Semicontinuous Monomer-Starved Emulsion Polymerization as a

Apr 16, 2013 - In this research, particle nucleation was decoupled from particle growth in the monomer-starved semicontinuous (micro)emulsion ...
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Semicontinuous Monomer-Starved Emulsion Polymerization as a Means to Produce Nanolatexes: Analysis of Nucleation Stage Yan Chen,† Fatemeh Jahanzad,‡ and Shahriar Sajjadi*,§ †

Division of Engineering, King’s College London, Strand, London WC2R 2LS, United Kingdom Chemical and Petroleum Engineering Group, London South Bank University, 103 Borough Road, London SE1 0AA, United Kingdom § Department of Physics, King’s College London, Strand, London WC2R 2LS, United Kingdom ‡

ABSTRACT: In this research, particle nucleation was decoupled from particle growth in the monomer-starved semicontinuous (micro)emulsion polymerization of styrene by close monitoring of the end of nucleation. This enabled us to exclude the effects of particle growth on nucleation and therefore unravel inherent features of nucleation in this process. Nanolatexes with average particle sizes as small as 15 nm were obtained. The average size of particles at the end of nucleation was found to be almost independent of surfactant concentration ([S]) but varied with the rate of monomer addition (Ra) to the 1/3 power. Nucleation time varied almost proportionally with [S]1.0. The sharpest particle size distribution was obtained at the lowest monomer feed rate used. The weight-average molecular weights (M̅ w) of the polymer produced decreased with decreasing Ra. A simple correlation was developed which shows that the number-average molecular weight (M̅ n) is proportional to Ra1.0 but independent of [S] (i.e., [S]0.0), which was in fair agreement with the experimental results. It is also shown that the polymer molecular weight is proportional to the average volume of particles; the smaller the particle, the lower the molecular weight.

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INTRODUCTION Micellar solutions are thermodynamically stable aqueous solutions of surfactant micelles swollen with oil. Each micelle usually can contain oil up to 1−3 times the number of surfactant molecules forming the micelle, implying that only a small amount of oil can be solubilized by such solutions. A fourth component, cosurfactant, is often used to reduce the interfacial tension and enlarge micelles (i.e., droplets) so that they can solubilize more oil. Such solutions are usually referred to as microemulsion. Microemulsion polymerization has been frequently used to produce nanolatexs1,2 but equally suffers from usage of a large amount of surfactant, in addition to cosurfactant, and is usually limited to low-solid content.3 Semicontinuous monomer-starved emulsion or microemulsion polymerization is a variant of conventional emulsion or microemulsion polymerization that can produce concentrated nanolatexes using a low concentration of surfactant simply via delayed addition of the monomer.4−13 The prerequisite for preparation of nanolatexes is to start with a micellar solution of a monomer (basically a 3- or even 4-component microemulsion), followed by addition of monomer at a low rate. Nanolatexes have been used in a number of applications, including ultrathin films, high performance coating materials, adhesives, semiconductors, and the drug delivery nanocarriers used for penetrating various biological barriers within the human body.14−17 Nanolatexes of different monomers, including butyl acrylate (BA), methyl methacrylate (MMA), © 2013 American Chemical Society

styrene (St), vinyl acetate (VA), methyl acrylate (MA), butyl methacrylate (BMA), and n-isopropylacrylamide (NIPAM) have been studied.6,8,9,12,18−23 Such nanolatexes usually have narrow size distributions.8,12,19 Furthermore, particles with core−shell structures and functionalized groups have been synthesized as well, using this technique.11,18,24 Nomura showed that if polymerization is carried out in the presence of a small quantity of monomer, particle formation is enhanced.25 Sajjadi pointed out that in a typical semicontinuous monomer-starved (microemulsion) polymerization, in which the volume growth rate of particles is controlled by the rate of monomer addition (Ra), particle formation is prolonged and particles with small and narrow size distribution can be produced.9,19 A few groups have studied particle nucleation in such systems and have indicated how particle size and number vary with reaction conditions. Puig’s group12,26 investigated the effects of surfactant concentration ([S]) and Ra on the particle size and number for MMA and BMA. The exponent values of particle size in terms of [S] and Ra were found to be around −0.20 ± 0.01 and 0.17 ± 0.03, respectively. The exponent values of Np with respect to [S] and Ra were around 0.68 ± 0.02 and −0.48 ± 0.01, respectively. Furthermore, the molar masses remained Received: January 7, 2013 Revised: April 16, 2013 Published: April 16, 2013 5650

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there was no reference to the end of particle nucleation. In some of the works, particle nucleation was completed by the end of monomer feeding,5,6,10,13,19,20,22,28,29,31 whereas in many others, particle nucleation continued well beyond monomer feeding time.6,8,9,11,12,21,23,24,26,27 Most, if not all, experimental data available in the literature are based on the particle size average and distribution at the end of monomer addition.5,8−10,12,23,26 Such data cannot truly reflect features of particle size and size distribution at the end of nucleation because of (1) the growth of particles during postnucleation, which can significantly change the broadness of particle size distribution5,29 and (2) possible coagulation in the course of monomer addition.19 From above, it can easily be inferred that the exact relations between particle size, PSD, and Np with respect to Ra and [S] are not really known. It comes as no surprise that there are such significant variations in reported particle size and Np with process formulations. In this work, particle nucleation in the semicontinuous monomer-starved emulsion polymerization of styrene has been closely monitored and is decoupled from particle growth. The effects of [S] and Ra on the kinetics, average particle size and size distribution, number of particles, and average molecular weights of polymers are reported and compared with the theoretical developments reported in the literature.

practically unchanged throughout the reaction for all [S] used but diminished as Ra decreased. Rempel’s group studied the effect of [S] and Ra on particle size for MMA and St.8,27 They found that the exponent value of particle size in terms of [S] was around −0.24 ± 0.01. Sajjadi and co-workers have conducted monomer-starved semicontinuous (micro)emulsion polymerization of several monomers under the conditions that utilization of surfactant micelles was assured. They found different exponent values of the final Np with respect to Ra for St, BA, MMA, and VA monomers, which are −0.65, −0.68, −1.98, and −2.36, respectively.5,10,22 The exponent values of Np in terms of [S] for St and MMA were also found to be around 0.79 ± 0.01.5,22 Sajjadi developed a model, which shows the effect of Ra and [S] on Np for styrenic monomers:28 Np = k(as[S])RI2/3R a−2/3

(1)

where k is a numerical constant, αs the adsorption area occupied by a molecule of emulsifier on the surface of polymer particles, and RI the rate of radical generation in the water phase. Good agreements with model predictions have been reported for St and BA semicontinuous (micro)emulsion polymerization.10,28 Very recently, Nunes and Asua13 showed that eq 1 could also predict their experimental Np for a wide range of polymerization conditions. Sajjadi also derived an equation for the nucleation period (tf) t f = k(as[S])RI−1/3R a−2/3

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Chemicals. Analytical grade potassium persulfate (Aldrich), sodium dodecyl sulfate (SDS; Aldrich), and sodium hydrogen carbonate (Aldrich) were used as initiator, emulsifier, and buffer, respectively. Styrene monomer was supplied at 99.9% purity by Aldrich, inhibited against thermal polymerization with trace quantities of an inhibitor. The inhibitor was removed from the monomer by an ion-exchange column (Aldrich) prior to use. The uninhibited monomer was stored at −20 °C and used within a week. Apparatus. Polymerizations were carried out in a 1 L glass reactor equipped with a four-bladed flat turbine-type impeller with a width of half the vessel diameter, a standard four-baffle plate with a width of 1/ 10 the vessel diameter located at 90° intervals, a thermocouple, a sampling device, a port for nitrogen purge, and an inlet for feeding ingredients. The stirrer rate was kept constant at 300 rpm. The temperature of the reactor content was controlled at 70 ± 1.0 °C, by pumping water with the appropriate temperature from a water bath through the jacket. Sampling was carried out at the desired time interval by the removal of an aliquot of 1−2 g of latex with a hypodermic syringe. Procedure. The reactor was initially charged with most of the deionised water (570 mL), a weighed quantity of surfactant and buffer (0.2016 g), and allowed to heat up to the reaction temperature (70 °C) while being purged with nitrogen under strong mixing (450 rpm). The purging was continued at the reaction temperature for another 15 min. Then the nitrogen line was lifted to sit well above the surface of the water to prevent evaporation. The nitrogen rate was turned down to provide only slight overpressure, and the agitation speed was then reduced to 300 rpm. An amount of 0.6489 g of KPS dissolved in 30 mL of deionised water from the overall recipe was then added to the vessel. The system was allowed to return to the reaction temperature. Prior to addition, the monomer was purged with nitrogen for 15 min to remove the dissolved oxygen. The monomer addition was started instantly with a dosing pump at a given rate. The onset of reaction was continuously monitored by sampling from the vessel and precipitating the sample in methanol. Generally, inhibition periods of 1−3 min were observed. The reaction time zero for the start of reaction was considered when the first droplet of monomer was added into the reactor. The micellar solution was initially clear and transparent but became translucent to opaque with the progress of polymerization. The viscosity of the emulsion increased with the progress of reaction as the solid content increased.

(2)

There is no report in the literature that concerns nucleation time in semicontinuous emulsion polymerization, which unlike that in conventional batch polymerization can be easily tracked and accurately measured because of slow dynamics in semicontinuous polymerizations. The volume-average diameter of particles at the end of nucleation and its relation to polymerization variables were derived as follows:29 Dv = (6/π )1/3 (R a /RI)1/3

EXPERIMENTAL SECTION

(3)

where Dv is the volume-average diameter of polymer particles at the end of nucleation. Equation 3 ignores the density difference between monomer and polymer in the particles. It is also valid under the conditions that only micelles could capture radicals, therefore a proportionality relation should be used for a broader range of conditions. Equations 2 and 3 show that although tf prolongs with increasing [S], the average size of latex particles at the end of nucleation is independent of [S], a prediction yet to be compared with experiments. On the other hand, the volumeaverage diameter of latex particles at the completion of nucleation is proportional to Ra1/3. The particle size distribution (PSD) of latexes at the end of nucleation was also shown to be independent of surfactant concentration if the rate of radical entry into micelles and particles was high.29 The prerequisite for application of eqs 1 and 3 to monomer-starved nucleation, in order to elucidate the underlying mechanisms of nucleation, is identification of the end of nucleation. This enables one to use properties of latexes obtained at the end of nucleation for comparison with the model. A check on nucleation time, in addition to the size and number of particles, also serves as a complementary test for a nucleation model. In the most previous works cited, however, the dependence of Dv and Np on Ra and [S] was reported regardless of nucleation stage, and 5651

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Measurements. Conversions were measured gravimetrically. There are two types of conversion; one is the overall conversion, xo, defined as the weight ratio of the polymer produced in the reactor to the total monomer in the recipe. The other is the instantaneous conversion at a given time t, xi, defined as the weight ratio of the polymer formed in the reactor to the total monomer fed into the reactor by the time t. The rate of polymerization (Rp) was calculated from the gradient of the linear part (ignoring the first few minutes of reaction) of the overall conversion−time curves during the nucleation stage for each experiment. In order to obtain a better understanding of particle growth, the z average diameter of particles (Dz) was measured in the course of polymerization using dynamic light scattering (Zetasizer, Malvern). Selected samples were examined using a transmission electron microscope (TEM; Nippon, 200kv). More than 2000 particles were counted for each sample. For the PSD presentation, all particles in the range of a size increment of 5 nm were assigned to the higher bound of the size increment for the bin. The volume average diameter of particles (Dv) was calculated using the following equation:

⎡ ∑n n D3 ⎤1/3 i i ⎥ Dv = ⎢ i =n1 ⎢⎣ ∑i = 1 ni ⎥⎦

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Table 1. Recipe for the Semicontinuous (Micro)Emulsion Polymerization of Styrene

(4)

(5)

where the densities of styrene (ρm) and polystyrene (ρp) are 0.909 g mL−1 and 1.044 g mL−1,9 respectively. Using the Dv data and total amount of emulsifier in the reaction mixture, it is possible to estimate the surface coverage ratio of particles (θ), using the Langmuir model:

θ=

K[S]w 1 + K[S]w

(6)

where [S]w is the equilibrium surfactant concentration in the water phase and K is the adsorption constant with a typical value of 6000 L mol−1 for SDS.30 The amount of adsorbed surfactant at the interface (Sd; mol/L) is given by ⎛ πD2 N θ(M /N ) ⎞ ps p SDS A ⎟ Sd = ⎜⎜ ⎟ αs ⎝ ⎠

(7)

where αs is the area occupied by an emulsifier molecule on the surface of particles, which is 0.43 nm2 for SDS on polystyrene particles.28 NA is Avogadro’s constant and MSDS is the molecular weight of SDS. Dps is the average diameter of the monomer-swollen particles, which can be calculated as ⎛ 6(mp /ρ + mm /ρ ) ⎞1/3 p m ⎟⎟ Dps = ⎜⎜ π N p ⎝ ⎠

ingredients

quantity

styrene (mL) water (mL) KPS (mmol Laq−1) buffer (mmol Laq−1) reaction temperature (°C)

50 600 4 4 70

Rate of Polymerization (Rp). The prerequisite for the starved nucleation to occur, and also the application of eqs 1−3, is that the rate of polymerization should be tightly controlled by the rate of monomer addition and the absence of monomer droplets in the reaction system be ensured. This could be one of the reasons for discrepancies reported in the literature regarding the exponents of Np with respect to [S] and Ra. Figure 1 shows the variations in the overall (xo) and instantaneous (xi) conversions versus reaction time for different Ra and [S]. Before polymerization can start, the added monomer has to dissolve in the water phase and diffuse into micelles. This imposes some delay time, which decreased with increasing Ra (a higher rate of monomer addition will saturate the water phase and micelles sooner). The instantaneous conversion, which can be considered as a measure of monomerpolymer ratio in the system at any given time, started at high values (∼40%) for all sets, except for the set with [S] = 17 mmol Laq−1 (see Figure 1a). Under monomer-starved conditions, monomer can reside in three locations: water phase, micelles, and polymer particles. Polystyrene particles can absorb styrene to one and half times their weight (the critical ratio of polymer in the particle is wcr = 0.40).9 Above this ratio, the particles are not fully swollen with the monomer and are monomer starved. Therefore, a high polymer ratio at the beginning of reactions can signify fully monomer-starved conditions. Micelles can serve to establish starved conditions by solubilizing some monomer.31 The instantaneous conversion represents monomer conversion in the whole reaction vessel and should not be confused with the polymer weight ratio in particles (wp). Therefore, the domain of monomer-starved conditions is wider than what can be envisaged by xins > wcr = 0.40 (i.e., an xins value of smaller than 0.40 can still represent monomer-starved conditions, depending on Ra and [S]). The monomer partitioning among different phases was estimated based on a model reported elsewhere.28,29 Calculations show that only for the experiments with [S] = 17 mmol Laq−1 with high Ra (Ra ≥ 12.3 mol min−1 Laq−1), where there was not sufficient micelles in the aqueous phase to solubilize all the monomer fed to the reactor, monomer droplets formed during the early reaction. In these runs, therefore, particle formation occurred partly under flooded conditions. For the same reason, the data points

ρm R atxi π /6Dv3ρp

(9)

RESULTS AND DISCUSSION The recipe for the polymerizations is shown in Table 1. The surfactant concentrations ([S]) of 17, 35, 52, and 69 mmol Laq−1 were chosen for this study. The rate of monomer addition (Ra) of 24.2, 12.3, 6.0, 3.2, and 2.1 mmol min−1 Laq−1 were also selected.

From the comparison of the z-average and volume-average diameters obtained by TEM with the z-average diameter obtained by dynamic light scattering (Dz,dls), the conversion factor of Cf = 0.90 ± 0.02 was found so that Dv = Cf Dz,dls. The number of particles was calculated according to the following equation:

Np =

⎛ 1 ⎞ KC =⎜ + 2A 2C⎟ Rθ ⎝ M̅ w ⎠

(8)

where mm is the mass of the unreacted monomer in the system. Equations 6 and 7 were simultaneously solved, using an overall mass balance equation for the surfactant as [S]in = [S]w + Sd, where [S]in is the initial concentration of surfactant in the water phase, to give θ. The weight-average molecular weight (M̅ w) of the polymer was measured by the static light scattering technique (SLS; Malvern), using the simplified Rayleigh equation: 5652

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Figure 1. Conversions of styrene versus time for different Ra at [S] values of (a) 17 mmol Laq−1, (b) 35 mmol Laq−1, (c) 52 mmol Laq−1, and (d) 69 mmol Laq−1. The closed and open symbols represent xo and xi, respectively.

depletion of emulsifier micelles and thus inversely affects the nucleation. As Ra decreases, the monomer starvation of particles becomes more significant. Monomer-starved particles have smaller outer surface areas and thus absorb fewer surfactant molecules.5 This can prolong the nucleation period by delaying the depletion of emulsifier micelles. Thus, the starvation of primary particles with the monomer is in fact the prerequisite for a prolonged nucleation period. In order to investigate the nucleation stage, the nucleation period of each experiment is needed to be known. A rule of thumb is that the nucleation period ends when the variations in Np with time become negligible (method one). However, one should note that according to the micellar nucleation mechanism, micelles are consumed via particle formation and growth. The end of micellar nucleation can practically be identified by the time that surface tension starts to rise.10 In the absence of such measurements, the end of micellar nucleation can be estimated by the time that all micelles are depleted. Therefore, the surface coverage ratio (θ) of polymer particles by surfactant molecules can give a reliable indication of the presence of micelles in the reaction mixture. A complete surface coverage, θ = 1.0, indicates that free micelles exist in the aqueous phase. Thus, the end of micellar nucleation can theoretically be predicted by the time θ starts to drop below 1.0 (method two). The nucleation periods calculated by the two methods (i.e., constant Np and θ < 1) were comparable to each other, as can be observed in Figure 2. However, method one was used to determine the nucleation periods. Typical calculations for θ are shown in Figure 3b. Figure 4 shows the variations in nucleation period (tf) with different Ra and [S]. The exponent value of tf with respect to [S] was about 0.99 ± 0.15 for all Ra. Experiment and theory are in fair accord (see eq 2). The Evolution of Particle Size and Size Distribution during Formation Stage. Figure 5 shows particle sizes versus relative nucleation time (tf,r) for runs with different [S] and Ra. The relative nucleation time for any run is defined as the ratio

from these experiments have been ignored for the calculation of the exponent values, as will be discussed later. Generally, a slightly higher monomer starvation in polymer particles was obtained by increasing [S] at a given Ra, as can be inferred from Figure 1. Figure 2 represents the relationship between the inverse of Ra and Rp for different [S] during nucleation. An equality

Figure 2. 1/Rp vs 1/Ra for different [S], during the nucleation stage.

relationship, Rp = Ra, exists when [S] and Ra are both high. It appears that at low Ra and [S], the rate of reaction becomes slower than the rate of addition, Rp < Ra, which indicates an accumulation of monomer in the particles with time, while the system remains starved. One possibility for a decrease in Rp with decreasing Ra could be the depletion of initiator with time (i.e, long addition time). However, this cannot explain why this effect is more effective if surfactant concentration is low. Nucleation Period. Figure 3 shows the variations in Np with time in the course of the reaction. At any given Ra, the number of particles was initially comparable for different values of [S], but the difference gradually became wider with time. The number of particles increased with either decreasing Ra or increasing [S]. In a typical emulsion polymerization, particle formation and growth occur simultaneously. Particle growth leads to the 5653

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Figure 3. Time evolution of number of particles for different [S] at Ra of (a) 12.3 mmol min−1 Laq−1, (b) 6.0 mmol min−1 Laq−1, (c) 3.2 mmol min−1 Laq−1, and (d) 2.1 mmol min−1 Laq−1. Figure 3b also shows typical variations in θ (as the 2nd y axis) in the course of nucleation and how the end of nucleation was identified.

Figure 4. Variations in the nucleation period versus (a) Ra and (b) [S] − [S]CMC, for different [S] and Ra, respectively.

Figure 5. Variations in Dv vs relative nucleation time with (a) [S] and (b) Ra at Ra= 3.2 mmol min−1 Laq−1 and [S] = 35 mmol Laq−1, respectively.

Figure 6. Variations in the size of particles at the end of nucleation vs (a) Ra and (b) [S] − [S]CMC, for different [S] and Ra, respectively.

of time over the nucleation time (the time at which particle nucleation is completed); tf,r = 1.0 indicates the end of

nucleation. It is interesting to note that during the nucleation stage, particles grew relatively (i.e.; in terms of relative 5654

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Figure 7. Number of particles at the end of nucleation versus (a) Ra and (b) [S] − [S]CMC, for different [S] and Ra, respectively.

nucleation time) at almost the same rate for various [S], as shown in Figure 5a. As a result, the average size of particles at the end of nucleation in polymerizations with different [S] was almost the same for a given rate of addition. The results imply that the relative rate of growth of particles is controlled by Ra but is independent of [S]. In order to compare the results with the theoretical predictions, the average size of particles at the end of nucleation was drawn against Ra and [S], and the governing exponents were calculated. Figure 6 shows the variations in the volume-average particle size at the end of nucleation with Ra and [S] − [S]CMC. The exponent values of Dv with respect to Ra and [S] are around 0.34 ± 0.01 and 0.01 ± 0.03, respectively. A constant Dv with increasing [S] is because the increase in the nucleation time with [S] is counterbalanced with a reduction in the instantaneous rate of growth (Ra/Np). These values are already in good agreement with the predictions from eq 3. The results can also resolve discrepancies reported in the literature regarding the Dv−[S] relation. Any deviation from relation Dv ∝ [S]0.0 is due to the growth of particles in the postnucleation stage (i.e., growth stage), which varies with polymerization conditions. The nucleation time prolongs with increasing [S], and hence, a larger mass of monomer is converted during nucleation (Ratf) and less is left for the subsequent growth which has to be shared by a larger number of particles at a given Ra. This is the reason that in most publications reported in the literature Dv varies inversely with [S]. Figure 7 shows the number of particles at the end of nucleation produced by various Ra and [S] − [S]CMC. The exponent value of Np with respect to Ra is −0.46 for [S] = 17 mmol Laq−1 but around −0.67 for the higher values of [S]. This is again in good agreement with the value of −2/3, predicted by eq 1. The exponent value of Np in terms of [S] is around 1.14 ± 0.08 for all Ra. This is in fair agreement with the value of 1.0 predicted by eq 1. Possible reasons for the deviations from model predictions have been discussed elsewhere.28 Particle Size Distribution (PSD). It was very interesting to see that polymer particles as small as 5−10 nm were formed (see Figure 8). This is only slightly larger than the size of micelles measured by DLS. The particle size distributions (PSD) of latexes were produced using images from TEM measurements. Figure 9a shows the PSD of latexes at the end of nucleation for different Ra. PSD is much sharper for the semicontinuous process than the batch process, and moreover, the lower the Ra for the semicontinuous process, the narrower was the particle size distribution. As explained later, particles continue to receive radicals in the course of polymerization and grow. Therefore, most probably, the smallest particles of the size distributions,

Figure 8. TEM images of the latexes 1 h into addition for Ra = 6.0 mmol min−1 Laq−1 with different [S]: (a) 35 mmol Laq−1 and (b) 69 mmol Laq−1.

having a size slightly larger than that of micelles, represent the last generation of nucleated micelles (the size distribution of micelles is shown in the inset of Figure 9a). PSD sharpening with decreasing Ra can be explained by the reduced rate of particle growth under monomer-starved conditions. One should note that the nucleation time increased with decreasing Ra, suggesting that a broader PSD should be expected as understood from conventional emulsion polymerization theory.29 However, particles formed under monomer-starved conditions grow slowly, allowing more micelles, which otherwise had to disintegrate to provide stability to growing polymer particles, to be nucleated. This means that despite longer nucleation times, particles formed using the semicontinuous approach became smaller with a sharper PSD with decreasing Ra. Figure 9b shows the PSD at the end of nucleation for two different [S], with the same rate of monomer addition (Ra = 6.0 mmol min−1 Laq−1). The average sizes of particles and their PSD at the end of nucleation were almost the same for the two runs. 5655

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Figure 9. PSD at the end of nucleation obtained by TEM for (a) different Ra at [S] = 35 mmol Laq−1 and (b) different [S] at Ra = 6.0 mmol min−1 Laq−1. The PSD of SDS micelles (inset) and particles obtained from corresponding batch polymerization are also shown in (a) for comparison.

Figure 10. Time (relative) evolution of PSD during nucleation obtained by TEM for feeding rates of (a) 6.0 mmol min−1 Laq−1 and (b) 2.1 mmol min−1 Laq−1, using [S] = 35 mmol Laq−1.

Figure 10 shows the evolution of PSD during the nucleation stage for two different rates of monomer addition (Ra = 6.0 and 2.1 mmol min−1 Laq−1) at [S] = 35 mmol Laq−1. It can be observed that the average particle size increased, and the PSD became broader as the polymerization reaction proceeded. This clearly suggests that particles continuously underwent growth during nucleation, which can also be found from Figure 5. This makes the current system different from some conventional microemulsion polymerization (in the presence of a high concentration of surfactant), in which micelles (or particles) might receive radicals only once.32 Polymer Molecular Weight. Figure 11 shows the evolution of the weight-average molecular weight (M̅ w) with relative nucleation time for polymer latexes from batch and semicontinuous processes at constant [S] = 35 mmol Laq−1. For all processes studied, the M̅ w values remained practically constant in the course of polymerization reactions. However, it is clear that the batch process produced the highest M̅ w. M̅ w

Figure 11. Evolution polymer produced in conversion for batch different feeding rates;

decreased with decreasing rate of monomer addition for the semicontinuous process. For a typical emulsion polymerization, the number-average molecular weight (M̅ n) can simply be calculated by M̅ n = MmonX̅ n = MmonRp/(Rt + Rtr,m). Rt and Rtr,m represent the rates for termination and chain transfer to the monomer in the particle, respectively. If M̅ n is controlled by chain transfer to monomer, Rt can be ignored; M̅ n = Mmonkp/ktr,m. For polystyrene particles (at 70 °C with kp = 480 L mol−1 s−1 and ktr,m = 9.3 × 10−3 L mol−1 s−128), the value of M̅ n is obtained as 5375 kDa. As M̅ n is always smaller than M̅ w, the polymer molecular weight, as shown in Figure 11, is much smaller than 5375 kDa, indicating that the molecular weight is governed by radical termination reactions via secondary entry of radicals into growing particles.22 It is also inferred that radical termination becomes progressively more dominant with a decreasing rate of monomer addition (increasing addition time). A lower monomer concentration in polymer particles, because of smaller rate of monomer addition, will allow for a slower but longer propagation in the particles. This can enhance the chance of termination by secondary radical entry during the lifetime of a propagating radical.29 For a typical free-radical chain polymerization, the average (instantaneous or accumulative) kinetic chain length (v) can be defined as the total number of monomer molecules consumed per each radical generated. For a semicontinuous emulsion polymerization, in which the rate of polymerization is controlled by the rate of monomer addition (i.e., Ra ≈ Rp), the number of monomer molecules consumed by the end of nucleation, tf, can be approximated by Ratf. Therefore, the number-average degree of polymerization (X̅ n) will be

of the weight-average molecular weight of the course of polymerization versus overall process and semicontinuous process with [S] = 35 mmol Laq−1.

X̅ n = v = 5656

R at f RIt f

=

Ra RI

(10)

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Figure 12. Weight-average molecular weight of polymer produced at the end of nucleation versus (a) Ra and (b) [S] − [S]CMC, with [S] = 35 mmol Laq−1 and Ra = 6.0 mmol min−1 Laq−1, respectively.

semicontinuous processes. The number of chains per particle, however, is much lower for the semicontinuous process than for the batch process. The number of chains per particle is in fact a reflection of the growth mechanism of particles. The results clearly indicate that single-chain particles once formed in the early stage of reactions underwent subsequent growth by secondary radical entry. Using a high [S], at a given Ra, for example, will not affect the average number of chains per particle (not shown) despite an increase in tf and Np because the growth rate of particles is proportionately reduced, leading to a constant particle size and molecular weight (see Figures 6b and 12b). Single chain particles form in the early stage of nucleation where only micelles exist in the reaction medium. The results, however, suggest that producing single small chain particles might only be possible at an infinitesimally low rate of monomer addition.

For microemulsion polymerization under the conditions that radicals are only captured by micelles (the higher-limit assumption), eq 10 coincides with that given in eq 3 for the average size of particles.29 This equation indicates that the number-average molecular weight of polymer produced is proportional to Ra but independent of [S]. Figure 11 clearly shows that M̅ w remained practically constant in the course of monomer addition, so the replacement of M̅ n (= MmonX̅ n) by M̅ w is justified. Figure 12 shows the variations in M̅ w of polymer produced at the end of nucleation with Ra and [S]. The exponent value of M̅ w in terms of Ra was found to be around 0.7 (Figure 12a). The exponent value of M̅ w with respect to [S] was almost zero (see Figure 12b), which is in good agreement with the prediction of eq 10. The growing particles are more likely to be terminated by the secondary entry of radicals, the lower-limit assumption. Because of the termination reaction type of small-large radicals being dominant in small particles encountered in microemulsion polymerization, there is always two radicals consumed per each polymer chain. So a similar relationship to that of eq 10 is obtained as X̅ n = 2v = 2Ra/RI. Note that these simple equations may not truly reflect features of the molecular weight development in the course of starved nucleation but can be used to find out the relation between molecular weights and formulation variables. The average number of chains in each particle, nc̅ , can be calculated by the average size of particles and M̅ w of the polymer chains, according to the following equation: nc̅ = (π Dv 3ρp )NA /6M̅ w

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CONCLUSIONS Despite all recent understanding of semicontinuous (micro)emulsion polymerization, there is no experimental data available in the literature on particle size at the end of nucleation. In most research on semicontinuous (micro)emulsion polymerization, particle size average and size distributions have been reported for the end of polymerization where the monomer is depleted but micelles do exist. In many others, kinetic data were reported for the conditions where micelles are fully depleted but particles have grown during the growth stage. This implies that the history of particle size evolution, which is required for a mechanistic study, is either lost due to incomplete nucleation or masked due to subsequent growth during postnucleation. No experimental data has been published to date have addressed nucleation time and its variations with polymerization parameters in such processes. Semicontinuous (micro)emulsion polymerization of styrene during the particle nucleation stage was investigated in this work with the aim of filling this gap in the literature. It has been found that particle nucleation period can be prolonged by either decreasing Ra or increasing [S]. The results indicate that the size of particles at the end of nucleation is almost independent of surfactant concentration but varies with Ra1/3. The PSD of particles at the end of nucleation showed only a slight change with [S] but significantly narrowed with decreasing Ra. The exponent dependence of the number of particles with respect to Ra and [S] were also confirmed to approach those theoretically predicted, −2/3 and 1, respectively. The molecular weight of polymer decreased with decreasing rate of monomer addition but remained unchanged with surfactant concentration. It follows from eqs 3 and 10 that the molecular weight of the particles is proportional to their size: M̅ n ∝ Vparticle. Secondary radical entry was found to be the

(11)

The results are shown in Figure 13. Single chain nanoparticles were predominantly produced during the early reaction (i.e., initial 10% conversion). The number of chains per particle increased during nucleation for both batch and

Figure 13. Average number of chains per particle during the nucleation period for batch process and semicontinuous process with different feeding rates; [S] = 35 mmol Laq−1. 5657

dx.doi.org/10.1021/la4000654 | Langmuir 2013, 29, 5650−5658

Langmuir

Article

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dominant mechanism of chain termination during nucleation. A simple correlation was developed which showed a fair success in the prediction of the pattern in which the polymer molecular weight changed with [S] and Ra.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/la4000654 | Langmuir 2013, 29, 5650−5658