. 10 Experimental and simulated 15N IRCP MAS NMR spectra recorded at short (top—5 ms) and longer (bottom—450 ms) inversion time

values on fibers derived from 15N-enriched poly[B-(methylamino)borazine] heated at (a) 200 uC and (b) 400 uC (0.8 uC min21; ammonia

atmosphere). Spectra help to differentiate each individual signal through its CP response.

Fig. 11 Experimental and simulated 13C CP MAS NMR spectra of:

(a) poly[B-(methylamino)borazine] and resulting fibers at various

stages of the thermal transformation process at: (b) 200 uC, (c) 400 uC,

and (d) 600 uC.


isolated at 400 uC too suggests an incorporation of nitrogen in

the material after the cross-linking step.

From the above predictions, it turns out that the material

formed at 400 uC must consist of a highly cross-linked network

composed of bridged NCH3 sites, B2NH and B3N environ-

ments. The overall structure of the cross-linked solid is

dominated by branching NB3 units (Table 1).

During the second weight loss, IR changes are the same as

those observed under nitrogen atmosphere. Therefore, only

solid-state NMR spectroscopy served to investigate the

structural changes which occurred between 400 and 1000 uC.

Between 400 and 600 uC, the amount of BxNH32x units

(x = 1,2) remains almost constant, while the B2N(CH3) bridges

almost disappear (Table 1). Besides, the increase in the relative

intensity of the signal attributed to N(H)CH3 bridges in the13C CP NMR spectra (d = +28 ppm; Fig. 11) at 600 uCindicates the reformation of peripheral N(H)CH3 groups,

although this is not observed by 15N MAS NMR (Fig. 9). This

suggests a very small amount compared to the other nitrogen

environments. Above 600 uC, 15N MAS NMR spectra (Fig. 9d

and e) indicate that the nitrogen environment is dominated

by NB3 sites, since they give a main sharpened signal centred

at 2285 ppm.

Given these observations, it is therefore reasonable to

speculate that the second weight loss implies important

structural rearrangements initiated by chain and borazine

bond scission leading to reformation of BxNH32x units (x =

1,2) and N(H)CH3 groups. It is even appropriate to suggest

that ammonia takes part in chain depolymerization through

the cleavage of N(CH3) bridges and the subsequent reforma-

tion of end groups of the type NH2 and N(H)CH3 (eqn (5)).

According to their reactivity, NH2 groups enable the

formation and consolidation of rings through condensation

reactions with appearance of B2NH sites generating ammonia

and methylamine.

The formation of NB3 sites present in the final boron nitride

phase requires the condensation of NH2 groups, N(H)CH3

groups and N(H) units. A consolidated BN planar structure

with a low excess of carbon and hydrogen (B3.0N2.9C0.2H0.7) is

therefore obtained at 1000 uC.

Changes in fiber morphology during ceramic transformation

SEM experiments were investigated at room temperature

to follow changes of the fiber morphology during ceramic

transformation up to 1000 uC in the selected nitrogen and

ammonia atmospheres. Pyrolysis of nitrogen- and ammonia-

treated fibers was conducted above 1000 uC under nitrogen to

investigate the morphology of the final fibers obtained at

1800 uC. It should be mentioned that pyrolysis at 1800 uC is

required to produce crystalline boron nitride.10–12

It is interesting to observe that annealing of green fibers

(Fig. 13a) under nitrogen at 450 uC (end of the first weight loss;

Fig. 13b) causes the appearance of a few defects on the fiber

surface. Loss of fiber integrity observed through the appear-

ance of inter-fiber fusion zone is even observed after heating at

1000 uC (end of the second weight loss; Fig. 13c). Upon further

heating from 1000 to 1800 uC, it is even surprising to observe

that crystallization, characterized by the development of a

coarse-grained microtexture, extends over several fibers due to

these inter-fiber fusion zones (Fig. 13d).

The incapability of fibers to withstand temperatures of 1000

and 1800 uC is caused by an inefficient cross-linking of the

green fibers at low temperature. Finally, it is most probable

that the poor thermal reactivity of N(H)CH3 groups caused by

steric and electronic effects is responsible for an incomplete

cross-linking step through eqn (2) and (3). In addition, forma-

tion of N(CH3) bridges in the polymer backbone (eqn (2))

would strongly prevent further thermally-induced condensa-

tion reactions during the first weight loss. Both effects yield a

solid with a poor cross-linking density at 450 uC. During the

second weight loss, this poorly cross-linked solid undergoes

depolymerization and volatilization and cannot withstand the

increase of temperature to 1000 uC, and then 1800 uC.

In contrast, exposure of green fibers to ammonia below

1000 uC represents an efficient way to retain the fiber integrity

(Fig. 14).

This appreciable curing treatment is clearly ascribed to the

high chemical reactivity of poly[B-(methylamino)borazines]

with ammonia, leading to the formation of highly reactive

NH2 groups. Hence, we assume that, in contrast to N(H)CH3

groups, NH2 groups formed during transamination reactions

represent an interesting source for initiating rapid condensa-

tion reactions in the polymer network with further heating

giving both an enhanced cross-linking density and the desired

infusibility to fibers at 400 uC (Fig. 14b). In addition, it is

suggested that the appreciable thermal reactivity of as-formed

N(H) bridges facilitates the occurrence of condensation

reactions depicted in eqn (4) yielding highly cross-linked

materials at 400 uC. Such a curable precursor offers a great

Fig. 12 IR spectra recorded during the polymer-to-ceramic trans-

formation of (a) poly[B-(methylamino)borazine] and resulting fibers

annealed in an ammonia atmosphere (0.8 uC min21) at: (b) 100 uC, (c)

150 uC, (d) 200 uC and (e) 300 uC.

(5)


simplicity of processing in the absence of catalyst and oxygen

to obtain fiber shape stabilization at low temperature. In spite

of structural rearrangements during the second weight loss,

the cured fiber retains its shape after annealing at 1000 uC(Fig. 14c). This means that the small polymer fragments

resulting from chain scission, i.e., rearrangements, during the

second weight loss display sufficient branched molecular

structure to withstand the increasing temperature.

Further heating from 1000 to 1800 uC of ammonia-treated

fibers was carried out in a nitrogen atmosphere. Annealing

resulted in the formation of ceramic fibers without surface

defects. The final fibers obtained at 1800 uC can be easily

separated from each other, and crystallization homogeneously

extends over the whole fiber cross-section (Fig. 14d).

Crystallization tendency of BN fibers

Fig. 15 shows XRD patterns of nitrogen- (Fig. 15a) and

ammonia- (Fig. 15b) treated fibers annealed from 1000 to

1800 uC in a nitrogen atmosphere. For crystallinity com-

parisons, a commercially-available h-BN (Fig. 15c; boron

nitride grade A from Carborundum Corporation, New York)

was taken as a reference.

Comparisons in the chemical compositions of the final fibers

were also made, but it should be mentioned that the chemical

composition is uncertain for fibers produced at 1800 uC due to

the refractory character of the well-crystallized materials.

Their thermal stability may produce variations in the chemical

composition due to an incomplete combustion leading to

misleading results using traditional analytical techniques.

However, we can refer to the chemical composition of the

less-crystallized fibers isolated at 1500 uC, since their com-

position is expected to be close to that of fibers produced

at 1800 uC in accordance with the absence of chemical

phenomena in this temperature range.

Fibers prepared at 1800 uC in a nitrogen atmosphere

(Fig. 15a) remain less crystallized that the commercially-

available h-BN most probably due to the presence of

carbon in the final nitrogen-poor grey product (B3.0N2.6C0.3).

Indeed, the excess of carbon in the material obtained at

1000 uC (B3.0N2.9C0.5H1.5) in flowing nitrogen (i) restricts

the crystallization progress and growth of BN crystals

during further heat-treatment from 1000 uC to 1800 uC and

(ii) generates a boron carbonitride material after annealing

at 1800 uC.

Fig. 14 SEM images of (a) green fibers heated in an ammonia atmosphere at (b) 400 uC and (c) 1000 uC. Heating rates (25–1000 uC): 0.8 uC min21.

SEM image of (d) ammonia-treated fibers obtained at 1800 uC in a nitrogen atmosphere. Heating rate (1000 to 1800 uC): 10 uC min21.

Fig. 13 SEM images of (a) green fibers heated in a nitrogen atmosphere at (b) 450 uC, (c) 1000 uC, and (d) 1800 uC. Heating rates (25–1000 uC):

0.8 uC min21 and (1000 to 1800 uC): 10 uC min21.


For chemical and structural comparison, ammonia-treated

fibers were subjected to the same crystallization heat-treatment

in a nitrogen atmosphere from 1000 to 1800 uC (Fig. 15b).

Based on the composition of fibers isolated at 1500 uC(B3.0N2.8), it is reasonable to consider that fibers prepared at

1800 uC are free of carbon and that the stoichiometry is

approaching that of h-BN, i.e., loss of nitrogen is restricted,

using ammonia from 25 to 1000 uC. It is also relevant to

mention here that another advantage of controlling the

environment of the curing and pre-pyrolysis by using ammonia

up to 1000 uC is that the crystallinity of the boron nitride

phase can be improved. A comparison of the linewidths in

Fig. 15a and b, shows that the crystallization tendency of the

material is improved at high temperature due to the small

amount of excess carbon and hydrogen that must be lost

between 1000 and 1800 uC. Given these observations,

the mechanical properties, which are closely related to the

crystallinity of the material,12 may be improved using

ammonia below 1000 uC. Comparison with the XRD pattern

of the commercially-available h-BN (Fig. 15c) shows that BN

fibers (Fig. 15b) remain less crystallized and display a

transitional structure between turbostratic and hexagonal as

previously demonstrated.12

Conclusions

In the present paper, we have described an access to boron

nitride fibers using a typical melt-spinnable poly[B-(methyl-

amino)borazine] as preceramic polymer.

First, we use the appreciable thermal stability of poly[B-

(methylamino)borazine] at moderate temperature in nitrogen

atmosphere to produce green fibers during a stable melt-

spinning process. Second, we use the reactivity of the derived

green fibers towards ammonia to obtain fibrous shape

stabilization by a chemical curing process, and to obtain

polycrystalline boron nitride fibers by a controlled pyrolysis

process. A polymer-to-ceramic conversion associated with five

chemical steps is identified up to 1000 uC using ammonia.

A curing process under ammonia is the most promising

process to retain fiber integrity, i.e., avoid melting of the

polymer fibers, during further pyrolysis. The ammonia atmo-

sphere acts in the initial stage of the polymer-to-ceramic

conversion through transamination reactions of the peripheral

N(H)CH3 groups into highly reactive NH2 groups. These

reactions, which initiate decomposition, provide a means of

facilitating the cross-linking reactions during the second and

third processes below 400 uC forming cross-linked NB3 sites in

the highly cross-linked polymer network.

In spite of important structural rearrangements through

chain scission, loss of fiber integrity is prevented during the

subsequent concurrent fourth and fifth processes from 400 to

1000 uC. Besides, ammonia acts at intermediate temperatures

to increase the loss of remaining carbon-containing groups

and reduce the amount of excess hydrogen. This (i) facilitates

the formation of the naphthalenic-type structure of the boron

nitride phase at 1000 uC, (ii) improves the crystallization

tendency of the boron nitride phase at higher temperatures

(1000–1800 uC), and (iii) restricts the loss of nitrogen in the

final product (1800 uC).

Acknowledgements

We thank the European Community who supported this work

through the Marie Curie Research Training Network

PolyCerNet (Contract MRTN-CT-2005-019601).

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