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Growth of polymer nanoparticles in microemulsion polymerization initiated with γ ray
PERGAMON
Radiation Physics and Chemistry Radiation Physics and Chemistry 54 (1999) 279±283
Growth of polymer nanoparticles in microemulsion polymerization initiated with g ray Xu Xiangling a, *, Ge Xuewu a, Ye Qiang a, Zhang Zhicheng a, Zuo Ju b, Niu Aizhen b, Zhang Manwei a a
Department of Applied Chemistry, University of Science and Technology of China, Hefei 230026, People's Republic of China Department of Chemistry, State Key Laboratory of Functional Polymeric Materials for Adsorption and Separation, Nankai University, Tianjin 300071, People's Republic of China
b
Received 15 April 1998
Abstract In microemulsion polymerization of styrene, butyl acrylate and methyl methacrylate initiated with gamma ray, growth of polymer nanoparticles was observed with photon correlation spectroscopy, and the conversion curve was recorded with a dilatometer. There is some similarity in the growth of polymer particles. The size of polymer particles rapidly increases up to their maximum at the early stage. With the increase of conversion, the large particles supply their monomer to newly formed particles and become smaller. In all these three microemulsion polymerizations, the evidence of continuous nucleation was observed. When monomer is styrene or butyl acrylate, a plateau of polymerization rate emerges. When monomer is methyl methacrylate, no plateau of polymerization is observed. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction During the past decade microemulsion polymerization, as a new branch of emulsion polymerization, has been extensively studied. However, most attention was paid to the polymerization rate. Only a few papers focus on the growth of polymer particles, and the variation of the distribution of particles during polymerization. This may be due in part to the diculties in directly obtaining the particle size using the most conventional analytic techniques, because of the high emulsi®er concentration and the various complications associated with microemulsions. It was reported that in W/O microemulsion, the nucleation is continuous throughout the polymerization (Candau et al., 1985). The early work on styrene O/W microemulsion polymerization assumed that the nucleation occurs in the microdroplets and that nucleation was limited below 20% conversion. After re-exam-
* Corresponding author.
ination of the same system, Guo et al. (1989, 1992) postulated that particle nucleation is continuous. Gan et al. (1994) investigated the formation of polystyrene nanoparticles in ternary cationic microemulsion through photon correlation spectroscopy (PCS) and TEM. It was reported that the initiation occurred mainly in the styrene swollen microemulsion droplets. The average hydrodynamic radius of swollen particles ®rst increased rapidly to a maximum at 4±7% conversion. Due to the continuous nucleation, the number of polymer particles and the polydispersity of particle size increased continuously throughout the polymerization process. The growth of PMMA particles in ternary cationic microemulsion had also been studied (Gan et al., 1995). In contrast to that of the styrene system, the sizes of methyl methacrylate (MMA)-swollen polymer particles increased continuously during polymerization, no maximum of hydrodynamic radius of swollen particles was observed. In a similar system (Bleger et al., 1994), a two-stage process of polymerization is observed. The ®rst stage, described by a very slow
0969-806X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 9 8 ) 0 0 2 5 4 - 0
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Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
increase in conversion, is attributed mainly to homogenous nucleation. The second stage, characterized by a much higher rate of conversion, involves continuous nucleation and is governed mainly by a micelle-entry mechanism. In this paper, microemulsion polymerization of three dierent monomers, styrene, butyl acrylate and methyl methacrylate, were initiated with g ray, since polymerization was very easy to stop at the desired conversion by removing g ray source. In this way, variation of the size and its distribution of monomer-swollen polymer particles with conversion was reported and discussed.
2. Experimental
Fig. 1. Growth of polymer particles in styrene microemulsion polymerization initiated with g ray at 338C. The microemulsion composition is: 20wt% St, 12wt% SBOA, 68wt% H2O. Dose rate is 28.5 Gy/min.
2.1. Materials Styrene (St), butyl acrylate (BA), and methyl methacrylate (MMA) were distilled and stored at ÿ108C. 12butinoyloxy-9-octadecenoic acid (BOA) was synthesized at 1308C by direct esteri®cation of butanoic acid with 12-oxy-9-octadecenoic acid (OOA). BOA was further neutralized with NaOH to give SBOA (sodium 12-butinoyloxy-9-octadecenate). 2.2. Preparation and polymerization of microemulsion Preparation and polymerization of microemulsion was described in a previous paper (Xu et al., 1997). 2.3. Particle size determination Particle sizes Dp (hydrodynamic diameter) of microemulsion latex were determined by PCS using a Brookhaven BI-200SM instrument. Before measuring, the latex was diluted to 100±150 times to minimize interparticle interaction.
3. Results and discussions 3.1. Polymerization of styrene At the early stage of polymerization, the newly formed particles are rapidly swollen with monomer (Guo et al., 1992). Therefore the observed diameter of polymer particles at about 5% conversion, is the largest (Fig. 1 and Table 1). When the conversion is increased from 14% up to 50%, the polydispersity of particles becomes smaller. The large particles serve as monomer reservoir, and supply monomer to small particles by diusion. In this way, the large particles become smaller, and the small particles become larger. When the conversion is higher than 50%, small particles (10±20 nm, diameter) were detected. They should be the newly nucleated particles. In microemulsion polymerization, the emulsi®er concentration is very high. During whole polymerization process, there are
Table 1 Variation of some physical values with polymerization conversion in microemulsion polymerization of styrene Conversion (%)
Dpa (nm)
Npb (1019/l)
Mvc (105)
npd
Rpe (mmol/l.s)
5.3 14.3 25.9 34.1 47.2 100
142 36.5 34.8 36.1 36.4 38.4
0.02 1.04 1.39 1.31 0.94 0.61
6.9 6.4 11.0 11.8 10.4 6.9
37.6 2.60 2.03 2.64 5.80 28.6
1.20 1.90 1.98 1.62 1.16 /
a
Hydrodynamic diameter of monomer-swollen polymer particles. Total number of monomer-swollen polymer particles in microemulsion. c Molecular weight of polymer at dierent conversion. d Average number of polymer chain per polymer particles. e Overall polymerization rate at dierent conversion b
Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
Fig. 2. The conversion and polymerization rate vs. polymerization time in St microemulsion initiated with g ray at 338C. Microemulsion composition is: 20wt% St, 12wt% SBOA, 68wt% H2O. Dose rate is 28.5 Gy/min.
always many micelles swollen with monomer. These micelles may capture radicals produced in the aqueous phase to nucleate and form growing polymer particles. When the conversion is low, the monomer content in the system is still high enough to quickly make these newly formed growing particles swollen with monomer. However, at high conversion, the monomer content in the system is much reduced, and the monomer content in polymer particles becomes lower than their saturation concentration. In this case, the swelling of newly formed particles is greatly restricted. Thus the small particles are observed. It was shown that the Np decreases at higher conversion. The much higher Np at 100% conversion (Table 1) indicates it was caused by the coagulation. Fig. 2 is the conversion curve of St microemulsion polymerization. There is an obvious plateau of polymerization rate. When the conversion is increased from 14% to 26%, the number of total particles in the system is increased to about 1.4 times. However, the polymerization rate is almost unchanged. It is in good agreement with the conclusion we proposed (Xu et al., 1997). In the plateau of Rp, the number of growing polymer particles achieves a balance value and is kept
281
Fig. 3. The diameter (Dp) and its distribution of swollen particles vs. conversion in MMA microemulsion polymerization. Microemulsion composition: 17wt% MMA, 12wt% SBOA, 71wt% H2O; dose rate, 32.5 Gy/min.
constant, though the number of total particles is still increasing. At higher conversion, the decrease of Rp is mainly due to the decrease of monomer concentration in the particles. 3.2. Polymerization of methyl methacrylate The polarity of MMA was much higher than that of St. Therefore it was expected that besides micelle nucleation, the homogeneous nucleation may play an important role in microemulsion polymerization of MMA. Table 2 and Fig. 3 show some information about the growth of PMMA particles. As observed in St microemulsion, Dp at low conversion (6%) is also the biggest among all samples. It coincides with the fact that the transparent microemulsion became translucent just after the polymerization started, and then the transparency increased slowly with conversion. At about 13% conversion, small particles appear and the large monomer-swollen particles shrink by transporting monomer to small particles. At about 33% conversion, the number of total polymer particles reaches its maximum, and so is the polymerization
Table 2 Variation of some physical values with polymerization conversion in microemulsion polymerization of methyl methacrylate Conversion (%)
Dpa (nm)
Npb (1018/l)
Mvc (105)
npd
Rpe (m mol/l.s)
5.6 13.4 33.8 59.4 100
179 55.9 38.5 39.4 50.7
0.09 2.00 7.50 6.22 3.05
6.82 8.15 7.92 8.28 7.86
91 8.0 5.5 11.2 40.5
1.5 2.2 3.6 2.4 /
a±e
See Table 1
282
Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
Fig. 4. Conversion and polymerization rate vs. conversion in MMA microemulsion polymerization initiated with g ray at 338C. Microemulsion composition: 17wt% MMA, 12wt% SBOA, 71wt% H2O; dose rate, 32.5 Gy/min.
rate. When conversion is higher than 50%, the number of total polymer particles decreases again, while the size of particles keeps on increasing (Table 2). Taking account of the high polarity of MMA and the huge number of swollen micelles, it was assumed that MMA microemulsion polymerization nucleates through both homogeneous nucleation or micellar nucleation at the early stage of polymerization. Once the growing polymer particles are formed, they will be quickly swollen with the monomer, and the size of particles increases rapidly. With further polymerization, there are more and more growing polymer particles formed, and the monomer will redistribute in the system. In this way, the large particles rapidly become smaller by supplying monomer to newly formed small particles. It is just the situation observed at 13% conversion (Fig. 3). It was shown in Table 2 that Np keeps on increasing up to above 30% conversion. Meanwhile, the smallest size of particles above 13% conversion keeps increasing (Fig. 3). The growth of the smallest size cannot be attributed to the coagulation of polymer particles. Otherwise, the total Np should decrease from 13 to 33% conversion. Now the question arises: where do the new particles come from? We assume that the nucleation mechanism is changed. On the one hand, when the conversion is high enough the MMA concentration in the aqueous phase and the emulsi®er layer should both decrease. Therefore the probability of homogeneous nucleation is minimized. If the new particles are still produced through homogeneous nucleation, the smallest size of particles will decrease with conversion as observed in St microemulsion polymerization, since the transferring
of monomer to these newly formed particles will be restricted at high conversion. On the other hand, MMA may act as cosurfactant and change the property of the emulsi®er layer because of its high polarity (Bleger et al., 1994). The interface tension should increase with conversion, because of the decrease of MMA concentration. It would further lead to the increase of the swollen micelle size. Since micellar nucleation predominates at high conversion, the size of newly formed growing polymer particles increased with conversion. Np decreases above 50% conversion, because of the coagulation of polymer particles, as observed in St microemulsion polymerization. It was shown in Fig. 4 that Rp reaches its maximum at about 30% conversion and no plateau of Rp is observed. In contrast to St microemulsion polymerization, the nucleation is slower, and the Np is only about half of that in St microemulsion polymerization. When conversion is increased from 13% to 33%, Np is increased by about 3.7 times. Two factors may lead to the disappearance of the plateau of Rp. First, the propagation constant (kp) of MMA is about 5 times the kp for St. Although the maximum Np is only about half of that for St, the overall polymerization rate is still about twice that in St microemulsion polymerization. As has been shown previously (Xu et al., 1997), the higher the polymerization rate, the shorter the plateau of Rp. Second, though the monomer concentration before polymerization was similar (1.9 M and 1.7 M for St and MMA), the monomer saturation of MMA in polymer particles (6.3 M) is higher than that of St (5.1 M) (Tidwell et al., 1970). Therefore, for MMA microemulsion, the monomer concentration in polymer particles will be
Fig. 5. Growth of particles in BA microemulsion polymerization initiated with g ray at 338C. Microemulsion composition: 25wt% BA, 12wt% SBOA, 63wt% H2O; dose rate, 32.5 Gy/min.
Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
283
Table 3 Variation of some physical values with polymerization conversion in microemulsion polymerization of butyl acrylate Conversion (%)
Dpa (nm)
Npb (1018/l)
Mvc (106)
npd
Rpe (m mol/l.s)
7.0 15.0 31.0 50.0 100
93.1 68.7 57.6 49.7 47.4
1.03 2.53 4.06 6.03 4.54
1.51 1.68 1.31 1.35 0.73
6.7 5.5 8.8 9.2 45.4
1.8 2.3 2.4 1.2 /
a±e
See Table 1
reduced below the saturation concentration at lower conversion. It will further lead to the disappearance of the plateau. 3.3. Polymerization of butyl acrylate Though there was polar group in BA, the polarity of BA was much smaller than that of MMA. As St, BA could be regarded as insoluble in aqueous phase. It was expected that the polymerization of BA in microemulsion should be similar to St, rather than to MMA. As shown in Fig. 5 and Table 3, the growth of particles in BA microemulsion polymerization is very similar to that in St and MMA microemulsion polymerization. At the early stage of polymerization, the sizes of particles quickly increase up to their maximum. With further increase of conversion, the sizes of polymer particles become smaller. Np keeps on increasing even up to 50% conversion, and Fig. 5 shows that there are still small newly formed particles. These facts indicate that the nucleation is really a continuous process. At higher conversion, the coagulation between polymer particles dominates. As a result, the Np decreases.
In contrast to MMA microemulsion polymerization, the smallest size in BA microemulsion polymerization keeps on decreasing with conversion (Fig. 6), when conversion is lower than 50%. This is very similar to St microemulsion polymerization (Fig. 1). In addition, the glass temperature of poly(butyl acrylate) was much lower than that of PSt, and the monomer in swollen polymer particles was easier to transport to polymerizing polymer particles. When conversion was bigger than 50%, there were no newly formed particles observed for BA microemulsion polymerization, while for St microemulsion polymerization, at high conversion, the monomer in polymer particles slowly diused out into micelles, and these, swollen with monomer, formed new particles when radicals diused in. As shown in Fig. 6, there is also an apparent plateau of Rp in 15±35% conversion, when the initial monomer concentration in microemulsion was 1.9 M. The plateau is longer than that in St microemulsion polymerization. It may be attributed to the lower saturation concentration (4.6 M) of BA in polymer particles. In comparison with St microemulsion, the monomer concentration in polymer particles could be kept equal to saturation at higher conversion. References
Fig. 6. Conversion and polymerization rate vs. time in BA microemulsion polymerization initiated with g ray at 338C. Microemulsion composition: 25wt% BA, 12wt% SBOA, 63wt% H2O; dose rate, 32.5 Gy/min.
Bleger, F., Murthy, A.K., Pla, F., Kaler, E.W., 1994 Macromolecules 27, 2559. Candau, F., Leong, Y.S., Fitch, J.R., 1985 J. Polym. Sci. Polym. Chem. Ed. 23, 193. Gan, L.M., Chew, C.H., Lee, K.C., Ng, S.C., 1994 Polymer 35, 2659. Gan, L.M., Lee, K.C., Chew, C.H., Tok, E.S., Ng, S.C., 1995 J. Polym. Sci. Part A: Polym. Chem. 33, 1161. Guo, J.S., El-Aasser, M.S., Vanderho, J.W., 1989 J. Polym. Sci. Polym. Chem. Ed. 27, 691. Guo, J.S., Sudol, E.D., Vanderho, J.W., El-Aasser, M.S., 1992 J. Polym. Sci. Polym. Chem. Ed. 30, 691. Tidwell, P.W., Mortimer, G.A., 1970 J. Macromol Sci. Rev. Macromol. Chem. 4, 281. Xu, X.L., Ge, X.W., Zhang, Z.C., Wu, Z.C., Zhang, Z.W., 1997 Radiation Physics and Chemistry 49, 469.
Radiation Physics and Chemistry Radiation Physics and Chemistry 54 (1999) 279±283
Growth of polymer nanoparticles in microemulsion polymerization initiated with g ray Xu Xiangling a, *, Ge Xuewu a, Ye Qiang a, Zhang Zhicheng a, Zuo Ju b, Niu Aizhen b, Zhang Manwei a a
Department of Applied Chemistry, University of Science and Technology of China, Hefei 230026, People's Republic of China Department of Chemistry, State Key Laboratory of Functional Polymeric Materials for Adsorption and Separation, Nankai University, Tianjin 300071, People's Republic of China
b
Received 15 April 1998
Abstract In microemulsion polymerization of styrene, butyl acrylate and methyl methacrylate initiated with gamma ray, growth of polymer nanoparticles was observed with photon correlation spectroscopy, and the conversion curve was recorded with a dilatometer. There is some similarity in the growth of polymer particles. The size of polymer particles rapidly increases up to their maximum at the early stage. With the increase of conversion, the large particles supply their monomer to newly formed particles and become smaller. In all these three microemulsion polymerizations, the evidence of continuous nucleation was observed. When monomer is styrene or butyl acrylate, a plateau of polymerization rate emerges. When monomer is methyl methacrylate, no plateau of polymerization is observed. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction During the past decade microemulsion polymerization, as a new branch of emulsion polymerization, has been extensively studied. However, most attention was paid to the polymerization rate. Only a few papers focus on the growth of polymer particles, and the variation of the distribution of particles during polymerization. This may be due in part to the diculties in directly obtaining the particle size using the most conventional analytic techniques, because of the high emulsi®er concentration and the various complications associated with microemulsions. It was reported that in W/O microemulsion, the nucleation is continuous throughout the polymerization (Candau et al., 1985). The early work on styrene O/W microemulsion polymerization assumed that the nucleation occurs in the microdroplets and that nucleation was limited below 20% conversion. After re-exam-
* Corresponding author.
ination of the same system, Guo et al. (1989, 1992) postulated that particle nucleation is continuous. Gan et al. (1994) investigated the formation of polystyrene nanoparticles in ternary cationic microemulsion through photon correlation spectroscopy (PCS) and TEM. It was reported that the initiation occurred mainly in the styrene swollen microemulsion droplets. The average hydrodynamic radius of swollen particles ®rst increased rapidly to a maximum at 4±7% conversion. Due to the continuous nucleation, the number of polymer particles and the polydispersity of particle size increased continuously throughout the polymerization process. The growth of PMMA particles in ternary cationic microemulsion had also been studied (Gan et al., 1995). In contrast to that of the styrene system, the sizes of methyl methacrylate (MMA)-swollen polymer particles increased continuously during polymerization, no maximum of hydrodynamic radius of swollen particles was observed. In a similar system (Bleger et al., 1994), a two-stage process of polymerization is observed. The ®rst stage, described by a very slow
0969-806X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 9 8 ) 0 0 2 5 4 - 0
280
Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
increase in conversion, is attributed mainly to homogenous nucleation. The second stage, characterized by a much higher rate of conversion, involves continuous nucleation and is governed mainly by a micelle-entry mechanism. In this paper, microemulsion polymerization of three dierent monomers, styrene, butyl acrylate and methyl methacrylate, were initiated with g ray, since polymerization was very easy to stop at the desired conversion by removing g ray source. In this way, variation of the size and its distribution of monomer-swollen polymer particles with conversion was reported and discussed.
2. Experimental
Fig. 1. Growth of polymer particles in styrene microemulsion polymerization initiated with g ray at 338C. The microemulsion composition is: 20wt% St, 12wt% SBOA, 68wt% H2O. Dose rate is 28.5 Gy/min.
2.1. Materials Styrene (St), butyl acrylate (BA), and methyl methacrylate (MMA) were distilled and stored at ÿ108C. 12butinoyloxy-9-octadecenoic acid (BOA) was synthesized at 1308C by direct esteri®cation of butanoic acid with 12-oxy-9-octadecenoic acid (OOA). BOA was further neutralized with NaOH to give SBOA (sodium 12-butinoyloxy-9-octadecenate). 2.2. Preparation and polymerization of microemulsion Preparation and polymerization of microemulsion was described in a previous paper (Xu et al., 1997). 2.3. Particle size determination Particle sizes Dp (hydrodynamic diameter) of microemulsion latex were determined by PCS using a Brookhaven BI-200SM instrument. Before measuring, the latex was diluted to 100±150 times to minimize interparticle interaction.
3. Results and discussions 3.1. Polymerization of styrene At the early stage of polymerization, the newly formed particles are rapidly swollen with monomer (Guo et al., 1992). Therefore the observed diameter of polymer particles at about 5% conversion, is the largest (Fig. 1 and Table 1). When the conversion is increased from 14% up to 50%, the polydispersity of particles becomes smaller. The large particles serve as monomer reservoir, and supply monomer to small particles by diusion. In this way, the large particles become smaller, and the small particles become larger. When the conversion is higher than 50%, small particles (10±20 nm, diameter) were detected. They should be the newly nucleated particles. In microemulsion polymerization, the emulsi®er concentration is very high. During whole polymerization process, there are
Table 1 Variation of some physical values with polymerization conversion in microemulsion polymerization of styrene Conversion (%)
Dpa (nm)
Npb (1019/l)
Mvc (105)
npd
Rpe (mmol/l.s)
5.3 14.3 25.9 34.1 47.2 100
142 36.5 34.8 36.1 36.4 38.4
0.02 1.04 1.39 1.31 0.94 0.61
6.9 6.4 11.0 11.8 10.4 6.9
37.6 2.60 2.03 2.64 5.80 28.6
1.20 1.90 1.98 1.62 1.16 /
a
Hydrodynamic diameter of monomer-swollen polymer particles. Total number of monomer-swollen polymer particles in microemulsion. c Molecular weight of polymer at dierent conversion. d Average number of polymer chain per polymer particles. e Overall polymerization rate at dierent conversion b
Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
Fig. 2. The conversion and polymerization rate vs. polymerization time in St microemulsion initiated with g ray at 338C. Microemulsion composition is: 20wt% St, 12wt% SBOA, 68wt% H2O. Dose rate is 28.5 Gy/min.
always many micelles swollen with monomer. These micelles may capture radicals produced in the aqueous phase to nucleate and form growing polymer particles. When the conversion is low, the monomer content in the system is still high enough to quickly make these newly formed growing particles swollen with monomer. However, at high conversion, the monomer content in the system is much reduced, and the monomer content in polymer particles becomes lower than their saturation concentration. In this case, the swelling of newly formed particles is greatly restricted. Thus the small particles are observed. It was shown that the Np decreases at higher conversion. The much higher Np at 100% conversion (Table 1) indicates it was caused by the coagulation. Fig. 2 is the conversion curve of St microemulsion polymerization. There is an obvious plateau of polymerization rate. When the conversion is increased from 14% to 26%, the number of total particles in the system is increased to about 1.4 times. However, the polymerization rate is almost unchanged. It is in good agreement with the conclusion we proposed (Xu et al., 1997). In the plateau of Rp, the number of growing polymer particles achieves a balance value and is kept
281
Fig. 3. The diameter (Dp) and its distribution of swollen particles vs. conversion in MMA microemulsion polymerization. Microemulsion composition: 17wt% MMA, 12wt% SBOA, 71wt% H2O; dose rate, 32.5 Gy/min.
constant, though the number of total particles is still increasing. At higher conversion, the decrease of Rp is mainly due to the decrease of monomer concentration in the particles. 3.2. Polymerization of methyl methacrylate The polarity of MMA was much higher than that of St. Therefore it was expected that besides micelle nucleation, the homogeneous nucleation may play an important role in microemulsion polymerization of MMA. Table 2 and Fig. 3 show some information about the growth of PMMA particles. As observed in St microemulsion, Dp at low conversion (6%) is also the biggest among all samples. It coincides with the fact that the transparent microemulsion became translucent just after the polymerization started, and then the transparency increased slowly with conversion. At about 13% conversion, small particles appear and the large monomer-swollen particles shrink by transporting monomer to small particles. At about 33% conversion, the number of total polymer particles reaches its maximum, and so is the polymerization
Table 2 Variation of some physical values with polymerization conversion in microemulsion polymerization of methyl methacrylate Conversion (%)
Dpa (nm)
Npb (1018/l)
Mvc (105)
npd
Rpe (m mol/l.s)
5.6 13.4 33.8 59.4 100
179 55.9 38.5 39.4 50.7
0.09 2.00 7.50 6.22 3.05
6.82 8.15 7.92 8.28 7.86
91 8.0 5.5 11.2 40.5
1.5 2.2 3.6 2.4 /
a±e
See Table 1
282
Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
Fig. 4. Conversion and polymerization rate vs. conversion in MMA microemulsion polymerization initiated with g ray at 338C. Microemulsion composition: 17wt% MMA, 12wt% SBOA, 71wt% H2O; dose rate, 32.5 Gy/min.
rate. When conversion is higher than 50%, the number of total polymer particles decreases again, while the size of particles keeps on increasing (Table 2). Taking account of the high polarity of MMA and the huge number of swollen micelles, it was assumed that MMA microemulsion polymerization nucleates through both homogeneous nucleation or micellar nucleation at the early stage of polymerization. Once the growing polymer particles are formed, they will be quickly swollen with the monomer, and the size of particles increases rapidly. With further polymerization, there are more and more growing polymer particles formed, and the monomer will redistribute in the system. In this way, the large particles rapidly become smaller by supplying monomer to newly formed small particles. It is just the situation observed at 13% conversion (Fig. 3). It was shown in Table 2 that Np keeps on increasing up to above 30% conversion. Meanwhile, the smallest size of particles above 13% conversion keeps increasing (Fig. 3). The growth of the smallest size cannot be attributed to the coagulation of polymer particles. Otherwise, the total Np should decrease from 13 to 33% conversion. Now the question arises: where do the new particles come from? We assume that the nucleation mechanism is changed. On the one hand, when the conversion is high enough the MMA concentration in the aqueous phase and the emulsi®er layer should both decrease. Therefore the probability of homogeneous nucleation is minimized. If the new particles are still produced through homogeneous nucleation, the smallest size of particles will decrease with conversion as observed in St microemulsion polymerization, since the transferring
of monomer to these newly formed particles will be restricted at high conversion. On the other hand, MMA may act as cosurfactant and change the property of the emulsi®er layer because of its high polarity (Bleger et al., 1994). The interface tension should increase with conversion, because of the decrease of MMA concentration. It would further lead to the increase of the swollen micelle size. Since micellar nucleation predominates at high conversion, the size of newly formed growing polymer particles increased with conversion. Np decreases above 50% conversion, because of the coagulation of polymer particles, as observed in St microemulsion polymerization. It was shown in Fig. 4 that Rp reaches its maximum at about 30% conversion and no plateau of Rp is observed. In contrast to St microemulsion polymerization, the nucleation is slower, and the Np is only about half of that in St microemulsion polymerization. When conversion is increased from 13% to 33%, Np is increased by about 3.7 times. Two factors may lead to the disappearance of the plateau of Rp. First, the propagation constant (kp) of MMA is about 5 times the kp for St. Although the maximum Np is only about half of that for St, the overall polymerization rate is still about twice that in St microemulsion polymerization. As has been shown previously (Xu et al., 1997), the higher the polymerization rate, the shorter the plateau of Rp. Second, though the monomer concentration before polymerization was similar (1.9 M and 1.7 M for St and MMA), the monomer saturation of MMA in polymer particles (6.3 M) is higher than that of St (5.1 M) (Tidwell et al., 1970). Therefore, for MMA microemulsion, the monomer concentration in polymer particles will be
Fig. 5. Growth of particles in BA microemulsion polymerization initiated with g ray at 338C. Microemulsion composition: 25wt% BA, 12wt% SBOA, 63wt% H2O; dose rate, 32.5 Gy/min.
Xu Xiangling et al. / Radiation Physics and Chemistry 54 (1999) 279±283
283
Table 3 Variation of some physical values with polymerization conversion in microemulsion polymerization of butyl acrylate Conversion (%)
Dpa (nm)
Npb (1018/l)
Mvc (106)
npd
Rpe (m mol/l.s)
7.0 15.0 31.0 50.0 100
93.1 68.7 57.6 49.7 47.4
1.03 2.53 4.06 6.03 4.54
1.51 1.68 1.31 1.35 0.73
6.7 5.5 8.8 9.2 45.4
1.8 2.3 2.4 1.2 /
a±e
See Table 1
reduced below the saturation concentration at lower conversion. It will further lead to the disappearance of the plateau. 3.3. Polymerization of butyl acrylate Though there was polar group in BA, the polarity of BA was much smaller than that of MMA. As St, BA could be regarded as insoluble in aqueous phase. It was expected that the polymerization of BA in microemulsion should be similar to St, rather than to MMA. As shown in Fig. 5 and Table 3, the growth of particles in BA microemulsion polymerization is very similar to that in St and MMA microemulsion polymerization. At the early stage of polymerization, the sizes of particles quickly increase up to their maximum. With further increase of conversion, the sizes of polymer particles become smaller. Np keeps on increasing even up to 50% conversion, and Fig. 5 shows that there are still small newly formed particles. These facts indicate that the nucleation is really a continuous process. At higher conversion, the coagulation between polymer particles dominates. As a result, the Np decreases.
In contrast to MMA microemulsion polymerization, the smallest size in BA microemulsion polymerization keeps on decreasing with conversion (Fig. 6), when conversion is lower than 50%. This is very similar to St microemulsion polymerization (Fig. 1). In addition, the glass temperature of poly(butyl acrylate) was much lower than that of PSt, and the monomer in swollen polymer particles was easier to transport to polymerizing polymer particles. When conversion was bigger than 50%, there were no newly formed particles observed for BA microemulsion polymerization, while for St microemulsion polymerization, at high conversion, the monomer in polymer particles slowly diused out into micelles, and these, swollen with monomer, formed new particles when radicals diused in. As shown in Fig. 6, there is also an apparent plateau of Rp in 15±35% conversion, when the initial monomer concentration in microemulsion was 1.9 M. The plateau is longer than that in St microemulsion polymerization. It may be attributed to the lower saturation concentration (4.6 M) of BA in polymer particles. In comparison with St microemulsion, the monomer concentration in polymer particles could be kept equal to saturation at higher conversion. References
Fig. 6. Conversion and polymerization rate vs. time in BA microemulsion polymerization initiated with g ray at 338C. Microemulsion composition: 25wt% BA, 12wt% SBOA, 63wt% H2O; dose rate, 32.5 Gy/min.
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