Water Absorption Behavior of Polystyrene Particles Prepared by

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Water Absorption Behavior of Polystyrene Particles Prepared by Emulsion Polymerization with Nonionic Emulsifier and Innovative Easy Synthesis of Hollow Particles Masayoshi Okubo, Hiroshi Kobayashi, Chujuan Huang, Eri Miyanaga, and Toyoko Suzuki Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00232 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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Water Absorption Behavior of Polystyrene Particles Prepared by Emulsion Polymerization with Nonionic Emulsifier and Innovative Easy Synthesis of Hollow Particles †

Masayoshi Okubo,*†‡ Hiroshi Kobayashi,‡ Chujuan Huang,† Eri Miyanaga,‡ and Toyoko Suzuki‡ †

Institute of Advanced Materials (IAM), Nanjing Tech University, 5 Xinmofan Road, Nanjing

210009, China ‡

Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan

†Part CCCLXVI of the series “Studies on suspension and emulsion”

ABSTRACT: Submicrometer-sized raspberry-like polystyrene (PS) particles, which were prepared by emulsion polymerization with polyoxyethylene nonylphenyl ether nonionic emulsifier (Emulgen 910, HLB 12.2) and potassium persulfate initiator, contained 8.5 vol% (relative to particle) of water and 5.5 wt% (relative to PS) of Emulgen 910 in the inside. The water absorption decreased the glass transition temperature of PS particles dispersed in an aqueous medium. The wt% (relative to PS) of incorporated Emulgen 910 was increased with increasing initial Emulgen 910 concentration in the emulsion polymerization but wt% (relative to total Emulgen 910 used) of the incorporated Emulgen 910 was constant at approximately 50% independently of the initial concentration. The vol% (relative to particle) of water was increased to 46% by heat treatment at 90 ˚C for 24 h, which was based on further water absorption, and resulted in spherical hollow particles. Where, the amount of incorporated Emulgen 910 remarkably decreased in a short treatment, and then remained almost constant during the heat treatment. After another 24 h treatment, the percentage of

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non-hollow particles increased gradually, which was based on an escaping of the water domain together with Emulgen 910 from the inside of the particles. On the other hand, spherical PS particles prepared by emulsifier-free emulsion polymerization did not contain water in the inside and were not changed to hollow ones by a similar heat treatment.

From

these results, an innovative easy method to synthesize hydrophobic hollow PS particles is proposed.

■ INTRODUCTION In a general emulsion polymerization, nonionic emulsifier has been often used for the preparation of polymer colloids, because it offers various merits compared with ionic emulsifiers such as higher chemical and freeze-thaw stability, higher pigment affinity and less effervescence of the emulsion. Nonionic emulsifier has been used for a long time about 5-10 times weight of anionic emulsifier in emulsion polymerization from the experience, where large polymer particles having more than about 400 nm were often formed even at the high emulsifier concentration. However, recently we found by chance the reason why we had to use such a large amount of nonionic emulsifier in emulsion polymerization.1 It was based on the partition of nonionic emulsifier to monomer phase and the incorporation in polymerizing particles during polymerization. In actual, surprisingly 75% of the polyoxyethylene nonylphenyl ether nonionic emulsifier (Emulgen 911, hydrophilic-lipophilic balances (HLB) 13.7) used in emulsion copolymerization of styrene and methacrylic acid was incorporated inside copolymer particles at the completion of the polymerization. The partitioning of nonionic emulsifier to a monomer phase and monomer-swollen polymerizing particles, in other words, to an aqueous medium greatly affected of the particle formation throughout 2


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emulsion polymerization of hydrophobic (styrene) or hydrophilic (methyl methacrylate) monomer.2 In this way, as for the incorporation, emulsion polymerization with nonionic emulsifier has a significant disadvantage. In a series of investigations, we have clarified there are several factors greatly affecting the incorporation in styrene-methacrylic acid copolymer (P(S-MAA))3,4 or polystyrene (PS)5-7 particles. On the basis of the fundamental studies, in the latter emulsion polymerization of styrene, we successfully proposed a method to depress the incorporation by applying monomer-feed addition.5 On the other hand, we also should emphasize such an incorporation of nonionic emulsifier inside polymer particles had an advantage for the synthesis of hollow polymer particles. In actual, it was demonstrated that submicrometer-sized (multi)hollow hydrophobic polymer particles were prepared by seeded emulsion polymerization of styrene using PS seed particles with incorporated Emulgen 911. Such (multi)hollow particles are widely used in the various applications such as weigh-saving thermal insulation, hiding and opacifying agents through light scattering8-12, and loss enhancers for paper coatings and microcapsules for controlled and sustained drug delivery systems in the pharmaceutical industry13-17, and various techniques for the preparation of such particles have been proposed18-31. In this article, it will be examined in detail the water absortion behavior of PS particles prepared by emulsion polymerization of styrene with persulfate initiator and nonionic emulsifier and then an innovative simple and easy method based on heat treatment or absorbing/releasing of toluene of an aqueous dispersion of the PS particles at room temperature will be proposed for the synthesis of submicrometer-sized, hollow PS particles.

■ MATERIALS AND METHODS 2.1. Materials. Styrene was purified by distillation under reduced pressure and potassium

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persulfate (KPS) of analytical grade (Nacalai Tesque Inc., Kyoto, Japan) was purified by recrystallization. Commercial grade polyoxyethylene nonylphenyl ether nonionic emulsifier (Emulgen 910, HLB 12.2) with averages of 7.8 ethylene oxides per molecule was used as received (Kao Co., Tokyo, Japan). Tetrahydrofuran (THF) and 2-propanol of guaranteed reagent grade, chloroform-d1 (CDCl3) of chemical reagent grade, and hexamethyldisiloxane (HMDS) of extra pure reagent grade were used as received (Nacalai Tesque Inc., Kyoto, Japan).

Water was purified using an Elix UV 3 system (Nihon Millipore K.K., Tokyo,

Japan). 2.2. Preparation of PS Particles. Two kinds of original PS particles (i.e., PS particles before heat treatment) were prepared by emulsifier-free emulsion polymerization and emulsion polymerization with Emulgen 910 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 under the conditions in Table 1. Water (120 g) and Emulgen 910 (0−10 wt% relative to monomer) were added to the reactor, and the solution was stirred with a half-moon type stirrer at 240 rpm under a nitrogen atmosphere. The distance between the lower edge of the blade of the stirrer and the bottom of the flask was set to be approximately 1 cm. After the solution was heated to 70 ˚C, styrene (4 g) was poured into the reactor. The mixture was deoxygenated with a stream of nitrogen gas for 30 min. Subsequently, a KPS aqueous solution (0.32 g of KPS was dissolved in 20 g of water) was added to the reactor to initiate polymerizations. The conversions were over 90% according to gravimetric measurements. It was confirmed with a gas chromatography (GC) (GC-18A, Shimadzu Corp., Kyoto, Japan) that there was no residual monomer in the final products.

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Table 1.

Preparation of PS particles by emulsifier-free emulsion polymerizationa) and emulsion a)

polymerization with Emulgen 910

Emulgen 910 [mmol/L (wt% relative to monomer)] Ingredients 0 (0)

0.5 (1)

1.5 (3)

2.5 (5)

3.5 (7)

5.1 (10)

Styrene

(g)

4.0

4.0

4.0

4.0

4.0

4.0

KPS

(g)

0.32

0.32

0.32

0.32

0.32

0.32

0.04

0.12

0.20

0.28

0.40

Emulgen 910

b)

Water Incorporationc)

(g)



(g)

140

(%)



140

140

140

140

140

0.6 (61)

1.5 (51)

2.7 (53)

3.8 (54)

5.5 (55)

a)

N2; 70 ˚C, 24 h; stirring rate, 240 rpm

b)

Ethylene oxide units = 7.8; HLB = 12.2

c)

Relative to total weight of polymer (weight of emulsifier); measured by H NMR

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Abbreviations: PS, polystyrene; KPS, potassium persulfate; Emulgen 910, polyoxyethylene nonylphenyl ether nonionic emulsifier

2.3. Heat Treatment of PS Particles. The original PS particles were centrifugally washed with water three times, and the solid content was adjusted to 1 wt% in order to prevent coagulation of particles and the washed dispersions were and shaken in glass tubes horizontally at 100 cycles/min at 120 ˚C for 30 min in an oil bath. When a variation of hollow structure with heating time was examined, heat treatment was carried out at 90 ˚C. After the heat treatment, the glass tubes were immediately cooled under running water.

2.4. Electron Microscopy. The particles were observed with a TEM (JEM-1230, JEOL Datum Ltd., Tokyo, Japan). Each original dispersion was centrifugally washed with water three times to remove excess emulsifier in the aqueous medium and small byproduct particles

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(no further washing for dispersions after heat treatment), and then TEM samples were prepared according to the previous article.6 The number-average particle diameter (dn) and the coefficient of variation (Cv) were determined using image analysis software (MacSCOPE, Mitani Co. Ltd., Fukui, Japan) for a Macintosh computer from data based on in excess of 200 particles. The particles were also observed with a scanning electron microscope (SEM, JSM-5610LV, JEOL Datum Ltd., Tokyo, Japan).

2.5. Quantitative Analysis of Nonionic Emulsifier inside Particle. The PS dispersions prepared by the emulsion polymerization with Emulgen 910 were centrifugally washed with 2-propanol three times to remove emulsifier completely from the particle surfaces according to the previous work.4,5 , and then dried at room temperature under reduced pressure for a few days. The dried particles were dissolved in CDCl3 containing 0.1 wt% HMDS as an internal standard. 1H NMR spectra were recorded on a Bruker AVANCE 500 NMR spectrometer (Karlsruhe, Germany) operating at 500 MHz with 16 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. Emulgen 910 concentration was obtained from 1H NMR relative intensity due to ethylene oxide protons.

2.6. Measurement of Glass Transition Temperatures in Dry State (TgD) and Emulsion State (TgE). TgD of dried PS particles was measured using approximately 10 mg of the dried particles (after washing with 2-propanol) on a sealed aluminum pan (5-mm diameter) with a heat flow-type differential scanning calorimeter (DSC, DSC 6200, Seiko Instruments Inc., Chiba, Japan) under a nitrogen atmosphere (the flow rate: 40 mL/min) at a scanning rate of 5 °C/min. TgE of the wet PS particles dispersed in the aqueous medium was measured using

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approximately 0.33 mL of the dispersion (after centrifugal washing with water three times, dispersed in water at the solid content of approximately 3 wt%, and then degassed in a desiccator under reduced pressure with stirring for 30 min) with a power compensative differential scanning calorimeter (nano-DSC, nano-DSC II 6100, Calorimetry Sciences Corp., Utah, USA) in the temperature range from 40 to 120°C at 1 °C/min. During the measurement, the dispersion was pressurized to 3 atm to prevent water evaporation. The TgD and TgE were determined as the temperature corresponding to the maximum on the second differential curves to avoid human error and are, respectively, shown as TgDd and TgEd according to a previous article32 .

2.7. Quantitative Analysis of the Amount of Water inside Particle. The evaporation behavior of water from PS dispersion was examined using 30 mg of the PS aqueous dispersion (the solid content, approx. 10 wt%; after centrifugal washing with water five times) with a TG/DTA (TG/DTA 6200, Seiko Instruments Inc., Chiba, Japan) on an aluminum pan (5-mm diameter) at 30 ˚C under a nitrogen atmosphere (the flow rate: 250 mL/min). The instantaneous evaporation rate of water was determined by differentiation of the weight-loss curve. The amount of water inside the particles was determined based on the curves of the instantaneous evaporation rate of water and the weight loss according to a previous work.32

■ RESULTS AND DISCUSSION 3.1. Preparation of Original PS Particles. In the previous articles,4,5 it was clarified that the higher the hydrophobicity of nonionic emulsifier (i.e., the level of hydration of emulsifier was lower), the larger was the amount of emulsifier incorporated. In addition, it was revealed that the preparation of hollow particles by seeded emulsion polymerization was favored by an

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Comment [大久保政芳 [大久保政芳1]: 大久保政芳1]:

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increase in the amount of nonionic emulsifier incorporated inside PS seed particles.6 However, the ability of the particles to absorb water would not be directly related to the hydrophilicity of emulsifier.7 Therefore, in this study, Emulgen 910, which is more hydrophobic than Emulgen 911, was used to prepare original PS particles with incorporated nonionic emulsifier under expectations for promoting the incorporation, that is, preparing hollow particles more effectively. However, the higher the hydrophobicity of emulsifier, the lower the cloud point. In fact, the cloud point of Emulgen 910 (23.8 ˚C) is much lower than the polymerization temperature (70 ˚C). This suggests that Emulgen 910 could hardly stabilize particles in this polymerization system, but electrostatic repulsion enough to prevent a coagulation between the particles would be achieved by relatively high initiator concentration (the number of sulfate end groups on the particle surface would be relatively large). Figure 1 shows TEM photographs of original PS particles prepared by emulsifier-free emulsion polymerization (a) and emulsion polymerization of styrene using Emulgen 910 (b) with KPS initiator (8.45 mmol/L-water) at 70 ˚C under the conditions in Table 1. In the case of the emulsifier-free emulsion polymerization, relatively monodisperse spherical particles were obtained. When 10 wt % (relative to monomer) of Emulgen 910 was used, nonspherical particles with a raspberry-like shape (dn = 642 nm; Cv = 7.3%), in which 5.5 wt% (relative to PS) of Emulgen 910 was incorporated, were obtained. This nonspherical shape was ascribed to a hetero-coagulation between large pre-existing particles and new small particles caused by an unincorporated portion of the emulsifiers released from the monomer droplets (i.e., emulsifier in the aqueous phase) like using Emulgen 911.4,5 However, the particles prepared using Emulgen 910 were less nonspherical than those using Emulgen 911, and no less contrast small domain was observed inside the particles.6 The amount (5.5 wt% relative to PS) of Emulgen 910 incorporated inside PS particles was higher than that (3.2 wt% relative to PS) of

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Emulgen 911 as expected in the above paragraph, although they were prepared under the same polymerization recipe.

Figure 1. TEM photographs of original PS particles without (a) and with (b) incorporated Emulgen 910 (5.5 wt% relative to PS), respectively, prepared by emulsifier-free emulsion polymerization (a) and emulsion polymerization using 5.1 mmol/L-water of Emulgen 910 (b), with KPS (8.45 mmol/L-water) at 70 ˚C.

Figure S1 shows DSC 2nd heating curves (in dry state) of the original PS particles without/with incorporated Emulgen 910. TgDd value (88 ˚C) of the particles with 5.5 wt% (relative to PS) of incorporated Emulgen 910 was approximately 7 ˚C lower than that (95 ˚C) of PS particles without incorporated Emulgen 910, wherein only a single peak was observed. In a previous article, 3 it was clarified that the incorporated emulsifiers were homogeneously mixed with P(S-MAA) to some extent (approximately 5wt%) by turbidity measurement of the film containing emulsifiers. Therefore, the incorporated emulsifiers would be compatible with PS within particle and act as a plasticizer in the dry state. Figure 2 shows Nano-DSC 2nd heating curves and differential curves of aqueous dispersions of the original PS particles without/with incorporated Emulgen 910. Both TgE values of the particles in emulsion states without/with the incorporated Emulgen 910 were,

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respectively, less than 10 ˚C than their TgD values shown in Figure 1S. There was no monomer inside the particles (confirmed with a GC). Therefore, this suggests that the PS particles were further plasticized by water absorbed during the polymerizations. Moreover, the degree of reduction in the TgE (88˚C→74˚C) of the particles with the incorporated Emulgen 910 was slightly larger than that (95˚C→84˚C) without incorporated emulsifier, which indicates that a larger amount of water was absorbed into PS particles with incorporated Emulgen 910 than the particles without emulsifier. This result was well in consist with a previous finding that incorporated nonionic Emulgen 911 emulsifiers promoted the absorption of water into the particles.6

Figure 2. Nano-DSC 2nd heating curves (A) and differential curves (B) of aqueous dispersions of original PS particles without (a) and with (b) incorporated Emulgen 910 (5.5 wt% relative to PS). The curves (a) and (b) correspond to the particles (a) and (b) in Fig. 1.

3.2. Heat Treatment of Aqueous Dispersions of the Original PS Particles. Figure 4 shows TEM (a, b) and SEM (c, d) photographs of PS particles with 5.5 wt% (relative to PS) of incorporated Emulgen 910 before (a, c) and after (b, d) heat treatment at 120 ˚C for 30 min. 10


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Spherical particles with a single less contrast region at the center were observed after the heat treatment as shown in Figure 3b. The disappearance of the nonspherical shape by the heat treatment inversely supports the idea proposed in the previous article6 that the nonspherical PS particles resulting from the coagulation of small particles onto pre-existing main large particles in the latter of the emulsion polymerization of styrene at 70 ˚C no longer change in shape at high conversion, at which Tg of PS particles seem to be near the polymerization temperature, because monomer concentration inside particles was not high enough to become thermodynamically favorable shape (i.e., spherical particle). From the SEM photograph (Figure 3d), the surface of the particles was basically smooth although some particles had a mild dimple that seems to be due to weakness of the shell. Therefore, the less contrast region in the particles was attributed to a hollow structure inside the particles, which were filled with water before drying. On the other hand, after the heat treatment of the original particles without incorporated emulsifier, the particles with homogeneous contrast were observed as shown in Figure 5a’.

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Figure 3. TEM (a, b) and SEM (c, d) photographs of PS particles with incorporated Emulgen 910 (5.5 wt% relative to PS) prepared by emulsion polymerization using Emulgen 910 (5.1 mmol/L-water) with KPS (8.45 mmol/L-water) at 70 ˚C before (a, c) and after (b, d) heat treatment at 120 ˚C for 30 min.

The TgE values of the particles with the incorporated Emulgen 910 (5.5 wt% relative to PS) after the heat treatment (i.e., hollow particles) slightly shifted to higher temperature (Figure 4). This may be because some incorporated emulsifiers escaped from the particles, which will be discussed in a near future, and/or the domain size of water inside particles increased due to coalescence caused by the heat treatment, which may lead to a reduction in the plasticization effect of the emulsifiers and water.

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Figure 4. Nano-DSC 2nd heating curves (A) and differential curves (B) of aqueous dispersions of PS particles with incorporated Emulgen 910 (5.5 wt% relative to PS) before (a) and after (b) heat treatment at 120 ˚C for 30 min.

3.3. Effect of Incorporated Emulsifier on Formation of Hollow Structure. Figure 5a-d shows TEM photographs of original PS particles with different amounts of incorporated emulsifier prepared by emulsion polymerization using Emulgen 910 (0-10 wt% relative to monomer) with KPS (8.45 mmol/L) at 70 ˚C under the conditions in Table 1. In the cases of the original PS particles with 0 – 1.5 wt% (relative to PS) of incorporated Emulgen 910, spherical particles without hollows were obtained after the heat treatment at 120˚C for 30 min. This coincides with the previous result that no hollow particles were formed by seeded emulsion polymerization of styrene when PS seed particles with a small amount of incorporated Emulgen 911 were used. 6 In the cases of 2.7 and 3.8 wt% of incorporated Emulgen 910, multihollow particles were observed after the heat treatment. In the case of 3.8 wt%, some particles with a single hollow, which predominantly appeared in smaller-sized particles, were obtained. This is reasonably explained by the fact that water domains can

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coalesce and become a single one more quickly in a smaller particle than in a larger particle. The size of hollows increased with increasing the amount of incorporation, and eventually a single hollow was observed at 5.5 wt%. As for the reason why the degree of coalescence of water domains increased with increasing the amount of incorporation, it is conceivable that a lot of water domains are formed when the level of incorporation is high, resulting in large total interface area between water domains and PS phase. The resultant high interfacial free energy causes faster coalescence of water domains, which decreases the total interfacial area. These suggest that control of the size and number of hollows can be achieved by the amount of the incorporated emulsifier. Figure 6 shows the amounts of incorporated Emulgen 910 inside the particles, which were measured by 1H NMR. The amount of incorporation (wt% relative to PS) linearly increased with increasing initial Emulgen 910 concentration. The percentages of incorporation (relative to total Emulgen 910 used in the polymerization) were almost a constant to be at approximately 50% independently of the initial emulsifier concentration. This result may suggest that the incorporation of Emulgen 910 inside PS particles is mainly determined by partitioning of emulsifier between monomer and water.

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Figure 5. TEM photographs of PS particles with different amounts of incorporated Emulgen 910 prepared by emulsion polymerization using Emulgen 910 with KPS at 70 ˚C under the conditions in Table 1 before (a-d) and after (a’-d’) heat treatment at 120 ˚C for 30 min. Incorporation (wt% relative to PS): (a) 0; (b) 2.7; (c) 3.8; (d) 5.5.

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Figure 6. Weight percentages [relative to PS () and total Emulgen 910 ()] of Emulgen 910 incorporated inside the original PS particles prepared by emulsion polymerizations as functions of Emulgen 910 concentration.

3.4. Formation of Hollow Structure by Heat Treatment. To make observation and measurement easy for considering a variation of hollow structure with heating time, heat treatment of the aqueous dispersion of the original PS particles with 5.5 wt% (relative to PS) of incorporated Emulgen 910 was carried out at 90 ˚C, where water domains was expected to coalesce slower than at 120 ˚C. Because the variation of hollow structure in the heat treatment is based on the thermodynamic variation from an unstable to a stable state, reduction in the heat temperature from 120˚C to 90˚C must affect only the variation rate but not the final hollow structure. Figure 7 shows TEM photographs of PS particles with the incorporated Emulgen 910 after the heat treatment at 90 ˚C for various heating times. At 1 h of the heating time, the nonspherical shape already disappeared and a lot of small domains, which were filled with water before drying, were formed. The size of hollows inside the particle increased and their number decreased with the heating time. This means that the water domains coalesce inside

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the particles during the heat treatment to minimize interfacial free energy, and the hollow structure can also be controlled by heating time as well as the amount of the incorporated emulsifier.

Figure 7. TEM photographs of PS particles with incorporated Emulgen 910 (5.5 wt% relative to PS) after heat treatment at 90 ˚C for various heating times.

The original PS particles were

prepared by emulsion polymerization using Emulgen 910 (5.1 mmol/L-water) with KPS (8.45 mmol/L-water) at 70 ˚C.

After 24 h, most particles had a single hollow, however, some particles without hollows were also observed and their percentage increased after another 24 h. The non-hollow particles result from an escaping of the water domain from the inside of the particles, which obviously indicates the hollow structure to be thermodynamically unstable. A similar phenomenon was observed in the preparation of hollow polymer particles consisting of styrene-methacrylic acid copolymer by the stepwise alkali/acid method.33 The water domain tends to escape earlier in smaller particles than in larger ones. That is, it is important to use monodisperse PS particles

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to get hollow PS particles at 100% yield. This must be carefully considered to synthesize hollow polymer particles.

Figure 8. Number-average particle diameter (dn) of PS particles with incorporated Emulgen 910 (5.5 wt% relative to PS) as a function of heating time at 90 ˚C. Only the particles with hollow(s) were measured except for at heating time = 0 h.

Figure 8 shows the number-average particle diameter (dn) of PS particles as a function of heating time at 90 ˚C, where only the particles with hollow(s) were measured except for the original particles. The dn surely increased from approximately 640 to 800 nm with the heating time, and reached a plateau more than 24 h. There are two possible reasons why dn increased during heat treatment: i) coalescence of particles due to a softening of particles by the heat treatment, because of decrease in stabilizing ability of nonionic emulsifier at the high temperature; ii) an increase in the amount of water inside particles due to further absorption of water into the particles. It is important to clarify this for considering formation of hollow structure during the heat treatment. Therefore, the amount of water inside particles during the heat treatment was focused attention on. If the coalescence of particles causes the increase in 18


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dn, the amount of water should remain constant during the heat treatment. Whereas, it should increase when the absorption of water into the particles occurs.

Figure 9. Volume percentages (relative to particles) of water inside PS particles with () and without () incorporated Emulgen 910 (5.5 wt% relative to PS) as functions of heating time at 90 ˚C.

Figure 9 shows volume percentages (relative to particles) of water inside PS particle without and with incorporated Emulgen 910 (5.5 wt% relative to PS) as a function of heating time at 90 ˚C, which were determined from TG/DTA data. When an aqueous dispersion of the original PS particles without incorporated emulsifier was treated at 90 ˚C, no water inside the particles was measured (or too little to be determined by the method). In the case of the original PS particles with the incorporated Emulgen 910, approximately 8.5 vol% (relative to particles) of water existed inside the particles before the heat treatment (i.e., inside the original PS particles). This value was much lower than that (23 vol% relative to particle) of PS particles prepared with incorporated Emulgen 911 (3.2 wt% relative to PS) in the previous article.7 This difference seems to be based on that hydrophobicity of Emulgen 910 is higher 19


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than Emulgen 911. In addition, another possible reason may be that the polymerization period, in which particles are plasticized, was shorter in emulsion polymerization with Emulgen 910 than that with Emulgen 911 (e.g., the period, in which relatively high monomer concentration inside particles was maintain, was shorter in the emulsion polymerization with Emulgen 910). The volume percentage of water inside the particle with Emulgen 910 increased to approximately 46 vol% with the heating time. The real volume percentage appears to be somewhat higher than 46 vol% because non-hollow particles also included in the TG/DTA measurements sample. The saturated water content (vol%) was well in accord with the volume percentage of final hollow in the particle (48.8 %) calculated using the initial and final dn values shown in Figure 8 and that (approximately 50 vol%) calculated from the TEM photograph shown in Figure 7f. These results indicate that the increase in dn was mainly caused by further absorption of water into the particles during the heat treatment (the reason for the further absorption will be discussed more in detail in a following article). However, the size of some non-hollow particles (Figure 7f) was smaller than that of the original particles (Figure 7a), although their sizes cannot be accurately compared because the latter particles had a nonspherical shape. This suggests the coalescence of particles may occur somewhat during the heat treatment. Figure 10 shows the results of quantitative analysis of Emulgen 910 incorporated inside the particles after the heat treatment at 90 ˚C for various times. The amount of incorporated Emulgen 910 remarkably decreased in a short treatment, and then remained almost constant during the heat treatment. This result is in consist with the finding shown above that TgE of particles after the heat treatment at 120 ˚C for 30 min shifted to higher temperature (74˚C→ 77˚C). The reason of the decrease in the amount of incorporated Emulgen 910 seems to be based on that they would repartition into a fresh aqueous medium, in which free emulsifiers were removed by centrifugal washing before the heat treatment, from PS particles softened by

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the heat treatment.

Figure 10. Weight percentage (relative to polymer) of Emulgen 910 incorporated inside PS particles as functions of heating time at 90 ˚C. Non-hollow particles also included in the measurement of incorporated emulsifier by 1H NMR.

From the above results, the formation of hollow structure by heat treatment of an aqueous dispersion of PS particles with the incorporated nonionic emulsifier is considered as follows. PS particles, which were prepared by the emulsion polymerization with Emulgen 910 and potassium persulfate, have already contained water as fine domains in the inside at the completion of the polymerization and absorbed further water from the aqueous medium at the heating process. During heat treatment above TgE of the particles, the water domains grow by further absorption of water into the particle and simultaneously coalesce, and eventually escape from the particle to minimize interfacial free energy. That is, the number and size of hollows are mainly determined by further absorption of water, a rate of coalescence (i.e., the time and temperature of heat treatment).

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To examine the validity of the above idea, the following experiments with regard to whether hollow particles can be prepared through plasticization of the particles by other means were carried out. Toluene was added to an aqueous dispersion of the original PS particles with the incorporated Emulgen 910 (Figure 1b), and the mixture was stirred in a covered glass cylindrical vessel for 1 h using a magnetic stirrer at room temperature. Toluene was then allowed to evaporate naturally from the mixture at room temperature. As a result, hollow polymer particles were obtained like plasticization by heat treatment (Figure S2c). When PS aqueous dispersion prepared by emulsifier-free emulsion polymerization (Figure S2a) was treated with toluene in the same manner as described above, no hollow particles were formed (Figure S2b). These results strongly support that both incorporation of nonionic emulsifier inside particles and plasticization of particles are important for the synthesis of hollow polymer particles. In a following article, effect of sulfate polymer end groups as KPS initiator fragment on the adsorption of water into PS particle will be examined in detail and comprehensively the formation mechanism of hollow structure inside PS particle will be discussed.

■ CONCLUSIONS Hollow PS particles were prepared only by heat treatment or absorbing/releasing of toluene at room temperature of an aqueous dispersion of PS particles with incorporated Emulgen 910, which were prepared by emulsion polymerization. The amount of incorporated Emulgen 910 inside the particles linearly increased with increasing its concentration in the emulsion polymerization. The size of hollows increased with increasing amount of incorporation. Eventually a single hollow was observed in the inside of PS particles with 5.5 wt% relative to PS of Emulgen 910 after the heat treatment at 120 ˚C for 30 min. Volume percentages of water inside PS particles, which corresponds to percentage of hollow, increased from 8.5 to

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46 vol% by further absorption of water with increasing heating time at 90˚C.

■ ASSOCIATED CONTENT s

Supporting Information Figure S1, S2, S3, and S4 as supporting references (PDF)

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This work was supported by Grant-in-Aid for Scientific Research (B) (Grant 25288054; given to M.O.) from the Japan Society for Promotion of Science (JSPS) and Start-Up Foundation (Grant 3980001602; given to M.O.) from the Institute of Advanced Materials of Nanjing Tech University.

■ REFERENCES (1) Okubo, M.; Furukawa, Y.; Shiba, K.; Matoba, T. Inclusion of nonionic emulsifier inside polymer particles produced by emulsion polymerization. Colloid Polym. Sci. 2003, 281, 182-186. (2) Matsusaka, N.; Suzuki, T.; Okubo, M. Effect of Partitioning of Monomer and Emulsifier in Aqueous Media on Particle Formation in Emulsion Homopolymerization of Hydrophobic and Hydrophilic Monomers with a Nonionic Emulsifier. Polymer J. 2013, 45, 153-159. (3) Okada, M.; Matoba, T.; Okubo, M. Influence of nonionic emulsifier included inside 23


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carboxylated polymer particles on the formation of multihollow structure by the alkali/cooling method. Colloid Polym. Sci. 2004, 282, 193-197. (4) Chaiyasat, A.; Yamada, M.; Kobayashi, H.; Suzuki, T.; Okubo, M. Incorporation of nonionic

emulsifiers

inside

styrene-MAA

copolymer

particles

during

emulsion

copolymerization. Polymer 2008, 49, 3042-3047. (5) Okubo, M.; Kobayashi, H.; Matoba, T.; Oshima, Y. Incorporation of nonionic emulsifiers inside particles in emulsion polymerization: Mechanism and methods of suppression. Langmuir 2006, 22, 8727-8731. (6) Kobayashi, H.; Miyanaga, E.; Okubo, M. Preparation of multihollow polymer particles by seeded emulsion polymerization using seed particles with incorporated nonionic emulsifier. Langmuir 2007, 23, 8703-8708. (7) Kobayashi, H.; Suzuki, T.; Moritaka, M.; Miyanaga, E.; Okubo, M. Preparation of multihollow polystyrene particles by seeded emulsion polymerization using seed particles with incorporated nonionic emulsifier: effect of temperature. Colloid Polym. Sci. 2009, 287, 251-257. (8) Kowalski, A.; Vogal, M.; Blankenship, R. Sequential heteropolymer dispersion and a particulate material obtainable therefrom, useful in coating compositions as a thickening and/or opacifying agent. US Patent 4 427 836, 1984. (9) Blankenship，R. M.; Novak, R. W.; Neyhart, C. J.; Vogel, M.; Kowalski, A. Polymeric particles. European Patent, 0 565 244, 1993. (10) Williams, E.; Fasana, D. M. Handbook of Coating Additives 2.; Calbo L., Ed., Marcel Dekker, Inc.: New York, 1992, pp 233. (11) Hultman, J. D.; Schuh, S. M. Opaque thermal transfer paper for receiving heated ink from a thermal transfer printer ribbon.US Patent 5 677 043, 1997. (12) Sang，H. I.; Jeong, U.; Xia, Y. Polymer hollow particles with controllable holes in their surfaces. Nature Materials 2005, 4, 671-675. (13) Oh, J. K.; Lee, D. I.; Park, J. M. Biopolymer-based microgels/nanogels for drug delivery applications. Progr. Polym. Sci. 2009, 34,1261-1282. (14) Yang, X. Y.; Chen, L.; Huang, B.; Bai, F.; Yang, X. L. Synthesis of pH-sensitive hollow polymer microspheres and their application as drug carriers. Polymer 2009, 50, 3556-3563. (15) Jungmann, N.; Schmidt, M.; Maskos, M. Synthesis of amphiphilic poly(organosiloxane) nanospheres with different core-shell architectures. Macromolecules 2002, 35, 6851-6857. (16) Damge, C.; Michel, C.; Aprahamian, M.; Couvreuer, P.; Devissaguet, J. Nanocapsules as carriers for oral peptide delivery. J. Controlled Release 1990, 13, 233-239. (17) Schuler, C.; Caruso, F. Decomposable hollow biopolymer-based capsules. Biomacromolecules. 2001, 2, 921-926. (18) Kim, J. W.; Joe, Y. G.; Suh, K. D. Poly(methyl methacrylate) hollow particles by 24


Page 25 of 36

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water-in-oil-in-water emulsion polymerization. Colloid Polym. Sci. 1999, 277, 252-256. (19) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Production of hollow polymeric microspheres by surface-confined living radical polymerization on Silica templates. Chem. Mater. 2000, 12, 3481-3487. (20) McDonald, C. J.; Bouck, K.; Chaput, A. B.; Stevens, C. J. Emulsion polymerization of voided particles by encapsulation of a nonsolvent. Macromolecules 2000, 33, 1593-1605. (21) Tiarks, F.; Landfester, K.; Antonietti, M. Preparation of polymeric nanocapsules by miniemulsion polymerization. Langmuir 2001, 17, 908-918. (22) Jang, J.; Ha, H. Fabrication of hollow polystyrene nanospheres in microemulsion polymerization using triblock copolymers. Langmuir 2002, 18, 5613-5618. (23) Xu, X. L.; Asher, S. A. Synthesis and utilization of monodisperse hollow polymeric particles in photonic crystals. J. Am. Chem. Soc. 2004, 126, 7940-7945. (24) Lee, J. E.; Kim, J. W.; Jun, J. B.; Ryu, J. H.; Kang, H. H.; Oh, S. G.; Suh, K. D. Polymer/Ag composite microspheres produced by water-in-oil-in-water emulsion polymerization and their application for a preservative. Colloid Polym. Sci. 2004, 282, 295-299. (25) Zhang, Y. W.; Jiang, M.; Zhao, J. X.; Wang, Z. X.; Dou, H. J.; Chen, D. Y. pH-responsive core-shell particles and hollow spheres attained by macromolecular self-assembly. Langmuir 2005, 21, 1531-1538. (26) Fu, G. D.; Shang, Z. H.; Hong, L.; Kang, E. T.; Neoh, K. G. Preparation of cross-linked polystyrene hollow nanospheres via surface-initiated atom transfer radical polymerizations. Macromolecules 2005, 38, 7867-7871. (27) Ali, M. M.; Stöver, H. D. H. Interfacial living radical copolymerization of oil- and water-soluble comonomers to form composite polymer capsules. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 156-171. (28) Kobašlija, M.; MaQuade, D. T. Polyurea microcapsules from oil-in-oil emulsions via interfacial polymerization. Macromolecules 2006, 39, 6371-6375. (29) Bédard, M.; Skirtach, A. G.; Sukhorukov, G. B. Optically driven encapsulation using novel polymeric hollow shells containing an azobenzene polymer. Macromol. Rapid Commun. 2007, 28, 1517-1521. (30) Lu, F.; Luo, Y.; Li, B. A facile route to synthesize highly uniform nanocapsules: Use of amphiphilic poly(acrylic acid)-block-polystyrene RAFT agents to interfacially confine miniemulsion polymerization. Macromol. Rapid Commun. 2007, 28, 868-874. (31) Zhang, F.; Ma, J. Z.; Xu, Q. N.; Zhou, J. H.; Simion, D.; Carmen, G. A facile method for fabricating room-temperature-film-formable casein-based hollow nanospheres. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2015, 484, 329-335. 25


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(32) Suzuki, T.; Inoue, M.; Okubo, M. Estimation of water absorption state within ionized carboxylated polymer particles with high sensitive differential scanning calorimetry. Colloid Polym. Sci. 2006, 284, 802-806. (33) Okubo, M.; Sakauchi, A.; Okada, M. Formation mechanism of multihollow structure within submicron-sized styrene-methacrylic acid copolymer particles by the alkali/acid method, Colloid Polym. Sci. 2002, 280, 303-309.

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Table of Contents Graphic

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: Water domain

Escape of Emulsifier

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: Nonionic emulsifier

Absorption of water

Coagulation of water domains

Heat treatment ( > Tg ) ACS Paragon Plus Environment

Escape of water domain

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Water Absorption Behavior of Polystyrene Particles Prepared by

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