Growth Processes of Charge

Aug 10, 2010 - Ten minutes later, with a conversion of 3.4%, the 65-nm particles had coalesced to form larger ones (535 nm), as shown in Figure 5b...
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J. Phys. Chem. B 2010, 114, 10970–10978

Direct Observations of Three Nucleation/Growth Processes of Charge-Stabilized Dispersion Polymerizations with Varying Water/Methanol Ratios Fen Zhang, Yuhong Ma, Lianying Liu, and Wantai Yang* Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, College of Material Science and Engineering, Beijing UniVersity of Chemical Technology, Beijing, 100029, P. R. China ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: July 25, 2010

This article presents observations of three polymerization modes of a self-developed cation-charge-stabilized styrene/water/methanol dispersion polymerization system: (1) a water/methanol (20/80) system, corresponding to a typical dispersion polymerization mode where the particle nucleation occurred in the solution phase and growth in the particle phase; (2) a pure CH3OH system, including a first nucleation in the solution phase with growth by absorption of the small particles and polymers formed in this phase, and a secondary nucleation with growth in the particle phase, when high molecular weight copolymers appeared in the solution phase; and (3) a water/methanol (5/95) system, similar to the conventional dispersion polymerization mode during the first 90 min, with subsequent epitaxial growth. Interestingly, the metastable state of the nucleation stage, including minuscule 6-nm particles, their aggregates, and the aggregating process, was first observed experimentally. By quantitatively following the relationship of the deposited molecular weight and the nucleation/growth process in the three systems, it was proposed that the molecular weight of the deposited polymer had to reach a specific high value before they could absorb or capture monomer to form smooth/ spherical nuclei or particles. Introduction Dispersion polymerization, defined as a heterogeneous polymerization by which latex particles are formed in the presence of a suitable stabilizer from an initially homogeneous reaction mixture,1 is a unique method for preparing monodisperse polymer particles with diameters in the 1-15 µm2-7 size range in a single polymerization step. The polymerization process can be divided into the nucleation (particle formation) stage and the growth stage. Initially, the nuclei are formed and aggregate with each other to form bigger particles until enough polymeric stabilizers are present on their surface to achieve steric or electrosteric stabilization. In the second step, the number of particles remains constant, and the polymerization or the particle growth occurs within the particles by absorption of monomer and polymer radicals from the continuous phase.8-11 The nucleation stage during the particle formation is known to be very complex and sensitive, and also very important for producing monodisperse particles.12,13 However, during this stage, due to a low monomer conversion (1-2%),8,14 small sizes of the typical particle nuclei (in the order of 1-5 nm),14 and large amounts of polymeric stabilizer (generally more than 5 wt %)15 remaining in the system, it is difficult to experimentally track the evolution of the nucleation stage by direct observations of the nucleation process. Paine10 proposed a multibin kinetic model for the aggregation of precipitated radicals or unstabilized particles in dispersion polymerization, and believed that the diffusion-controlled particle aggregation would stop when the particle surfaces were completely covered with poly(vinyl pyrrolidone)-graft-polystyrene (PVP-g-PS) produced by the chain transfer reaction with the stabilizer, and that no new nuclei would be formed after this. Yasuda et al.11 described a mode to simulate the particle * To whom correspondence should be addressed. E-mail: yangwt@ mail.buct.edu.cn. Phone: +86-10-64432262. Fax: +86-10-64416338.

formation stage in dispersion polymerization of styrene, and assumed that PS chains with a polymerization degree of jrc would precipitate out to form nuclei. These nuclei would then aggregate with each other through Brownian diffusion and the shear stress of the fluid until all the particle surfaces were covered with PVP-g-PS copolymer. El-Aasser et al.16 employed dynamic light scattering (DLS) measurements on a methyl methacrylate (MMA)/methanol/PVP-K30 system, and found some very impressive results that small particles (nuclei) with average diameters of 15-20 nm appeared 3-4 min after the start of the reaction and quickly grew to 200 nm within 2 min. This work was the first indirect experimental evidence demonstrating an aggregation of the nuclei in the particle formation stage, but there is still no direct proof of the existence of unstable nuclei. Mandal et al.17 investigated a pyrrole oxidation dispersion polymerization using FeCl3 as the oxidant in pure or 70% ethanol and observed nuclei smaller than 20 nm by transmission electron microscopy after the reaction proceeded for about 30 min; in addition, some of these nuclei were isolable and could coexist with larger particles when the reaction was over. Hu et al.18 used an optical microscope to observe polydisperse nuclei in the nucleation stage of an MMA dispersion polymerization in a hexane/dodecane mixture using a polyhydroxyl-stearic acid graft poly(MMA) (PMMA) copolymer (PHSA) as the stabilizer. In this case, however, the observed nuclei were larger than 1 µm, which was too big to simulate the routine dispersion polymerization. In a previous study,19,20 we have reported on a simple and highly efficient in situ self-stabilized dispersion polymerization system by copolymerization of charged monomer methacryloxyethyltrimethyl ammonium chloride (MATMAC) or sodium styrene sulfonate (NaSS) with St in a methanol/water mixture. For such a system, the composition of the polymerization medium affected not only the monomer partitioning behavior,21 but also the copolymerization reactivity of the charged monomer,

10.1021/jp102936n  2010 American Chemical Society Published on Web 08/10/2010

Three Charge-Stabilized Dispersion Polymerizations SCHEME 1: Chemical Structure of MATMAC

e.g., MATMAC, due to the strong interaction of quarternary ammonium cations with water. This, in turn, seriously influenced the polymerization mechanism and particle growth mode. Moreover, for the St/MATMAC/MeOH/H2O system, the stabilizer was a small monomer and was added in a lower amount, which provides a possibility to directly track the particle nucleation/growth process experimentally, and avoids the effect of the polymeric stabilizer mentioned above in the conventional dispersion polymerization system. The present article describes a further investigation of the effect of the water content on a H2O/MeOH polymerization system with 0 vol % (pure methanol), 5 vol % (low water content), and 20 vol % (high water content) of water respectively. The objective was not only to observe different particle nucleation/growth modes under these three H2O/MeOH ratios, but also to directly observe the formation of several minuscule nanometer-sized particles and sequential metastable aggregates as the nucleation stage proceeded.

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Np )

6 · M 0 · Xm π · Dn3 · Fp

where Dn is the number-average diameter, Xm is the fractional monomer conversion, M0 is the amount of monomer initially charged per cubic centimeter of solvent, and Fp is the density of the polymer. A value of 1.05 g/cm3 was adopted for Fp.22 The molecular weights of the polymers were determined by gel permeation chromatography (GPC) with a Waters 2410 refractive index detector. THF was utilized as the mobile phase at a flow rate of 1.0 mL/min at 35 °C. The samples were dissolved in THF (2 mg/mL) and 100 µL amounts were injected into the chromatograph. The calibration curve was obtained with polystyrene (PS) standards. Results and Discussion Water/Methanol (20/80) System. In order to clarify the nucleation and growth process of the particles, the polymerization process was followed by extracting samples from the reaction mixture at various reaction times. Subsequently, each sample was characterized with regard to conversion, the number average molecular weight, the particle morphology, and the particle diameter, while the number of particles was counted according to the formula in experiment part. The results are presented in Figures 1 and 2, where Figure 1 shows the evolution

Experimental Section Materials. Styrene (Beijing Chemical Reagent Company; 98%) was purified by distillation at reduced pressure. 2,2′Azobisisobutyronitrile (AIBN) (Beijing Fine Chemical Plants; chemically pure), MATMAC (Shanghai Hengyi; 75%, see Scheme 1), methanol (Beijing Modern Eastern Fine Chemical Plants; 99.5%), and tetrahydrofuran (THF; Beijing Modern Eastern Fine Chemical Plants; analytical grade) were used as received. The deionized (DI) water that was used in the chemical reactions was obtained from a Pine-tree water purification system. Polymerization. The dispersion polymerization was carried out in a three-necked 250-mL, round-bottomed flask equipped with a reflux condenser and a mechanical stirrer (200 rpm) in a thermoregulated water bath. For a typical synthesis, 5.0 mL of styrene, 0.0906 g of AIBN initiator, 0.0906 g of MATMAC, 47.5 mL of methanol, and 2.5 mL of water were charged to the flask. Subsequently, the reactants were heated to start the polymerization and were then kept under a gentle reflux (ca. 75 °C). For the kinetic studies, aliquots were sampled from the reactor with a syringe at various reaction times and the polymerization was terminated by adding a drop of hydroquinone solution (0.2‰ g/mL), rendering it possible to monitor the conversions by gravimetry. The polymerization was allowed to proceed for a total of 8 h. Characterization. Scanning electron microscopy (SEM) was carried out with a Hitachi S-4700 field-emission scanning electron microscope (FESEM). A drop of the dispersion sample was placed on a glass substrate, and upon evaporation of methanol, the samples were sputtercoated with gold. Micrographs were acquired at an accelerating voltage of 20.0 kV. The particle size was examined by FESEM. In general, the diameters of 100 particles were used to calculate the average diameter (Dn). Furthermore, the number of particles per cubic centimeter of water (Np) was calculated according to the following formula:

Figure 1. (A) The evolution of the conversion and particle diameter versus the polymerization time, and (B) the number average molecular weight and number of particles versus the polymerization time in a methanol-water mixture with 20 vol % of water. Other reaction parameters: 10 vol % St, 2 wt % MATMAC, and 2 wt % AIBN.

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Figure 2. SEM images at various reaction times of the nucleation and growth of the PS particles in the methanol-water mixture containing 20 vol % of water. Other reaction parameters: 10 vol % St, 2 wt % MATMAC, and 2 wt % AIBN. (a) 5 min, (b) 12 min, (c) 30 min, (d) 60 min, (e) 75 min, (f) 90 min, (g) 120 min, (h) 150 min, (i) 180 min, (j) 240 min, and (k) 480 min.

of the conversion, particle diameter, number average molecular weight, and number of particles versus the polymerization time, while Figure 2 illustrates SEM images of the PS particles at various polymerization times. Among the main results, it was found from the curve of the number of particles versus time that when the reaction had proceeded for 30 min, the number of particles decreased to a constant amount. At this moment, the conversion increased to 6.9% and the particle diameter was 230 nm. Subsequently, the particles started to grow faster, reaching 560 nm, and the conversion steeply increased to 79%, after which it increased very slowly. At this stage, the particle diameter did not change much. The molecular weight curve showed a similar trend to that of the conversion. All of these results were characteristics of a typical dispersion polymerization mode. When combined with the SEM images shown in Figure 2, the particle nucleation and growth process could be discussed in detail, as done in the following text. Nucleation Stage. When the polymerization began, St homopolymerized and copolymerized with MATMAC to form PS and PS-poly(MATMAC) (PMATMAC) or the corresponding chain radicals in solution. When the polymerization had proceeded for 5 min, the transparent solution became slightly turbid, which signified the appearance of nuclei. From Figure 2a, it can be seen that a large number of minuscule particles of a few nanometers (about 6 nm) were formed and that some of them had aggregated into bigger soft particles with a diameter of 95 nm. At this stage, the conversion was 2.2% and the deposited polymer molecular weight was 0.46 × 104 g/mol (Figure 1), while the number of bigger particles was 42.30 × 1014 L-1. From these results, the following points could be made: (1) the molecular weight at which PS and PS-PMATMAC formed

in solution began to precipitate out, namely, the critical molecular weight, was low, (2) big amount of minuscule particles were formed, (3) the minuscule particles could aggregate to form bigger particles. Twelve minutes after the start of the reaction, the dispersion became light white, a similar SEM image (Figure 2b) was obtained and the conversion increased to 2.9%. As compared to Figure 2a, a larger number of aggregated particles that seemed more rigid and of bigger size (approximately 107 nm) were observed. These results suggested that a metastable polymerization-association-aggregation process was ongoing. It is worth mentioning an interesting phenomenon here. When the sampled dispersion was centrifuged, the solution became transparent, and no solid could be obtained. This indicated that the nascent aggregated particles were not solid enough and would break into small particles at a high centrifugation rate, thereby dissolved in the reaction solution. When the reaction had proceeded for 30 min, the conversion increased to 6.9%, and spherical particles with a diameter of 230 nm (Figure 2c) could be observed. Almost all of the minuscule particles and small nuclei had disappeared, which implied that the metastable aggregation process had come to an end. The particle number decreased to 9.87 × 1014 L-1, and did not change much in the later stages of the polymerization, which indicated that the nucleation step was over. The number average molecular weight of the particles was increased to 0.74 × 104 g/mol, signifying that a certain amount of higher molecular weight polymer, i.e., over 1.00 × 104 g/mol, had appeared. It was clear from Figure 2c that spherical particles with smooth surfaces were formed at this moment. The basic driving forces for the formation of the smoothsurface spherical particles would be attributed to the soft St-

Three Charge-Stabilized Dispersion Polymerizations swollen higher molecular weight polymer chains. We here tentatively proposed a mechanistic explanation: the polymer chains of higher molecular weight had the ability to carry/absorb St monomer, thereby becoming St-solvated soft polymer coils. These coils would prefer to first stick to the “boundary” area of the coagulated particles, where the surface curvature changed abruptly. In other words, the St-swollen soft polymer coils tended to fill the “gap” between the particles that just aggregated with each other by preference. Therefore, particles with smooth surface were formed. Growth Stage. Half an hour after the start of the reaction, a serious gel effect could be observed (cf. Figure 1). The conversion steeply increased to 79% during the next 90 min, the particles grew from 230 to 560 nm, and the number average molecular weight also increased very fast, from 0.74 × 104 to 3.00 × 104 g/mol. Accompanying the gel effect was the phenomenon that particles were liable to become distorted by pressing against each other, as seen from Figure 2c,d. It can be explained in the following way. In the polymerization system with a large portion of water, the monomer partitioning coefficient was high, as reported by Guillot,21 and therefore they could be easily swollen by the absorbed St monomer, which resulted in the distortion of the particles due to the fact that they were not solid enough at this moment. During this stage, the particles grew by absorbing St, oligomer radicals, and AIBN from the solution, and the small particles grew faster because of their high surface area.3,10,23 MATMAC was water-soluble and difficult to penetrate into the interior of the particles, and thus mainly remained in the solution phase to copolymerize. After the reaction had proceeded for 2 h, the increase in particle diameter became very slow and no obvious changes could be seen from the SEM images. This was due to the less remaining monomer and initiator as the reaction proceeded, which led to the slow increase in conversion and molecular weights at this stage. When the reaction was terminated after 8 h, smooth particles with diameters of 620 nm were obtained. Nucleation and Growth Mode. On the basis of the experimental results and the phenomenon observed above, a plausible particle nucleation process is schematically shown in Figure 3. In polymerization systems with high water contents, the interaction between MATMAC and water was much stronger, and thus MATMAC showed lower copolymerization ability and mainly stayed in the solution phase. In the early period of the reaction, the majority of the product was St homopolymer with only small amounts of PS-PMATMAC copolymer. These homopolymer chains self-associated to form several minuscule nanometer-sized particles, while MATMAC-hydrated groups in the PS-PMATMAC copolymer partly stabilized these particles (Figure 3a). Meanwhile, the minuscule particles tended to coagulate with each other to form loosely aggregated particles of about 100 nm in diameter (Figure 3b). However, the coupling termination of the charged microradicals was believed to be difficult because of the charge repulsion, which had been studied by Kabanov et al.24 in the copolymerization of MMA and acrylic acid (AA) in aqueous solution. They found that the molecular weight of the polymer increased with the increasing of the solution pH, and they ascribed it to the electrostatic repulsion between the already charged macroradicals. So in the methanol/water solvent with high polarity, a similar phenomenon was supposed to exist. After MATMAC was copolymerized with St, the copolymer chain radicals had some difficulties in carrying out the coupling termination

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Figure 3. A schematic of the dispersion polymerization mode for a system with 20 vol % water.

Figure 4. (A) The evolution of the conversion and particle diameter versus the polymerization time, and (B) the number average molecular weight and the number of particles versus the polymerization time in a pure methanol system. Other reaction parameters: 10 vol % St, 2 wt % MATMAC, and 2 wt % AIBN.

because of the charge repulsion, which increased the life of the oligomer radicals, and thus the molecular weight. This effect would become stronger as the copolymerization proceeded. When the copolymer molecular weight reached a certain value, e.g., 1.00 × 104 g/mol, the coiled copolymer chains in the solution were able to carry/absorb St (oil soluble composition in the system) and adhere to the aggregates to form smooth/ spherical particles (cf. Figure 3c). At this point, the nucleation stage came to an end. Subsequently, the polymerization location transferred to the particle phase, and the particles grew by absorbing monomer, oligomer radicals, and initiator into the interior of the particles. Consequently, the polymerization rate was very fast and the molecular weight of the polymer increased continuously.

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Figure 5. SEM images at various reaction times of the nucleation and growth of the PS particles during polymerization in a pure methanol system. Other reaction parameters: 10 vol % St, 2 wt % MATMAC, and 2 wt % AIBN. (a) 12 min, (b) 22 min, (c) 30 min, (d) 60 min, (e) 90 min, (f) 120 min, (g) 180 min, (h) 240 min, (i) 360 min, (j) 480 min.

MATMAC was preferable to stay in the solution phase, and the ionized charges on the copolymer mainly remained on the particle surface. Thus, the efficiency of MATMAC in the polymerization system was exceptionally high, as reported in a previous paper;19 0.025 wt % was enough to prepare a stable latex with monodisperse PS particles. Pure Methanol System. For a pure MeOH system, when no MATMAC was added, the reaction could not proceed normally, and after polymerization for 8 h, which meant that AIBN had been almost consumed up, a final conversion of only 20% was reached, and most of the polymer adhered to the wall of the flask. The number average molecular weight of the PS polymer existing in the solution phase was 0.21 × 104 g/mol, while the polymer adhering to the flask was 0.52 × 104 g/mol. These results implied that the termination of the primary radicals and the short chain radicals were predominant in this solution polymerization, and the pure PS chains or PS particles were unable to stably remain in the solution. When a certain amount of MATMAC, e.g., 2 wt %, was added to the reaction system, the polymerization could proceed normally, and stable latex was obtained after polymerization for 8 h. The final conversion was in this case 74%. By using the same method as used in the former system, the evolution of the conversion, particle diameter, number of particles, number average molecular weight, and particle morphology versus the polymerization time was followed, and the main results are given in Figures 4 and 5. It was found that this polymerization system had different characteristics, compared to the former one. When it came to the polymerization rate, and the molecular weight of the formed polymer, the polymerization could be divided into two stages: during the first 240 min, both the polymerization rate and the molecular weight

of the formed polymer increased slowly, while after 240 min, the increase was much faster. Regarding the particle growth, aggregated particles consisting of minuscule particles of several nanometers could be observed after a reaction time of 12 min, and the particle size increased very rapidly, particularly during the first 30 min, indicating that the particles grew mainly by aggregation. At a reaction time of 180 min, some small particles began to appear, signifying the start of a secondary nucleation. Nevertheless, the bigger particles continued to grow, and their surface became more and more smooth. The polymerization process will be discussed in detail in the following section. First Nucleation and Growth. After the polymerization had proceeded for 12 min, the system was slightly white with a conversion of 2.6%. The precipitated polymer had a number average molecular weight of 0.41 × 104 g/mol (Figure 4), which was higher than the 0.21 × 104 g/mol in the system without MATMAC. This was because the termination became difficult when electrical charges were present on the polymer chains, which has been explained in the above paragraphs. From the SEM image in Figure 5a, a large number of minuscule particles (shown in the inset), several nanometers in size, and some bigger particles about 65 nm in diameter (the number of large particles was 1.49 × 1016 L-1 according to Figure 4) were discernible. It was obvious that these minuscule particles came from the association of the precipitated polymer with a number average molecular weight of 0.41 × 104 g/mol, and that the relatively large particles came from the aggregation of their minuscule counterparts. Ten minutes later, with a conversion of 3.4%, the 65-nm particles had coalesced to form larger ones (535 nm), as shown in Figure 5b. Compared to the above system (20/80), the separated larger particles seemed not too loose, as shown

Three Charge-Stabilized Dispersion Polymerizations

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Figure 6. A schematic of the polymerization mode of a system with pure methanol.

in the inset in Figure 5b, and presumptively, they consisted of the 65-nm particles and polymers formed in solution. Meanwhile, the number of particles decreased drastically to 0.36 × 1014 L-1. When the reaction had proceeded for 30 min, the SEM image displayed round particles with 65-nm particles attached to their surface (Figure 5c). Furthermore, the particle diameter increased to 750 nm and the number of particles decreased to 0.15 × 1014 L-1 (Figure 4), while the conversion only increased to 3.9% and the number average molecular weight was merely 0.43 × 104 g/mol. All of these results indicated that a considerable particle aggregation had occurred during the first 30 min of the polymerization, and that the particles grew mainly through the coagulation of smaller ones (about 65 nm in size). Particles of several nanometers in size aggregated into 65 nm-sized particles, and then further aggregated into larger ones, as seen from the polymerization mode in Figure 6a,b. The significant coagulation could be attributed to the nearly absence of water in the polymerization system, and thus less occurrence of ionization of the quarternary ammonium cations in methanol, causing the repulsion force among the nuclei to be weak. This, in turn, led to the fast aggregation of the nuclei. Then the number of particles did not decrease and the nucleation stage came to an end. Following this, the particles entered the growth stage. From 30 to 120 min after the start of the reaction, as shown in Figure 4, the particle diameter increased from 750 to 1150 nm and the particle surfaces attached with 65-nm particles remained observable, as illustrated in the magnified images in Figure 5d-f. So it was considered that the particles grew mainly by adsorbing the unstable particles and the polymers formed in the solution, and the schematic of the polymerization mode was shown in Figure 6c. Secondary Nucleation and Growth. When the reaction had proceeded for 180 min, small particles began to appear, as seen in Figure 5g, which indicated the start of secondary nucleation. Simultaneously, the bigger particles continued to grow as the reaction went on. After having proceeded for 240 min, corresponding to a 22% conversion and a number average molecular weight 0.83 × 104 g/mol, a significant turning point appeared: more small particles could be observed and both the conversion and the molecular weight curves began to increase quickly, as seen in Figures 4 and 5. At the same time, the larger particles continued to grow from 1600 nm at 180 min to 2400 nm at 480 min, while the small particles grew from 360 to 900 nm during this 300-min period. The number of particles in the system increased to 1.04 × 1014 L-1 from the initial 0.15 × 1014 L-1. Moreover, an interesting phenomenon could be observed: after the turning point, both big and small particles showed quite smooth surfaces. The polymerization process could be interpreted as followed: (1) As compared with the above polymerization system or the

Figure 7. (A) The evolution of the conversion and particle diameter versus the polymerization time, and (B) the number average molecular weight and number of particles versus the polymerization time in a methanol-water mixture with 5 vol % of water. Other reaction parameters: 10 vol % St, 2 wt % MATMAC, and 2 wt % AIBN.

conventional dispersion polymerization, where the number of particles was nearly 10 × 1014 L-1, the corresponding amount after the first nucleation stage in this system was much lower (merely 0.15 × 1014 L-1) due to the serious aggregation of the nascent particles. Therefore, as the reaction proceeded, more polymer chains were produced in the solution, and the particle surface area was insufficient to adsorb a large number of polymer chains. (2) As the polymerization proceeded, the ratio of MATMAC/St in the solution increased continuously so that the molecular weight of the PS-PMATMAC copolymer became increasingly large due to the above-mentioned repulsion of the electrical charges, which reduced the coupling termination of the copolymer chain radicals. When the number average molecular weight was over 1.00 × 104 g/mol, the polymer chains could carry/absorb enough St to form monomer-swollen polymer chains, which were not easily adsorbed by the large particles, as mentioned previously. Instead, these chains could easily associate with each other, thus precipitating out to form new nuclei. The formed nuclei grew by absorbing monomer, oligomer radicals and initiator to their interior. In other words, the polymerization location moved to the inside of these new particles, resulting in the fast increase of the polymerization rate as well as the molecular weight. At the same time, some of the swollen polymer chains could be adsorbed by the large particles, and the soft monomer-swollen polymer chains had

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Figure 8. SEM images at various reaction times of the nucleation and growth of the PS particles during the polymerization in a methanol-water mixture containing 5 vol % of water. Other reaction parameters: 10 vol % St, 2 wt % MATMAC, and 2 wt % AIBN. (a) 7 min, (b) 12 min, (c) 30 min, (d) 60 min, (e) 75 min, (f) 90 min, (g) 120 min, (h) 150 min, (i) 180 min, (j) 240 min, (k) 480 min.

enough time to exclude the solvent and “patch” the uneven sites on the particle surfaces, thereby leading to the formation of smooth particle surfaces. Water/Methanol (5/95) System. In order to investigate a polymerization system with water content between the two already studied, i.e., between those with 20 vol % water and pure methanol, a system comprised of 5 vol % water was prepared. With such a solution mixture, a very special dispersion polymerization system was created, as shown in Figures 7 and 8. On the basis of the figures, the following features could be observed: (1) the nucleation time was shorter, i.e., only 12 min; (2) the polymerization rate was moderate, and the reaction was nearly complete in 3 h; (3) the molecular weight of the precipitated out polymer was relatively high (0.71 × 104 g/mol) and increased linearly to 8.96 × 104 g/mol in 90 min; (4) the particle diameter increased steadily, and its distribution gradually became uniform; and (5) interestingly, the particle surface became increasingly coarse as the polymerization proceeded. From the SEM images taken in the nucleation stage (Figure 8a,b), no metastable minuscule particles or their aggregates could be observed, as had been the case in the former two systems. When the reaction had proceeded for only 7 min (1.2% conversion), sheet-like particles (96 nm and their aggregates 2or 3-fold larger) could be seen. At this stage, the number of particles was 21.70 × 1014 L-1. When the polymerization had proceeded for 12 min, the conversion increased to 2.0%, and spherical particles with a diameter of 260 nm were observed. The particle number decreased drastically to 1.98 × 1014 L-1 and did not change much afterward, as seen in Figure 7. This indicated that the nucleation stage was over. Subsequently, the reaction proceeded as the conventional dispersion polymerization. The key point for the present system was the formation of the deposited copolymer of relatively high molecular weight

(0.71 × 104 g/mol). The system contained only small amounts of water, which would mainly exist in the form of partly hydrated quarternary ammonium cations, and would not have much significant effect on the polarity of the polymerization medium. The partly hydrated quarternary ammonium cations would endow MATMAC with a moderate copolymerization activity, between that observed in 20 vol % water and pure methanol, resulting in, on the one hand, an increase of the copolymer molecular weight due to the repulsion between the copolymer chain radicals, as mentioned before, and on the other hand, an increase of the solubility of the copolymer in the solution due to more MATMAC units present in the copolymer as well as the lower solvent polarity (as compared with 20 vol % water system). These led to the molecular weight of the critical polymer chains being significantly higher than that in the system with 20 vol % water or pure methanol. As has been pointed out in the discussion of the two former systems, these long copolymer chains could carry/absorb monomer to form soft polymer coils, and meanwhile be absorbed by the nuclei to form larger and smooth particles, as depicted in Figure 9a. Moreover, when the amount of MATMAC groups on the particle surface became large enough, the particles would become stable, and thus the nucleation stage would be terminated. After the nucleation stage, the polymerization proceeded in the particle phase by absorbing monomer, AIBN and oligomers, as illustrated in Figure 9b. Since the system only contained small amounts of water, the ratio and rate of the St/MATMAC monomer and the AIBN initiator entering into the interior of the particles would be significantly lower than those of the 20 vol % water system. The conversion increased to 45%, and the number average molecular weight of the polymer linearly increased to 8.96 × 104 g/mol during the following 80 min, while the particle diameter gradually increased from 260 to 840

Three Charge-Stabilized Dispersion Polymerizations

Figure 9. A schematic of the dispersion polymerization mode in a system with 5 vol % water.

nm. The latter indicated that we were dealing with a conventional dispersion polymerization but with an insignificant gel effect. At the same time, the polymerization in the solution phase also proceeded, and because of the charge expulsion, it became increasingly difficult for the MATMAC monomer to enter the particles, therefore the ratio of St to MATMAC in solution became smaller. This would inevitably result in more MATMAC copolymerizing onto the polymer chains, consequently rendering it more and more difficult for the copolymer chains or chain radicals to go inside the particles. In addition, when the nucleation stage was over, the particle number was 1.98 × 1014 L-1, which was much larger than that of the pure methanol system (0.15 × 1014 L-1), so the particle surface was enough to adsorb the polymer chains formed in the solution phase. Therefore, after a certain polymerization time, i.e., 90 min, an epitaxial growth mode (shown in Figure 9c) was believed to occur. This mode constituted to grow by absorption of copolymer chains or chain radicals formed in the solution phase, corresponding to a particle surface polymerization. Direct evidence to support the hypothesis of such a polymerization mode was the appearance of coarse particle surfaces after 90 min, as displayed in Figure 8f. Moreover, as the polymerization proceeded, the roughness of the particle surface became more and more significant. Capek et al.25 proposed that a third polymerization locus, viz., particle surface layer, might exist when using a methacryloyl-terminated poly(ethylene oxide) macromonomer as the stabilizer in polar media, but they did not provide any evidence. Nevertheless, that hypothesis corresponded well with what was observed in the present dispersion polymerization system containing a small amount of water. Conclusions In this novel cation-charge-stabilized dispersion polymerization system, the development of the polymerization process, especially during the nucleation stage, and the locus of the particle propagation were found to change significantly with varying water/methanol ratios: (1) The polymerization in water/methanol (20/80) took place like a typical dispersion polymerization, but during the nucleation stage, in solution, PS/PS-PMATMAC associated into minuscule particles (about 6 nm in size), which then aggregated into 95-nm loose bigger particles. Subsequently, with the help of the high molecular weight of PS-PMATMAC, which could carry/absorb the St monomer, stable and spherical nuclei particles were formed. (2) In the pure MeOH system, the PS/PS-PMATMAC polymers formed in methanol, via the minuscule particles as the intermediate step, first coagulated to form particles of about 65 nm, and then quickly aggregated into bigger particles and grew by absorbing the 65-nm-sized particles and polymers

J. Phys. Chem. B, Vol. 114, No. 34, 2010 10977 formed in the solution phase. When the PS-PMATMAC produced in solution reached a high molecular weight, i.e., 0.83 × 104 g/mol, the secondary nucleation occurred, and the growth of the new particles took place in the particle phase. (3) In the water/methanol (5/95) system, a very nice dispersion polymerization with a moderate polymerization rate, high molecular weight, as well as uniform and size-controllable spherical particles could be obtained. The reason behind this polymerization behavior was the formation of high molecular weight PS-PMATMAC, acted as a polymeric stabilizer, during early stages of the reaction. After 90 min, along with a decrease in monomer concentration inside the particles, the solution phase gradually became the main polymerization location, giving rise to an epitaxial growth mode. In the systems with pure methanol and water/methanol (20/ 80), we not only observed primitive minuscule particles produced in solution but also witnessed their intermediate aggregation. To the best of our knowledge, this is the first time the nucleation process has been directly and experimentally observed. Undoubtedly, the obtained results will be very helpful to further understand mechanistic aspects of dispersion polymerization as well as for the development of new dispersion polymerization systems to fabricate particles of varying functions and shapes. Acknowledgment. We thank the Programme of Introducing Talents of Discipline to Universities (B08003), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) (IRT 0706) and BMEC (XK100100640) for financial support of this work. References and Notes (1) Kawaguchi, S.; Winnik, M. A. Dispersion Copolymerization of n-Butyl Methacrylate with Poly(ethylene oxide) Macromonomers in Methanol-Water. Macromolecules 1995, 28, 1159–1166. (2) Lok, K. P.; Ober, C. K. Particle Size Control in Dispersion Polymerization of Polystyrene. Can. J. Chem. 1985, 63, 209–216. (3) Pain, A. J.; Luymes, W.; McNulty, J. Dispersion Polymerization of Styrene in Polar Solvents. 6. Influence of Reaction Parameters on Particle Size and Molecular Weight in Poly(N-vinylpyrrolidone)-Stabilized Reactions. Macromolecules 1990, 23, 3104–3109. (4) Shen, S.; Sudol, E. D.; El-Aasser, M. S. Control of Particle Size in Dispersion Polymerization of Methyl Methacrylate. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 1393–1402. (5) Lacroix-Desmazes, P.; Guyot, A. Reactive Surfactants in Heterophase Polymerization. 2. Maleate Based Poly(ethylene oxide) Macromonomers as Steric Stabilizer Precursors in the Dispersion Polymerization of Styrene in Ethanol-Water Media. Macromolecules 1996, 29, 4508–4515. (6) Song, J. S.; Tronc, F.; Winnik, M. A. Two-Stage Dispersion Polymerization toward Monodisperse, Controlled Micrometer-Sized Copolymer Particles. J. Am. Chem. Soc. 2004, 126, 6562–6563. (7) Song, J. S.; Chagal, L.; Winnik, M. A. Monodisperse MicrometerSize Carboxyl-Functionalized Polystyrene Particles Obtained by Two-Stage Dispersion Polymerization. Macromolecules 2006, 39, 5729–5737. (8) Barrett, K. E. J.; Thomas, H. R. Kinetics of Dispersion Polymerization of Soluble Monomers. I. Methyl Methacrylate. J. Polym. Sci. Part A: Polym. Chem. 1969, 7, 2621–2650. (9) Tseng, C. M.; Lu, Y. Y.; El-Aasser, M. S.; Vanderhoff, J. W. Uniform Polymer Particles by Dispersion Polymerization in Alcohol. J. Polym. Sci., Part A 1988, 24, 2995–3007. (10) Paine, A. J. Dispersion Polymerization of Styrene in Polar Solvents. 7. A Simple Mechanistic Model to Predict Particle Size. Macromolecules 1990, 23, 3109–3117. (11) Yasuda, M.; Seki, H.; Yokoyama, H.; Ogino, H.; Ishimi, K.; Ishikawa, H. Simulation of a Particle Formation Stage in the Dispersion Polymerization of Styrene. Macromolecules 2001, 34, 3261–3270. (12) Song, J. S.; Winnik, M. A. Cross-Linked, Monodisperse, MicronSized Polystyrene Particles by Two-Stage Dispersion Polymerization. Macromolecules 2005, 38, 8300–8307. (13) Thomson, B.; Rudin, A.; Lajoie, G. Dispersion Copolymerization of Styrene and Divinylbenzene. II. Effect of Crosslinker on Particle Morphology. J. Appl. Polym. Sci. 1996, 59, 2009–2028.

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