Continuous Flow Synthesis of Polystyrene Nanoparticles via Emulsion

Aug 14, 2017 - Conducting emulsion polymerization in continuous flow mode for polymer ... Many attempts have been made to overcome this drawback, whic...
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Continuous Flow Synthesis of Polystyrene Nanoparticles via Emulsion Polymerization Stabilized by a Mixed Nonionic and Anionic Emulsifier Xiaojing Liu, Yangcheng Lu,* and Guangsheng Luo State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Conducting emulsion polymerization in continuous flow mode for polymer nanoparticle synthesis has the potential to improve productivity and reliability but has to face the fact that the emulsion is difficult to remain stable without stirring. In this work, a mixed nonionic− anionic emulsifier TX-100/SDBS (4:1) was found to perform much better in stabilizing pre-emulsion than anionic emulsifier SDBS and then was exploited in the microflow system to achieve reliable operation, controllable conversion, and continuous synthesis of nanoparticles with uniform size (PDI < 0.09). The reaction temperature could be elevated to 95 °C, and the emulsifier concentration could be decreased to 8.515 mM. The average size of the nanoparticles was facilely adjusted from 52 to 92 nm by changing the emulsifier concentration.

1. INTRODUCTION Environmentally friendly waterborne polymer nanoparticles are experiencing intensive research and have a great market in a broad range of fields, from adhesives to inks, paints, coatings, drug delivery systems, and cosmetics.1−3 Emulsion polymerization is the most widely used way of producing polymer nanolatexes.4 The commercial emulsion polymerization process is commonly conducted in a continuous stirred tank. Since the highly exothermic nature of free-radical polymerization, it usually adopts a low temperature and slow feeding rate to avoid uncontrolled acceleration of polymerization as well as fatal thermal runaway reactions,5,6 which suffer from low productivity and high risk in scaling up.7 Therefore, continuous flow reaction is worth considering for emulsification process intensification.8 Recently, continuous emulsion polymerization in a tubular reactor has been receiving significant attention.9−11 Compared with continuously stirred tank reactors (CSTRs),12 the tubular reactor has the distinct advantage of the control of the residence time distribution. Besides, tubular reactors, especially the tubular microreactors (diameter of 1000 μm or less), have perfect heat transfer performance due to a large surface-to-volume ratio, which is beneficial for the control of polymerization temperature. The precise control of the residence time and polymerization temperature may endow the products with high uniformity.13,14 Other advantages of the tubular reactor include high safety related to high pressure resistance of tube, strict airtightness to avoid the influence of oxygen, and easy-to-scaling-up for industrial application.15 However, phase separation and clogging are prone to take place during emulsion polymerization in tubular reactors © XXXX American Chemical Society

without stirring, especially at high polymerization temperature and low emulsifier concentration. Many attempts have been made to overcome this drawback, which can be classified into two categories. One is to enhance the turbulence in the tubular reactor by adjusting the flow regime,16 introducing additional power sources like a pulsation source in the reactor system,17 etc. Nevertheless, these methods may bring nonideal flow or increase difficulty and unreliability in reactor design and operation. For instance, the input of pulsation in the tubular reactor will broaden the residence time distribution much, even similar to that of CSTRs;18 the turbulent flow in the tubular reactor could give rise to the formation of a precoagulum.19 The other is to enhance the stability of pre-emulsion. The intense agitation of pre-emulsion20 and the ultrasonic homogenization of pre-emulsion21 could cut down the diameter of monomer droplets to improve the stability of pre-emulsion,22 resulting in stable miniemulsion polymerization in tubular reactors.11,23 However, the preparation of miniemulsion is usually a hard and energy intensive process.24 Ouzined et al.25 indicated that the unstable operation of emulsion polymerization in tubular reactors mainly originates from the emerging of large monomer droplets in the early stage of the polymerization due to droplets coalescence. Both the droplets coalescence26 and emulsion polymerization are highly dependent on the type of emulsifier, so the selection and optimization of emulsifier deserves attention. Chern and coworkers27 investigated the batch emulsion polymerization of Received: Revised: Accepted: Published: A

June 8, 2017 August 1, 2017 August 14, 2017 August 14, 2017 DOI: 10.1021/acs.iecr.7b02352 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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styrene using SDS/NP-40 as the mixed surfactant system. They found that, when the content of NP-40 in the surfactant mixture is 80 wt %, it results in the most stable latex particles and greatest polymerization rate. They proposed that anionic emulsifiers form mixed micelles with nonionics and increase the space charge repulsion between micelles, thus leading to their stabilization.28,29 Besides, the nonionic−anionic surfactant mixtures could help to elevate the clouding point (CP) of nonionic surfactants and decrease the kraft point (KP) of anionic surfactants,30 and sodium dodecyl benzenesulfonate (SDBS) plus Triton X-100 (TX-100) is an example reported by Goel.31 Therefore, we envision that the usage of a mixed nonionic−anionic emulsifier is promising to guarantee the operating stability of emulsion polymerization in the tubular reactor, although there is no related report until now to the best of our knowledge. In this study, the mixed emulsifier, consisting of anionic emulsifier SDBS and nonionic emulsifier TX-100, was used to obtain pre-emulsion for polystyrene nanoparticle synthesis via emulsion polymerization. After the optimization of their ratio according to the stability of pre-emulsion, TX-100/SDBS (4:1) was first exploited in flow emulsion polymerization in a tubular microreactor to achieve fast and sufficient conversion as well as stable operation. Monodispersed polystyrene nanoparticles were successfully prepared within the residence time as short as 5 min at 90 °C. The effect of emulsifier concentration was also carefully investigated to facilely adjust the size of nanoparticles.

TSI =

∑h = 0 |scan n(h) − scan n − 1(h)|

(1) H where n means the number of scanning, scan the light intensity, h the height of monitor position, and H the total height of the sample in the cell. The value of TSI is ranged from 0 to 100. A lower TSI value indicates higher emulsion stability. 2.3. Tubular Microreactor Setup. The tubular microreactor setup is shown in Figure 1, which consists of a

Figure 1. Setup for emulsion polymerization.

polytetrafluoroethylene (PTFE) tubing with 1 mm inner diameter, 3 mm outer diameter, and 6.36 or 12.72 m length. The microreactor was rolled into a helical coil of 90 mm in diameter and immerged into a thermostat. A metering pump (Beijing Satellite Co., Ltd.) was used to deliver the emulsion through the microreactor from the feed tank. 2.4. Flow Polymerization in the Microreactor. The emulsion polymerization performance in the continuous tubular reactor was tested using two different emulsifiers: one is the commonly used anionic emulsifier SDBS and the other is the mixed anionic-nonionic emulsifier TX-100/SDBS with a molar ratio of 4:1. First, the monomer solution of styrene and DVB (with volume ratio of 10/1), the emulsifier, and the ultrapure water were added into a three-necked flask. The preemulsion was homogenized by continuous mechanical stirring for 30 min at nitrogen atmosphere. The initiator solution was prepared by dissolving KPS in water, and added to the feed tank a few minutes before the reaction. The formulation of preemulsion in feed tank is shown in Table 1. The pre-emulsion

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Styrene (St, Sinopharm) and divinylbenzene (DVB, J&K) were washed with 5 wt % NaOH and water sequentially to remove the polymerization inhibitors and then purified by distillation under reduced pressure. Besides, before use, St, DVB, as well as water were stripped with nitrogen gas to remove dissolved oxygen for at least 1 h. The water-soluble initiator potassium persulfate (KPS, Sinopharm), the quencher hydroquinone (J&K), the emulsifiers SDBS (Sinopharm), and TX-100 (Xilong Chemical) were used as received. The water used throughout the experiments was ultrapure water produced from a water purification machine (Center 120FV-S). 2.2. Emulsion Preparation and Stability Analysis. The O/W emulsions were prepared according to the recipe of an oil-to-water volume ratio of 1:6, 8.515 mM of mixture of TX100/SDBS (molar ratio: 8/1, 4/1, 2/1, 1/1, 1/2, 1/4, and 0/1) as emulsifier. In a 100 mL three-necked flask, 30 mL of ultra pure water and the required amount of emulsifier were added under stirring until complete dissolution. The monomer solution of styrene and DVB (with volume ratio of 10/1) was then added to the emulsifier solution, and the mixture was magnetically stirred at 400 rpm for 60 min at room temperature. The stability of the obtained emulsions was determined by a Turbiscan Lab apparatus (Formulaction, France) with a near-infrared light source (λ = 880 nm).32 The prepared emulsions (20 mL) were transferred into a glass tube and then inserted into the chambers. The variation of backscattering signals reflecting creaming or coalescence phenomenon were monitored by the backscattering detector along the cell height every 1 min. The stability of emulsion was indicated by turbiscan stability index (TSI).33 The TSI value can be calculated with the special computer program using eq 1:

Table 1. Formulation of Pre-emulsions entry ES1 ES2 ES3 ES4 ES5 ES6

TX-100 (mM) 6.812 27.25 13.62 40.87

SDBS (mM)

monomer/water (V/V)

KPS/monomer (wt %)

8.515 1.703 34.06 6.812 3.406 10.22

1:6 1:6 1:6 1:6 1:6 1:6

1 1 1 1 1 1

was pumped using a pump at flow rate varied from 0.5 to 2.5 mL/min to realize different residence time in microreactor. The nominate residence time was calculated from the volume of the tubing immersed in thermostat divided by flow rate. The samples were collected from the fifth residence time to ensure that the steady state had been achieved. The hydroquinone solution (1 wt %) was dropped into the collecting vials in advance to quench the polymerization. Gravimetric measurements on the remaining emulsion confirmed that no polymerization occurred in the feed tank. B

DOI: 10.1021/acs.iecr.7b02352 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 2.5. Batch Polymerization. The pre-emulsions used in the batch reactor were the same as those used in the microreactor. The pre-emulsion in the feed tank was pumped through a stainless steel coil emerged in the thermostat to reach the desired reaction temperature rapidly and injected into a 250 mL three-necked flask with stirring at nitrogen atmosphere. The total injection time was less than 30 s, and the initial polymerization time was taken once the injection was finished. The schematic of the batch polymerization setup is shown in Figure 2. Since the flask had a small volume and the stirring was

coalescence of the emulsions. For a uniform emulsion just prepared under violent stirring, the backscattering intensity should be consistent throughout the sample. When the stirring is canceled, the creaming and coalescence may take place and lead to the backscattering intensity decreasing at the lower part and increasing at the upper part for the working system. The quicker the changing is, the poorer the emulsion stability is. Figure 4a corresponds to the emulsion stabilized by pure SDBS at 25 °C. It is clear that the backscattering curve changes with time quickly and remarkably, indicating poor stability of the emulsion stabilized by pure SDBS and rapid phase separation due to serious droplet creaming and coalescence. Figure 4b corresponds the emulsion stabilized by TX-100/SDBS (4:1) at 25 °C. Compared with what is shown in Figure 4a, the changing of the backscattering curve within 60 min in Figure 4b is only focused on the very top and the very bottom of the sample. It indicates that the monomer droplets only migrate slightly from the bottom to the top of the sample vial.34 Figure 4c corresponds the emulsion stabilized by TX-100/SDBS (4:1) at 80 °C, in which the evolution of the backscattering curve with time is similar to that in Figure 4b. It indicates that the TX-100/SDBS (4:1) can stabilize the emulsion effectively in a wide temperature range, at least from 25 to 80 °C, while the stabilization effect of pure SDBS is poor. It should be noticed that a so-called stable emulsion in this work just does not eliminate but slows the creaming and coalescence with the removal of stirring. A sufficient polymerization before achieving serious creaming and coalescence is necessary to realize controlled emulsion polymerization. In a tubular reactor, with the increasing of the residence time to conduct polymerization, the possibility of clogging will increase since the monomer droplets and polymer particles may adhere to the reactor wall.23 Therefore, a high polymerization rate is also demanded in flow emulsion polymerization besides of stable emulsion, and the effect of emulsifier type on polymerization rate is worth noticing. Herein, using TX-100/ SDBS (4:1) and TX-100/SDBS (8:1) showing similar stabilization effects on pre-emulsion as the emulsifier, we investigated the polymerization rates in the batch reactor comparatively. The corresponding monomer conversion− residence time relationships are provided in Figure S1 of the Supporting Information. As seen, the monomer conversion using TX-100/SDBS (4:1) is higher than that using TX-100/ SDBS (8:1), indicating a higher polymerization rate. Therefore, we selected TX-100/SDBS (4:1) as the emulsifier to conduct further studies in this work. 3.2. Polymerization Performance in the Microreactor. SDBS is a commonly used anionic emulsifier in emulsion polymerization. To reveal what the usage of mixed TX-100/ SDBS (4:1) can bring to the emulsification polymerization in the microreactor, the polymerization of pre-emulsion stabilized by SDBS was conducted as the control group. When using two types of emulsifiers at 8.515 mM, the time profiles of monomer conversion at 90 °C are shown in Figure 5a. The monomer conversion of using SDBS is above that of using TX-100/SDBS (4:1) initially but is surpassed soon after 10 min. The former may be explained that SDBS can provide a higher nucleation rate as well as polymerization rate than TX-100/SDBS (4:1) as long as the emulsion remains uniform, like what Figure S1 demonstrates. The latter is related to the difference in the stability of pre-emulsions. For the pre-emulsion stabilized by SDBS with poor stability, severe phase separation may occur quickly as the pre-emulsion was injected into the microreactor,

Figure 2. Schematic of the batch polymerization setup.

violent, we supposed that the batch polymerization experienced a strictly controlled emulsion polymerization process in this work, and the results in batch polymerization were taken as a benchmark to judge whether a polymerization process is under control or not. 2.6. Analysis and Characterization. For latex particles, the morphology was characterized with a scanning electron microscope (SEM, JSM 7401F, JEOL); the size distribution of the latex particles was characterized by dynamic light scattering (DLS, SZ-100, HORIBA). The monomer conversion was measured with the gravimetric method. The average conversion of the three paralleled samples was taken for each run.

3. RESULTS AND DISCUSSION 3.1. Pre-emulsion Stability. Figure 3 shows the time profiles of TSI values of emulsions stabilized by different

Figure 3. TSI of emulsions stabilized by mixtures of TX-100/SDBS with different molar ratios. T = 25 °C.

emulsifiers with various ratios of TX-100 to SDBS. In the experiments, the temperature was 25 °C, and the timing was started once the pre-emulsion was transferred into the measuring tube. As seen, the emulsion stabilized by pure SDBS has the poorest stability, and the emulsions stabilized by TX-100/SDBS (4:1) and TX-100/SDBS (8:1) have similar stabilities, much superior to other emulsions. Figure 4 shows the backscatter curves of various emulsions at specific time intervals, which can reflect the creaming and C

DOI: 10.1021/acs.iecr.7b02352 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Time evolution of backscattering intensity profiles along the sample height of emulsions. (a) Pure SDBS, T = 25 °C; (b) TX-100/SDBS (4:1), T = 25 °C; (c) TX-100/SDBS (4:1), T = 80 °C.

Figure 5. Kinetic comparison of emulsion polymerization under various conditions. (a) Flow polymerization, pre-emulsions ES1 and ES2; (b) flow polymerization, pre-emulsions ES3 and ES4; (c) batch polymerization and flow polymerization, pre-emulsion ES4; (d) batch polymerization and flow polymerization, pre-emulsion ES3. T = 90 °C. D

DOI: 10.1021/acs.iecr.7b02352 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research especially at high polymerization temperature (90 °C) and low emulsifier concentration. This will result in a sharp decrease of mass transfer area and polymerization rate. For the preemulsion stabilized by TX-100/SDBS (4:1) with much better stability, the decrease of the mass transfer area will be not remarkable and the sharp decrease of the polymerization rate can not take place. The advantage of TX-100/SDBS (4:1) in stability will become more obvious with the increasing of the residence time in the microreactor. After 20 min, the monomer conversion when using TX-100/SDBS (4:1) could reach over 80%, almost twice that when using SDBS. As shown in Figure 5b, when increasing the concentration of two types of emulsifiers to 34.06 mM, the polymerization rate increased since more micelles existed in the aqueous phase. Similarly, the monomer conversion of the emulsion stabilized by SDBS was higher than that stabilized by TX-100/SDBS (4:1) at the primary stage but got lower when the residence time was greater than 5 min with a final conversion of around 68%. We further made comparisons on reaction performances between flow polymerization and batch polymerization to explore the stability of the flowing system in the microreactor, since the emulsion in the stirring batch reactor could be taken as a stable one. When using 34.06 mM TX-100/SDBS (4:1) as emulsifier, the conversion−residence time curves in two kind of reactors are plotted together in Figure 5c. The two curves are nearly overlapped. It indicates that a stable emulsion system as well as controllable conversion was achieved in the tubular reactor. In contrast, when using 34.06 mM SDBS as the emulsifier, the conversion−residence time curves in two kind of reactors using 34.06 mM SDBS are plotted together in Figure 5d. The monomer conversion in the tubular reactor is far lower than that in the batch reactor at the same residence time, which is because this emulsion system may be deteriorated by serious phase separation in the tubular reactor. Once it happened, the mass transfer of monomer from droplets to micelles or colloids will be slowed down, and the polymerization rate decreases dramatically. The stability of the emulsion system not only has influence on the polymerization rate but also affects the particle morphology. Figure 6 shows the SEM images of products obtained in microreactor using different emulsifiers. Figure 6a,b shows the polymerization products using 8.515 and 34.06 mM SDBS. In these two images, bulk of aggregates can be observed with opaque contours of nanoparticles. The aggregation phenomenon reflects that SDBS could not stabilize the growing particles effectively, and serious coagulation occurred during the polymerization process in tubular reactor at 90 °C. Figure 6c,d shows the products of polymerization stabilized by 8.515 and 34.06 mM TX-100/SDBS (4:1), respectively. In these two images, almost all of the particles have a clear spherical shape and are separate with each other. DLS results show that the polydispersity index (PDI) of particles is 0.09 and 0.101, respectively, indicating good uniformity in size. In evidence, monodispersed particles were successfully synthesized by using TX-100/SDBS (4:1) in the tubular microreactor at 90 °C, since the mixed emulsifier can greatly improve the latex stability via the synergetic effects provided by both electrostatic and steric stabilization mechanisms. The long-term operation stability is important for a commercial emulsion polymerization process. Therefore, we carried out a long-term experiment of emulsification polymerization stabilized by TX-100/SDBS (4:1) in the microreactor to determine the fluctuation of monomer conversion and particle

Figure 6. SEM images of products obtained by flow emulsion polymerization. (a) Pre-emulsion ES1, 20 min; (b) pre-emulsion ES3, 10 min; (c) pre-emulsion ES2, 20 min; (d) pre-emulsion ES4, 10 min. T = 90 °C.

size. Neither clogging nor reactor plugging was observed for more than 150 min of operation. All of the results are shown in Figure 7. As seen, the deviations of these two parameters are both quite small, reflecting very reliable operation performance in the microreactor with the residence time of 10 min.

Figure 7. Conversion and the average particle size of products at the outlet of the microtubular reactor during long-term operation. The hollow symbols are for average size and the solid symbols are for monomer conversion. Pre-emulsion ES4; residence time, 10 min; T = 90 °C.

3.3. Potentials of Fast and Controllable Emulsion Polymerization in the Microreactor. For emulsion polymerization, it is well-known that the concentration of emulsifier and temperature have significant effects on the polymerization rate and particle size. The perfect mass/heat transfer performance of the microreactor may improve the adaptability on diverse reaction conditions and bring more potentials of fast and controllable emulsion polymerization. 3.3.1. Effect of Emulsifier Concentration. The emulsion polymerizations stabilized by different concentrations of TX100/SDBS (4:1) were carried out in tubular microreactors. Figure 8 shows that the polymerization rate increases with the increasing of the emulsifier concentration from 8.515 to 51.09 mM. As seen, the conversion reaches a plateau around 90% at 5 min when emulsifier concentration is 51.09 mM, while the E

DOI: 10.1021/acs.iecr.7b02352 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. Time profiles of monomer conversion under different polymerization temperatures. The pre-emulsion was ES4.

Figure 8. Effects of emulsifier concentration on polymerization rate. The pre-emulsions were ES2, ES5, ES4, and ES6. T = 90 °C.

residence time needed to be 20 min to reach the final conversion when the emulsifier concentration is 8.515 mM. As for measuring the mean size of the final latex particles, the residence time was set at 10 min as the emulsifier concentrations were 34.06 and 51.09 mM and 20 min as the emulsifier concentrations were 17.03 and 8.515 mM. Table 2

plausible explanation is that severe phase separation occurred at a temperature over the cloud point. For the microreactor capable for high pressure and high temperature, the emulsifier with a high cloud point will be welcome, and the cloud point of a mixed nonionic and anionic emulsifier, like TX-100/SDBS (4:1), needs more investigation.

Table 2. Mean Diameter and PDI of Nanoparticles under Different Conditions temperature (°C)

pre-emulsion

mean diameter (nm)

PDI

90 90 90 90 95

ES2 ES5 ES4 ES6 ES4

92 86 76 52 70

0.09 0.113 0.101 0.109 0.105

4. CONCLUSION In this work, we first exploited the mixed emulsifier TX-100/ SDBS for continuous emulsion polymerization in the tubular microreactor and achieve rapid and reliable flow synthesis of polystyrene nanospheres. The mixed emulsifier TX-100/SDBS with the molar ratio of 4:1 performed much better in stabilizing pre-emulsion than anionic emulsifier SDBS. The pre-emulsion stabilized by TX-100/SDBS (4:1) could keep stable within 1 h at a temperature lower than 80 °C, and the polymerization kinetics of this pre-emulsion at 90 °C in the microreactor was similar to that in the batch. It guarantees reliable operation as well as fast and controllable conversion in the microflow system to prepare monodispersed spherical nanoparticles with PDI of a size as small as 0.09. The almost constant monomer conversion and particle size corresponding to specific residence time confirmed the long-term stability of the emulsion polymerization system with neither clogging nor reactor plugging. Toward a reliable emulsion polymerization process and monodispersed products, the reaction temperature could be elevated to 95 °C and the emulsifier concentration could be decreased to 8.515 mM. The average size of the nanoparticles was facilely adjusted from 52 to 92 nm by changing the emulsifier concentration. All of the results indicate that the mixed nonanionic and anionic emulsifier, TX-100/SDBS (4:1) as a sample, can provide enough emulsion and operation stability in polymerizations conducted in the tubular microreactor and will push the development of continuous flow synthesis for desired polymer nanoparticles.

shows that changing the emulsifier concentration from 8.515 to 51.09 mM causes the mean size of the final particles to decrease from 92 to 52 nm. As is well-known, the increasing of the emulsifier concentration results in the increasing of the number density of micelles in water, making it possible to generate more colloids and decrease the size of each particle. The SEM images of the obtained final nanoparticles are shown in Figures 6 and S2 in the Supporting Information. The size of latex particles could be adjusted facilely and monotonically by changing the emulsifier concentration, which is in favor of latex product customization. All of the final particles show good monodispersity with PDI values around 0.1. 3.3.2. Effect of Polymerization Temperature. The polymerization temperature was further raised to explore the upper limit of the operational temperature for emulsion polymerization stabilized by TX-100/SDBS (4:1) in the tubular microreactor. As shown in Figure 9, increasing the temperature to 95 °C is of benefit for the increasing of the polymerization rate, because of the increasing of the number density of micelles in water and the decomposition rate of the initiator. The SEM image of the obtained particles at 95 °C is provided in Figure S3 in the Supporting Information. Table 2 shows that mean diameter of the particles has a slight decrease from 76 to 70 nm as the temperature increased from 90 to 95 °C, and the PDI is almost the same. Besides, the necessary residence time to reach the final conversion was shortened to around 7 min. When the temperature is increased to 98 °C, close to the boiling point of water, the polymerization rate decreased dramatically. A

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02352. Time profiles of monomer conversion in batch, SEM images, and DLS results. (PDF) F

DOI: 10.1021/acs.iecr.7b02352 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(16) Rollin, A. I.; Patterson, W. I.; Archambault, J.; Bataille, P. Continuous-emulsion polymerization of styrene in a tubular reactor. In Polymerization Reactors and Processes; Henderson, J. N., Bouton, T. C., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979; Vol. 104, p 113. (17) Paquet, D. A.; Ray, W. H. Tubular reactors for emulsion polymerization. 1. Experimental investigation. AIChE J. 1994, 40, 73. (18) Xue, W.; Yoshikawa, K.; Oshima, A.; Sato, S.; Nomura, M. Continuous emulsion polymerization of vinyl acetate. II Operation in a single Couette−Taylor vortex flow reactor using sodium lauryl sulfate as emulsifier. J. Appl. Polym. Sci. 2002, 86, 2755. (19) Rollin, A. L.; Patterson, I.; Huneault, R.; Bataille, P. The effect of flow regime on the continuous emulsion polymerization of styrene in a tubular reactor. Can. J. Chem. Eng. 1977, 55, 565. (20) Ghosh, M.; Forsyth, T. H. Continuous emulsion polymerization of styrene in a tubular reactor. ACS Symp. Ser. 1976, 24, 367. (21) Yadav, A. K.; de la Cal, J. C.; Barandiaran, M. J. Feasibility of tubular microreactors for emulsion polymerization. Macromol. React. Eng. 2011, 5, 69. (22) Yadav, A. K.; Barandiaran, M. J.; de la Cal, J. C. Synthesis of water-borne polymer nanoparticles in a continuous microreactor. Chem. Eng. J. 2012, 198−199, 191. (23) Daniloska, V.; Tomovska, R.; Asua, J. M. Designing tubular reactors to avoid clogging in high solids miniemulsion photopolymerization. Chem. Eng. J. 2013, 222, 136. (24) Asua, J. M. Challenges for industrialization of miniemulsion polymerization. Prog. Polym. Sci. 2014, 39, 1797. (25) Ouzineb, K.; Graillat, C.; McKenna, T. Continuous tubular reactors for latex production: Conventional emulsion and miniemulsion polymerization. J. Appl. Polym. Sci. 2004, 91, 2195. (26) Celis, M. T.; Contreras, B.; Forgiarini, A.; Rosenzweig, P.; Garcia-Rubio, L. H. Effect of emulsifier type on the characterization of O/W emulsions using a spectroscopy technique. J. Dispersion Sci. Technol. 2016, 37, 512. (27) Chern, C. S.; Lin, S. Y.; Chen, L. J.; Wu, S. C. Emulsion polymerization of styrene stabilized by mixed anionic and nonionic surfactants. Polymer 1997, 38, 1977. (28) Sadaghiania, A. S.; Khan, A. Clouding of a nonionic surfactant: The effect of added surfactants on the cloud point. J. Colloid Interface Sci. 1991, 144, 191. (29) Yang, B. X.; Cao, W. X. Interaction of diphenylamine diazonium salt with sodium dodecyl sulfate in aqueous solution. J. Colloid Interface Sci. 1999, 212, 190. (30) Thakkar, K.; Bharatiya, B.; Ray, D.; Aswal, V. K.; Bahadur, P. Molecular interactions involving aqueous Triton X-100 micelles and anionic surfactants: Investigations on surface activity and morphological transitions. J. Mol. Liq. 2016, 223, 611. (31) Goel, S. K. Critical phenomena in the clouding behavior of nonionic surfactants induced by additives. J. Colloid Interface Sci. 1999, 212, 604. (32) Wisniewska, M.; Terpilowski, K.; Chibowski, S.; Urban, T.; Zarko, V. I.; Gun’ko, V. M. Effect of polyacrylic acid (PAA) adsorption on stability of mixed alumina-silica oxide suspension. Powder Technol. 2013, 233, 190. (33) Sun, C. C.; Wu, T.; Liu, R.; Liang, B.; Tian, Z. J.; Zhang, E. Q.; Zhang, M. Effects of superfine grinding and microparticulation on the surface hydrophobicity of whey protein concentrate and its relation to emulsions stability. Food Hydrocolloids 2015, 51, 512. (34) Mengual, O.; Meunier, G.; Cayre, I.; Puech, K.; Snabre, P. Characterisation of instability of concentrated dispersions by a new optical analyser: the TURBISCAN MA 1000. Colloids Surf., A 1999, 152, 111.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+86)010-62773017. ORCID

Xiaojing Liu: 0000-0002-5264-1462 Guangsheng Luo: 0000-0002-0498-0224 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (21422603, U1662120) and China Scholarship Council.

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REFERENCES

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DOI: 10.1021/acs.iecr.7b02352 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX