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Melt-photografting polymerization of maleic anhydride onto LDPE film
European Polymer Journal 38 (2002) 1449–1455 www.elsevier.com/locate/europolj
Melt-photografting polymerization of maleic anhydride onto LDPE film Jian-Ping Deng
a,b
, Wan-Tai Yang
a,b,*
, Bengt R anby
c
a
b
Department of Polymer Science, Beijing University of Chemical Technology, P.O. Box 63, Beijing 100029, China Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing 100029, China c Department of Polymer Technology, Royal Institute of Technology, S-10044 Stockholm, Sweden Received 22 February 2001; received in revised form 26 September 2001; accepted 22 November 2001
Abstract Maleic anhydride (MAH) was photografted onto low density polyethylene substrates at temperatures above the melting point of MAH. The effects of some principal factors including irradiation temperature, photoinitiators, the intensity of UV radiation, and the far UV radiation on the grafting polymerization were investigated in detail. Percent conversion and grafting efficiency of the polymerizations were determined by the gravimetric method. The contact angles of the grafted film PE-g-PMAH against water and the FTIR spectrum of the grafted film were measured as characterization. The results show that the photografting polymerization of MAH can proceed smoothly at temperatures higher than the melting point of MAH; the far UV radiation and the intensity of the UV radiation affect the grafting polymerization greatly; the photoinitiators also have influence on the polymerization. According to the FTIR spectra, it is clearly confirmed that the grafted film samples contain anhydride groups. The contact angles demonstrate that the wettability of the grafted films is enhanced obviously, especially to those grafted film samples through hydrolysis. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Photografting polymerization; LDPE film; Maleic anhydride
1. Introduction It has been assumed that maleic anhydride (MAH) polymerizes with much difficulty because of high steric hindrance resulting from disubstitution. In contrast, MAH is a powerful electron acceptor, readily proceeding co-polymerization with electron donors, such as vinyl acetate [1,2], styrene [3,4], N-vinylpyrrolidone [5], 2-vinylnaphalene [6] and so on. Therefore, in the field of photografting polymerization relevant to MAH, most part of the studies were centered on binary monomer systems [7,8]. In our proceeding papers, different monomers, including MAH, were grafted onto low density polyethylene (LDPE) film initiated by UV radiation
[9–12]. According to the investigations, MAH can be photografted onto LDPE films with considerable grafting yields, but all of these previous grafting polymerizations were performed in solution, i.e. using acetone as the solvent. During our experiments, it was unexpectedly found that MAH can also be photografted onto LDPE films in the absence of solvents. Therefore, in the present work, a series of studies are undertaken to study photografting polymerization of MAH in the absence of solvent in detail, in hope to develop a more effective and practicable method by which MAH can be grafted onto LDPE films. 2. Experimental 2.1. Materials
*
Corresponding author. Address: Department of Polymer Science, Beijing University of Chemical Technology, P.O. Box 63, Beijing 100029, China. Fax: +86-10-6443-2262. E-mail address: [email protected] (W.-T. Yang).
Monomer: MAH (chemically pure grade, mp: 52–54 °C, from Tianjian Chemical Reagent Plant No. 6, China), was purified by recrystallization in acetone.
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 0 0 4 - 6
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Photoinitiator: Benzophenone (BP, chemically pure grade, from Shanghai Reagent Plant No. 1, China), benzoyldimethyldital (Irgacure 651, analytically pure grade, from Ciba, Switzerland) and thioxanthane (ITX, analytically pure grade, from PHT, USA) were used as received. Substrates: Commercial LDPE film, 63 lm in thickness, was cut into circular samples with diameter of 7 cm, and then subjected to extraction in Soxhlet with acetone for 5 h to get rid of the additives and impurities before use. Solvent: Acetone and ethyl acetate, analytically pure grade, without further purification before use.
2.2. Preparing films containing MAH A drop of acetone solution (unless otherwise noted) containing predetermined amounts of MAH and photoinitiator was deposited between two film samples with a micro-syringe. An appropriate pressure was given to make the solution liquid a thin and even layer. The samples were dried at ambient temperature. The amounts of MAH and photoinitiator were determined by the volume of the solution.
CP ¼ ðWP =WM Þ 100%
ð1Þ
GE ¼ ðWG =WP Þ 100%
ð2Þ
where WM is the weight of monomer added between the two films; WP is the weight of polymer formed, including homopolymer and grafted polymer; WG is the weight of the grafted polymer, which was obtained after extracting the homopolymer with acetone. 2.4. Hydrolysis The grafted film samples PE-g-PMAH were placed in distilled water for 10 min at ambient temperature and then the water containing the film samples was heated and kept at about 100 °C for another 10 min. The films were taken out and dried again at about 50 °C. 2.5. FTIR Fourier transform infrared spectrometer (Nicolet 50 DXC FTIR Spectrometer) was used to measure the IR spectrum of the grafted film sample PE-g-PMAH. Pure LDPE film was also measured as reference sample under the same conditions.
2.3. Grafting procedure The photografting polymerization of MAH onto LDPE films was carried out as follows. The films containing MAH and photoinitiator were laid on the holder of the irradiation equipment, which was illustrated elsewhere [10], and a piece of quartz plate was used to cover the films. The polymerization systems were irradiated by UV radiation (UV lamp: high-pressure mercury lamp, 1000 W) at a given temperature controlled by a thermocouple thermometer. The polymerization degree was decided by the irradiation time. When the effects of the intensity of UV radiation on grafting polymerization were investigated, the distance between the films and the UV lamp was adjusted to obtain the appropriate intensity; when the effects of the far UV radiation (200–300 nm) on the grafting polymerization were investigated, a piece of polyethylene terephthalate was used to cover the films to exclude the far UV radiation. After polymerization, the samples were taken out and separated and then dipped in a large amount of benzene at ambient temperature for 2 min to remove the residual MAH. The films were taken out and dried at about 50 °C to constant weight. Then the grafted films were subjected to Soxhlet extraction with acetone to exclude the homopolymer of PMAH. Percent conversion (CP) and grafting efficiency (GE) were calculated according to the following definitions:
2.6. Contact angles Contact angles of pure LDPE film, PE-g-PMAH film and hydrolyzed PE-g-PMAH film against water were measured as follows. A piece of the films (2 1 cm2 ) was laid on the sample holder of the Instrument of Measuring Contact Angle (JJC-I, from Changchun Optical Instrument Plant, China), about 10 ll distilled water was deposited on the film with a micro-syringe. Then the instrument was adjusted as quickly as possible until the number of contact angle was read clearly. The procedure was repeated for five times and the average value was obtained for each sample.
3. Results and discussion 3.1. Photografting polymerization of MAH in molten phase The effects of the irradiation temperatures on polymerization are presented in Fig. 1. The irradiation temperature was varied in a wide range from 35 to 85 °C (Fig. 1), which is indicated by the horizontal axis. From Fig. 1, it is clear that the irradiation temperature has great influence on the polymerization. At lower temperatures (<50 °C), MAH exists
J.-P. Deng et al. / European Polymer Journal 38 (2002) 1449–1455
Fig. 1. The effects of irradiation temperature on the grafting polymerization: ( ) CP; ( ) GE. Reaction conditions: irradiation time, 4 min; concentration of MAH, 2 wt.% of the film; concentration of BP, 3 wt.% of MAH; intensity of UV radiation, 5800 lw/cm2 .
mainly in the phase of solid. The polymerization can occur, but CP and GE of the polymerization systems are at a low level. Take the polymerization at 45 °C for example, both CP and GE are just about 35%. CP and GE increase slowly initially as the temperature goes up, but around the melting point of MAH, i.e. 52 °C, both of them perform a rapid increase. Then, when the temperature is elevated higher than 65 °C, the increase of CP and GE becomes slower again; when it exceeds 75 °C, the temperature affects the polymerization slightly. The feature of the curves of CP and GE suggests that the grafting polymerization of MAH could proceed smoothly at temperatures higher than its melting point. When the polymerization is completed at higher temperatures (>55 °C), MAH exists in molten phase instead of solid phase, which is expected to make the molecules of MAH active. Therefore, the opportunity for MAH to take part in the polymerization increases markedly. On the basis of the above results, it is clarified that the polymerization of MAH still proceeds smoothly without solvent but necessarily at higher temperatures than its melting point. These results are full of importance because the related polymerization system is simplified when the solvent of MAH is excluded during photografting polymerization, which makes this technology more practicable. On the other hand, these results are also unexpected, due to the fact that, according to the literature, nearly all of the photografting polymerizations of MAH were carried out in vapor phase and meanwhile in the presence of another co-monomer [7,8]. In fact, these results obtained here are quite reasonable. When grafting polymerization is performed at temperatures higher than MAHs melting point (52–53 °C), MAH melts and
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permeates gradually into deep layers of the LDPE film, which is favorable to the grafting polymerization, much similar to the hetero-phase grafting polymerization in the presence of solvent. In addition, it is still important to note that, when MAH is introduced onto LDPE film in the form of solution, part of MAH is distributed to the deep regions of the film, not just limited on the surface of the film, which is also helpful to make MAH not screen the UV light seriously and make the molecules of MAH contact sufficiently with macro-molecules of LDPE. When grafting polymerization is carried out at temperatures below 52 °C, based on Fig. 1, grafting polymerization of MAH still takes place. This should be attributed to the fact that, in this system, MAH actually took part in grafting co-polymerization, not homopolymerization. Therefore, it is assumed that the grafted chains of MAH are not long, which is very effective to improve the surface hydrophilicity of the substrate. This point can be proved by the drastic decrease performed by the contact angles of the grafting films against water. Besides, during the process of UV irradiation, molecules of MAH absorbs heat from the radiation to make the solid MAH melt, especially when irradiated for long time, which makes the grafting polymerization possible to a certain degree. 3.2. The evolution of the grafting polymerization The effects of irradiation time on the polymerization are illustrated in Fig. 2. It is seen from Fig. 2 that when the irradiation time is lengthened from 1 to 5 min, CP and GE of the polymerization systems increase steadily, and then level off, which shows that most of the monomer takes part in polymerization. Another impressive phenomenon is that CP and GE cannot go up to 100%, according to the curves in Fig. 2. During the experiments, it was observed that after polymerization, some MAH was found on the edge of the film samples, which may come from the easy sublimation of MAH. This phenomenon probably accounts for the results that CP of the polymerization system cannot reach 100%, even under the conditions that the polymerization time is lengthened. Besides, it was already confirmed that dimer of MAH can be generated under UV irradiation [13], which should be responsible for the limit of increasing GE. That is, GE cannot reach 100% either, just like CP. 3.3. Effects of photoinitiators Three types of photoinitiators, BP, Irgacure 651 and ITX, were used to initiate the grafting polymerization of MAH, and their effects on the grafting polymerization are shown in Figs. 3 and 4.
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Fig. 2. The evolution of grafting polymerization of MAH: ( ) CP; ( ) GE. Reaction conditions: irradiation temperature, 65 °C; concentration of MAH, 2.5 wt.% of the film; concentration of BP, 3 wt.% of MAH; intensity of UV radiation, 4700 lw/ cm2 .
Fig. 3 shows that although the initiation reactivity and mechanism of the three photoinitiators are very different, CP of the polymerization systems performs no apparent difference. This may be assigned to that MAH itself can absorb energy from UV radiation, form excimer and further proceed polymerization [13]. But from Fig. 4, it could be seen that GE has a certain difference, and decreases in the following order, BP > ITX > Irgacure 651. These results are similar to those of the grafting polymerization of VAC, and the explanations could be given by their different initiation mechanism. Another surprising and interesting phenomenon should
Fig. 3. The effects of photoinitiators on CP of the grafting polymerization: ( ) BP; ( ) Irgacure 651; ( ) ITX. Reaction conditions: irradiation time, 3 min; irradiation temperature, 60 °C; concentration of MAH, 3 wt.% of the film; intensity of UV radiation, 4500 lw/cm2 .
Fig. 4. The effects of photoinitiators on GE of the grafting polymerization: ( ) BP; ( ) Irgacure 651; ( ) ITX. Reaction conditions: irradiation time, 3 min; irradiation temperature, 60 °C; concentration of MAH, 3 wt.% of the film; intensity of UV radiation, 4500 lw/cm2 .
be given much attention, i.e. the fact that GE of the polymerization systems is markedly at a high level in the absence of photoinitiators. When no photoinitiators is added, GE of the LDPE/MAH polymerization system is about 60%, which suggests that MAH itself can abstract hydrogens from LDPE macro-molecules and further undergo grafting polymerization, which has been reported on a separate paper [14].
3.4. Effects of UV radiation The effects of UV radiation, including the intensity of UV radiation and the far UV radiation on the grafting polymerizaiton of MAH were investigated, and the results are shown in Figs. 5 and 6. Fig. 5 demonstrates that increasing the intensity of UV radiation is desired to the polymerization of MAH, not only to the homopolymerization, but to the grafting polymerization also. For example, when the intensity is about 2800 lw/cm2 , CP and GE of the polymerization system are just 32% and 61%, respectively; when the intensity is elevated to 5300 lw/cm2 , about 82% of the MAH participates in polymerization, among which about 85% is the grafted polymer. When the far UV radiation (200–300 nm) was excluded, the polymerization results are presented in Fig. 6. From Fig. 6, it is observed that the grafting results are not satisfactory. When the far UV radiation is removed, although the irradiation time was lengthened to 180 s, only 38% of the MAH proceeded polymerization (PMAH), and lower than half of the PMAH is proved to be grafted polymer of MAH. Based on the above investigations, it is confirmed that it is the far UV ra-
J.-P. Deng et al. / European Polymer Journal 38 (2002) 1449–1455
Fig. 5. The effects of the intensity of UV radiation on the grafting polymerization: ( ) CP; ( ) GE. Reaction conditions: irradiation time, 3 min; irradiation temperature, 65 °C; concentration of MAH, 2 wt.% of the film; concentration of BP, 3 wt.% of MAH.
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Fig. 7. The contact angles of the films against water ( ) before hydrolysis and ( ) after hydrolysis.
before hydrolysis, e.g. containing PMAH about 80 lg/ cm2 , the contact angle drops to 62°; to the sample containing the same amount of PMAH but through hydrolysis, the contact angle drops to 40°. In addition, the decrease of contact angles confirms indirectly the formation of LDPE-g-PMAH.
ð3Þ
Fig. 6. The grafting results excluding the far UV radiation: ( ) CP; ( ) GE. Reaction conditions: irradiation temperature, 65 °C; concentration of MAH, 2.5 wt.% of the film; concentration of BP, 3 wt.% of MAH; intensity of UV radiation before removing the far UV radiation, 5200 lw/cm2 .
diation that plays a big part in the process of polymerization. 3.5. Contact angles The contact angles of the grafted film PE-g-PMAH against water are illustrated in Fig. 7. From Fig. 7, it is obvious that the wettability of the LDPE film is improved greatly by grafting MAH onto it, especially to those film samples through hydrolysis. Before grafting, the contact angle of the controlled LDPE film is about 94°. As to the grafted film samples
Through hydrolysis, some anhydride groups change into carboxylic groups (Reaction (3)). While the grafted film samples PE-g-PMAH are dipped in boiling water, some PMAH chains embedded in the film may unfold to the surface layers of the film; besides, by hydrolysis, one anhydride group turns into two carboxylic groups. These two reasons may account for the different contact angles performed by the grafted film PE-g-PMAH before and after hydrolysis.
3.6. FTIR spectra The FTIR spectra of the grafted LDPE film PEg-PMAH before hydrolysis and the controlled LDPE film were measured and recorded in Fig. 8. Compared with the FTIR spectrum of pure LDPE film (Fig. 8(a)), the new-appeared sharp absorption peaks at around 1780, 1850 and 1215 cm1 (characteristic absorptions of cyclic anhydride groups) in Fig. 8(b) indicate that the grafted LDPE film sample contains PMAH chains. Because the homopolymer of PMAH
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Fig. 8. The FTIR spectra of the LDPE films: (a) pure LDPE film; (b) grafted LDPE film PE-g-PMAH.
has been extracted totally in advance, the examined film sample is verified to be the product of PE-g-PMAH.
Acknowledgements This research has been supported by Special Funds for Major State Basic Research Projects and Chinese
State Outstanding Youth Foundation, which are gratefully acknowledged. References [1] Seymour RB, Garner DP, Sanders LJ. Alternalting and random copolymers of vinyl acetate and maleic anhydride. J Macromol Sci-Chem 1979;A13(2):173–81.
J.-P. Deng et al. / European Polymer Journal 38 (2002) 1449–1455 [2] Caze C, Loucheux C. Mechanism of alternalting copolymerization of vinyl acetate and maleic anhydride. J Macromol Sci-Chem 1975;A9(1):29–43. [3] Hill DJT, O’Donnell JH, O’Sullivan PW. Analysis of the mechanism of copolymerization of styrene and maleic anhydride. Macromolecules 1985;18:9–17. [4] Deb PC, Meyerhoe G. Study on kinetics of copolymerization of styrene and maleic anhydride in dioxane. Eur Polym J 1984;20:713–9. [5] Georgiev G, Konstantinov C, Kabaivanov V. Role of the charge-transfer complex during the copolymerization of Nvinylpyrrolidone and maleic anhydride. Macromolecules 1992;25:6302–8. [6] Li T, Luo B, Li SJ, Chu GB. Charge-transfer and energytransfer in the photo-induced copolymerization of 2vinylnaphthalene with maleic anhydride. Chin J Polym Sci 1990;8(2):170–6. [7] Hayakawa K, Kawase K, Yamakita H. Vapor-phase graft copolymerization of binary solid monomers onto poly (ethylene-co-vinyl acetate) film by ultraviolet irradiation. J Polym Sci 1979;A17:3337–48. [8] Kubota H, Yoshino N, Ogiwara Y. Vapor phase photografting on low-density polyethylene film in bi-
[9]
[10]
[11]
[12]
[13] [14]
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nary monomer systems. J Appl Polym Sci 1990;39: 1231–9. Deng JP, Yang WT, R anby B. Surface photograft polymerization of vinyl acetate on low density polyethylene film. Effects of solvent. Polym J 2000;32:834–7. Deng JP, Yang WT, R anby B. Surface photografting polymerization of vinyl acetate (VAC), maleic anhydride (MAH), and their charge transfer complex. I. VAC (1). J Appl Polym Sci 2000;77:1513–21. Deng JP, Yang WT, R anby B. Surface photografting polymerization of vinyl acetate (VAC), maleic anhydride (MAH), and their charge transfer complex. II. VAC (2). J Appl Polym Sci 2000;77:1522–31. Deng JP, Yang WT, R anby B. Photografting polymerization of maleic anhydride on to low density polyethylene. Chin J Polym Sci, submitted for publication. Gaylord NG. Poly(maleic anhydride). J Macromol Sci-Rev Macromol Chem 1975;C13(2):235–61. Deng JP, Yang WT. Self-initiating performance of maleic anhydride on surface photografting polymerization. J Polym Sci: Part A, Polym Chem 2001;39:3246– 9.
Melt-photografting polymerization of maleic anhydride onto LDPE film Jian-Ping Deng
a,b
, Wan-Tai Yang
a,b,*
, Bengt R anby
c
a
b
Department of Polymer Science, Beijing University of Chemical Technology, P.O. Box 63, Beijing 100029, China Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing 100029, China c Department of Polymer Technology, Royal Institute of Technology, S-10044 Stockholm, Sweden Received 22 February 2001; received in revised form 26 September 2001; accepted 22 November 2001
Abstract Maleic anhydride (MAH) was photografted onto low density polyethylene substrates at temperatures above the melting point of MAH. The effects of some principal factors including irradiation temperature, photoinitiators, the intensity of UV radiation, and the far UV radiation on the grafting polymerization were investigated in detail. Percent conversion and grafting efficiency of the polymerizations were determined by the gravimetric method. The contact angles of the grafted film PE-g-PMAH against water and the FTIR spectrum of the grafted film were measured as characterization. The results show that the photografting polymerization of MAH can proceed smoothly at temperatures higher than the melting point of MAH; the far UV radiation and the intensity of the UV radiation affect the grafting polymerization greatly; the photoinitiators also have influence on the polymerization. According to the FTIR spectra, it is clearly confirmed that the grafted film samples contain anhydride groups. The contact angles demonstrate that the wettability of the grafted films is enhanced obviously, especially to those grafted film samples through hydrolysis. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Photografting polymerization; LDPE film; Maleic anhydride
1. Introduction It has been assumed that maleic anhydride (MAH) polymerizes with much difficulty because of high steric hindrance resulting from disubstitution. In contrast, MAH is a powerful electron acceptor, readily proceeding co-polymerization with electron donors, such as vinyl acetate [1,2], styrene [3,4], N-vinylpyrrolidone [5], 2-vinylnaphalene [6] and so on. Therefore, in the field of photografting polymerization relevant to MAH, most part of the studies were centered on binary monomer systems [7,8]. In our proceeding papers, different monomers, including MAH, were grafted onto low density polyethylene (LDPE) film initiated by UV radiation
[9–12]. According to the investigations, MAH can be photografted onto LDPE films with considerable grafting yields, but all of these previous grafting polymerizations were performed in solution, i.e. using acetone as the solvent. During our experiments, it was unexpectedly found that MAH can also be photografted onto LDPE films in the absence of solvents. Therefore, in the present work, a series of studies are undertaken to study photografting polymerization of MAH in the absence of solvent in detail, in hope to develop a more effective and practicable method by which MAH can be grafted onto LDPE films. 2. Experimental 2.1. Materials
*
Corresponding author. Address: Department of Polymer Science, Beijing University of Chemical Technology, P.O. Box 63, Beijing 100029, China. Fax: +86-10-6443-2262. E-mail address: [email protected] (W.-T. Yang).
Monomer: MAH (chemically pure grade, mp: 52–54 °C, from Tianjian Chemical Reagent Plant No. 6, China), was purified by recrystallization in acetone.
0014-3057/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 2 ) 0 0 0 0 4 - 6
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Photoinitiator: Benzophenone (BP, chemically pure grade, from Shanghai Reagent Plant No. 1, China), benzoyldimethyldital (Irgacure 651, analytically pure grade, from Ciba, Switzerland) and thioxanthane (ITX, analytically pure grade, from PHT, USA) were used as received. Substrates: Commercial LDPE film, 63 lm in thickness, was cut into circular samples with diameter of 7 cm, and then subjected to extraction in Soxhlet with acetone for 5 h to get rid of the additives and impurities before use. Solvent: Acetone and ethyl acetate, analytically pure grade, without further purification before use.
2.2. Preparing films containing MAH A drop of acetone solution (unless otherwise noted) containing predetermined amounts of MAH and photoinitiator was deposited between two film samples with a micro-syringe. An appropriate pressure was given to make the solution liquid a thin and even layer. The samples were dried at ambient temperature. The amounts of MAH and photoinitiator were determined by the volume of the solution.
CP ¼ ðWP =WM Þ 100%
ð1Þ
GE ¼ ðWG =WP Þ 100%
ð2Þ
where WM is the weight of monomer added between the two films; WP is the weight of polymer formed, including homopolymer and grafted polymer; WG is the weight of the grafted polymer, which was obtained after extracting the homopolymer with acetone. 2.4. Hydrolysis The grafted film samples PE-g-PMAH were placed in distilled water for 10 min at ambient temperature and then the water containing the film samples was heated and kept at about 100 °C for another 10 min. The films were taken out and dried again at about 50 °C. 2.5. FTIR Fourier transform infrared spectrometer (Nicolet 50 DXC FTIR Spectrometer) was used to measure the IR spectrum of the grafted film sample PE-g-PMAH. Pure LDPE film was also measured as reference sample under the same conditions.
2.3. Grafting procedure The photografting polymerization of MAH onto LDPE films was carried out as follows. The films containing MAH and photoinitiator were laid on the holder of the irradiation equipment, which was illustrated elsewhere [10], and a piece of quartz plate was used to cover the films. The polymerization systems were irradiated by UV radiation (UV lamp: high-pressure mercury lamp, 1000 W) at a given temperature controlled by a thermocouple thermometer. The polymerization degree was decided by the irradiation time. When the effects of the intensity of UV radiation on grafting polymerization were investigated, the distance between the films and the UV lamp was adjusted to obtain the appropriate intensity; when the effects of the far UV radiation (200–300 nm) on the grafting polymerization were investigated, a piece of polyethylene terephthalate was used to cover the films to exclude the far UV radiation. After polymerization, the samples were taken out and separated and then dipped in a large amount of benzene at ambient temperature for 2 min to remove the residual MAH. The films were taken out and dried at about 50 °C to constant weight. Then the grafted films were subjected to Soxhlet extraction with acetone to exclude the homopolymer of PMAH. Percent conversion (CP) and grafting efficiency (GE) were calculated according to the following definitions:
2.6. Contact angles Contact angles of pure LDPE film, PE-g-PMAH film and hydrolyzed PE-g-PMAH film against water were measured as follows. A piece of the films (2 1 cm2 ) was laid on the sample holder of the Instrument of Measuring Contact Angle (JJC-I, from Changchun Optical Instrument Plant, China), about 10 ll distilled water was deposited on the film with a micro-syringe. Then the instrument was adjusted as quickly as possible until the number of contact angle was read clearly. The procedure was repeated for five times and the average value was obtained for each sample.
3. Results and discussion 3.1. Photografting polymerization of MAH in molten phase The effects of the irradiation temperatures on polymerization are presented in Fig. 1. The irradiation temperature was varied in a wide range from 35 to 85 °C (Fig. 1), which is indicated by the horizontal axis. From Fig. 1, it is clear that the irradiation temperature has great influence on the polymerization. At lower temperatures (<50 °C), MAH exists
J.-P. Deng et al. / European Polymer Journal 38 (2002) 1449–1455
Fig. 1. The effects of irradiation temperature on the grafting polymerization: ( ) CP; ( ) GE. Reaction conditions: irradiation time, 4 min; concentration of MAH, 2 wt.% of the film; concentration of BP, 3 wt.% of MAH; intensity of UV radiation, 5800 lw/cm2 .
mainly in the phase of solid. The polymerization can occur, but CP and GE of the polymerization systems are at a low level. Take the polymerization at 45 °C for example, both CP and GE are just about 35%. CP and GE increase slowly initially as the temperature goes up, but around the melting point of MAH, i.e. 52 °C, both of them perform a rapid increase. Then, when the temperature is elevated higher than 65 °C, the increase of CP and GE becomes slower again; when it exceeds 75 °C, the temperature affects the polymerization slightly. The feature of the curves of CP and GE suggests that the grafting polymerization of MAH could proceed smoothly at temperatures higher than its melting point. When the polymerization is completed at higher temperatures (>55 °C), MAH exists in molten phase instead of solid phase, which is expected to make the molecules of MAH active. Therefore, the opportunity for MAH to take part in the polymerization increases markedly. On the basis of the above results, it is clarified that the polymerization of MAH still proceeds smoothly without solvent but necessarily at higher temperatures than its melting point. These results are full of importance because the related polymerization system is simplified when the solvent of MAH is excluded during photografting polymerization, which makes this technology more practicable. On the other hand, these results are also unexpected, due to the fact that, according to the literature, nearly all of the photografting polymerizations of MAH were carried out in vapor phase and meanwhile in the presence of another co-monomer [7,8]. In fact, these results obtained here are quite reasonable. When grafting polymerization is performed at temperatures higher than MAHs melting point (52–53 °C), MAH melts and
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permeates gradually into deep layers of the LDPE film, which is favorable to the grafting polymerization, much similar to the hetero-phase grafting polymerization in the presence of solvent. In addition, it is still important to note that, when MAH is introduced onto LDPE film in the form of solution, part of MAH is distributed to the deep regions of the film, not just limited on the surface of the film, which is also helpful to make MAH not screen the UV light seriously and make the molecules of MAH contact sufficiently with macro-molecules of LDPE. When grafting polymerization is carried out at temperatures below 52 °C, based on Fig. 1, grafting polymerization of MAH still takes place. This should be attributed to the fact that, in this system, MAH actually took part in grafting co-polymerization, not homopolymerization. Therefore, it is assumed that the grafted chains of MAH are not long, which is very effective to improve the surface hydrophilicity of the substrate. This point can be proved by the drastic decrease performed by the contact angles of the grafting films against water. Besides, during the process of UV irradiation, molecules of MAH absorbs heat from the radiation to make the solid MAH melt, especially when irradiated for long time, which makes the grafting polymerization possible to a certain degree. 3.2. The evolution of the grafting polymerization The effects of irradiation time on the polymerization are illustrated in Fig. 2. It is seen from Fig. 2 that when the irradiation time is lengthened from 1 to 5 min, CP and GE of the polymerization systems increase steadily, and then level off, which shows that most of the monomer takes part in polymerization. Another impressive phenomenon is that CP and GE cannot go up to 100%, according to the curves in Fig. 2. During the experiments, it was observed that after polymerization, some MAH was found on the edge of the film samples, which may come from the easy sublimation of MAH. This phenomenon probably accounts for the results that CP of the polymerization system cannot reach 100%, even under the conditions that the polymerization time is lengthened. Besides, it was already confirmed that dimer of MAH can be generated under UV irradiation [13], which should be responsible for the limit of increasing GE. That is, GE cannot reach 100% either, just like CP. 3.3. Effects of photoinitiators Three types of photoinitiators, BP, Irgacure 651 and ITX, were used to initiate the grafting polymerization of MAH, and their effects on the grafting polymerization are shown in Figs. 3 and 4.
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Fig. 2. The evolution of grafting polymerization of MAH: ( ) CP; ( ) GE. Reaction conditions: irradiation temperature, 65 °C; concentration of MAH, 2.5 wt.% of the film; concentration of BP, 3 wt.% of MAH; intensity of UV radiation, 4700 lw/ cm2 .
Fig. 3 shows that although the initiation reactivity and mechanism of the three photoinitiators are very different, CP of the polymerization systems performs no apparent difference. This may be assigned to that MAH itself can absorb energy from UV radiation, form excimer and further proceed polymerization [13]. But from Fig. 4, it could be seen that GE has a certain difference, and decreases in the following order, BP > ITX > Irgacure 651. These results are similar to those of the grafting polymerization of VAC, and the explanations could be given by their different initiation mechanism. Another surprising and interesting phenomenon should
Fig. 3. The effects of photoinitiators on CP of the grafting polymerization: ( ) BP; ( ) Irgacure 651; ( ) ITX. Reaction conditions: irradiation time, 3 min; irradiation temperature, 60 °C; concentration of MAH, 3 wt.% of the film; intensity of UV radiation, 4500 lw/cm2 .
Fig. 4. The effects of photoinitiators on GE of the grafting polymerization: ( ) BP; ( ) Irgacure 651; ( ) ITX. Reaction conditions: irradiation time, 3 min; irradiation temperature, 60 °C; concentration of MAH, 3 wt.% of the film; intensity of UV radiation, 4500 lw/cm2 .
be given much attention, i.e. the fact that GE of the polymerization systems is markedly at a high level in the absence of photoinitiators. When no photoinitiators is added, GE of the LDPE/MAH polymerization system is about 60%, which suggests that MAH itself can abstract hydrogens from LDPE macro-molecules and further undergo grafting polymerization, which has been reported on a separate paper [14].
3.4. Effects of UV radiation The effects of UV radiation, including the intensity of UV radiation and the far UV radiation on the grafting polymerizaiton of MAH were investigated, and the results are shown in Figs. 5 and 6. Fig. 5 demonstrates that increasing the intensity of UV radiation is desired to the polymerization of MAH, not only to the homopolymerization, but to the grafting polymerization also. For example, when the intensity is about 2800 lw/cm2 , CP and GE of the polymerization system are just 32% and 61%, respectively; when the intensity is elevated to 5300 lw/cm2 , about 82% of the MAH participates in polymerization, among which about 85% is the grafted polymer. When the far UV radiation (200–300 nm) was excluded, the polymerization results are presented in Fig. 6. From Fig. 6, it is observed that the grafting results are not satisfactory. When the far UV radiation is removed, although the irradiation time was lengthened to 180 s, only 38% of the MAH proceeded polymerization (PMAH), and lower than half of the PMAH is proved to be grafted polymer of MAH. Based on the above investigations, it is confirmed that it is the far UV ra-
J.-P. Deng et al. / European Polymer Journal 38 (2002) 1449–1455
Fig. 5. The effects of the intensity of UV radiation on the grafting polymerization: ( ) CP; ( ) GE. Reaction conditions: irradiation time, 3 min; irradiation temperature, 65 °C; concentration of MAH, 2 wt.% of the film; concentration of BP, 3 wt.% of MAH.
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Fig. 7. The contact angles of the films against water ( ) before hydrolysis and ( ) after hydrolysis.
before hydrolysis, e.g. containing PMAH about 80 lg/ cm2 , the contact angle drops to 62°; to the sample containing the same amount of PMAH but through hydrolysis, the contact angle drops to 40°. In addition, the decrease of contact angles confirms indirectly the formation of LDPE-g-PMAH.
ð3Þ
Fig. 6. The grafting results excluding the far UV radiation: ( ) CP; ( ) GE. Reaction conditions: irradiation temperature, 65 °C; concentration of MAH, 2.5 wt.% of the film; concentration of BP, 3 wt.% of MAH; intensity of UV radiation before removing the far UV radiation, 5200 lw/cm2 .
diation that plays a big part in the process of polymerization. 3.5. Contact angles The contact angles of the grafted film PE-g-PMAH against water are illustrated in Fig. 7. From Fig. 7, it is obvious that the wettability of the LDPE film is improved greatly by grafting MAH onto it, especially to those film samples through hydrolysis. Before grafting, the contact angle of the controlled LDPE film is about 94°. As to the grafted film samples
Through hydrolysis, some anhydride groups change into carboxylic groups (Reaction (3)). While the grafted film samples PE-g-PMAH are dipped in boiling water, some PMAH chains embedded in the film may unfold to the surface layers of the film; besides, by hydrolysis, one anhydride group turns into two carboxylic groups. These two reasons may account for the different contact angles performed by the grafted film PE-g-PMAH before and after hydrolysis.
3.6. FTIR spectra The FTIR spectra of the grafted LDPE film PEg-PMAH before hydrolysis and the controlled LDPE film were measured and recorded in Fig. 8. Compared with the FTIR spectrum of pure LDPE film (Fig. 8(a)), the new-appeared sharp absorption peaks at around 1780, 1850 and 1215 cm1 (characteristic absorptions of cyclic anhydride groups) in Fig. 8(b) indicate that the grafted LDPE film sample contains PMAH chains. Because the homopolymer of PMAH
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Fig. 8. The FTIR spectra of the LDPE films: (a) pure LDPE film; (b) grafted LDPE film PE-g-PMAH.
has been extracted totally in advance, the examined film sample is verified to be the product of PE-g-PMAH.
Acknowledgements This research has been supported by Special Funds for Major State Basic Research Projects and Chinese
State Outstanding Youth Foundation, which are gratefully acknowledged. References [1] Seymour RB, Garner DP, Sanders LJ. Alternalting and random copolymers of vinyl acetate and maleic anhydride. J Macromol Sci-Chem 1979;A13(2):173–81.
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