- Email: [email protected]
Essential work of fracture analysis of poly(propylene carbonate) with varying molecular weight
Polymer Testing 24 (2005) 699–703 www.elsevier.com/locate/polytest
Material Properties
Essential work of fracture analysis of poly(propylene carbonate) with varying molecular weight X.L. Wanga, R.K.Y. Lib, Y.X. Caoc, Y.Z. Menga,c,* a
Institute of Energy and Environmental Materials, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, People Republic of China b Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong c Guangzhou Institute of Chemistry, Chinese Academy of Sciences, P.O. Box 1122, Guangzhou 510650, People Republic of China Received 26 March 2005; accepted 10 May 2005
Abstract Fracture toughness of poly(propylene carbonate) (PPC) with various molecular weights was determined by the essential work of fracture concept (EWF) using double-edge notched (DENT) specimens. The thermal properties and thermomechanical properties were studied by thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and modulated differential scanning calorimetry (MDSC), respectively. The essential fracture work (we) of PPC tends to increase with increasing molecular weight. The TGA, DMA and MDSC results showed that the decomposition temperature and glass transition temperature of PPC increase with increasing molecular weight. It was indicated that the fracture toughness, thermal stability and glass transition temperature can be improved by increasing the molecular weight of PPC. q 2005 Elsevier Ltd. All rights reserved. Keywords: Essential work of fracture; Polycarbonate; Fracture; Toughness
1. Introduction Plastics have become an important part of modern life and are used in many application sectors, such as packaging, consumer products and many more. Each year about 100 m tons of plastics are produced worldwide. Most of synthesized plastics and synthetic polymers are produced from petrochemicals. As conventional plastics are persistent in the environment, improperly disposed of plastic materials are a significant source of environmental pollution, potentially harming our life. Poly(propylene carbonate) (PPC) synthesized from carbon dioxide and propylene oxide has been proved to be a biodegradable polymer owing to the ester bonds on its * Corresponding author. Tel./fax: C862 84114113. E-mail address: [email protected] (Y.Z. Meng).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.05.005
backbone. In this regard, it has attracted considerable attention recently [1–9]. In previous work [1,2], we have synthesized high molecular weight PPC with alternating structure by using zinc glutarate as catalyst under optimized reaction conditions. Since then, a set of pilot-scale experimental apparatus has been built up in our laboratory with a main autoclave of 500 L. In order to explore the potential application of the new polymer as a packaging material, it is necessary to study its properties comprehensively. The essential work of fracture (EWF) method is presently widely used to characterize the fracture of a material. The EWF method was originally Broberg’s idea [10] and then developed by Cotterel and Reddel for metals [11]. It has been recently used to investigate the toughness of different polymers [12–23]. The EWF analysis can also be used to investigate the fracture behavior of PPC [24]. The
700
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703
main aim of this study was to determine the EWF parameters of PPC with different molecular weights.
2. Experimental 2.1. Materials The PPC used in this work was prepared in our pilotscale experimental apparatus. It is an alternating copolymer derived from propylene oxide and carbon dioxide. The purities for the propylene oxide and carbon dioxide used were 99.5 and 99.9% respectively. The copolymerization method and conditions have been described in previous reports [1,2]. The polymerization product was dried directly without further treatment. The dried PPC polymer was compression molded into plates. The mold was heated to 150 and kept for 5 min before the PPC was loaded. The molding temperature was 150 and the molding cycle time was 5 min. The samples for thermal analysis and mechanical testing were cut out from the compression molded plates.
2.4. Mechanical properties Fracture toughness measurements were carried out using a universal tensile testing machine (Model 4206 Instron). The testing temperature was 158C and the cross-head speed of the tensile tester was 1 mm/min for all tests. The fracture toughness of the PPC was measured using the EWF analysis method. The double-edge-notchedtension (DENT) specimen geometry was employed. As demonstrated in previous studies [13,25–27], the specific essential fracture work is independent of specimen geometry. The DENT specimen geometry is used in the ESIS TC-4 Testing Protocol [28]. The sample gauge length, width and thickness were equal to 70, 25 and 1 mm respectively. Pre-notching was done by cutting with a bandsaw, followed by sharpening with a fresh razor blade. Dynamic mechanical analysis of the PPC was studied with a dynamic mechanical analyzer (DMA 2980, TA Instruments). A sample with dimensions 35!10!3 mm3 was cut out from the compression molded plates, and tested under the single cantilever mode using a fixed frequency of 1 Hz and a heating rate of 3/min.
3. Results and discussion 2.2. Molecular weight determination
2.3. Thermal analysis Thermal stability of the PPC was determined by simultaneous differential thermal analysis/thermal gravimetric analysis (DTA/TGA). Measurements were carried out using a thermal gravimetric analyzer (Seiko model SSC/5200) under helium/oxygen (50/50 ratio) gas flow conditions. The weight loss as a function of temperature was measured at a heating rate of 5/min. A DSC trace of the PPC was determined in a modulated thermal analysis MDSC instrument (model 2910) at a heating rate of 10/min under a nitrogen flow of 50 mL/min. Table 1 PPC material with varying number average molecular weights (Mn) PPC
PPC-1
PPC-2
PPC-3
PPC-4
Mn, Da
141 000
112 000
58 000
29 000
3.1. Thermal and thermomechanical behavior Fig. 1 shows the TG curves for PPC with varying number average molecular weights (Mn). The 5% weight loss temperature (TK5%) and 50% weight loss temperature (TK50%) are listed in Table 2. It is evidenced from Fig. 1 Weight remaining (%)
The Number Average Molecular Weight (Mn) value of the PPC was determined with a gel permeation chromatography (GPC) system consisting of a Waters 515 pump and a Waters 410 refractive-index detector. Chloroform was used as a solvent. The mobile phase was tetrahydrofuran. Calibration was performed with polystyrene standards with molecular weights in the range of 2000–1,950, 000 g/mol. Mn of the 4 samples ranged from 29 000 to 141 000 (see Table 1).
100 80 60 40
PPC-1 PPC-2 PPC-3 PPC-4
20 0 100
150
200
250
300
Temperature (˚C) Fig. 1. Thermal decomposition behavior for the PPC with varying molecular weights. Table 2 Thermal properties of PPC with varying molecular weights PPC
Mn, Da
TK5
TK50%
PPC-1 PPC-2 PPC-3 PPC-4
141 000 112 000 58 000 29 000
216 204 188 178
231 220 208 208
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703 7
4.0
Tan Delta
3.0 2.5
PPC-1 PPC-2 PPC-3 PPC-4
PPC-1 PPC-2 PPC-3 PPC-4
6
Heat flow
3.5
701
2.0 1.5 1.0
5
Tg
4 3
0.5 0.0
2 0
20
40
0
60
20
Temperature (˚C)
40
60
80
100
Temperature (˚C)
and Table 2 that both TK5% and TK50% of PPC increase steadily with increasing molecular weight. The result is the same as that reported in previous work [4]. However, the absolute value is smaller than that reported for carefully purified PPC samples. The lower value results from the fact that PPC dried directly without further treatment exhibits lower decomposition temperature than that dried with the further treatment of removing the residual catalyst. Fig. 3 presents the tan delta versus temperature curves for PPC with varying molecular weights, and Fig. 2 is the DSC traces for the PPC with varying molecular weights. It can be seen that the glass transition temperature (Tg) increases with increasing molecular weight of PPC. From Table 3 it can also be seen that the Tgs obtained from DMA and MDSC are different because of the different methods used, but the trends are the same.
Fig. 3. DSC trace for PPC with varying molecular weights.
(a) 0.30 0.25
Force (KN)
Fig. 2. Tan delta versus temperature for PPC with varying molecular weights.
PPC-1
0.20 L = 5.0
0.15
0.10 L = 9.4 0.05
0.00
3.2. Essential work of fracture analysis
0
1
2
3
4
5
6
5
6
Displacement (mm)
Wf Z We C Wp
(1)
We is surface-related, whereas Wp is volume-related. Wf can be rewritten by the related specific work terms: Table 3 Glass transition temperature for PPC with varying molecular weights PPC PPC-1 PPC-2 PPC-3 PPC-4
Mn, Da 141 000 112 000 58 000 29 000
Tg, (from DMA)
Tg, (from MDSC)
36 36 32 30
37 27 25 24
(b) 0.30 0.25
Force (KN)
According to the essential work of fracture (EWF) theory, the total work of fracture (Wf) is composed of the essential work required to fracture the polymer in its process zone (We) and the plastic work consumed by various deformation mechanisms in the plastic zone (Wp):
PPC-2
0.20
0.15 L = 5.1 0.10
L = 9.9
0.05
0.00 0
1
2
3
4
Displacement (mm) Fig. 4. Load-displacement curves for PPC with varying molecular weights.
Specific total fracture work (KJ/m2)
702
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703
200 180 160 140 120 100 80 60 40 20 0
indicating that the fracture toughness of PPC can be improved by increasing molecular weight. It has been reported in literature that the we values for polypropylene, rigid polyvinyl chloride and polyethylene are respectively as follows: polypropylene: 15 kJ/m2 [29]; rigid poly(vinyl chloride); 12–14 kJ/m2 [30]; polyethylene: 20 kJ/m2 [31]. It can be seen that the high molecular weight PPC possess a we value very close to polypropylene and poly(vinyl chloride).
PPC-1 PPC-2 PPC-3 PPC-4
0
2
4
6
8
10
12
4. Conclusions
Ligament (mm) Fig. 5. Plot of the specific total fracture work against ligament for the PPC with varying molecular weights.
Wf Z we tl C bwp tl2
(2)
Wf Z Wf =tl Z we C bwp l
(3)
where l is the ligament length, t the thickness of the specimen and b a shape factor related to the form of the plastic zone. Since both we and wp are material constants, and b is independent of l, wf will vary in a linear manner with l. By extrapolating the curve of wf vs l to zero ligament length, we can be easily determined. More recently, we have reported that the EWF method can be used for the characterization of PPC copolymer [24]. Fig. 4 depicts the load-displacement curves for PPC with varying molecular weights. It was found that the loaddisplacement curves at different ligament length are similar to one another for every PPC sample, so that one of the basic requirements of the EWF theory is met. Because of the different molecular weight and different molecular distribution, the load-displacement curves for different PPC are not similar. During the EWF measurement, it had been found that a significant amount of plastic deformation took place around the initial ligament region. A plot of the specific total fracture work against ligament length is shown in Fig. 5. It can be seen that a good linear relationship exists between specific total fracture work and ligament length, conforming to Eq. (3). From Fig. 5 the specific essential fracture work (we) and bwp can be obtained as listed in Table 4. It can be seen that we increases with increasing the molecular weight of PPC, Table 4 Specific essential fracture work for PPC with varying molecular weights PPC
Mn, Da
w2ekJ/m
bw2pkJ/m
PPC-1 PPC-2 PPC-3 PPC-4
141 000 112 000 58 000 29 000
12.6 10.5 8.9 9.1
1.9 4.3 8.1 11.9
The thermal and thermomechanical behavior of PPC with varying molecular weight was studied. The ductile fracture behavior of PPC with varying molecular weight was studied by the essential work of fracture (EWF) method using double edge notched tensile (DENT) specimens. Based on this study the following conclusions may be drawn: The specific essential fracture work of PPC increases with increasing the molecular weight. This indicates that the fracture toughness for the PPC can be improved by increasing molecular weight. The thermal and thermomechanical measurements revealed that increasing molecular weight led to an increase in both thermal decomposition and glass transition temperature of the PPC. Finally, the high molecular weight PPC possesses a we value very close to polypropylene and poly(vinyl chloride).
Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (Key Project, Grant No. 29734120), Key Strategic Project of Ministry of Sci and Tech of China (Grant No. 2002BA653C), and the Strategic Research Grant of the City University of Hong Kong, China (Project No. 7001566).
References [1] S.J. Wang, S.C. Tjong, L.C. Du, X.S. Zhao, Y.Z. Meng, J. Appl. Polym. Sci. 85 (11) (2002) 2327. [2] Q. Zhu, Y.Z. Meng, S.C. Tjong, X.S. Zhao, Y.L. Chen, Polym. Int. 51 (10) (2002) 1079. [3] Y.Z. Meng, L.C. Du, Q. Zhu, A.S. Hay, J. Polym. Sci. Part A, Polym. Chem. 40 (21) (2002) 3579. [4] X.H. Li, S.C. Tjong, Q. Zhu, Y.Z. Meng, J. Appl. Polym. Sci. 89 (12) (2003) 3301. [5] Q. Zhu, Y.Z. Meng, S.C. Tjong, W. Wan, Polym. Int. 52 (5) (2003) 799. [6] X.H. Li, S.C. Tjong, Y.Z. Meng, Q. Zhu, J. Polym. Sci. Part B, Polym. Phys. 41 (15) (2003) 1806. [7] X.H. Li, Y.Z. Meng, S.J. Wang, A.V. Rajulu, S.C. Tjong, J. Polym. Sci. Part B, Polym. Phys. 42 (4) (2004) 666.
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703 [8] L.C. Du, Y.Z. Meng, S.J. Wang, S.C. Tjong, J. Appl. Polym. Sci. 92 (3) (2004) 1840. [9] M.H. Chisholm, D. Navarro-Liobet, Z.P. Zhou, Macromolecules 35 (17) (2002) 6494. [10] K.B. Broberg, Int. J. Frac. 4 (1968) 11. [11] B. Cotterell, J.K. Reddel, Int. J. Frac. 13 (1977) 267. [12] J.S. Wu, Y.W. Mai, B. Cotterell, J. Mater. Sci. 28 (1993) 3373. [13] J.S. Wu, Y.W. Mai, Polym. Eng. Sci. 36 (1996) 2275. [14] S. Hashemi, J. Mater. Sci. 32 (1997) 1563. [15] S. Hashemi, J.G. Williams, Plast. Rub. Comp. 29 (2000) 294. [16] D. Ferrer-Balas, M.L. Maspoch, A.B. Martinez, E. Ching, R. K.Y. Li, Y.W. Mai, Polymer 42 (2001) 2665. [17] D.E. Mouzakis, N. Papke, J.S. Wu, J. Karger-Kocsis, J. Appl. Polym. Sci. 79 (2001) 842. [18] A. Arkhireyeva, S. Hashemi, Plast. Rub. Comp. 30 (7) (2001) 333. [19] A. Arkhireyeva, S. Hashemi, Polymer 43 (2) (2002) 289. [20] A. Arkhireyeva, S. Hashemi, Eng. Fract. Mech. 71 (4–6) (2004) 789.
703
[21] J. Karger-Kocsis, T. Barany, E.J. Moskala, Polymer 44 (19) (2003) 5696. [22] T. Barany, J. Karger-Kocsis, T. Czigany, Polym. Degrad. Stab. 82 (2) (2003) 271. [23] C.H. Ho, T. Vu-Khanh, Theor. Appl. Fract. Mech. 41 (1–3) (2004) 103. [24] K.L. Fung, H.X. Zhao, J.T. Wang, Y.Z. Meng, S.C. Tiong, R. K.Y. Li, Polym. Eng. Sci. 44 (3) (2004) 580. [25] E.C.Y. Ching, W.K.Y. Poon, R.K.Y. Li, Y.W. Mai, Polym. Eng. Sci. 40 (2000) 2558. [26] Y.W. Mai, P.J. Powell, J. Polym. Sci. Part B: Polym. Phys. 29 (1991) 785. [27] S. Hashemi, Polym. Eng. Sci. 37 (1997) 912. [28] Testing Protocol for Essential Work of Fracture, ESIS TC-4 Group, 1997. [29] M.N. Bareau, F. Perrin-sarazin, M.T. Ton-that, Polym. Eng. Sci. 44 (6) (2004) 1142. [30] G. Levita, Polym. Eng. Sci. 36 (1996) 2534. [31] B. Fayolle, J. Verdu, Polym. Eng. Sci. 45 (3) (2005) 424.
Material Properties
Essential work of fracture analysis of poly(propylene carbonate) with varying molecular weight X.L. Wanga, R.K.Y. Lib, Y.X. Caoc, Y.Z. Menga,c,* a
Institute of Energy and Environmental Materials, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, People Republic of China b Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong c Guangzhou Institute of Chemistry, Chinese Academy of Sciences, P.O. Box 1122, Guangzhou 510650, People Republic of China Received 26 March 2005; accepted 10 May 2005
Abstract Fracture toughness of poly(propylene carbonate) (PPC) with various molecular weights was determined by the essential work of fracture concept (EWF) using double-edge notched (DENT) specimens. The thermal properties and thermomechanical properties were studied by thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and modulated differential scanning calorimetry (MDSC), respectively. The essential fracture work (we) of PPC tends to increase with increasing molecular weight. The TGA, DMA and MDSC results showed that the decomposition temperature and glass transition temperature of PPC increase with increasing molecular weight. It was indicated that the fracture toughness, thermal stability and glass transition temperature can be improved by increasing the molecular weight of PPC. q 2005 Elsevier Ltd. All rights reserved. Keywords: Essential work of fracture; Polycarbonate; Fracture; Toughness
1. Introduction Plastics have become an important part of modern life and are used in many application sectors, such as packaging, consumer products and many more. Each year about 100 m tons of plastics are produced worldwide. Most of synthesized plastics and synthetic polymers are produced from petrochemicals. As conventional plastics are persistent in the environment, improperly disposed of plastic materials are a significant source of environmental pollution, potentially harming our life. Poly(propylene carbonate) (PPC) synthesized from carbon dioxide and propylene oxide has been proved to be a biodegradable polymer owing to the ester bonds on its * Corresponding author. Tel./fax: C862 84114113. E-mail address: [email protected] (Y.Z. Meng).
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.05.005
backbone. In this regard, it has attracted considerable attention recently [1–9]. In previous work [1,2], we have synthesized high molecular weight PPC with alternating structure by using zinc glutarate as catalyst under optimized reaction conditions. Since then, a set of pilot-scale experimental apparatus has been built up in our laboratory with a main autoclave of 500 L. In order to explore the potential application of the new polymer as a packaging material, it is necessary to study its properties comprehensively. The essential work of fracture (EWF) method is presently widely used to characterize the fracture of a material. The EWF method was originally Broberg’s idea [10] and then developed by Cotterel and Reddel for metals [11]. It has been recently used to investigate the toughness of different polymers [12–23]. The EWF analysis can also be used to investigate the fracture behavior of PPC [24]. The
700
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703
main aim of this study was to determine the EWF parameters of PPC with different molecular weights.
2. Experimental 2.1. Materials The PPC used in this work was prepared in our pilotscale experimental apparatus. It is an alternating copolymer derived from propylene oxide and carbon dioxide. The purities for the propylene oxide and carbon dioxide used were 99.5 and 99.9% respectively. The copolymerization method and conditions have been described in previous reports [1,2]. The polymerization product was dried directly without further treatment. The dried PPC polymer was compression molded into plates. The mold was heated to 150 and kept for 5 min before the PPC was loaded. The molding temperature was 150 and the molding cycle time was 5 min. The samples for thermal analysis and mechanical testing were cut out from the compression molded plates.
2.4. Mechanical properties Fracture toughness measurements were carried out using a universal tensile testing machine (Model 4206 Instron). The testing temperature was 158C and the cross-head speed of the tensile tester was 1 mm/min for all tests. The fracture toughness of the PPC was measured using the EWF analysis method. The double-edge-notchedtension (DENT) specimen geometry was employed. As demonstrated in previous studies [13,25–27], the specific essential fracture work is independent of specimen geometry. The DENT specimen geometry is used in the ESIS TC-4 Testing Protocol [28]. The sample gauge length, width and thickness were equal to 70, 25 and 1 mm respectively. Pre-notching was done by cutting with a bandsaw, followed by sharpening with a fresh razor blade. Dynamic mechanical analysis of the PPC was studied with a dynamic mechanical analyzer (DMA 2980, TA Instruments). A sample with dimensions 35!10!3 mm3 was cut out from the compression molded plates, and tested under the single cantilever mode using a fixed frequency of 1 Hz and a heating rate of 3/min.
3. Results and discussion 2.2. Molecular weight determination
2.3. Thermal analysis Thermal stability of the PPC was determined by simultaneous differential thermal analysis/thermal gravimetric analysis (DTA/TGA). Measurements were carried out using a thermal gravimetric analyzer (Seiko model SSC/5200) under helium/oxygen (50/50 ratio) gas flow conditions. The weight loss as a function of temperature was measured at a heating rate of 5/min. A DSC trace of the PPC was determined in a modulated thermal analysis MDSC instrument (model 2910) at a heating rate of 10/min under a nitrogen flow of 50 mL/min. Table 1 PPC material with varying number average molecular weights (Mn) PPC
PPC-1
PPC-2
PPC-3
PPC-4
Mn, Da
141 000
112 000
58 000
29 000
3.1. Thermal and thermomechanical behavior Fig. 1 shows the TG curves for PPC with varying number average molecular weights (Mn). The 5% weight loss temperature (TK5%) and 50% weight loss temperature (TK50%) are listed in Table 2. It is evidenced from Fig. 1 Weight remaining (%)
The Number Average Molecular Weight (Mn) value of the PPC was determined with a gel permeation chromatography (GPC) system consisting of a Waters 515 pump and a Waters 410 refractive-index detector. Chloroform was used as a solvent. The mobile phase was tetrahydrofuran. Calibration was performed with polystyrene standards with molecular weights in the range of 2000–1,950, 000 g/mol. Mn of the 4 samples ranged from 29 000 to 141 000 (see Table 1).
100 80 60 40
PPC-1 PPC-2 PPC-3 PPC-4
20 0 100
150
200
250
300
Temperature (˚C) Fig. 1. Thermal decomposition behavior for the PPC with varying molecular weights. Table 2 Thermal properties of PPC with varying molecular weights PPC
Mn, Da
TK5
TK50%
PPC-1 PPC-2 PPC-3 PPC-4
141 000 112 000 58 000 29 000
216 204 188 178
231 220 208 208
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703 7
4.0
Tan Delta
3.0 2.5
PPC-1 PPC-2 PPC-3 PPC-4
PPC-1 PPC-2 PPC-3 PPC-4
6
Heat flow
3.5
701
2.0 1.5 1.0
5
Tg
4 3
0.5 0.0
2 0
20
40
0
60
20
Temperature (˚C)
40
60
80
100
Temperature (˚C)
and Table 2 that both TK5% and TK50% of PPC increase steadily with increasing molecular weight. The result is the same as that reported in previous work [4]. However, the absolute value is smaller than that reported for carefully purified PPC samples. The lower value results from the fact that PPC dried directly without further treatment exhibits lower decomposition temperature than that dried with the further treatment of removing the residual catalyst. Fig. 3 presents the tan delta versus temperature curves for PPC with varying molecular weights, and Fig. 2 is the DSC traces for the PPC with varying molecular weights. It can be seen that the glass transition temperature (Tg) increases with increasing molecular weight of PPC. From Table 3 it can also be seen that the Tgs obtained from DMA and MDSC are different because of the different methods used, but the trends are the same.
Fig. 3. DSC trace for PPC with varying molecular weights.
(a) 0.30 0.25
Force (KN)
Fig. 2. Tan delta versus temperature for PPC with varying molecular weights.
PPC-1
0.20 L = 5.0
0.15
0.10 L = 9.4 0.05
0.00
3.2. Essential work of fracture analysis
0
1
2
3
4
5
6
5
6
Displacement (mm)
Wf Z We C Wp
(1)
We is surface-related, whereas Wp is volume-related. Wf can be rewritten by the related specific work terms: Table 3 Glass transition temperature for PPC with varying molecular weights PPC PPC-1 PPC-2 PPC-3 PPC-4
Mn, Da 141 000 112 000 58 000 29 000
Tg, (from DMA)
Tg, (from MDSC)
36 36 32 30
37 27 25 24
(b) 0.30 0.25
Force (KN)
According to the essential work of fracture (EWF) theory, the total work of fracture (Wf) is composed of the essential work required to fracture the polymer in its process zone (We) and the plastic work consumed by various deformation mechanisms in the plastic zone (Wp):
PPC-2
0.20
0.15 L = 5.1 0.10
L = 9.9
0.05
0.00 0
1
2
3
4
Displacement (mm) Fig. 4. Load-displacement curves for PPC with varying molecular weights.
Specific total fracture work (KJ/m2)
702
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703
200 180 160 140 120 100 80 60 40 20 0
indicating that the fracture toughness of PPC can be improved by increasing molecular weight. It has been reported in literature that the we values for polypropylene, rigid polyvinyl chloride and polyethylene are respectively as follows: polypropylene: 15 kJ/m2 [29]; rigid poly(vinyl chloride); 12–14 kJ/m2 [30]; polyethylene: 20 kJ/m2 [31]. It can be seen that the high molecular weight PPC possess a we value very close to polypropylene and poly(vinyl chloride).
PPC-1 PPC-2 PPC-3 PPC-4
0
2
4
6
8
10
12
4. Conclusions
Ligament (mm) Fig. 5. Plot of the specific total fracture work against ligament for the PPC with varying molecular weights.
Wf Z we tl C bwp tl2
(2)
Wf Z Wf =tl Z we C bwp l
(3)
where l is the ligament length, t the thickness of the specimen and b a shape factor related to the form of the plastic zone. Since both we and wp are material constants, and b is independent of l, wf will vary in a linear manner with l. By extrapolating the curve of wf vs l to zero ligament length, we can be easily determined. More recently, we have reported that the EWF method can be used for the characterization of PPC copolymer [24]. Fig. 4 depicts the load-displacement curves for PPC with varying molecular weights. It was found that the loaddisplacement curves at different ligament length are similar to one another for every PPC sample, so that one of the basic requirements of the EWF theory is met. Because of the different molecular weight and different molecular distribution, the load-displacement curves for different PPC are not similar. During the EWF measurement, it had been found that a significant amount of plastic deformation took place around the initial ligament region. A plot of the specific total fracture work against ligament length is shown in Fig. 5. It can be seen that a good linear relationship exists between specific total fracture work and ligament length, conforming to Eq. (3). From Fig. 5 the specific essential fracture work (we) and bwp can be obtained as listed in Table 4. It can be seen that we increases with increasing the molecular weight of PPC, Table 4 Specific essential fracture work for PPC with varying molecular weights PPC
Mn, Da
w2ekJ/m
bw2pkJ/m
PPC-1 PPC-2 PPC-3 PPC-4
141 000 112 000 58 000 29 000
12.6 10.5 8.9 9.1
1.9 4.3 8.1 11.9
The thermal and thermomechanical behavior of PPC with varying molecular weight was studied. The ductile fracture behavior of PPC with varying molecular weight was studied by the essential work of fracture (EWF) method using double edge notched tensile (DENT) specimens. Based on this study the following conclusions may be drawn: The specific essential fracture work of PPC increases with increasing the molecular weight. This indicates that the fracture toughness for the PPC can be improved by increasing molecular weight. The thermal and thermomechanical measurements revealed that increasing molecular weight led to an increase in both thermal decomposition and glass transition temperature of the PPC. Finally, the high molecular weight PPC possesses a we value very close to polypropylene and poly(vinyl chloride).
Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (Key Project, Grant No. 29734120), Key Strategic Project of Ministry of Sci and Tech of China (Grant No. 2002BA653C), and the Strategic Research Grant of the City University of Hong Kong, China (Project No. 7001566).
References [1] S.J. Wang, S.C. Tjong, L.C. Du, X.S. Zhao, Y.Z. Meng, J. Appl. Polym. Sci. 85 (11) (2002) 2327. [2] Q. Zhu, Y.Z. Meng, S.C. Tjong, X.S. Zhao, Y.L. Chen, Polym. Int. 51 (10) (2002) 1079. [3] Y.Z. Meng, L.C. Du, Q. Zhu, A.S. Hay, J. Polym. Sci. Part A, Polym. Chem. 40 (21) (2002) 3579. [4] X.H. Li, S.C. Tjong, Q. Zhu, Y.Z. Meng, J. Appl. Polym. Sci. 89 (12) (2003) 3301. [5] Q. Zhu, Y.Z. Meng, S.C. Tjong, W. Wan, Polym. Int. 52 (5) (2003) 799. [6] X.H. Li, S.C. Tjong, Y.Z. Meng, Q. Zhu, J. Polym. Sci. Part B, Polym. Phys. 41 (15) (2003) 1806. [7] X.H. Li, Y.Z. Meng, S.J. Wang, A.V. Rajulu, S.C. Tjong, J. Polym. Sci. Part B, Polym. Phys. 42 (4) (2004) 666.
X.L. Wang et al. / Polymer Testing 24 (2005) 699–703 [8] L.C. Du, Y.Z. Meng, S.J. Wang, S.C. Tjong, J. Appl. Polym. Sci. 92 (3) (2004) 1840. [9] M.H. Chisholm, D. Navarro-Liobet, Z.P. Zhou, Macromolecules 35 (17) (2002) 6494. [10] K.B. Broberg, Int. J. Frac. 4 (1968) 11. [11] B. Cotterell, J.K. Reddel, Int. J. Frac. 13 (1977) 267. [12] J.S. Wu, Y.W. Mai, B. Cotterell, J. Mater. Sci. 28 (1993) 3373. [13] J.S. Wu, Y.W. Mai, Polym. Eng. Sci. 36 (1996) 2275. [14] S. Hashemi, J. Mater. Sci. 32 (1997) 1563. [15] S. Hashemi, J.G. Williams, Plast. Rub. Comp. 29 (2000) 294. [16] D. Ferrer-Balas, M.L. Maspoch, A.B. Martinez, E. Ching, R. K.Y. Li, Y.W. Mai, Polymer 42 (2001) 2665. [17] D.E. Mouzakis, N. Papke, J.S. Wu, J. Karger-Kocsis, J. Appl. Polym. Sci. 79 (2001) 842. [18] A. Arkhireyeva, S. Hashemi, Plast. Rub. Comp. 30 (7) (2001) 333. [19] A. Arkhireyeva, S. Hashemi, Polymer 43 (2) (2002) 289. [20] A. Arkhireyeva, S. Hashemi, Eng. Fract. Mech. 71 (4–6) (2004) 789.
703
[21] J. Karger-Kocsis, T. Barany, E.J. Moskala, Polymer 44 (19) (2003) 5696. [22] T. Barany, J. Karger-Kocsis, T. Czigany, Polym. Degrad. Stab. 82 (2) (2003) 271. [23] C.H. Ho, T. Vu-Khanh, Theor. Appl. Fract. Mech. 41 (1–3) (2004) 103. [24] K.L. Fung, H.X. Zhao, J.T. Wang, Y.Z. Meng, S.C. Tiong, R. K.Y. Li, Polym. Eng. Sci. 44 (3) (2004) 580. [25] E.C.Y. Ching, W.K.Y. Poon, R.K.Y. Li, Y.W. Mai, Polym. Eng. Sci. 40 (2000) 2558. [26] Y.W. Mai, P.J. Powell, J. Polym. Sci. Part B: Polym. Phys. 29 (1991) 785. [27] S. Hashemi, Polym. Eng. Sci. 37 (1997) 912. [28] Testing Protocol for Essential Work of Fracture, ESIS TC-4 Group, 1997. [29] M.N. Bareau, F. Perrin-sarazin, M.T. Ton-that, Polym. Eng. Sci. 44 (6) (2004) 1142. [30] G. Levita, Polym. Eng. Sci. 36 (1996) 2534. [31] B. Fayolle, J. Verdu, Polym. Eng. Sci. 45 (3) (2005) 424.