Study on thermal decomposition of calix[6]arene and calix[8]arene

Article abstract of DOI:10.1007/s10973-010-1065-5

Thermal decomposition kinetics of calix[6]arene (C6) and calix[8]arene (C8) were studied by Thermogravimetry analysis (TG) and Differential thermal analysis (DTA). TG was done under static air atmosphere with dynamic heating rates of 1.0, 2.5, 5.0, and 10.0 K min^-1. Model-free methods such as Friedman and Ozawa-Flynn-Wall were used to evaluate the kinetic parameters such as activation energy (E _a) and pre-exponential factors (ln A). Model-fitting method such as linear regression was used for the evaluation of optimum kinetic triplets. The kinetic parameters obtained are comparable with both the model-free and model-fitting methods. Within the tested models, the thermal decomposition of C6 and C8 are best described by a three dimensional Jander's type diffusion. The antioxidant efficiency of C6 and C8 was tested for the decomposition of polypropylene (PP).

Full text of DOI:10.1007/s10973-010-1065-5

J Therm Anal Calorim (2011) 103:853–862
DOI 10.1007/s10973-010-1065-5
Study on thermal decomposition of calix[6]arene
and calix[8]arene
•
K. Chennakesavulu M. Raviathul Basariya
•
•
G. Bhaskar Raju S. Prabhakar
Received: 9 August 2010 / Accepted: 16 September 2010 / Published online: 9 October 2010
´
´
Ó Akademiai Kiado, Budapest, Hungary 2010
Abstract Thermal decomposition kinetics of calix[6]ar-
ene (C6) and calix[8]arene (C8) were studied by Thermo-
gravimetry analysis (TG) and Differential thermal analysis
(DTA). TG was done under static air atmosphere with
Calix[n]arenes are extensively used in selective extraction
of metals, nuclear waste treatment, catalysis, complexa-
tion of fullerenes and neutral molecules. Synthesis of
calix[n]arenes and characterizing them by their physical
properties such as melting point, solubility and acid dis-
sociation constant (pKa) is a challenging task [1, 2]. The
higher melting points of calix[n]arenes necessitate a careful
study of their thermal behaviour. Thermo kinetic analysis
has been extensively applied in the field of polymers, solid/
liquid interface, carbohydrate chemistry, minerals, ener-
getic materials, pharmaceutical and biochemistry [3–5].
The TG analysis is a useful tool for the determination of
inclusion behaviour of calix[n]arenes with guest molecules
such as toluene, xylene, chloroform, acetone and alkyl
ammonium [6, 7]. Calix[n]arenes are good antioxidants for
the oxidation of polyolefins such as polypropylene, low-
density polyethylene and high-density polyethylene [8–12].
It is reported that in USA alone, nearly twenty thousand
tonnes of phenolic antioxidants were consumed during
1983 for the stabilization of plastics, it clearly reveals the
importance of antioxidants in polymer industry [13]. The
present study includes the results of the work carried out on
the synthesis of calix[n]arenes and their thermal decom-
position kinetics.
dynamic heating rates of 1.0, 2.5, 5.0, and 10.0 K min-1
.
Model-free methods such as Friedman and Ozawa–Flynn–
Wall were used to evaluate the kinetic parameters such as
activation energy (Ea) and pre-exponential factors (ln A).
Model-fitting method such as linear regression was used for
the evaluation of optimum kinetic triplets. The kinetic
parameters obtained are comparable with both the model-
free and model-fitting methods. Within the tested models,
the thermal decomposition of C6 and C8 are best described
by a three dimensional Jander’s type diffusion. The anti-
oxidant efficiency of C6 and C8 was tested for the
decomposition of polypropylene (PP).
Keywords Calix[n]arenes ꢀ Thermal decomposition ꢀ
Activation energy ꢀ Model-free and model-fitting methods ꢀ
Jander’s type diffusion
Introduction
Calix[n]arenes are macrocyclic baskets derived from con-
densation of phenol and formaldehyde in alkali media.
Experimental
Materials
K. Chennakesavulu (&) ꢀ M. Raviathul Basariya ꢀ
G. Bhaskar Raju ꢀ S. Prabhakar
The p-tert-butyl phenol, formaldehyde and para-formal-
dehyde used in the study are of analytical grade chemicals,
whilst chloroform, toluene and xylene are of Merk Speci-
ality grade.
National Metallurgical Laboratory Madras Centre, CSIR Madras
Complex, Taramani, Chennai 600113, India
e-mail: chennanml@yahoo.com;
chenna_velpumadugu@yahoo.com
123

854
K. Chennakesavulu et al.
Synthesis of calixarenes
Sample preparation
5,11,17,23,29,35-Hexa-tert-butyl-37,38,39,40,41,42-hexa
hydroxy calix[6]arene and 5,11,17,23,29,35,41,47-Octa-
tert-butyl-49,50,51,52,53,54,55,56-octa hydroxy calix[8]-
arene were synthesized as per the established procedure
[14]. The calixarenes obtained were further de-tert-butyl-
ated using AlCl3/Toluene media [15].
In order to study the thermograms of pure samples, prior to
the TG experiments, the C6 and C8 were degassed at
500 K under vacuum (10-5 Torr) for 48 h. The removal of
traces of solvent in calix[n]arenes, results in controlled TG
signal as evidenced by a constant mass before the start of
the melting of the sample.
37,38,39,40,41,42-Hexa hydroxy calix[6]arene (C6):
Melting point: 688–690 K, LCMS-ESI (M - 1)?: 635.
Molecular Formulae: C42H36O6, Calculated: (C, 79.22%)
Thermal analysis
1
(H, 5.70%), Found: (C, 79.12%), (H, 5.29%). H NMR
TG/DTG experiments were performed with Versa Therm
Cahn thermo balance TG-151 attached to an analytical
thermal system. Thermal experiments were conducted in
the temperature range of 300–1,100 K with samples
weighing around 20 mg. The analyses were carried out at
(500 MHz, CDCl3) d (ppm): 10.3 (s, 6H, –OH), 7.1, 6.8
(m, 18H, Ar–H), 3.8 (s, 12H, Ar–CH2–). 13C NMR
(500 MHz, pyridine-d5) d (ppm): 152.9, 128.8, 123.2,
120.3, 32.7. DEPT-NMR (500 MHz, pyridine-d5) d (ppm):
152, 128, 123.2, 120.3 (Aromatic –CH), 32.7 (Bridged
methylene, –CH2–). 13C NMR CP-MAS (300 MHz) d
(ppm): 150, 128, 125, 120, 29.5.
four nominal heating rates (1.0, 2.5, 5.0 and 10.0 K min-1
)
under static air atmosphere. DTA was carried out using
Netzsch 409C model instrument in static air atmosphere
with heating rate of 10.0 K min-1 and with 20 mg of
sample. The Netzsch Thermo kinetics software was used
for kinetic analysis of experimental data.
49,50,51,52,53,54,55,56-Octa hydroxy calix[8]arene
(C8): Melting point: 646–648 K, LCMS-ESI (M - 1)?:
847. Molecular Formulae: C42H36O6, Calculated: (C,
79.22%), (H, 5.70%), Found: (C, 79.21%), (H, 5.98%).
1H NMR (500 MHz, DMSO-d6) d (ppm): 8.8 (s, 8H,
–OH), 6.9, 6.6 (m, 24H, Ar–H), 3.8 (s, 16H, Ar–CH2–).
13C NMR (500 MHz, pyridine-d5) d (ppm): 152.9,
128.8, 123.5, 120.4, 32.3. DEPT-NMR (500 MHz,
pyridine-d5) d (ppm): 149.9, 128.8, 123.5, 120.4 (Aro-
matic –CH), 32.3 (Bridged methylene, –CH2–). 13C
NMR CP-MAS (300 MHz) d (ppm): 148.6, 128.5, 120.4,
29.8.
Theoretical approach
The kinetics of solid single step thermal decomposition
[16] is governed by the Eq. 1.
da
¼ kðTÞfðaÞ
ð1Þ
dt
where a is the conversion of the reaction, da/dt rate of
conversion, f(a) reaction model and k(T) is the temperature-
dependent rate constant, respectively.
Characterization
The fractional conversion a which is applicable to TG
data is expressed (m0 - mt)/m0 - mf), where m0, mf are
the initial and final mass of the sample and mt is the mass of
the sample at time t. The explicit temperature dependence
of the rate constant is described according to the Arrhenius
Eq. 2.
Bruker Avance-500 MHz NMR spectrometer was used to
record the 1H NMR, 13C NMR, DEPT-NMR of the
sample. Bruker Avance 300 MHz NMR spectrometer was
used to record the 13C NMR Cross Polarization Magic
Angle Spinning Spectrum (CP-MAS). The samples were
taken in 2.5 mm diameter zircon rotors and spun at
7 kHz. The elemental analysis of the sample was per-
formed using Euro Vectors Elemental analyzer. FTIR
spectra were recorded with Perkin Elmer FTIR instru-
ment. HPLC and LCMS–ESI analyses were carried out
using Schimadzu and Agilent technology instruments
respectively. EPR spectra were recorded on a Bruker
X-Band EMX-EPR instrument at room temperature with
the modulation frequency set at 100 kHz and microwave
power at 3.2 mW. The g value were measured against
DPPH, g = 2.0045 0.0002, which was used as the
external reference. The error in the reported g value is
0.0002.
ꢀ
ꢁ
Ea
kðTÞ ¼ A exp ꢁ
ð2Þ
RT
By combining the Eqs. 1 and 2 the following is obtained.
ꢁ
ꢀ
da
dt
ꢁEa
¼ A exp
fðaÞ
ð3Þ
RT
where A is the pre-exponential factor (rate constant at
infinite temperature), R is the gas constant, Ea is the
apparent activation energy and
T is the absolute
temperature. According to non-isothermal kinetic theory,
the fractional conversion (a) can be expressed as a function
of temperature, which is dependent on the time of heating.
123

Study on thermal decomposition of calix[6]arene and calix[8]arene
855
The explicit time dependence of Eq. 3 can be eliminated
through the trivial transformation and is expressed by
Eq. 4, which is valid only for constant heating rate.
100
80
60
40
20
0
(a)
ꢀ
ꢁ
da
dt
1
ꢁEa
¼ A exp
fðaÞ
ð4Þ
b
RT
–1
–1
Exp. 1.0 K min
Exp. 2.5 K min
Exp. 5.0 K min
where b = dT/dt is the heating rate.
–1
Kinetic parameter calculations based on TG data can be
obtained from Eqs. 3 and 4. In the present study, two dif-
ferent approaches namely Model-free methods [17–19] and
Model-based methods [20–22] were used to calculate the
kinetic parameters. The kinetic parameters obtained using
non-isothermal TG was observed to be advantageous over
conventional isothermal studies [23].
–1
Exp. 10.0 K min
–1
Simu. 1.0 K min
–1
Simu. 2.5 K min
Simu. 5.0 K min
–1
–1
Simu. 10.0 K min
500
600
700
800
900
1000
Temperature/K
Model-free or isoconversional method
100
80
(b)
In order to gain more information regarding the decom-
position mechanism, model-free methods which are rec-
ommended as a trustworthy were adopted for kinetic
analysis. The basic assumption of the model-free method is
that the kinetic triplet is independent on the heating rate.
The activation energy values calculated by this method are
independent of any implicit reaction model, which allows
one to explore multi-step kinetics [24]. The most common
model-free methods such as Friedman and Ozawa–Flynn–
Wall were employed for the thermo-kinetic analysis of
calix[n]arenes.
–1
–1
Exp. 1.0 K min
Exp. 2.5 K min
60
–1
Exp. 5.0 K min
–1
40
Exp. 10.0 K min
–1
Simu. 1.0 K min
–1
Simu. 2.5 K min
Simu. 5.0 K min
20
–1
–1
Simu. 10.0 K min
0
400
500
600
700
800
900
1000
Friedman method
Temperature/K
The method [17] based on the inter comparison of the rate
Fig. 1 Experimental and simulated non-isothermal TG curves of
a C6, b C8
of conversion da/dt for a given degree of conversion a
Table 1 Algebraic expression for f(a) and g(a) for the various kinetic reaction models
S. no.
Reaction model
Code
f(a)
g(a)
1
First order
F1
(1 - a)
-ln (1 - a)
2
Second order
F2
(1 - a)2
(1 - a)3
2(1 - a)[-ln(1 - a)]1/2
3(1 - a)[-ln(1 - a)]2/3
n(1 - a)[-ln(1 - a)](1-1/n)
a(1 - a)
(1 - a)-1 - 1
0.5(1 - a)-2 - 1
[-ln(1 - a)]1/2
[-ln(1 - a)]1/3
[-ln(1 - a)]1/n
ln[a/(1 - a)]
3
Third order
F3
4
Avrami Erofeev eq.
Avrami Erofeev eq.
Avrami Erofeev eq.
Prout-Tomkins eq
1D diffusion
A2
A3
An
B1
D1
D2
D3
D4
R2
R3
5
6
7
8
1/2a
[-ln(1 - a)]-1
(3/2)(1 - a)2/3[1 - (1 - a)1/3 -1
(3/2)[(1 - a)-1/3 - 1]-1
2(1 - a)1/2
(a)2
9
2D diffusion
a ? [(1 - a) ln(1 - a)]
10
11
12
13
Jander’s equation
Ginstling-Brounshtein
2D phase boundary
3D phase boundary
]
[1 - (1 - a)1/3 2
[1 - 2a/3] - (1 - a)2/3
]
[1 - (1 - a)1/2
[1 - (1 - a)1/3
]
]
3(1 - a)2/3
123

856
K. Chennakesavulu et al.
determined using dynamic heating rate b is considered as
the most general derivative technique.
After taking logarithms of Eq. 4 and rearranging, we
obtain
The activation energy (Ea) values over a wide range of
conversions was obtained by plotting ln bda versus
ꢂ
ꢃ
dT
1/T for a constant a value.
ꢀ
ꢁ
da
dT
Ea
ln b
¼ ln½AfðaÞꢂ ꢁ
ð5Þ
Ozawa–Flynn–Wall method
RT
This method [18, 19] is an integral method which can
determine the activation energy without the knowledge of
reaction order for given values of conversion.
4
C6
C8
Integrating Eq. 4 and rearranging, we obtain
Endo
2
0
AEa
0:4567Ea
log b ¼ log
ꢁ 2:314 ꢁ
ð6Þ
gðaÞR
RT
where
a
Z
–2
da
gðaÞ ¼
ð7Þ
fðaÞ
0
–4
–6
–8
By plotting log b against 1/T at certain conversion rate, the
slope -Ea/R indicate the activation energy.
Model-fitting method
400
600
800
Temperature/K
1000
1200
The model-based kinetic analysis depends on the reaction
type and reaction model. The reaction model may take
Fig. 2 DTA curves of C6 and C8 (heating rate: 5 K min-1
)
Fig. 3 Model-free plots of
a Friedman analysis of C6,
b Ozawa–Flynn–Wall analysis
of C6, c Friedman analysis of
C8, d Ozawa–Flynn–Wall
analysis of C8
0.02
(a)
(b)
0.98
1.0
–3.0
0.8
0.6
0.4
0.2
–3.5
–4.0
–4.5
–5.0
0.02
0.98
0
1.7
1.3
1.1 1.2 1.3
1.5
–1
1.1
1.5
–1
1.4
1.6
1.7
1000 KT
1000 KT
0.98
0.02
(c)
(d)
–3.0
1.0
0.8
–3.5
–4.0
–4.5
–5.0
0.6
0.4
0.2
0.98
0.02
0
–5.5
1.0
1.4
1.6
1.2
1.6
1.0
1.2
1.4
–1
–1
1000 KT
1000 KT
123

Study on thermal decomposition of calix[6]arene and calix[8]arene
857
various forms based on nucleation and nucleus growth,
phase boundary reaction, diffusion and chemical reaction.
The model with selected reaction types gives quite reliable
and consistent kinetic parameters. The unknown parame-
ters were found from the fitting of measured data with the
simulated curves for the given model and reaction types.
The different kinetic reaction models [25] used for the
analysis to identify the best reaction model is represented
in Table 1.
Results and discussion
TG/DTA analysis
The experimental TG curves corresponding to the thermal
mass loss of C6 and C8 are shown in Fig. 1. The thermal
decomposition of C6 occurred between 660 and 900 K
with a mass loss of 95%. In the case of C8, the decom-
position occurred between 630 and 900 K with a mass loss
Fig. 4 a Energy plot of
Friedman for C6, b energy plot
of Ozawa–Flynn–Wall for C6,
c energy plot of Friedman for
(a)
(b)
8
195
7.8
180
180
C8, d energy plot of Ozawa–
Flynn–Wall for C8. The values
160
7.2
7
165
150
are evaluated using F1 kinetic
model
6.6
6.0
Ea
In A
Ea
In A
140
135
6
1.0
0.0
150
0.2
0.4
0.6
0.8
0.5
0.0
1.0
6
5
4
150
135
120
105
(c)
(d)
135
6.0
4.5
120
105
90
Ea
In A
Ea
In A
90
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
Table 2 Kinetic triplets of C6 and C8 for the tested models with linear regression
Code
Calix[6]arene
Calix[8]arene
Ea/kJ mol-1
ln A/s-1
r2
r
Ea/kJ mol-1
ln A/s-1
r2
r
Ea
ln A
Ea
ln A
F1
111
136
126
98
4.6
6.7
5.9
3.7
3.9
7.5
5.9
5.6
6.1
6.6
5.8
3.8
3.8
0.9954
0.9964
0.9969
0.9802
0.9704
0.9979
0.9529
0.9985
0.9990
0.9998
0.9992
0.9918
0.9930
5.1
6.2
6.0
6.1
4.8
4.4
7.6
0.9
1.2
0.5
2.1
4.2
5.0
0.27
0.32
0.31
0.32
0.25
0.23
0.43
0.08
0.06
0.02
0.11
0.22
0.26
99
124
110
85
3.7
5.7
4.5
2.7
2.7
5.2
4.7
4.5
5.0
5.5
4.7
2.8
2.8
0.9980
0.9982
0.9987
0.9869
0.9787
0.9980
0.9634
0.9987
0.9991
0.9999
0.9992
0.9957
0.9967
2.8
3.1
2.7
4.4
4.2
2.1
2.7
1.2
1.5
0.7
2.2
3.1
3.2
0.14
0.16
0.14
0.23
0.22
0.11
0.14
0.10
0.08
0.06
0.11
0.16
0.16
F2
F3
A2
A3
An
B1
D1
D2
D3
D4
R2
R3
99
86
152
113
132
141
165
145
105
107
121
98
118
127
127
132
92
94
123

858
K. Chennakesavulu et al.
Model-free analysis
of 95%. It is interesting to note that both C6 and C8 follows a
single stage decomposition. The three step decomposition
mechanism was observed for thermal decomposition of
calix[4]arene, azacalix[4]arene and p-tert-butyl calix[4]
arenes. The first step corresponds to the loss of entrapped
methanol molecules and melting of compounds with
simultaneous two step decomposition process [26]. The
DTA of C6 and C8 are shown in Fig. 2. In the case of C6,
an endothermic peak was observed around 670 K indicat-
ing the melting of the C6. The exothermal peak above
700 K indicates the cleavage of methylene group from
phenolic arene groups. Similarly, an endothermic peak
observed around 630 K indicates the melting of sample and
the exothermal peak above this temperature shows cleav-
age of C8 cavity. The strong influence of the atmospheric
condition on exothermal decomposition of calix[4]arene
was reported [26].
The Friedman and Ozawa–Flynn–Wall analysis for the
thermal decomposition reaction of C6 and C8 and their
corresponding energy plots were shown in Figs. 3 and 4
respectively. These plots shows good linearity with little
scattering at the beginning and end of the plots. The dif-
ferential Friedman method indicates an average Ea value of
161 kJ mol-1 for C6 with standard deviation (r) of 4.1 and
121 kJ mol-1 (r = 5.7) for C8. Similar results with an
average Ea value of 159 kJ mol-1 (r = 4.6) for C6 and
119 kJ mol-1 (r = 4.7) for C8 were observed while using
Ozawa–Flynn–Wall method. From the results shown in
Fig. 3, it is clear that Ozawa–Flynn–Wall straight lines
fitted at different conversion rates are almost parallel
indicating the best fit of this method to our system [27].
Figure 4 shows the plots of activation energy and pre-
exponential factor as a function of fractional conversion
where a = 0.1–0.9 for C6 and C8. It reveals the linear
dependence of activation energy with the pre-exponential
factor. As the thermal decomposition proceeds, the acti-
vation energy remains almost same with an error limit of
(a)
0.0
5 kJ mol-1
.
–0.2
Model-fitting analysis
–1
Exp. 1.0 K min
–0.4
–0.6
–0.8
–1.0
–1
Exp. 2.5 K min
Linear regression models were used for the evaluation of
kinetic parameters. Most probable kinetic model is ana-
lyzed by choosing best fit based on the value of the
–1
Exp. 5.0 K min
–1
Exp. 10.0 K min
–1
Simu. 1.0 K min
–1
Simu. 2.5 K min
Simu. 5.0 K min
–1
–1
Simu. 10.0 K min
700
Temperatur/K
500
900
1000
800
600
(b)
(a)
0.0
–0.2
–0.4
–0.6
–0.8
–1
Exp. 1.0 K min
Exp. 2.5 K min
Exp. 5.0 K min
–1
–1
–1
(b)
Exp. 10.0 K min
–1
Simu. 1.0 K min
–1
Simu. 2.5 K min
Simu. 5.0 K min
–1
–1
Simu. 10.0 K min
600
400
800
Temperatur/K
900
1000
500
700
3340 3360 3380 3400 3420
G
Fig. 5 Fit of experimental and simulated DTG curves obtained for
the reaction type D3 a C6, b C8
Fig. 6 EPR spectrum of TGA quenched a C6 at 745 K, b C8 at
715 K
123

Study on thermal decomposition of calix[6]arene and calix[8]arene
859
correlation coefficient r2 which is close to one. The acti-
vation energies and correlations corresponding to different
mechanism for linear regression method for C6 and C8 are
shown in Table 2. Among the tested models, the C6 and C8
obeyed the diffusion mechanism namely D3, (model 10) the
EPR analysis
The ESR spectra of TG quenched solid C6 and C8 were
shown in Fig. 6. The symmetrical singlets of ESR spectra
could be explained due to the formation of isotropic free
radicals. The g factor was measured to be 2.00324 for C6
and 2.0362 for C8. The effect of pyrolysis temperature on
EPR signal of primary radicals formed during the pyrolysis
of tobacco was reported [28]. The results confirm that at
lower temperature the most resolved spectra were obtained,
where at higher temperatures the broadened spectra were
obtained. The free radicals of water soluble calix[n]arenes
do not exhibit their hyperfine interaction, because the
radical centre is in the –OH groups of calix[n]arenes cavity
[29]. Generation of free radical was observed when
5,11,17,23-tetrabromo-25,26,27,28-tetramethoxy calix[4]-
arene was subjected to UV irradiation [30]. In the present
study, the quenched samples were kept for 1 week and
Jander’s type with integral form g(a) = [1 - (1 - a)1/3 2
]
and differential form (3/2)(1 - a)2/3[1 - (1 - a)1/3 -1
]
.
Figure 5 shows the fitted curve for linear regression of the
DTG kinetic data for a single step degradation of C6 and
C8, which invokes 3D diffusion. In order to check the
validation of kinetic parameters the experimental TG curves
are compared with simulated TG curves obtained using D3
reaction model as shown in Fig. 1. Using the activation
energy data, we found the values obtained using model-free
and model-based methods were close to each other.
Therefore, the determined activation energies are reason-
able and can be applied to arrive at the best kinetic models
for C6 and C8.
(a)
(a)
8
860 K
6
810 K
745 K
PP
PP + 0.5% C6
4
PP + 1.0% C6
2
RT
0
200
160
120
80
40
0
600
700
800
PP
900
/ppm
δ
Temperature/K
8
6
(b)
(b)
850 K
770 K
715 K
PP + 0.5% C8
PP + 1.0% C8
PP + 1.5% C8
4
2
0
RT
200
160
120
80
/ppm
40
0
700
800
500
600
δ
Temperature/K
Fig. 7 13CNMR CP-MASS of TGA quenched a C6 and b C8 at
various temperatures
Fig. 8 DTG of polypropylene with C6 and C8 as a antioxidant
additive (heating rate: 5 K min-1
)
123

860
K. Chennakesavulu et al.
Antioxidant efficiency of C6 and C8
their EPR spectra were taken. The EPR signals were
noticed even after 1 week. Thus it is apparent that the free
radicals generated are very stable.
TG of polypropylene was done in the absence and presence
of C6 and C8, the results were presented in the form of
DTG in Fig. 8. The DTG curve of polypropylene (PP)
suggests the decomposition of PP occurs in single step in
the range of 600–800 K. In the presence of C6 the single
step DTG curve of PP shows two step decomposition. It is
evident that during decomposition of PP the C6 is exhib-
iting its antioxidant activity. Similarly, in the presence of
C8 the single step DTG curve of PP becomes multi-step
decomposition. The FTIR coupled with mass spectroscopy
study revealed the formation of less stable primary and
secondary radicals, when PP was subjected to decomposi-
tion [32]. These radicals may gain hydrogen atom from
phenolic groups of C6 or C8 and form paraffins. Thus the
presence of C6 and C8 prevents the decomposition of PP.
Generally, the efficiency of antioxidant capacity depends
on the ring size due to the effect induced by the phenolic
groups, the spatial protections of double bond by delocal-
ization of p-electrons and the interaction of oxygen atoms
of OH units and the trapped radicals [10]. The decompo-
sition mechanism of PP in the presence of calix[n]arene
13CNMR CP-MAS analysis
The 13CNMR CPMAS of C6 and C8 at room temperature
and quenched at various temperatures were shown in
Fig. 7. It is apparent that at room temperature, the aromatic
carbon signals were observed from 150 to 120 ppm where
as signals corresponding to bridged methylene carbons
were observed at 29.5 ppm. In the case of quenched C6 and
C8 at various temperatures, the signals corresponding to
aromatic carbons were broadened drastically. This could be
attributed to the formation of stable phenoxy free radical.
The delocalization of free electron throughout the arene
ring may lead to the broadening of signal in 13CNMR CP
MAS. Qin et al. reported that the formation of free radicals
within apocynum fiber, results in broader peaks in
13CNMR CP MAS [31]. The bridged methylene signals
were shifted and almost disappeared at higher tempera-
tures. This indicates the cleavage of methylene bridge from
its phenolic groups.
Fig. 9 Decomposition
prevention mechanism of PP in
presence of calix[n]arene
CH
2
CH
2
CH
CH
3
3
CH
3
PoIypropylene
c - c scission
H C•
2
CH
2
CH
CH•
CH
CH
3
3
CH
3
OH
OH
OH
HO
Hydrogen
abstraction
n
Calix[n]arene
H3C
CH
C•
2
CH
2
CH
CH2
2
•
o
OH
n-1
CH
CH
3
CH
3
3
Ethylene + Propylene = Heptene + Hexane + Pentane + pentene
123

Study on thermal decomposition of calix[6]arene and calix[8]arene
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was shown in Fig. 9. The Ea values of water soluble
calix[n]arenes revealed lowering of Ea value with
increasing the ring size [33]. The Ea value varies with the
addition of PP during the pyrolysis of oil shale indicates the
accelerated decomposition of oil shale [34]. The Ea values
help us to estimate the efficiency of antioxidant capacity.
Based on the ring size and its lower Ea, C8 was observed to
be better antioxidant compared to C6.
Conclusions
Thermal decomposition of C6 and C8 follows the single
stage with a Jander’s type diffusion mechanism. The
kinetic study showed that the Ea values of C6 obtained by
Friedman and Ozawa–Flynn–Wall methods were 161 and
159 kJ mol-1 respectively, with ln A of 6.8 s-1 and in the
case of C8 the Ea values are 121 and 119 kJ mol-1
respectively, with ln A of 5.1 s-1. The EPR and 13CNMR
CP-MAS confirm the formation of open chain poly phe-
nolic free radicals during decomposition of C6 and C8. The
higher antioxidant efficiency of C8 with PP was observed
when compared to C6. In presence of calix[n]arene the
decomposition mechanism of polypropylene was proposed.
Acknowledgements The authors K. Chennakesavulu and M. Ra-
viathul Basariya Senior Research Fellows are grateful to Council of
Scientific and Industrial Research (CSIR), New Delhi (India) for
financial support. The authors are grateful to SAIF-NMR Facility,
Indian Institute of Technology, Madras (India), NMR Centre-Indian
Institute of Science, Bangalore (India) for providing the necessary
spectral and analytical data.
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