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MMA/DVB Emulsion Surface Graft Polymerization Initiated by UV Light Yongxin Wang† and Wantai Yang* Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing 100029, China, and Department of Polymer Science, Beijing University of Chemical Technology, Beijing 100029; China Received March 9, 2004. In Final Form: April 27, 2004 Methyl methacrylate/1,2-divinylbenzene (MMA/DVB) in an opaque emulsion were successfully grafted onto the surface of polymeric substrate under the irradiation of UV light with benzophenone (BP) as a photoinitiator that was previously coated on the substrate surface. Monomer conversion, grafting efficiency, and grafting yields were determined by the gravimetric method. ATR-IR, AFM, and TEM were used to characterize the surface composition, to observe the topography of the grafted substrates, and to view inter-film colloid particles formed by cross-linking. The results reveal that, with the opaque MMA/DVB emulsion system and CPP film as substrate, the monomer conversion is in the range of 15-55%, the grafting efficiency is about 80%, the grafting yield reaches 5%, and the thickness of the graft layer can be controlled in the range 0.09-1.5 µm. Images of AFM show that the graft layer is pilled up by nanoparticles (about 30-50 nm in diameter), which are linked together and tied to the substrate surface with covalent bonds. A possible model of surface graft polymerization including surface initiating, nucleation, and shish kebab growing is put forward to interpreting the above results.
Introduction UV-initiated surface photografting polymerization is a facile and efficient method in polymer surface modification. With the distinct advantages, forming fast covalent bonds between the new graft layers and the substrate surface but doing little damage to the material’s bulk properties, this method has attracted much attention since the pioneering work of Oster in 1957.1 Up to now, it has been realized in many systems, i.e., vapor phase,2-5 batch phase,6 continuous phase,7 sandwich process,8 and bulk process.9 By the graft method, the surface of polymeric materials can be planted a layer of new polymer chains with various functional groups in very short time according to the requirements for modification applications. Recently, the study of functionalized or patterned surface further stimulates the exploration of various graft methods because it brings together supermolecule science, material science,microelectronics,optoelectronics,andbiotechnology10-12 in an unprecedented way. However, this kind of study has just been introduced into polymer science and still limited in the piling structures formed by various graft * Author for correspondence. Fax: +86 010 64416338. E-mail:
[email protected]. † E-mail:
[email protected]. (1) Oster, G.; Shibata, O. J. Polym. Sci. 1957, 26, 233. (2) Wright, A. N. Nature (London) 1967, 215, 953-955. (3) Ogiwara, Y.; Kanda, M.; Takumi, M.; Kubota, H. J. Polym. Sci., Polym Lett. Ed. 1981, 19, 457-462. (4) Tazuke, S.; Kimura, H. Makromol. Chem. 1978, 179, 2603-2612. (5) Allmer, K.; Hult, A.; Rånby, B. J. Polym. Sci., Polym Chem 1988, 26, 2099-2111. (6) Tazuke, S.; Kimura, H. Makromol Chem, 1978, 179, 2603-2612. (7) Rånby, B.; Gao, Z. M.; Hult, A.; Zhang, P. Y. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), 1986, 27, 38. (8) Yang, W. T.; Rånby, B. Polym. Bull. (Berlin) 1996, 37, 89. (9) Yang, W. T.; Rånby, B. J. Appl. Polym. Sci. 1997, 62, 533&545. (10) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550-575. (11) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823-1848. (12) Lvov, Y.; Moehwald, H. Protein architecture: Interfacing molecular assemblies and immobilization biotechnology; Marcel Dekker: New York, 1999.
densities. 13-15 If some special structure, such as a sphere, is located on polymer surfaces, it is certain to promote the patterning study on polymer surfaces. Additionally, the combination of colloid and interface science will open up new interest for the further study of these two fields. Emulsion polymerization is one of the direct approaches used to synthesize spherical particles, while the emulsion’s opacity makes it difficult for it to be used in the surface photografting polymerization; therefore, no such reports have been seen yet. In the past several years, our laboratory has emphasized the development of novel photopolymerization systems and obtained some significant accomplishments, e.g., UV-initiated gradient polymerization,16 photoinduced inverse emulsion polymerization,17 and living photografting polymerization.18 On the basis of the consideration above, we designed the strategy to directly photograft opaque emulsions onto the surface of polymer materials in order to obtain a surface with colloid particles. Consequently, the emulsion system is proved not only suitable for surface photografting but also a rapid way to construct a special topography, layers of spherical nanoparticles, on a polymer surface. As the first report of one series study, this paper will focus on the photografting of the general emulsion, which contains low graft activity monomer methyl methacrylate (MMA), on a casting polypropylene film (CPP) surface. Experimental Section Materials. Commercial casting polypropylene (CPP) films with a diameter of 6 cm and a thickness of approximate 30 µm are used as substrates in the experiments after extracted by (13) Zhao, W.; Krausch, G.; Rafailovich, M. H.; Sokolov, J. Macromolecules 1994, 27, 2933-2935. (14) Uchida, E.; Ikada, Y. Macromolecules 1997, 30, 5464-5469. (15) Pientka, Z.; Oike, H.; Tezuka, Y. Langmuir 1999, 15, 31973201. (16) Yin, M. Z.; Liu, T.; Yang, W. T. Chem. J. Chin. Univ. 2001, 22, 345-347. (17) Liu, L. Y.; Yang, W. T. J. Polym. Sci., Polym Chem. 2004, 42, 846-852. (18) Xing, C. M.; Deng, J. P.; Yang, W. T. Polym. J. 2003, 35, 613621.
10.1021/la0493924 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/16/2004
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Figure 1. Profile of the Sandwich setup for photografting: I1, UV intention at interface 1; I2, UV intention at interface 2. acetone for 36 h to remove the additives and impurities. Methyl methacrylate (MMA) was purchased from Beijing Yili Chemical Co. Ltd., Beijing, China, and purified by vacuum distillation before use. Benzophenone (BP) and 1, 2-divinylbenzene (DVB) were used as received from Shanghai Chemical Regents Co. Ltd., Shanghai,China.Hexadecyltrimethylammoniumbromide(CTAB), acetone and methanol were used as received from Beijing Chemical Regents Co. Ltd., Beijing, China. Deionized water has a conductance of 0.5 µs‚cm-1. Preparation of (MMA/DVB)/O/W Ternary Emulsion. Under vigorous stirring, emulsions were prepared by adding MMA and DVB (molar ratio, 30:1) into aqueous solutions containing CTAB (30 wt % of monomer concentration) at room temperature. Surface Photografting Polymerization. A two-step photografting polymerization method was used in the present work.In the first step, BP was fabricated evenly on the surface of two CPP films by coating BP acetone solution and subsequently evaporating the solvent. The face density of BP was 3.84 × 10-4 mol‚m-2. In the second step, 20 µL of MMA/DVB emulsion was added and spread into a liquid layer in above unit, which was then fixed between two quartz plates and irradiated under UV light (1 kW high-pressure mercury lamp) for 1-15 min at room temperature (Figure 1). After that, the top and bottom films were split off and extracted with methanol for 6 h to remove the emulsifier. It was further extracted with acetone for 6 h and then oscillated with ultrasound to remove the unreacted BP, homopolymer, and ungrafted particles. Following each step, the films were dried at room temperature to constant weight and weighed. The Second Grafting. The grafted films that experienced cleaning treatments above were reassembled again and coated with the same emulsion. The second step was repeated to allow the films to undergo grafting once more. Solution Photografting (Comparative Experiment). A 20 µL MMA/DVB acetone solution (containing 20 wt % MMA, 7% DVB and 5 wt % BP) was added into the middle of two CPP films. Then this unit was irradiated under UV light for 1-15 min in the same setup. After that, the films were extracted with acetone to remove the unreacted BP and homopolymers and dried to constant weight. The monomer conversion (C), grafting yields (GY), and grafting efficiency (GE) on the grafted films were calculated by the following formulas:
C % ) (Wp/Wm) × 100%; GY % ) (Wg/Wo) × 100%; GE % ) (Wg/Wp) × 100% where Wo is the weight of blank film, Wm is the weight of monomer, Wp is the weight of the whole polymers, obtained after removing the emulsifier, and Wg is the weight of graft polymers, obtained after removing other attachments. Characterization. The characteristic groups of the films were detected by FT-IR spectrometer (Nicolet Nexus 670) with variable angle horizontal ATR accessory, on which a 45° rectangle ZnSe crystal was used. The topography of the films was observed with atomic force microscopy (AFM) (Nanoscope III, Digital Instruments), and all of the images were captured under the tapping mode. The inter-film emulsions were collected after irradiation and observed with transmission electron microscopy (TEM) (HITACHI H-800; accelerated voltage, 200 kV).
Figure 2. Relationship between conversion of MMA/DVB emulsion and irradiation time under various monomer concentrations: BP, 5%; UV intensity 7 kµw/cm2; room temperature.
Results and Discussion Photografting Polymerization of MMA/DVB Emulsion. Using a two-step method, BP is, first, adhered to the surface of CPP films, and then the MMA/DVB emulsion is added. By controlling irradiation time, we investigated the polymerization evolution of this system in detail, and the results are shown in Figure 2, Table 1, and Figure 4, separately. Figure 2 illustrates the relationship between the monomer conversions and the irradiation time over the monomer concentration from 20% to 60%. Table 1 lists the grafting efficiency on both top and bottom films under those conditions, and Figure 4 shows the dependence of surface grafting yields on irradiation time. Comparing and analyzing the data and the curves shown in these figures and table, the following features are to be noted. (1) High reaction rate and no induction period at the early stage of polymerization are observed. In Figure 2, no induction period can be observed. The reaction rate is high in the first few minutes and then levels off. With the irradiation toward sandwich setup under UV light, a great deal of surface free radicals are yielded in a short time and used to initiate the graft polymerization of MMA/ DVB, while the amount of oxygen resolved in the system is negligible to and does not have any effect on the initial retardation. Therefore, the induction period appearing in normal free radical polymerization is replaced by a highspeed conversion period in this system. After this stage, the polymerization rate decreases gradually and a final conversion is reached. To elucidate the character of this emulsion grafting system, under the same conditions, a comparative experiment is done where an acetone solution of MMA/DVB is used (Figure 3). It is clear that, in the solution grafting system, the conversion behavior is very different from that in the emulsion grafting system. Without a high rate period, it is a stable linear increase with irradiation time. These results are likely the reflections of the following features. Using the two-step method, BP just adheres on the inner surface of the films, and its hydrophobicity renders it difficult for it to diffuse into the inter-film emulsion. In the early several minutes, most of them experience photoreduction via abstracting surface active hydrogen and leave amounts of surface free radicals to initiate the monomers’ polymerization. Additionally, the remarkable character of emulsion polymerization increases the reaction rate, too. Of course, the graft living chains, which just come from the surface free radicals, have an average dynamic length. Along with the reaction, all of them end gradually, and a final conversion is reached.
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Table 1. Grafting Efficiency of MMA/DVB Emulsion on CPP Filmsa grafting efficiency %(monomer concentration) 20%
30%
40%
50%
60%
time (min)
top
bottom
top
bottom
top
bottom
top
bottom
top
bottom
1 2 4 6 8 10 15
91.40 86.57 75.00 80.24 81.70 76.19 75.90
94.34 92.11 84.23 87.27 83.33 80.67 77.27
85.00 69.00 69.00 82.00 62.50 71.20 64.50
93.08 92.00 91.56 85.08 77.70 83.86 76.04
87.50 68.80 77.40 75.40 69.30 68.40 64.50
89.40 88.95 88.00 80.50 73.20 81.20 62.10
87.70 81.40 72.90 86.50 87.10 75.99 62.30
88.70 82.40 83.20 82.70 75.60 78.80 79.30
85.19 76.92 84.50 86.01 56.90 60.36 51.85
86.40 88.00 81.30 84.00 79.00 64.40 61.20
a
UV intensity, 7 kµw/cm2; room temperature.
Figure 3. Relationship between conversion of MMA/DVB acetone solution and irradiation time: MMA, 20%; BP, 5%; DVB, 7%; UV intensity, 7 kµw/cm2; room temperature.
(2) The higher the monomer concentration, the lower the final conversion (Figure 2) is. The reason is described above: when the amount of BP that coated on the films in the first step is certain, the surface free radicals that can be yielded are certain. Though the absolute amount of monomers involved in the polymerization will increase, the relative conversion will decrease with the increasing monomer concentration in the emulsion system. In fact, this influence is quite great as shown in the following example: in a system with a monomer concentration of 20%, at 4 min, the conversion reaches approximately 45% and finally goes beyond 50%, yet in a system with a concentration of 60%, at 4 min, the conversion is only approximately 20% and finally reaches 25%. (3) There is a difference in grafting yields between the top and bottom films. Examining the two groups of curves shown in Figure 4, it can be found that the grafting yields on both top and bottom films are all increasing with the irradiation time, but their degrees are very different. First, the grafting yields of top films are higher than those of the bottom films. Second, their changes in grafting yields against the monomer concentration are just the converse, and the difference is amplified with the increased concentration. For instance, in a system with 20% of monomer, the GYs are 1.5-2.1% for top films and 0.45-1.12% for bottom films; their difference is a factor of 2-3. However, to a system with 60% monomer, the GYs are 2.22-5.52% for top films and 0.33-0.55% for bottom films; their difference is a factor of 7-10. In addition, as a contrast, the GYs of top and bottom films of a solution graft system are shown in Figure 5. It could be found that to a transparent graft system, this difference (2 times) is much less than that in the opaque emulsion system. Those results reveal the following facts that are associated with the reaction assembly shown in Figure 1: UV-initiated
photografting polymerization mainly takes place in two areas, interface 1 and interface 2, where the reaction rates are determined by the light intensity, BP concentration, and the monomer concentration. Since the BP and monomer concentrations are the same for both interfaces, it should be the distinction of the light intensity that causes the rate difference. This difference, which comes from the inter-film emulsion’s absorption and reflection to the UV light, makes I2 (UV light intensity at interface 2) lower than I1 (UV light intensity at interface 1). The higher the monomer concentration is in the emulsion, the greater the difference is between I1 and I2, and also the greater the difference is in grafting yields. (4) The grafting efficiency is quite high. A previous report showed that MMA was not easy to surface photograft.9 For the MMA/DVB acetone solution graft, the graft efficiency is just about 50-70%. However, for the emulsion photograft, the results are remarkable. In Table 1, the GEs of both top and bottom films are grouped by monomer concentration and listed against the irradiation time. Analyzing this table, the following results are obtained. (1) Overall, GEs are rather high for all of the systems, in the range 60-90%. (2) The monomer concentration has little effect on the GE; namely, with the increase of monomer concentration, GE has a small decrease, but even for a system with a high monomer concentration of 60%, GE still reaches approximately 90% after 1 min irradiation. (3) GE decreases a little during the irradiation. The maximum appears at an early stage; in particular, in the first minute, it can be 85-95%. After 2 min, the change of GE is not obvious and the decrease of GE emerges at about 10-15 min. (4) The GEs of the bottom films are much higher than those of the top films. All of the results indicate that the polymerization in this system starts with the surface free radicals’ yielding by BP’s abstracting surface hydrogen. These radicals proceed into the emulsion. In the whole process, the percentage of surface grafting is much higher than that of homopolymerization and plays a major role. The thickness of the surface graft layer allows to be calculated by the matched GY and its value is in the range from 90 nm to 1.5 µm. In fact, the thickness can be successively increased by subsequent reinitiating of the surface graft polymerization as shown in Figure 6. This result is helpful to the surface modification. The second surface photografting in Figure 5 is conducted in the following way: the first grafted films are thoroughly cleaned from any remaining BP, and then pure MMA/ DVB emulsions that have the same MMA concentration of 20% were added. Under the second irradiation, the GY is reincreased and has a similar evolution to that in the first irradiation. Since no new photoinitiator, BP, is added, the results here showed the existence of re-photoinitiatable groups that will be further discussed in the mechanism section below.
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Figure 4. Relationship between grafting yields of emulsions and irradiation time under various MMA concentrations: room temperature; BP concentration, 5%; UV intensity, 7 kµw/cm2.
Figure 5. Relationship between grafting yields of MMA/ Acetone solution and irradiation time: MMA, 20%; BP, 5%; DVB, 7%; UV intensity, 7 kµw/cm2; room temperature.
Figure 6. Comparison of GY in the first and second photografting of MMA/DVB emulsion: MMA concentration, 20%; UV intensity, 7 kµw/cm2; room temperature.
The spectra of characteristic groups of blank and the grafted films are obtained by ATR-IR. As shown in Figure 7, in contrast with the blank one, there is an obvious peak of υ(CdO) at a wavenumber of 1731.79 cm-1 after the grafting, which indicates the existence of the PMMA. Moreover, this peak’s intensity is further increased after the second grafting. In the subtraction spectrum, the first and second grafting spectra are compared and the peak of υ(CdO) still presents at 1731.79 cm-1. Topography and Morphology of the Grafted Films. The topographies of the grafted and ungrafted films are
investigated by AFM (Figure 8). All of the films are strictly treated by extraction and supersonic oscillation to remove the components adhered on their surfaces before observation. Compared with the blank film (Figure 8a), whose surface is almost smooth and where no special morphology can be seen, the film grafted with MMA solution for 15 min in a comparative experiment displays a surface with lots of irregular blocks but yet no spherical structures (Figure 8b). Representative images of the samples, which have been grafted with the emulsion system, are shown in parts c-f. Figure 8c provides a clear view of the topography of the grafted film, which has been grafted with GY of 2.23% (matching with a grafting thickness about 690 nm). It is apparent that the surface has been covered with the spherical nanoparticles, whose diameters are about 30 nm. Corresponding to the lower places (with the deeper color) in the height image (left one in Figure 8c), in the phase image (right one), they are covered with the similar nanoparticles as well. This reveals that it is the piles of PMMA spherical nanoparticles that form the thick graft layer. On the other hand, in Figure 8d (the section analysis of a part in Figure 8c, which has the greatest undulation of height), the maximum altitude of the section is about 20 nm and close to the diameters of the graft particles. Besides the valley, the undulation of the height is regular and the width of the peaks seems alike and matches with the diameters of the outer graft particles on the surface. In the valley, a similar undulation of the height also exists. These further indicate that the surface graft layer is not made up of just one layer of nanoparticles, and on each layer the distribution of the particles’ diameters is almost uniform. The other evidence is shown in Figure 8e, where the grafting assembly is just irradiated for 1 min and the graft layer has a thickness of about 300 nm with the same emulsion. Similarly, the surface of the film is covered with great amounts of nanoparticles with diameters of about 15 nm (smaller than that in the case of 15 min irradiation), which, to some extent, cohered to each other. Furthermore, matched with the lower places in the height image, the phase image is covered with particles, too. Therefore, we think that the film’s surface has been covered with layers of growing nanoparticles in the first minute. Figure 8f shows the topography of a film sample which has been grafted for 15 min with an emulsion of 60% MMA. It is clear that apart from a small amount of bigger particles, the size of most particles is about 50 nm, which is bigger than that of 20% monomer concentration at the same
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Figure 7. ATR-IR spectra of the PP film: sub, the subtraction spectrum of two times (first and second) grafting MMA/DVB emulsion; 1st, GY, 1.01%; 2nd, GY, 1.23%; MMA concentration 30%; BP concentration, 5%; room temperature; UV intensity, 7 kµw/cm2.
irradiating time. At this time, the thickness of the graft layer is about 1.5 µm (calculated by the GY of the grafted film). Table 2 lists the data of the mean diameter of the graft particles under various monomer concentrations. Along with the increase of the monomer concentration, the mean size of the particles is increasing and its distribution gets wide. Generally, the colloid particles gained from emulsion polymerization have diameters ranging from hundreds of nanometers to several micrometers. But in this grafting system, the diameter of the grafted particles is just about 30 nm and out of the range. In contrast, we collect and observe the inter-film particles, which are formed by crosslinking homopolymerization of MMA/DVB between two films under UV radiation. On the basis of the TEM images shown in Figure 9, the inter-film colloid particles have a bigger size, ranging from 60-130 nm to 60-200 nm along with the increase of the monomer concentration from 30% to 60%, than the graft particles, but still less than the value (hundreds of nanometers) obtained in general MMA emulsion photopolymerization systems.19 These results indicate that the inter-film colloid particles have a more free condition for growing than the graft particles so that their shapes are more complete and their sizes are bigger. However, their growing is still confined by the graft particles’ competition because of the limited inter-film space, meanwhile the ungraft polymerization is not significant in the system. As a result, the colloid particles’ growing is not so sufficient as that in the real general emulsion. Possible Mechanism and Discussion A series of interesting results are obtained in the study of the photoinitiated surface graft polymerization with MMA/DVB emulsion, such as the high surface GE, nanometer scale of surface graft particles, and the formed thick graft layers, which tie on the substrate and have a thickness that is dozens of times more than the particles’ diameter. To give an acceptable explanation for the results (19) Shim, S. E.; Shin, Y.; Jun, J. W.; Lee, K.; Jung, H.; Choe, S. Macromolecules 2003, 36, 7994-8000.
above, here, we describe a possible mechanism which includes the process of surface initiating, grafting combined with cross-linking, special termination, and shish kebab growing, as shown in Scheme 1. Surface Initiation. According to the present grafting procedure, BP is previously coated on the inner surfaces of top and bottom films, and it has not enough time and no ability (hydrophobic) to diffuse into the aqueous phase. Therefore, under UV irradiation, the abstracting hydrogen reactions of BP (being reduced into semipinacol radicals) only take place on the surfaces of both top and bottom films and leave great amounts of surface free radicals to initiate the surface graft polymerization. On the other hand, MMA is a monomer with poor hydrophilicity;20-23 therefore, those dissolved in the aqueous phase will concentrate on the lipophilic substrate surfaces, and a highly effective surface graft polymerization is induced at an early stage. Propagation with 3D Cross-Linking. Going on with the process, some graft particles form on the PP films’ surface (described later) and the graft chains’ propagation is confined in them, as well as another component, DVB, which also participates in the polymerization. Thereby, the reactions in the particles include the polymer chains’ propagation and their cross-linking. In fact, the addition of amounts of DVB into the system is helpful to the formation of particles in such a limited polymerization space. As the chain length increases rapidly, they also cross-link with each other to form the three-dimensional network to enhance the particle’s shape. Termination with BPH Free Radicals. In conventional emulsion polymerization, the termination reaction takes place mainly between the chain radicals in the micelles and primary radicals coming in. In the present surface grafting system, there are three unique features: (1) there are almost no mobile primary radicals existing (20) Fitch, R. M.; Tsai, C. H. Polymer Colloids; New York: Plenum: 1971, p 73. (21) Ming, W.; Jones, F. N.; Fu, S. Macromol. Chem. Phys. 1998, 199, 1075-1079. (22) Pilcher, S. C.; Ford, W. T. Macromolecules 1998, 31, 3454. (23) He, G. W.; Pan, Q. M.; Garry, L. R. Macromol. Rapid Commun. 2003, 24, 585.
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Figure 8. Tapping mode AFM images of CPP film before and after grafting of PMMA nanoparticles: (a) blank film; (b) image of grafting with acetone solution of MMA; irradiation time, 15 min; Gy, 1.22%; (c) image of grafting with MMA/DVB emulsion; MMA concentration, 30%; Gy, 2.23%; (d) section analysis of image c; (e) image of grafting with MMA/DVB emulsion; MMA concentration, 30%; irradiation time 1 min; Gy, 1.04%; (f) image of grafting with MMA/DVB emulsion; MMA concentration, 60%; Gy 5.60%. Irradiation time for parts c and f: 15 min. Table 2. Average Diameters of Graft Particles in Different Concentration Emulsiona monomer concn in emulsion (%) 20 30 40 50 60 diameters of graft particles (nm) ∼30 ∼30 30-40 30-50 ∼50 a All of the above results were obtained after 15 min irradiation at room temperature.
except surface radicals; (2) the chain propagating and cross-linking reactions proceed simultaneously so that the graft polymerization rapidly transforms into the growing of 3D cross-linking particles; (3) each particle is bounded to the substrate surface. Therefore, the coupling of two graft chain radicals or the chain radicals with primary radicals is quite difficult. However, the coupling of chain radicals with the semipinacol free radicals (from BP photoreduced) to form the C-BPH dormant groups is possible. The formation and reinitiation of this dormant
group has been previously reported in several publications.18,24,25 The existence of the dormant groups not only makes the particles grow continuously possible but also provides a number of new ways to further functionalize the grafted surfaces. The reinitiated graft polymerization shown in Figure 6, in fact, supported this supposition. Formation and Growth of Particles. With the growing of the graft chains on the substrates’ surfaces, to keep the stable state of the system, the emulsifier is induced to surround the chains to form the precursory particles; meanwhile, the DVB existing in the system induces cross-linking among those chains to form the three-dimensional networks. Thus, a layer made of living (24) Yang, W. T.; Rånby, B. Macromolecules 1996, 29, 3308. (25) Ma, H.; Davis, R. H.; Bowman, C. N. Macromolecules 2000, 33, 331.
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Figure 9. TEM images of the inter-film un-graft cross-linking particles after 15 min irradiation: (a) MMA concentration, 30%; (b) MMA concentration, 60%. Scheme 1. Chemical Formation of the Graft Chains and the Growing Mechanism of the Graft Particles.
graft particles is tied to each top and bottom interface. The particles then grow in size with the replenishment of the monomers from the outer phase. On the other hand, the space used for polymerization in the present reaction system is rather limited, so that the propagating graft chains may penetrate into the micelles directly to form the growing particles. When the growing chain radicals meet the semipinacol radicals, the dormant groups are formed and the growth of the particles stops temporarily. Shish Kebab Growth. Under the UV irradiation, the dormant groups in the precursory particles can be recleft, and some reactivated chain radicals are formed on the particles’ surfaces to initiate the graft polymerization once again. In the meantime, with the helps of the emulsifier and the cross-linking agent, new particles are formed on the former ones. Repeating those procedures, strings of particles are formed on the substrate surface like shish kebabs. In our view, the rapid graft of such thick layers of particles is very interesting and important. First, a new type of rapid modification method on polymer surface is discovered; second, the introduction of a regular structure (spherical nanoparticles) onto the polymer surface may provide the possibility for polymer surface patterning.
Conclusion For the first time, opaque MMA/DVB emulsion is successfully used in surface photografting polymerization. In this system, the polymerization rate is higher than that in the solution system, which has the same ratio of MMA/BP, but the final conversion is just in the range 15%-55%. In addition, the GE reaches about 80%; the GY reaches about 5% associated with a thickness of about 1.5 µm. The AFM images show that the graft PMMA/ DVB is in the shape of nanoparticles with a diameter of 30-50 nm, which is much smaller than that of the interfilm colloid particles observed by TEM, 60-200 nm. To interpret the above results, a possible mechanism is proposed with a model that graft PMMA/DVB is forced to undergo a shish kebab type of growing and form a thick layer on the film surface based on the changing limitation factors in the polymerization. Supporting Information Available: Table S1, grafting efficiency of MMA/DVB solutions on CPP films, Figure S1, tapping mode AFM images of CPP film before and after grafting, and Scheme S1, chemical formation of the graft chains and the growing mechanism of the graft particles. This material is available free of charge via the Internet at http://pubs.acs.org. LA0493924
