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Kinetic analysis of thermal decomposition of poly(propylene carbonate)
Polymer Degradation and Stability 89 (2005) 282e288 www.elsevier.com/locate/polydegstab
Kinetic analysis of thermal decomposition of poly(propylene carbonate) X.L. Lua, Q. Zhub, Y.Z. Menga,* a
Institute of Energy and Environmental Materials, School of Physics and Engineering Science, Sun Yat-Sen University, Guangzhou 510275, PR China b School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China Received 21 June 2004; received in revised form 29 November 2004; accepted 8 December 2004 Available online 26 February 2005
Abstract The dynamic thermal degradation of poly(propylene carbonate)s (PPC)s with alternating molecular structure and varying molecular weights was studied using TGeDTA. Because of the thermal hysteresis and the dissolved air for bulk samples, it is beneficial to employ film samples for kinetic analysis. It was found that the DTA curve for low molecular weight PPC had both an endothermic peak and an exothermic peak, whilst the DTA curve for high molecular weight PPC exhibited only the exothermic peak. PPC with high molecular weight showed better thermal stability than that with low molecular weight. Kinetic analysis of the dynamic TG curves for PPCs with varying molecular weights was carried out using the Ozawa method. The decomposition activation energy of PPC was calculated to range from 105 to 118 kJ/mol. The influence of nitrogen flow rate on the decomposition of PPC was also investigated and is discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polycarbonate; Carbon dioxide; Thermal decomposition; Ozawa method
1. Introduction The key to dynamic analysis of thermal decomposition is to determine mechanism of solid reactions and to obtain kinetic parameters. Theoretical analysis methods include differential method, integral method and the integration of the both methods. Experimental techniques include isothermal methods and the non-isothermal methods which are more popular than the former [1]. It should be noted that the dynamic parameters derived from varying method are sometimes quite different [2]. The Ozawa method is well documented to be the most useful one to obtain decomposition activation energy [3,4]. Using this method, there is no
* Corresponding author. Tel./fax: C8620 84114113. E-mail address: [email protected] (Y.Z. Meng). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.12.025
need to consider any reaction models when using the specified theory to calculate other parameters. Carbon dioxide is the main factor to cause green house effect, nevertheless, it is also an important carbon resource. Inoue [5] successfully co-polymerized carbon dioxide with propylene oxide to get poly(propylene carbonate) (PPC) using organometallic compounds as catalysts in 1969. Because of the superior mechanical properties and complete biodegradability, it has attracted much attention of scientists all over the world in recent years. Meng et al. [6] found the highest catalytic activity of 126 g polymer/g cat. for alternating PPC by using supported zinc catalyst. Thereafter, much work has been performed to investigate the thermal properties of the new polymer because the polymer has been reported to be thermally unstable [7]. Inoue and Tsurata [7] described that its thermal decomposition consisted of two different mechanisms, i.e., upzipping and random
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X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
chain scission. Yan et al. [8] have investigated the thermal decomposition behaviour of PPC in the presence of ceramic powder. Li et al. [9] have also studied the thermal decomposition of alternating PPC using TG/IR and Py-GC/MS techniques. The results showed that chain scission occurred at relatively lower temperature than unzipping reaction; the final pyrolysates were cyclic propylene carbonate and 1,2 propanediol. Peng et al. [10] demonstrated that PPC had the same degradation temperature in both nitrogen and air, i.e., there was no oxidation reaction. However, owning to the complexity of thermal decomposition, different decomposition mechanisms, decomposition products and the different function of some factors such as surrounding atmosphere, end-capping has been reported. Because of these difficulties, very little information is available for the thermal decomposition of PPC in literature. In the present study, the thermal decomposition activation energies for PPCs with varying molecular weights were investigated and calculated using Ozawa method. The decomposition mechanism and some factors influencing the thermal decomposition were discussed.
[6,11]. To remove the catalyst residues, the as-made PPC was dissolved in chloroform, and then the solution was introduced into a funnel containing 5% HCl, followed by washing three times with deionised water. Subsequently, PPC was precipitated from the solution using alcohol, and dried under vacuum at 80 C for 48 h. The film specimen of PPC with thickness of about 60 mm was obtained by solution casting in chloroform. Other reagents were analytical grade and used without further treatment. The structure of PPC is shown as below:
2. Theoretical analysis
3.2. TG measurement
The basic equation of thermal decomposition can be expressed as follows: dx E ZA exp ð1Þ W n; dt RT
The non-isothermal decomposition of PPC was carried out using a PerkineElmer Thermogravimetric/ Differential Thermal Analyzer (TG/DTG) instrument against alumina as a reference. For each measurement 10 mg sample was used. The experiments were performed under protection of nitrogen atmosphere at different flow rates. The heating rates were adopted as 2, 4, 6, 8 and 10 C/min.
where x is the fraction of a structural quality, t time, R the gas constant, A the pre-exponential factor, E the activation energy of the decomposition reaction, n the order of the thermal reaction and W the decomposition residue. For the majority of solid state reactions, if the heating rate b is a constant, the following equation can be derived from Eq. (1) according to Ozawa method [3], log b 0:4567
E Zconstant: RT
ð2Þ
Eq. (2) indicates that the plot of log b versus 1/T is a straight line, from whose slope the activation energy E can be obtained.
O O
C
O
CH CH2 CH3
n
The number average molecular weight (Mn ) of PPC was measured with gel permeation chromatography (GPC) system including a Waters 515 pump and a Waters 2414 refractive index detector with tetrahydrofuran (THF) as eluent solvent. Calibration was performed with polystyrene standards with molecular weight ranging from 1260 to 3,850,000. The Mn data of used PPCs are listed in Table 1.
4. Results and discussion 4.1. Thermal hysteresis The thermal decomposition behaviour of a polymer is influenced by many factors, including heating rate, surrounding atmosphere and sample weight. Moreover, it has been reported that the sample shape also influenced on the thermal degradation behaviour. Fig. 1 shows the DTG and DTA curves of a block (size:
3. Experimental
Table 1 Molecular weights of used PPC specimens
3.1. Material
Specimen
Mn
MW
MW =Mn
1 2 3
42,000 56,100 86,300
85,300 119,700 523,900
2.03 2.13 6.07
Poly(propylene carbonate) (PPC) with alternating structure was prepared according to previous work
284 0
0
-5
-10
DTG(%/min)
DTG(%/min)
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
-10 -15 -20
-40 50 100 150 200 250 300 350 400
5 0 -5
DTA
DTA
-30
-50
50 100 150 200 250 300 350 400 4 2 0 -2 -4 -6 -8 -10 -12
-20
-10 -15 -20 -25
50 100 150 200 250 300 350 400
-30
50 100 150 200 250 300 350 400
Temperature(°C)
Temperature(°C)
(a)
(b)
Fig. 1. DTG and DTA curves for (a) the film sample and (b) the block sample, at heating rate of 10 C/min and N2 flow rate of 50 mL/min.
1 mm ! 3 mm ! 3 mm/thickness ! width ! length) and a film of specimen 2 measured at a heating rate of 10 C/ min and at a flow rate of 50 mL/min of nitrogen. It was evident that the block and the film samples exhibited different thermal decomposition characteristics. From Fig. 1a, it can be seen that the DTG curve for block sample was coarse and wide, while that for film was smooth and narrow (Fig. 1b). Several peak shoulders appeared randomly in both block and film samples under the same experimental conditions. The rough peak and peak shoulders resulted from the dissolved air or other gases produced during the thermal decomposition of block sample. Increasing temperature, the heat was removed by gases resulting in the fluctuation of weight and heat. This behaviour was also seen from some disturbances of DTA curves. The main peak of the DTG curve for block sample located at 263 C, but it appeared at 250 C for film sample. This was attributed to the larger mass of block sample that led to the difficulty in heat conduction within the sample and resulted in thermal hysteresis. The hysteresis resulted in the wide peak of DTG curve for block sample. The phenomenon can result in difficulty in kinetic analysis for block sample. Thus, film samples were used for all the investigations in this work. 4.2. Influence of protective gas flow rate The peak decomposition temperatures obtained from both DTG and TGA, and 5% weight loss temperatures at different flow rates of nitrogen for specimen 2 were tabulated in Table 2. The heating rate was set as 10 C/ min, and the flow rates of nitrogen were 50, 100, 150,
250 mL/min in turn. It can be seen that the peak decomposition temperatures varied from 249 to 251 C, and 5% weight loss temperature varied from 241 to 243 C. One can conclude that the flow rate showed little influence on the thermal decomposition of PPC.
4.3. Thermal decomposition behaviour of PPC with varying molecular weight Fig. 2 shows the DTG, TG and DTA curves of PPC samples with varying molecular weights at heating rate of 10 C/min under nitrogen. From Fig. 2a, there were an endothermic peak at 221 C and an exothermic platform from 255 to 317 C as shown in the DTA curve; accordingly in the DTG curve, there existed two peaks located at 221 and 333 C, respectively. From Fig. 2b, an endothermic peak at 251 C and an exothermic peak at 308 C were observed, while the DTG curve had two peaks at 251 and 336 C, respectively. Finally, Fig. 2c showed only one endothermic Table 2 Thermal decomposition temperatures of specimen 2 at different nitrogen flow rates Flow rate (mL/min)
T1a ( C)
T2b ( C)
T3c ( C)
50 100 150 250
251 250 249 249
251 250 250 250
244 241 241 242
a b c
Peak temperature in DTG curve. Peak temperature in DTA curve. 5% weight loss temperature.
285
DTG
40
-30
20
-40
0
-50
60
-10
TG
-20
DTG
40
-30
20
-40
0
-50
50 100 150 200 250 300 350
5 -5
DTA
50 100 150 200 250 300 350
-10
80
DTG
-20
40
-30
20 0
-40 50 100 150 200 250 300 350
-5 DTA
-15
-15 -20
-25
-25 50 100 150 200 250 300 350
DTA
-10
-20 -30
-10
TG
60
0
0
DTA
5 0 -5 -10 -15 -20 -25 -30 -35
50 100 150 200 250 300 350
80
0
100
DTG(%/min)
-20
Weight remaining(%)
TG
0
100
DTG(%/min)
60
DTG(%/min)
80
-10
Weight remaining(%)
0
100
DTA
DTA
Weight remaining(%)
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
-30
50
100 150 200 250 300 350
Temperature(°C)
Temperature(°C)
Temperature(°C)
(a)
(b)
(c)
Fig. 2. DTG, TG and DTA curves for PPC specimens with varying molecular weights. (a) Specimen 1, Mn Z42; 000; (b) specimen 2, Mn Z56; 100; (c) specimen 3, Mn Z86; 300.
peak at 254 C for the DTA curve and a corresponding peak at 252 C for the DTG curve. As discussed above, the exothermic DTA peak became smaller and smaller with increasing molecular
weight. The peak cannot be detected for specimen 3 with the greatest molecular weight. Furthermore, with the increase of molecular weight, the exothermic peak shifted to high temperature as can be seen in TG curve
Fig. 3. TG curves of PPC specimens with varying molecular weights obtained at heating rates of 2, 4, 6, 8 and 10 C/min, respectively. (a) Specimen 1, Mn Z42; 000; (b) specimen 2, Mn Z56; 100; (c) specimen 3, Mn Z86; 300. A: b Z 2 C/min; B: b Z 4 C/min; C: b Z 6 C/min; D: b Z 8 C/min; E: b Z 10 C/min.
286 1.1
1.1
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
logβ
logβ
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15
0.2
1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05
1000/T(K-1)
1000/T(K-1)
(a)
(b) 1.1 1.0 0.9
logβ
0.8 0.7 0.6 0.5 0.4 0.3 0.2 1.75
1.80
1.85
1.90
1.95
2.00
2.05
1000/T(K-1)
(c) Fig. 4. Heating rate versus the reciprocal absolute temperature for PPC specimens with varying molecular weights. (a) Specimen 1, Mn Z42; 000; (b) specimen 2, Mn Z56; 100; (c) specimen 3, Mn Z86; 300. C: W Z 10%; B: W Z 30%; :: W Z 50%; 6: W Z 70%; -: W Z 80%; ,: W Z 90%.
for specimen 2. The endothermic peak accounted for thermal decomposition of PPC as reported in previous work [9]. From the DTG curves in Fig. 2, the maximum decomposition rate of PPC was as fast as 50%/min, and the decomposition ended at about 350 C. This process implied that PPC was extremely sensitive to temperature with dramatically fast thermal decomposition. Furthermore, TG curves showed that PPC decomposed completely at a maximum temperature of !350 C. The decomposition products were released in the form of gas. The composition of pyrolysates was investigated using TGeIR technique in previous work [9]. 4.4. Weight loss at various heating rates Fig. 3 shows the weight loss curves of PPC specimens with different molecular weights from dynamic measurements at heating rates of 2, 4, 6, 8 and 10 C/min, respectively. The flow rate of nitrogen was set as 50 mL/ min. The changing tendency for all the specimens with varying molecular weights behaved the same. With increasing the heating rate b from 2 to 10 C/min, the
Table 3 Decomposition activation energies derived with Ozawa method W (%)
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
E (kJ/mol) Specimen 1 ðMn Z42; 000Þ
Specimen 2 ðMn Z56; 100Þ
Specimen 3 ðMn Z86; 300Þ
102 106 106 105 105 105 104 103 103 103 102 99.8 99.1 99.0 98.5 111 122 138 149
104 110 114 117 119 121 122 122 122 121 119 116 112 111 108 106 109 133 149
113 114 116 117 115 112 108 102 97 91 87 83 81 80 82 87 95 117 192
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X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
O O O O H H H H H O CH CH2 O C O n1 C C O C O C C O C O C C O C On2 C C O H H H H H CH3 2 CH3 2 CH3 2 CH3 2 CH3
main chain random scission
O H O CH CH2 O C On1C CH2 + CH3 CH3
O O H H + H2C C C O C O C C O C On2 C C O H H H2 CH3H2 CH3H2
CO2
Scheme 1.
in Scheme 1. This is because reactive end groups in the aliphatic polycarbonates can lead to a chain unzipping reaction. Polycarbonates with high molecular weights or with alternating structure, however, contain very few these end groups. Accordingly, the unzipping decomposition mechanism of PPC can be depicted in Scheme 2. It is apparent from Scheme 2 that the polycarbonates with uncapped end groups and with lower molecular weight favour the chain unzipping decomposition. Because of the formation of the stable 5-member ring compound, 4-methyl-1,3-dioxolan-2-one, the chain unzipping reaction has low apparent activation energy. Therefore, chain unzipping decomposition takes place preferably compared with chain scission decomposition for the uncapped and low molecular weight polycarbonates. Comparing the activation energy at 5% weight loss, it was found that this value increased greatly with increasing molecular weight. This means that the content of end groups in PPC obviously affected the thermal stability. Increasing molecular weight provided an effective way to improve the thermal stability of PPC. However, PPC exhibited poor thermal stability when compared with other polymers [12,13]. This can be seen from the low decomposition energy (Table 2).
decomposition curves shifted to higher temperature. For specimen 1 (Fig. 3a), the temperatures of 5% weight loss were 187, 199, 206, 214 and 212 C, respectively, corresponding to different heating rates; while for specimen 3 (Fig. 3c), they were 216, 228, 236, 240 and 246 C, respectively. Moreover, the temperatures of maximum decomposition rate also increased with increasing b value. This phenomenon resulted from the temperature retardancy. 4.5. The decomposition activation energy Fig. 4 shows Ozawa plots for PPC specimens with varying molecular weights at weight loss W value ranging from 0.05 to 0.95. The calculated results from the dynamic TG data are listed in Table 3. It can be seen that the decomposition activation energy increased, then decreased and again increased in this turn with increasing weight loss. Presumably, this was due to the changes in mechanism during the thermal decomposition. The thermal decomposition process of main chain scission reaction can be expressed as in Scheme 1. Generally, aliphatic polycarbonates with high molecular weights or with alternating structure mainly exhibit the thermal decomposition mechanism shown O H
O
CH
CH2 O
C
O
CH C
O n
CH3
CH3
H2C
O
H C
C O H2
CH3
O
H C
O
CH3 O
+
H
O
CH
O
CH3
continuted
H2C
cyclization
C
( n +1 )
O
CH2 O
C
O
n-1
H C CH3
CH C
C O H2
heat
H
O C O H2
C
O
H C CH3
C O H2
CH3
O
O Scheme 2.
+
H
O
CH CH3
C O H2
H
H
288
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
As discussed in previous work [9,11], the glass transition temperature of synthesized PPC is slightly higher than room temperature. However, the starting thermal decomposition temperature is generally greater than 250 C. Based on these data, it can be concluded that PPC can be melt processed at a temperature lower than 180 C. Because of relatively low activation energy, it is also important to reduce the melt processing time. The increase in molecular weight is crucial for stabilizing the PPC, therefore, it is believed that end-capping should further improve its thermal stability.
Acknowledgements We thank the Ministry of Science and Technology of China (Grant No. 2002BA653C), Natural Science Foundation of Guangdong Province (Excellent Team Project, Grant No. 015007), Key Strategic Project of Chinese Academy of Sciences (Grant No. KJCX2-206B) and Key Project of Guangzhou Science and Technology Bureau (Grant No. 2001-z-114-01) for financial support of this work.
5. Conclusions References The dynamic thermal decomposition of poly(propylene carbonate) (PPC) was investigated under various conditions. The results indicated that the film and block samples exhibited distinct decomposition behaviours. Due to the coarse curve nature for block sample, the film sample was used for the measurement. Except for the exothermic peak, the PPC with Mn ! 60 kDa showed the endothermic peak in DTA examination. Both 5% weight loss temperature and the peak decomposition temperature increased with the increase in molecular weight and heating rate. Based on the results of the decomposition activation energy examination, we concluded that the content of end group affected greatly the thermal stability of PPC. Moreover, the PPC exhibited lower thermal stability as shown by its lower decomposition activation energy.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Flynn JH. Thermochim Acta 1992;203:519e26. House Jr JE, Manquardt LA. J Solid State Chem 1990;89:155. Ozawa T. Bull Chem Soc Jpn 1965;38:1881e6. Flynn JH. Thermochim Acta 1996;282/283:35e42. Inoue S, Koinuma H, Tsurata T. J Polym Sci Poly Lett Ed 1969;7(4):287e92. Meng YZ, Du LC, Tjong SC, Zhu Q, Hay AS. Polym Sci Part A Polym Chem 2002;40:3579e91. Inoue S, Tsurata T. J Appl Polym Symp 1975;26:257e67. Yan HW, Cannon WR, Shanefield DJ. Ceram Int 1998;24:433e9. Li XH, Meng YZ, Zhu Q, Tjong SC. Polym Degrad Stab 2003;81:157e65. Peng SW, An YX, Chen C, Fei B, Zhuang YG, Dong LS. Polym Degrad Stab 2003;80:141e7. Zhu Q, Meng YZ, Tjong SC, Zhao XS, Chen YL. Polym Int 2002;51:1079e85. Rychly J, Rychla L. J Therm Anal 1989;35:77e90. Vaslle C, Costea E. Polym Bull 1990;23:413e38.
Kinetic analysis of thermal decomposition of poly(propylene carbonate) X.L. Lua, Q. Zhub, Y.Z. Menga,* a
Institute of Energy and Environmental Materials, School of Physics and Engineering Science, Sun Yat-Sen University, Guangzhou 510275, PR China b School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China Received 21 June 2004; received in revised form 29 November 2004; accepted 8 December 2004 Available online 26 February 2005
Abstract The dynamic thermal degradation of poly(propylene carbonate)s (PPC)s with alternating molecular structure and varying molecular weights was studied using TGeDTA. Because of the thermal hysteresis and the dissolved air for bulk samples, it is beneficial to employ film samples for kinetic analysis. It was found that the DTA curve for low molecular weight PPC had both an endothermic peak and an exothermic peak, whilst the DTA curve for high molecular weight PPC exhibited only the exothermic peak. PPC with high molecular weight showed better thermal stability than that with low molecular weight. Kinetic analysis of the dynamic TG curves for PPCs with varying molecular weights was carried out using the Ozawa method. The decomposition activation energy of PPC was calculated to range from 105 to 118 kJ/mol. The influence of nitrogen flow rate on the decomposition of PPC was also investigated and is discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polycarbonate; Carbon dioxide; Thermal decomposition; Ozawa method
1. Introduction The key to dynamic analysis of thermal decomposition is to determine mechanism of solid reactions and to obtain kinetic parameters. Theoretical analysis methods include differential method, integral method and the integration of the both methods. Experimental techniques include isothermal methods and the non-isothermal methods which are more popular than the former [1]. It should be noted that the dynamic parameters derived from varying method are sometimes quite different [2]. The Ozawa method is well documented to be the most useful one to obtain decomposition activation energy [3,4]. Using this method, there is no
* Corresponding author. Tel./fax: C8620 84114113. E-mail address: [email protected] (Y.Z. Meng). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.12.025
need to consider any reaction models when using the specified theory to calculate other parameters. Carbon dioxide is the main factor to cause green house effect, nevertheless, it is also an important carbon resource. Inoue [5] successfully co-polymerized carbon dioxide with propylene oxide to get poly(propylene carbonate) (PPC) using organometallic compounds as catalysts in 1969. Because of the superior mechanical properties and complete biodegradability, it has attracted much attention of scientists all over the world in recent years. Meng et al. [6] found the highest catalytic activity of 126 g polymer/g cat. for alternating PPC by using supported zinc catalyst. Thereafter, much work has been performed to investigate the thermal properties of the new polymer because the polymer has been reported to be thermally unstable [7]. Inoue and Tsurata [7] described that its thermal decomposition consisted of two different mechanisms, i.e., upzipping and random
283
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
chain scission. Yan et al. [8] have investigated the thermal decomposition behaviour of PPC in the presence of ceramic powder. Li et al. [9] have also studied the thermal decomposition of alternating PPC using TG/IR and Py-GC/MS techniques. The results showed that chain scission occurred at relatively lower temperature than unzipping reaction; the final pyrolysates were cyclic propylene carbonate and 1,2 propanediol. Peng et al. [10] demonstrated that PPC had the same degradation temperature in both nitrogen and air, i.e., there was no oxidation reaction. However, owning to the complexity of thermal decomposition, different decomposition mechanisms, decomposition products and the different function of some factors such as surrounding atmosphere, end-capping has been reported. Because of these difficulties, very little information is available for the thermal decomposition of PPC in literature. In the present study, the thermal decomposition activation energies for PPCs with varying molecular weights were investigated and calculated using Ozawa method. The decomposition mechanism and some factors influencing the thermal decomposition were discussed.
[6,11]. To remove the catalyst residues, the as-made PPC was dissolved in chloroform, and then the solution was introduced into a funnel containing 5% HCl, followed by washing three times with deionised water. Subsequently, PPC was precipitated from the solution using alcohol, and dried under vacuum at 80 C for 48 h. The film specimen of PPC with thickness of about 60 mm was obtained by solution casting in chloroform. Other reagents were analytical grade and used without further treatment. The structure of PPC is shown as below:
2. Theoretical analysis
3.2. TG measurement
The basic equation of thermal decomposition can be expressed as follows: dx E ZA exp ð1Þ W n; dt RT
The non-isothermal decomposition of PPC was carried out using a PerkineElmer Thermogravimetric/ Differential Thermal Analyzer (TG/DTG) instrument against alumina as a reference. For each measurement 10 mg sample was used. The experiments were performed under protection of nitrogen atmosphere at different flow rates. The heating rates were adopted as 2, 4, 6, 8 and 10 C/min.
where x is the fraction of a structural quality, t time, R the gas constant, A the pre-exponential factor, E the activation energy of the decomposition reaction, n the order of the thermal reaction and W the decomposition residue. For the majority of solid state reactions, if the heating rate b is a constant, the following equation can be derived from Eq. (1) according to Ozawa method [3], log b 0:4567
E Zconstant: RT
ð2Þ
Eq. (2) indicates that the plot of log b versus 1/T is a straight line, from whose slope the activation energy E can be obtained.
O O
C
O
CH CH2 CH3
n
The number average molecular weight (Mn ) of PPC was measured with gel permeation chromatography (GPC) system including a Waters 515 pump and a Waters 2414 refractive index detector with tetrahydrofuran (THF) as eluent solvent. Calibration was performed with polystyrene standards with molecular weight ranging from 1260 to 3,850,000. The Mn data of used PPCs are listed in Table 1.
4. Results and discussion 4.1. Thermal hysteresis The thermal decomposition behaviour of a polymer is influenced by many factors, including heating rate, surrounding atmosphere and sample weight. Moreover, it has been reported that the sample shape also influenced on the thermal degradation behaviour. Fig. 1 shows the DTG and DTA curves of a block (size:
3. Experimental
Table 1 Molecular weights of used PPC specimens
3.1. Material
Specimen
Mn
MW
MW =Mn
1 2 3
42,000 56,100 86,300
85,300 119,700 523,900
2.03 2.13 6.07
Poly(propylene carbonate) (PPC) with alternating structure was prepared according to previous work
284 0
0
-5
-10
DTG(%/min)
DTG(%/min)
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
-10 -15 -20
-40 50 100 150 200 250 300 350 400
5 0 -5
DTA
DTA
-30
-50
50 100 150 200 250 300 350 400 4 2 0 -2 -4 -6 -8 -10 -12
-20
-10 -15 -20 -25
50 100 150 200 250 300 350 400
-30
50 100 150 200 250 300 350 400
Temperature(°C)
Temperature(°C)
(a)
(b)
Fig. 1. DTG and DTA curves for (a) the film sample and (b) the block sample, at heating rate of 10 C/min and N2 flow rate of 50 mL/min.
1 mm ! 3 mm ! 3 mm/thickness ! width ! length) and a film of specimen 2 measured at a heating rate of 10 C/ min and at a flow rate of 50 mL/min of nitrogen. It was evident that the block and the film samples exhibited different thermal decomposition characteristics. From Fig. 1a, it can be seen that the DTG curve for block sample was coarse and wide, while that for film was smooth and narrow (Fig. 1b). Several peak shoulders appeared randomly in both block and film samples under the same experimental conditions. The rough peak and peak shoulders resulted from the dissolved air or other gases produced during the thermal decomposition of block sample. Increasing temperature, the heat was removed by gases resulting in the fluctuation of weight and heat. This behaviour was also seen from some disturbances of DTA curves. The main peak of the DTG curve for block sample located at 263 C, but it appeared at 250 C for film sample. This was attributed to the larger mass of block sample that led to the difficulty in heat conduction within the sample and resulted in thermal hysteresis. The hysteresis resulted in the wide peak of DTG curve for block sample. The phenomenon can result in difficulty in kinetic analysis for block sample. Thus, film samples were used for all the investigations in this work. 4.2. Influence of protective gas flow rate The peak decomposition temperatures obtained from both DTG and TGA, and 5% weight loss temperatures at different flow rates of nitrogen for specimen 2 were tabulated in Table 2. The heating rate was set as 10 C/ min, and the flow rates of nitrogen were 50, 100, 150,
250 mL/min in turn. It can be seen that the peak decomposition temperatures varied from 249 to 251 C, and 5% weight loss temperature varied from 241 to 243 C. One can conclude that the flow rate showed little influence on the thermal decomposition of PPC.
4.3. Thermal decomposition behaviour of PPC with varying molecular weight Fig. 2 shows the DTG, TG and DTA curves of PPC samples with varying molecular weights at heating rate of 10 C/min under nitrogen. From Fig. 2a, there were an endothermic peak at 221 C and an exothermic platform from 255 to 317 C as shown in the DTA curve; accordingly in the DTG curve, there existed two peaks located at 221 and 333 C, respectively. From Fig. 2b, an endothermic peak at 251 C and an exothermic peak at 308 C were observed, while the DTG curve had two peaks at 251 and 336 C, respectively. Finally, Fig. 2c showed only one endothermic Table 2 Thermal decomposition temperatures of specimen 2 at different nitrogen flow rates Flow rate (mL/min)
T1a ( C)
T2b ( C)
T3c ( C)
50 100 150 250
251 250 249 249
251 250 250 250
244 241 241 242
a b c
Peak temperature in DTG curve. Peak temperature in DTA curve. 5% weight loss temperature.
285
DTG
40
-30
20
-40
0
-50
60
-10
TG
-20
DTG
40
-30
20
-40
0
-50
50 100 150 200 250 300 350
5 -5
DTA
50 100 150 200 250 300 350
-10
80
DTG
-20
40
-30
20 0
-40 50 100 150 200 250 300 350
-5 DTA
-15
-15 -20
-25
-25 50 100 150 200 250 300 350
DTA
-10
-20 -30
-10
TG
60
0
0
DTA
5 0 -5 -10 -15 -20 -25 -30 -35
50 100 150 200 250 300 350
80
0
100
DTG(%/min)
-20
Weight remaining(%)
TG
0
100
DTG(%/min)
60
DTG(%/min)
80
-10
Weight remaining(%)
0
100
DTA
DTA
Weight remaining(%)
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
-30
50
100 150 200 250 300 350
Temperature(°C)
Temperature(°C)
Temperature(°C)
(a)
(b)
(c)
Fig. 2. DTG, TG and DTA curves for PPC specimens with varying molecular weights. (a) Specimen 1, Mn Z42; 000; (b) specimen 2, Mn Z56; 100; (c) specimen 3, Mn Z86; 300.
peak at 254 C for the DTA curve and a corresponding peak at 252 C for the DTG curve. As discussed above, the exothermic DTA peak became smaller and smaller with increasing molecular
weight. The peak cannot be detected for specimen 3 with the greatest molecular weight. Furthermore, with the increase of molecular weight, the exothermic peak shifted to high temperature as can be seen in TG curve
Fig. 3. TG curves of PPC specimens with varying molecular weights obtained at heating rates of 2, 4, 6, 8 and 10 C/min, respectively. (a) Specimen 1, Mn Z42; 000; (b) specimen 2, Mn Z56; 100; (c) specimen 3, Mn Z86; 300. A: b Z 2 C/min; B: b Z 4 C/min; C: b Z 6 C/min; D: b Z 8 C/min; E: b Z 10 C/min.
286 1.1
1.1
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
logβ
logβ
X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15
0.2
1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05
1000/T(K-1)
1000/T(K-1)
(a)
(b) 1.1 1.0 0.9
logβ
0.8 0.7 0.6 0.5 0.4 0.3 0.2 1.75
1.80
1.85
1.90
1.95
2.00
2.05
1000/T(K-1)
(c) Fig. 4. Heating rate versus the reciprocal absolute temperature for PPC specimens with varying molecular weights. (a) Specimen 1, Mn Z42; 000; (b) specimen 2, Mn Z56; 100; (c) specimen 3, Mn Z86; 300. C: W Z 10%; B: W Z 30%; :: W Z 50%; 6: W Z 70%; -: W Z 80%; ,: W Z 90%.
for specimen 2. The endothermic peak accounted for thermal decomposition of PPC as reported in previous work [9]. From the DTG curves in Fig. 2, the maximum decomposition rate of PPC was as fast as 50%/min, and the decomposition ended at about 350 C. This process implied that PPC was extremely sensitive to temperature with dramatically fast thermal decomposition. Furthermore, TG curves showed that PPC decomposed completely at a maximum temperature of !350 C. The decomposition products were released in the form of gas. The composition of pyrolysates was investigated using TGeIR technique in previous work [9]. 4.4. Weight loss at various heating rates Fig. 3 shows the weight loss curves of PPC specimens with different molecular weights from dynamic measurements at heating rates of 2, 4, 6, 8 and 10 C/min, respectively. The flow rate of nitrogen was set as 50 mL/ min. The changing tendency for all the specimens with varying molecular weights behaved the same. With increasing the heating rate b from 2 to 10 C/min, the
Table 3 Decomposition activation energies derived with Ozawa method W (%)
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
E (kJ/mol) Specimen 1 ðMn Z42; 000Þ
Specimen 2 ðMn Z56; 100Þ
Specimen 3 ðMn Z86; 300Þ
102 106 106 105 105 105 104 103 103 103 102 99.8 99.1 99.0 98.5 111 122 138 149
104 110 114 117 119 121 122 122 122 121 119 116 112 111 108 106 109 133 149
113 114 116 117 115 112 108 102 97 91 87 83 81 80 82 87 95 117 192
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X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
O O O O H H H H H O CH CH2 O C O n1 C C O C O C C O C O C C O C On2 C C O H H H H H CH3 2 CH3 2 CH3 2 CH3 2 CH3
main chain random scission
O H O CH CH2 O C On1C CH2 + CH3 CH3
O O H H + H2C C C O C O C C O C On2 C C O H H H2 CH3H2 CH3H2
CO2
Scheme 1.
in Scheme 1. This is because reactive end groups in the aliphatic polycarbonates can lead to a chain unzipping reaction. Polycarbonates with high molecular weights or with alternating structure, however, contain very few these end groups. Accordingly, the unzipping decomposition mechanism of PPC can be depicted in Scheme 2. It is apparent from Scheme 2 that the polycarbonates with uncapped end groups and with lower molecular weight favour the chain unzipping decomposition. Because of the formation of the stable 5-member ring compound, 4-methyl-1,3-dioxolan-2-one, the chain unzipping reaction has low apparent activation energy. Therefore, chain unzipping decomposition takes place preferably compared with chain scission decomposition for the uncapped and low molecular weight polycarbonates. Comparing the activation energy at 5% weight loss, it was found that this value increased greatly with increasing molecular weight. This means that the content of end groups in PPC obviously affected the thermal stability. Increasing molecular weight provided an effective way to improve the thermal stability of PPC. However, PPC exhibited poor thermal stability when compared with other polymers [12,13]. This can be seen from the low decomposition energy (Table 2).
decomposition curves shifted to higher temperature. For specimen 1 (Fig. 3a), the temperatures of 5% weight loss were 187, 199, 206, 214 and 212 C, respectively, corresponding to different heating rates; while for specimen 3 (Fig. 3c), they were 216, 228, 236, 240 and 246 C, respectively. Moreover, the temperatures of maximum decomposition rate also increased with increasing b value. This phenomenon resulted from the temperature retardancy. 4.5. The decomposition activation energy Fig. 4 shows Ozawa plots for PPC specimens with varying molecular weights at weight loss W value ranging from 0.05 to 0.95. The calculated results from the dynamic TG data are listed in Table 3. It can be seen that the decomposition activation energy increased, then decreased and again increased in this turn with increasing weight loss. Presumably, this was due to the changes in mechanism during the thermal decomposition. The thermal decomposition process of main chain scission reaction can be expressed as in Scheme 1. Generally, aliphatic polycarbonates with high molecular weights or with alternating structure mainly exhibit the thermal decomposition mechanism shown O H
O
CH
CH2 O
C
O
CH C
O n
CH3
CH3
H2C
O
H C
C O H2
CH3
O
H C
O
CH3 O
+
H
O
CH
O
CH3
continuted
H2C
cyclization
C
( n +1 )
O
CH2 O
C
O
n-1
H C CH3
CH C
C O H2
heat
H
O C O H2
C
O
H C CH3
C O H2
CH3
O
O Scheme 2.
+
H
O
CH CH3
C O H2
H
H
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X.L. Lu et al. / Polymer Degradation and Stability 89 (2005) 282e288
As discussed in previous work [9,11], the glass transition temperature of synthesized PPC is slightly higher than room temperature. However, the starting thermal decomposition temperature is generally greater than 250 C. Based on these data, it can be concluded that PPC can be melt processed at a temperature lower than 180 C. Because of relatively low activation energy, it is also important to reduce the melt processing time. The increase in molecular weight is crucial for stabilizing the PPC, therefore, it is believed that end-capping should further improve its thermal stability.
Acknowledgements We thank the Ministry of Science and Technology of China (Grant No. 2002BA653C), Natural Science Foundation of Guangdong Province (Excellent Team Project, Grant No. 015007), Key Strategic Project of Chinese Academy of Sciences (Grant No. KJCX2-206B) and Key Project of Guangzhou Science and Technology Bureau (Grant No. 2001-z-114-01) for financial support of this work.
5. Conclusions References The dynamic thermal decomposition of poly(propylene carbonate) (PPC) was investigated under various conditions. The results indicated that the film and block samples exhibited distinct decomposition behaviours. Due to the coarse curve nature for block sample, the film sample was used for the measurement. Except for the exothermic peak, the PPC with Mn ! 60 kDa showed the endothermic peak in DTA examination. Both 5% weight loss temperature and the peak decomposition temperature increased with the increase in molecular weight and heating rate. Based on the results of the decomposition activation energy examination, we concluded that the content of end group affected greatly the thermal stability of PPC. Moreover, the PPC exhibited lower thermal stability as shown by its lower decomposition activation energy.
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