Curing kinetics and thermal properties of imide containing phthalonitrile resin using aromatic amines

Article abstract of DOI:10.1016/j.tca.2020.178749

Due to their outstanding properties, phthalonitrile based polymers plays an important role in high performance advanced materials evaluated depending upon their curing process and mechanism. Henceforth it is necessary to understand the curing kinetics of

Full text of DOI:10.1016/j.tca.2020.178749

ThermochimicaActa693(2020)178749
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Thermochimica Acta
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Curing kinetics and thermal properties of imide containing phthalonitrile
resin using aromatic amines
Devendra Kumara,b, Veena Choudharya,*
a Department of Materials Science and Engineering, Indian Institute of Technology, Hauz Khas, New Delhi, 110016, India
b Department of Chemistry, D.S. College, Aligarh, Uttar Pradesh, 202001, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Due to their outstanding properties, phthalonitrile based polymers plays an important role in high performance
advanced materials evaluated depending upon their curing process and mechanism. Henceforth it is necessary to
understand the curing kinetics of bisphthalonitrile monomer having imide linkage using 4, 4’-diaminodiphe-
nylsulfone (DDS) as curing agent for further investigations. Thus, curing kinetics was investigated by recording
non-isothermal differential scanning calorimetric (DSC) scans at heating rates of 5 °C, 10 °C, 15 °C and 20 °C/
min. Activation energy for bisphthalonitrile/4, 4’-diaminodiphenylsulfone (DDS) system was calculated using
Starink method and results give a substantial evidence that Arrhenius activation energy (Ea) are certainly de-
pendent on the structure of monomer and curing agent. The cured FIPN/DDS, ODPA-PN/ DDS systems show
comparable high thermal stability and elevated glass transition temperature as compared to Bis-ADPN/ DDS
composition. Subsequently, this composition exhibits comparative heat resistance and improved moisture re-
sistance.
Phthalonitrile
Curing kinetics
Thermal properties
1. Introduction
process-ability. So the perfect use of phthalonitrile resins is to under-
stand and control the curing process. Hence it is essential to choose the
Phthalonitrile resins are an accomplished class of high performance
thermosetting materials with wide range of potential applications [1,2]
for being utilized in diversified industries such as aerospace, ships and
microelectronics [3]. Phthalonitrile polymers have superior thermal
and thermo-oxidative stability, good mechanical properties, low water
absorption, corrosion resistance, fire resistance properties [4] and
negligible presence of Tg before the thermal decomposition tempera-
ture. Due to these characteristics, phthalonitrile have been used as
potential matrices for advanced composites [5], adhesive, carbon pre-
cursor for demanding applications [4,6–8].
correct process conditions and parameters to obtain the desired prop-
erties. The curing process for phthalonitrile resins is complicated since
it leads to various processes together [16]. The resulting properties of
cured bisphthalonitrile polymers are highly dependent on the curing
kinetic parameters of polymerization determined with the extent of
curing degree, the curing conditions and curing rate [10,16]. Therefore,
there is a need to understand the curing kinetics of phthalonitrile resins
to get the desired structure for improved performance.
Diversified characterization have been reported to explore the
curing kinetics of thermosetting resins such as differential scanning
calorimetry (DSC) [1,7,16,17], Fourier Transform Infrared Spectro-
scopy (FTIR) [18] and rheo-kinetic measurements [6]. Out of all, DSC is
the least difficult and most utilized method to determine the curing
kinetic parameters for the polymerization of phthalonitrile resins, since
it is easy to operate, requires minimum amount of samples (approxi-
mately 5–10 mg) and kinetic data can be obtained in a relatively short
period of time. Henceforth, the obtained kinetic parameters by dynamic
DSC scans were labeled to agree well by other techniques [1,16,19].
Non-isothermal DSC scans has been accomplished at different heating
rates to calculate the curing kinetic parameters. Since in a short period
of time the kinetics data can be achieved which makes the method more
Previous several works on phthalonitrile polymers have been fo-
cused on synthesis and curing of bisphthalonitrile with/without dif-
ferent curing agent, self-catalyzed phthalonitrile monomers [3,6,7] and
phthalonitrile based composites [8,9]. Nevertheless, the curing of
phthalonitrile monomer is slow at temperature in excess of 300 °C,
which limited their use for further applications [10]. To solve this
problem, lot of appreciable research has been published on the devel-
opment for efficient additions to promote the curing rate, such as salts
of strong organic acids/ amines, metallic salts, active hydrogen con-
taining heterocyclic compound have been used [4,11–15].Other re-
quirements of phthalonitrile resins are the ease of polymerization and
⁎ Corresponding author.
E-mail addresses: veenac@polymers.iitd.ac.in, veenach@hotmail.com (V. Choudhary).
https://doi.org/10.1016/j.tca.2020.178749
Received 2 February 2020; Received in revised form 6 August 2020; Accepted 7 August 2020
Availableonline23August2020
0040-6031/©2020ElsevierB.V.Allrightsreserved.

D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
demanding in nature [7,19–22].
In the present work, 4,4’-oxydiphthalic anhydride (ODPA) based
imide
containing
bisphthalonitrile
(ODPA-PN),
4,4’-(hexa-
fluoroisopropylidene) diphthalic anhydride (6FDA) based imide con-
taining bisphthalonitrile (FIPN) and bisphenol-A dianhydride based
imide containing bisphthalonitrile (Bis-ADPN) were synthesized and
characterized using FT-IR,1H-NMR,13C-NMR. The curing behavior of
bisphthalonitrile (ODPA-PN, Bis-ADPN, FIPN) using 4,4’-diaminodi-
phenylsulfone (DDS) as curing agent was evaluated by recording DSC
scans at varying heating rate. The addition of DDS as a curing agent to
Bis-ADPN, ODPA-PN and FIPN increased the enthalpy of reaction and
activation energy of the system which helps to improve the quality of
cured product (Bis-ADPN/DDS, ODPA-PN/DDS and FIPN/DDS). The
kinetic parameters were calculated using Starink method. The study of
curing kinetics of a phthalonitrile resin has significant importance to
optimize the curing cycles in manufacturing process to have an im-
proved thermal property of final product.The thermal stability of finally
cured resins was investigated by thermo-gravimetric analysis and the
char yield was used to evaluate the flaming characteristics. The water
uptake tests were performed to study the % water uptake in boiling
water over 24 h.
Fig. 1. 1H-NMR spectrum of Bis-ADPN in DMSO-d6.
and then the solution of 4-(4-aminophenoxy) phthalonitrile (4.7 wt% in
dry DMF, 4.7 g, 0.02 mol) was added drop wise. After complete addi-
tion, the reaction mixture was stirred for 1 h at 90 °C followed by ad-
dition of toluene. The solution was then heated to reflux for 12 h in
nitrogen atmosphere. The water as a byproduct was removed azeo-
tropically over a period of 12 h. After complete removal of water, to-
luene was distilled under reduced pressure. The product was obtained
by precipitation using ethanol as non-solvent. The resulting light
creamy precipitates were collected by filtration, washed thoroughly
with water and until the filtrate was neutral. The precipitate thus ob-
tained was then dried in vacuum oven at 80 °C. Melting point = 223 °C,
(Fig. 6A DSC scan) yield = 78 %.
2. Experimental
2.1. Monomer synthesis
2.1.1. Synthesis of 4-(4-aminophenoxy) phthalonitrile
4-(4-aminophenoxy) phthalonitrile [APPH] was synthesized ac-
cording to the procedure reportedelsewhere [3].
2.1.2. Procedure for synthesis of bisphthalonitrile monomers
Phthalonitrile monomers were synthesized by reacting APPH with
corresponding dianhydride according to reaction Scheme 1.
2.1.3. Characterization of Bis-ADPN
1H-NMR (400 MHz, DMSO-d6), δ (ppm): 1.63 (s, 6 H), 7.06 (d, 4 H),
7.26 (d, 8 H), 7.31(d, 4 H), 7.35 (s, 2 H), 7.36 (d, 2 H), 7.44 (d, 2 H),
7.46 (d, 2 H), 7.78 (s, 2 H), 7.79 (d, 2 H) [Fig. 1].
13C-NMR (400 MHz, DMSO-d6), δ (ppm): 31.20, 36.21, 109.14,
112.01, 115.81, 116.31, 117.26, 120.20, 120.89, 122.99, 123.77,
125.76, 126.39, 129.09, 129.52, 129.82, 134.70, 136.86, 147.57,
153.03, 153.84, 160.99, 163.23, 163.53 (Fig. 2).
(i) Synthesis of FDA-based phthalonitrile monomer (FIPN) [3]
Synthesis and characterization of phthalonitrile monomer FIPN
(synthesized from 4, 4’-(hexafluoroisopropylidene) diphthalic anhy-
dride and APPH) has been reported earlier [3] (yield 78.72 %; melting
point 143.8 °C).
FT-IR (KBr, cm−1): 3076, 2970 (aromatic CHe stretching), 2878,
2924 (aliphatic CeH stretching) 1774, 1727 (C]O stretching in imide
group), 2232 (CN^ stretching vibration in nitrile), 1248 and 1082
(COC stretching), 1375 and 730 (CNCeeee stretching and bending in
imide group) [Fig. 5].
(ii) Synthesis of Bis-phenol-A dianhydride (BPADA)based imide con-
taining bisphthalonitrile (Bis-ADPN)
BPADA (5.20 g, 0.01 mol) and DMF (100 mL) was added to a 500
mLthree-necked round bottom flask equipped with a dean stark appa-
ratus and reflux condenser. The solution was stirred on magnetic stirrer
at 70 °C in nitrogen atmosphere until the BPADA dissolved completely
(iii) Synthesis of ODPA based imide containing bisphthalonitrile
(ODPA-PN)
Scheme 1. Synthesis of FIPN, ODPA-PN and Bis-ADPN monomers.
Fig. 2. 13C-NMR spectrum of Bis-ADPN in DMSO-d6.
2

D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
Fig. 3. 1H-NMR spectrum of ODPA-PN in DMSO-d6.
Fig. 5. FT-IR spectra of (a) APPH, (b) Bis-ADPN, (c) FIPN, and (d) ODPA-PN.
Monomer ODPA-PN was synthesized using appropriate reagents
adopting similar procedure as given above for Bis-ADPN.
Melting point = 249.5 °C, yield = 84 %.
2.1.4. Characterization of ODPA-PN
1H-NMR (400 MHz, DMSO-d6), δ (ppm): 7.35 (d, 4 H), 7.44 (d, 2 H),
7.46 (s, 2 H), 7.86 (s, 2 H), 8.06 (d, 2 H), 8.11 (d, 2 H) [Fig. 3].
13C-NMR (400 MHz, DMSO-d6), δ (ppm): 109.11, 114.18, 115.02,
117.25, 120.90, 122.97, 123.64, 125.52, 126.65, 127.87, 134.84,
136.89, 153.80, 160.96, 166.69 (Fig. 4).
FT-IR (KBr, cm−1): 3076 (aromatic CHe stretching), 1780, 1714
(C]O stretching in imide group), 2232 (stretching vibration of nitrile
group), 1248, 1089 (CeOCe stretching), 1375 and 716 (CeNCe
stretching and bending in imide group) [Fig. 5].
2.1.5. Preparation of the monomers/aromatic diamine blends
Monomer/ aromatic diamine (90:10 w/w) blends were prepared by
mixing solution of appropriate monomer with DDS using acetone as
solvent. The mixture was stirred at room temperature using magnetic
stirrer for 3 h to get a homogenous mixture. Acetone was then removed
using rotary evaporator at 50 °C.The powder then obtained was used for
sequential kinetic studies and curing studies.
2.1.6. Isothermal curing and casting of monomers/aromatic diamines
mixture
The bisphthalonitrile monomers /DDS prepolymer were prepared
by mixing synthesized bisphthalonitrilemonomers: DDS (90:10 w/w)
above melting temperature of monomers/ aromatic amine system under
Fig. 6. (A) DSC scans of(a) Bis-ADPN, (b) ODPA-PN and (c) FIPN monomers
and (B) DSC scans of monomers in the presence of DDS before isothermal
curing.
vigorous mechanical stirring until the formation of homogeneous
mixture. The mixture was poured directly in to preheated alumina
crucible and then crucible was transferred into a muffle furnace at 100
°C to drive off entrapped bubbles. The isothermal curing of bisphtha-
lonitrile/DDS mixture was completed by heating in a muffle furnace in
nitrogen atmosphere at (a)280 °C/6 h; (b) 280 °C/6 h +350 °C/6 h; (c)
280 °C/6 h +350 °C/6 h +400 °C/4 h [3].
Fig. 4. 13C-NMR spectrum of OPDA-PN in DMSO-d6.
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D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
3. Results and discussion
due to either the system becomes less reactive or less homogeneous at
higher heating rates. Similar behavior has also been reported by
Domínguez et al. in the study of non-isothermal curing kinetics of a
polyfurfuryl alcohol bio-resin [23].
The FIPN, Bis-ADPN and ODPA-PN monomers were synthesized
according to reaction Scheme 1 using the procedure reported elsewhere
[3]. Figs. 1–4 represents the 1H-NMR and 13C-NMR of Bis-ADPN and
ODPA-PN. In the 1H-NMR spectra of ODPA-PN and Bis-ADPN aromatic
proton detected at the range of 7.06–8.11 ppm. All these structure does
not show the amino proton at around 5.50 ppm, confirming the com-
pletion of reaction. Furthermore, in the 13C-NMR spectra, the signals at
109.14–166.69 ppm are assigned to the aromatic carbon and nitrile
carbon. The chemical shifts at 31.20 and 36.21are denotedto qua-
ternary carbon atoms in the isopropyl group and resonance at 115.60
ppm arising due to CN carbon signal. Therefore, the obtained spectra
confirmed well as expected structure of monomers.
The curing enthalpy was dependent on the reactivity of monomer
and DDS(reaction between amine and nitrile group) and it was lowest
in case of Bis-ADPN/DDS mixture. This could be due to the higher
equivalent weight of Bis-ADPN as compared to ODPA-PN and FIPN
monomers.
3.2. Curing kinetics theory
The extent of conversion (degree of cure), α at any particular
temperature (Tα) was determined as the ratio of the area under the
exothermic peak in DSC and depicted as:
The structure of monomers was also confirmed by FT-IR spectra as
depicted in Fig. 5. The strong absorption band at 2232 cm−1 (Fig. 5)
corresponding to the cyano group stretching vibration. Appreance of
peaks at 3076 and 2970 cm−1 are due to the aromatic CHe stretching.
In addition, new absorption bands at 1774 and 1727 cm−1 is assigned
HT
Htotal
=
(1)
Where HT the enthalpy of partially cured samples (reaction up to time
due to asymmetric and symmetric vibration of C]O stretching of imide
Tα) and Htotal is the total enthalpy of curing. In non-isothermal system
−1
group correspondingly and peak located at 1248 and 1082 cm
ap-
proved the COCee stretching vibration in aromatic region of mono-
mers. The characteristic stretching vibration of amino (-NH2) com-
pletely disappears in IR spectrum, indicating the successful synthesis of
imide containing FDA, ODPA and Bis-A dianhydride based bisphtha-
lonitrile monomer.
HT
Htotal dt
d
dt
d
dT
=
=
(2)
Where = dT/dt
Kinetics of curing reaction of thermosetting resins is generally ex-
plained by a single step reaction, which presents the relationship be-
tween reaction rate and temperature, T; extent of conversion, α,
[24,25].The curing reaction rate dα/dt is generally expressed as Eq. (3)
3.1. Curing behavior of FIPN, ODPA-PN and Bis-ADPN
The effect of monomer structure (FIPN, Bis-ADPN and ODPA-PN) on
curing behavior using 4, 4’-diaminodiphenyl sulfone in ratio of
monomer: DDS (90:10) was evaluated using DSC. Fig. 6A shows the
DSC scans of FIPN, Bis-ADPN and ODPA-PN monomers at a heating rate
of 10 °C/min in nitrogen atmosphere. An endothermic transition due to
melting was observed with a peak temperature at 144, 223.0 and 249 °C
for FIPN, Bis-ADPN and ODPA-PN respectively. In the DSC scans of pure
monomers, exothermic peak was not observed. This could be due to
very slow and sluggish reaction of neat monomers which did not show
any exothermic transition [3,10]. However, in the presence of DDS, all
the monomers showed an endothermic transition (melting point) as
well as exothermic transition (curing) in the DSC scans of monomer:
amine (90:10).From these studies, it can be concluded that all three Bis-
ADPN, ODPA-PN,FIPN systems can be cured using DDS as curing agent.
. Non-isothermal DSC was performed to understand the curing be-
havior and curing kineticsof bisphthalonitrile/aromatic diamine at
heating rate of 5 °C, 10 °C, 15 °C and 20 °C/min. The DSC scans of
samples (a) Bis-ADPN/DDS (b) ODPA-PN/DDS and (c) FIPN/DDS are
shown in Fig. 7. From these plots, the curing exotherm was analyzed by
noting the following temperatures:
d
dt
= k(T)f( )
(3)
where α is the degree of conversion/curing; t is the time; k (T) is the
rate constant which depends on the temperature; and f (α) is the pro-
cess mechanism function. The function of k (T) can be described by the
Arrhenius equation [25].
Ea
RT
K(T) = Aexp
(4)
where Ea is activation energy and A are pre-exponential factor and
activation energy for curing, respectively, R is the universal gas con-
stant and T is the absolute temperature.
The kinetic equation for non-isothermal curing can be described as:
d
dt
d
dT
Ea
RT
=
= Aexp
f( )
(5)
Frequently used method for analyzing the curing activation energy
) are Kissinger, Ozawa and Friedman methods. All methods are
(
E
based on the fact that the reaction rate depends on the temperature at
constant extent of curing [25,26]. To determine the activation energy of
curing for phthalonitrile/aromatic diamines, several approaches have
been applied to evaluate the kinetics ofphthalonitrile resins [1,6,26].
According to ICTAC (International confederation for thermal ana-
lysis and calorimetry) kinetic project, [24,27] use of Starink [28,29]
(Eq. (6)) method is suggested due to simplicity and accuracy of acti-
vation energy.
Initial temperature where the first heat evolved was detected (Ti),
•
Onset temperature (Tonset)was obtained by drawing a tangent to the in-
•
itial portion of exotherm
Peak exotherm temperature (Tp),
•
End temperature (Tend).
•
The results of curing exotherm are summarized in Table 1. Peak
exothermic temperature gradually increased and peak become sharp as
we increase the heating rate for recording DSC scans [10]. The exo-
thermic enthalpy (ΔH) gathered in Table 1 was calculated from the area
under the exothermic peak, expressing the total heat of curing of
samples. For all the three samples, the fastest heating rate (15 and 20
°C/min.) show much lower heat of curing than the lowest heating rate
(5 °C/min). It is expected that ΔH should be dependent on the structure
of monomer, curing agent and its amount. However we observed a
significant decrease in ΔH with increasing heating rate. This could be
Ea
RT ,i
ln
= Constant 1.0008
T1.92
,i
(6)
Where
is the heating rate and T ,i is the temperature at which the
degree of conversion; α is reached at a given ith heating rate.
The various curing mechanisms are of different systems. According
to previous work, there are two usual models for cuing reaction me-
chanism; autocatalytic model and nth- order model [28,29]. The
equations are as shown in Eqs. (7) and (8)
4

D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
Fig. 7. Dynamic DSC scans for bisphthalonitrile monomers/DDS (90/10) mixture at heating rates of 5 °C, 10 °C, 15 °C and 20 °C under nitrogen atmosphere (a) Bis-
ADPN/DDS, (b) ODPA-PN/DDS and (c) FIPN/DDS.
calculated by Starink method. For the evaluation of activation energy
by the Starink method, ln(β/Tα1.92) vs. 1000/Tα was plotted for
bisphthalonitrile/DDS samples in the range of 0.05−0.95 conversion
with a step of 0.05. Fig. 9 shows Starink plots of bisphthalonitrile/ DDS
mixture in the range of 0.05−0.95 conversion. In this figure, due to the
high R2value the plotted data was accurately fit to the line, which
proves that Starink method is suitable for the present studies.
The activation energy observed from slopes of various heating rate
values for α = 0.05−0.95 displayed a little difference, and the average
value of Bis-ADPN/ DDS, FIPN/DDS and ODPA-PN/DDS systems are 68,
80, and 85 KJ/mol, respectively. From the calculated results, we ob-
served that the values of average apparent activation energy iscom-
parable to the values reported earlier (87–105 KJ mol−1)[7, 16, and
28].
Table 1
Curing characteristics of bisphthalonitrile monomers using DDS as curing agent.
Sample
Heating rate
Ti
Tonset
Tp
Tend
ΔH
(°C/min.)
(°C)
(°C)
(°C)
(°C)
(J/g)
● Bis-ADPN/DDS
5
244
264
269
280
239
242
249
258
222
238
242
251
258
280
294
302
244
258
268
275
241
258
270
275
266
289
305
315
252
268
279
288
251
270
283
291
313
330
340
355
295
323
337
342
320
335
337
342
53.8
40.7
35.4
26.3
69.5
65.5
55.6
48.2
97.2
81.0
75.5
62.8
●
10
15
20
5
●
●
● ODPA-PN /DDS
●
10
15
20
5
●
●
● FIPN/DDS
●
●
●
10
15
20
In Fig. 10, we observe the small change in activation energy at the
middle stage of curing (between 20–70 % conversion). It has been
found that activation energy of Bis-ADPN/ DDS decreased from 78 to 68
KJmol−1 before 30 % conversion and after that remains constant. The
activation energy for ODPA-PN/DDS samples also show the same trend
by which activation energy decreased from 103 to 86 KJmol−1, while
the activation energy of FIPN/DDS show a small change before 70 %
conversion, which could be due to the change of curing extent with the
mobility of reacting molecule. But the activation energy of FIPN/DDS
after 70 % conversion suddenly increased with low degree of curing.
The average activation energy of different samples (Bis-ADPN/DDS,
FIPN/DDS and ODPA-PN/DDS) blends in the conversion range
0.05−0.95 was found to be dependent on structure of monomer and
minimum in case of Bis-ADPN/DDS and maximum in case of ODPA-PN/
DDS. It is worth noting that, in the higher conversion region increase in
activation energy was observed for FIPN/DDS and decrease is observed
for ODPA-PN/DDS. This could be due to decrease in free volume and
the chain segment movement at higher percent conversion, and hence
● Initial temperature where first heat evolved was detected (Ti).
● Onset temperature (Tonset) obtained by drawing tangent to the initial portion
of exotherm.
● Peak exotherm temperature (Tp).
● End temperature (Tend).
d
dt
Ea
RT
n
= Aexp
= Aexp
m(1
)
(Autocatalytic model)
(7)
(8)
d
dt
Ea
RT
n
(1
)
(nth order model)
Where, m and n are an order of reaction, m + n is the total order.
As shown in Fig. 8, the degree of conversion increases with tem-
perature for a Bis-ADPN/DDS, ODPA-PN/DDS and FIPN/DDS for par-
ticular heating rate (5oC, 10oC, 15oC and 20 oC/ min).
As explained in kinetic theory section, the activation energy of
bisphthalonitrile monomers in presence of 10 wt. % DDS were
5

D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
Fig. 8. Plots of conversion(α) as a function of temperature for (a) Bis-ADPN/DDS, (b) ODPA-PN/DDS, and (c) FIPN/DDS blends.
Fig. 9. Starink method plot at degree of conversion between 0.05 and 0.95 for (a) Bis-ADPN/DDS (b) ODPA-PN/DDS and (c) FIPN/DDS.
6

D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
rearranging and logarithms in both sides of Eq. (7).
d
dt
d
dT
Ea
RT
ln
= ln
= lnA
+ mln + n(1
)
(9)
From Eq. (9), we can obtain thepre-exponential factor, A, reaction
order m and n through the multiple linear regression using several
heating rate data at the same time. Therefore, the values of A, m and n
can be obtained by average activation energy using Starink method and
the kinetic parameters calculated for all heating rates are listed in
Table 2. It can be observed from Table 2, different heating rate show a
little effect on the values of pre-exponential factor and reaction order.
Therefore, average values can be used to express the kinetic parameters
of different system. The experimental curves and predicted curve ob-
tained from kinetic parameters of curing reaction are shown in Fig. S1
(see Supporting information). It can be seen that experimental values
are in good agreement with calculated results.
Fig. 10. Activation energy vs. degree of conversion for bisphthalonitrile: DDS
To examine the effect of monomer and DDS structure on the glass
transition temperature of cured products, DSC scans were recorded
after isothermal curing of bisphthalonitrile monomer: DDS (90:10).
Fig. 12(A–C) shows that no exothermic transition was observed in the
DSC scans of isothermally cured bisphthalonitrile resins. A shift in
baseline was observed which was used to calculate glass transition
temperature as midpoint inflextion. As predicted, the glass transition
temperature (Tg) increased as the isothermal curing temperature or
time increased. However, it was found to be dependent on the structure
formed during the curing. The FIPN/DDS resin has the highest Tg as
compared to another monomer/DDS resin when cured up to [(a) 280
°C/6 h]. Although FIPN was more reactive with DDS, the DDS was
mixed very well which enhance the rigidity of developed structure.
However, at the next stage of cuing [(b)280 °C/6 h + 350 °C/6 h],
ODPA-PN/DDS cured resins showed a higher Tg. This could be due to
(90:10) mixture (Starink method).
activation energy increases. [29]
Fig. 11 show the plots of ln (dα/dt) + Ea/RT vs. ln(1-α). As shown
in. In Fig. 11, the peak point corresponds to the peak point of the DSC
curve. It can be observed that there is a non linear increase before peak
point followed by a linear decrease. The plot expressed a maximum of
ln (dα/dt) + Ea/RT at ln(1-α)at around −0.51 to −0.22, which is due
to the autocatalytic reactions which is equivalent to the degree of
conversion at about 0.2−0.4 [28,29]. Autocatalytic reactions are those
in which reaction intermediate or products also work as catalyst for
further reaction.
In the study of auto catalytic reaction, Eq. (9) obtained by
Fig. 11. Plots of ln (dα/dt) + Ea/RT vs. ln(1-α) at different heating rates for (A) Bis-ADPN/DDS, (B) FIPN/DDS and (C) ODPA-PN system.
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D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
Table 2
Kinetics parameters demonstrated for bisphthalonitrile system.
Sample
β
In A (sec−1
)
m
n
Ea (KJ/mol)
Correlation coefficient
(R2)
(oC /min)
FIPN/ DDS
5
14. 23 0.032
13.55 0.058
13.09 0.022
12.85 0.032
11.07 0.079
10.41 0.025
10.03 0.016
09.78 0.019
15.23 0.061
14.74 0.048
14.25 0.036
14.00 0.019
0.27 0.014
0.23 0.015
0.18 0.006
0.16 0.008
0.17 0.020
0.14 0.006
0.11 0.004
0.14 0.005
0.17 0.014
0.25 0.013
0.25 0.009
0.16 0.005
0.85 0.011
0.84 0.018
0.77 0.005
0.81 0.009
0.91 0.022
0.89 0.007
0.86 0.004
0.85 0.005
0.84 0.018
0.84 0.013
0.84 0.009
0.83 0.004
79.83 4.42
67.89 3.16
84.78 5.14
0.9980
0.9943
0.9998
0.9980
0.9955
0.9947
0.9978
0.9991
0.9987
0.9974
0.9982
0.9997
10
15
20
5
Bis-ADPN/DDS
ODPA-PN/DDS
10
15
20
5
10
15
20
β: heating rate, o C/min; A: pre-exponential factor, s−1 ; m, n : reaction order.
Fig. 12. DSC scans of Bis-ADPN, FIPN and ODPA-PN after isothermal heating with DDS: (A) ODPA/DDS, (B) Bis-ADPN/DDS, (C) FIPN/DDS.
(where a, b and c mean the isothermal curing stage).
(a) 280 oC/ 6h.
(b) 280 oC/ 6h + 350 oC/6h.
(c) 280 oC/6h + 350 oC/6h + 400 oC/4h.
the formation of a more compact structure and formation of a cross-link
network. This is supported by ΔH value. No shift in baseline after final
heating [stage (c)].DDS cured phthalonitrile monomer (Bis-ADPN,
FIPN, ODPA-PN) displayed the glass transition temperature (Tg) in the
range of 217–353 °C as shown in Fig. 12, which are higher than re-
ported values (Tg = 225–286 °C) [30,31] and the results are summar-
ized in Table 3.
Table 3
Glass transition temperature of cured Bis-ADPN/DDS, ODPA-PN/DDS and
FIPN/DDS samples at different stages of heating.
Cure procedure and isothermal
Glass transition temperature (oC)
heating stage
Bis-ADPN/
DDS
ODPA-PN/
DDS
FIPN/DDS
(a) 280 °C/6 h
217
219
239
(b) 280 °C/6 h + 350°C/6 h
(c) 280 °C/6 h + 350 °C/6 h + 400
°C/4 h
240
363
283
3.3. Thermal stability
> 400
> 400
> 400
The thermal stability of synthesized bisphthalonitrile monomers
cured isothermally with DDS (280 °C/6 h +350 °C/6 h +400 °C/4 h)
was evaluated over the temperature ranging from 50−800 °C in
8

D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
Table 5
Results of water uptake in cured phthalonitriles.
Sample Water uptake (%)
12 h
24 h
36 h
48 h
72 h
Bis-ADPN + DDS
ODPA-PN + DDS
FIPN + DDS
2.36
2.48
0.89
2.56
2.74
0.93
2.72
2.88
1.64
2.80
2.92
2.80
2.84
3.10
2.86
LOI= 17.5 + 0.4CR
Where CR- Char yield
The results are given in Table 4. All the samples had LOI greater
than 40 which is much above the self-extinguishing value i.e. 27. On the
bases of LOI values, such materials can be established as self-extin-
guishing thermoset and they will be having good fire resistance prop-
erties.
Fig. 13. TG traces of neat FIPN, ODPA-PN, Bis-ADPN monomers before and
after curing isothermally with DDS in nitrogen atmosphere.
3.4. Moisture absorption
Table 4
For water uptake studies, the cured samples were immersed in
boiling water for scheduled time and weighed after drying. Water up-
take was determined by following equation and the results are sum-
marized in Table 5.
Results of thermal stability parameters for the cured polymers.
Sample
Degradation temperature (°C) at%
Char yield at
LOI (%)
loss
800 °C
5
10
Water uptake (%) = (M2-M1)/M1*100
Bis-ADPN/DDS 478
ODPA-PN/DDS 494
505
546
563
62
60
61
43
41
42
Where M1and M2 are the weight of polymer before immersion and after
immersion in boiling water
FIPN/DDS
523
Rapid water uptake was observed in initial stage followed by gra-
dual increase in sample bis-ADPN/DDS and ODPA-PN system where as
it was very low initially in FIPN/DDS and then showed a sudden in-
crease after 36 h and 48 h. Due to the lower degree of hydrophilic
nature, the polymer shows the lower water uptake.
*Isothermal heating at 280 °C/6 h +350 °C/6 h +400 °C/4 h.
nitrogen atmosphere at heating rate of 20 °C/min using thermo-gravi-
metry (TG) and derivative thermo-gravimetry. Thermo-gravimetric
traces are shown in Fig. 13. All the cured samples display single step
decomposition. The relative thermal stability was correlated by com-
The lower water uptake is a significant advantage of these polymers
over the other higher performance polymer for the application of
polymers in aqueous medium or under high temperature and humid
conditions. As shown in Table 5, the cured polymers Bis-ADPN/DDS,
FIPN/DDS, and ODPA-PN/DDS showed a maximum water uptake of
around 2.84, 2.86, and 3.10, respectively, after 72 h immersion in
boiling water, which is lower than that of other phthalonitrile polymers
(3 % in 50 h) [33]. The ODPA-PN/DDS sample had higher water uptake
as compared to FIPN/DDS and Bis-ADPN/DDS under the same condi-
tions. This could be due to the structures of cured resins developed
during curing.
paring degradation-temperature at mass loss of 5% (T5) and 10 % (T10
and char yield at 800 °C. The results are summarized in Table 4.
)
When heated in N2 atmosphere the completely cured Bis-ADPN,
ODPA-PN and FIPN polymer showed 5% weight loss (T5) at478, 494
and 523 °C, respectively. The temperatures of 10 % weight loss i.e. T10
are in the range of 505–561 °C. With isothermal heating time or tem-
perature, decomposition temperature at 5 % and 10 % weight loss in-
creased. The char yields of cured product at 800 °C were found to be
dependent on the developed cross-link structure which is in the range
from 60 to 62 %. The char yields also increased with increasing heating
time and temperature. The thermal stability of cured resins was affected
by the nature of substituent in its backbone (Fig. 13, Table 4). The
cured Bis-ADPN shows the higher char yield of 62 % whereas FIPN and
ODPA-PN showed highest T5 and T10 degradation temperature.
These results may be associated to the small free volume of cured
polymer which marks that these polymers have good flame-resistance
property. The polymer chain of ODPA-PN and FIPN, T5 and T10values of
the FIPN/DDS and ODPA-PN/DDS (523−563 °C and 494−546 °C re-
spectively) are much higher than those of Bis-ADPN/DDS. Furthermore,
the values of T5and T10 are also much higher than that of other
bisphthalonitrile polymer as reported [5]. The introduction of flexible
isopropylidene group in to polymer chain of Bis-ADPN suppressed the
thermal stability of cured polymer at values of T5and T10The relatively
high char yield of Bis-ADPN could be due to the higher crosslink density
and condensed aromatic rings which facilitate the formation of com-
paratively high char by controlling the evolution of volatiles during
degradation.
4. Conclusion
The imide-group containing bisphthalonitrile monomers have been
successfully synthesized and characterized. The curing behavior and
thermal stability of cured bisphthalonitrile were investigated. The
curing kinetics of a bisphthalonitrile using 4, 4’-diaminodiphenyl sul-
fone (10 wt %), was investigated by non-isothermal DSC analysis.
Starink method was used to calculate the activation energy of mono-
mers with DDS. Activation energy was found to be dependent on the
structure of bisphthalonitrile monomer and was minimum in case of Bis
ADPN/DDS and maximum in case of ODPA/-PN/DDS. Thermal stability
of cured bisphthalonitrile resin was excellent in nitrogen atmosphere.
Particularly ODPA-PN/DDS and FIPN/DDS cured resins showed higher
thermal stability. The degradation temperature of cured polymer for 5
% weight loss (T5)and 10 % weight loss (T10) are in range of 478–523
and 505−561 °C, respectively. LOI values calculated from char yield
are much higher than the threshold value of 27 and thus we can say that
such materials are fire resistant. The glass transition temperature of
cured bisphthalonitrile polymer was found to exceed 400 °C after
curing. The water uptake was negligible which made these materials a
An attempt has been made to calculate limiting oxygen index for
such systems from the knowledge of char yield using Van Krevelen and
Hoftyzer equation [32] (Eq. (1)).
9

D. Kumar and V. Choudhary
ThermochimicaActa693(2020)178749
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