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Incorporation of Nonionic Emulsifiers Inside Particles in Emulsion Polymerization: Mechanism and Methods of Suppression† Masayoshi Okubo,*,‡,§ Hiroshi Kobayashi,‡ Takumi Matoba,‡ and Yoshiteru Oshima‡ Graduate School of Science and Technology, and Department of Chemical Science and Engineering, Faculty of Engineering, Kobe UniVersity, Kobe 657-8501, Japan ReceiVed May 22, 2006. In Final Form: July 31, 2006 Emulsion polymerizations of styrene were carried out using two kinds of polyoxyethylene lauryl ether nonionic emulsifiers having different hydrophilic-lipophilic balances (HLB): Emulgen 109P (HLB 13.6); and Emulgen 150 (HLB 18.3). In both cases, incorporation of emulsifier inside polystyrene (PS) particles was clearly observed, as previously reported for the emulsion polymerization of styrene and methacrylic acid using polyoxyethylene nonyl phenyl ether (Emulgen 911, HLB 13.7) nonionic emulsifier. The generality of the incorporation phenomenon of nonionic emulsifier inside polymer particles in emulsion polymerization was clarified. In the case of Emulgen 109P, which is more hydrophobic than Emulgen 150, about 30% of the total amount was incorporated inside the PS particles, higher than for Emulgen 150 (15%). The difference seemed to be ascribed to the difference in the affinities between the nonionic emulsifiers and styrene, which cause the incorporation of emulsifier. On the basis of this idea, suppression of the incorporation was achieved by decreasing the polymerization temperature and the monomer-feed rate. This strongly supports the proposed incorporation mechanism.
Introduction In a general emulsion polymerization, emulsifiers play significant roles by forming micelles as the polymerization loci for the nucleation of particles as well as stabilizing the particles. In the emulsion polymerization with nonionic emulsifier, some unusual behaviors were reported. Pirrma et al. observed two constant rate regions and obtained bimodal particle size distribution in the emulsion polymerization of styrene using (tridecyloxy)poly(ethyleneoxy)ethanol.1 They suggested that the formation of oligomers-emulsifier mixed micelles, consisting of oligomeric species and emulsifier having escaped from the vanishing monomer phase with the disappearance of monomer phase, was the main reason for this phenomenon. Chern et al. reported that emulsion polymerization of styrene using sodium dodecyl sulfate (SDS)/nonylphenol tetracontylethoxylate (NP-40) led to significant deviation from the Smith-Ewart theory2 at a relatively high level of NP-40.3,4 O ¨ zdeger et al. investigated emulsion polymerization using polyoxyethylene octyl phenyl ether nonionic emulsifier with an average of 40 ethylene oxides per molecule with a reaction calorimeter.5-7 They pointed out that most of the nonionic emulsifier (91%) existed in the styrene phase before addition of initiator. In the polymerization, two separate nucleation periods were observed, which resulted in a bimodal particle size distribution similar to that reported by Pirrma et al.1 In a general emulsion polymerization, the nucleation period (Interval I) finishes at low conversion. They proposed that the first and second nucleations were based on homogeneous and micellar nucleation †
Part CCLXXV of the series “Studies on Suspension and Emulsion”. * Corresponding author. Phone/fax: +81-78-803-6161. E-mail:
[email protected]. ‡ Graduate School of Science and Technology. § Faculty of Engineering. (1) Piirma, I.; Chang, M. J. Polym. Sci.: Polym. Chem. Ed. 1982, 20, 489. (2) Smith, W. V.; Ewart, R. H. J. Chem. Phys. 1948, 16, 592. (3) Chern, C. S.; Lin, S. Y.; Chen, L. J.; Wu, S. C. Polymer 1997, 38, 1977. (4) Chern, C. S.; Lin, S. Y.; Chang, J.; Lin, J. Y.; Lin, F. Y. Polymer 1998, 39, 228. (5) O ¨ zdeg _er, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3813. (6) O ¨ zdeg _er, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3827. (7) O ¨ zdeg _er, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3837.
mechanisms, respectively. The latter was caused by the continuous release of emulsifier to the aqueous phase from the monomer droplets. We have prepared submicrometer-sized polymer particles having one or more hollows by posttreatments, the “stepwise alkali/acid method”8,9 and the “alkali/cooling method”,10,11 for carboxylated polymer particles prepared by emulsion copolymerization with methacrylic acid (MAA). Throughout these studies, it was observed that polyoxyethylene nonyl phenyl ether nonionic emulsifier (Emulgen 911), used in the emulsion copolymerization for the preparation of the styrene-MAA copolymer [P(S-MAA)] particles, was incorporated inside the polymer particles during the polymerization.12 As a result of quantitative analysis, surprisingly, 75% of the total amount of emulsifier was incorporated inside the P(S-MAA) (MAA, 10 mol %) particles. To our knowledge, the incorporation of nonionic emulsifier inside polymer particles during emulsion polymerization had not been reported. Incorporation of emulsifier induces low efficiency of the function in the emulsion polymerization and has adverse effects on polymer properties. On the other hand, the nonionic emulsifier incorporated inside the P(S-MAA) particles operates effectively to form a multihollow structure inside the particles by the alkali/cooling method described above.13 In these ways, in both negative and positive aspects, it is very important to clarify the incorporation phenomenon. The objectives of this Article are to show that the incorporation of nonionic emulsifier in emulsion polymerization is a general phenomenon, to clarify the incorporation mechanism, and finally to propose ways of suppressing the incorporation. Experimental Section Materials. Styrene was purified by distillation under reduced pressure. Potassium persulfate (KPS) of analytical grade (Nacalai Tesque Inc., Kyoto, Japan) was purified by recrystallization. (8) Okubo, M.; Kanaida, K.; Fujimura, M. Chem. Express 1990, 5, 797. (9) Okubo, M.; Sakauchi, A.; Okada, M. Colloid Polym. Sci. 2002, 280, 303. (10) Okubo, M.; Ito, A.; Kanenobu, T. Colloid Polym. Sci. 1996, 274, 801. (11) Okubo, M.; Ito, A.; Okada, M.; Suzuki, T. Colloid Polym. Sci. 2002, 280, 574. (12) Okubo, M.; Furukawa, Y.; Shiba, T.; Matoba, T. Colloid Polym. Sci. 2003, 281, 182. (13) Okada, M.; Matoba, T.; Okubo, M. Colloid Polym. Sci. 2004, 282, 193.
10.1021/la061429s CCC: $33.50 © 2006 American Chemical Society Published on Web 09/07/2006
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Table 1. Batch Emulsion Polymerizationsa with n-Polyoxyethylene Lauryl Ether Nonionic Emulsifiers (C12EO9,b C12EO47c) and Emulsifier-Free Emulsion Polymerization of Styrene
C12EO9-Hd C12EO9-Le C12EO47-Hd C12EO47-Le E-freef
styrene (g)
KPS (g)
20.0 20.0 20.0 20.0 10.5
0.08 0.312 0.08 0.312 0.7
Na2S2O3‚5H2O (g)
emulsifier (g)
water (g)
conv.g (%)
coagulationg (%)
Dhh (nm)
1.0 1.0 1.0 1.0
180 180 180 180 350
97 96 99 95 100
3.2 0.05 0.6 2.1 1.7
470 960 240 120 430
0.705 0.705
a N2, 24 h. b EO units 9.2; HLB 13.6. c EO units 47.4; HLB 18.3. d 70 °C, stirring rate, 200 rpm. e 40 °C, stirring rate, 200 rpm. f 70 °C, stirring rate, 240 rpm. g Gravimetry. h Hydrodynamic diameter measured by dynamic light scattering.
Table 2. Monomer-Feed Emulsion Polymerizationsa of Styrene with C12EO9 and C12EO47
C12EO9 C12EO47 a
feed rate (g/h)
styrene (g)
KPS (g)
emulsifier (g)
water (g)
conv.b (%)
coagulationb (%)
Dhc (nm)
1.0 2.0
20.0 20.0
0.08 0.08
1.0 1.0
180 180
98 100
0.3 5.8
100 95
N2; 70 °C, 24 h; stirring rate, 200 rpm. b Gravimetry. c Hydrodynamic diameter measured by dynamic light scattering.
Commercial grade n-polyoxyethylene lauryl ether nonionic emulsifiers with averages of 9.2 (Emulgen 109P; C12EO9, HLB 13.6) and 47.4 (Emulgen 150; C12EO47, HLB 18.3) ethylene oxides per molecule were used as received (Kao Co., Tokyo, Japan). 2-Propanol and sodium thiosulfate pentahydrate of guaranteed reagent grade and hexamethyldisiloxane (HMDS) of extra pure reagent grade were used as received (Nacalai Tesque Inc., Kyoto, Japan). Pyridine-d5 was used as received (Wako Pure Chemicals Industries, Ltd., Osaka, Japan). Deionized water with a specific resistance of 5 × 106 Ω cm was distilled before use. Preparation of PS Particles. PS particles were prepared by (i) emulsifier-free, (ii) batch, and (iii) monomer-feed emulsion polymerizations in a four-necked 500-mL reactor equipped with an inlet of N2, a reflux condenser, and a half-moon type stirrer under the conditions in Tables 1 and 2. In all cases, the conversions were over 95% according to gravimetric measurements. Electron Microscopy. The PS particles were observed with a Hitachi H-7500 transmission electron microscope (TEM). Each emulsion was diluted to about 50 ppm, and a drop was placed onto a carbon-coated copper grid and allowed to dry at room temperature in a desiccator. Measurement of Particle Diameter. The hydrodynamic diameters (Dh) of the PS particles were measured by dynamic light scattering (DLS) (DLS-700, Otsuka Electronics Co. Ltd., Kyoto, Japan) with the data taken at a light-scattering angle of 90° at room temperature. Sample emulsions were diluted to 10 ppm. Quantitative Analysis of Nonionic Emulsifier Inside PS Particles. The PS emulsions prepared by batch emulsion polymerizations under the conditions in Tables 1 and 2 were centrifugally washed with 2-propanol three times to remove emulsifier from the particle surfaces, and subsequently dried at room temperature under reduced pressure for 1 week. The dried particles were dissolved in pyridined5 containing 0.1 wt % HMDS as an internal standard. 1H NMR spectra were obtained with a Bruker DPX 250 NMR spectrometer operating at 250 MHz with 128 scans with repetition delay 2 s. Chemical shifts were obtained relative to the methyl groups of HMDS at 0 ppm. The normalized NMR integrals were determined by normalizing the integral intensity of each peak due to the emulsifier to that due to the methyl groups of HMDS. C12EO9 and C12EO47 concentrations were obtained from 1H NMR relative intensity due to ethylene oxide protons. Partitioning of Emulsifier between Styrene and Water. Styrene/ emulsifier/water mixtures of the same composition as in the polymerization recipe were shaken vigorously, and then left at various temperatures until complete separation of the aqueous and the organic phases. The amount of emulsifier in the aqueous layer was determined by gravimetry, and the amount in styrene was calculated by mass balance. Partitioning of Emulsifier between Styrene-Swollen PS Particles and Water. PS particles containing different amounts of styrene were prepared as follows. PS emulsion prepared by emulsifier-free
emulsion polymerization under the conditions in Table 1 was centrifugally washed with distilled water three times. Various amounts of styrene were added into the washed emulsion, where the total weight percentage of PS and styrene was adjusted to 10 wt %. The mixtures were stirred at room temperature for 24 h, and all monomer was absorbed by the PS particles. Next, 5 wt % of emulsifier based on styrene-swollen PS particles was added into each styrene-swollen PS emulsion followed by heating at 70 °C for 3 h. After the heat treatment, styrene in the PS particles was evaporated quickly under reduced pressure at 70 °C. Each PS emulsion was centrifugally washed with 2-propanol and dried. The amounts of the emulsifier inside the PS particles were measured via 1H NMR as outlined above. Estimation of Monomer Content in Polymer Particles during Batch and Monomer-Feed Emulsion Polymerizations. Emulsion samples were taken from the bottom of the reactor at various conversions of styrene after stopping the stirring and were centrifuged to remove the monomer droplets. The concentrations of styrene and the mass of polymer particles in the emulsion were measured by gas chromatography and gravimetry, respectively.
Results and Discussion Incorporation of Nonionic Emulsifier Inside Polymer Particles. To completely remove the emulsifier from the particle surfaces, a preliminary experiment was carried out as follows. The same amount of emulsifiers as used in the polymerizations was separately added to PS colloids prepared by emulsifier-free emulsion polymerization under the conditions in Table 1. These PS colloids were kept at room temperature for 1 day for the emulsifier to adsorb at the particle surfaces. More than 50% of the emulsifier still remained on the particles even after centrifugal washing with water five times, as estimated by 1H NMR. However, complete removal of emulsifier was achieved by centrifugal washing with 2-propanol three times. In both emulsion polymerizations of styrene using C12EO9 and C12EO47, 30% and 15% of the total mass were, respectively, incorporated inside the PS particles after the centrifugal washing, although the levels of incorporation were lower than for Emulgen 911 (polyoxyethylene nonyl phenyl ether) (HLB 13.7) incorporated inside P(S-MAA) particles (75%).12 The difference is probably due to the difference in the affinity of the emulsifiers to styrene; C12EO9 (HLB 13.6) is more hydrophobic than is C12EO47 (HLB 18.3). This idea is supported by experimental results described later. The result that nonionic emulsifiers were incorporated inside the PS particles in the emulsion polymerization of styrene as well as the emulsion copolymerization of S-MAA suggests the emulsifier incorporation is a general phenomenon. To clarify the reason for incorporation of the nonionic emulsifiers inside the PS particles, the partitioning of the
Nonionic Emulsifiers in Emulsion Polymerization
Figure 1. Percentages (relative to total weight of emulsifier) and concentrations in the styrene phase of C12EO9 (O) and C12EO47 (b) (styrene/emulsifier/water, 1/0.05/9) as functions of temperature. Index: WES ) weight of emulsifier in the styrene phase; WET ) total weight of added emulsifier; WS ) weight of styrene phase.
emulsifiers between styrene and the aqueous phase was measured, at different temperatures (Figure 1). In both cases, approximately 90% of the emulsifiers initially dissolved in the aqueous phase were partitioned to the styrene at 70 °C, although the styrene content was only 10 wt % of the total weight. It is well known that nonionic emulsifiers partition between oils and water and these results are also consistent with the experimental results reported by O ¨ zdeg _er et al. that 91 wt % of Triton X-405 was partitioned to styrene before starting the emulsion polymerization.5 However, C12EO47 unlike C12EO9 preferentially partitioned to the aqueous phase below 45 °C. This indicates that the affinity of C12EO47 to water (hydrogen bonding) is stronger below than above 45 °C. The cloud points (CP) of C12EO9 and C12EO47 are approximately 83 and above 100 °C, respectively (according to the manufacturer). The drastic change in partitioning for C12EO47 is thus not directly related to the cloud point, even if one considers a decrease in CP due to the salting-out effect of the monomer. During the emulsion polymerization, polymerizing particles are swollen with styrene that diffuses through the aqueous medium from styrene droplets. Most nonionic emulsifiers, which are dissolved in the styrene droplets, would also diffuse into the inside of polymer particles swollen with styrene. As a result, the emulsifier would be located inside the PS particles after the polymerization. To further clarify this proposed mechanism, it was separately examined whether the nonionic emulsifiers can be absorbed by styrene-swollen PS particles. Figure 2 shows the weight percentages of C12EO9 and C12EO47 incorporated (absorbed) inside styrene-swollen PS particles at 70 °C as a function of weight fraction of styrene. Both emulsifiers were absorbed in the styrene-swollen PS particles, and the amounts increased with increasing weight fraction of styrene. This result supports the above idea that the nonionic emulsifiers are absorbed inside the PS particles swollen with styrene during the emulsion polymerization because of the strong affinity to styrene. The amount of absorbed C12EO9 was always higher than that of C12EO47. This result is as expected on the basis of the HLB values. Emulsifiers with low HLB value, that is, shorter poly(ethylene oxide) chains, are less stable in water because of less hydrogen bonding than those with high HLB value (longer poly(ethylene oxide) chains). Consequently, the affinity of C12EO9 to styrene was stronger than for C12EO47. This trend is consistent with C12EO9 being incorporated inside PS particles during emulsion polymerization to a greater extent than C12EO47. However, the amounts of both emulsifiers partitioned to the monomer phase are almost the same (Figure 1). This is because the influence of the difference in HLB value
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Figure 2. Percentages (relative to total weight of emulsifier) and weight percentages (relative to total mass) of C12EO9 (O) and C12EO47 (b), which were, separately, post-added (5 wt % based on styrene-swollen PS particles), absorbed into styrene-swollen PS particles (10 wt % in emulsion) at 70 °C as functions of wt % styrene in the swollen particles. Index: WESP ) weight of emulsifier in styrene-swollen PS particles; WET ) total weight of added emulsifier; WSP ) weight of styrene-swollen PS particles. Scheme 1. Incorporation of Nonionic Emulsifiers Inside Polymer Particles in Emulsion Polymerization
on partitioning is small when the emulsifier concentration is (sufficiently) low relative to the monomer concentration. The mechanism proposed for the incorporation of nonionic emulsifiers inside polymer particles in emulsion polymerization is illustrated in Scheme 1. We also measured the amount of incorporated emulsifier by gel permeation chromatography (method described in detail in ref 14), which gives only the amount of noncovalently incorporated emulsifier. The results of both measurements were very similar, and thus the amount of grafted emulsifier is negligible. Suppression of Incorporation. To suppress the incorporation of nonionic emulsifiers during emulsion polymerization, the following two strategies were carried out, which strongly support the mechanism of the incorporation phenomenon from the inverse viewpoint. The first strategy is to reduce the affinity between the nonionic emulsifier and styrene. Batch emulsion polymerization of styrene using C12EO47 was carried out at 40 °C under the conditions in Table 1, because most of the emulsifier partitioned to the aqueous phase below 45 °C, unlike C12EO9 (Figure 1). For comparison, a polymerization was also carried out using C12EO9 at 40 °C. Figure 3 shows TEM photographs of PS particles prepared by batch emulsion polymerizations using C12EO9 and C12EO47 at 40 and 70 °C under the conditions in Table 1. In both emulsifiers, bimodal and/or polydisperse particle size distribution were obtained at 70 °C (Figure 3c,d), which is often seen in the emulsion polymerization of styrene with nonionic emulsifier because of secondary nucleation.5 The particles with C12EO47 were relatively smaller than those with C12EO9. This might be caused by an (14) Chaiyasat, A.; Kobayashi, H.; Okubo, M. Colloid Polym. Sci., in press.
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Figure 3. TEM photographs of PS particles prepared by batch emulsion polymerizations with C12EO9 (a,c) and C12EO47 (b,d) at 40 °C (a,b) and 70 °C (c,d).
intrinsic difference in the stabilization ability between these emulsifiers due to their different chemical structures. In addition, the amounts of emulsifiers able to participate in stabilization are not the same in each case due to different levels of incorporation. It cannot be excluded that the rate of radical generation is to some extent influenced by accelerated decomposition of KPS in the presence of ethylene oxide units,15 which would in turn affect the nucleation process and the particle size. A redox initiator was used in the polymerizations at 40 °C. In the case of C12EO9, unexpectedly, the particles obtained at 40 °C were larger than those at 70 °C despite almost the same level of incorporation (30%). The stabilization ability of C12EO9 at 40 °C was superior to that at 70 °C; therefore, one would expect that the particles prepared at 40 °C must be smaller than those at 70 °C, but the opposite tendency was observed. This would be explained in terms of the number of particles prepared in the nucleation stage, which depends on the difference of generation rate of initiator radicals because emulsifier concentrations in the aqueous phase were almost the same at both temperatures. On the other hand, the particles obtained with C12EO47 at 40 °C were much smaller than those at 70 °C. This indicates that most of the C12EO47 effectively operated as micelles for the particle formation in the early stage of the emulsion polymerization (Interval I) at 40 °C, which is based on the fact that more than 90% of emulsifier existed in the aqueous phase at 40 °C. The percentage of C12EO47 incorporated in the particles at 40 °C was less than 3%, much less than 15% at 70 °C as expected. Moreover, the nonspherical shape and the regions with less contrast were observed in Figure 3a. Tauer et al. mentioned that the interfacial area should be increased and/or the particle shape should deviate from the spherical shape if the interfacial tension is very small in a dispersion of PS particles consisting of poly(ethylene glycol)-PS-poly(ethylene glycol) triblock or diblock copolymers.16 The nonspherical shape may be related to this explanation, but a sufficient understanding needs further inves(15) Okubo, M.; Suzuki, T.; Tasaki, S. In Studies in Surface Science and Catalysis Vol. 132: The International Conference on Colloid and Surface Science; Iwasaki, Y., Oyama, N., Kunieda, H., Eds.; Elsevier B. V.: Amsterdam, 2001; p 347.
Figure 4. Styrene wt % in PS particles prepared by batch (O) and monomer-feed (4, 0) emulsion polymerizations at 70 °C with C12EO9 (a) and C12EO47 (b) as a function of styrene conversion. Monomer feed rates (g/h): 4, 2.0; 0, 1.0.
tigations. O ¨ zdeg _er et al. pointed that the emulsion polymerization of styrene with ethoxylated nonionic emulsifier under certain conditions proceeds in the homogeneous nucleation mechanism.5 Moreover, some authors reported that nonspherical particles with voids (regions with less electron density in TEM observations) were obtained in the emulsifier-free emulsion polymerization of styrene with KPS.17-21 In comparison with these results, the formation of regions in Figure 3a with less contrast will be discussed in the near future. The above results indicate that the incorporation exerts a significant influence on emulsion polymerization. Suppression of the incorporation was achieved by decreasing the polymerization temperature in the case of the emulsifier with high HLB value, and this supports our incorporation mechanism. Although this method of suppression requires relatively low temperature, it is believed to be industrially viable in its simplicity. The second strategy was to reduce the amount of monomer absorbed in the polymer particles during polymerization. Emulsion polymerizations were carried out using the monomer-feed method at low feed rates of 1.0 g/h (C12EO9) and 2.0 g/h (C12EO47) under the conditions in Table 2. In these polymerizations, no monomer droplets nor monomer layer were observed and the styrene concentration in the particles remained at a low level during the polymerizations (Figure 4). Therefore, a monomer-starved condition was achieved in these monomerfeed systems. (16) Tauer, K.; Mu¨ller, H.; Rosengarten, L.; Riedelsberger, K. Colloids Surf. 1999, 153, 75. (17) Goodall, A. R.; Wilkinson, M. C.; Hearn, J. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 2193. (18) Cox, R. A.; Wilkinson, M. C.; Creasey, J. M.; Goodall, A. R.; Hearn, J. J. Polym. Sci.: Polym. Chem. Ed. 1977, 15, 2311. (19) Tauer, K.; Deckwer, R.; Ku¨hn, I.; Schellenberg, C. Colloid Polym. Sci. 1999, 277, 607. (20) Tauer, K.; Riedelsberger, K.; Deckwer, R.; Zimmermann, A. Macromol. Symp. 2000, 155, 95. (21) Tauer, K. Macromolecules 2006, 39, 2007.
Nonionic Emulsifiers in Emulsion Polymerization
Figure 5. TEM photographs of PS particles prepared by batch (a,b) and monomer-feed (c,d) emulsion polymerizations at 70 °C with C12EO9 (a,c) and C12EO47 (b,d). Monomer feed rates (g/h): (c) 1.0; (d) 2.0.
Figure 5 shows TEM photographs of PS particles prepared by batch (a) and monomer-feed (c) emulsion polymerizations with C12EO9. At a monomer-feed rate of 1.0 g/h, the PS particles were much smaller (ca. 100 nm) than those prepared by batch emulsion polymerization (ca. 470 nm). In the case of C12EO47, batch and monomer-feed (2.0 g/h) polymerization resulted in ca. 240 and ca. 95 nm, respectively (Figure 5b,d). Monomodal particle size distributions were obtained in both cases in the feed systems, suggesting that only micellar nucleation (or only homogeneous nucleation) was occurring.5 However, the final particle size formed by homogeneous nucleation is usually 200-500 nm in the case of styrene, and it is unlikely that less than 100-nm particles were obtained by homogeneous nucleation. This indicates that one nucleation period (miceller nucleation) might exist. These results show that both emulsifiers acted effectively as stabilizer in the emulsion polymerization under monomer-starved conditions as expected. Figure 6 shows percentages of C12EO9 and C12EO47 incorporated inside the PS particles in batch and monomer-feed emulsion polymerizations. In both emulsifiers, suppression of incorporation could thus be achieved by keeping the monomer concentration at a low level with suitable adjustment of the monomer-feed rate. Especially, in the case of C12EO9, effective suppression was achieved. These results strongly support the incorporation mechanism that the affinity of nonionic emulsifier to monomer causes the incorporation of emulsifier inside polymer particles.
Conclusions In this work, the incorporation of nonionic emulsifier inside polymer particles in emulsion polymerization was investigated.
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Figure 6. Percentages (relative to total weight of emulsifier) and weight percentages (relative to total mass) of C12EO9 (a) and C12EO47 (b) incorporated inside PS particles in batch and monomerfeed emulsion polymerizations. Index: WE109PP,WE150P ) weight of emulsifiers in the PS particles; WE109PT,WE150T ) total weight of added emulsifiers; WPS ) weight of PS particles.
Emulsion polymerizations of styrene were carried out using two kinds of nonionic emulsifiers having different HLB values: C12EO9 (13.6) and C12EO47 (18.3). In both cases, incorporation of emulsifier inside PS particles was clearly observed. It is concluded that the incorporation of nonionic emulsifiers inside polymer particles during emulsion polymerization is a general phenomenon, and it is caused by high affinity between monomer and nonionic emulsifier. The incorporation of emulsifier greatly influences emulsion polymerization, especially particle formation, particle size, and its distribution. Approximately 30% of the total C12EO9, which is more hydrophobic than C12EO47, was incorporated inside the PS particles, higher than for C12EO47 (15%). The difference seemed to be ascribed to the difference in the affinities between the nonionic emulsifiers and styrene. On the basis of this idea, suppression of the incorporation was achieved by two methods: (i) decreasing the polymerization temperature, and (ii) keeping the monomer concentration at a low level with suitable adjustment of the monomer-feed rate. This achievement of suppression strongly supports the proposed incorporation mechanism. Acknowledgment. This work was partially supported by the Creation and Support Program for Start-ups from Universities (No. 1509) from the Japan Science and Technology Agency (JST). LA061429S
