J Therm Anal Calorim (2013) 111:1627–1632 DOI 10.1007/s10973-012-2351-1
Thermal decomposition kinetics of polypyrrole and its star shaped copolymer Metin Ak • Gu¨lbanu Koyundereli C ¸ ılgı Ferah Diba Kuru • Halil Cetis¸ li
•
CEEC-TAC1 Conference Special Issue Ó Akade´miai Kiado´, Budapest, Hungary 2012
Abstract Thermal behavior of 2,4,6-tris(4-(1H-pyrrol-1yl)phenoxy)-1,3,5-triazine monomer, polypyrrole, and their star shaped copolymer, were investigated using TG and DTA methods. It was found that Tria melts at 517 K and after than it starts to decompose. Decomposition proceeded in two stages which were corresponding to removal of branched groups and remaining core structure degradation, respectively. Polypyrrole and copolymer showed similar thermal behaviors. These compounds decomposed in three stages which are removal of solvent, removal of dopant anion and rest of structure decomposition. The calculation of activation energies of all reactions were realized using model-free (KAS and FWO) methods. The graphs were prepared which show the alteration of activation energy with decomposition ratio. Thermal analysis results showed that dopant anion and solvent removal activation energy values for copolymer are lower than polypyrrole. Star shaped loose-packed novel structure greatly facilitates solvent and dopant anion removal from copolymer. It can be concluded also that thermal analysis can be used as predict package structure of conducting polymers. Keywords FWO KAS Star shaped copolymer Activation energy Model-free methods
Introduction The growth in the intensive study of highly conducting polymers began in 1977 with the discovery of the change in M. Ak G. K. C¸ılgı (&) F. D. Kuru H. Cetis¸ li Department of Chemistry, Faculty of Science and Arts, Pamukkale University, Denizli, Turkey e-mail:
[email protected]
the electrical conductivity of poly(acetylene) on doping with Br2, I2, and AsFs [1]. Other conjugated polymers which exhibit interesting electrical and electrochemical properties associated with their extended p-bonding system are now known. Polymers containing heterocyclic units in the backbone were found to have notable electrical conductivities and to offer increased stability and processability in both the doped and neutral states when compared with poly(acetylene)s. Among the many poly(heterocyclic)s, polypyrrole (PPy) and its derivatives have aroused great interest. The synthesis, structure, electrochemical, electrical, and physicochemical properties and applications of PPys and their derivatives have been investigated deeply [2] to solve many questions, such as structure–properties relationships, increasing of stability and processability. Building super-structured conducting polymers (CPs) is of great interest because of the novel properties that could arise from such structures. Branched CPs with electronically connected nodes are excellent candidates among this family of super-structured CPs; with such polymers, there should be no need for inter-chain coupling or inter-chain electronic transfer to insure high electronic conductivity [3, 4]. Moreover, this type of material possesses a three dimensional structure which could also improve the conductivity. For these reasons, we have chosen to synthesize 3D star-shaped molecules with pyrrole branches and aromatic and non-aromatic connecting-up. For the future utilization of CPs, the knowledge of the thermal behavior and stability is important. Although there are an enormous number of papers related to the preparation conditions of PPy with high electric conductivity or mechanical stability [5], there are only a few reports dealing with the thermal stability of PPy [6, 7]. High thermal stability and electrical conductivity will provide CPs with extremely wide applications for the practical
123
123
31.82
507.20 488.42
30.26 30.13
507.08 346.49
15.44 14.73
350.14 350.7
14.57 23.17
527.67 543.99
23.88 26.01
526.66 356.67
4.86 4.96
350.9 349.9
4.43 60.47
670.3
63.80 Dm (%mg)
61.60
653.93 Tp/K
639.23
431.68
620.21 592.5
406.59 310.99
419.03 414.36
306.07 301.31
377.25 606.05
503.00 490.00
603.02 600.07
483.00 317.34
378.97 378.9
323.34 311.30
359.97 713.99
570.00 557.00 550.00
711.91
Ti/K
Tf/K
8 6 b/°C/min
712.08
8 6 8 6 10 8 10 8 6
Removal of ACN First decomposition
10
PPy Tria
where a corresponds to the degree of conversion, A stands for the pre-exponential factor, E denotes the activation energy, f(a) represents the differential conversion function, and R is the gas constant. For an a constant, the plot of lnb/S 2 versus 1/T (for KAS equation), and the plot of lnb versus 1/T (for FWO equation) are obtained from TG graphs. The lnb/S 2 versus 1/T plots and lnb versus 1/T plots, which are recorded at different heating rates, should form a straight line with a slope that allows an evaluation of the activation energy.
Compound
ð2Þ
Table 1 The thermal analysis results of all compounds
and FWO equation: A:E E 1 ln b ¼ 5:3305 1:05178 : ; R:gðaÞ R T
6
Removal of perchlorate
Theory of kinetic analysis In this study we selected the non-isothermal multiple-scan methods for studying of kinetics, isoconversional methods are also called model-free method because no kinetic model was set before the calculation of energy. KAS and FWO methods are two representative ones of model-free methods, which are convenient to calculate the activation energy. The final equations of these methods as follow [9– 14]: KAS equation: b AR E 1 ln 2 ¼ ln : ; ð1Þ T gðaÞE R T
Removal of ACN
Copolymer
10
Removal of perchlorate
10
operation of solid-state electronic devices. Differential scanning calorimetric (DSC) and thermogravimetric (TG) studies of PPy have been reported and a number of papers have reported a temperature dependence of the conductivity of oxidized PPy [8]. The most important and reliable factor in the study of heat stable polymers is the measurement or evaluation of thermal stability. Thermal properties and interaction between the polymers can also be noted from the oxidative degradation curves through TG and DSC studies. DSC is most commonly used to determine transition temperatures such as glass transitions, melting cross-linking reactions, and decomposition. In this study, we investigated thermal and kinetic behavior of PPy and its copolymer with a star shaped pyrrole (Tria-Py) monomer. Electrochemical polymerizations of PPy and P(Tria-co-Py) were performed in acetonitrile solvent and using sodium perchlorate as the supporting electrolyte. Thermal analysis results showed that dopant anion and solvent removal activation energy values for copolymer are lower than PPy. This is due to the loose-packed structure of P(Tria-co-Py). Star shaped novel structure greatly facilitates solvent and perchlorate removal.
436.44
M. Ak et al. 629.95
1628
Thermal decomposition kinetics of polypyrrole
1629
Experimental
Results and discussions
The star shaped monomer, 2,4,6-tris(4-(1H-pyrrol-1yl)phenoxy)-1,3,5-triazine (Tria), was synthesized according to literature method [15]. PPy and P(Tria-co-Py) were synthesized by using electrochemical polymerization method. The IVIUM Compactstat model potentiostat/galvanostat was used for all of the electrochemical synthesis. Polymerization of PPy was performed in acetonitrile solvent (20 ml) and using 3 ml pyrolle monomer. Sodium perchlorate (0.05 M) was selected as the supporting electrolyte and constant potential of 1.2 V was applied under nitrogen atmosphere. P(Tria-co-Py) was synthesized in electrochemical cell which contains 20 ml acetonitrile, 20 mg Tria, and 5 ml pyrrole. Pt foil was used as working and counter electrodes and Ag wire was selected as pseudo reference electrode. Constant potential of 1.6 V was applied under nitrogen atmosphere. The surface morphologies of the PPy and P(Tria-co-Py) were analyzed using a JEOL JSM-6400 scanning electron microscope. All of the TG, DTG, and differential thermal analysis (DTA) curves were obtained simultaneously by using a Shimadzu DTG-60H Thermal Analyzer. The measurements were carried out in flowing nitrogen (100 ml/min.) atmosphere and temperature ranged from 25 to 1,000 °C in platinum crucible. The heating rate (b) varied as 6–8 and 10 °C/min and highly sintered Al2O3 was used as the reference material. All experiments were performed three times for repeatability and the results showed good reproducibility with the smaller variations in the kinetic parameters.
Thermal analysis
Kinetic analysis The kinetic analyses were realized for only defined stages in thermal analysis section. These defined stages are
a 100
b 100
60
30
TG/%
DTA/uV
20
50
80
20 10
60
DTA/uV
40
0
0
–10
40
–20 0
–20 1000
1500
400
Temperature/K
c
600
800
1000
1200
Temperature/K
100 30 20 10
50
DTA/uV
500
TG/%
TG/%
Fig. 1 TG and DTA curves at 10 °C/min. a Tria, b PPy and c P(Tria-co-Py)
The thermal analysis results from the evaluation of curves obtained at all heating rates are summarized in Table 1. However, only the curves that are obtained at heating rate of 10 °C/min are presented in Fig. 1 as an example. It was found that Tria melts at 517 K and after that it starts to decompose. Decomposition proceeded in two stages. First decomposition stage occurs in 550–714 K temperature range and corresponds to removal of branched groups and core structure remains intact. Theoretical mass loss is (63.41%) compatible with average experimental value (61.96%). Decomposition regions are signed at Fig. 2. Second decomposition stage which corresponds to rest of structure degradation occurs in 714–1,473 K temperature ranges. Thermal behaviors of PPy and P(Tria-co-Py) are similar. Decomposition proceeded in three stages which are correspond removal of acetonitrile, removal of perchlorate anion and rest of structure decomposition, respectively. The removal of perchlorate ion is an exothermic process while the removal of acetonitrile is an endothermic process. The average peak temperatures of exothermic reaction (removal of perchorate anion) are 532.8 and 500.9 K for PPy and P(Tria-co-Py) compounds, respectively.
0 –10 0 400
600
800
1000
1200
Temperature/K
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M. Ak et al.
I O
N N
I
I
O
N O
N
I N
Fig. 2 First decomposition regions of Tria monomer
700
Ea/kJ/mol
FWO KAS 350
0 0.0
0.2
0.4
0.6
0.8
1.0
α
Fig. 3 Variation of activation energy values with decomposition ratio-a of monomer
removal of solvent (acetonitrile) and dopand (perchlorate anion) from PPy and copolymer, and first decomposition stage of Tria. The kinetic analysis of last decomposition stages could not performed due to the fast reaction and irregular data in TG curves. The activation energy values of first decomposition stage of Tria are calculated by using the two model-free
a
60
b
300
150
FWO KAS
Ea/kJ/mol
Fig. 4 Variation of activation energy values with decomposition ratio-a for removal of ACN reaction (a) and for removal of perchlorate anion reaction (b) of PPy
methods and their variation behavior with respect to the decomposition ratio are presented in Fig. 3. As seen from Fig. 3, KAS and FWO methods results are compatible with each other. The activation energy values are nearly constant to a:0.7 decomposition ratio and then rise suddenly. Finally the activation energy value reaches (311.28 kJ/mol) to the three times of the value at the beginning of the reaction (105.26 kJ/mol). This result proves that Tria decomposes in two stages and second stage needs higher activation energy. Figure 4a, b show the variation of activation energy with decomposition ratio for removal of acetonitrile solvent and removal of perchlorate anion reactions for PPy, respectively. Activation energy values of acetonitrile removal increases to a:0.42 and then decreases gradually. Modeling studies have not been realized yet. However, this activation energy trend shows that nucleation might be an effective model for this reaction. It is observed that the activation energy values, which are calculated using these two methods, correlate with each other for removal of perchlorate anion reaction. Activation energy increases persistently with the increase of decomposition ratio especially after a:0.8. This result shows last reaction corresponds to decomposition of the rest of the structure and have the highest activation energy. The activation energy values variation graphs for the removal of acetonitrile and removal of perchlorate anions from P(Tria-co-Py) with respect to the decomposition ratio are presented with Fig. 5a, b, respectively. Activation energy persistently decreases with the increase of decomposition ratio for removal of acetonitrile reaction. At the beginning of the reaction activation energy value is 37.76 kJ/mol averagely and at the end of the reaction activation energy value is 21.19 kJ/mol averagely. It is suggested that nucleation models are effective for removal of perchlorate anion reaction due to the peak observation at a:0.40 and after the decrease of activation energy in Ea-a graph.
Ea/kJ/mol
N
40
20 FWO KAS 0
0 0
123
0.2
0.4
α
0.6
0.8
1
0
0.2
0.4
α
0.6
0.8
1
Fig. 5 Variation of activation energy values with decomposition ratio-a for removal of ACN reaction (a) and for removal of perchlorate anion reaction (b) of P(Tria-co-Py)
1631
a
40
b
100
Ea/kJ/mol
Thermal decomposition kinetics of polypyrrole
50
Ea/kJ/mol
FWO KAS
20
FWO KAS 0
0 0
0.2
0.4
0.6
0.8
0
1
0.2
0.4
α
0.6
0.8
1
α
Table 2 The activation energy values at different decomposition degrees for all reactions of Tria, PPy, and P(Tria-co-Py) compounds a
Tria
PPy
P(Tria-co-Py)
Removal of ACN
Removal of per chlorate
Removal of ACN
Removal of per chlorate
FWO
FWO
KAS
FWO
KAS
FWO
KAS
FWO
KAS
0.1
120.296
115.901
42.937
39.742
37.230
30.807
34.247
30.787
34.408
28.859
0.2
137.577
133.789
49.139
46.155
42.138
35.773
30.529
26.688
74.439
70.478
0.3 0.4
146.615 152.110
143.141 148.813
50.268 51.295
47.252 48.246
50.213 57.688
44.093 51.808
28.713 26.932
24.646 22.658
91.361 99.170
87.958 95.914
0.5
154.922
151.684
48.330
45.051
68.134
62.662
24.908
20.421
87.296
83.211
0.6
160.108
157.059
46.106
42.637
83.190
78.361
22.788
18.079
76.313
71.441
0.7
169.309
166.663
43.429
39.744
101.417
97.387
19.836
14.858
67.118
61.521
0.8
184.662
182.740
35.241
31.066
126.913
124.023
14.251
8.878
56.457
50.025
0.9
216.321
215.947
25.510
20.777
155.469
153.828
12.161
6.498
45.048
37.666
Fig. 6 a Schematic representatives of P(TriaPy-coPy) and PPy b SEM micrographs of P(TriaPy-co-Py) and PPy
KAS
a
N
b
O N
N O
N O
N
N H
N
N H
The activation energies of all reactions (in kJ/mol unit) were given in Table 2. It was demonstrated that the redox properties of the conducting polymer depend strongly on the nature of the anion incorporated during synthesis [16, 17]. In conducting polymer films, charge is compensated by insertion of the
anions during oxidation (doping) and release of the same anions during reduction. Potential applications of CPs depend upon the dramatic property changes which occur during this redox process. Due to p–p interactions, conducting polymer chains show densely packed structure (pstacking). The spectro-electrochemical studies show that
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redox properties of super-structured CPs are much better than that of ordinary CPs. The great improvement can be attributed to the more accessible doping sites and the facile ion movement during the redox switching, brought by the loose packing of the CPs chains [18]. When surface morphologies of PPy and P(Tria-co-Py) were investigated, due to the unique molecular geometry of P(Tria-co-Py), the dense packing of the rigid PPy is prohibited, while a loosely packed structure is formed (Fig. 6). Thermal analysis results are compatible with surface morphology micrographs. As can be seen in Table 2 activation energy values of acetonitrile and perchlorate removal from P(Tria-co-Py) are lower than from PPy. It can be concluded that loose-packed structure of P(Tria-coPy) greatly facilitates solvent and perchlorate removal.
Conclusions It was found that Tria monomer melts at 517 K and after that it starts to decompose. Decomposition proceeded in two stages. These stages correspond to removal of branched groups and rest of structure decomposition, respectively. The activation energy values of first decomposition stage increased with an increase in decomposition ratio a continuously, which indicated that the second decomposition stage requires more activation energy. The thermal behaviors of PPy and copolymer are similar. These polymers decomposed in three stages which are removal of solvent, removal of dopant anion, and rest of structure decomposition. However, the kinetic behaviors of these compounds are different. Copolymer need lower energy for removal of solvent, removal of dopant anion reactions. This is due to the loose-packed structure of Tria-Py. Star shaped novel structure greatly facilitates solvent and perchlorate removal.
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