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pH Effects on the Synthesis of Nanocapsules via Interfacial Miniemulsion Polymerization Mediated by Amphiphilic RAFT Agent with the R Group of Poly(methyl acrylic acid-ran-styrene) Fujun Lu, Yingwu Luo,* and Bogeng Li Department of Chemical and Bioengineering, The State Key Laboratory of Chemical Engineering, Zhejiang UniVersity, Hangzhou 310027, China
Interfacial radical miniemulsion polymerization was constructed to be a general process for nanoencapsulation and the preparation of nanocapsules by using an amphiphilic reversible addition-fragmentation transfer (RAFT) agent with the R group of poly(methyl acrylic acid-co-styrene) as a reactive surfactant. Styrene and n-nonadecane were used as a model of monomer and core materials, respectively. It was found that the fragmental R radicals, which exited out of the droplets into the water phase, could trigger the homogeneous nucleation. When the homogeneous nucleation rather than the monomer droplet nucleation became the dominant mechanism of the particle formation, a large fraction of solid particles would be coformed with the nanocapsules. By decreasing pH values, we could decrease the hydrophilicity of R radicals and thus suppress the formation of the solid particles from the homogeneous nucleation. For the current model system, when values were decreased from pH 8.12 to 6.45, the fraction of nanocapsules linearly increased from 18 to 94%. In the middle pH values very uniform nanocapsules were obtained accompanying a few of the solid particles. Introduction Nanoencapsulation of functional substances like catalyst, dye, and drug within a polymeric shell hold promise in a large variety of application fields.1-3 On the other hand, the hollow polymeric nanocapsules, an emerging important advanced material, could conveniently be prepared by removing liquid core materials when a liquid material was encapsulated into a polymeric shell. Miniemulsion polymerization could be well-suited for nanoencapsulation or preparation of polymeric nanocapsules, considering that the polymeric nanoparticles are directly transformed from the nanodroplets of monomer dispersed in water by initiating the polymerization in a highly efficient and “green” way. Encapsulation of inorganic solids or carbon black by miniemulsion polymerization was reported.4-6 Miniemulsion polymerization has also been extensively explored to encapsulate small liquid organic molecules.7,8 It turned out that a welldefined core-shell structure was often difficult to obtain due to the strict thermodynamics and kinetic requirements by the regular miniemulsion polymerization.9 Reversible addition-fragmentation transfer (RAFT) polymerization both in the homogeneous polymerization systems, such as bulk and solution polymerization, and in the heterogeneous systems, such as miniemulsion polymerization, has been attracted tremendous attention in the past decade.10-12 RAFT polymerization is a kind of radical polymerization mediated by a reversible chain transfer agent, which is called a RAFT agent. The RAFT agent has a general structure of SdC(Z)sSR, where R is a leaving and reinitiating group and Z is an activating group. As one of three major approaches of controlled living radical polymerization, RAFT polymerization allows us to synthesize the polymer with a preset molecular weight and narrow molecular weight distribution, and to complex chain structures such as block and gradient copolymers.13-15 Very recently, an interfacially confined polymerization system was constructed by using an amphiphilic RAFT agent as a reactive surfactant
in the miniemulsion polymerization.16,17 Such a novel technique demonstrated an excellent process to synthesize nanocapsules.16,17 The amphiphilic RAFT agent molecules (for example the ammonolyzed styrene/maleic acid adduct of phenyldithioacetate,
denoted SMA-RAFT) would self-assemble on the interface between water and nanodroplets of miniemulsion due to their amphiphilic nature. Once a nanodroplet or particle has a radical, the radical would quickly transfer to the amphiphlic RAFT agent molecules anchored on the interface via the highly reactive RAFT reactions, as demonstrated in Scheme 1. A dormant chain
and a radical from the original R group (R radical) are formed. The dormant chain, a RAFT agent with a different leaving/ reinitiating group, could be reactivated and propagate with monomer later. The R radical would remain on the interface due to its amphiphilicity and propagate with monomer, transfer to other RAFT agent molecules and dormant chains on the interface. In such a way, the radicals would mainly stay on the interface during polymerization. The polymer chains, most of Scheme 1. RAFT Reactions
* To whom correspondence should be addressed. Fax: 86-57187951612. E-mail:
[email protected]. 10.1021/ie901515t 2010 American Chemical Society Published on Web 01/19/2010
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Scheme 2. Synthesis and Structure of the Random Amphi-RAFT
which derived from the original R groups, grow inward gradually to form a polymeric shell during hours of polymerization in a living manner and leave the core material in the core. In principle, this methodology of interfacially confined polymerization would offer a robust route for nanoencapsulation, which allows a flexible design in nanocapsule size, shell thickness, polymeric shell properties, and the functionality of the surface. Also, the process is green, highly efficient, and easy to scale up. This interfacially confined miniemulsion polymerization has been demonstrated by using SMA-RAFT.18 The SMA-RAFT was insoluble in water. Thus, the SMA-RAFT agent had to dissolve in the oil phase (styrene and hexadecane) and then transformed to be amphiphilic by amonalysis during miniemulsification. It is very desirable that the amphiphilic RAFT agent is able to dissolve in water at the beginning, just like most of the surfactant used in the miniemulsion polymerization, so that we could have more flexibility on the oil-phase compositions. For this end, poly(styrene-b-acrylic acid) has been designed to be the R group of the amphiphilic RAFT agent.19 The capsules with uniform size could be synthesized. In these previous studies, the well-defined nanocapsules were often accompanied with more or less solid particles. The mechanism has not been clear yet. We hypothesize that the formation of the solid particles should be related to the second nucleation. In the current paper, we would like to test this hypothesis and explore a simple method to remove the solid particles. The R group of the amphiphilic RAFT agent (amphi-RAFT) was designed to be a random copolymer of styrene/methyl acrylic acid rather than a block copolymer, considering that (i) the amphiphilic RAFT agent of the random copolymer was easily synthesized in one pot, (ii) the hydrophilicity of the R groups could be finely tuned by pH values, and (iii) the amphiphilic random copolymer radicals derived from the R group might be bound on the interface by numerous hydrophobic segments at the same time, which might prevent them from desorption off the minidroplets or particles during the polymerization. Experimental and Characterization Material. Methacrylic acid (MAA, AR) and styrene (St, AR) were dried via activated molecular sieve and distilled under reduced pressure to remove the inhibitor before use. Azobis(isobutyronitrile) (AIBN) was recrystallized twice from anhydrous ethanol. 4-Cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid (CDPA), as the original RAFT agent to prepare the amphi-RAFT, was synthesized according to the literature.20 4,4′-Azobis(4-cyanopentanoic acid) (V-501, 98%) was purchased from Aldrich. Sodium hydroxide (NaOH, AR), dioxane (AR), ethyl ether (AR), and n-nonadecane (ND) were received as AR grade and were used without further purification. Synthesis of the Amphi-RAFT. The amphi-RAFT was synthesized via RAFT solution polymerization mediated by 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl pentanoic acid (CDPA). CDPA (1.5 g, 3.7 mmol) and V-501 (0.1 g, 0.37 mmol, initiator) were dissolved in dioxane (15 g, solvent) and then mixed with purified MAA (6.4, 74.4 mmol) and St (3.9 g, 37.2 mmol). The solution was transferred into a three-necked round
flask and stirred magnetically with highly pure nitrogen bubbling for 30 min to remove any dissolved oxygen. The polymerization lasted 6 h at 80 °C. A drop of reaction mixture was extracted to determine the final conversion of about 70% by gravimetry. The amphi-RAFT was collected by precipitating twice from icy ethyl ether and dried in a vacuum oven at 30 °C. Characterization of the Amphi-RAFT. The initial molar ratio of MAA/St was set to be 2:1 in the synthesis of amphiRAFT. As estimated from 1H NMR, the average DPn,MAA of MAA was 16 and DPn,St of St was 7 for the collected copolymer, in good agreement with theoretical prediction by theoretical equation (DPn,St ) ([Mtotal]0x/[RAFT]0)FSt, DPn,MAA ) ([Mtotal]0x/ [RAFT]0)FMAA, where DPn,St and DPn,MAA are the degrees of polymerization of St and MAA, [Mtotal]0 is the initial total monomer concentration, [RAFT]0 is the initial concentration of the RAFT agent, and x is the total monomer conversion). The copolymer composition estimated from 1H NMR was FSt ) 0.31, in excellent agreement with that predicted by the Mayor-Lewis equation with the reactivity ratios of rMAA ) 0.66 and rSt ) 0.2.21 The compositional drifting could be negligible before 70% conversion, as predicted by the Mayo-Lewis equation [F1 ) (r1f12 + f1f2)/(r1f12 + 2f1f2 + r2f22)].22 The molecular weight of the Amphi-RAFT was 1260 Da, and PDI (polydispersity index) was 1.18 by GPC (Gel Permeation Chromatograph). Since the molecular weight of GPC was relative to polystyrene, some errors were reasonable. The structure of the amphi-RAFT is shown in Scheme 2. Miniemulsion Polymerization. Miniemulsions were prepared at various pH values. The amphi-RAFT (0.6 g) was dissolved in pure water (80 g) and fully neutralized with sodium hydroxide (0.11 g, equimolar to MAA units of the amphi-RAFT). Then the pH value of the amphi-RAFT aqueous solution was tuned with the diluted vitriol aqueous solution (0.1 mol/L) to the given value. The oil phase of AIBN (0.05 g, initiator), purified St (16 g), and ND (4 g, core material) was added into the amphi-RAFT aqueous solution, and the mixture was stirred magnetically for 30 min to form a coarse emulsion. The coarse emulsion was then subject to sonication using an ultrasonic processor (JOYII) at a power output of 600 W for a period of 10 min to obtain the miniemulsion. During the sonication, the emulsion was bathed in 0 °C water to prevent polymerization. The prepared miniemulsion was transferred to a four-neck 250 mL roundbottom flask with a mechanical stirrer, a reflux cooler, a rubber plug, and a N2 inlet. The polymerization started by placing the flask in a 68 °C water bath after having purged high-purity N2 for 10 min. During the polymerization, N2 was slowly purged through the flask. Samples were taken periodically and quenched by adding hydroquinone solution to determine the conversion by gravimetry, molecular weight by GPC, and the particle size by dynamic light scattering (DLS). The polymerization lasted for about 8 h, and the final latex was collected. pH Value Measurement. The pH values of the amphi-RAFT aqueous solution were determined by a pH meter (LEICI PHS2C) equipped with the electrode E-201-C. 1 H NMR Spectrometry. The composition of the amphiRAFT was determined by an Avance 400 MHz NMR spectrometer with DMSO-d6 as the solvent.
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a m b p,f
d (nm) d (nm) Nd × 10-17c (L-1) Np × 10-17c (L-1) Np/Nd
pH 8.12
pH 6.92
pH 6.75
pH 6.45
123.8 54.2 2.28 24.4 10.7
101.1 109 4.20 3.0 0.72
104.2 116 3.84 2.46 0.64
151.9 146 1.24 1.25 1.0
a The initial droplet size was determined by DLS. b The particle size (on volume average) was counted from the TEM images. c The number of droplets and final particles in per liter aqueous are calculated using the equations Nd ) 6(MSt + MND)/(πdm,i3Fm,i) and Np ) 6(Mp + MND)/ (πdm,f3Fm,f), where Nd is the number of droplets and Np is the number of final particles. [In these equations, MSt is the concentration of styrene per liter of water (gSt/L), Mp is the concentration of polystyrene per liter of water (gpolystyrene/L), MND is the concentration of ND per liter of water (gND/L), dm,i is the initial droplet size by volume, dp,f is the final particle size, Fm,i is the initial mixing density of the styrene and ND, and Fp,f is the final mixing density of polystyrene and ND.]
Figure 1. Comparison of droplet size distribution with particle size distribution at various monomer conversions.
Monomer Conversion. The samples taken out during the polymerization were dried in a vacuum oven at 180 °C to remove water, ND, and residual monomers to collect the polymer. The monomer conversion was calculated by gravimetry. GPC Analysis. Molecular weight and distribution of the polymer were characterized by Waters 1525/2414/2487 GPC with Waters styragel columns HR 5, 4, smf 3. For the amphiRAFT, Waters styragel columns HR 4, 3, and 1 were used. Tetrahydrofuran (THF) was used as the eluent. A flow rate was 1 mL min-1. Molar masses were calculated by using the calibration curve based on polystyrene standards (molar mass range, 100-710000 g/mol for columns HR 4, 3, and 1 and 500-4000000 g/mol for columns HR 5, 4, and 3). Particle Size and Distribution. The initial droplet size and particle size during the polymerization process were measured by the DLS with a Malvern ZETASIZER 3000 at a fixed scattering angle of 90°. The miniemulsion samples were diluted to 0.5 wt % by using deionized water with 0.5 wt % sodium dodecyl sulfate (SDS). The particle size and size distribution were obtained by averaging the results of 10 measurements. TEM Observations. Transmission electron microscopy (JEOL JEMACRO-1230) was used to observe the morphology of the particles at the operating voltage of 80 kv. The diluted collected latex was mounted onto 400-mesh carbon-coated copper grids and dried at room temperature overnight. More than 500 particles were counted to measure the fraction of nanocapsule by number and weight. Results and Discussion The ionization degree and the hydrophilicity of the amphiRAFT could be largely tuned by pH values, which is able to offer some clues to the mechanism of solid particle formation. A series of experiments at various pH values were carried out. The results are presented in the following. Evolution of Droplet/Particle Size Distributions. The droplet size distribution before polymerization and the particle size distributions at various monomer conversions were monitored by DLS. The results are presented in Figure 1. The droplet size and particle size of the final latex on volume average are summarized in Table 1. At pH ) 8.12 and 6.45, the droplet size distributions are broader than those at pH ) 6.92 and 6.75, as indicated by the appearance of a minor peak of 300-500 nm. Accordingly, the average droplet sizes at pH ) 8.12 and
6.45 are larger than those at pH ) 6.92 and 6.75. It is likely that the ionization degree was too low so that some coalescence of droplets occurred in the case of pH ) 6.45. The reason for the larger droplet size at pH ) 8.12 is not clear yet, possibly due to the badly matched HLB (hydrophilic-lipophilic balance) value. In an ideal miniemulsion polymerization, the monomer droplet nucleation should be the dominant nucleation mode. In such a case, the particle size distribution should be a copy of the droplet size distribution, if we ignore the shrinkage caused by polymerization. As seen in Figure 1, the particle size distribution curve dramatically moves to the smaller size in the early stage of the polymerization at pH ) 8.12, indicating that a large number of particles were actually formed by the second nucleation. With a decrease of pH values to 6.92 and 6.75, the particle size distribution becomes more and more similar to the droplet size distribution, suggesting that the majority of particles were nucleated from monomer droplets as desired. In the case of pH ) 6.45, the particle size distribution becomes pronouncedly bimodal. DLS is not suitable for measuring the broad particle size distribution. Therefore, the particle size distributions at pH ) 6.45 could not be exactly true. From the droplet size and the particle size of the final latex, the number of the droplets and the particles are estimated and listed in Table 1. It is obvious that most of the particles are actually formed from the second nucleation at pH ) 8.12 since the ratio of the number of particles to that of droplets is much larger than the unit. In the rest of the cases, the ratios are close to the unit. At pH ) 6.92 and 6.75, the measurement errors could contribute to Np/Nd lower than the unit, considering that much diluting of the miniemulsion during DLS measurement could underestimate the droplet size. It is also likely due to some coalescence that occurred during the polymerization. It has been well-documented that RAFT miniemulsion polymerization nucleation efficiency could be lower than that of the regular miniemulsion polymerization.23-25 Particle Morphology from TEM Observations. The typical TEM images of the final particles are presented in Figure 2. As seen in Figure 2a, most of particles are “solid” particles at pH ) 8.12, while a fraction of particles are nanocapsules with welldefined core/shell structures. The solid particles look smaller than those nanocapsules. With a decrease of pH values, most of particles become nanocapsules. In the cases of pH ) 6.92 and 6.75, the particle size looks uniform with the diameter around 100 nm. Two populations of nanocapsules and solid particles are well-separated in the size distribution spectrum at pH ) 8.12, as seen in Figure 3. The average diameter of solid
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followed by the fastest polymerization rate. The polymerization ends within 150 min. With a decrease of pH values, the polymerization rates decrease. The early polymerization rate at pH ) 6.45 was unexpectedly high. The reason might be ascribed to the existence of some large droplets in the miniemulsion. Since the oil-soluble AIBN was used as the initiator, the initiation efficiency in these large droplets was much higher than those small droplets,26,27 leading to a faster polymerization rate. When the nucleation ends, the number of particles would become the determining factor for the overall polymerization rate. The polymerization rate at pH ) 6.45 becomes the lowest. Discussion
Figure 2. TEM images for the final latex with every polymerization. Image a was the final latex of the experiment at (a) pH ) 8.12, (b) pH ) 6.92, (c) pH ) 6.75, (d) pH ) 6.45.
particles is around 49.5 nm, while the average diameter of the nanocapsules is 84.8 nm. In the cases of lower pH values, the solid particles are not well-separated from the nanocapsules in terms of particle size, while the size of the solid particles is smaller than that of the nanocapsules on average. The statistical data from TEM images are presented in Figures 3 and 4. At pH ) 6.92, the particle size distribution is very narrow. The coefficient of variation (CV) is 3.2%. With further decrease of pH values, the particle size distribution becomes increasingly broader. At pH ) 6.75, Dn ) 119.0 nm and CV ) 5.7%, while, at pH ) 6.45, Dn ) 144.3 nm and CV ) 14%. Interestingly, the volume fraction of nanocapsules increases linearly from 0.18 to 0.94 with the decrease of values from pH 8.12 to 6.45, as seen in Figure 4. As listed in Table 2, the shell thickness from TEM statistics increases from 19.8 to 40.2 nm with a decrease of pH values partly because of the formation of solid particles and partly because of an increase of particle sizes (i.e., less particles). Molecular Weight and Molecular Weight Distribution. The molecular weights and molecular weight distribution at various pH values are presented in Figure 5 and Figure 6. The molecular weight grows linearly in all cases, in excellent agreement with the theoretical prediction data, as seen in Figure 6. At pH ) 8.12, GPC curves are bimodal since the early stage polymerization. The peak of the higher molecular weight has a molecular weight of 720000 g/mol, much higher than the preset value (Mn,th ) 68000 g/mol). The final latex was subjected to centrifugation. Unfortunately, we could not separate well the solid particles from the nanocapsules. The top layer was capsulerich, while the bottom layer was rich in solid particles. The average molecular weight of the top layer was about Mn ) 24900 g/mol, while that of the bottom layer was about Mn ) 110100 g/mol. At lower pH values, GPC curves become monomodal and move to higher molecular weight with an increase of the monomer conversion, showing a typical feature of living polymerization. The molecular weight distribution as indicated by PDI in Figure 6 steadily narrows with decrease of pH values. Polymerization Kinetics. Monomer conversions at different polymerization time were recorded and plotted in Figure 7. It is obvious that the polymerization kinetics curve at pH ) 8.12 is very different from the other curves. The polymerization at pH ) 8.12 has a clear inhibition period of about 35 min,
In the absence of the second nucleation mode other than the monomer droplet nucleation, the interfacially confined RAFT miniemulsion polymerization would be a robust process to encapsulate the functional materials and to synthesize welldefined nanocapsules. The homogeneous nucleation is a wellaccepted particle formation mechanism in emulsion polymerization for those monomers with relative high water solubility, such as methyl methacrylate and vinyl acetate.28,29 However, it is well-accepted that the emulsion polymerization of styrene should follow a micelle nucleation mode.30 In a miniemulsion polymerization, the micelle is absent.31 In such a case, monomer droplet nucleation is believed to be the dominant nucleation mode if the monomer droplet diameter is small enough.31-33 In the emulsion polymerization of styrene initiated by potassium persulfate (KPS), the homogeneous nucleation is less likely to occur due to the small solubility of styrene in water. It is also well-known that replacing water-soluble initiator with the oilsoluble initiator helps to suppress the homogeneous nucleation. In the current study, an oil-soluble initiator, AIBN, was used to suppress the possible homogeneous nucleation. The droplet diameter was as small as around 120 nm at pH ) 8.12. Modeling results suggested that it should be unlikely to have the homogeneous nucleation under such conditions.34 Surprisingly, the results presented previously strong support that the nucleation was actually bimodal at pH ) 8.12. DLS measurement indicated that the system had many more particles than the original droplets since the early stage of polymerization. TEM observations clearly showed that there were two types of particles: “solid” particles with smaller size and nanocapsules with larger size. Considering that the core material of ND is extremely insoluble in water, little ND molecules would transport from the monomer droplets or those particles converted from the original droplets to those particles from the second nucleation. The solid particles should come from the second nucleation. GPC curves were also bimodal since low conversion. One of the two peaks had a molecular weight much higher than the theoretical value. It is suggested that very few of the RAFT agent molecules were in those particles from the second nucleation. According to the above results, we believe that the second nucleation should be the homogeneous nucleation triggered by the RAFT agent, which could be demonstrated in Scheme 3. Prior to the polymerization, the RAFT agent molecules were self-assembled on the interface of monomer droplet/water phase. Once the initiation was triggered by raising the temperature, the radicals either born in the droplets or entered from the aqueous phase would quickly transfer to the RAFT agent since the transfer reaction is highly active. In the meanwhile, poly(MAA-co-St) radicals were released. At pH ) 8.12, the carboxylic acid groups on poly(MAA-co-St) radicals were completely neutralized, so poly(MAA-co-St) radical could exit
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Figure 3. Size distribution of all particles and nanocapsules counted statistically from TEM images.
in water to form a surface-active radical, which is able to enter the particles. However, in the current case, the surface-active adducts of poly(MAA-co-St) with styrene might be much larger in size. The diffusion rate could be much slower, as suggested by D)
Figure 4. Variation of the fraction of nanocapsules with pH values, statistically counted and calculated from the TEM images: 2, averaged in terms of number; 4, averaged in terms of volume. Table 2. Shell Thickness of Nanocapsules from TEM Statistics pH values shell thickness, nm
8.12 19.8
6.92 33.0
6.75 35.0
6.45 40.2
out of the droplet with a quite high probability. This radical exit caused by RAFT reactions led to the inhibition period, as we experimentally observed in the kinetic curve as shown in Figure 7. Poly(MAA-co-St) radical would react with styrene dissolved in the water and precipitated out to form new particles, which were self-stabilized. During the homogeneous nucleation, only a small fraction of mobile RAFT agent molecules transport to the new-born particles. So, these particles would have the polymers of much higher molecular weight, attributing to the higher molecular weight peak in GPC curves. It is well-accepted that in the normal (non-RAFT) emulsion polymerization of styrene initiated by KPS, the homogeneous nucleation is less likely to occur. It is believed that the water-soluble radicals decomposed from KPS could be added with styrene dissolved
kT 6πηRh
where D is the diffusion rate coefficient, k is the Boltzmann constant, T is temperature, η is water viscosity, and Rh is the hydrodynamic volume. Before they re-enter droplets or particles, they could terminate each other or propagate further with styrene to precipitate out. Thus, self-stabilized solid particles were formed. With the decrease of pH values, the hydrophilicity of poly(MAA-co-St) radicals decreased due to the lower dissociation degree of carboxylic acid groups. As a result, the desorption probability of poly(MAA-co-St) radical decreased, which was evident in the disappearance of the inhibition period, as referred to in Figure 7. The homogeneous nucleation became less likely to occur; the monomer nucleation was dominant. Therefore, both GPC curves and PSD curves became monomodal. Most of particles were nanocapsules. In these cases, the fraction of polymer from the solid particles was small, so we did not observe a separate GPC peak of high molecular weight like at pH ) 8.12. However, the polymer of high molecular weight from solid particles did broaden the molecular weight distribution. Accordingly, the PDI decreased with the decrease of pH values, as seen in Figure 6. Obviously, the homogeneous nucleation mainly occurred in the early stage of the polymerization, where the poly(MAA-
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Figure 5. GPC curves at different conversions.
Scheme 3. Homogeneous Nucleation Triggered by the Amphiphilic RAFT
Figure 6. Molecular weight (Mn) and PDI evolution against monomer conversion. (Mn,theo ) MRAFT + ([M]0MStx)/([RAFT]0), where MRAFT is the molar mass of the RAFT agent, MSt is the molar mass of styrene, x is the fractional monomer conversion, [M]0 is the initial monomer concentration, [RAFT]0 is the initial Amphi-RAFT concentration.)
poly(MAA-co-St) radicals, rapidly decreasing the water solubility of the releasing radicals so that the homogeneous nucleation stops. Conclusion The effect of pH values on the interfacial miniemulsion polymerization using a RAFT agent, with amphiphilic random copolymer being the R group, as a reactive surfactant was investigated. It was found that the homogeneous nucleation was more likely to occur than the regular miniemulsion polymerization, triggered by the high hydrophilicity of the R group of the RAFT agent. The homogeneous nucleation led to the formation of a large fraction of solid particles. Fortunately, we could suppress the homogeneous nucleation and then remove the solid particles by decreasing the pH values of the systems. The process promises a general method for synthesizing the nanocapsules with well-defined structures in a “green” and highly efficient way.
Figure 7. Kinetics of miniemulsion polymerization at different pH values: -2-, pH ) 8.12; -9-, pH ) 6.92; -1-, pH ) 6.75; -b-, pH ) 6.45.
co-St) radicals released from amphi-RAFT could dissolve in the water. The polymer chains grow by adding monomers to
Acknowledgment We thank the National Science Foundation of China (NSFC) for Award Nos. 20474057 and 20836007 and the Ministry of Education for New Century Excellent Talent in University,
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ReceiVed for reView September 27, 2009 ReVised manuscript receiVed December 24, 2009 Accepted January 1, 2010 IE901515T
