thompson, s., howlin, b. j., stone, c. a. , & hamerton, i. (2018). exploring · thompson, s.,...
TRANSCRIPT
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Thompson, S., Howlin, B. J., Stone, C. A., & Hamerton, I. (2018).Exploring the thermal degradation mechanisms of somepolybenzoxazines under ballistic heating conditions in helium and air.Polymer Degradation and Stability, 156, 180-192.https://doi.org/10.1016/j.polymdegradstab.2018.09.002
Peer reviewed versionLicense (if available):CC BY-NC-NDLink to published version (if available):10.1016/j.polymdegradstab.2018.09.002
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https://doi.org/10.1016/j.polymdegradstab.2018.09.002https://doi.org/10.1016/j.polymdegradstab.2018.09.002https://research-information.bris.ac.uk/en/publications/b8fcfc90-16a2-4092-9a21-d9f150a522c6https://research-information.bris.ac.uk/en/publications/b8fcfc90-16a2-4092-9a21-d9f150a522c6
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1
Exploring the Thermal Degradation Mechanisms of Some
Polybenzoxazines Under Ballistic Heating Conditions in Helium and Air
Scott Thompson1,†, Brendan J. Howlin1, Corinne A. Stone2, Ian Hamerton3*
1 Department of Chemistry, Faculty of Engineering and Physical Sciences, University of
Surrey, Guildford, Surrey, GU2 7XH, UK
2 Dstl, Porton Down, Salisbury, SP4 0JQ, UK
3 Bristol Composites Institute (ACCIS), Department of Aerospace Engineering, School of
Civil, Aerospace, and Mechanical Engineering, Queen’s Building, University of Bristol,
University Walk, Bristol, BS8 1TR, UK
† Present address: Hexcel Composites Ltd., Duxford, Cambridge, CB22 4QD, UK
*Corresponding author [email protected]
ABSTRACT: The degradation behaviour of five polybenzoxazines (PBZs) is studied using
pyrolysis-GC/MS. Upon heating to 800 °C in helium the PBZs generate a variety of similar
pyrolysis products including aniline (the major product in all cases), substituted phenols,
acridine, and 9-vinylcarbazole. During the initial stages of heating (200 to 300 °C) aniline is
the dominant pyrolysis product; from 350 °C onwards substituted phenols are released,
particularly 2-methylphenol and 2,6-dimethyl phenol. The same major species are produced
on heating in air, but in addition isocyanatobenzene is observed which results from the
oxidation of Mannich bridges, along with a number of sulphurous species from the monomer
containing a thioether bridge. This suggests that sulphur is more likely to be retained in the
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2
char in a helium atmosphere, but takes part in oxidative reactions to form pyrolysis fragments
in air. During the ramped temperature cycles in both air and helium atmospheres the release
of aniline was observed to rise, fall and then to rise again. This may be due a combination of
the very high heating rate, poor thermal conduction of the polymer and the availability of the
Mannich bridges to undergo breakdown.
Keywords: Polybenzoxazines, thermal stability, pyrolysis, degradation mechanisms.
INTRODUCTION
Polybenzoxazines (PBZs) form a comparatively new family of thermosetting resins that are
being explored1 as potential higher performance replacements for phenolic2 or epoxy resins3
and, whilst they are not currently widely used in civil aviation, are being evaluated in this
application. PBZs are formed through step growth ring-opening polyaddition from bis-
benzoxazine monomers (Fig. 1), which are in turn the products of the Mannich reaction
between a mono- or bis-phenol, formaldehyde, and a primary amine4. Unlike many other
commercial thermosetting resins, which evolve condensation products such as water or
ammonia, benzoxazine monomers react relatively cleanly to form a polymer with few
reaction by-products5, although the exact mechanism of the polymerisation reaction to form a
network has not been fully elucidated. Our group has examined the influence of additives on
the nature of the polymerisation mechanism, the formation of the polymer network structure
and the resulting final properties6.
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3
Figure 1. Schematic showing polymerisation of bisbenzoxazines through ring opening and
crosslinking.
The synthetic route employed (in which both polyphenol and amine might be varied) offers
the potential to yield polymers with improved toughness over conventional phenolics.
Cured PBZs offer a combination of favourable thermal and mechanical performance (e.g. dry
Tg values of 246 C, hot/wet Tg = 207 C are possible7, coupled with low moisture uptake)
that gives an attractive property profile. Although requiring toughening for some engineering
applications, PBZs can be combined with epoxy resins 8 or inherently tough engineering
thermoplastics (e.g. oligomeric polyethersulphone, KIC = 0.99 MPa m1/2)9 to yield impressive
enhancements.
The thermal degradation of the same series of PBZs was studied in air and nitrogen using
thermogravimetric analysis (TGA) and reported in a previous publicationError! Bookmark not
defined.. The influence of particle size, and the structure of the bisphenyl unit, on the manner in
which the crosslinked polymer undergoes degradation was examined. Although the method
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4
indicated the degree of degradation and allowed distinction of degradation stages it did not
yield direct mechanistic information about the chemical processes which are occurring during
the thermal degradation. In contrast, a technique such as pyrolysis-GC/MS allows the
products of thermal degradation to be identified and quantified via gas chromatography
coupled with mass spectroscopy. The nature of the identified pyrolysis products can then be
used to postulate the reactions which have given rise to these species. Pyrolysis uses high
heating rates (in this work a heating rate of 20000 K s-1 is used) which reduces the likelihood
of secondary reactions, resulting in pyrolysis fragments which should reflect the original
structure of the sample more closely than those formed at slower heating rates.
EXPERIMENTAL
Materials. All monomers were supplied by Huntsman Advanced Materials (Basel),
characterised fully, and used as supplied. The characterisation data are included as
supplementary information (Table S1 and Figs. S1 – S5). The BF-a monomer (a mixture of
isomers) is known to contain ca. 1 wt % of aniline, arising from the production process.
Cure conditions. All monomers (Fig. 2), except the monomer containing the phenolphthalein
moiety (BP-a), were degassed at approximately 90 °C for one hour using a vacuum oven to
reduce void formation during the subsequent curing process. BP-a was found to be very
difficult to control under degassing conditions (excessive void formation) and so was simply
melted and held at 120 °C over the same time period that the other samples were degassed.
Following degassing, all samples were placed in an air circulating oven at 120 °C and the
following cure schedule was applied: heating 2 K min-1 to 180 °C (isothermal two hours)
followed by heating at 2 K min-1 to 200 °C (isothermal two hours).
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5
Figure 2. Structures of the monomers examined in this study.
Characterisation and Measurements. Pyrolysis-gas chromatography-mass spectrometry
(Py-GC-MS) was performed on samples prepared by shaving pieces from a cured puck using
a micro-plane. Samples were weighed in a quartz pyrolysis boat before analysis and after the
final pyrolysis event. Sample mass (100–600 µg) was chosen such that products could be
identified without overloading the analytical system. Pyrolysis was carried out using a CDS
5200 pyrolyser (Analytix Ltd, Boldon, Tyne and Wear, UK) in trap mode with a Tenax TA™
sorbent trap. The rest temperatures for the interface and trap were set at 50 °C and the trap
desorb temperature was set to 280 °C for 2 minutes and the valve oven and transfer line held
at 310 °C. The final temperature of the pyroprobe was held for 15 seconds with heating rate
set to maximum (20000 K s-1). The interface was held at the final temperature for 2 minutes
for each method. The GC-MS analysis of the volatile organic components was performed on
a PerkinElmer AutoSystem XL gas chromatograph coupled to a PerkinElmer Clarus 560S
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6
mass spectrometer. Separation was performed on a Zebron ZB-1701 chromatographic column
(dimensions 30 m x 0.25 mm ID x 0.25 µm film thickness). The carrier gas was helium and
constant pressure of 8 psi (0.8 MPa) was applied. Split mode was used with an injector
temperature of 310 °C. The oven temperature program was 45 °C for 4.5 minutes, 10 K min-1
to 130 °C, hold for 3 min, 10 K min-1 to 280 °C, hold for 2 minutes; the total run time was
33 minutes. Mass spectrometric detection was performed with electron impact (EI) ionisation
at 70 eV in full scan mode (m/z 34 to 450) with a solvent delay of 2 minutes. The source
temperature was 230 °C with inlet line temperature of 280 °C. Analysis of the pyrolytic
products was performed following pyrolysis. Data were compared against a series of
standards: phenol, o-cresol, p-cresol, 2,4-dimethylphenol, 2,6-dimethylphenol, 2,4,6-
trimethylphenol, o-xylene, p-xylene, 1,3,5-trimethylbenzene, aniline, o-toluidine, p-toluidine,
formanilide, 9H-xanthene, phenanthrene, and acridine.
RESULTS AND DISCUSSION
Selection of Experimental Parameters. In our previous work, TGA was used over the
range room temperature to 800 °C and so it is across this range that pyrolysis-GC/MS has
been applied. Initially, a small sample of ground poly(BA-a) was heated to 800 °C under an
atmosphere of helium which was subsequently repeated for each PBZ. The chromatograms
produced for each of these analyses are presented in Fig. 3, from which it is immediately
apparent that the various PBZs generate many of the same pyrolysis products but that there
are also numerous peaks in the chromatograms which are unique to one sample.
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7
Figure 3 - GC data produced from pyrolysis-GC/MS of various PBZs (RT to 800 °C).
05-Jul-2012 09:30:50800 ballistic, 273.1 ug
2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00Time0
100
%
0
100
%
0
100
%
0
100
%
0
100
%
0
100
%
LMB6490r01 Scan EI+ TIC
5.17e911.76
3.87 4.92
14.0113.31
14.3426.7124.7420.92
26.24
LME10140r01 Scan EI+ TIC
5.47e911.84
2.124.96
13.36
13.14 14.0415.9114.75 16.45
22.26
35900r01 Scan EI+ TIC
3.43e911.80
13.36
13.14
13.05
14.04
26.2714.3721.63 24.7623.79
26.74
27.62
35800r01 Scan EI+ TIC
3.08e911.75
3.863.02 4.91
13.29
13.1013.99
26.2114.32
24.7220.8919.8415.85 23.74
26.6827.56
35700r01 Scan EI+ TIC
7.24e911.84
13.35
13.04 14.03 15.9114.7527.9616.45 21.18
35600r01 Scan EI+ TIC
7.21e911.78
8.259.23
13.31
13.09 13.98
20.8420.1015.03 26.22
26.69
poly(BA-a)
poly(BF-a)
poly(BP-a)
poly(BT-a)
poly(BD-a)
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8
The mass spectra of the peaks were obtained and compared with the NIST database10 for
identification, as well as comparison with standards run under the same conditions. In
particular these chromatograms all share many of the intense peaks: the major peak in all of
the analyses represents aniline, with the peaks in the range 13-17 minutes representing
substituted phenolic and aniline species. In all, twelve pyrolysis products were shared by all
the PBZs (see Supplementary Fig. S6). The origins of many of these molecules are relatively
straightforward to identify through bond scission within the PBZ structure. For instance, an
aniline moiety is pendant from the Mannich bridge that connects the benzoxazine repeat units
and cleavage of the two C-N bonds surrounding the nitrogen atom will result in release of
aniline. This situation has already been postulated by Low and Ishida following work using
TGA-FTIR11. Similar scenarios can be suggested for the production of the remaining smaller
molecules in Table 1, the production of indole, acridine, and 9-vinylcarbazole might arise
from secondary reactions where smaller molecules have recombined or are released at high
temperatures when carbonaceous char decomposes. The pyrolysis products reported in the
present work also tend to share similar structural features, however there is no evidence of
Mannich base or biphenyl compounds being produced by the PBZs. It is possible that these
molecules had formed but broken down further or recombined and thus their production
might have been masked.
The analysis carried out in this work is not strictly quantitative, but due to the overwhelming
abundance of aniline in all of the chromatograms, presenting the integration values of the
peaks relative to the value of their respective aniline peaks allows a useful interpretation of
the data (Fig. 4) and this presentation of results underlines the dominance of phenol and
aniline based species. This is to be expected as the three major stages of PBZ thermal
degradation are: breakdown of Mannich bridges that are at the chain ends or side chains (this
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9
structure is abundant in monomeric benzoxazine derived polymers), and breakdown of the
Mannich bridges that are in the main chain discrete from the chain ends (this structure
dominates polymers that are derived from the main-chain type precursor). The first
degradation peak observed in the TGA around 300 °C nearly disappears in the main-chain
type PBZs and thus this degradation process is attributed to the degradation of the chain ends.
Both of these processes release aniline; they are followed by bisphenol linkage breakdown
(releasing phenols) and finally char degradation12.
It is perhaps surprising that aniline dominates the chromatograms, for in the data acquired
under nitrogen using TGAError! Bookmark not defined. the bisphenol linkage breakdown stage is the
major degradation process for many of these PBZs. The larger species, such as acridine, are
produced at such a relatively low level, when it has been observed that by approximately
600 °C the polymer samples have become very blackened and carbonaceous in nature. It is
possible that both of these anomalies can be explained by the speed of the pyrolysis.
Mannich bridge breakdown is thought to occur at relatively low temperatures, therefore when
heated quickly it may be that only this early stage of degradation has the opportunity to take
place fully whereas there is less time for the latter stages (e.g. char breakdown) to proceed.
The other major pyrolysis product, which is specific to only one of the PBZs, is butanol,
which is added as a processing aid to at least one commercial formulation of the BP-a
monomer.
Predictably, sulphurous molecules are observed in the pyrolysis products of poly(BT-a), but
the amounts (of meta-methoxybenzenethiol and 1-methoxy-3-(methylthio)benzene) released
are very low compared with the other products. Considering that the levels of phenolic type
-
10
pyrolysis products (which are assumed to be released upon bisphenol linkage breakdown) are
not dissimilar for the other PBZs it is inferred that the sulphur in the PBZ structure remains in
-
11
Figure 4 - Pyrolysis products of PBZs under helium when heated to 800 °C (integrated values relative to aniline)
0
20
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-
12
the char as the material is heated rather than being released. This may, in turn, contribute to
the fact that the char yield of poly(BT-a) has been found to be one of the highest of this PBZ
set. Alternatively, thiols may undergo reaction with some PBZs, particularly aliphatic
polymers, the extremely high temperature might facilitate this although the authors have no
evidence to support this possible pathway.
Examination of thermal degradation mechanism. To build a greater understanding of how
the thermal degradation of PBZ proceeds with an increase in temperature a stepwise
pyrolysis-GC/MS programme was applied. For this procedure the same experimental set up
described in the previous section was used, but the sample was heated to 200 °C, then to 250
°C and subsequently incrementally to 300, 350, 400, 450, 500, and 600 °C. The upper
temperature was selected from examination of the TGA data, which indicated that by that
temperature the major degradation events had ceased. Figs 5a to 5f demonstrate how the
concentrations of degradation products vary with PBZ backbone at various stages during the
stepwise heating programme to 450 °C under helium.
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13
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l-
Ph
eno
l, 3
,5-D
imet
hy
l-
Ph
eno
l, 2
,4,6
-Tri
met
hy
l
Qu
inaz
oli
ne
Qu
ino
line,
2-M
eth
yl
Ben
zen
emet
han
ol,
4-(
1,1
-
Ph
enol,
2-(
2-M
ethy
l-2-
5H
-1-P
yri
nd
ine
Form
amid
e, N
-Phen
yl
Ben
zen
e, H
exam
eth
yl
Ben
zofu
ran
, 2
,3-
Inte
grati
on
Rel
ati
ve
to A
nil
ine (
%)
PBA-
a PBF-a
PBP-a
PBT-a
(e)
-
16
Figure 5 - Pyrolysis-GC/MS products relative to aniline, under helium at (a) 200 °C, (b) 250 °C, (c)
300 °C, (d) 350 °C, (e) 400 °C, (f) 450 °C.
At lower temperatures (200 to 300 °C) the major pyrolysis product found in all the polymers
tested is aniline, followed by aniline derivatives, whose release accompanies breakdown of
the Mannich bridges. Although there are two types of cleavage site available along this
Mannich bridge structure (one N-C and one aromatic C-C) the results presented here would
suggest that cleavage of both C-N bonds is favourable to the cleavage of one, or both of the
C-C bonds, presumably reflecting the polarity of the C-N bond supporting movement of the
electrons to reside on the nitrogen. This is supported by the relative amounts of products: the
yield of aniline vastly outweighs that of n-methylaniline; this is true throughout the entire
0
20
40
60
80
100
120
1,3
-Cycl
open
tad
iene
Anil
ine
Ben
zenam
ine,
N,N
-Dim
ethy
l-A
nil
ine,
N-M
ethy
l-P
hen
ol
Ben
zenam
ine,
3-M
ethy
l-P
-Am
ino
tolu
ene
Phen
ol,
2-M
ethyl
Phen
ol,
2,6
-Dim
ethyl-
N-M
ethyl-
O-T
olu
idin
eP
hen
ol,
3-M
ethyl-
Ben
zenam
ine,
N,4
-Dim
ethy
l-B
enze
nam
ine,
2,5
-Dim
eth
yl-
Phen
ol,
3,4
-Dim
ethyl-
Phen
ol,
2,4
-Dim
ethyl-
Phen
ol,
2,3
-Dim
ethyl
Phen
ol,
2,4
,6-T
rim
ethyl-
Quin
azoli
ne
Quin
oli
ne,
2-M
ethy
lP
hen
ol,
2-M
ethyl-
5-(
1-M
ethyle
thy
l)-
Ben
zenem
eth
anol,
4-(
1,1
-Dim
ethy
leth
yl)
-5H
-1-P
yri
din
eP
-Iso
pro
pen
ylp
hen
ol
Phen
ol,
2-(
2-M
eth
yl-
2-P
ropen
yl)
-B
enze
ne,
Hex
amet
hyl
Quin
azoli
ne
Indole
Form
amid
e, N
-Ph
eny
l-1H
-Ind
en-1
-one,
2,3
-Dih
ydro
-3,3
-Dim
ethy
l-1H
-Ind
en-1
-one,
2,3
-Dih
ydro
-3,4
,7-…
Nap
hth
o[2
,1-B
]Fura
n, 1,2
-Dim
ethy
l-A
crid
ine,
9-M
ethyl-
Imin
ost
ilben
eB
enz[
J]Is
oqu
ino
line,
3,6
,8,-
Tri
met
hyl-
3-H
ydro
xy
-3-(
1H
-Im
idaz
ol-
2-y
l)-2
-…
3,3
’-D
iam
inodip
hen
ylm
eth
ane
Inte
gra
tio
n R
ela
tiv
e to
An
ilin
e (%
)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
(f)
-
17
temperature cycle, not only at these lower temperatures. The reported bond energies (C-C,
348 kJ/mol.; C-N 293 kJ/mol) [13] are also consistent with this suggestion.
Low and Ishida11 postulated from TGA-GC/MS data that a conjugated Schiff base would be
produced in competition with n-methylaniline, particularly in the case of poly(BA-a),
although the present work shows no sign of Schiff base throughout the analysis of any of the
PBZs (see Supplementary Figs. S1 - S3). This may be a result of differing experimental
parameters between the TGA-GC/MS and pyrolysis-GC/MS configurations, in particular a
far more rapid heating rate is employed during pyrolysis (20000 K s-1) compared to the TGA
(10 K min-1). n-Phenylformamide is also produced at these lower temperatures, although it is
more difficult to identify the origin of this product (unless one assumes the degradation of an
unreacted monomer unit, to form a new double bond resulting from the O-C cleavage). The
hypothesis for this event is supported by DSC data, which have shown the PBZs in this work
cure to approximately 90 %, leaving a significant quantity of unopened heterocyclic rings
available to react in this way. The pyrolysis product plots confirm that this n-
phenylformamide production is low compared to aniline at approximately 5-10 %, which
agrees with the 90 % degree of cure values. It has also been postulated that monomer units
can be liberated during thermal degradation of PBZs14 and so it is possible that this has
occurred followed by further degradation.
The major difference at these temperatures between the PBZs is the production of butanol
from poly(BP-a). Butanol production has previously been seen when raising the temperature
directly and rapidly to 800 °C, where the principal release of the alcohol occurs between 200
and 250 °C and dominates the plot for poly(BP-a). Release at these relatively low
temperatures supports the notion that this is free butanol remaining in the cured resin. With
-
18
only aniline being released for all of the other PBZs it is suggested that at these temperatures
Mannich bridge breakdown is occurring, which should not be particularly influenced by the
various bisphenol linkages in this set of materials. Therefore, as butanol is the only additional
species apparent at this temperature, it is logical to assume that this merely results from
evaporation rather than degradation of the PBZ structure.
As the temperature increases from 300 to 450 °C other species, such as phenols, begin to be
released. Again, this is to be expected as this is the temperature range over which bisphenol
linkage degradation occurs. The appearance of 2,6-dimethylphenol and 2-methylphenol
supports the hypothesis that the second stage in PBZ degradation is bisphenol linkage
breakdown. Thus, following aniline release, the cleavage between the bisphenol group and
phenolic moiety to release both 2,6-dimethylphenol and 2-methylphenol can be envisaged.
These two phenolic compounds are seen first in PBA-a followed by PBP-a, suggesting that
the bisphenol linkages in these two structures are weaker than in the other PBZs.
Furthermore, in this temperature range 2,6-dimethylphenol is more prevalent than 2-
methylphenol, which again suggests that C-N cleavage is preferred to C-C cleavage, until
higher temperatures are reached when the trend is reversed.
Of the PBZs studied, poly(BA-a) generates more complex molecules at a significantly earlier
stage of the degradation process compared with the other materials. For instance, at 350 °C
poly(BA-a) releases 2-methylquinoline, 4-(1,1-dimethylethyl)benzenemethanol, 2-(2-methyl-
2-propenyl)phenol and hexamethylbenzene. It follows that if the backbone structure of
poly(BA-a) has started breaking down before the other materials, that secondary reactions
and recombinations would occur first for poly(BA-a). These four species may be split into
two categories where 2-methylquinoline and hexamethylbenzene are products of secondary
reactions, and 4-(1,1-dimethylethyl)benzenemethanol and 2-(2-methyl-2-propenyl)phenol are
-
19
generated from more simple cleavage of the primary structure. Following Mannich bridge
breakdown, the generation of these two molecules from the poly(BA-a) structure by
bisphenol linkage breakdown is a reasonable hypothesis. However, it is true that the
production of 2-(2-methyl-2-propenyl)phenol might arise more logically through cleavage
from the poly(BA-a) structure if the (2-methyl-2-propenyl) group was located in the 4-
position on the phenolic ring rather than 2-position. As this molecule has been identified by
comparison of mass spectra it is possible that isomers exist, which is suggested by the 1H
NMR spectrum6. These two molecules are only apparent for poly(BA-a) throughout the
whole temperature cycle, which confirms that the nature of the monomer backbone
influences the pyrolysis pathway. Later in the analysis, 2-methyl-5-(1-methylethyl)phenol is
also found to be generated solely by poly(BA-a) and this again fits this structure well,
particularly if the (1-methylethyl) group is attached at position 4 instead of 5.
By 450 °C the degradation products from the remaining four PBZs contain more complex
molecules and by 600 °C, in most cases, aniline is no longer the biggest contributor to the
pyrolysis products. At these higher temperatures there are subtle differences in the species
generated where bisphenol linkage breakdown is expected to occur in all of the PBZs. If one
considers just the simple phenolic species: 2,4-dimethylphenol dominates the degradation
products of poly(BF-a) at 450 and 500 °C, as it arises from a simple cleavage of the
methylene bridge; whereas it is not a major degradation product for the remaining PBZs.
Indeed, unlike poly(BF-a) and poly(BA-a), it is difficult to find any strong evidence of
release of molecules containing the bisphenol linkage group for poly(BP-a), poly(BT-a), and
poly(BD-a). This is most obvious for poly(BT-a), owing to an almost a complete lack of
pyrolysis products containing sulphur (aside from the more complex 4-nitro-N-(2,6-
xylylbenzene)sulfonamide that occurs at a low level, 0.3 %). When the analysis is run
-
20
directly to 800 °C (see previous section) two or three sulphur containing molecules are
formed, albeit at relatively low magnitude (~10 %). These observations lead to two
hypotheses: (a) stopping the pyrolysis procedure at various temperature intervals has an effect
on the thermal degradation processes occurring in the sample; (b) sulphur is more likely to
remain in the sample by contributing to char than to be released as a pyrolysis product (see
the previous comments relating to thiol reactivity towards benzoxazine monomers). Similarly,
for poly(BP-a) and poly(BD-a) there are no signs of phenolphthalein nor dicyclopentadienyl
moieties being found in the respective pyrolysis products. This is not surprising as it is likely
that these large groups would themselves undergo breakdown. It is likely that, in the case of
the dicyclopentadiene group in poly(BD-a), this gives rise to a large range of more complex
pyrolysis products (e.g. dicyclopentadiene is prone to undergo a retro Diels-Alder reaction to
yield two equivalents of cyclopentadiene)15. For example, in the 500 °C plot (Fig. 6a) it is
poly(BD-a) which dominates the central portion of the graph displaying a variety of indole-
and indenone-related molecules.
-
21
0
20
40
60
80
100
120
1,3
-Cy
clo
pen
tad
ien
e
An
ilin
e
An
ilin
e, N
-Met
hy
l- (
smal
l)
Ph
eno
l
Ben
zenam
ine,
3-M
eth
yl-
Ben
zen
amin
e, 3
-Met
hy
l- /
P-A
min
oto
luen
e
Ph
eno
l, 2
-Met
hy
l
Ph
enol,
2,6
-Dim
eth
yl-
Ph
eno
l, 4
-Met
hy
l
Ph
eno
l, 3
-Met
hy
l-
Ben
zen
eam
ine,
2,4
-Dim
eth
yl-
Ben
zenam
ine,
2,5
-Dim
eth
yl-
Ph
enol,
2,4
-Dim
eth
yl-
Phen
ol,
2,3
-Dim
ethy
l
Ph
enol,
2,4
-Dim
eth
yl-
Ph
enol,
2,4
,6-T
rim
eth
yl
Ph
eno
l, 2
,3,5
-Tri
met
hy
l-
Ben
zofu
ran
, 2
,3-D
ihy
dro
-
Ph
eno
l 3
-Eth
yl-
5-M
eth
yl-
Qu
inoli
ne,
2-M
eth
yl
Ph
enol,
2-M
eth
yl-
5-(
1-M
eth
yle
thyl)
-
Ben
zen
emet
han
ol,
4-(
1,1
-Dim
eth
yle
thyl)
-
2-E
thyl-
3-M
eth
yle
ne-
Ind
an-1
-on
e
Indo
le
P-I
sop
rop
eny
lph
eno
l
Ph
eno
l, 2
-(2
-Met
hyl-
2-P
ropen
yl)
-
1(3
H)-
Isob
enzo
fura
no
ne
Ben
zen
e, H
exam
eth
yl
1H
-In
do
le,
3-M
eth
yl-
Form
amid
e, N
-Ph
enyl-
1H
-In
den
-1-o
ne,
2,3
-Dih
ydro
-3,3
-Dim
eth
yl-
1H
-In
den
-1-o
ne,
2,3
-Dih
yd
ro-3
,4,7
-Tri
met
hyl-
1-N
apth
ol-
Dim
eth
yl
2H
-In
dol-
2-o
ne,
1,3
-Dih
yd
ro-
Cy
clo
pro
pan
ol,
2,2
-Dim
eth
yl-
3-(
2-P
hen
yle
thy
nyl)
-
9H
-Xan
then
e
Nap
hth
o[2
,1-B
]Fura
n,
1,2
-Dim
eth
yl-
Ph
thal
imid
e
2,5
-Dim
eth
ylb
enzo
ph
enon
e
Ind
eno
[2,1
-C]P
yri
din
e, 1
,4,6
-Tri
emth
yl-
Acr
idin
e
1,8
,10
-Tri
emet
hylp
yra
zin
o[1
,2-A
]In
do
le
cycl
opro
pan
ol,
2,2
-Dim
eth
yl-
3-(
2-P
hen
yle
thy
nyl)
-
9-V
iny
lcarb
azo
le
Imin
ost
ilb
ene
7H
-Py
rro
lo[2
,3-H
]Qu
ino
line,
2,4
,8,9
-Tet
ram
eth
yl
1-M
eth
yl-
3-P
hen
yli
nd
ole
Acr
idin
e, 9
-Met
hyl-
7H
-Pyrr
olo
[2,3
-H]Q
uin
oli
ne,
2,4
,8,9
-Tet
ram
eth
yl-
9-V
iny
lcarb
azo
le
2-M
eth
yl-
7-P
hen
yli
nd
ole
1-M
eth
yl-
3-P
hen
yli
nd
ole
2,3
,5,7
,9-P
enta
met
hy
l-1
H-P
yrr
olo
[2,3
-F]Q
uin
oli
ne
3,3
'-D
iam
ino
dip
hen
ylm
ethan
e
Ben
z[J]
Iso
quin
oli
ne,
3,6
,8,-
Tri
met
hyl-
2-S
tilb
azo
le,
2'H
yd
roxy
dih
yd
ro-
Ben
zen
amin
e, 4
,4’-
Met
hy
len
ebis
-
3-H
yd
rox
y-3
-(1
H-I
mid
azo
l-2
-yl)
-2-P
hen
yl-
2,3
-Dih
yd
ro-I
soin
do
l
1H
-In
dole
, 2
-T-B
uty
l-3
-Ph
enyl-
Inte
gra
tio
n R
ela
tive t
o A
nil
ine
(%)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
(a)
-
22
Figure 6 Pyrolysis-GC/MS products relative to aniline, under helium at (a) 500 °C, (b) 600 °C.
0
100
200
300
400
500
600
1,3
-Cycl
op
enta
die
ne
3-V
iny
l-1-C
ycl
ob
ute
ne
Ben
zene
1-B
uta
no
l T
olu
ene
Ben
zen
e, 1
,3-D
iem
ethyl-
P
-Xyle
ne
O-X
yle
ne
Ben
zen
e, 1
,2,4
-Tri
met
hyl-
B
enze
ne,
1,2
,4-T
rim
ethyl-
B
enzo
fura
n
An
ilin
e B
enzo
fura
n, 7
-Met
hyl-
A
nil
ine,
N-M
ethyl-
P
hen
ol
Ben
zenam
ine,
3-M
eth
yl-
B
enze
nam
ine,
3-M
eth
yl-
/ P
-Am
inoto
luen
e P
hen
ol,
2-M
ethy
l P
hen
ol,
2,6
-Dim
ethy
l-
Ph
eno
l, 4
-Met
hyl-
P
hen
ol,
3-M
eth
yl-
B
enze
nam
ine,
2,4
-Dim
ethy
l-
Ben
zenam
ine,
2,6
-Dim
ethy
l-
Ben
zenam
ine,
3,4
-Dim
ethy
l-
Ben
zenam
ine,
2,5
-Die
mth
yl-
P
hen
ol,
2-E
thyl-
P
hen
ol,
2,3
-Dim
eth
yl
Ph
eno
l, 2
,4-D
imet
hyl
Ph
eno
l, 2
,5-D
imet
hyl
Phen
ol,
2,4
,6-T
rim
ethy
l P
hen
ol,
2,3
,6-T
rim
ethyl-
P
hen
ol,
4-E
thy
l
Phen
ol
3-E
thy
l-5-M
ethyl-
P
hen
ol,
2-E
thy
l-4
,5-D
imet
hy
l-
Qu
inoli
ne,
2-M
ethyl
1H
-Pyra
zole
, 4
-Phen
yl-
Q
uin
oli
ne,
7-M
ethyl-
2,5
-Die
thy
lph
enol
2-E
thyl-
3-M
eth
yle
ne-
Ind
an-1
-one
Ph
enol,
2-M
eth
yl-
5-(
1-M
eth
yle
thy
l)-
Ben
zen
emet
han
ol,
4-(
1,1
-Dim
eth
yle
thy
l)-
Indole
P
hen
ol,
2-(
2-M
ethy
l-2-P
rop
eny
l)-
1(3
H)-
Isoben
zofu
rano
ne
Ben
zen
e, 3
-Eth
yl-
1,2
,4,5
-Tet
ram
ethyl-
1
H-I
ndole
, 4-M
eth
yl-
1
H-I
ndole
, 3-M
eth
yl-
1
H-I
ndole
, 3-M
eth
yl-
1
H-I
ndole
, 2-M
eth
yl-
1
H-I
nd
en-1
-on
e, 2
,3-D
ihy
dro
-3,3
-Dim
ethyl-
1
H-I
nd
en-1
-on
e, 2
,3-D
ihy
dro
-3,3
-Dim
ethyl-
1
H-I
nden
-1-o
ne,
2,3
-Dih
yd
ro-3
,4,7
-Tri
met
hyl-
A
ceta
mid
e, N
-Ph
enyl-
F
luo
rene
2H
-In
dol-
2-o
ne,
1,3
-Dih
ydro
-
Cy
clop
rop
ano
l, 2
,2-D
imet
hy
l-3
-(2
-Ph
enyle
thyny
l)-
9H
-Xan
then
e D
iphen
yla
min
e N
aph
tho
[2,1
-B]F
ura
n, 1
,2-D
imet
hyl-
P
hth
alim
ide
Nap
hth
o[2
,1-B
]Fura
n,1
,2-D
imet
hy
l-
2,5
-Dim
eth
ylb
enzo
ph
eno
ne
Nap
haz
oli
ne
1,8
,10
-Tri
met
hy
lpy
razi
no
[1,2
-A]I
ndole
1
,8,1
0-T
rim
rth
ylp
yra
zin
o[1
,2-A
]ind
ole
A
crid
ine
1,8
,10
-Tri
emet
hy
lpy
razi
no[1
,2-A
]Ind
ole
A
crid
ine,
9-M
eth
yl-
9
-Vin
ylc
arb
azo
le
Imin
ost
ilben
e 7
H-P
yrr
olo
[2,3
-H]Q
uin
oli
ne,
2,4
,8,9
-Tet
ram
ethyl-
P
yri
din
e, 2
-(2
-(4
-Dim
ethy
lam
inop
hen
yl)
ethen
yl)
- T
ran
s 1
-Met
hy
l-3
-Ph
eny
lin
dole
9
-Vin
ylc
arb
azo
le
Acr
idin
e, 9
-Met
hyl-
2
-Met
hy
l-7
-Ph
eny
lin
dole
1
H-I
ndo
le,
1-M
eth
yl-
2-P
hen
yl-
1
-Met
hy
l-3
-Ph
eny
lin
dole
1
H-I
ndo
le,
1-M
eth
yl-
2-P
hen
yl-
2
-Met
hy
l-7
-Ph
eny
lin
dole
1
-Met
hy
l-3
-Ph
eny
lin
dole
2
,3,5
,7,9
-Pen
tam
eth
yl-
1H
-Py
rro
lo[2
,3-F
]Qu
ino
lin
e 5
H-D
iben
z[B
,E]a
zep
ine,
6,1
1-D
ihydro
- B
enz[
J]Is
oq
uin
oli
ne,
3,6
,8,-
Tri
met
hyl-
1
H-I
nd
ole
, 1
-Met
hyl-
2-P
hen
yl
Ben
z[J]
Isoq
uin
oli
ne,
3,6
,8,-
Tri
met
hyl-
1
H-I
ndo
le,
2-T
-Buty
l-3
-Phen
yl-
A
crid
ine,
9-T
-Buty
l-
Inte
gra
tio
n R
ela
tive t
o A
nil
ine
(%)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
(b)
-
23
The data for poly(BP-a) show that a number of phenolic species are produced upon pyrolysis,
supporting the view that bisphenol linkage breakdown occurs, although at higher
temperatures the species tend to occur for all of the PBZs studied and are no longer unique to
poly(BP-a). For example, acridine is a large molecule which is common to all five PBZs, but
poly(BP-a) generates 170 % relative to aniline, whereas the other four materials produce at
most only 10 % at 600 °C. The presence of the phenolphthalein moiety in the original
structure may allow the material to contribute an extra benzene ring to the reaction mixture,
which then gives rise to an increase in the larger polyaromatic pyrolysis products. At 600 °C
a variety of larger species, common to all the PBZs, are produced (Fig. 6b). Thus, the likes
of acridine, quinolone, and vinylcarbazoles suggest that the final stage of thermal degradation
involves degradation of char, which has formed from secondary reactions or recombinations.
Ishida et al.16 observed large fragments, including both rings and bridging moieties such as
C6H4NHCH2C6H4NHCH2-, from the thermal degradation of PBZ when using direct pyrolysis
mass spectrometry (DP-MS). Interestingly, no such fragments have been found in any of the
pyrolysis analyses carried out within this work. It may be that these fragments are being
produced but, owing to the less direct nature of the pyrolysis-GC/MS, secondary reactions are
taking place before they can be detected.
Fig. 7 depicts a plot of the integrated values of the pyrolysis products against the temperature
of the analysis for poly(BA-a). Here a number of molecules arising from the pyrolysis have
been selected and the trends for this material are also seen in the other PBZs. The most
striking aspect of this plot is the bimodal trend for aniline production.
-
24
This graph shows that the amount of aniline being released by poly(BA-a) increases between
200 and 250 °C and, as expected as the Mannich base structures start to breakdown at these
low temperatures, and then decrease from 250 to 350 °C. However, counter-intuitively, this
trend is then reversed from 350 to 500 °C as aniline production increases once again. The
breakdown of the bisphenol linkage to release phenolic species has been shown to occur in
this temperature range and this is supported by this graph, which confirms that the release of
2-methylphenol and 2,6-dimethylphenol both increase over this range.
Figure 7- Selected pyrolysis products of poly(BA-a) tracked through a temperature cycle (helium).
TGA data15 show that the initial release of aniline should end at around 350 °C, which
matches well with the first peak in this graph. At this stage, no definitive attribution can be
made for the presence of the second peak centred around 500 °C. Consequently, this is
0
20000000
40000000
60000000
80000000
100000000
120000000
200 250 300 350 400 450 500 550 600
Inte
gra
tio
n
Temperature of Analysis (°C)
Aniline
Aniline, N-Methyl-
Phenol
Benzenamine, 3-Methyl- / P-Aminotoluene
Phenol, 2,6-Dimethyl-
Phenol, 2-Methyl
Formamide, N-Phenyl
Acridine
-
25
attributed to the experimental technique: the extremely high heating rates are likely to
promote the growth of the first aniline peak representing degradation of the easily accessible
Mannich bridges, while the second peak represents degradation of Mannich bridges within
the bulk polymer, that become exposed following further degradation. Alternatively, this may
reflect the relative stabilities of the Mannich bridges located at the chain ends or side chains
and those present in the main chains. The same effect and trends in species are seen for all
other PBZs using this analytical technique.
Pyrolysis under Air. The Pyrolysis-GC/MS analysis previously carried out under an
atmosphere of helium was repeated in an atmosphere of air. All five PBZs were heated to
800 °C, before applying a temperature cycle. Fig. 8 shows a plot of all the pyrolysis products
relative to their respective aniline peak, from which it can be seen that a wide variety of
molecules are produced by the pyrolysis process. Furthermore, similar molecules are
generally produced by each PBZ, particularly in the case of aniline and substituted phenols.
-
26
Figure 8 - Pyrolysis products of PBZs under air when heated to 800 °C (integrated values relative to aniline).
0
20
40
60
80
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TH
AN
ON
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XIM
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Per
cen
tag
e R
ela
tiv
e to
An
ilin
e (%
)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
-
27
Some 15 molecules (substituted benzenes or polycyclics) are found to be pyrolysis products
common to all of the PBZs studied in this work (see Supplementary Fig. S6). The origin of
many of these species is easy to discern within the PBZ structure, e.g. 2-methylphenol is
likely to be produced from cleavage of Mannich bridges and bisphenol linkages. The
formation of many of these species has been discussed in detail when discussing pyrolysis
under helium. There are also larger shared molecules, which suggests that the secondary
reactions, recombinations and char formation in all of the PBZs are similar.
There are almost one hundred more molecules produced via pyrolysis under air than under
helium, but many of these compounds are produced at a very low level. This is seen in Fig. 8
which, although it has a much longer x-axis, shows that the major pyrolysis products are very
similar to those under helium. Once again, aniline dominates the plots with phenolic, aniline
and benzene derivatives and acridine and 9-vinycarbazole being the other major contributors.
In common with the analysis performed in helium, although there are differences between the
materials, these are generally more subtle than the molecules that predominate, suggesting
that the PBZ backbone has a subtle effect on thermal degradation. Butanol production is
again the obvious difference in the plots, apparent only in the pyrolysis of poly(BP-a). The
other result of note is that poly(BT-a) produces a much higher level of acridine, which is
consistently the most common of the more complex molecules with longer retention times,
than the other materials. From an integration analysis, poly(BT-a) produces almost four
times as much acridine as the next PBZ, poly(BP-a), which perhaps shows a greater ability of
poly(BT-a) to produce char material.
Temperature Cycles under Air. The same experimental protocol was applied to the
pyrolysis of the PBZs in air. Plots showing the integrated values of the products of pyrolysis
-
28
relative to aniline are presented in Figs 10a-10h. In general, the pyrolysis process seems to
proceed in much the same way as under nitrogen (previously reportedError! Bookmark not defined.)
with aniline and aniline related molecules being produced at lower temperatures before the
rise of phenolic species from 350 °C and subsequently larger aromatics like acridine are
produced above 450 °C. This suggests that the thermal degradation pathways of PBZs are
very similar in both nitrogen or air. Initially, Mannich bridge breakdown occurs, releasing
aniline, which coincides with the start of bisphenol linkage breakdown, finally followed by
products of secondary reactions or degradation of char. Under TGA conditions it has been
found that there is generally an extra degradation stage from approximately 500 °C, which is
believed to be breakdown of char with release of carbon dioxide.
-
29
0
50
100
150
200
250
1-But
anol
Pent
anal
, 3-M
ethy
l-
Tolu
ene
1-Ben
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But a
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A
ceta
mid
e, N
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yl-
-
31
0
50
100
150
200
But a
nal
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Benz
ene ,
Isoc
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nilin
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0
20
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An
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)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
(e)
-
32
0
20
40
60
80
100
120
1,3
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Rela
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nil
ine
(%)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
(f)
-
33
0
20
40
60
80
100
120
1,3
-Cy
clop
enta
die
ne
Ben
zen
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To
luen
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Ben
zen
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zene,
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zenea
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,3-D
ihyd
ro-2
-Met
hy
l-
1H
-In
do
le,
3-M
ethy
l-
1H
-In
den
-1-o
ne,
2,3
-Dih
yd
ro-3
,3-D
imet
hy
l-
1H
-In
den
-1-o
ne,
2,3
-Dih
yd
ro-3
,3-D
imet
hy
l-
1H
-In
den
-1-o
ne,
2,3
-Dih
yd
ro-3
,4,7
-Tri
met
hy
l-
Flu
ore
ne
O-C
yan
oben
zoic
Aci
d
Dib
enzo
fura
n, 4
-Met
hy
l
N-M
eth
yl-
1H
-Ben
zim
idaz
ol-
2-A
min
e
Phen
ol,
2-(
2-M
ethy
l-2
-Pro
pen
yl)
-
Ben
zen
e, H
exam
ethy
l
1H
-Ben
zotr
iazo
le,
5-M
ethy
l-
9H
-Xanth
ene
Nap
hth
o[2
,1-B
]Fu
ran
, 1,2
-Dim
ethy
l-
Acr
idin
e
9-V
inylc
arb
azole
Acr
idin
e, 9
-Met
hy
l-
Xanth
on
e
9-V
inylc
arb
azole
Acr
idin
e, 9
-Met
hy
l-
1-M
ethy
l-3
-Phenyli
nd
ole
Ben
z[J]
Isoq
uin
oli
ne,
3,6
,8,-
Tri
met
hy
l-
1H
-In
do
le, 1
-Eth
yl-
2-P
hen
yl
Acr
idin
e, 9
-T-B
uty
l-
1H
-Iso
indo
le-1
,3(2
H)-
Dio
ne,
2-P
hen
yl
Benze
nam
ine,
4,4
'-M
ethyle
neb
is-
Inte
gra
tio
n R
ela
tiv
e to
An
ilin
e (%
)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
(g)
-
34
Figure 10 - Pyrolysis-GC/MS products relative to aniline, under air at (a) 200 °C, (b) 250 °C, (c) 300 °C, (d) 350 °C, (e) 400 °C, (f) 450 °C (g) 500 °C, h 600 °C.
0
50
100
150
200
250
300
Ben
zene
1,3
,5-C
ycl
oh
epta
trie
ne/
To
luen
e
To
luen
e
P-X
yle
ne
O-X
yle
ne
1,3
,5-C
ycl
oh
epta
tien
e
Ben
zen
e, 1
,3-D
imet
hyl-
Ben
zene,
Iso
cyan
ato
Ben
zofu
ran
An
ilin
e
Ben
zofu
ran,
7-M
eth
yl-
Ben
zen
eam
ine,
N,-
Sulf
iny
l-
Ben
zene,
1-I
socy
anat
o-2
-Met
hyl-
Anil
ine,
N-m
ethyl
Phen
ol
Py
ridin
e, 2
,5-D
imet
hyl
P-A
min
oto
luen
e
Ben
zen
amin
e, 3
-Met
hy
l-
Phen
ol,
2-M
ethy
l
Ph
eno
l, 2
,6-D
imet
hyl-
Ph
enol,
3-M
ethy
l-
Ph
enol,
4-M
ethy
l-
Ben
zene,
Iso
thio
cyan
ato-
Ben
zen
amin
e, 2
,4-D
imet
hyl-
Ben
zen
amin
e, 2
,4-D
imet
hyl-
Ph
eno
l, 2
-Eth
yl
Ph
eno
l, 3
,4-D
imet
hy
l
Ph
eno
l, 2
,4-D
imet
hy
l
Ph
eno
l, 3
,4-D
imet
hyl-
Ph
eno
l, 2
-Eth
yl-
4-M
ethy
l-
Ben
zoth
iazo
le
Phen
ol,
2,4
,6-T
rim
eth
yl
Ph
eno
l, 2
-Eth
yl-
6-M
ethy
l-
Qu
inoli
ne
2-Q
uin
oli
nec
arb
oxy
lic
Aci
d
Ph
eno
l, 4
-Eth
yl
Ph
eno
l 3
-Eth
yl-
5-M
eth
yl-
1H
-Ind
ene,
2,3
-Dih
yd
ro-1
,1,5
,6-T
etra
met
hyl
Phen
ol,
4-(
1-M
eth
yle
thy
l)-
2,5
-Die
thylp
hen
ol
Ph
eno
l, 2
-Met
hy
l-5
-(1-M
ethy
leth
yl)
-
Ben
zene,
1-M
eth
ox
y-4
-Met
hy
l-2
-(1
-Met
hy
leth
yl)
-
2-E
thy
l-3
-Met
hy
len
e-In
dan
-1-o
ne
5H
-1-P
yri
din
e
Indole
1(3
H)-
Iso
ben
zofu
ranon
e
1,2
-Ben
zen
edic
arb
oxy
lic
Aci
d
Ben
zen
e, H
exam
eth
yl-
1H
-In
do
le,
3-M
ethy
l-
Ben
zofu
ran,
2,3
-Dih
yd
ro-2
-Met
hyl-
1H
-In
den
-1-o
ne,
2,3
-Dih
yd
ro-3
,3-D
imet
hy
l-
1H
-In
den
-1-o
ne,
2,3
-Dih
yd
ro-3
,3-D
imet
hy
l-
1H
-Ind
en-1
-on
e, 2
,3-D
ihy
dro
-3,4
,7-T
rim
eth
yl-
2H
-In
do
l-2
-on
e, 1
,3-D
ihy
dro
-
Flu
ore
ne
Phth
alim
ide
Dib
enzo
fura
n, 4
-Met
hyl
2H
-Ind
ol-
2-O
ne,
1,3
-Dih
ydro
-
1H
-Ben
zotr
iazo
le,
5-M
ethy
l-
Flu
ore
ne
9H
-Xan
then
e
Xan
then
e, 9
,9-D
iem
thyl-
Nap
hth
o[2
,1-B
]Fura
n,
1,2
-Dim
eth
yl-
Phth
alim
ide
Nap
hth
o[2
,1-B
]fu
ran
,1,2
-Dim
eth
yl-
1,1
'-B
iphen
yl,
3,3
',4,4
'-T
etra
met
hyl-
Acr
idin
e
9-V
inylc
arb
azole
Acr
idin
e, 9
-Met
hyl-
Xan
thon
e
1-M
ethy
l-3
-Phen
ylI
nd
ole
9-V
inylc
arb
azole
Acr
idin
e, 9
-Met
hyl-
2-M
ethy
l-7
-Phen
yli
nd
ole
1-M
ethy
l-3
-Phen
yli
nd
ole
1H
-In
do
le,
1-M
eth
yl-
2-P
hen
yl-
1H
-Ind
ole
, 1
-Met
hy
l-2-P
hen
yl
2-E
thyla
crid
ine
Ben
z[J]
Isoq
uin
oli
ne,
3,6
,8,-
Tri
met
hyl-
1H
-In
dole
, 1
-Eth
yl-
2-P
hen
yl-
Ben
z[J]
Isoq
uin
oli
ne,
3,6
,8,-
Tri
met
hyl-
1H
-In
dole
, 1
-Eth
yl-
2-P
hen
yl-
Acr
idin
e, 9
-T-B
uty
l-
Tolu
ene,
3-(
2-C
yan
o-2
-Ph
enyle
then
yl)
Ben
zam
ide,
2-M
ethyl-
N-P
hen
yl-
1H
-Iso
indo
le-1
,3(2
H)-
Dio
ne,
2-P
hen
yl
Inte
grati
on
Rel
ati
ve
to A
nil
ine (
%)
PBA-a
PBF-a
PBP-a
PBT-a
PBD-a
(h)
-
35
35
A similarity in degradation products under inert and oxidative atmospheres has been observed
before11, in fact the only difference was the production of a small amount of isocyanatobenzene
(~1%) when heated in air. Isocyanatobenzene has also been found to be produced in this work,
albeit at very low levels, which is believed to arise from the oxidation of Mannich base (Scheme 1).
Scheme 1- Oxidation of PBZ Mannich base in the presence of hydrogen bonding (redrawn from Low
and Ishida11).
The production of isocyanatobenzene is of a very low level compared with n-methylaniline and
aniline, suggesting that thermal cleavage of the Mannich base in the absence of hydrogen bonding
still predominates. When run in an atmosphere of helium only one pyrolysis product of poly(BT-a)
was found to contain sulphur, although this was a particularly complex molecule and was present at
approximately 0.3 %. However, in air three sulphurous molecules are produced: n-
sulphinylbenzenamine, isocyanatobenzene and benzothiazole. The latter are present at a low level
-
36
36
(0.5-1.5 %), but are more prevalent than in helium. Furthermore, they appear more than once
throughout the cycle, suggesting that under a reactive atmosphere the sulphur in the PBZ structure
is more likely to react and form fragments than to become incorporated in the resulting char.
n-Sulphinylbenzenamine, whose structure suggests that it is produced from a secondary reaction
between aniline- and sulphur-containing fragments following oxidation, was first observed at
400 °C, at which point phenolic species are also detected, suggesting that this is facilitated by the
breakdown of bisphenol linkages. Furthermore, there is no evidence of n-methylaniline following
degradation of PBT-a throughout the temperature cycle, despite evidence for the same molecule
arising from the remaining PBZs at multiple temperature stages. However, n-methylaniline is
produced by poly(BT-a) under helium, which suggests that its production is replaced by n-
sulphinylbenzenamine under air, for which a more complex mechanism would evidently be
involved.
The level of n-methylaniline produced by the other PBZs is much lower in air (maximum level is
8.5 %) than helium (maximum level is 33 %.) and the competitive production of isocyanatobenzene
(previously discussed) may contribute to this, although this seems to be present at too low a level to
be the major contributor. Aside from isocyanatobenzene, there doe