Studies on structurally different benzoxazines based on diphenols and diamines: Kinetics of thermal degradation and TG-FTIR studies

Article abstract of DOI:10.1016/j.tca.2015.09.006

Structurally different bisbenzoxazines (QB, RB, pHBA-pd and mHBA-pd) are synthesized using quinol, resorcinol, p-phenylenediamine and m-phenylenediamine and are thermally cured. The thermal stability of the materials was studied using TGA. The plate like

Full text of DOI:10.1016/j.tca.2015.09.006

Accepted Manuscript
Title: Studies on structurally different benzoxazines based on
diphenols and diamines: kinetics of thermal degradation and
TG-FTIR studies
Author: S. Shamim Rishwana A. Mahendran C.T.
Vijayakumar
PII:
S0040-6031(15)00362-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.tca.2015.09.006
TCA 77341
To appear in:
Thermochimica Acta
Received date:
Revised date:
Accepted date:
2-6-2015
4-8-2015
5-9-2015
Please cite this article as: S.S. Rishwana, C.T. Vijayakumar, Studies on
structurally different benzoxazines based on diphenols and diamines: kinetics
of thermal degradation and TG-FTIR studies, Thermochimica Acta (2015),
http://dx.doi.org/10.1016/j.tca.2015.09.006
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*Highlights (for review)
Highlights
1. Aromatic diphenol and diamine based bisbenzoxazines were prepared and
polymerized
2. Polybisbenzoxazines from Quinol and p-phenylenediamine results in ladder
structures
3. Meta diphenol and diamine based materials show intramolecular hydrogen
bonding
4. Para oriented materials show higher thermal stability than meta oriented
materials
5. Phenols are the major degradation products from diphenol based
polybisbenzoxazines
Page 1 of 59

Graphical Abstract (for review)
Page 2 of 59

*Manuscript
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Studies on structurally different benzoxazines based on diphenols and diamines: kinetics of
thermal degradation and TG-FTIR studies.
S. Shamim Rishwana1, A.Mahendran2, C. T. Vijayakumar3*
1Department of Chemistry, Kamaraj College of Engineering and Technology,
K. Vellakulam Post 625 701, India
2Competence Center for Wood Composites and Wood Chemistry Ltd, Area Wood Carinthian
Competence Center (W3C), Klagenfurterstrasse 87-89, St. Veitan der Glan, Austria.
3Department of Polymer Technology, Kamaraj College of Engineering and Technology,
K. Vellakulam Post 625 701, India
Paper submitted for publication in
Thermochimica Acta
*Author to whom correspondence should be addressed.
Tel.: +91 94420 57674
E-mail: ctvijay22@yahoo.com
Page 3 of 59

Abstract
Structurally different bisbenzoxazines(QB, RB, pHBA-pd and mHBA-pd) are
synthesized using quinol, resorcinol, p-phenylenediamine and m-phenylenediamine
and are thermally cured. The thermal stability of the materials was studied using
TGA. The plate like structure resulting from the interlinking of the ladders by
intermolecular hydrogen bonding provides thermal stability to poly(pHBA-pd).
The kinetics of thermal degradation is followed using Vyazovkin and Friedman
methods. The diphenol based polybisbenzoxazines show higher apparent activation
energy for thermal degradation (Ea-D) (PQB =252 to 330 kJ/mol) than the diamine
based polybisbenzoxazines (poly(pHBA-pd) = 98 to188 kJ/mol). Aromatic amines
are found to be released in considerable quantities from the diamine based
polybisbenzoxazines whereas phenolics are released from diphenol based
polybisbenzoxazines.
Keywords
Bisphenols, aromatic diamines, bisbenzoxazines, FTIR, TGA, Model free kinetics,
TG-FTIR.
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1. Introduction
Phenolic resins [1] are playing a key role on the thermosetting resin market since
they appeared in the early twentieth century. They are widely used for a variety of
applications including high performance materials for aerospace industry. Phenolic
resins possess such important advantages as good mechanical strength,
dimensional stability, chemical resistance and flame retardance. However, they are
associated with the drawbacks of traditional phenolic resins such as release of
volatiles upon curing which results in micro voids in composite matrix and need
for strong acid as a crosslinking catalyst. Thus, phenolic resins provide some
challenges for the development of the products with desirable properties and
industrial production.
Due to the reasons described above, polybisbenzoxazines, a newly developed class
of thermosetting resins, recently are getting strong attention since they combine the
advantages of phenolic resins and it is one of the very few new classes of polymers
which were successfully commercialized in the last three decades.
Benzoxazine monomers are heterocyclic compounds obtained via condensation of
an amine, formaldehyde and a phenol and were first synthesized by Holly and
Cope [2]. Benzoxazine resins [3-5]have a wide range of mechanical and physical
properties that can be tailored to various needs, reflecting the extremely rich
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molecular design flexibility. The material property balance of polybenzoxazines is
excellent. Good thermal, chemical, electrical, mechanical and physical properties
make them attractive alternatives to existing applications. Additional new
applications can be developed by utilizing unique properties of polybenzoxazines
that have not been often observed by other well known polymers. Those properties
include near-zero shrinkage, very high char yield, fast development of mechanical
properties as a function of conversion, glass transitions, much higher than curing
temperatures, excellent electrical properties and low water uptake despite having
many hydrophilic groups.
Bisbenzoxazines can be synthesized from inexpensive raw materials and
polymerized by a ring-opening addition reaction, yielding no reaction by-product.
The superb molecular design flexibility of the polybenzoxazines allows the
properties of the polymerized materials to be tailored in a wide range of properties
for the specific requirements of individual applications. Reiss et al.[6]studied the
reaction kinetics of benzoxazine oligomer formation using, mostly the mono
functional benzoxazines, showing that large molecular weight linear
polybenzoxazines cannot be obtained from mono functional benzoxazines. Several
monomers studied showed the molecular weights roughly between a few hundreds
to a few thousands upon polymerization.
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In the present investigation the authors studied the properties of different
bisbenzoxazines having similar molecular formula by choosing hydroquinone (Q),
resorcinol (R), p-phenylenediamine (PPDA) and m-phenylenediamine (MPDA) as
the reactants. The hydroquinone and resorcinol based bisbenzoxazines have been
prepared by solvent method whereas p-phenylenediamine and m-phenylene-
diamine based bisbenzoxazines are prepared by three step process [7]. The full
names and abbreviated names of the materials used are given in Table 1
Table 1
Thermogravimetric studies (TG) provide information regarding composition of
multi-component materials or blends, thermal stability, oxidative stability,
estimation of product life time, effect of reactive atmosphere on material, filler
content in materials, moisture and volatiles content and decomposition kinetics of
the polymeric materials [3]. Non-isothermal method is an easy method to derive
the degradation kinetic parameters [4] provided the TG curves recorded at different
heating rates are available. In this process, the data necessary for the kinetic
analysis can be obtained in a relatively short period of time, although getting the
kinetic parameters from these data need complicated mathematical analysis[5].
Generally different mathematical approximations developed Vyazovkin (VYZ) [8-
13] and Friedman (FRD) [14] is employed by various authors to derive kinetic
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parameters from TG data. The use of the expressions developed by Kissinger [15]
and Coats-Redfern [16] are also found in the literature.
Thermogravimetric analyzer (TGA) interfaced with Fourier transform infrared
spectrophotometer (FTIR) provide information on thermal degradation products
[17–20]. Further details about the type and functionality of the degradation
products as a function of time and/or temperature [17, 18] can be obtained from
TG-FTIR studies. The thermal degradation processes of various polybenzoxazines
were studied and the degradation mechanisms were proposed in the literature [21-
25]. In the previous investigation [26, 27], bisphenol-A bisbenzoxazine (BAB),
indane bisphenol bisbenzoxazine (IBPB) and spirobiindane bisphenol
bisbenzoxazine (SBIB) were prepared. The curing of the monomers and thermal
stabilities of the cured compounds has been studied. The presence of different
structural entities in between the benzoxazine units and the choice of the
temperature for the thermal curing have significant effects on the thermal stability
of the cross linked polymer.
The structural effects of diphenol and diamine based polybisbenzoxazine on the
thermal degradation of polybisbenzoxazines were not investigated systematically
and discussed. Hence the authors studied the thermal curing kinetics of these
compounds and reported in the previous paper [7]. The results showed that the para
substituted aromatic compounds based bisbenzoxazines show higher apparent
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activation energy for curing (Ea-C) in the initial curing stage. This reveals that, not
only the structure and molecular formula of the compound determines the
characteristics of the polymers, the nature of the oxazine ring formed will also play
a vital role in the curing of the monomers. Here in the present investigation, the
authors intended to investigate in detail the apparent activation energies for the
degradation (Ea-D) of the different polybisbenzoxazine thermosets and the results
will be discussed in light of the structure of the materials. The volatile products
formed during the thermal degradation of these compounds have also been studied
using TG-FTIR and the results are presented and discussed.
2. Experimental
2.1 Materials
Bisphenol-A and paraformaldehyde were purchased from SISCO Research
Laboratory Pvt. Ltd., Mumbai-400099. Sodium hydroxide, resorcinol, quinol,
ethanol and 1,4-dioxane were purchased from Lobachemie Pvt. Ltd, Mumbai-
400002. Sodium borohydride was purchased from S.D. fine chem Ltd., Mumbai-
400025. Aniline, m-phenylenediamine, p-phenylenediamine, 2-hydroxybenz-
aldehyde (HB), N,N’-dimethylformaldehyde (DMF), chloroform and diethylether
were purchased from MERCK Specialist Pvt. Ltd, Mumbai-400018. Aniline and
1,4-dioxane were distilled and used.
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2.2
Preparation
of
3,3′-(p-Phenylene)bis(3,4-dihydro-2H-1,3-
3,3′-(m-Phenylene)bis(3,4-dihydro-2H-1,3-
benzoxazine)(pHBA-pd)
benzoxazine) (mHBA-pd)
and
An aromatic diamine PPDA was employed as a material to form pHBA-pd by
using the 3-step process [28].
Step (I): 2-Hydroxybenzaldehyde (0.12 mol = 14.6 g) and 0.06 mol (6.5 g) of
PPDA were taken in a 250mL round bottomed flask and dissolved in 100mL of
DMF with efficient stirring. The solution was kept at room temperature and stirred
for three hours; the formed precipitate was filtered and dried for 24 h in an air oven
kept at 80 °C to obtain the first intermediate (PPDA-HB) in powder form.
Step (II): Nitrogen was introduced into a reactor to remove the humidity for 30
minutes and a balloon with hydrogen was assembled on the reactor. The first
intermediate (PPDA-HB, 0.03 mol = 9.5 g) was dissolved in 30mL of ethanol and
added to the reactor. Sodium borohydride (0.075 mol = 2.5 g) was divided into 3
batches and added into the reactant and the contents were kept at room temperature
and stirred for 10 hours. The mixture was added into water to precipitate. The
precipitate is filtered and dried for 24 h in an air oven kept at 80 °C to obtain the
second intermediate (PPDA-HB-r).
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Step (III): In a 250 mL round flask the second intermediate (PPDA-HB-r, 0.03 mol
= 9.6 g) was dissolved in 85mL of chloroform. Formaldehyde solution (0.06 mol
= 4.6 mL) was added to the above solution in drop wise and the mixture was stirred
for 4 hours at room temperature. The temperature was increased and the solution
was refluxed for 5 hours. The solution was added into the ethanol solution
(ethanol:water = 1:1) to precipitate the bisbenzoxazine (pHBA-pd) Scheme 1. The
m-phenylenediamine bisbenzoxazine (mHBA-pd) was also prepared by employing
the same procedure. The materials were dried for 24 h in an air oven kept at 80 °C.
Scheme 1
2.3 Preparation of 3,7-Diphenyl-3,4,7,8-tetrahydro-2H,6H-1,5-dioxa-3,7-diaza-
phenanthrene (RB) and 3,6-Diphenyl-2,3,4,5,6,7-hexahydro-1,8-dioxa-3,6-
diaza-phenanthrene (QB).
In a 100 mL flask, aniline (0.06 mol, 5.9 g) was dissolved in 25 mL of 1,4-dioxane
at room temperature. The solution was cooled in an ice bath, followed by the
portion wise addition of paraformaldehyde (0.12 mol, 3.8 g) with stirring for 10
min. Then quinol (0.03 mol, 3.5 g) was added to this cold solution. The
temperature was raised and refluxed for 24 h. After removing 1,4-dioxane under
vacuum, the resulting crude product was purified by dissolving in 150 mL of
diethyl ether and washing several times with 0.1N sodium hydroxide, and finally
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two times with distilled water. After drying with anhydrous sodium sulfate, the
mass was dried under vacuum at 60 °C for 24 h to afford solid [29]. The reaction is
presented in Scheme 2
Scheme 2
2.4 Thermal curing
The bisbenzoxazines, QB, RB, pHBA-pd and mHBA-pd were taken in separate
micro test tubes and flushed with dry oxygen-free-nitrogen and thermally
polymerized (QB = 210 °C, RB = 225 °C , pHBA-pd = 250 °C, and mHBA-pd =
140 °C) for 6 hours. After the polymerization, the samples were removed from the
micro test tubes, ground to coarse powder, packed and stored for further analysis.
2.5 Methods
Shimadzu (S8400) FTIR spectrophotometer was used for recording the FTIR
spectra of bisbenzoxazines and polymerized materials. The KBr disc technique was
employed.
2.6 Thermogravimetric analysis (TGA)
The degradation behavior of the polymerized bisbenzoxazines was examined using
TGA Model Q50 supplied by TA Instruments. The measurements were carried out
using approximately 4-5 mg sample in nitrogen atmosphere and the flow of
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nitrogen to the balance area was 40 mL/min and sample was swept with a nitrogen
flow of 60 mL/min and the materials were heated from 40 to 800°C using different
heating rates (5, 10, 20 and 30°C/min).
2.7 TG-FTIR
The TG-FTIR studies were carried out in the instrument TGA Q5000 V3.10 Build
258 purchased from TA Instruments employing a heating rate of 10°C/min from
ambient to 900°C under nitrogen atmosphere. The nitrogen flow was 10 mL/min.
2.8 Kinetic studies
The rate of solid-state reactions can be described as
where dα/dt is the rate of the reaction, k(T) is the rate constant, f(α) is the reaction
model[30]. According to Arrhenius’s equation, the temperature-dependent rate
constant, k(T) is defined as
where A is the pre-exponential factor, Ea is the apparent activation energy, R is the
gas constant and T is the temperature.
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The reaction extent (α) for the degradation reaction is shown in the following
equation
2.8.1 Vyazovkin method (VYZ)
According to Vyazovkin’s model free kinetic method, the activation energy can be
determined at any particular degree of conversion by finding the value of Ea in Eq.
(4) for which the objective function Ω is minimized.
with
Theory and application of model free kinetic approaches, starting from basic rate
equations and ending in activation energy prediction is discussed in the literature
[31-38]. Since every kinetic method has different error, the use of more than one
method can give a range of values for the activation energy at very particular value
of the extent of reaction (α).
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2.5.3 Friedman method (FRD)
This is one of the differential methods used to calculate Ea and the equation [14] is
given below:
From the slope (-Ea/R) of the linear plot between ln(dα/dt) vs. 1/T, the apparent
activation energy (Ea) of the system can be calculated.
3. Results and discussion
3.1 FTIR Studies
The FTIR spectra recorded for the compounds pHBA-pd, mHBA-pd, QB and RB
is presented in Fig 1. The absence of the band at 2908 cm-1 in pHBA-pd and
mHBA-pd indicates the -NH2 group present in PPDA and MPDA have been used
for the formation of oxazine rings. The condensation reaction of the phenolic
(quinol and resorcinol) compounds with aniline and paraformaldehyde resulted in
the formation of QB and RB respectively. The absence of the broad absorption
peak typical for the –OH group (3500 cm-1) in QB and RB confirms the
consumption of the –OH groups and leads to the formation of the monomer. The
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band noted at 948 cm-1 [39-42] is useful to recognize the oxazine ring structure.
This band is due to the benzene ring mode of the benzene to which oxazine is
attached. Further the presence of the oxazine rings in the prepared compounds was
supported by the appearance of new absorption bands for the oxazine asymmetric
and symmetric stretching at 1230 and 1500 cm-1 respectively. The presence of
C6H5-N group in QB and RB is confirmed by presence of an absorption band at
2908 cm-1.
Fig 1
The FTIR spectra recorded for the polymers (poly(pHBA-pd), poly(mHBA-pd),
PQB and PRB) are also presented in Fig. 1. The presence of an intense band at
3361 cm-1 indicates the formation of phenolic groups during the ring opening
polymerization of the benzoxazines. When oxazine ring opens, the band noted at
948 cm-1 disappears. It is important to note that disappearance of this mode is not
the evidence of oxazine polymerization, but it is a simple indication that the
oxazine ring opened [39]. Further, the complete disappearance of the peaks
specific for the oxazine group in the FTIR spectrum of the polymers obtained by
thermally polymerizing indicates the completion of the thermal polymerization.
Before going into further details, it is necessary to have a look at the different
structures resulting from the polymerization of QB, RB, mHBA-pd and pHBA-pd.
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Polymerization of QB results in a ladder like structure (Scheme 3) incorporating
the benzene ring in the ladder. The appropriate orientation of the phenyl groups
present in the nitrogen atoms is a must to have intermolecular hydrogen bonding
(Scheme 3) between the -OH groups present in the periphery of the ladders
resulting in a structure that favors much thermal stability.
Polymerization of RB will lead to the formation of the thermoset having the
structure shown in Scheme 4. The ring opening followed by polymerization leads
to intramolecular hydrogen bonding between the phenolic -OH group and the
nitrogen atom of the ortho placed -CH2-N(-CH2-)-C6H5 unit. Owing to this
favorable structure, the polymerization proceeds at the ortho position existing in
between the two OH groups. Comparison of the structures of PQB and PRB, one
can easily recognize the higher thermal stability of PQB.
Scheme 3
Scheme 4
The ladder structure resulting from the polymerization of pHBA-pd is shown in
Scheme 5. But the structure is much different from that of the structure noted for
PQB. Here the poles of the ladder are joined by phenyl rings and this ladder is
much flexible compared to that of the ladder resulting from the polymerization of
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QB. As in the case of PQB, the structure resulting from the polymerization of
pHBA-pd is also amenable for intermolecular hydrogen bonding (Scheme 5)
between the ladders through the favorably oriented phenolic -OH groups. Hence
poly pHBA-pd is comparatively less stable than that of PQB.
Scheme 5
Scheme 6
The structure of the polymer resulting from the polymerization of mHBA-pd is
presented in Scheme 6. As in the case of PRB, here also the intramolecular
hydrogen bonding is present. In this system, the possibilities of the attack at both
the ortho and the para positions are shown in Scheme 6. Hence the structures of
both mHBA-pd and PRB are having considerable amounts of intramolecular
hydrogen bonds whereas pHBA-pd and PQB are having intermolecular hydrogen
bonds.
The materials PQB and poly(pHBA-pd) show a band centered at 3418 and 3416
cm-1 (Fig 1) respectively and proves the existence of –OH---N intermolecular
hydrogen bonding. The polymers PRB and poly(mHBA-pd) show significant
intramolecular hydrogen bonding as demonstrated by the presence of a strong band
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at 3400 cm-1 (Fig 1). Further the presence of free hydroxyl groups (3615 cm-1) is
also noted for these materials.
3.2 TGA and DTG studies:
The TG and DTG traces for poly(pHBA-pd), poly(mHBA-pd), PQB and PRB
recorded at multiple heating rates (10, 20 and 30°C/min) in nitrogen atmosphere
are shown in Fig 2. All the thermograms are shifted to higher temperatures with
increasing heating rates. The onset, maximum, end set temperatures for the
degradation and the char residue obtained at 800 °C for all the samples noted at a
heating rate 10 °C/min are tabulated in Table 2.
Fig 2
Table 2
In the previous study [7], the authors reported that the difference in the structure of
the bisbenzoxazine influenced the curing behavior of poly(pHBA-pd),
poly(mHBA-pd), PQB and PRB as evidenced by the change noted in the apparent
activation energies for the curing of the bisbenzoxazines. Hence, it is reasonable to
expect the structural differences must influence the degradation behavior of these
materials (poly(pHBA-pd), poly(mHBA-pd), PQB and PRB).
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The detailed observations of the thermogravimetric data for the samples obtained
at a heating rate of 10 °C/min are discussed below. The thermally cured pure
poly(pHBA-pd) shows the onset degradation temperature at 331°C attains
maximum at 430°C and ends at 544°C. The DTG curves of thermally cured pure
poly(pHBA-pd) shows single degradation maximum at 430 °C. The temperature
specific for 5% mass loss for poly(pHBA-pd) is 369 °C, which is much higher than
that of PBAB. According to previous studies, the 5% mass loss temperature of
conventional polybenzoxazines rarely exceeds 350 °C and the release of aromatic
amines (e.g. aniline) is responsible for the low thermal stability of
polybenzoxazines [43-46]. Therefore, the introduction of a cross linkable site in
this aromatic amine part, such as ethylnyl [46,47], propargyl ether [48], and nitrile
group[49] has been proven to be an effective way to enhance thermal stability of
polybenzoxazines. However, an enhanced crosslinking density also indicates a
sacrificial toughness .As in Scheme 5, the nitrogen present in poly(pHBA-pd) are
bonded to the other repeating units and this makes the release of aromatic amines
difficult and causing poly(pHBA-pd) to be thermally more stable than PBAB.
The poly(mHBA-pd) shows three degradation stages and the maxima at 278, 380
and 530 °C. This may be attributed due to the release of different compounds
specifically at these stages. This material shows a very low char yield compared to
the other materials (poly(pHBA-pd), PQB and PRB). The PQB shows a single
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degradation step. The onset degradation temperature is at 268 °C, attains maximum
at 380 °C and ends at 539 °C. The PRB shows three degradation maxima at 310,
330 and 430 °C respectively. Hence the degradation of poly(mHBA-pd)and PRB
are much complex compared to poly(pHBA-pd) and PQB. Further poly(pHBA-pd)
and PQB are found to be comparatively thermally more stable than poly(mHBA-
pd)and PRB.
The high initial degradation temperature shown by the material poly(pHBA-pd) is
mainly due to the structurally favoured intermolecular hydrogen bonding resulting
in an interlinking of the ladders leading to a plate like structure. The breaking of
the hydrogen bonds followed by further chain scissions leading to the evolution of
volatiles is difficult when compared to the intermolecular hydrogen bonding that is
present in the case of PQB. Due this fact, the initial degradation temperature of
PQB is low. But comparison of the char yield (Table 2) showed that the compound
PQB yields slightly higher char than poly(pHBA-pd). This may be due to the
presence of much flexible ladder structure present in poly(pHBA-pd), a situation
which favours easy release of volatiles.
3.3 Degradation Kinetic Studies
The apparent activation energy for the thermal degradation of thermally cured
materials, poly(pHBA-pd), poly(mHBA-pd), PQB and PRB, were calculated using
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three isoconversional model free kinetic methods such as VYZ, and FRD methods.
The plots between the apparent activation energy(Ea-D) for thermal degradation
and the reaction extent (α) values for all the samples investigated using VYZ, and
FRD method are shown in Fig 3.
Fig 3
The trends noted in the variation of apparent activation energy for the degradation
of thermally cured samples obtained by the three kinetic methods are same and the
values are in good agreement. The slight deviation noted in the apparent activation
energy values between these methods is due to the different approximation used in
the kinetic methods. The apparent activation energies for thermal degradation
obtained using VYZ method for thermally cured poly(pHBA-pd), poly(mHBA-pd),
PQB and PRB is considered for the discussion. The apparent activation energy for
the degradation of poly(pHBA-pd)increases progressively from 98 to188 kJ/mol.
This implies that the degradation of the poly(pHBA-pd) follows a similar
mechanism throughout the thermal degradation. The apparent activation energy for
the degradation of poly(mHBA-pd) varies from 92 to 308 kJ/mol. Initially both the
compound (poly(pHBA-pd) and poly(mHBA-pd)) shows approximately same
activation energies (α = 0.6) and then the poly(mHBA-pd) shows a steep increase
in the apparent activation energy. This may be attributed to the operation of
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different degradation mechanism leading to the formation different products. This
is reflected in the TG curve of poly(mHBA-pd), which more complex compared to
the TG curve of poly(pHBA-pd). Similar explanation is given by Lee et al [50] in
estimation of cure and thermal degradation kinetics of epoxy organoclay
nanocomposite. When the selected conversion became higher, the apparent
activation energy increased, because as thermal degradation was proceeding, the
molecules easily broken from the epoxy network were volatile at low temperature,
however with the increasing degradation conversion, the stable residue that is
present needs much higher temperature for further degradation. The average
activation energy for the thermal degradation was 171 kJ/mol.
In the case of PQB, the apparent activation energy values vary from 252 to 330
kJ/mol (α= 0.1–0.9). Initially α value decreased upto the reaction extent level of
0.3. After reaction extent of 0.3, the Ea-D value increased steadily for higher
values of α. The initial decrease in Ea-D values was attributed to the existence of
weak points in the polymer chains whereas the higher Ea-D values at the later
stages of degradation was associated with high degrees of random scission of the
main chain. Krongauz [51] reported similar results for the crosslink density
dependence of polymer degradation kinetics in photo crosslinked acrylates. The
rotation of polymer segments is also consistent with low apparent activation energy
of polyacrylate degradation and with an increase in activation energy with increase
Page 23 of 59

in crosslink density. Change from degradation due to rotation of polymer segments
at low temperature to degradation through direct bond scission at higher
temperatures may also explain increase in apparent activation energy with
temperature.
Pitchaimari et al [52] reported similar results in studies on thermal degradation
kinetics of thermal and UV cured N-(4-hydroxy phenyl) maleimide derivatives.
The apparent activation energy values for the degradation of poly (N-(4-
methacryloyloxyphenyl) maleimide (MAX) with N-vinyl-2-pyrrolidone (NVP)) as
a comonomer (TDMAX) increased from 84 to 156 kJ/mol gradually with
increasing extent of reaction (α= 0.1–0.9). The initial lower value of the apparent
activation energy is most likely associated with the initiation process that occurs at
these weak links. As these weak links are consumed, the limiting step of
degradation shifts toward the degradation initiated by random scission, which
typically has greater activation energy. The very high values of Ea-D calculated at
large extents of degradation explain the large amount of solid residue remained. An
increase of Ea-D with α has been also reported in literature for other types of
polymers [53,54]. Achilias et al. [55] report similar result for the thermal
degradation of light cured bisphenol-A glycidyldimethacrylate (Bis-GMA),
bisphenol-A ethoxylated dimethacrylate (Bis-EMA), urethane dimethacrylate
(UDMA) and triethylene glycol dimethacrylate (TEGDMA).
Page 24 of 59

Initially for PRB, the apparent activation energy value increases with (Ea-D)
increasing reaction extent values (α). After that the Ea-D values shows a
continuous decrease for further reaction extent levels. This may be due to the
difference in the structure of the net work formed and the way in which the
degradation is initiated and propagated. Further as in the case of diamine based
polybisbenzoxazines, in the diphenol based polybisbenzoxazines, the meta
substituted system is showing much complexity compared to the para substituted
system.
According to the statistical procedure proposed by Vyazovkin and Wight [8] the
realistic confidence intervals for the activation energy determined are estimated for
all the cases. The estimated activation energy and the Fisher confidence intervals
for the three heating rates (10, 20, and 30 ºC/min) for the four samples are
presented in Fig. 4.
Fig 4
3.4 TG-FTIR-Studies
The combination technique TG-FTIR provides information on neither the exact
structure of all the decomposition products nor the separation of different
decomposition products coming off at the same time. The thermal degradation
Page 25 of 59

processes of various polybenzoxazines were studied and the degradation
mechanisms were proposed in the literature [21-24]. The structural effects of
phenols and amines on the thermal degradation of polybenzoxazines were
investigated systematically [56, 57].
The TG-FTIR studies of diphenol and diamine based polybisbenzoxazines have
been carried out and the results are presented in Fig. 5 and 6 respectively.
Fig 5
Fig 6
The FTIR spectra shown in the Fig. 5 and 6 are for the mixture of products
resulting from the thermal degradation of the chosen compounds.
The FTIR spectra of the volatiles formed from the polymers at the temperature
where the rate of degradation is maximum are shown in Fig. 7.
Fig 7
The initial degradation for PQB starts around 200 °C and the degradation
maximum was found to be at around 400 °C. In the initial stage of degradation,
aromatic amine was the volatile product that has been released. Any two bond
cleavages are sufficient to produce aromatic amines and N-alkyl substituted amines
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from PQB (Scheme 7). The cleavage of CH2-N bond is necessary for the release of
amines from degrading polybisbenzoxazines. The breakage of Ar-CH2 bond is
essential for the formation of substituted amines.
Scheme 7
The C–N bond dissociation energy (bond energy = 72 Kcal/mol) [58] is lower than
the C–C bond dissociation energy (bond energy = 82.6 Kcal/mol). Hence it is
reasonable to expect the initial degradation at the CH2-N bond. A Schiff base
(C=N) is formed when the Mannich base is cleaved. So, the next likely cleavage
point is the C–C bond as the C–C bond (bond energy = 82.6 Kcal/mol) has a much
lower bond energy than a C=N bond (bond energy = 147 Kcal/mol). In the
bisphenol based polybisbenzoxazine structure, there are two equivalents C–N
bonds that have equal probability of thermal cleavage. Aromatic amine is evolved
as a degradation product which is favored at lower temperatures. A small
-1
absorption band observed at 3700 cm (Fig. 7) in the evolved gas proves in the
initial stage of degradation proves the foregoing argument.
The degradation products of PQB and PRB are mainly various types of aromatic
Schiff bases and substituted phenolic compounds, depending on the corresponding
benzoxazine monomer structure. In addition, substituted benzene modes at1600,
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1480, and 747 cm-1 were also detected in the FTIR spectra. These findings
suggested that substituted phenols are the degradation products at this stage.
Formation of compounds having strong absorption band at 1670 cm -1 was detected
at around 380 °C for PQB and PRB. This may be attributed to the evolution of
Schiff bases from the degrading PQB and PRB. The possible degradation
mechanism of this compound is shown in the Scheme 7.
The formation of phenols from PQB needs a minimum of four bond cleavages. At
around 430 °C, a significant band at 3050 cm-1 was detected from degrading PQB
(Fig. 7). This band is due to the free OH group of phenol and it is accompanied by
-1
a band at 1180 cm , which is due to the C–O bond of phenol or substituted
phenols. Hence the primary or the initial degradation of PQB and PRB is the
formation of volatile aromatic amines followed by the evolution of phenolic
compounds and this represents the temperature of the maximum rate of weight loss
in the TG curves of both PRB and PQB. Thus, the degradation observed at higher
temperatures in TG curves can be assigned to the evolution of phenolic compounds
from the diphenol based polybisbenzoxazines.
Based on the TGA and FTIR evolved gas analyses (EGA), the thermal degradation
mechanism of poly(pHBA-pd)have been proposed and is presented in Scheme 8.
Scheme 8
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From the Fig. 7 one can conclude that aromatic amine and substituted aromatic
amines were the major degradation products at the degradation stage where the rate
degradation is found to be the maximum. The Schiff base was also detected as a
degradation product from this polybisbenzoxazine at around 380 and 430 °C, for
poly(mHBA-pd) and poly(pHBA-pd).The appearance of a band at 3650 cm-1 was
significant. This is mainly attributed to the evolution of aromatic diamines. This
temperature corresponds to the temperature of the maximum rate of weight loss in
the TGA curves of poly(pHBA-pd) and poly(mHBA-pd).
4. Conclusion
It was found that the orientation of the polymerizable groups is having significant
influence on the thermal degradation parameters. The material PQB (58%) shows
relatively higher char value compared to materials poly(mHBA-pd) (34%),
poly(pHBA-pd) (47%) and PRB( 50%). The materials PQB and PRB show higher
apparent activation energies for the thermal degradation process (Ea-D) (252 and
196 kJ/mol) when compared to poly(pHBA-pd) and poly(mHBA-pd) (98 and 95
kJ/mol) at the initial degradation stage. Aromatic amines are found to be released
in considerable quantities from the diamine based polybisbenzoxazines whereas
phenolic compounds are released from diphenol based polybisbenzoxazines. The
thermal stability and the products formed during the thermal degradation may be
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