Mechanism of Narrowly Dispersed Latex Formation in a Surfactant

Article pubs.acs.org/Macromolecules

Mechanism of Narrowly Dispersed Latex Formation in a SurfactantFree Emulsion Polymerization of Styrene in Acetone−Water Mixture Zhiyong Li, He Cheng,* and Charles C. Han* State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ABSTRACT: The mechanism of narrowly dispersed latex formation in a surfactant-free emulsion polymerization (SFEP) of styrene in acetone−water was studied by a combination of transmission electron microscopy (TEM) and dynamic light scattering (DLS). The critical nuclei were experimentally observed and the formation of narrowly dispersed PS latex is proved to be originated from competitive growth kinetics. Spherical nuclei were regenerated via a microphase inversion of PS oligomer in a 50% volume fraction acetone−water mixture at 70 °C. They followed a polydispersed log-normal distribution with R̅ s ∼ 10.6 nm and δ ∼ 0.298, and the smallest nucleus with Rs ∼ 1.1 nm was similar to critical nuclei, with PS backbone (average chain length ∼6−7) repeat units) inside and sulfate groups to stabilize it at oligomer chain ends. Note that the spherical nuclei are not necessarily narrowly dispersed. Competitive growth kinetics makes smaller nuclei grow much faster than large nuclei in the subsequent polymerization process, resulting in narrowly dispersed PS latex. Time resolved dynamic light scattering (DLS) was used to verify this. Two kinds of PS seed particles which had already grown to the dimension of ⟨Rh⟩ ∼ 300 nm and ∼400 nm were added, separately, into two parallel surfactant-free emulsion polymerization batches of styrene in acetone−water mixture at 70 °C, when the average hydrodynamic radius of PS latex was about ∼20−30 nm. It was found that the size of ∼300 or 400 nm seed particles almost did not change, but the small size PS latex grew rapidly. Narrowly dispersed PS latex was finally obtained in the SFEP system, which supports the competitive growth mechanism proposed by Vanderhoff and co-workers. difficult, if not impossible. On one hand,13 microscopy in real space could not monitor the time dependent process of the emulsion polymerization; on the other hand, most of scattering techniques in reciprocal space failed to discriminate the generation of critical nuclei, as well. This is because that in the early stage of nucleation, monomer droplets (∼μm) and critical nuclei (∼nm) coexisted, the scattering from the former was overwhelming. The origin of narrowly dispersed latex formation in surfactant-free emulsion polymerization is another puzzle. It was proposed in 1950 that if all particles are nucleated in a negligible short time, they can grow at the same rate, then narrowly dispsersed latex particles can be developed.14 In fact, the nucleation process is not negligibly short. Although it was reported that the number of particles in SFEP gets a burst of ∼1013 mL−1 within a time gap of less than 1 s,15 the data may be overestimated. There are two ways to calculate the particle numberÑ . In the first approximation, the particle numberÑ was calculated according to

1. INTRODUCTION The mechanism of surfactant-free emulsion polymerization (SFEP) is a fundamental and classical problem in heterophase polymerization. The possibility of carrying out emulsion polymerization in the absence of emulsifiers stimulated the search for a general particle nucleation model beyond the assumption of micellar nucleation1,2 for which no experimental proof exists. Harkins,3−6 Priest,7,8 and Goodwin9,10 et al. have presented the homogeneous nucleation, coagulative nucleation and micellar nucleation model, which clearly indicated that the radical oligomers with hydrophobic chain backbone and charged end-groups precipitate or associate, absorb new radical oligomers and monomers to form primary particles, and aggregate until certain stability can be reached. In 1995, Tauer and Kuhn11 proposed a completely different approach, that is, nucleation can be considered as multichain event and not as single chain event. At last, the SFEP process often leads to the formation of monodisperse latexes, which depends on the reaction recipe and conditions. However, two main puzzles remain, i.e., the critical dimension of nucleation was experimentally difficult to observe; and the origin of narrowly dispersed latex formation was not experimentally clarified either. Although Vanderhoff et al.12 predicted that the size of critical nuclei should be about 1.4 nm as early as 1985 (SFEP of styrene in water with potassium persulfate, KPS, as initiator), the experimental observation of critical nuclei is extremely © 2012 American Chemical Society

Ñ = C /ρ(πD3/6)

(1)

Received: November 19, 2011 Revised: March 14, 2012 Published: March 28, 2012 3231

dx.doi.org/10.1021/ma202535j | Macromolecules 2012, 45, 3231−3239

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where C is polymer concentration, ρ is polymer density, and D is diameter of particles measured from DLS or TEM. However, eqs 1 cannot be verified in the early stage of surfactant-free emulsion polymerization, because neither TEM nor DLS can measure the nuclei particle size, as demonstrated previously. In the second way15 τ/Δκ = FD4

> 3, the distribution becomes broader as larger particles grow faster than smaller ones. However, if x = 3, particle growth does not depend on particle size.23,24 So x should be a function of the particle radius (or volume). But for simplicity of discussion, x is often approximated as a volume independent parameter. In this study, polydispersed spherical nuclei were observed experimentally after microphase inversion of styrene oligomer in 50% volume fraction of acetone−water mixture. They have a polydispersed log-normal distribution, and the smallest nucleus with a radius of ∼1.1 nm is similar to the critical nuclei size.12 According to competitive growth kinetics, the polydispersed spherical nuclei can grow into monodispersed spherical PS latex after competing with one another for available monomer and radicals. In order to prove this, PS seed particles, with two different sizes (300 and 400 nm), were added into two parallel reaction batches of surfactant-free emulsion polymerization of styrene in 50% volume fraction of acetone−water mixture at 70 °C when the average hydrodynamic radius of growing latex is about 20−30 nm. Styrene monomers and KPS initiators were fed continuously afterward to support the competitive growth of these two different sized particles. It was found that the size distribution of such different-sized latexes grow to become narrowly distributed, eventually.

(2)

where τ is turbidity, scaling with τ ∝ nD (n is the particle concentration, and D is the particle diameter), Δκ is the difference between the calculated and experimental conductivity, and F is a complex constant that is fitted from off-line measured particle sizes which is only possible in the late stage of polymerization. Note that the error connected with the application of eqs 2 is greater in the early stage of SFEP (because of smaller τ). Therefore, both methods cannot accurately determine Ñ in the early stage of SFEP, and we cannot conclude that the small nuclei do not exist just because we cannot see them by DLS and TEM. The present manuscript tries to contribute to better understanding of particle nucleation and growth in SFEP. A microphase inversion (that is, PS oligomer was first dissolved in DMF, then added dropwise into an excess of acetone−water mixture, resulting in a transparent stable colloidal dispersion) of PS oligomer into 50% volume fraction of acetone−water mixture at 70 °C was used to generate spherical nuclei, experimentally. Wu and co-workers16−18 have found that block copolymers or polymers with low amount and random distribution of charges along the chain (ionomers) can self-assemble in a selective solvent to form surfactant-free nanoparticles, which can be induced by chemical reaction, chain−chain complexation, and microphase inversion in addition to the normal controllable parameters (such as temperature and etc.). It was proved in these systems that the self-assembled structure is similar to its collapsed state in polymerization process.19−21 Ganeva et al.22 used amphiphilic RAFT (reversible addition−fragmentation chain transfer)capped, acrylic acid and styrene block copolymers as stabilizers in emulsion polymerization. And they found that, further polymerization of these amphiphilic diblocks generates the final latex particles. These may suggest that it is possible to regenerate the nuclei formation process by directly selfassembly of oligomer in its polymerization environment. On the other hand, narrowly dispersed PS latex may be grown from polydispersed PS nuclei via competitive growth kinetics which was pioneered by Vanderhoff and co-workers, with some further development by Morrison et al.23−25 The volume growth of a particle is given by eq 3a, where K is assumed to be independent of particle size. 6

dV / dt = K ·Rx (a);

2. EXPERIMENTAL SECTION 2.1. Chemicals. In the present work, all chemicals were bought in analytical grade. Styrene monomer was purified by a standard procedure, that is, distilled at reduced pressure to remove stabilizers and stored in a refrigerator (−18 °C) before use. Potassium persulfate (KPS) was recrystallized from water, and stored at 4 °C. Analyticalgrade acetone, and N,N-dimethylformamide (DMF) were used as received unless stated otherwise. Deionized water from a Millipore ultrapure water system with a resistivity of 18 MΩ·cm was used in all of the experiments. To obtain degassed water, oxygen was removed from water by bubbling high purity Argon (≥99.99%) through the liquids at ambient temperature. 2.2. Polymerization. The different procedures of surfactant-free emulsion polymerization are summarized in Table 1. The molar ratio of KPS and styrene ([KPS]/[St]) were listed for comparison and further discussions. PS-1 was used to measure the kinetics of surfactant-free emulsion polymerization of St in 50% volume fraction acetone−water mixture. PS-2 and PS-3 were used to prepare surface-

Table 1. Typical Procedures of SFEP through KPS Initiated Polymerization of Styrene in Water−Acetone Mixture (50% Volume Fraction of Acetone at 70°C) sample −1

cKPS/g·L cSt/g·L−1 [KPS]/[St] ⟨jGPC⟩a ⟨jMS⟩a purpose

dR / dt = K ′·Rx − 2 (b);

R3 − x − R 03 − x = K ″·Δt (c)

(3)

For the case that the size growth follows eqs 3b for spherical particles, it leads to the assumption that all Ks are independent of time (i.e., K, K′, K″ only depend on polymerization conditions) after integration to eqs 3c. Note, the particle size is always the unswollen size; thus, using eqs 3a means the size dependence of the monomer concentration is neglected. D and D0 are the sizes at the beginning and the end of polymerization with duration Δt, respectively. A closer inspection of eqs 3b and 3c reveals that if x < 3, smaller particles will grow faster and the particle size distribution is self-sharpening. Contrary to that, if x

PS-1 0.6 5 0.12 ∼115 − monitor the SFEP process

PS-2

PS-3

0.6 0.6 1 2 0.6 0.3 ∼6 ∼12 ∼7 ∼9 prepare PS oligomer

PS-4b

PS-5c

0.12 0.12 10 10 0.012 0.012 − − − − observe the competitive growth kinetics

a The chain length of ⟨jGPC⟩ and ⟨jMS⟩ was obtained from the numberaveraged molecular weight Mn measured by GPC and MALDI−TOF− MS techniques, respectively. Here, ⟨jGPC⟩ was used for comparison with the critical chain length ⟨jcr⟩ addressed in the model of Tauer and Kuhn.11 bWhen ⟨Rh,PSlatex⟩ ∼ 20−30 nm, seed particles with ⟨Rh,seed⟩ ∼ 300 nm were added. The initial cSt was fixed at 10 g·L−1. After 3 h of reaction, residual monomer was fed into the reacting system continuously at ∼5.0 g/(L·h). cWhen ⟨Rh,PSlatex⟩ ∼ 20−30 nm, seed particles with ⟨Rh,seed⟩ ∼ 400 nm were added.

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active oligomers. PS-4 and PS-5 were used to observe the competitive growth kinetics. In a typical SFEP, a four-neck flask was assembled inside the oven. The center neck was connected with a water reflux condenser with a coaxial glass stirrer. The other three necks were used for an argon inlet, an in situ sample drawing, and a monomer feeding head, respectively. The reaction mixture with a proper composition was weighted in the flask and was bubbled with argon for 30 min at room temperature. The flask was thermostated with silicone oil bath at 70 °C. After that, the reaction was started by injecting the corresponding stoichiometric amount of KPS initiator dissolved in degassed water. All polymerizations were carried out without the addition of surfactant. 2.3. Competitive Growth Experiments. Two parallel competitive growth kinetics experiments were carried out (PS-4 and PS-5). In a typical experiment, 100 μL of 10% mass fraction seed particles emulsion with monodispersed seed particles (the two batches of emulsion with 300 and 400 nm narrowly dispersed latex were prepared by SFEP in water with cKPS ∼ 0.1 g·L−1, cSt ∼ 80−100 g·L−1) were added into the reacting system when the small particles grew to ∼20− 30 nm. Because the relative scattering intensity contribution from the seed particles is decreasing with the growth of small PS latex, DLS signal from seed particle will vanish after ∼3 h.26 Because the size of seed particles in the SFEP are still not changed much, a fixed amount of the large sized seeds emulsion (10 mL of a 10% mass fraction) were added into the reacting system again (we added two times of seed particles in each experiment) to make the seed particles remained measurable in DLS. The initial cSt was fixed at 10 g·L−1. After 3 h, residual monomers were fed continuously at ∼5.0 g/(L·hr) to maintain the growth of latex particles. During the SFEP, a small amount of the emulsion was sampled and diluted to do DLS experiments to determine the size distribution. 2.4. Oligomers Preparation and Characterization. A series of experiments were carried out with different procedures (PS-2 and PS3). The reacting mixture was kept at 70 °C for 48 h. Then the solution was evaporated. The raw product was washed to remove salt, residual KPS and water-soluble oligomers, then the product was collected by centrifugation at 12000 rpm. After that, it was freeze-dried for 48 h. The apparent molecular weight of oligomers were measured by gel permeation chromatography (GPC), matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI−TOF−MS), and dynamic light scattering (DLS). GPC was carried on Waters 1515 equipped with HR1, HR0.5 Waters Styragel columns, using tetrahydrofuran (THF) as eluant and PS as standard. All MALDI-TOF-MS were acquired on a Bruker Autoflex mass spectrometer (Bruker Daltonics, Bremen, Germany), equipped with a 355 nm nitrogen laser. The details of DLS will be introduced in the next section. 2.5. Latex Characterization. Latex samples were taken from the reacting system at different times and diluted for particle size distribution analysis by DLS and transmission electron microscopy (TEM). A commercial LS spectrometer (ALV/DLS/SLS-5022F) equipped with a cylindrical 22 mW He−Ne laser (Uniphase, at λ = 632.8 nm) was used. The spectrometer has a high coherence factor of β ∼ 0.95 because of a novel single-mode fiber optics coupled with an efficient avalanche photodiode. The LS cell was held in a thermostat refractive index matching vat filled with purified and dust-free toluene. In static laser light scattering (SLS), the angular dependence of the excess time-averaged scattered intensity was acquired. In DLS, intensity−intensity correlation function G(2)(τ,q) was acquired with an ALV-5000 multitau digital time correlator; a relaxation rate (Γ) distribution f(Γ) was obtained by Laplace inversion.27,28 For a pure diffusive relaxation, τi is related to the translational diffusion coefficient Di by τi = 1/Γi and Di = Γi/q2 or to a hydrodynamic radius Rh = kBT/ 6πηD with kB, T and η as the Boltzmann constant, the absolute temperature, and solvent viscosity, respectively. The details of LS instrumentation and theory can be found elsewhere.29−34 TEM was performed using a Hitachi H-800 instrument (spatial resolution is ∼0.5 nm), with an acceleration voltage of 100 kV. For TEM examination, the solution was diluted to the order of magnitude μL/L and then dropped on a copper grid which is covered by carbon

membrane. Subsequently, the TEM grid was dried for at least 24 h to remove the solvent.

3. RESULTS AND DISCUSSION 3.1. Kinetics of Surfactant-Free Emulsion Polymerization. Fitch et al. first used SLS to investigate the kinetics of water-solvable monomers.25,35−37 However, in a typical surfactant-free emulsion polymerization system of hydrophobic monomers with KPS as initiator, micrometer-size styrene droplets can only be stabilized in aqueous medium by mechanical stirring, which makes it very difficult to monitor the time-resolved kinetics evolution. Fortunately, Homola et al. and Okubo et al..38,39 reported the surfactant-free emulsion polymerization of styrene in water with the addition of methanol or acetone. Wu and co-workers30,40 further prepared uniform surfactant-free microlatex with the average radius down from 300 to 35 nm in a 50% volume fraction acetone−water mixture. Ngai et al.40 explained that, the mixed solvent with equal amounts of acetone and water can reduce the surface tension of system, resulting in the formation of smaller particles. All of the previous efforts made it possible to study the timeresolved kinetics in surfactant-free emulsion polymerization of styrene because styrene monomer droplets can stabilize themselves in a mixed solvent without mechanical stirring. Figure 1 is the time-resolved intensity−intensity time

Figure 1. Evolution of (a) the intensity−intensity time correlation function (G(2)(q, τ)) and (b) the corresponding hydrodynamic radius distribution (the scattering angle θ = 30°) with time. Here the procedure corresponds to sample PS-1.

correlation function from the DLS and the corresponding hydrodynamic radius distribution of PS-1. At the very beginning (0 min), there is only one slow mode with a hydrodynamic radius ∼300 nm existed, which corresponds to the styrene monomer droplet in the mixed solvent. Then KPS initiates the emulsion polymerization in acetone−water mixture phase, and PS radical oligomers with hydrophobic PS chain 3233


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Figure 2. Typical TEM images of polystyrene latex obtained through surfactant-free emulsion polymerization of styrene in 50% volume fraction of acetone−water mixture (experiment PS-1).

merge them together. We will discuss this in the next section again. Figure 2 is the TEM images of polystyrene latex obtained through SFEP of styrene in 50% volume fraction of acetone− water mixture at different polymerization stages. At the very beginning, nothing can be observed because the size and population of nuclei are too small. After 1.5 h, nanoscaled PS latex can be observed. At 5 h (3.5 h later), lots of narrowly dispersed nanoscaled PS latex are generated. A longer reaction time makes essentially no difference on TEM image. Note that we cannot observe critical nuclei in real space, either, because their size may be too small or their number density is too low to be monitored in the early stage of emulsion polymerization. And the large 300 nm styrene droplets observed in the DLS (as the slow mode) cannot be observed (dried out) in the TEM image. Therefore, eqs 1 cannot be verified by measuring particle numbers in the early stage of SFEP, because neither DLS nor TEM can observe the diameter of particles at that stage. 3.2.1. Microphase Inversion of PS Oligomer. Synthesis of PS Oligomers. In order to study the nucleation process in the early stage of surfactant-free emulsion polymerization, various methods have been used to prepare PS oligomers. Pohlein et al.42 added water-soluble inhibitor in acrylic acid-styrene emulsion copolymerization systems, in order to obtain the critical oligomer size for particle entry in 1993. In 1994,43 they further added chain transfer agent to the system and low molecular weight species were formed when water-born oligomeric radicals diffused to the surface of these seed particles. Tompson et al.44 centrifuged the PMMA latex at 12 000 rpm and filtered the clear aqueous phase through a membrane (100 nm pore size) in a serum replacement cell to ensure removal of any debris present. The water-soluble MMAbased oligomers remaining were isolated and analyzed both asproduced and after hydrolysis. Yuan et al.45 prepared watersoluble oligomers in the emulsion polymerization of styrene− butadiene-acrylic acid through ultracentrifugation (37 000 rpm for 36 h). Tauer et al.15 summarized in their work in 1999 considering the isolation of oligomers from dispersion, that is, all efforts to obtain amounts of oligomers that is large enough can be handled have failed. In this manuscript, we would like to present a new route, which is achieved by increasing [KPS]/[St] (batches PS-2 and PS-3 in Table 1), it can increase initiation and termination but decrease propagation in reacting process. A large amounts of

backbone and sulfate end groups may form. The PS radical oligomer may propagate until a critical chain length is reached. They may then collapse and coagulate to form the critical nuclei. Unfortunately, the nucleation process cannot be observed by DLS, because the scattered light from critical nuclei is too small to be monitored due to the existence of the large monomer droplet, and there is only one slow mode of monomer droplet before 1.5 h in DLS. As we all know, the scattered light intensity comes from two parts, which is the summation from the critical nuclei and monomer droplet, respectively, i.e., I ∝ ndMd2 + ncMc2, where n and M are number density and molar mass of monomer droplet and critical nuclei, respectively. If we assume that ρd = ρc and M = 4πρR3/3, n M 2 n R 6 Id /Ic = d d2 = d d ncMc ncR c 6

(4)

When critical nuclei are born, Rd ∼ 100Rc, Id is overwhelmingly larger, the generation of critical nuclei cannot be directly monitored. After the formation of nuclei, they will absorb new radical oligomers and monomers, aggregate and grow (polymerize) to form latex particles with certain stability. Both nc and Rc grow simultaneously, when the scattering from latex particles and monomer droplets are comparable, a fast mode of ∼27 nm appears. The amplitude and size of the fast mode increases, while the amplitude of the slow mode decreases with time, which means that the PS latex from the critical nuclei grow in size and consume styrene monomers from the styrene droplets. Finally, stable PS latex particles with ⟨Rh⟩ ∼ 35 nm form, which are stabilized by sulfate end groups. [KPS]/[St] is crucial for the formation of nanoscaled stable PS latex because it controls the average occupied surface area per sulfate groups on the PS nanoscaled latex periphery. Wu used LS to determine the surfactant interface thickness of spherical PS microlatices, and found the average occupied surface area per surfactant molecule on the surface of polymer core is a fundamental parameter for governing the final size of microlatices.41 When [KPS]/[St] is very low, the growth of PS latex makes the averaged area occupied by each sulfate group on the latex periphery too large to stabilize itself. These latex will percolate and aggregate with each other, and only large PS latex can be obtained.39 Therefore, a proper [KPS]/ [St] ratio is necessary to make a nanoscaled PS latex polymerizable.40 In PS-1, [KPS]/[St] ∼ 0.12, the average area per sulfate end group is reasonable to stabilize the latex particle in the polymerization process, and collision cannot 3234


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radical oligomers are generated in the early stage of emulsion polymerization, which have hydrophobic chain backbone and charged end group and can form micelles after a critical concentration is reached,10 or precipitate to form primary particles for chain length above a critical value jcr.46 The further polymerization of primary particles is constrained because no sufficient monomers are provided.8 Because a large amount of initiator were added ([KPS]/[St] is large), it is expected that the double-end termination is dominating. However, in the preparation process of oligomers, we have washed the raw products with water to remove salt, those water-soluble oligomer, that is, one or two charged groups with shorter PS backbone have been removed. Therefore, it is reasonable to believe that our sample is mainly composed of short (slightly larger) oligomers with one end charged groups and longer oligomers with two end charged groups. It was expected that through such method, samples with chain length j, which is just equal to or above jcr, dominates, where jcr is the critical chain length. 3.2.2. Characterization of PS Oligomers. In light of these, PS oligomers were synthesized in 50% volume fraction acetone−water mixture with higher [KPS]/[St] ratio (PS-2 and PS-3 in Table 1). Figure 3 is the GPC results of these two

Figure 4. Comparisons of molecular weight distribution of PS-2 measured by (a) DLS, (b) GPC, and (c) MALDI−TOF mass spectrum.

molecular weight distribution of PS-2 oligomer in DMF solution by GPC. The result of Mz ∼ 2,300 is consistent with DLS result. Figure 4c is the MALDI−TOF mass spectrum of PS-2. Mass resolved signals in Figure 4c allow the assignment of each peak to a specific oligomer which can be used to identify the structure of the oligomer and charged groups attached to the chain ends. The mass difference between the peaks is ∼104 which is equal to the mass of one styrene monomer. Seven mass series are present in the mass spectrum in Figure 4c. The number-averaged molecular weight Mn ∼ 760 and weightaveraged molecular weight Mw ∼ 790. Both of them are reasonably consistent with GPC results (Mn ∼ 670, Mw ∼ 880). In the persulfate-initiated polymerization system of styrene in water, the critical value of chain length ⟨j⟩ ∼ (6 to 7) (⟨j⟩ = (Mn − MSO4−)/MSt); while the reported experimental results by Tauer and Kuhn is jcr,exp ∼ 5.11 Fitch and Carro et al.11,13,46 also suggested that during the early stage of emulsifier-free emulsion polymerization of styrene in water, oligomers with chain length from several to dozens of repeat units could be found for precursor particles. In this study, the addition of acetone into water enhances the solubility of styrene; with the combination of Figure 3 and Figure 4, we can show that the averaged chain length is ⟨j⟩ ∼ 6−7. 3.2.3. Microphase Inversion of PS Oligomers. The microphase inversion in our study is a redispersion process, namely, dissolving oligomer (oligomer prepared in PS-2 or PS3) in a small amount of DMF to make a mother solution, with cp = 20 mg/mL. Then the DMF mother solution is added dropwise to a large amount of 50% volume fraction acetone− water mixture under vigorous stirring at 70 °C for 12 h. As expected, DMF immediately mixed with acetone−water mixture, and PS oligomers self-assembled themselves, and should have the ionic sulfate chain end located on the periphery

Figure 3. GPC results of two procedures of PS oligomer using THF as eluant: PS-2 and PS-3. The inset shows the log-normal distribution fitted curves of the two peaks of PS-3.

procedures of PS oligomer, PS-2 ([KPS]/[St] = 0.6) and PS-3 ([KPS]/[St] = 0.3), respectively. It was found that, bimodal molecular weight distribution appears when [KPS]/[St] increases to 0.3 in case of PS-3. Each peak has a log-normal distribution, as demonstrated in the inset in Figure 3. The further increase of [KPS]/[St] to ∼0.6 (PS-2) leads to the formation of only one peak. With the increase of [KPS]/[St], it may switch from dispersion to solution, which is the origin of bimodal distribution in PS-3 and single log-normal distribution in PS-2. Figure 3 clearly shows that PS-2 oligomer has a lognormal distribution with Mw = 880 and Mw/Mn = 1.3. It will be better if we compare GPC results with DLS and MALDI-TOF mass spectrum results for PS-2 oligomer, as shown in Figure 4. Figure 4a comes from the hydrodynamic radius distribution of PS-2 oligomer in DMF solution at room temperature. Adam and Delsanti studied the chain conformation of PS in good solvent by DLS and obtained the scaling law between hydrodynamic radius and z-averaged molecular weight, i.e., Rh = A·Mzv,47,48 where v ∼ 0.55, and A ∼ 0.15. Thereby, the zaveraged molecular weight distribution of PS-2 oligomer in DMF can be directly transformed from its hydrodynamic radius distribution. Figure 4b demonstrates the number-averaged 3235


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Figure 5. Typical TEM images of polystyrene latex obtained through the microphase inversion of PS-2 in 50% volume fraction acetone−water mixture. The temperature for self-assembly experiments is fixed at 70 °C. The pictures on the right (B1−B3) show the enlarged images of different regions from the middle image.

after the microphase inversion.16 The self-assembled structure formed by oligomers during microphase inversion process may be similar (or comparable) to the nucleus at the early stage of SFEP. Figure 5 is the typical TEM images of polystyrene latex obtained through the microphase inversion of PS-2 in 50% volume fraction acetone−water mixture. Polydispersed PS nuclei can be vividly seen. These particles often show anomalous morphology, which can be attributed to the swelling of particles by monomers,49−51 or hydrophilic regions inside latex particles in water.21 In this study, there are no monomers in coexistence, and polydispersed homospheres are obtained. These polydispersed PS nuclei particles act as the “nanoreactor” in a real emulsion polymerization system. Monodispersed PS particles may be obtained after further reaction according to the competitive growth model by Vanderhoff and co-workers,23,24 which we will discuss later. The average size Rs and size distribution f(Rs) can be obtained from the TEM result, as shown in Figure 6. As

distribution of nuclei from microphase inversion quite well. Both R̅ s ∼ 10.6 nm and ⟨jcr⟩ are consistent with the critical values in the calculation of Tauer and Kuhn. Antonitti52,53 and Wu41 studied the structure of hydrophobic latex stabilized by charged groups on the surface. They all concluded that the average area occupied by each charged group is crucial to stabilize the latex. The sulfate groups packed inside hydrophobic styrene segments or inside the nuclei particle cannot contribute to stabilize the nuclei particle. So the nuclei with R̅ s ∼ 10.6 nm looks like certain grow-up structure from the original critical nuclei, and encloses lots of polystyrene chains inside. The smallest nucleus of ∼1.1 nm (the smallest ones in the TEM picture, B3) in this study may be attributed to the critical nuclei, because these oligomers in self-assembled structure is similar to that in the parent collapsed globular state in which they were formed during polymerization.20,54 The inset of Figure 6 also shows the number of oligomers inside each nucleus, Nc. And it was calculated as40 Nc =

(4πR s3/3)ρ Mc /NA

(5)

Here, ρ is the density of bulk PS (ρ ∼ 1.05 g/cm3), Mc is the number-average molecular weight of oligomer from GPC measurement (Mc ∼ 670 is used). When the size of particles is small, all the chain contribute end groups to the surface, hence, N = Nc, where N is the number of oligomers at the nuclei surface. When Rs increases from ∼1.1 nm to ∼35 nm, more oligomers aggregate, so Nc increase from ∼13 to ∼105. Note that in practice, some end groups are trapped inside the nuclei, that is, N < Nc, when oligomers begin to aggregate. The more end groups inside can come to the surface when aggregation continues until all end groups are consumed. In this study, the size of the critical nuclei, Rs ∼ 1.1 nm, is about the end-to-end distance of critical oligomer, ⟨j⟩ ∼ 6−7. 3.3. Competitive Growth Kinetics. In order to test the model that the competitive growth kinetics is the origin of the narrow dispersed latex formation in surfactant-free emulsion polymerization, a small amount of ∼300 nm PS seed particles were introduced to a SFEP in the early stage, and DLS was used to follow kinetics of particle size distribution variation, as shown in Figure 7 (PS-4). In DLS, an ill-defined Laplace transformation was used to obtain the hydrodynamic radius distributions of particles with different sizes. In order to

Figure 6. Size distribution f(Rs) of spherical PS nuclei particles obtained from TEM image in Figure 5. The solid line showed the fitted results from the log-normal distribution function, and the inset showed the plot in linear axis and the number of oligomer chains (Nc) at the corresponding particle radius.

mentioned earlier, Tauer and Kuhn11 have studied this problem of particle nucleation in emulsion polymerization. Their results indicated that at the number of oligomers nolig ∼ 887 and the average chain length jcr ∼ 6, particles can be self-assembled with a critical diameter of ∼11.8 nm. The data in Figure 6 was obtained through counting and averaging of particles in Figure 5. A log-normal distribution with the mean value of radius R̅ s ∼ 10.6 nm and the standard deviation δ ∼ 0.298 fits the size 3236


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Figure 7. Evolution of the hydrodynamic radius distribution (θ = 30°) in SFEP when seed particle was introduced in the early stage. 100 μL 10% mass fraction emulsion with ⟨Rh⟩ = 300 nm seed particles was added into the system (200 mL) when small latex particles grow to ⟨Rh⟩ ∼ 20−30 nm. After 3 h, 10 mL of 10% mass fraction large-size seeds emulsion was added again (in total, two batches of seeds were added in an experiment) to keep the scattering contribution from the slow mode “visible” by DLS.26

Figure 8. Evolution of the hydrodynamic radius distribution observed by DLS at three typical angles (θ = 30°, 75°, 120°) in SFEP after seed particles were introduced. A small amount of 100 μL 10% mass fraction seed particles with ⟨Rh⟩ ∼ 400 nm in emulsion was added to a SFEP batch when the small latex particles grow to ⟨Rh⟩ ∼ 20−30 nm. After 3 h, another 10 mL of 10% mass fraction seeds emulsion was added again (in total, two batches of seeds were introduced in an experiment) to make the scattering contribution from the slow mode “visible” by DLS.26

discriminate the size differences between two, or more modes, they have to be within about one decade in size from each other. Figure 7 shows that at very beginning there are two modes from the DLS measurements before the introduction of seed particles, one is the monomer droplet, with ⟨Rh⟩ ∼ 300 nm; the other is the small PS latex, with ⟨Rh⟩ ∼ 20−30 nm. Because of this measurement constraint, the size of the seed particles prepared was also ∼300−400 nm, which is similar to that of monomer droplets, in order to observe their competitive growth process by DLS. After the ∼300 nm seed particles were added, they compete with the small ∼20−30 nm latex for available monomer and initiator. However, after the addition of seed particles, there is only one narrowly ∼300 nm seed peak at scattering angle of 30° in DLS as illustrated in Figure 7 (no distinguishable monomer droplets can be determined separately). It may be caused by the lower intensity contribution from the smaller amount of styrene droplets in the system (most styrene has been consumed or was dissolved in the acetone−water medium), or the ill-defined Laplace inversion technique cannot discriminate the contribution from monomer droplet and seed particles. So further investigation should be made, but it is not the focus of this manuscript. The timeresolved DLS measurement clearly proves the small ∼20−30 nm PS latex grows much faster than the seed particles in Figure 7. After 28 h, these two peaks begin to merge into a broader one, and after 48 h, only a single narrowly dispersed, 350 nm PS latex peak remained in DLS measurement, which means that these two different sized particles grow to become the same size. To clarify the result that the diminish of fast mode with ⟨Rh⟩ ∼ 30 nm in DLS at the scattering angle θ = 30° once a small amount of seed particles was added is caused by its relative small contribution to scattered light, a multiangle time-resolved DLS observation is essential. Figure 8 shows the multiangle DLS observation of particle size evolution in a SFEP with a small amount of ∼400 nm PS seed particles added at θ = 30°, 75°, and 120°, respectively. Before adding seed, there are two modes in DLS. The fast mode, ∼20−30 nm, corresponds to small latex particles, while the slow mode, ∼300 nm should be attributed to styrene monomer droplets, which are all consistent with Figure 7. The

larger the scattering angle, the larger contribution (relatively) from small particle to the scattering intensity. Therefore, the peak of small ∼20−30 nm PS latex, which diminishes at θ = 30° after the large 400 nm seed particles were added, emerges at higher scattering angle (75 and 120°). And its peak area increases with scattering angle. The competitive growth begins after the introduction of seed particle. DLS data from different scattering angles demonstrate the same fact that the small particle grows much faster than the large one, their size difference becomes smaller with growth time. After 19 h, two peaks merged and became one single broad peak. Finally, a narrowly dispersed PS latex is obtained, and DLS cannot distinguish whether the final monodispersed particles were coming from the large seeds or the small particles. One of the reasons may be due to the fact that the final particle distribution is more narrowed; therefore, the constrained Laplace transformation cannot distinguish between the two original peaks anymore. The phenomena above are easily understood, because the volume is a third power to the radius, V ∝ R3, while the mass increase is proportional to its surface area, that is, Δm ∝ R2 dR, then Δm/Δt should be scaled to R2(dR/dt). If both the small and large particles increase the same amount of mass, the small particle will of course grow much faster.55 If there are two particles with different sizes in the system, and x in eqs 3 should be a size-dependent parameter in a particular competitive growth process. Vanderhoff et al. derived the following equation in 195623 R large/R small = (R small /R small ,0)−1 × [(R small /R small ,0)3 − x + (R large ,0/R small ,0)3 − x − 1]1/(3 − x)

(6)

with the assumption that x is a constant. Here Rsmall,0 is the latex radius before the seed particles were added, Rlarge,0 is the original 3237


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size of seed particles; Rsmall and Rlarge are the radius of small PS particles and large sized seeds as reaction continues, respectively. Parts a and b of Figure 9 show the Rsmall/Rsmall,0 dependence of large/Rsmall corresponding to the procedure of PS-4 and PS-5

Therefore, we can conclude that the size difference between the large seed and small latex becomes smaller with the growth of PS latex, then the growth rates of both particles also tend to be similar. But the small particle is always growing a little faster than the large particles because x < 3. Figure 9b further confirms the above assumption with the measurement of PS-5 series; that is, the competitive growth of latex particles of different sizes favors the smaller particles in the specific SFEP system. Thus, the initially polydispersed latexes in SFEP will decrease the polydispersity with particle growth when x < 3. Note that a necessary condition in competitive growth experiment is that the particle number does not change during the growth.8,10,15,56 In the SFEP system, we assumed it is true because pure DLS cannot distinguish particle number variation from the increase of scattering light intensity.

4. SUMMARY The manuscript is to reveal the structure of critical nuclei in SFEP, and demonstrate the origin of the narrowly dispersed latex formation. In a typical microphase inversion process of PS oligomer in 50% volume fraction of acetone−water mixture, polydispersed spherical nuclei with log-normal distribution were formed. The average nuclei size, ∼10.6 nm, is too large for the average end-to-end distance of PS oligomer, so we proposed that the smallest nuclei, ∼1.1 nm, might be the critical nuclei, which is stabilized by sulfate groups. Note the spherical nuclei are not necessarily narrowly dispersed. Competitive growth kinetics makes small nuclei grow much faster than large nuclei in the subsequent polymerization process, resulting in a narrowly dispersed PS latex distribution. Time-resolved DLS was used to verify this mechanism. Large seeds with ⟨Rh⟩ ∼ 300 or 400 nm were added into two different reacting systems, when the average hydrodynamic radius of PS latex is ∼20−30 nm at 70 °C. It was found that the size of 300 nm seed particles only changed a little, but the small ⟨Rh⟩ ∼ 20−30 nm PS latex grows rapidly in size. At last, a narrowly dispersed PS latex is obtained. This is rationalized according to the treatments of Vanderhoff and co-workers. The results suggest that even when the size distribution of particles at early stage is broad, the resultant latex can be narrowly distributed. This study provides us with better understanding of the formation of narrowly distributed ultrafine particles in emulsifier-free emulsion polymerization.

Figure 9. Rsmall/Rsmall,0 dependence of Rlarge/Rsmall in the procedure of (a) PS-4 and (b) PS-5 when the small latex particles grow to ⟨Rh⟩ ∼ 20−30 nm, respectively. The lines are the theoretically calculated results according to eqs 6. Different x-axes indicate the starting points (Rsmall/Rsmall,0 = 1, Rsmall/R′small,0 = 1, Rsmall/R″small,0 = 1) for different calculations. Here Rsmall,0, R′small,0 and R″small,0 are different particle sizes obtained after each time step as new initial sizes for the next growth step, and x is assumed to be a constant for a fixed Rlarge,0/Rsmall,0 in a fixed Rsmall/Rsmall,0 range.

(DLS data in Figure 7 and Figure 8) when the small latex particles grow to ⟨Rh⟩ ∼ 20−30 nm. Theoretical calculations according to eqs 6 are shown for comparison, where x is the only variable, and is assumed to be constant for a fixed Rlarge,0/ Rsmall,0 in a fixed Rsmall/Rsmall,0 range. Figure 9a shows that at the very beginning, Rsmall,0 ∼ 20−30 nm, Rsmall/Rsmall,0 = 1 and Rlarge,0/Rsmall,0 ∼ 13.5, when ∼300 nm seed particles and small PS latex began to compete for available monomer and initiator resources. Rlarge/Rsmall decreases with Rsmall/Rsmall,0. All of the data are consistent with 0 < x < 1 very well. Poehlein and Vanderhoff24 also proposed that 0 < x < 1 because of the poorer accuracy of the competitive-growth mechanism in the small particle-size range. Interestingly, if the particle sizes obtained after each time step are new initial sizes for the next growth step, different starting points with SFEP time is chosen, that is, R′small,0 = 2Rsmall,0 (medium x-axis) and R″small,0 = 5Rsmall,0 (upper x-axis), as shown in the inset bar in Figure 9 (a). Different x and different Rlarge,0/Rsmall,0 values are needed to analyze the kinetics at different growth step. It shows that the latex growth rate x increases to be 2.0 when Rlarge,0/R′small,0 = 7, and gets to be 2.5 when Rlarge,0/R″small,0 = 3. Vanderhoff and coworkers23 also reported that for particle sizes larger than about 150 nm (e.g., initial sizes of 264 and 557 nm), the calculated value of x = 2.5 is consistent to the experimental data in a KPS initiated SFEP of styrene in water system.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86 010 82618089. Fax +86 010 62521519. Email: (C.C.H) [email protected]; (H.C.) [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support of the National Basic Research Program of China (973 Program, 2012CB821500), and National Natural Scientific Foundation of China (No. 21174152, and Young Scientists Fund No. 20804052).

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REFERENCES

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