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The investigation of argon plasma surface modification to polyethylene: Quantitative ATR-FTIR spectroscopic analysis
EUROPEAN POLYMER JOURNAL
European Polymer Journal 42 (2006) 1625–1633
www.elsevier.com/locate/europolj
The investigation of argon plasma surface modification to polyethylene: Quantitative ATR-FTIR spectroscopic analysis Lai-Shun Shi *, Lu-Yan Wang, Yu-Na Wang School of Chemistry and Chemical Engineering, South Campus, Shandong University, Jinan 250061, PR China Received 31 August 2005; received in revised form 11 January 2006; accepted 11 January 2006 Available online 23 March 2006
Abstract This paper studied the surface modification of argon plasma to polyethylene by using ATR-FTIR analysis. The mass loss ratio has maximum value at discharge time of 70–120 s or discharge power of 62 W by using argon plasma treatment for polyethylene. New surface structure was formed after polyethylene was treated by argon plasma. The peroxide bond peak area also has maximum value at discharge time of 70–120 s or discharge power of 62 W. The C@C nonsaturated double bond absorb peaks were appeared at 1640 cm1, 1549 cm1 and 1528 cm1 after polyethylene treated by argon plasma. The C@C nonsaturated double bond absorb peak area has minimum value at discharge time of 60–70 s and the power of 65 W. The C@C nonsaturated double bond absorb peak area has maximum value at discharge power of 62–72 W and the discharge time of 2 min. The absorption peak intensity of 2916 cm1 methylene nonsymmetry stretch vibration, 2848 cm1 methylene symmetry stretch vibration, 1463 cm1 methylene nonsymmetry changing angle vibration, and 719 cm1 methylene swing in plane vibration was decreased greatly. The four absorption peaks intensity has maximum value at discharge time of 120 s or discharge power of 62 W. 2006 Elsevier Ltd. All rights reserved. Keywords: Polyethylene; Plasma; Argon; Surface modification; ATR-FTIR
1. Introduction Cold plasma is a new technology which can be used to modify the surface properties of macromolecular materials to meet certain requirements, e.g., adhesion, printing ability of polymer film, dyeing ability, hydrophilicity and biocompatability. Zhao Yaqiu studied maleic anhydride and carboxylic acid groups can be chemically bonded to poly(vinylidene fluoride) (PVDF) surfaces with the *
Corresponding author. Fax: +86 531 88392980. E-mail address: [email protected] (L.-S. Shi).
use of microwave plasma energy [1]. Maleic anhydride reacts with the PVDF surface through a C@C bond opening of the maleic anhydride ring, and its hydrolysis results in chemically attached carboxylic acid groups on the PVDF surface. The extent of the surface reactions can be monitored using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Heung Kim and Urban studied the effect of discharge gases on microwave plasma reactions of imidazole on poly(dimethylsiloxane) surfaces by quantitative ATR-FTIR spectroscopic analysis [2]. Schiller studied the chemical structure and properties of
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.01.007
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L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
plasma-polymerized maleic anhydride films [3]. The film chemical structures were obtained using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Surface derivatization reactions using decylamine and benzylamine were used to demonstrate their surface reactivity toward nucleophilic moieties and to change the surface free energy of the plasma polymer films. Matthews et al. [4] investigated the etching mechanism of polyethylene terephthalate (PET) films treated in helium and oxygenated-helium atmospheric plasmas. Differential scanning calorimetry (DSC) was used to characterize pre- and post-exposure films. Weight changes and the degree of solubility were also determined. A model was developed to predict the change in the degree of solubility for polyphase surfaces considering the etching rate per phase. In previous articles [5,6], the surface flame retardancy of polypropylene by crosslinking of methane plasma has been studied. Also, we have reported the characteristics of surface modification of polypropylene and poly(ethylene terephthalate) with the CH4, CF4 and CF4/CH4 plasmas to impart flame retardancy by using burning rate and TGA analysis, and its influence to flammability [7–11]. The result indicates that the flame retardancy behavior of polymers was influenced by the polymer’s structures. The likely degradation mechanism of polymer films treated by plasma was proposed based on the experiment results. This paper reports the effect of discharge time and power on the mass loss ratio and surface composition of polyethylene treated by argon plasma by using ATR-FTIR analysis. 2. Experimental 2.1. Sample preparation Linear low density polyethylene (PE) was used in this study. Specimen thicknesses of 1.0 mm was processed first by a two-roll mixing mill and then reduced to 0.8 mm thickness through a platform vulcanizing press at a pressure of 10 MPa and at 160 C for 10 min. Before mounting the three 0.8 mm PE films (40 mm length with different width) on the ground electrode, it was necessary (a) to wash the films with a nonionic detergent solution, (b) to rinse them with deionized water and chloroform, and then (c) to dry them overnight in air. Argon (Jinan DEYANG Gas Factory) supplied in 99.99% purity was used without further purification.
2.2. Low temperature RF plasma reactor A bell-shaped plasma reactor of U200 · 250 mm chamber was supplied by Changzhou SHITAI Plasma Technology Factory. The glow discharge was generated inner-capacitively, operating at 13.56 MHz. An RF generator of 13.56 MHz was connected to the upper electrode; the lower was grounded. The polyethylene samples were placed at the lower grounded electrode to prevent the charging of the insulated polyethylene. Argon was fed to the chamber through a needle valve. Pump-out was usually at the base of the bell jar. The flow rate was measured with a mass flow meter (Model D07-7/ZM, Jianzhong Machinery Factory, Beijing) and the vacuum in the chamber was controlled by a vacuum pressure gauge (Model ZD0-2). The reaction system was pumped down to 0.8 Pa or lower. Argon, adjusted to a desired flow rate, was introduced into the chamber. Once the pressure remained constant of 25–30 Pa, the glow was initiated at the desired power for certain treated time. After plasma treatment, the vacuum chamber simply vents to the atmosphere. Mass loss ratio Dm (mg/cm2) can be calculated based on the mass changes per square of centimeter. The treated PE samples were measured by ATR-FTIR analysis after placed in air for 48 h. 2.3. ATR-FTIR spectrum ATR-FTIR spectra were collected on a Bruker Tensor-27 FTIR single beam spectrometer having a 4 cm1 resolution. A KRS-5 crystal with 60 angle 50 · 20 · 5 mm was used. Each spectrum represents 64 co-added scans ratioed against a reference spectrum obtained by recording 64 co-added scans of an empty ATR cell. 3. Results and discussion 3.1. Effect of argon plasma discharge time on the mass loss ratio Fig. 1 gives the mass loss ratio of PE plotted against argon plasma discharge time at the condition of working pressure of 25–30 Pa and power of 65 W. It was found that the mass loss ratio increased along with the increasing of discharge time (t). Then the mass loss ratio got maximum value at discharge time of 70–120 s. Further prolong the discharge time, the mass loss ratio decreased and remained constant.
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
0.10
0.050
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ATR Units
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1
0.020
2 0.00
0.015 0.010 0
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Δm / (mg/cm2)
0.040
0.06
0.08
0.045
100
200
300
400
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3500
600
1500
1000
500
Fig. 2. The ATR-FTIR spectra of PE.
vibration, methylene nonsymmetry changing angle vibration, and methylene swing in plane vibration, respectively. Fig. 3 gives the local amplificatory ATR-FTIR spectra of PE. Curve a represents original PE. Curve b represents the ATR-FTIR spectrum of PE treated by argon plasma at the condition of pressure 28 Pa, power 32 W, and discharge time 2 min. As shown in Fig. 3, the new absorption peak of 3398 cm1 was attributed to peroxide bond. It is obvious that peroxide bond was formed by the reaction of free radicals, which was formed by argon plasma, with oxygen and water in air. The reaction is as follows:
0.10
Fig. 4 represents the influence of argon plasma discharge time to 3398 cm1 peroxide bond absorb peak area (A) at the condition of working pressure
0.06 0.04
(a)
0.02
ATR Units
0.08
3.2. Effect of argon plasma discharge time on the surface chemical composition of PE The polyethylene sample was analyzed by ATRFTIR method at the same condition after it was treated by argon plasma and then placed in air for 48 h. Fig. 2 gives the ATR-FTIR spectra of PE. Curve 1 represents original PE. Curve 2 represents the ATR-FTIR spectrum of PE treated by argon plasma at the condition of pressure 28 Pa, power 32 W, and discharge time 2 min. As shown in Fig. 2, the absorption peaks of 2916, 2848, 1463, and 719 cm1 were attributed to methylene nonsymmetry stretch vibration, methylene symmetry stretch
2000
(b)
3398
0.00
As shown in Fig. 1, the mass loss ratio increased along with the increasing of discharge time when the discharge time was below 120 s. This is because argon plasma belongs to etching atmosphere. The active species in the plasma, e.g., electron and ion, bombard the surface macromolecular chains of PE film, which lead to the breakage of C–H and C–C bond and produce more oligomers and small organic molecules. Therefore, the mass loss ratio increases. The etching effect is the main reaction process in the region. When the discharge time is greater than 120 s, more radicals are produced at the surface. The crosslinking among the radicals becomes main reaction process. A highly crosslinking graphite-like carbon membrane is formed, which prevents the further etching effect of argon plasma to PE surface [5]. Therefore, the mass loss ratio remained constant at prolonged treatment time.
2500
Wavenumber cm-1
t/s
Fig. 1. The plot of mass loss ratio versus argon plasma discharge time.
3000
3400
3300
3200
3100
3000
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2800
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Wavenumber cm-1
Fig. 3. The local amplificatory ATR-FTIR spectra of PE.
0.020 0.015
1640
1549 1528 (b)
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ATR Units
0.010
(a)
1800
1700
1600
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1400
1300
Wavenumber cm-1
Fig. 5. The local amplificatory ATR-FTIR spectra of PE.
0.100
0.095
0.090
A
of 25–30 Pa and power of 65 W. It was found that the peroxide bond absorb peak area increased along with the increasing of discharge time. Then the peak area got maximum value at discharge time of 70– 120 s. Further prolong the discharge time, the peak area decreased and remained constant. It is obvious that the changing trend of peroxide bond absorb peak area with plasma discharge time is similar to that of mass loss ratio. It gives that the best plasma discharge time is 70–120 s. Fig. 5 gives the local amplificatory ATR-FTIR spectra of PE. Curve a represents original PE. Curve b represents the ATR-FTIR spectrum of PE treated by argon plasma at the condition of pressure 28 Pa, power 32 W, and discharge time 2 min. As shown in Fig. 5, the new absorption peaks of 1640, 1549, and 1528 cm1 were attributed to C@C nonsaturated double bond on the treated PE surface. It can be concluded that C@C nonsaturated double bond was formed by the reaction of border free radicals, which were formed by argon plasma’s etching and hydrogen abstraction. The reaction is as follows:
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L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
0.000
1628
0.085
Fig. 6 represents the influence of argon plasma discharge time to 1640 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the power of 65 W. Fig. 7 represents the influence of argon plasma discharge time to 1549 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the power of 65 W. It is obvious that the changing trend
0.080
0.075 0
100
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300
400
500
600
t/s
Fig. 6. 1640 cm1C@C nonsaturated double bond absorb peak area versus discharge time.
0.07
0.034
0.032
0.06
A
A
0.030
0.05
0.028
0.026
0.04
0.024
0.03 0
100
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400
500
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t/s
Fig. 4. The plot of 3398 cm1 peroxide bond absorb peak area versus discharge time.
0
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t/s
Fig. 7. 1549 cm1 C@C nonsaturated double bond absorb peak area versus discharge time.
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
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of C@C nonsaturated double bond absorb peak area with plasma discharge time is reverse to that of peroxide bond peak area. It was found that the C@C nonsaturated double bond absorb peak area decreased along with the increasing of discharge time. Then the peak area got minimum value at discharge time of 60–70 s. Further prolong the discharge time, the C@C nonsaturated double bond absorb peak area increased. By looking at the formation mechanism of C@C nonsaturated double bond and peroxide bond, it belongs to competition reactions. The surface free radical concentration is constant after PE is treated by argon plasma. The more the peroxide bond formed, the less the C@C nonsaturated double bond formed. It is obvious that the surface structure is favorable to form peroxide bond, go against to form C@C nonsaturated double bond at the condition of argon plasma discharge time approximately 70 s. That is to say, it has the following structure and reaction:
By contrast the peak intensity in Fig. 2, the absorption peak intensity of 2916 cm1 methylene nonsymmetry stretch vibration, 2848 cm1 methylene symmetry stretch vibration, 1463 cm1 methylene nonsymmetry changing angle vibration, and 719 cm1 methylene swing in plane vibration was decreased greatly after PE was treated by argon plasma. It further proves that argon plasma is a continuously etching and hydrogen abstraction process. This is because the concentration of CH2 group decreases at the constant sampling depth after PE is treated by argon plasma. Therefore, the four absorption peaks’ intensity decreases. In Fig. 2, the absorption peaks of 2916 and 2848 cm1 were attributed to methylene nonsymmetry stretch vibration and methylene symmetry stretch vibration, respectively. It was integrated for the two peaks at the range of 2984–2780 cm1 by using ATR-FTIR software. The result is shown in Fig. 8. As shown in Fig. 8, the absorption peak area has maximum value at discharge time of
When the plasma discharge time is greater than 120 s, the surface structure is favorable to form C@C nonsaturated double bond and crosslinking (–CH h and i C h), go against to form peroxide bond. It has the following structure and reaction:
120 s. This phenomenon was unexpected, and need further investigation. Fig. 9 represents the influence of argon plasma discharge time to 1463 cm1 methylene nonsymmetry changing angle vibration absorb peak area at the
or
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L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633 10
0.5
9 8
0.4
6
A
A
7
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5 4
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3 2 0
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600
0
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Fig. 8. CH2 stretch vibration absorption peak area versus plasma discharge time.
Fig. 10. 719 cm1 methylene swing in plane vibration absorb peak area versus plasma discharge time.
0.9
0.030
0.8
0.025
0.7
Δm(mg/cm2)
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A
0.6 0.5 0.4 0.3
0.015
0.010
0.005
0.2 0
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400
500
600
t/s
Fig. 9. 1463 cm1 methylene nonsymmetry changing angle vibration absorb peak area versus plasma discharge time.
condition of working pressure of 25–30 Pa and the power of 65 W. Fig. 10 represents the influence of argon plasma discharge time to 719 cm1 methylene swing in plane vibration absorb peak area at the condition of working pressure of 25–30 Pa and the power of 65 W. As shown in Figs. 9 and 10, the absorption peak area also has maximum value at discharge time of 120 s. 3.3. Effect of argon plasma discharge power on the mass loss ratio Fig. 11 gives the mass loss ratio of PE plotted against argon plasma discharge power at the condition of working pressure of 25–30 Pa and the discharge time of 2 min. It was found that the mass
0.000 30
40
50
60
70
80
90
100
110
P/W
Fig. 11. The plot of mass loss ratio versus argon plasma discharge power.
loss ratio increased along with the increasing of discharge power. Then the mass loss ratio got maximum value at discharge power of 62 W. Further increasing the discharge power, the mass loss ratio decreased rapidly and remained constant. As shown in Fig. 11, the mass loss ratio increased along with the increasing of discharge power when it was below 62 W. This is because argon plasma belongs to etching atmosphere. The active species in the plasma, e.g., electron and ion, gain more energy and bombard the surface macromolecular chains of PE film, which lead to the breakage of C–H and C–C bond and produce more oligomers and small organic molecules. Therefore, the mass loss ratio increases. The etching effect is the main reaction process in the region. When the discharge
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633 0.14
0.12
0.10
A
power is greater than 62 W, more radicals are produced at the surface. The crosslinking among the radicals becomes main reaction process. A highly crosslinking graphite-like carbon membrane is formed, which prevents the further etching effect of argon plasma to PE surface[5]. Therefore, the mass loss ratio remained constant at increased treatment power.
0.04
0.035
0.030
A
0.025
0.020
0.015
50
60
70
80
90
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110
P/W
Fig. 12. The plot of 3398 cm1 peroxide bond absorb peak area versus discharge power.
0.02 30
40
50
60
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110
P/W
Fig. 13. 1640 cm1 C@C nonsaturated double bond absorb peak area versus discharge power.
0.06
0.05
0.04
A
The polyethylene sample was analyzed by ATRFTIR method at the same condition after it was treated by argon plasma and then placed in air for 48 h. Fig. 12 represents the influence of argon plasma discharge power to 3398 cm1 peroxide bond absorb peak area at the condition of working pressure of 25–30 Pa and discharge time of 2 min. It was found that the peroxide bond absorb peak area increased along with the increasing of discharge power. Then the peak area got maximum value at discharge power of 62–72 W. Further increasing the discharge power, the mass loss ratio decreased and remained constant. It is obvious that the changing trend of peroxide bond absorb peak area with plasma discharge power is similar to that of mass loss ratio. It proves that the best plasma discharge power is 62–72 W. Fig. 13 represents the influence of argon plasma discharge power to 1640 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the discharge time of 2 min. Fig. 14 represents the influence of
40
0.08
0.06
3.4. Effect of argon plasma discharge power on the surface chemical composition of PE
0.010 30
1631
0.03
0.02
0.01 30
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60
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P/W
Fig. 14. 1549 cm1 C@C nonsaturated double bond absorb peak area versus discharge power.
argon plasma discharge power to 1549 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the discharge time of 2 min. It is obvious that the changing trend of C@C nonsaturated double bond absorb peak area with plasma discharge power is similar to that of peroxide bond peak area. It was found that the C@C nonsaturated double bond absorb peak area increased along with the increasing of discharge power. Then the peak area got maximum value at discharge power of 62–72 W. Further increasing the discharge power, the C@C nonsaturated double bond absorb peak area decreased. It is obvious that the influence of plasma discharge time and power to C@C nonsaturated double bond absorb peak area is different. The
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
surface structure is favorable to form peroxide bond and C@C nonsaturated double bond at the condition of argon plasma discharge power 62–72 W. In Fig. 2, the absorption peaks of 2916 and 2848 cm1 were attributed to methylene nonsymmetry stretch vibration and methylene symmetry stretch vibration, respectively. It was integrated for the two peaks at the range of 2984–2780 cm1 by using ATR-FTIR software. The result is shown in Fig. 15. As shown in Fig. 15, the absorption peak area has maximum value at discharge power of 62 W. This phenomenon was unexpected, and also need further investigation. Fig. 16 represents the influence of argon plasma discharge power to 1463 cm1 methylene nonsym-
0.28 0.26 0.24 0.22
A
1632
0.20 0.18 0.16 0.14 30
40
50
60
70
80
90
100
110
P/W
Fig. 17. 719 cm1 methylene swing in plane vibration absorb peak area versus plasma discharge power.
4.4
metry changing angle vibration absorb peak area at the condition of working pressure of 25–30 Pa and discharge time of 2 min. Fig. 17 represents the influence of argon plasma discharge power to 719 cm1 methylene swing in plane vibration absorb peak area at the condition of working pressure of 25–30 Pa and discharge time of 2 min. As shown in Figs. 16 and 17, the absorption peak area also has maximum value at discharge power of 62 W.
4.2 4.0 3.8
A
3.6 3.4 3.2 3.0 2.8 2.6 2.4 30
40
50
60
70
80
90
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110
P/W Fig. 15. CH2 stretch vibration absorption peak area versus plasma discharge power.
0.42 0.40 0.38 0.36
A
0.34 0.32 0.30 0.28 0.26 0.24 0.22 30
40
50
60
70
80
90
100
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P/W
Fig. 16. 1463 cm1 methylene nonsymmetry changing angle vibration absorb peak area versus plasma discharge power.
4. Conclusions (1) The mass loss ratio increased along with the increasing of discharge time when PE was treated by argon plasma. Then the mass loss ratio got maximum value at discharge time of 70–120 s. Further prolong the discharge time, the mass loss ratio decreased and remained constant. (2) New surface structure was formed after polyethylene was treated by argon plasma. The influence effect of peroxide bond peak area is the same as that of mass loss ratio. The peroxide bond peak area also has maximum value at discharge time of 70–120 s. (3) The C@C nonsaturated double bond absorb peaks were appeared at 1640 cm1, 1549 cm1 and 1528 cm1 after polyethylene treated by argon plasma. It was found that the C@C nonsaturated double bond absorb peak area decreased along with the increasing of discharge time. Then the peak area got min-
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
imum value at discharge time of 60–70 s. Further prolong the discharge time, the C@C nonsaturated double bond absorb peak area increased. It is obvious that the changing trend of C@C nonsaturated double bond absorb peak area with plasma discharge time is reverse to that of peroxide bond peak area. (4) The absorption peak intensity of 2916 cm1 methylene nonsymmetry stretch vibration, 2848 cm1 methylene symmetry stretch vibration, 1463 cm1 methylene nonsymmetry changing angle vibration, and 719 cm1 methylene swing in plane vibration was decreased greatly. It indicates that argon plasma is a continuously etching and hydrogen abstraction process. The four absorption peaks’ intensity has maximum value at discharge time of 120 s or discharge power of 62 W. (5) The mass loss ratio increased along with the increasing of discharge power. Then the mass loss ratio got maximum value at discharge power of 62 W. Further increasing the discharge power, the mass loss ratio decreased rapidly and remained constant. (6) The influence effect of plasma discharge power to peroxide bond, C@C nonsaturated double bond absorb peak area is the same as that of mass loss ratio. The peroxide bond and C@C nonsaturated double bond absorb peak area increased along with the increasing of discharge power. Then it got maximum value at discharge power of 62–72 W. Further increasing the discharge power, it decreased rapidly and remained constant.
1633
References [1] Zhao YQ, Urban MW. Spectroscopic studies of microwave plasma reactions of maleic anhydride on poly (vinylidene fluoride) surfaces: crystallinity and surface reactions. Langmuir 1999;15:3538–44. [2] Heung Kim, Urban MW. Effect of discharge gases on microwave plasma reactions of imidazole on poly(dimethylsiloxane) surfaces: quantitative ATR-FTIR spectroscopic analysis. Langmuir 1999;15:3499–505. [3] Schiller S, Hu J, Jenkins ATA, et al. Chemical structure and properties of plasma-polymerized maleic anhydride films. Chem Mater 2002;14:235–42. [4] Matthews SR, Hwang YJ, McCord MG, Bourham MA. Investigation into etching mechanism of polyethylene terephthalate(PET) films treated in helium and oxygenatedhelium atmospheric plasmas. J Appl Polym Sci 2004;94:2383–9. [5] Shi LS, Wang JQ. An approach of CF4/CH4 plasmas induced surface modification to impart flame retardancy to polypropylene. Chin Sci Bull 1998;43(7):775–9 [in Chinese]. [6] Shi LS, Wang JQ. The investigation of flame retardancy for polyethylene treated by plasma grafting of methylacrylic acid. Chem J Chin Univ. 2001;22(3):489–93 [in Chinese]. [7] Shi LS, Xie YF. An approach of CF4/CH4 plasmas induced surface modification to impart flame retardancy to polypropylene and poly (ethylene terephthalate). J Polym Eng 2003;23(6):457–69. [8] Shi LS. Investigation of surface modification and reaction kinetics of PET in RF CF4–CH4 plasmas. J Polym Eng 1999;19(6):445–55. [9] Shi LS. An approach using CF4/CH4 plasma-induced surface modification to impart flame retardancy to polypropylene. J Polym Eng 2000;20(5):313–28. [10] Shi LS. The investigation of polymer surface dynamics modified by plasma using dynamic contact angle method. Eur Polym J 2000;36:987–92. [11] Shi LS. Characterization of the flame retardancy of EVA copolymer by plasma grafting of acrylic acid. Eur Polym J 2000;36:2611–5.
European Polymer Journal 42 (2006) 1625–1633
www.elsevier.com/locate/europolj
The investigation of argon plasma surface modification to polyethylene: Quantitative ATR-FTIR spectroscopic analysis Lai-Shun Shi *, Lu-Yan Wang, Yu-Na Wang School of Chemistry and Chemical Engineering, South Campus, Shandong University, Jinan 250061, PR China Received 31 August 2005; received in revised form 11 January 2006; accepted 11 January 2006 Available online 23 March 2006
Abstract This paper studied the surface modification of argon plasma to polyethylene by using ATR-FTIR analysis. The mass loss ratio has maximum value at discharge time of 70–120 s or discharge power of 62 W by using argon plasma treatment for polyethylene. New surface structure was formed after polyethylene was treated by argon plasma. The peroxide bond peak area also has maximum value at discharge time of 70–120 s or discharge power of 62 W. The C@C nonsaturated double bond absorb peaks were appeared at 1640 cm1, 1549 cm1 and 1528 cm1 after polyethylene treated by argon plasma. The C@C nonsaturated double bond absorb peak area has minimum value at discharge time of 60–70 s and the power of 65 W. The C@C nonsaturated double bond absorb peak area has maximum value at discharge power of 62–72 W and the discharge time of 2 min. The absorption peak intensity of 2916 cm1 methylene nonsymmetry stretch vibration, 2848 cm1 methylene symmetry stretch vibration, 1463 cm1 methylene nonsymmetry changing angle vibration, and 719 cm1 methylene swing in plane vibration was decreased greatly. The four absorption peaks intensity has maximum value at discharge time of 120 s or discharge power of 62 W. 2006 Elsevier Ltd. All rights reserved. Keywords: Polyethylene; Plasma; Argon; Surface modification; ATR-FTIR
1. Introduction Cold plasma is a new technology which can be used to modify the surface properties of macromolecular materials to meet certain requirements, e.g., adhesion, printing ability of polymer film, dyeing ability, hydrophilicity and biocompatability. Zhao Yaqiu studied maleic anhydride and carboxylic acid groups can be chemically bonded to poly(vinylidene fluoride) (PVDF) surfaces with the *
Corresponding author. Fax: +86 531 88392980. E-mail address: [email protected] (L.-S. Shi).
use of microwave plasma energy [1]. Maleic anhydride reacts with the PVDF surface through a C@C bond opening of the maleic anhydride ring, and its hydrolysis results in chemically attached carboxylic acid groups on the PVDF surface. The extent of the surface reactions can be monitored using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Heung Kim and Urban studied the effect of discharge gases on microwave plasma reactions of imidazole on poly(dimethylsiloxane) surfaces by quantitative ATR-FTIR spectroscopic analysis [2]. Schiller studied the chemical structure and properties of
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.01.007
1626
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
plasma-polymerized maleic anhydride films [3]. The film chemical structures were obtained using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Surface derivatization reactions using decylamine and benzylamine were used to demonstrate their surface reactivity toward nucleophilic moieties and to change the surface free energy of the plasma polymer films. Matthews et al. [4] investigated the etching mechanism of polyethylene terephthalate (PET) films treated in helium and oxygenated-helium atmospheric plasmas. Differential scanning calorimetry (DSC) was used to characterize pre- and post-exposure films. Weight changes and the degree of solubility were also determined. A model was developed to predict the change in the degree of solubility for polyphase surfaces considering the etching rate per phase. In previous articles [5,6], the surface flame retardancy of polypropylene by crosslinking of methane plasma has been studied. Also, we have reported the characteristics of surface modification of polypropylene and poly(ethylene terephthalate) with the CH4, CF4 and CF4/CH4 plasmas to impart flame retardancy by using burning rate and TGA analysis, and its influence to flammability [7–11]. The result indicates that the flame retardancy behavior of polymers was influenced by the polymer’s structures. The likely degradation mechanism of polymer films treated by plasma was proposed based on the experiment results. This paper reports the effect of discharge time and power on the mass loss ratio and surface composition of polyethylene treated by argon plasma by using ATR-FTIR analysis. 2. Experimental 2.1. Sample preparation Linear low density polyethylene (PE) was used in this study. Specimen thicknesses of 1.0 mm was processed first by a two-roll mixing mill and then reduced to 0.8 mm thickness through a platform vulcanizing press at a pressure of 10 MPa and at 160 C for 10 min. Before mounting the three 0.8 mm PE films (40 mm length with different width) on the ground electrode, it was necessary (a) to wash the films with a nonionic detergent solution, (b) to rinse them with deionized water and chloroform, and then (c) to dry them overnight in air. Argon (Jinan DEYANG Gas Factory) supplied in 99.99% purity was used without further purification.
2.2. Low temperature RF plasma reactor A bell-shaped plasma reactor of U200 · 250 mm chamber was supplied by Changzhou SHITAI Plasma Technology Factory. The glow discharge was generated inner-capacitively, operating at 13.56 MHz. An RF generator of 13.56 MHz was connected to the upper electrode; the lower was grounded. The polyethylene samples were placed at the lower grounded electrode to prevent the charging of the insulated polyethylene. Argon was fed to the chamber through a needle valve. Pump-out was usually at the base of the bell jar. The flow rate was measured with a mass flow meter (Model D07-7/ZM, Jianzhong Machinery Factory, Beijing) and the vacuum in the chamber was controlled by a vacuum pressure gauge (Model ZD0-2). The reaction system was pumped down to 0.8 Pa or lower. Argon, adjusted to a desired flow rate, was introduced into the chamber. Once the pressure remained constant of 25–30 Pa, the glow was initiated at the desired power for certain treated time. After plasma treatment, the vacuum chamber simply vents to the atmosphere. Mass loss ratio Dm (mg/cm2) can be calculated based on the mass changes per square of centimeter. The treated PE samples were measured by ATR-FTIR analysis after placed in air for 48 h. 2.3. ATR-FTIR spectrum ATR-FTIR spectra were collected on a Bruker Tensor-27 FTIR single beam spectrometer having a 4 cm1 resolution. A KRS-5 crystal with 60 angle 50 · 20 · 5 mm was used. Each spectrum represents 64 co-added scans ratioed against a reference spectrum obtained by recording 64 co-added scans of an empty ATR cell. 3. Results and discussion 3.1. Effect of argon plasma discharge time on the mass loss ratio Fig. 1 gives the mass loss ratio of PE plotted against argon plasma discharge time at the condition of working pressure of 25–30 Pa and power of 65 W. It was found that the mass loss ratio increased along with the increasing of discharge time (t). Then the mass loss ratio got maximum value at discharge time of 70–120 s. Further prolong the discharge time, the mass loss ratio decreased and remained constant.
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
0.10
0.050
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ATR Units
0.030 0.025
1
0.020
2 0.00
0.015 0.010 0
0.04
0.035
0.02
Δm / (mg/cm2)
0.040
0.06
0.08
0.045
100
200
300
400
500
3500
600
1500
1000
500
Fig. 2. The ATR-FTIR spectra of PE.
vibration, methylene nonsymmetry changing angle vibration, and methylene swing in plane vibration, respectively. Fig. 3 gives the local amplificatory ATR-FTIR spectra of PE. Curve a represents original PE. Curve b represents the ATR-FTIR spectrum of PE treated by argon plasma at the condition of pressure 28 Pa, power 32 W, and discharge time 2 min. As shown in Fig. 3, the new absorption peak of 3398 cm1 was attributed to peroxide bond. It is obvious that peroxide bond was formed by the reaction of free radicals, which was formed by argon plasma, with oxygen and water in air. The reaction is as follows:
0.10
Fig. 4 represents the influence of argon plasma discharge time to 3398 cm1 peroxide bond absorb peak area (A) at the condition of working pressure
0.06 0.04
(a)
0.02
ATR Units
0.08
3.2. Effect of argon plasma discharge time on the surface chemical composition of PE The polyethylene sample was analyzed by ATRFTIR method at the same condition after it was treated by argon plasma and then placed in air for 48 h. Fig. 2 gives the ATR-FTIR spectra of PE. Curve 1 represents original PE. Curve 2 represents the ATR-FTIR spectrum of PE treated by argon plasma at the condition of pressure 28 Pa, power 32 W, and discharge time 2 min. As shown in Fig. 2, the absorption peaks of 2916, 2848, 1463, and 719 cm1 were attributed to methylene nonsymmetry stretch vibration, methylene symmetry stretch
2000
(b)
3398
0.00
As shown in Fig. 1, the mass loss ratio increased along with the increasing of discharge time when the discharge time was below 120 s. This is because argon plasma belongs to etching atmosphere. The active species in the plasma, e.g., electron and ion, bombard the surface macromolecular chains of PE film, which lead to the breakage of C–H and C–C bond and produce more oligomers and small organic molecules. Therefore, the mass loss ratio increases. The etching effect is the main reaction process in the region. When the discharge time is greater than 120 s, more radicals are produced at the surface. The crosslinking among the radicals becomes main reaction process. A highly crosslinking graphite-like carbon membrane is formed, which prevents the further etching effect of argon plasma to PE surface [5]. Therefore, the mass loss ratio remained constant at prolonged treatment time.
2500
Wavenumber cm-1
t/s
Fig. 1. The plot of mass loss ratio versus argon plasma discharge time.
3000
3400
3300
3200
3100
3000
2900
2800
2700
Wavenumber cm-1
Fig. 3. The local amplificatory ATR-FTIR spectra of PE.
0.020 0.015
1640
1549 1528 (b)
0.005
ATR Units
0.010
(a)
1800
1700
1600
1500
1400
1300
Wavenumber cm-1
Fig. 5. The local amplificatory ATR-FTIR spectra of PE.
0.100
0.095
0.090
A
of 25–30 Pa and power of 65 W. It was found that the peroxide bond absorb peak area increased along with the increasing of discharge time. Then the peak area got maximum value at discharge time of 70– 120 s. Further prolong the discharge time, the peak area decreased and remained constant. It is obvious that the changing trend of peroxide bond absorb peak area with plasma discharge time is similar to that of mass loss ratio. It gives that the best plasma discharge time is 70–120 s. Fig. 5 gives the local amplificatory ATR-FTIR spectra of PE. Curve a represents original PE. Curve b represents the ATR-FTIR spectrum of PE treated by argon plasma at the condition of pressure 28 Pa, power 32 W, and discharge time 2 min. As shown in Fig. 5, the new absorption peaks of 1640, 1549, and 1528 cm1 were attributed to C@C nonsaturated double bond on the treated PE surface. It can be concluded that C@C nonsaturated double bond was formed by the reaction of border free radicals, which were formed by argon plasma’s etching and hydrogen abstraction. The reaction is as follows:
0.025
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
0.000
1628
0.085
Fig. 6 represents the influence of argon plasma discharge time to 1640 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the power of 65 W. Fig. 7 represents the influence of argon plasma discharge time to 1549 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the power of 65 W. It is obvious that the changing trend
0.080
0.075 0
100
200
300
400
500
600
t/s
Fig. 6. 1640 cm1C@C nonsaturated double bond absorb peak area versus discharge time.
0.07
0.034
0.032
0.06
A
A
0.030
0.05
0.028
0.026
0.04
0.024
0.03 0
100
200
300
400
500
600
t/s
Fig. 4. The plot of 3398 cm1 peroxide bond absorb peak area versus discharge time.
0
100
200
300
400
500
600
t/s
Fig. 7. 1549 cm1 C@C nonsaturated double bond absorb peak area versus discharge time.
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
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of C@C nonsaturated double bond absorb peak area with plasma discharge time is reverse to that of peroxide bond peak area. It was found that the C@C nonsaturated double bond absorb peak area decreased along with the increasing of discharge time. Then the peak area got minimum value at discharge time of 60–70 s. Further prolong the discharge time, the C@C nonsaturated double bond absorb peak area increased. By looking at the formation mechanism of C@C nonsaturated double bond and peroxide bond, it belongs to competition reactions. The surface free radical concentration is constant after PE is treated by argon plasma. The more the peroxide bond formed, the less the C@C nonsaturated double bond formed. It is obvious that the surface structure is favorable to form peroxide bond, go against to form C@C nonsaturated double bond at the condition of argon plasma discharge time approximately 70 s. That is to say, it has the following structure and reaction:
By contrast the peak intensity in Fig. 2, the absorption peak intensity of 2916 cm1 methylene nonsymmetry stretch vibration, 2848 cm1 methylene symmetry stretch vibration, 1463 cm1 methylene nonsymmetry changing angle vibration, and 719 cm1 methylene swing in plane vibration was decreased greatly after PE was treated by argon plasma. It further proves that argon plasma is a continuously etching and hydrogen abstraction process. This is because the concentration of CH2 group decreases at the constant sampling depth after PE is treated by argon plasma. Therefore, the four absorption peaks’ intensity decreases. In Fig. 2, the absorption peaks of 2916 and 2848 cm1 were attributed to methylene nonsymmetry stretch vibration and methylene symmetry stretch vibration, respectively. It was integrated for the two peaks at the range of 2984–2780 cm1 by using ATR-FTIR software. The result is shown in Fig. 8. As shown in Fig. 8, the absorption peak area has maximum value at discharge time of
When the plasma discharge time is greater than 120 s, the surface structure is favorable to form C@C nonsaturated double bond and crosslinking (–CH h and i C h), go against to form peroxide bond. It has the following structure and reaction:
120 s. This phenomenon was unexpected, and need further investigation. Fig. 9 represents the influence of argon plasma discharge time to 1463 cm1 methylene nonsymmetry changing angle vibration absorb peak area at the
or
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L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633 10
0.5
9 8
0.4
6
A
A
7
0.3
5 4
0.2
3 2 0
100
200
300
400
500
0.1
600
0
t/s
100
200
300
400
500
600
t/s
Fig. 8. CH2 stretch vibration absorption peak area versus plasma discharge time.
Fig. 10. 719 cm1 methylene swing in plane vibration absorb peak area versus plasma discharge time.
0.9
0.030
0.8
0.025
0.7
Δm(mg/cm2)
0.020
A
0.6 0.5 0.4 0.3
0.015
0.010
0.005
0.2 0
100
200
300
400
500
600
t/s
Fig. 9. 1463 cm1 methylene nonsymmetry changing angle vibration absorb peak area versus plasma discharge time.
condition of working pressure of 25–30 Pa and the power of 65 W. Fig. 10 represents the influence of argon plasma discharge time to 719 cm1 methylene swing in plane vibration absorb peak area at the condition of working pressure of 25–30 Pa and the power of 65 W. As shown in Figs. 9 and 10, the absorption peak area also has maximum value at discharge time of 120 s. 3.3. Effect of argon plasma discharge power on the mass loss ratio Fig. 11 gives the mass loss ratio of PE plotted against argon plasma discharge power at the condition of working pressure of 25–30 Pa and the discharge time of 2 min. It was found that the mass
0.000 30
40
50
60
70
80
90
100
110
P/W
Fig. 11. The plot of mass loss ratio versus argon plasma discharge power.
loss ratio increased along with the increasing of discharge power. Then the mass loss ratio got maximum value at discharge power of 62 W. Further increasing the discharge power, the mass loss ratio decreased rapidly and remained constant. As shown in Fig. 11, the mass loss ratio increased along with the increasing of discharge power when it was below 62 W. This is because argon plasma belongs to etching atmosphere. The active species in the plasma, e.g., electron and ion, gain more energy and bombard the surface macromolecular chains of PE film, which lead to the breakage of C–H and C–C bond and produce more oligomers and small organic molecules. Therefore, the mass loss ratio increases. The etching effect is the main reaction process in the region. When the discharge
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633 0.14
0.12
0.10
A
power is greater than 62 W, more radicals are produced at the surface. The crosslinking among the radicals becomes main reaction process. A highly crosslinking graphite-like carbon membrane is formed, which prevents the further etching effect of argon plasma to PE surface[5]. Therefore, the mass loss ratio remained constant at increased treatment power.
0.04
0.035
0.030
A
0.025
0.020
0.015
50
60
70
80
90
100
110
P/W
Fig. 12. The plot of 3398 cm1 peroxide bond absorb peak area versus discharge power.
0.02 30
40
50
60
70
80
90
100
110
P/W
Fig. 13. 1640 cm1 C@C nonsaturated double bond absorb peak area versus discharge power.
0.06
0.05
0.04
A
The polyethylene sample was analyzed by ATRFTIR method at the same condition after it was treated by argon plasma and then placed in air for 48 h. Fig. 12 represents the influence of argon plasma discharge power to 3398 cm1 peroxide bond absorb peak area at the condition of working pressure of 25–30 Pa and discharge time of 2 min. It was found that the peroxide bond absorb peak area increased along with the increasing of discharge power. Then the peak area got maximum value at discharge power of 62–72 W. Further increasing the discharge power, the mass loss ratio decreased and remained constant. It is obvious that the changing trend of peroxide bond absorb peak area with plasma discharge power is similar to that of mass loss ratio. It proves that the best plasma discharge power is 62–72 W. Fig. 13 represents the influence of argon plasma discharge power to 1640 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the discharge time of 2 min. Fig. 14 represents the influence of
40
0.08
0.06
3.4. Effect of argon plasma discharge power on the surface chemical composition of PE
0.010 30
1631
0.03
0.02
0.01 30
40
50
60
70
80
90
100
110
P/W
Fig. 14. 1549 cm1 C@C nonsaturated double bond absorb peak area versus discharge power.
argon plasma discharge power to 1549 cm1 C@C nonsaturated double bond absorb peak area at the condition of working pressure of 25–30 Pa and the discharge time of 2 min. It is obvious that the changing trend of C@C nonsaturated double bond absorb peak area with plasma discharge power is similar to that of peroxide bond peak area. It was found that the C@C nonsaturated double bond absorb peak area increased along with the increasing of discharge power. Then the peak area got maximum value at discharge power of 62–72 W. Further increasing the discharge power, the C@C nonsaturated double bond absorb peak area decreased. It is obvious that the influence of plasma discharge time and power to C@C nonsaturated double bond absorb peak area is different. The
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
surface structure is favorable to form peroxide bond and C@C nonsaturated double bond at the condition of argon plasma discharge power 62–72 W. In Fig. 2, the absorption peaks of 2916 and 2848 cm1 were attributed to methylene nonsymmetry stretch vibration and methylene symmetry stretch vibration, respectively. It was integrated for the two peaks at the range of 2984–2780 cm1 by using ATR-FTIR software. The result is shown in Fig. 15. As shown in Fig. 15, the absorption peak area has maximum value at discharge power of 62 W. This phenomenon was unexpected, and also need further investigation. Fig. 16 represents the influence of argon plasma discharge power to 1463 cm1 methylene nonsym-
0.28 0.26 0.24 0.22
A
1632
0.20 0.18 0.16 0.14 30
40
50
60
70
80
90
100
110
P/W
Fig. 17. 719 cm1 methylene swing in plane vibration absorb peak area versus plasma discharge power.
4.4
metry changing angle vibration absorb peak area at the condition of working pressure of 25–30 Pa and discharge time of 2 min. Fig. 17 represents the influence of argon plasma discharge power to 719 cm1 methylene swing in plane vibration absorb peak area at the condition of working pressure of 25–30 Pa and discharge time of 2 min. As shown in Figs. 16 and 17, the absorption peak area also has maximum value at discharge power of 62 W.
4.2 4.0 3.8
A
3.6 3.4 3.2 3.0 2.8 2.6 2.4 30
40
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60
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80
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P/W Fig. 15. CH2 stretch vibration absorption peak area versus plasma discharge power.
0.42 0.40 0.38 0.36
A
0.34 0.32 0.30 0.28 0.26 0.24 0.22 30
40
50
60
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P/W
Fig. 16. 1463 cm1 methylene nonsymmetry changing angle vibration absorb peak area versus plasma discharge power.
4. Conclusions (1) The mass loss ratio increased along with the increasing of discharge time when PE was treated by argon plasma. Then the mass loss ratio got maximum value at discharge time of 70–120 s. Further prolong the discharge time, the mass loss ratio decreased and remained constant. (2) New surface structure was formed after polyethylene was treated by argon plasma. The influence effect of peroxide bond peak area is the same as that of mass loss ratio. The peroxide bond peak area also has maximum value at discharge time of 70–120 s. (3) The C@C nonsaturated double bond absorb peaks were appeared at 1640 cm1, 1549 cm1 and 1528 cm1 after polyethylene treated by argon plasma. It was found that the C@C nonsaturated double bond absorb peak area decreased along with the increasing of discharge time. Then the peak area got min-
L.-S. Shi et al. / European Polymer Journal 42 (2006) 1625–1633
imum value at discharge time of 60–70 s. Further prolong the discharge time, the C@C nonsaturated double bond absorb peak area increased. It is obvious that the changing trend of C@C nonsaturated double bond absorb peak area with plasma discharge time is reverse to that of peroxide bond peak area. (4) The absorption peak intensity of 2916 cm1 methylene nonsymmetry stretch vibration, 2848 cm1 methylene symmetry stretch vibration, 1463 cm1 methylene nonsymmetry changing angle vibration, and 719 cm1 methylene swing in plane vibration was decreased greatly. It indicates that argon plasma is a continuously etching and hydrogen abstraction process. The four absorption peaks’ intensity has maximum value at discharge time of 120 s or discharge power of 62 W. (5) The mass loss ratio increased along with the increasing of discharge power. Then the mass loss ratio got maximum value at discharge power of 62 W. Further increasing the discharge power, the mass loss ratio decreased rapidly and remained constant. (6) The influence effect of plasma discharge power to peroxide bond, C@C nonsaturated double bond absorb peak area is the same as that of mass loss ratio. The peroxide bond and C@C nonsaturated double bond absorb peak area increased along with the increasing of discharge power. Then it got maximum value at discharge power of 62–72 W. Further increasing the discharge power, it decreased rapidly and remained constant.
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References [1] Zhao YQ, Urban MW. Spectroscopic studies of microwave plasma reactions of maleic anhydride on poly (vinylidene fluoride) surfaces: crystallinity and surface reactions. Langmuir 1999;15:3538–44. [2] Heung Kim, Urban MW. Effect of discharge gases on microwave plasma reactions of imidazole on poly(dimethylsiloxane) surfaces: quantitative ATR-FTIR spectroscopic analysis. Langmuir 1999;15:3499–505. [3] Schiller S, Hu J, Jenkins ATA, et al. Chemical structure and properties of plasma-polymerized maleic anhydride films. Chem Mater 2002;14:235–42. [4] Matthews SR, Hwang YJ, McCord MG, Bourham MA. Investigation into etching mechanism of polyethylene terephthalate(PET) films treated in helium and oxygenatedhelium atmospheric plasmas. J Appl Polym Sci 2004;94:2383–9. [5] Shi LS, Wang JQ. An approach of CF4/CH4 plasmas induced surface modification to impart flame retardancy to polypropylene. Chin Sci Bull 1998;43(7):775–9 [in Chinese]. [6] Shi LS, Wang JQ. The investigation of flame retardancy for polyethylene treated by plasma grafting of methylacrylic acid. Chem J Chin Univ. 2001;22(3):489–93 [in Chinese]. [7] Shi LS, Xie YF. An approach of CF4/CH4 plasmas induced surface modification to impart flame retardancy to polypropylene and poly (ethylene terephthalate). J Polym Eng 2003;23(6):457–69. [8] Shi LS. Investigation of surface modification and reaction kinetics of PET in RF CF4–CH4 plasmas. J Polym Eng 1999;19(6):445–55. [9] Shi LS. An approach using CF4/CH4 plasma-induced surface modification to impart flame retardancy to polypropylene. J Polym Eng 2000;20(5):313–28. [10] Shi LS. The investigation of polymer surface dynamics modified by plasma using dynamic contact angle method. Eur Polym J 2000;36:987–92. [11] Shi LS. Characterization of the flame retardancy of EVA copolymer by plasma grafting of acrylic acid. Eur Polym J 2000;36:2611–5.