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Incorporation of Nonionic Emulsifier Inside Carboxylated Polymer Particles during Emulsion Copolymerization: Influence of Methacrylic Acid Content† Hiroshi Kobayashi, Amorn Chaiyasat, Yoshiteru Oshima, Toyoko Suzuki, and Masayoshi Okubo* Graduate School of Engineering, Kobe UniVersity, Kobe 657-8501, Japan ReceiVed July 3, 2008. ReVised Manuscript ReceiVed October 3, 2008 The influence of the methacrylic acid (MAA) content (0-10 mol %) on the incorporation of polyoxyethylene nonylphenyl ether (Emulgen 911, HLB 13.7) nonionic emulsifier inside polymer particles during emulsion copolymerization of styrene (S) and MAA was investigated. The amount of incorporated emulsifier after centrifugal washing with 2-propanol to remove adsorbed emulsifier from the surfaces was directly measured by gel permeation chromatography (GPC) and 1H NMR. The level of incorporation increased with increasing MAA content and reached 74% of the total amount of emulsifier at 10 mol % MAA. At lower MAA contents (0-3 mol %), the particle size distribution was bimodal because of formation of new particles by secondary nucleation. However, only limited secondary nucleation occurred at higher MAA contents (6-10 mol %), and monodisperse particles were thus obtained.
Introduction Nonionic emulsifiers have been widely used in the preparation of polymer colloids by emulsion polymerization. Nonionic emulsifiers offer various advantages compared to ionic emulsifiers, for example, higher chemical (i.e., high tolerance to acids, alkalis, and inorganic salts) and freeze-thaw stabilities, higher pigment affinity, and lower effervescence of the emulsion. Most nonionic emulsifiers consist of a hydrophobic portion, such as an alkylphenol or alkyl moiety, and a hydrophilic polyethylene oxide chain, which gives the emulsifiers water solubility due to hydrogen bonding with water. Nonionic emulsifiers phase separate from aqueous solutions at a certain temperature (the so-called cloud point, CP) because the degree of hydration of the polyethylene oxide chains decreases with increasing temperature. It is well-known that the CP is strongly affected by various additives. Salting-out compounds (e.g., most inorganic salts, benzene, oleic acid, etc.) decrease the CP by dehydrating the emulsifier molecules.1-4 Salting-in compounds (e.g., anionic emulsifier, KSCN, etc.) operate in the opposite manner.1-5 Some characteristic behaviors, not observed in ionic emulsifier systems, have been reported in emulsion polymerization with nonionic emulsifiers, e.g., two constant polymerization rate regions and bimodal particle size distribution due to formation of oligomer-emulsifier mixed micelles6 and/or two separate nucleation periods based on homogeneous and micellar nucleation mechanisms.7-9 Moreover, significant deviations from the †
Part CCCXVI of the series “Studies on Suspension and Emulsion”. * To whom correspondence should be addressed. Phone and Fax: +8178-803-6161. E-mail:
[email protected]. (1) Maclay, W. N. J. Colloid Sci. 1956, 11, 272–285. (2) Schott, H.; Han, S. K. J. Pharm. Sci. 1975, 64, 658–664. (3) Sadaghiania, A. S.; Khan, A. J. Colloid Interface Sci. 1991, 144, 191–200. (4) Goel, S. K. J. Colloid Interface Sci. 1999, 212, 604–606. (5) Saito, S. Colloids Surf. 1986, 19, 351–357. (6) Piirma, I.; Chang, M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 489–498. ¨ zdegˇer, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part (7) O A: Polym. Chem. 1997, 35, 3813–3825. ¨ zdegˇer, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part (8) O A: Polym. Chem. 1997, 35, 3827–3835. ¨ zdegˇer, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part (9) O A: Polym. Chem. 1997, 35, 3837–3846.
Smith-Ewart theory10 were observed at relatively high emulsifier concentration.11,12 Emulsion copolymerization of hydrophobic monomer with MAA is commonly conducted to improve the colloidal stability of polymer colloids and produce particles having chemical reactivity. Shoaf and Poehlein developed a model for predicting the partitioning of styrene (S) and methacrylic acid (MAA) in emulsion copolymerization systems.13 Guillaume et al.,14 Shoaf and Poehlein,15 and Santos et al.16 reported the effect of pH on polymerization kinetics and/or distribution of poly(methacrylic acid) (PMAA) throughout the emulsion (or emulsifier-free) copolymerization containing MAA. Recently, throughout our studies on the preparation of submicrometer-sized (multi)hollow polymer particles by posttreatments, the “stepwise alkali/acid method”17 and the “alkali/ cooling method”,18 for styrene-methacrylic acid copolymer [P(S-MAA)] particles, incorporation of polyoxyethylene nonylphenyl ether nonionic emulsifier [Emulgen 911, C9H19-C6H4-O(CH2CH2O)10.9H, hydrophilic-lipophilic balance (HLB) 13.7] was observed in emulsion copolymerization for preparation of P(S-MAA) (MAA, 10 mol %) particles.19 Surprisingly, 75% of the emulsifier was incorporated inside the P(S-MAA) (MAA, 10 mol %) particles, which was revealed by indirect quantitative analysis as follows: the amount of the incorporated emulsifier inside the particles was obtained by subtracting the amounts of emulsifiers in the medium and on the particle surfaces from the total amount. (10) Smith, W. V.; Ewart, R. H. J. Chem. Phys. 1948, 16, 592–599. (11) Chern, C. S.; Lin, S. Y.; Chen, L. J.; Wu, S. C. Polymer 1997, 38, 1977– 1984. (12) Chern, C. S.; Lin, S. Y.; Chang, J.; Lin, J. Y.; Lin, F. Y. Polymer 1998, 39, 2281–2289. (13) Shoaf, G. L.; Poehlein, G. W. Ind. Eng. Chem. Res. 1990, 29, 1701–1709. (14) Guillaume, J. L.; Pichot, C.; Guillot, J. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 1937–1959. (15) Shoaf, G. L.; Poehlein, G. W. J. Appl. Polym. Sci. 1991, 42, 1239–1257. (16) Santos, A. M. D.; Mckenna, T. F.; Guillot, J. J. Appl. Polym. Sci. 1997, 65, 2343–2355. (17) Okubo, M.; Sakauchi, A.; Okada, M. Colloid Polym. Sci. 2002, 280, 303–309. (18) Okubo, M.; Okada, M.; Shiba, K. Colloid Polym. Sci. 2002, 280, 822– 827. (19) Okubo, M.; Furukawa, Y.; Shiba, K.; Matoba, T. Colloid Polym. Sci. 2003, 281, 182–186.
10.1021/la8021003 CCC: $40.75 2009 American Chemical Society Published on Web 11/21/2008
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The incorporation of nonionic emulsifier was also clearly observed in the emulsion homopolymerization of S using two kinds of polyoxyethylene lauryl ether nonionic emulsifiers having different HLB values, Emulgen 109P [C12H25O(CH2CH2O)9.2H, HLB 13.6] and Emulgen 150 [C12H25O(CH2CH2O)47.4H, HLB 18.3],20 where the amount of incorporation was directly measured by 1H NMR after removal of emulsifiers in the medium and on the particle surfaces by centrifugal washing with 2-propanol. Amounts of 30% and 15% of the total mass of Emulgen 109P and Emulgen 150 were, respectively, incorporated inside the polystyrene (PS) particles, i.e., the levels of incorporation were lower than for Emulgen 911 as mentioned above. The mechanism of incorporation of nonionic emulsifier inside particles during emulsion polymerization was proposed as follows.20 Most nonionic emulsifiers are dissolved in the monomer droplets due to high affinity between emulsifier and monomer in the early stage of the polymerization. Throughout the polymerization, the emulsifiers continuously diffuse into monomer-swollen particles through the aqueous medium from monomer droplets until the monomer droplets disappear. As a result, some emulsifier would be located inside the polymer particles after completion of the polymerization. On the basis of the above mechanism, suppression of the incorporation can be achieved by creating conditions such that no monomer droplets or monomer layer exist, and the monomer concentration in the particles remains at a low level during the polymerization with suitable adjustment of the monomer feed rate. The incorporation of two emulsifiers (Emulgen 911, Emulgen 109P) having a different hydrophobic portion in the emulsion homopolymerizations of methacrylic monomers (i.e., methyl methacrylate, ethyl methacrylate, and i-butyl methacrylate) was investigated.21 In the case of Emulgen 911, incorporation occurred in all cases. However, the incorporation of Emulgen 109P was not observed inside poly(ethyl methacrylate) and poly(i-butyl methacrylate) particles. Incorporation of emulsifier causes several problems such as a reduction in colloidal stability of the polymer colloids because of decrease in the amount of emulsifiers used for stabilizing particles, a lack of information about the required amount of emulsifier to stabilize the particles, waste of emulsifier, and making control of particle size distribution and polymerization kinetics difficult even if the particle surface is fully covered with emulsifier. In addition, in film applications, the existence of emulsifier in the final polymer particles (i.e., incorporation of emulsifier) causes a reduction in water resistance. However, we also found that the nonionic emulsifier incorporated inside P(S-MAA) particles promotes formation of multihollow structure by the alkali/cooling method.22 Furthermore, multihollow PS particles were formed during seeded emulsion polymerization of styrene using PS seed particles with incorporated emulsifier.23 Therefore, considering both negative and positive aspects, it is very important to clarify the incorporation phenomenon in more detail. The present article is aimed at increasing the understanding of incorporation of nonionic emulsifier inside particles during emulsion polymerization. Specifically, the influence of MAA content on the incorporation of Emulgen 911 nonionic emulsifier inside P(S-MAA) particles during emulsion copolymerization is clarified in connection with particle formation behavior (20) Okubo, M.; Kobayashi, H.; Matoba, T.; Oshima, Y. Langmuir 2006, 22, 8727–8731. (21) Chaiyasat, A.; Kobayashi, H.; Okubo, M. Colloid Polym. Sci. 2007, 285, 557–562. (22) Okada, M.; Matoba, T.; Okubo, M. Colloid Polym. Sci. 2003, 282, 193– 197. (23) Kobayashi, H.; Miyanaga, E.; Okubo, M. Langmuir 2007, 23, 8703– 8708.
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(secondary nucleation). Herein, the amount of incorporated emulsifier is directly measured by gel permeation chromatography (GPC) and 1H NMR. It is demonstrated that the incorporation is strongly affected by the level of hydration of emulsifier, which is examined by measuring a cloud point of the emulsifier in an aqueous solution containing dissolved monomers. Moreover, we discuss the occasion of secondary nucleation and resultant particle size distribution at lower MAA content.
Experimental Section Materials. Styrene and MAA were purified by distillation under reduced pressure. Potassium persulfate (KPS) of analytical grade (Nacalai Tesque Inc., Kyoto, Japan) was recrystallized using distilled water. Commercial grade polyoxyethylene nonylphenyl ether nonionic emulsifier with an average of 10.9 ethylene oxides per molecule (Emulgen 911, HLB 13.7; Kao Co., Tokyo, Japan), tetrahydrofuran (THF), 2-propanol of guaranteed reagent grade, and hexamethyldisiloxane of extrapure reagent grade were used as received (Nacalai Tesque Inc.). Superhigh molecular weight PS (Mw ) 5.48 × 106 g/mol; Tosoh Co., Tokyo, Japan) and pyridine-d5 were 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. Emulsion Polymerizations. Emulsion polymerizations were carried out at 70 °C for 24 h under a nitrogen atmosphere in a four-necked 300 mL round-bottom flask equipped with an inlet of nitrogen gas and a reflux condenser. Water (170 g) and Emulgen 911 (1.3 g) were added to the reactor, and the solution was stirred with a half-moon type stirrer at 120 rpm under a nitrogen atmosphere. After the solution was heated to 70 °C, a mixture of S and MAA (20 g), where MAA content was changed (0, 1, 3, 6, and 10 mol %), was poured into the reactor. After purging with nitrogen gas for 30 min, a solution of KPS (0.08 g) dissolved in water (10 g) was added to the reactor to initiate polymerization. In all cases, the conversions calculated from solid contents obtained by gravimetric measurements were over 94%. The solid contents were averages of three measurements, where the polymer dispersions were poured into disposable aluminum trays and subsequently dried in oven at 70 °C for 1 day. The obtained PS and P(S-MAA) particles were observed with a transmission electron microscope (TEM, H-7500, Hitachi Ltd., Tokyo, Japan). Each sample was diluted to approximately 50 ppm, and a drop was placed onto a carbon-coated copper grid and allowed to dry at room temperature in a desiccator. The weight-average particle diameters and particle size distributions were determined by dynamic light scattering (DLS, FPAR-1000RK, Otsuka Electronics Co. Ltd., Osaka, Japan). Quantitative Analysis of Emulsifier Incorporated Inside Particles. PS and P(S-MAA) particles were centrifugally washed with 2-propanol three times in order to remove the emulsifiers in the medium and on the particle surfaces and subsequently dried at room temperature under reduced pressure for 1 day.20,21 The dried particles (50 mg) were dissolved in THF (4.0 g). The sample solutions were sonicated for 7 h to reduce the molecular weight of PS (obtained by emulsion polymerization at 0% MAA content, not standard PS) and P(S-MAA), filtered with poly(tetrafluoroethylene) membranes (pore size, 0.45 µm), and subsequently a 0.5 wt % THF solution (1.0 g) of superhigh molecular weight PS was added as an internal standard. The samples were analyzed by GPC using two S/divinylbenzene gel columns [Tosoh Corporation, TSKgel GMHHR-H, 7.8 mm i.d. × 30 cm; separation range per column: approximately 50-4 × 108 g/mol (exclusion limit)] using THF as eluent at 40 °C at a flow rate of 1.0 mL/min employing refractive index (RI) detection (Tosoh RI-8020/21). The absolute amounts of nonionic emulsifier located inside particles were obtained by use of the internal standard. A calibration curve for quantitative analysis of the incorporated emulsifier was constructed as follows. THF solutions (containing 0.1 wt % of superhigh molecular weight PS) of emulsifier with concentrations ranging from 0.08 to 0.65 mg of emulsifier/g of THF were prepared. These samples were injected into the GPC column,
Influence of MAA on Emulsifier Incorporation
Figure 1. GPC charts (RI) of P(S-MAA) after three centrifugal washing of the P(S-MAA) particles with water (a and b) or 2-propanol (c). The GPC samples were ultrasonicated for 7 h before GPC measurement in (b and c).
and the areas of emulsifier and superhigh molecular weight PS of the RI versus time traces were determined. The area ratios of emulsifier to superhigh molecular weight PS were proportional to the mass concentration of emulsifier in the injected sample. The procedure of quantitative analysis by 1H NMR was the same as in previous work.20 Partitioning of Emulsifier. Styrene/MAA/emulsifier/water mixtures of the same compositions as the polymerization recipes (without KPS) were left at 70 °C for 1.5 h. In all cases, the emulsifier was initially dissolved in the aqueous phase. The amount of emulsifier in the aqueous phase was determined by gravimetry, and the amount in the monomer phase was calculated by mass balance. Cloud Point Measurements. To measure CP of emulsifier under the almost same conditions as in the actual polymerizations, the following procedures were conducted. S/MAA/water mixtures of the same compositions as the polymerization recipes (without KPS) were shaken at 70 °C for 3 h. After the stirring was stopped, portions of the aqueous solutions (10 g) containing the dissolved monomers were carefully withdrawn by syringe from the mixtures. Subsequently, the emulsifier (0.1 g, 1 wt % based on the aqueous solution) was added to the solutions. CP was determined as the temperature at which the transmittance of the aqueous solution was 90%,24 which was measured using a spectrophotometer (UV-2500, Shimadzu Co., Kyoto, Japan) at 550 nm with a temperature controller (S-1700, Shimadzu Co., Kyoto, Japan) at a heating rate of 1.0 °C/min from 30 to 85 °C.
Results and Discussion Quantitative Analysis of Nonionic Emulsifier Incorporated Inside Particles by GPC. To completely remove the emulsifier from the particle surfaces, a preliminary experiment was carried out as follows. Emulsifier-free emulsion polymerization of S-MAA was carried out, and subsequently an aqueous solution of Emulgen 911 (containing the same amount of the emulsifier as in the actual polymerization) was added to the thus obtained dispersion in order for the emulsifier to adsorb on the particle surface (the dispersion was kept at room temperature for 1 day) before the centrifugal washing. Figure 1 shows GPC charts (RI) of P(S-MAA) particles after centrifugal washing with water (Figure 1, curves a and b) or 2-propanol (Figure 1c) three times. Curves a and b show that emulsifiers still remained on the particle surfaces even after centrifugal washing with water three times. However, curve c shows that the emulsifiers adsorbed on the particles were completely removed by centrifugal washing with 2-propanol. When ultrasonication pretreatment of the sample (24) Boutris, C.; Chatzi, E. G.; Kiparissides, C. Polymer 1997, 38, 2567– 2570.
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Figure 2. Weight percentages (relative to total weight of emulsifier and weight of polymer in the particles) of Emulgen 911 incorporated inside P(S-MAA) particles in emulsion copolymerizations as a function of MAA content. The amount of incorporated emulsifier was measured by GPC (O) and 1H NMR (0). Indexes: WEP ) weight of emulsifier incorporated inside the particles; WET ) total weight of added emulsifier; WP ) weight of polymer in the particles.
solution was not conducted before the GPC measurement, the peaks of P(S-MAA) and a superhigh molecular PS used as an internal standard overlapped as shown in curve a. Ultrasonication pretreatment for the GPC samples was conducted in order to reduce the molecular weight of PS and P(S-MAA) by breaking the polymer chains25-27 and thus avoid overlapping. The molecular weight of Emulgen 911 is 700, and the ultrasonic treatment had little, if any, influence on the emulsifier peak. The superhigh molecular PS used as an internal standard was added to the sample solutions after ultrasonication/filtration to avoid chain degradation. The peaks of the ultrasonicated P(S-MAA) samples and the internal standard PS were clearly separated as shown in the curves b and c. Incorporation of Emulsifier Inside Particles during Emulsion Copolymerization of S-MAA. Emulsion copolymerizations of S-MAA were carried out with various MAA contents at 70 °C for 24 h (the conversions >94%). Figure 2 shows the weight percentages of Emulgen 911 incorporated inside the P(S-MAA) particles in emulsion copolymerizations as a function of MAA content. The amount of incorporated emulsifier was measured by GPC and 1H NMR. The experimental errors in these measurements are approximately (5% for the weight percentage of incorporated emulsifier (relative to total weight of emulsifier). GPC was employed to estimate the amount of emulsifier noncovalently incorporated inside particles. 1H NMR analysis gives the total amount of covalently (i.e., arising from chain transfer to nonionic emulsifier) and noncovalently incorporated emulsifier. The results obtained by GPC were essentially identical with those obtained by 1H NMR, revealing that the amount of grafted emulsifier covalently (i.e., chain transfer to nonionic emulsifier) is negligible. The amount of the emulsifier incorporated inside the P(S-MAA) particles increased with increasing MAA content. Approximately 74% of the emulsifier (relative to total weight of emulsifier) was incorporated inside the particles at 10 mol % of MAA content at the final conversion. This result well corresponds to that obtained by the indirect measurement as described in the Introduction part (approximately 75%).19 In our previous article,20 it was concluded that the dominant partition of nonionic emulsifier to the monomer due (25) Gro¨nroos, A.; Pirkonen, P.; Heikkinen, J.; Ihalainen, J.; Mursunen, H.; Sekki, H. Ultrason. Sonochem. 2001, 8, 259–264. (26) Miyazaki, T.; Yomota, C.; Okada, S. Polym. Degrad. Stab. 2001, 74, 77–85. (27) Vijayalakshmi, S. P.; Madras, G. Polym. Degrad. Stab. 2005, 90, 116– 122.
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Figure 3. Weight percentages (relative to total weight of emulsifier and weight of monomer phase) of Emulgen 911 partitioned to the monomer phase (monomer/emulsifier/water, 1/0.065/9) at 70 °C in 1.5 h without stirring as a function of MAA content. Indexes: WEM ) weight of emulsifier in the monomer phase; WET ) total weight of added emulsifier; WM ) weight of monomer phase.
to the high affinity to monomer causes the incorporation of emulsifier inside polymer particles during emulsion polymerization. Therefore, in order to investigate the influence of the MAA content on the affinity of emulsifier to monomer, the emulsifier partitioning between the monomer and aqueous phases was measured at different MAA contents. Emulsifier Partitioning between the Monomer and Aqueous Phases at Different MAA Contents. Most of the nonionic emulsifiers are located in the monomer droplets before starting polymerization. After formation of particles, because the particles are swollen with monomer, the emulsifiers repartition to the monomer inside the particles through the aqueous medium from the monomer droplets. Therefore, the partitioning of the emulsifier in a simple monomer/water system will provide useful information for considering the repartitioning of the emulsifier to the monomer inside the particles. When the monomer/emulsifier/water mixtures were stirred at the same rate as the polymerization mixture (120 rpm), more than 95% of the emulsifier partitioned to the monomer phase (the partitioning of the emulsifier reached near equilibrium) at all the MAA contents investigated, and the partitioning did not depend significantly on the MAA content (the differences were the same order as the experimental error in gravimetry). However, when the monomer was gently added to the aqueous solution of emulsifier and the resultant mixture was left at 70 °C for 1.5 h without stirring, an increase in the amount of emulsifier partitioned to the monomer phase with increasing MAA content was clearly observed (Figure 3), indicating that the nonstirred system did not reach equilibrium. This trend is very similar to the variation in the amount of emulsifier incorporated inside the particles with increasing the MAA content (Figure 2). This indicates the possibility that the repartitioning of the emulsifier to the monomer inside the particles is less likely to reach equilibrium in the actual polymerization even with stirring. This may be attributed to a variety of factors such as continuous monomer consumption by polymerization, surface activity of the emulsifier, migration of emulsifier and monomer through two interfaces (i.e., emulsifier and monomer migrate from monomer droplets to monomer-swollen particles through the aqueous phase), and the stirring conditions in the present work not being so vigorous. The MAA concentration in the aqueous phase increases with increasing MAA content in the monomer mixture because MAA is water-soluble (cf., the aqueous phase was saturated with S in all cases). Thus, it appears that MAA dissolved in the aqueous phase promotes the partitioning (migration) of the emulsifier to the monomer phase. Generally,
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Figure 4. Transmittances of 1 wt % Emulgen 911 aqueous solutions, saturated with S and containing different amounts of dissolved MAA, as functions of temperature. The aqueous solutions prior to emulsifier addition were obtained from the aqueous phase after mixing monomers and water to obtain the same composition as in the actual polymerizations (except for KPS and Emulgen 911). MAA contents in the monomer mixtures (mol %): (a) 0; (b) 1; (c) 3; (d) 6; (e) 10. The CP is defined as the temperature at which the transmittance of the aqueous solution reaches 90%.
polar and polarizable compounds (e.g., fatty acids and alcohols of moderate chain length, phenol, or benzene), decrease the level of hydration of nonionic emulsifier and consequently cause a salting-out effect (CP decreases).28 Maclay reported that oleic acid, which contains both a double bond and a carboxyl group, significantly reduces the CP.1 It is conceivable that the dehydration of the emulsifier due to MAA causes the promotion of partitioning as described above. In order to confirm the dehydration of the emulsifier due to MAA, CP was measured at different MAA contents. CP Measurement. Figure 4 shows the transmittances of 1 wt % Emulgen 911 aqueous solutions, saturated with S and containing different amounts of dissolved MAA, as functions of temperature. The aqueous solutions employed for the CP measurement were obtained as described in the Experimental Section. The aqueous solution thus consists of MAA, as dictated by the partitioning between S and water, and S as dictated by the water solubility. As expected, the CP of the emulsifier decreased with increasing MAA content. When the MAA content were 0 and 10 mol %, the CPs were 70.2 and 61.0 °C, respectively. These values were lower than that in pure water (73.6 °C). In addition, KPS as an initiator also decreases the CP. Furthermore, sulfuric acid as a decomposition byproduct of KPS also decrease the CP under the present experimental conditions (it causes a salting-out effect at a low concentration and a salting-in effect above a certain concentration2). Therefore, CP during the actual emulsion polymerization would be likely to be ever lower. It can be concluded that the affinity of the nonionic emulsifier to water decreases (the affinity to monomer increases) with increasing MAA content because of dehydration of the emulsifier (i.e., the decrease of CP) due to MAA dissolved in the aqueous phase. Consequently, the emulsifier dissolved in the aqueous phase becomes thermodynamically unstable and preferentially partitions to the monomer phase. This might be one of the reasons why the amount of incorporated emulsifier increased with increasing MAA content. This notion is consistent with previous reports that, the higher the emulsifier hydrophobicity (i.e., the level of hydration of emulsifier was lower), the larger the amount of incorporated emulsifier.20,21 Moreover, these findings are potentially useful for preparation of (multi)hollow particles, because (28) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989; p 194.
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Figure 5. TEM photographs of P(S-MAA) particles having various MAA contents (mol % corresponds to the recipe) prepared by emulsion copolymerizations using Emulgen 911.
the formation of a hollow structure is favored by an increase in the amount of incorporated nonionic emulsifier.23 However, another possible explanation is as follows: it is wellknown that polymeric acids form complexes (aggregates) with ethoxylated nonionic emulsifiers due to hydrogen bonding between nonionized carboxyl groups and oxygen atoms of ethylene oxide chains and due to hydrophobic interaction between alkyl chains of polymeric acids and emulsifiers.29-34 Furthermore, these complexes are only observed in a narrow pH range. In the case of poly(acrylic acid) (PAA)-nonionic emulsifier systems, complex formation does not occur above pH 5 and a waterinsoluble complex forms below pH 3, which redissolves by addition of excess nonionic emulsifier.35,36 In the case of PMAA, the systems are also turbid or partially precipitated below pH 2.837 and there is no interaction above pH 8 as evidenced by PMAA being almost completely ionized and very hydrophilic.38 In particular, in PMAA-nonionic emulsifier systems, the emulsifiers are bound to the hydrophobic microdomains of the PMAA coil in water below the emulsifier concentration where complexes are formed.37 Robb and Stevenson34 reported that in polar oil/nonionic emulsifier/water systems, addition of polymeric acid (i.e., formation of water-insoluble complex) caused an increase in the percentage of nonionic emulsifier partitioned to polar oil. Moreover, in nonpolar oil/nonionic emulsifier/water systems where the emulsifier preferentially partitions to water, the polymeric acid and the emulsifier partitioned mainly to oil after addition of polymeric acid. In the present study, the pH of the system during polymerization lies in the acidic range where the complex forms; there is thus a good possibility that interactions between MAA units in the P(S-MAA) copolymer and Emulgen 911 occurs. Consequently, partitioning of the nonionic emulsifier (29) Saito, S. J. Am. Oil Chem. Soc. 1989, 66, 987–993. (30) Baranovsky, V. Y.; Shenkov, S.; Borisov, G. Eur. Polym. J. 1993, 29(8), 1137–1142. (31) Saito, S. J. Colloid Interface Sci. 1994, 165, 505–511. (32) Vasilescu, M.; Anghel, D. F. Langmuir 1997, 13, 6951–6955. (33) Anghel, D. F.; Saito, S.; Ba˜ran, A. Langmuir 1998, 14, 5342–5346. (34) Robb, I. D.; Stevenson, P. Langmuir 2000, 16, 7168–7172. (35) Anghel, D. F.; Winnik, F. M.; Galatanu, N. Colloids Surf., A 1999, 149, 339–345. (36) Ladhe, A. R.; Radomyselski, A.; Bhattacharyya, D. Langmuir 2006, 22, 615–621. (37) Anghel, D. F.; Saito, S.; Ba˜ran, A.; Iovescu, A.; Cornitescu, M. Colloid Polym. Sci. 2007, 285, 771–779. (38) Chu, D. Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270–6276.
Figure 6. Particle size distributions (weight base) obtained by DLS for the emulsion copolymerizations of S and MAA using Emulgen 911 at five different MAA contents (mol % corresponds to the recipe).
to the monomer phase increase and/or the nonionic emulsifier directly forms a complex with copolymer inside the particles, resulting in an increase of the amount of the incorporated emulsifier. Particle Formation Behavior. Figures 5 and 6, respectively, show TEM photographs and particle size distributions obtained by DLS for P(S-MAA) particles prepared by emulsion copolymerizations of S and MAA using Emulgen 911 at five different MAA contents. At lower MAA contents (0-3 mol %), new particles and bimodal particle size distributions were obtained due to secondary nucleation. Such secondary nucleation is often observed in emulsion polymerizations with nonionic emulsifier.7,9,20 However, few new particles were obtained at higher MAA contents (6-10 mol %), resulting in relatively monodisperse particles. (In fact, a small amount of new particles exist in the TEM observations for 6-10 mol % MAA contents. However, it is negligible based on the DLS data.) As described above, the amount of incorporated emulsifier increased with increasing MAA content. These results suggest that the level of secondary nucleation decreases with an increase in the amount of incorporated emulsifier. The monomer droplets gradually decrease in size, and emulsifier located in the monomer droplets is gradually released to the aqueous phase as polymerization proceeds, and it is thus possible that the emulsifier concentration in the aqueous phase exceeded the critical micelle concentration
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(cmc) after primary nucleation. According to the previous work,19 the percentage of emulsifier located in the aqueous phase after polymerization was approximately 5% (incorporation 75%, adsorbed at particle surface 20%), which is nearly equivalent to 0.5 mmol/L of water. In general, ethoxylated nonionic emulsifier is a commercial material with a distribution of ethoxylate chain lengths. Therefore, the cmc is affected by partitioning of the emulsifier between monomer and water during polymerization. For example, the cmc of a long ethoxylate chain portion of emulsifier partitioned to water (10-1 mmol/L of water) is higher than that of the original emulsifier (10-2 mmol/L of water).39 It is thus expected that the secondary nucleation based on micellar nucleation mechanism might occur in the present polymerization system. Thus, if the level of incorporation of the emulsifier inside the particles is high, secondary nucleation is limited. However, in the case of a low level of incorporation, the emulsifier concentration in the aqueous phase is high and consequently secondary nucleation occurs. To examine the validity of the above idea (i.e., secondary nucleation was mainly caused by emulsifier), other mechanisms for secondary nucleation were also investigated. Morrison and Gilbert40 investigated the conditions for secondary particle formation in emulsion polymerization where the ionic emulsifier concentration was below the cmc (this can be considered as emulsion polymerization after nucleation) using a seeded system. Secondary nucleation occurred if the overall rate of radical entry was significantly lower than that of formation of new particles, and this depended on initiator concentration, particle number, particle size, and monomer composition. That is, the ability of existing polymer particles to capture radicals (capture efficiency) plays a crucial role with regards to radical nucleation events in the aqueous phase. It thus seems unlikely that secondary nucleation occurs if there are no significant variations in the decomposition rate of initiator and/or particle number (these factors are important for the rate of radical entry) during the polymerization. In fact, no significant coagulation of particles was observed in the present work, and it is unlikely that the decomposition rate of initiator was accelerated. An increase in the MAA content in the system enhances homogeneous nucleation and, consequently, can affect primary nucleation resulting in a decrease in size and an increase in the number of particles (i.e., total surface area of the particles increases). One may consider that the decrease in the fraction of newly generated particles is mainly caused by the increase in capture efficiency of the primary nucleated particles because of the increase in their total surface area with MAA content. However, this scenario is unlikely in this work for the following reasons. The size of the particles prepared by emulsifier-free emulsion polymerization with the same recipe as the emulsion polymerization was somewhat larger than that by the emulsion polymerization with emulsifier present. Thus, one might anticipate, on the basis of capture efficiency, that secondary nucleation occurs to a greater extent in the emulsifier-free system. However, no secondary nucleation was observed in the emulsifier-free emulsion polymerization. This suggests that the secondary nucleation observed in the emulsion polymerization conducted in this work is not caused by the decrease in the capture efficiency of the primary nucleated particles with decreasing MAA. If the variation in the capture efficiency is important with regards to secondary nucleation, (39) Chaiyasat, A.; Yamada, M.; Kobayashi, H.; Suzuki, T.; Okubo, M. Polymer 2008, 49, 3042–3047. (40) Morrison, B. R.; Gilbert, R. G. Macromol. Symp. 1995, 92, 13–30.
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new particles should be observed in the emulsifier-free emulsion polymerization with the same recipe. In a previous article, we studied emulsifier-free seeded emulsion polymerization of styrene using KPS in the presence of emulsifierfree poly(ethyl acrylate) (PEA) seed particles.41 New particles were observed when a large number of ionized carboxyl groups, which were introduced by hydrolysis at the particle surface, existed on the surfaces of PEA seed particles. This may be caused by long residence times of oligomer radicals in the aqueous medium due to electrostatic repulsion between the oligomer radicals and the carboxyl groups on the surface. This suggests that MAA units on the particle surfaces may induce the formation of new particles. However, in this study, the number of new particles oppositely decreased with increasing MAA content as described above. Therefore, it is unlikely that the formation of new particles was directly caused by MAA in the present work. From the above results, it was concluded that an unincorporated portion of the emulsifiers released from the monomer droplets (i.e., emulsifier in the aqueous phase) caused the formation of new particles in the present polymerization.
Conclusions The influence of MAA on the incorporation of nonionic emulsifier inside particles in emulsion polymerization was investigated. Emulsion copolymerizations of S and MAA were carried out using Emulgen 911 nonionic emulsifier at different MAA contents. The amount of emulsifier incorporated inside the particles was directly measured by GPC and 1H NMR. The amount of the emulsifiers incorporated inside the particles increased with increasing MAA content. Seventy-four percent of the total amount of the emulsifier was incorporated inside the particles at 10 mol % of MAA, consistent with previous results obtained by indirect quantitative analysis. The partitioning of Emulgen 911 between the monomer and aqueous phases exhibited the same trend as the increase of incorporated emulsifier with increasing MAA content. Cloud point measurements of Emulgen 911 at different MAA concentrations clearly showed the dehydration of the emulsifier by MAA dissolved in the aqueous phase (i.e., CP decreased). The increase in the amount of incorporated emulsifier with increasing MAA content can be explained by the increase in the amount of emulsifier partitioned to monomer arising from dehydration of emulsifier due to MAA dissolved in the aqueous phase and/or interactions between MAA units in the copolymer and the emulsifier due to hydrogen bonding and hydrophobic interaction. At lower MAA contents, new particles and bimodal particle distribution due to secondary nucleation were obtained. However, at higher MAA contents, the extent of secondary nucleation was close to negligible, resulting in relatively monodisperse particles. The present results suggest that there is a need to consider incorporation of nonionic emulsifier inside polymer particles in order to quantitatively and qualitatively explain secondary nucleation. Acknowledgment. This work was supported by a Grant-Aid for Scientific Research (No. 17201025) from the Japan Society for the Promotion of Science (JSPS) and by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (given to H.K.). LA8021003 (41) Matsumoto, T.; Okubo, M.; Onoe, S. Kobunshi Ronbunshu 1976, 33, 565–574.
