Role of Poly(ethylene glycol) in Surfactant-Free Emulsion

Feb 22, 2013 - Through zeta potential and surface tension measurements and a series of polymerization experiments, the role of poly(ethylene glycol) (...
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Role of Poly(ethylene glycol) in Surfactant-Free Emulsion Polymerization of Styrene and Methyl Methacrylate Yiming Shi, Guorong Shan,* and Yue Shang State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: Through zeta potential and surface tension measurements and a series of polymerization experiments, the role of poly(ethylene glycol) (PEG) in the process of surfactant-free polymerization of styrene (St)/methyl methacrylate (MMA) has been investigated experimentally. Nanoscale and stable copolymer particles were formed after an abnormal process, in which the nucleation and growth of particles was different from that in previously proposed mechanisms. It has been observed that PEG can exist in both the monomer and the aqueous phases at high temperature. PEG in the aqueous phase could form copolymer particles with a loose structure, making them prone to enter the monomer phase. Entry of these copolymer particles into the monomer phase would introduce excess PEG. From the ternary phase diagram, a solubility curve could be delineated in the ternary system of PEG/monomer/copolymer. The system used the ternary solubility property to regenerate copolymer particles in the monomer phase, which maintained their morphology until the end of the polymerization. At the end, consumption of the monomer resulted in the volume contraction of the particles, and the surface potential increased. This increasing potential is a driving force to prevent particles from stacking, leading to the formation of nanoscale and stable particles.

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INTRODUCTION Poly(ethylene glycol) (PEG) is a versatile polymer. With its derivatives or modification, PEG can be employed as macroinitiator,1 stabilizer,2 or dispersant3 in the field of emulsion polymerization. However, it is infrequent that PEG is directly used in the emulsion polymerization. Although literatures that PEG can play a role of dispersant in dispersion polymerization has been reported, it was still not known clearly whether simple PEG can be applied in the traditional emulsion polymerization or surfactant-free emulsion polymerization, and what effect PEG has on the particle nucleation and growth. It has been known that water-borne polymer has been widely used in the emulsion industry. For example, Kiehlbauch and Tsaur4 had reported a kind of styrene/maleic anhydride/acrylic acid resin, which can be used as polymer surfactant or dispersant for emulsion polymerization to obtain excellent performance industrial emulsion products for coating. PEG is a wonderful hydrophilic polymer and has many interesting properties, such as environmental friendliness, biocompatibility, unique solvent solution, and so on. If PEG can be directly applied in the emulsion polymerization, the end product will be endowed with these special properties; for instance, PEG can improve the fluidity of water-based ink. Therefore, it is valuable to study the role of PEG in emulsion polymerization for both academic research and industry application. Studies of the surfactant-free emulsion polymerization of styrene (St)/methyl methacrylate (MMA) have revealed that when the polymerization is carried out in aqueous poly(ethylene glycol) (PEG) solution, a unique process occurs, whereby the particle size first increases and then decreases.5 © 2013 American Chemical Society

This particle formation process is very different from the nucleation mechanism proposed previously. Accepted particle nucleation theory can be classified into three main groups: (1) micelle nucleation,6,7 which predominates when the surfactant concentration is above the CMC; (2) homogeneous nucleation,6,8−10 which predominates when the concentration is below the CMC; and (3) coagulative nucleation.11−14 These theories of nucleation all readily account for particles becoming larger, but none of them can explain why the particle size in the St/MMA copolymerization system in PEG aqueous solution shows the unusual behavior of first increasing and then decreasing. This abnormal process can be essentially attributed to two features of PEG. On the one hand, from a microscopic perspective, the properties of PEG influence the phases in the polymerization system, such as the monomer phase, the aqueous phase, and the polymer phase produced during the polymerization process, and the interaction between these phases. PEG is a special kind of hydrophilic material,15,16 being soluble both in water and in many organic solvents such as toluene, dichloromethane, ethanol, and acetone.17,18 PEG also has the unusual property of possessing a lower consolute temperature, or cloud point, of approximately 100 °C in water.19−21 That is to say, raising the temperature beyond 100 °C will result in insolubility and the formation of two phases. PEG in aqueous solution behaves as a highly mobile molecule Received: August 22, 2012 Revised: February 8, 2013 Published: February 22, 2013 3024

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Table 1. Ingredients and Conditions of Dispersion Model Experiments content of copolymer (wt %) monomer phase copolymer (g) St (g) MMA (g) PEG (×10−2) (g) aqueous phase PEG (g) water (g) temp (°C)

a-1

a-2

b-1

b-2

c-1

c-2

d-1

d-2

e-1

e-2

e-3

e-4

5 1 9.5 9.5 9.5 15 65 85

5 1 9.5 9.5 0 0 80 85

10 2 9 9 9 15 65 85

10 2 9 9 0 0 80 85

20 4 8 8 8 15 65 85

20 4 8 8 0 0 80 85

30 6 7 7 7 15 65 85

30 6 7 7 0 0 80 85

50 10 5 5 5 15 65 85

50 10 5 5 0 0 80 85

50 10 5 5 5 0 80 85

50 10 5 5 0 15 65 85

where ΔGt is the total energy at work in the system. This implies that all factors will influence the actual stability of the system. Through a unique process, the system studied can get a stable, well-distributed, nanometer scale dispersion product. Yet this process cannot be explained by any known nucleation and particle growth mechanism. Therefore, for understanding the process, it is very important to study how the PEG affects the aqueous, monomer, and copolymer phases in polymerization system, what role PEG plays during the polymerization process, and how the different phases interact. Here, we study the role of PEG in the process of polymerization of St/MMA by researching the distribution of PEG in monomer/aqueous phase, the effects of PEG on copolymer dispersion, PEG/ monomer/copolymer ternary phase system, online viscosity of system during polymerization, and surface tension between phases. The research into the St/MMA/PEG system can get a new viewpoint to understand how the PEG affects the nucleation of particle and can provide a new simple way to get a stable nanometer scale colloid system. For industry application, this technology can be applied in the preparation of stable and environment friendly water-borne ink, paint, and coating.

with a large exclusion volume. Relaxation-time studies show rapid motion of the polymer chain.22 All of these unique properties of PEG can affect the nucleation of the system in certain ways. On the other hand, the development of the morphology of a latex during the polymerization process and various colloid stability mechanisms have a great influence on the abnormal nucleation in aqueous PEG solution. In the field of the morphologies of copolymer particles, there has been much pioneering research. Torza and Mason23 studied the morphologies of suspensions of two kinds of incompatible liquid drops in a third incompatible liquid. However, their experiments were limited to liquid drops of low viscosity, and hence their prediction model for the morphology of particles is not applicable to polymer systems. On the basis of the method of Torza and Mason, Sundberg et al.24 developed a model to predict the morphologies of particles based on interfacial freeenergy changes of the three phases of polymer/oil/water (with surfactant). Chen et al.25 predicted the morphologies of emulsion polymerization of MMA on seed polystyrene and obtained results in good agreement with actual experiments. González-Ortiz and Asua26−28 studied latex particle morphologies and concluded that structured latex particles may generally be formed. In the field of colloid stability, two opposite interaction energies are at work in the system:29 (1) attraction energy, with attractive van der Waals potential ΔGa, and (2) repulsive energy, which encompasses repulsive electrostatic potential, ΔGe, and repulsive steric energy, provided by the polymer adsorbed on the surface of the colloid particle. Vincent et al.30 carried out a quantitative study of the steric stabilization effect. When two colloid particles bearing adsorbed polymer approach, two types of net repulsive forces are produced between their surfaces. One is the osmotic contribution, ΔGosm, which operates before the polymer layers actually interpenetrate. The local concentration of polymer chains between colloid particles increases above the bulk equilibrium value, causing an osmotic pressure effect that makes solvent molecules move into the area between the surfaces of particles, thereby pushing them apart.31 The other is the coil compression contribution, ΔGvr, which arises as a result of some polymer molecules being forced to undergo elastic compression. Thermodynamically, this compression corresponds to a net loss in configurational entropy. This effect gives rise to a new repulsion potential related to the restriction of the movement of the hydrophilic coils extended toward the solvent. The system stability will depend on the net form of the interaction energy curve, that is, the sum of the attractive and repulsive energies according to eq 1, as a function of the separation between the particles. ΔGt = ΔGe + ΔGosm + ΔGvr − ΔGa

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EXPERIMENTAL SECTION

Materials. Monomers, St (Aldrich, 99%) and MMA (Aldrich, 99%), were washed with 10 wt % aqueous NaOH (Fisher Scientific) solution followed by deionized water until the washings became neutral and then distilled under vacuum. The middle distillation fraction was collected and stored in a refrigerator before use. Buffer, NaHCO3 (Aldrich), and the initiator K2S2O8 (FMC Corp.) were of analytical grade. PEG (molecular weight about 2000) and hydroquinone were used as received. Tetrahydrofuran (THF) and methanol were of analytical grade. Deionized water was used throughout. Synthesis. Emulsion polymerizations were carried out in a 250 mL four-necked round-bottomed glass reactor equipped with a Teflon impeller. The reactor was first charged with water, NaHCO3, and PEG, followed by a mixture of MMA and St monomer. The reactor was flushed with nitrogen for about 20 min and then sealed. The speed of agitation was set at 300 rpm. The reactor was immersed in a water bath and left to rise to the reaction temperature for 10 min. K2S2O8 solution was then added to start the reaction. Samples were taken by means of a syringe at timed intervals. The whole polymerization process required about 120 min. A copolymer sample was prepared by the following method. The withdrawn emulsion sample was separated by centrifugation at 20 000 rpm for 2 h. The sediment was dissolved in a small amount THF, and this solution was poured into a large volume of methanol to precipitate the copolymer to eliminate PEG, residual monomer, and initiator. The copolymer was then dried. This process of dissolution and precipitation was repeated more than three times to obtain the pure copolymer. Distribution of PEG in the Aqueous and Monomer Phases at Different Temperatures. In an effort to determine the distribution

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Figure 1. Photos of system status during polymerization process (H2O = 64%, St = 10%, MMA = 10%, PEG = 15%, KPS = 0.8%, NaHCO3 = 0.2%, T = 85 °C, stirrer speed = 300 rpm). Conversion: (a) = 0%, (b) = 5.4%, (c) = 20.2%, (d) = 45.3%, (e) = 54.5%, (f) = 95.3%. of PEG in the aqueous and monomer phases, a designed extraction experiment was conducted. The monomer phase, a mixture of 50:50 (w/w) of MMA to St containing a small quantity of hydroquinone as inhibitor, was prepared. The aqueous phase was a PEG solution with a PEG to water ratio of 18.75:81.25 (w/w). The monomer and aqueous phases were divided into two equal batches. In the first batch, a mixture consisting of 2:8 (w/w) of monomer phase to PEG aqueous phase was stirred at 20 °C for 15 min. After standing for 5 min, the upper part was withdrawn. In the second batch, the monomer mixture was heated to 85 °C and immediately transferred to the aqueous PEG solution at the same temperature. After stirring for 15 min, the mixture was left to stand for 5 min without cooling. The upper part of the solution (monomer part) was withdrawn without cooling. The samples were placed in a weighing bottle to determine the weight precisely, then heated in an oven at 120 °C. After complete evaporation of the monomer and inhibitor, the involatile residue was accurately weighed. The samples withdrawn from the two batches were coated on potassium bromide plates and dried, and then measured by FTIR. To avoid the influence of film thickness on the FTIR results, the sample volumes coated on the plates and the spreading area were kept as consistent as possible. Dispersion Model Experiments. Dispersion model experiments were designed to model the real dispersion of the polymerization system. Details of the dispersion composition and experimental conditions are provided in Table 1. As described above, the copolymer was obtained by surfactant-free copolymerization of MMA/St without PEG, and washed three times to remove the residual monomer and initiator. The monomer phase was heated to 85 °C, maintaining this temperature until the copolymer and PEG dissolved in the monomers, and then immediately added to the aqueous phase at the same temperature with stirring. After the sample was stirred for 15 min at the same temperature, the size distribution of the dispersed sample was measured. Cloud Point Titration. A mixture consisting of monomer St/ MMA = 1:1, PEG, and copolymer of St/MMA can form a ternary solid−liquid system. As before, the copolymer was obtained by surfactant-free copolymerization of MMA/St without PEG, and washed three times to remove the residual monomer and PEG. The miscibility of such a system can be estimated by cloud point measurement. To this end, a small amount of the copolymer/ monomer mixture and a magnetic stirrer were placed in a sealable glass bottle with a rubber cover. The bottle was heated to 85 °C, and the magnetic stirrer was started. The ratio of copolymer/monomer

mixture was controlled within the range in which the viscosity of the copolymer solution is as high as possible but does not hinder the rotation of the stirrer. As the copolymer dissolved in the monomer mixture when the temperature was kept at 85 °C, PEG, which melts at 85 °C, was added dropwise by means of a syringe. When the copolymer solution first displayed cloudiness, the weight of PEG added was precisely weighed. An equilibrium data point in the triangular phase diagram was obtained. Further monomer mixture was then added to the bottle until the copolymer solution became translucent once more, and the weight of the monomer mixture was accurately measured. The above process was repeated until the bottle was filled. Through this method, the weight ratio of the ternary system was continuously varied, and an equilibrium line was formed by a series of cloud points. Ternary Composition during the Polymerization Process. The samples extracted from the polymerization system were added to centrifuge tubes and centrifuged on a Hitachi CR 22G high-speed refrigerated centrifuge until the aqueous phase completely separated from the polymerization system. The aqueous phase was removed by means of a syringe, transferred to a weighing bottle, and heated in an oven at 120 °C. As the weight of the bottle was constant, the weight of PEG left in the aqueous phase could be determined. This is based on the following hypothesis: (1) copolymer and monomer, which are insoluble in the aqueous phase, can be completely separated through high-speed centrifuging; (2) PEG, which is adsorbed on the surface of the copolymer, can be separated from the aqueous phase with the copolymer by centrifuging; and (3) PEG, which is soluble in the aqueous phase, remains in the aqueous phase after centrifugation. At the same time, the conversion of the extracted sample was measured. Using these data, the weight composition of the ternary system of copolymer/PEG/monomer during the polymerization process could be attained. Characterization. The monomer conversion was determined by a gravimetric method. At first, the samples withdrawn were accurately weighed, and 5% hydroquinone solution was added to inhibit the reaction. The samples were then dried in an oven to constant weight. Here, conversion = (GH − GSPP)/GSPM × 100%, where GS is the weight of the sample, GH is the weight of the dried sample, PM is the percentage of monomer originally introduced, and PP is the percentage of the involatile materials. The samples withdrawn from the polymerization system were diluted and then coated on a copper grid for TEM analysis, which was carried out on a Philips 400T transmission electron microscope operating at 100 kV. The particle size distribution was measured with a 3026

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Figure 2. Curves of conversion versus time and comparison of TEM images of particle from the system with PEG in the aqueous phase at different conversion (H2O = 64%, St = 10%, MMA = 10%, PEG = 15%, KPS = 0.8%, NaHCO3 = 0.2%, T = 85 °C). Conversion: (a) = 4.9%, (b) = 25%, (c) = 55.4%.

Figure 3. TEM images of particles of the final products with different content of PEG. Beckman Coulter LS 13 320 laser sizer. The particle potential was obtained by means of a Beckman Delsa Nano apparatus. The samples were diluted with water to give a concentration of about 100 mg L−1. FTIR spectra were measured on a Bruker FTIR spectrometer. The copolymer was dissolved in THF, and the solution was coated on a potassium bromide plate. The viscosity of the system during polymerization was measured with a Haake Rheo Stress 6000 apparatus. The contact angle and pendant drop were measured using a USA Kino SL200A model contact angle and pendant drop tension meter.

part) becomes transparent. At this moment, the conversion of polymerization system is about 15%. Stage III is a process of flocculation and finer. In Figure 1c, it can be seen that the copolymer flocculated in the monomer phase. Also, in the process from (c) to (e) in Figure 1, the appearance of system feels finer gradually, even there is no feeling of flocculation in Figure 1e. The microscopic morphology of the copolymer particles during stage III is shown in Figure 2. It can be seen that the copolymer particles assemble to form a stacked particle pile. Stage IV is a rapid process that can be completed in several minutes. As regards the stability of the system, the final product shows excellent resistance to solvents, electrolytes, and freeze thawing. These properties are not emphasized in this Article, but are just mentioned without details. It is worthy of note that while the system was transformed from particle pile to independent stable nanoscale particles during this stage, the conversion of the system increased rapidly from 55% to 95%. Figure 3 shows TEM images of the final products with different contents of PEG. It can be found that whether or not PEG is present in the system can greatly affect the particle size of the final product. Without the participation of PEG, the final particle size is about 300 nm. With the participation of PEG, the content of PEG is a key factor that decides whether the system can be stable. When the content of PEG is 1%, the system begins to coagulate in a short time after initiating, and cannot be redispersed anyway as shown in Figure 3. When the content of PEG is more than 5%, even reaching 30%, the polymerization systems all can be stable, and the size of particle

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RESULTS AND DISCUSSION Polymerization Process. Figure 1 shows photographs of the surfactant-free polymerization process of St/MMA in aqueous PEG solution. The process can be clearly classified into four stages: stage I as shown from (a) to (b), stage II as shown from (b) to (c), stage III as shown from (c) to (e), and stage IV as shown from (e) to (f). In stage I, it can be seen in Figure 1a that the monomer and aqueous phases without initiator in the system at 85 °C still separate almost completely after 10 min of mixing at 300 rpm, thus indicating that PEG is not an effective surfactant for emulsifying the monomer in the aqueous phase as monomer swelling micelles. In Figure 1b, the monomer phase (upper part) and aqueous phase (bottom part) are still separated, and the copolymer particles are present predominantly in aqueous phase. In stage II, it can be seen that the place where copolymer particles are present changes gradually from the bottom part into the upper part. In Figure 1c, the aqueous phase (bottom 3027

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is less than 100 nm. At the same time, in this range, the effect of PEG content on the size of the final particle is not obvious. During the polymerization process, another interesting phenomenon is the viscosity change of the system. Figure 4

Figure 5. FTIR spectrum comparison of PEG and residue material withdrawn from monomer phase at different temperature.

FTIR spectrum, as shown in Figure 5. It was thus proved that the ability of PEG to enter into the monomer phase is enhanced at higher temperature. The content of PEG extracted by the monomer phase was about 0.45% (w/w) [PEG/(PEG +monomer)] at 85 °C. The distribution of PEG in both phases at different temperatures shows that its effect on the polymerization comes not only from the aqueous phase, but also from the monomer phase. Effect of PEG on Interfacial Tension between Phases. The described polymerization system consists of an aqueous phase, a monomer phase, and a copolymer phase. The interfacial tension between phases is a very important parameter affecting the morphology of copolymer particles. Table 2 lists the contact angles or interfacial tensions between the various phases. From pendant drop data, it was

Figure 4. Viscosity change of system with polymerization time (H2O = 64%, St = 10%, MMA = 10%, PEG = 15%, KPS = 0.8%, NaHCO3 = 0.2%, T = 85 °C).

shows the curve of viscosity change of the system with polymerization time. The letters in Figure 4 correspond to the various periods shown in Figure 1. It can be seen that as the polymerization reaches stage IV, which is just the period of abnormal transformation, the viscosity of the system increases drastically from 180 to 1281 mPa s, but then quickly reverts to a level of about 220 mPa s. It takes only a few minutes for the whole transformation process. The polymerization rate of the system with PEG is obviously lower than that of the system without PEG (see Figure 2). When the conversion of the system with PEG reaches 50%, the conversion of the system without PEG is more than 95%. However, when the conversion increases beyond 64%, the polymerization rate is accelerated abruptly, and the conversion reaches more than 95% in a very short time, about one-tenth of the time needed for the previous polymerization. Concomitantly, the system releases a considerable amount of energy, and the temperature increases from 85 to 92 °C. From the above description of the polymerization process, it can be seen that in the presence of PEG, the St/MMA surfactant-free emulsion polymerization system undergoes an abnormal process, which differs not only from traditional surfactant-free emulsion polymerization, but also from traditional emulsion polymerization. It is obvious that PEG plays an important role in the polymerization process. Propensity for PEG To Enter the Monomer Phase. PEG is a special kind of hydrophilic copolymer, having a large amount of EO units. As a result, it shows some characteristics of a nonionic surfactant bearing EO units, such as a phase inversion temperature (PIT). However, the PIT of PEG is above 100 °C,15−17 and phase inversion does not take place for the system under the present polymerization conditions. Nevertheless, PEG still shows a tendency to enter into the monomer phase from the aqueous phase with increasing temperature. Figure 5 shows the propensity for PEG to enter the monomer phase at different temperatures. From Figure 5, it is evident that no residual material was left in the monomer phase at 25 °C, but when the temperature was increased to 85 °C, the monomer phase contained a large amount of involatile material. This was identified as PEG by comparison with its standard

Table 2. Interfacial Tension between Phases drop phase monomer mixture of St/MMA monomer mixture of St/MMA with 0.9% (w/w) PEG water water with 2.28% (w/ w) PEG water with 19.46% (w/ w) PEG water water with 2.28% (w/ w) PEG water with 19.46% (w/ w) PEG

plate or ambient phase

method

contact angle (deg)

copolymer of St/MMA copolymer of St/MMA

contact angle contact angle

29.6

copolymer of St/MMA copolymer of St/MMA copolymer of St/MMA monomer mixture of St/MMA monomer mixture of St/MMA monomer mixture of St/MMA

contact angle contact angle contact angle pendant drop

99.1

surface tension (mN/m)

22.7

74 73.4 19.1

pendant drop

15.9

pendant drop

10.4

observed that PEG decreased the interfacial tension between the monomer and aqueous phases from 19.1 to 10.4 mN/m, although this reduction was not large enough to give rise to an obvious decrease in the energy required to deform and disrupt the monomer droplets for emulsification.32 Furthermore, PEG can be dissolved in both the monomer and the aqueous phases, which weakens the Gibbs−Marangoni effect, preventing the 3028

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droplets from coalescing. Therefore, it is easy to understand that PEG cannot effectively emulsify the monomer in the aqueous phase at the beginning of the polymerization. The contact angle data in Table 2 prove the following points: (1) the copolymer of St/MMA is hydrophobic because the contact angle between the copolymer and water is greater than 90°; (2) PEG in the aqueous phase can be adsorbed on the surface of the copolymer, thereby rendering the latter more hydrophilic, because the adsorbed PEG makes the contact angle between the copolymer and water less than 90°; (3) the monomer wets the surface of the copolymer much more easily than water; and (4) PEG in the monomer phase can also be adsorbed on the surface of the copolymer, decreasing the interfacial tension between copolymer and monomer. Effect of PEG on the Size and Size Distribution of the Copolymer/Monomer Dispersion in the Aqueous Phase. The propensity for PEG to adsorb on the surface of the copolymer and change the interfacial tension between the copolymer phase and other phases can easily affect the dispersion of the copolymer in the aqueous phase. To study how PEG in the different phases affects nucleation of the copolymer with increasing conversion, a dispersion model experiments experiment was designed as described in the Experimental Section. Figure 6 shows the size distributions of the dispersion model experiments samples.

Figure 7. Dispersion photos of copolymer and St/MMA monomers mixture dispersed in water.

phase and just a copolymer ball in the bottom for e-2 and e-3 systems, which have no PEG in the aqueous phase. When system has PEG in the aqueous phase, the phenomena are completely different. As shown in Figure 7, there are a large number of particles dispersing in the aqueous phase for e-1 and e-4 systems. From Figure 6, it can be found that the PEG in the monomer phase has a great effect on both the number-average and the volume-average size of particle dispersed in the aqueous phase. It can be found that e-1, which has PEG in the monomer phase, has more and smaller particles than e-4. This confirms that PEG in the monomer phase can make more copolymer dispersed in the aqueous phase. From the experimental results, the following conclusions can be drawn: (1) PEG in the aqueous phase can improve the dispersion of the copolymer therein. (2) At low conversion, the viscosity of the copolymer/monomer mixture is relatively low. Therefore, the gentle shear of stirring can break the mixture into tiny pieces and disperse them in the aqueous phase. Under these conditions, the presence of PEG is not the key factor to make the copolymer disperse in the aqueous phase. (3) At high conversion, the percentage of copolymer in the copolymer/ monomer mixture is relatively high, which results in high viscosity of the mixture. When such a mixture is dropped into the aqueous phase, it is difficult to break it into tiny pieces by the gentle shear of stirring. Even if some pieces of the mixture are separated from the bulk by stirring, they will easily agglomerate due to the absence of a stabilizer. Under these conditions, PEG in the aqueous phase is a key factor in determining whether the copolymer can be dispersed therein. (4) PEG in the monomer phase has a great effect on breakage of a polymer/monomer mixture when such a mixture is dropped into a stirred aqueous phase. Because PEG in the monomer phase can be adsorbed on the surface of the copolymer in the mixture, it is easier for the copolymer to separate from the viscous mixture matrix through stirring due to the wedge effect of the PEG. Therefore, a sample with PEG in both phases can disperse more copolymer into the aqueous phase than one in which PEG resides only in the aqueous phase. Effect of PEG on the Morphology of Copolymer Particles Formed in the Aqueous Phase. As mentioned above, there are three primary phases in the polymerization system, and PEG is soluble in both the monomer and the aqueous phases. PEG in different phases can exert different effects on the morphologies of copolymer particles. In this section, we consider how PEG in the aqueous phase affects the morphology of copolymer particles formed in this phase at the beginning of the polymerization.

Figure 6. Particle size and size distribution of St/MMA copolymer and their monomers mixture dispersed in water with or without PEG (solid line is number average size distribution; dotted line is volume average size distribution).

It can be seen that when the conversion was less than 50%, the dispersion model experiments samples have a lot of detectable particles in the aqueous phase irrespective of whether or not there is PEG in the system. However, the presence of PEG usually makes dispersed particles smaller. It is easy to understand that PEG can improve the dispersion because of its better wettability. When conversion exceeds 50%, the importance of PEG in the aqueous phase to nucleation becomes apparent, because there were no detectable particles in the dispersion sample when PEG was not present in the aqueous phase. To understand how PEG in different phases helps polymer/ monomer mixture dispersing into aqueous phase at 50% conversion, four dispersion model experiments were designed as shown in Table 1 (from e-1 to e-4). Figure 7 shows the state of four dispersion model experiments. There are no detectable particles in the aqueous 3029

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maximum surface potential repulsion during the unique transformation. Also, this repulsion is a beneficial force to help particle separate from particle pile. Effect of PEG on the Morphology of the Copolymer in the Monomer Phase. When a copolymer particle enters the monomer phase, it can either dissolve or maintain its particulate morphology. Experimentally, we found that the copolymer is completely soluble in the monomer phase, even when the solid content is more than 50%. However, it should be noted that the monomer phase in the polymerization system not only contains the copolymer but also PEG. The copolymer, PEG, and monomer form a ternary phase system. A triangular phase diagram provides a simple method to predict the form that will be adopted by the copolymer in the monomer phase. As shown in Figure 9, the line formed by a series of open circles is a cloud

Although in PEG-containing systems the copolymer particles are predominantly produced in the aqueous phase at the beginning, the morphology of these particles differs from that of particles formed by traditional surfactant-free emulsion polymerization. As shown in Figure 8, for the 15% case, when

Figure 8. Comparison of zeta potential between system with and without PEG at different conversion (H2O = 64%, St = 10%, MMA = 10%, PEG = 15%, KPS = 0.8%, NaHCO3 = 0.2%, T = 85 °C).

the conversion was less than 5%, the particle surface potential of the system with PEG was very low, even less than −10 mV. However, for a system without PEG at the same conversion, the particle surface potential was higher than −50 mV. This phenomenon can be explained in terms of the adsorption of PEG onto the surface of the copolymer. While copolymer chains separate from the aqueous phase after reaching a certain critical length, PEG in the aqueous phase can be adsorbed on the surface of the copolymer chains, affecting their properties. The adsorbed PEG will hinder the aggregation rate of the unstable copolymer chains because of its steric hindrance, extending the formation period for stable particles. However, unstable copolymer chains will still aggregate into particles because their surface potential increases to more than −40 mV with increasing conversion. It should be noted that the surface potential of a system with PEG is still lower than that of a system without PEG. These features show that the adsorbed PEG imparts the copolymer particles with a loose structure. As a result, the surface charge density of particle of the system with PEG is less than that without PEG before the conversion is more than 60%. As compared to the system without PEG, the particles with loose structure prefer to enter into the monomer phase because the PEG adsorbed on their surfaces can improve the wettability of the monomer on the surface of the copolymer, as indicated in Table 2, and decrease the energy required for the copolymer to enter the monomer phase. At the same time, the content of PEG in the aqueous phase decreases, and increases the surface tension between the copolymer and water. As a result, the copolymer chains bearing adsorbed PEG can migrate more easily from the aqueous phase into the monomer phase. This just explains how the copolymer particles migrate from the aqueous phase into the monomer phase in stage II. The systems with different content of PEG have the same zeta potential trend with the process of polymerization. Especially, when the conversion is more than 50%, the potential trend curves with different content of PEG appear concave. Also, it should be noted that the time when the concave spot appears is just the moment when the unique transformation of the polymerization system occurs. Therefore, it can be confirmed that the polymerization system can have a

Figure 9. Ternary phase diagram of copolymer−PEG−monomer.

point line. The line shows that PEG can be considered as a sedimentation agent for the copolymer/monomer binary system. The region under the line is a ternary phase coexistence region, while the region above the line shows the conditions under which the copolymer will separate from the monomer phase. From this triangular phase diagram, it is clear that the content of PEG in the monomer phase is a key factor in determining whether the copolymer can exist in the monomer phase as particles. To study the compositional change of the ternary system of PEG/monomer/copolymer during the polymerization process, we designed a series of experiments as mentioned in the Experimental Section. The solid symbols in Figure 9 are points showing the equilibrium weight composition of the ternary system with increasing conversion. For the 15% PEG case, it can be seen that at a very low conversion of less than 11%, the point is under the cloud point line, which means that a copolymer particle entering the monomer phase in this region would dissolve rather than remain as a particle. Therefore, in stage II, the low conversion period, the monomer phase is translucent as shown in Figure 1b. With increasing conversion, however, the point in the ternary phase diagram is above the cloud point line, which means that the copolymer begins to separate from the monomer phase because of the deposition of PEG. Therefore, in stage III, the monomer phase is opaque. It should be noted that there is a great difference in the PEG content in the monomer phase between the binary phase system that is just composed of monomer and PEG as described previously, in which the PEG content in the monomer phase is just 0.45% (w/w) [PEG/(PEG+monomer)], and the ternary phase system composed of monomer, 3030

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Figure 10. TEM images of particle morphology as conversion is 43.1% with PEG = 15%.

PEG, and copolymer, in which the PEG content in the monomer phase is more than 28% (w/w) [PEG/(monomer +PEG+copolymer)]. The excess PEG content mainly arises in two ways: (1) The first is from particles formed in the aqueous phase. As mentioned above, PEG can be adsorbed on the surface of copolymer chains, lowering the surface potential of the particles. Therefore, when copolymer particles enter the monomer phase, a large amount of excess PEG also enters the monomer phase. (2) Second, there is also diffusion of PEG into the monomer phase. Although the capacity of the monomer to extract PEG from the aqueous phase is not great, any PEG in the monomer will be adsorbed on the copolymer dissolved in the monomer phase. Therefore, the concentration of PEG in the monomer phase decreases greatly, and the disruption of the equilibrium drives the PEG in the aqueous phase to enter the monomer phase. Of course, an appropriate stirring speed can accelerate the rate of entry of PEG because mixing will greatly increase the area of interaction between the aqueous and monomer phases. The process of PEG entry is very fast. When the conversion reaches 2%, the PEG content accounts for more than 28% of the entire ternary system. However, an equilibrium of mass transfer between the monomer phase and the aqueous phase for PEG is soon reached. Therefore, the increase in PEG content in the ternary system becomes slow after 2% conversion. As Figure 9 shows, except for 1% PEG case, the polymerization systems with different content of PEG have the same trend that the ternary equilibrium composition line with polymerization will be across the cloud point line at the end, and have the same experimental phenomena of unique transformation in polymerization. The system with 1% PEG, the equilibrium composition of which cannot be across the cloud point line, has been coagulated completely in 10 min. These results show that the phase separation is the key for unique transformation, and PEG is the key for phase separation. From the above experiments, it can be concluded that copolymer particles entering the monomer phase can retain their particulate morphology when the conversion of the polymerization system exceeds 11%. Figure 10 shows the TEM images of a sample withdrawn from the polymerization system at 43.1% conversion. It can be observed that a great number of nanoscale particles were aggregated into a particle pile. These TEM images verify that the copolymer that had entered the monomer phase retained its particulate morphology. Role of PEG during the Polymerization Process. In traditional emulsion polymerization, a surfactant can help the polymerization system emulsify the monomer in the aqueous

phase to form tiny micelles, and particle formation and growth take place in these micelles. At the same time, a surfactant helps the particles to disperse stably in the aqueous phase. In the present polymerization system, however, PEG cannot effectively emulsify the monomer in the aqueous phase to form micelles. In other words, PEG in this polymerization system does not serve as a surfactant. However, at the end of the polymerization, the system with PEG forms a more stable, finer emulsion than the system without PEG. From the triangular phase diagram, it can be seen that the critical point to discern the difference between stage II and stage III is whether the copolymer separates from the ternary coexisting system of copolymer/PEG/monomer. In stage II, the conversion is very low, and the composition point of the ternary system is under the equilibrium curve, and so any copolymer that enters into the monomer phase will dissolve to form a translucent copolymer solution. The copolymer only exists in the form of particles in the aqueous phase. Because PEG cannot stabilize the monomer phase in the aqueous phase, and the viscosity of the system shows no significant change at the beginning of the polymerization, as shown in Figure 4, the monomer phase in polymerization system is easy to separate out without stirring. With increasing conversion, more copolymer particles enter the monomer phase, and so the weight composition of the ternary system is changed. As the point in the triangular phase diagram is beyond the equilibrium line, the copolymer begins to deposit; the copolymer can exist in the monomer phase in the form of particles, and the polymerization system proceeds to stage III. Because of copolymer sedimentation, the monomer phase becomes cloudy and opaque as shown in Figure 1c. At the beginning of stage III, the copolymer/monomer ratio is low, so the copolymer particles can disperse freely in the monomer phase, and the viscosity of the system does not change greatly. PEG in the aqueous phase still cannot stabilize the monomer phase therein, as shown in Figure 1c. It should be noted that the deposition is a process of reforming of copolymer particles, which are different from the particles formed in the aqueous phase. In the middle of stage III, the copolymer/monomer ratio becomes larger, even exceeding 4:6, and the PEG content needed for sedimentation is less than that in the monomer phase. Therefore, most of the copolymer is separated from the monomer phase as dispersed particles. As more copolymer separates, the monomer phase becomes crowded and packed. Under the mechanical force of stirring, the monomer drop will be deformed and elongated,33−39 and large monomer drops 3031

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to escape from the particle pile. Therefore, from the macroscopic view, an abnormal process whereby the size of the particles changes from large to small can be observed. At the same time, the volume contraction of the particles provides more space for movement, and so the viscosity of the system returns to a relatively low level, as shown in Figure 4. Because the size of a particle separated from the particle pile is much smaller, as shown in Figure 12, the number of particles in the

containing particles break into small ones, as illustrated in Figure 11. This cracking process gradually reduces the size of

Figure 11. Schematic illustration of the mechanism of the particle pile breakup process.

the monomer drops. This trend can be observed from (d) to (e) in Figure 1, in which the appearance of the system becomes finer and smoother. It should be noted that PEG will hinder the rate of polymerization. There are two reasons for this: (1) the copolymer in the system with PEG is mostly transferred to the monomer phase, so the number of copolymer particles in the aqueous phase is few; and (2) PEG cannot emulsify the monomer phase in the aqueous phase; the monomer phase is just dispersed in the aqueous phase as large drops. In other words, the probability of a radical entering a copolymer particle or a monomer drop is very small. In comparison, in the system without PEG, the copolymer particles dispersed in the aqueous phase are smaller than those in the system with PEG, so there are more of them. Therefore, the polymerization rate in the system without PEG is greater than that in the system with PEG. The viscosity of the system will increase with decreasing size of the monomer drops for three reasons: (1) The droplet breakup can provide larger specific area, thus increasing the interaction between them. (2) The PEG between the monomer and the aqueous phase facilitates hydration of the monomer drops. Therefore, water will be adsorbed on the surface of the monomer drops to form a hydrated surface. The hydrated surfaces on droplet make the space more packed after droplet breakup, eventually hindering their free movement. In this regime, the viscosity will increase drastically, as shown in Figure 4. (3) As shown in Figure 2c, the particle size distribution is clear polydispersity. The packing of the small and large particles could produce an increase in viscosity. However, the packed situation does not mean that the copolymer particles will coagulate because the PEG coated on their surfaces prevents this. As conversion reaches a certain value, the viscosity of the system will reach a maximum value, as shown in Figure 4. At some point, the monomer phase around copolymer particles will vanish, and the remaining monomer only exists in the monomer swelling particles. Water will replace the monomer to fill the gaps between the particles. Further reaction leads to contraction of the copolymer particles, leading to an increase in the charge density on their surfaces, as shown in Figure 8. The increase in potential imparts the particles with sufficient energy

Figure 12. Particle size curve with conversion (H2O = 64%, St = 10%, MMA = 10%, PEG = 15%, KPS = 0.8%, NaHCO3 = 0.2%, T = 85 °C).

system will increase geometrically, and as a result the probability of a radical entering a particle increases exponentially. Therefore, the rate of polymerization accelerates greatly as shown in Figure 2, and a great deal of thermal energy is released in a short time, resulting in a rapid increase in the temperature of the system.

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CONCLUSION PEG can exist in both the monomer and the aqueous phases at high temperature. The PEG in the different phases can play different roles during the polymerization process. At the beginning of the polymerization, the PEG in the aqueous phase will affect the morphology of particles formed therein, imparting them with a loose structure. Such particles can enter into the monomer phase because they have a low surface potential and better wettability between the monomer and copolymer. Entry of the copolymer will also transfer excess PEG into the monomer phase. The PEG, monomer, and copolymer will form a ternary phase system. This system has a coexistence region, which is under the equilibrium curve. When conversion of the polymerization system reaches a critical point, the composition point in the triangular phase diagram will be beyond the curve, and the copolymer entering the monomer phase will separate to form particles. This process involves reformation of the particles, leading to a new size distribution. These newly formed particles retain their morphologies until the end of the polymerization. With increasing conversion, more copolymer particles will separate from the monomer phase, and these gradually pack the monomer phase. However, the adsorbed PEG will prevent particles from coagulating. The packed monomer phase results in an increase in viscosity, and increasing viscosity will break the monomer drops into tiny droplets. When the viscosity of the system reaches a maximum value, the monomer is present in the swollen particles. Water will replace the monomer to fill the space around the particles. Further consumption of the monomer reduces the volume of the particles, thereby increasing their surface potential. The increased surface 3032

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potential is a driving force to push the copolymer particles away from the particle pile. These particles are very stable because of the PEG adsorbed on their surfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Scientific Foundation of China (No. 21176210) and the Outstanding Youth Foundation of Zhejiang Province (R4110199).

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