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Particle kinetics and physical mechanism of microemulsion polymerization of octamethylcyclotetrasiloxane
Powder Technology 201 (2010) 146–152
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Particle kinetics and physical mechanism of microemulsion polymerization of octamethylcyclotetrasiloxane Ya-Qing Zhuang a, Xiang Ke a, Xiao-Li Zhan b,⁎, Zheng-Hong Luo a,⁎ a b
Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Department of Chemical and Biochemical engineering, Zhejang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 21 November 2009 Received in revised form 11 March 2010 Accepted 12 March 2010 Available online 20 March 2010 Keywords: Octamethylcyclotetrasiloxane Microemulsion polymerization Kinetics Physical model Nucleation mechanism
a b s t r a c t In the present study, the particle kinetics and physical mechanism of microemulsion polymerization of octamethylcyclotetrasiloxane (D4) were investigated by using dodecyldimethylbenzyl ammonium bromide (DBDA) as a surfactant and n-pentane as a cosurfactant. The light transparence of the emulsion, oil–water interfacial tension and the polymerization conversion as functions of the polymerization time were recorded. Furthermore, the particle sizes and their distributions in the process were measured by using dynamic light scattering technique (DLS). The results show that there does not exist constant-rate reaction period during the polymerization. The results of DLS show that the microemulsion polymerization can be distinguished as four steps, namely (I) the dispersion period, (II) the colloid formation and reaction period, (III) the colloid reaction period, and (IV) the agglomeration period. Corresponding physical models for each period were discussed. It has been found that the nucleation occurs mostly in the swollen-micelles and the polymerization occurs mostly in the new microlatex particles for the microemulsion polymerization of D4. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The ring-opening polymerization of octamethylcyclotetrasiloxane (D4) and/or the copolymerization of D4 with functional siloxane monomers can be used to prepare a wide variety of silicones. Silicones with various microstructures and architectures are useful in industry mainly due to their absence of toxicity, high chain flexibility and low surface energy [1–5]. Furthermore, emulsions based on silicones have been prepared for industrial applications [1–3]. A very simple way to synthesize silicones is the direct polymerization of the monomer in emulsion, namely emulsion polymerization. For instance, the emulsion polymerization of D4 can be used to prepare polydimethylsiloxane (PDMS) directly. In addition, PDMS, one kind of silicone, can be used to manufacture a series of industrial and civil products, such as lubricant, leather glossy agent, paint film polishing agent, etc. [6]. For the preparation of the emulsions based on PDMS, there are mainly three emulsion-polymerization methods, namely, conventional emulsion polymerization, miniemulsion polymerization and microemulsion polymerization [1,2,7–12]. PDMS latexes prepared by the conventional emulsion polymerization are usually unstable due to their large particle sizes in the order of 1000 nm [2]. However, by imposing an external power on the emulsion system prior to the polymerization, the latexes with particle sizes in the order of 50–500 nm prepared by the miniemulsion polymerization are stable [9,10]. Compared with the
⁎ Corresponding author. Tel.: + 86 592 2187190; fax: + 86 592 2187231. E-mail address: [email protected] (Z.-H. Luo). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.03.017
above two latexes, the latexes in microemulsion are the most transparent and their particle sizes are in the order of 0–100 nm [11,12]. In practice, the microemulsion polymerization needs a higher emulsifier concentration instead of the external power used in the miniemulsion polymerization. In short, the microemulsions are transparent and thermodynamically stable mixtures of oil and water, and are stabilized by surfactant and usually with a cosurfactant [12,13]. Accordingly, the microemulsion polymerization has attracted more and more concerns and been widely applied in industry. Up to now, there were many reports on the microemulsion polymerization [8,11–15]. Full et al. [14] succeeded in preparing stable microemulsion via the microemulsion polymerization of styrene (St) by the addition of cosurfactant and investigated the effects of the addition of cosurfactant on the microemulsion polymerization behaviour. Xu et al. [11] reported the copolymerization of St with butyl acrylates (BA) in emulsion and microemulsion. It was found that the copolymerization behaviour of St/BA in microemulsion is obviously different from that in emulsion copolymerization. The result of St/BA copolymerization strongly suggests that the real reaction position of the microemulsion polymerization is the inter-side of the emulsifier layer. Xu et al. [15] also investigated the microemulsion polymerization of methyl methacrylate (MMA) initiated with Benzoyl Peroxide (BPO) in 1999. In their research, the growth of monomer-swollen polymer particles was studied by using photo correlation spectroscopy, and the polymerization kinetics was studied with a dilatometer. Shi et al. [13] investigated the kinetics of and the mechanism of microemulsion copolymerization of St and acrylonitrile in oil-in-water microemulsions by using n-butanol as a cosurfactant. Recently, Yu and Zhan [8] studied the microemulsion
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polymerization of D4. However, in their work, the reaction recipe of the microemulsion polymerization was not reported and the microemulsion polymerization system was only confirmed via measuring the light transmittance of the system, corresponding polymerization kinetics was ignored. Based on the above discussions, it is clear that previous studies on the microemulsion polymerization concentrate on the issues of the emulsification systems, the nucleation mechanisms, and the process kinetics. Furthermore, it is also clear that past studies are limited to the microemulsion ho/co-polymerizations of unsaturated monomers and the resultant products are mainly the polymers with flexible hydrocarbon bond [16]. Less attention was paid to organosilicone polymers in aqueous solution because their microemulsions are usually difficult to be prepared. Moreover, the theory of microemulsion on organosilicone polymer was rarely reported. On the other hand, there were also reports regarding the ring-opening polymerization of D4 in bulk or miniemulsion. Recently, Hémery et al. [1,7] investigated the anionic polymerization kinetics and mechanism of D4 in miniemulsion. Chen et al. [17] investigated the cationic polymerization of D4 initiated by acid-treated bentonite in bulk. However, the above reports are not regarding the microemulsion polymerization of D4, corresponding studies can still throw some light on this work. In this work, we investigate the kinetics and physical mechanism of microemulsion polymerization of D4 by using dodecyldimethylbenzyl ammonium bromide (DBDA) as a surfactant and n-pentane as a cosurfactant. The light transparence of the emulsion and the polymerization conversion as the functions of the polymerization time are recorded. Furthermore, the particle sizes and their distributions in the process are also measured by dynamic light scattering technique (DLS). 2. Experimental details 2.1. Materials D4 (Union Carbide Co., USA) was used as the polymerization monomer. Chromatographic analysis indicated that no other cyclic or linear matter was present. DBDA (Xuanguang Science and Technology Company of Xiamen, China) was used as the surfactant, and n-pentane (Chemical Reagent Company of Shanghai, China) was used as the cosurfactant. The polymerization catalyst was sodium hydroxide (NaOH, Chemical Reagent Company of Xiamen, China). Acetic acid (Chemical Reagent Company of Shanghai, China) was selected as the pH regulator, and ethanol and acetone (Chemical Reagent Company of Xiamen, China) were chosen as the separation solvents. All reagents were used without further purification. Deionized water as the aqueous phase was used as the aqueous phase throughout this experiment. 2.2. Polymerization At first, DBDA, n-pentane and deionized water were successively added into a 250 ml reactor equipped with a four-blade agitator, a thermometer and a reflux condenser. The mixture was stirred at 400 rpm to be well dispersed; at the same time the mixture temperature was raised from room temperature to 353 K. After the emulsifier (DBDA + n-pentane) was dissolved in water entirely, D4 was added to the reactor and emulsified for 10 min. Then NaOH was added into the reactor, meanwhile, the reaction was designated as t = 0 min. This reaction would last about 650 min wherein the sampling was withdrawn from the reactor in a regular interval. Finally, the reaction was stopped by adding acetic acid. 2.3. FTIR analysis The product, polymer latex, obtained in Section 2.2 was precipitated with a large amount of ethanol, and the polymer was then dissolved in acetone and precipitated again with ethanol so as to completely
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remove the emulsifier. After that, the obtained polymer was dried under vacuum. The polymer thus obtained was hot-pressed under 170 °C into a piece of thin film and then analyzed by FTIR (Nicolet FTIR-8201). 2.4. Determination of light transmittance and conversion The clarity of the emulsions diluted to 70 g/l was determined by measuring the light transmittance with a visible light spectrophotometer (Model 721, Precision & Scientific Instrument of Shanghai, China) at a wavelength of 500 nm. The polymer conversion was recorded as a function of time and the polymerization rate was calculated by further differentiation. The conversion of monomer was measured by gravimetry by drying the sampled emulsion to constant weight in vacuum at 120 °C [13,17]. 2.5. Transmission electron microscopy (TEM) analysis The obtained latexes were stained by using phosphotungstic acid as a staining agent and then examined by using a transmission electron microscope (Tecnai F30) at an accelerating voltage of 300 kV. 2.6. Interfacial tension analysis Interfacial tensions were measured according to the spinning drop method [18] using a SITE LP-10 Interfacial Tensiometer (Institute of Petroleum Engineering, Clausthal, Germany). 2.7. Determination of particle size and size distribution The emulsion particle size and size distribution were measured by dynamic light scattering technique (DLS technique) from the Malvern Zeta-sizer 3000 (Malvern Instruments GmbH, Herrenberg, Germany) at ambient temperature. 3. Results and discussion 3.1. Microemulsion polymerization conditions In order to obtain the typical microemulsion polymerization recipe, the effect of the emulsifier (DBDA + n-pentane) concentration on the light transmittance and average particle size of the emulsion were firstly investigated. The results show that the average particle size of the emulsion decreases with the increase of the emulsifier concentration [3,8,19]. When the ratio of (DBDA + n-pentane)/mixture (wt./wt.) is no less than 12.5%, the light transmittance of the emulsion is close to 80% and the appearance of the emulsion is transparent. TEM results of latexes prove that the emulsion is microemulsion. It shows that the emulsion exists exactly as the microemulsion state. The experimental results on the light transmittance of the microemulsions at different emulsifier concentrations are shown in Fig. 1. A typical TEM result of the latex is given in Fig. 2. Furthermore, a typical IR-spectrum of the polymer produced is given in Fig. 3. According to Figs. 1 and 2, the emulsifier concentration in the aqueous should be larger than 12.5% in order to form a microemulsion to perform a microemulsion polymerization of D4. This emulsifier concentration required is much higher than that of a conventional emulsion polymerization system, which is consistent with what was observed in Refs. [8,12–14,16,20–22]. In addition, Fig. 2 shows that the particles in the microemulsion polymerization system are at the nanoscale, which is different from that reported in Refs. [8,19]. Furthermore, as shown in Fig. 3, the bands at 1021–1090, 800–870, and 1260, 2926 cm− 1 are the characteristic bands of Si–O, Si–C and –CH3, respectively, which confirms that PDMS latex has been produced in the microemulsion polymerization.
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Fig. 1. The light transmittance histories of the microemulsion at different emulsifier (DBDA + n-pentane) concentrations. (DBDA:n-pentane= 8:3(wt./wt.); H2O:D4:NaOH = 530:120:1 (wt./wt./wt.); T = 353 K).
Fig. 3. A typical FTIR spectrum of the PDMS latex obtained by the microemulsion polymerization of D4 (recipe for the polymerization is the same as that in Table 1; T = 353 K; t = 360 min).
Based on the above experimental results, one can obtain that an approximately transparent PDMS microemulsion can be prepared by the microemulsion polymerization of D4 at a high emulsifier (DBDA + npentane) concentration (N12.5%). In the following experiments, the mass concentration of emulsifier is about 14.45% and corresponding recipe is listed in Table 1. In addition, three microemulsion polymerization experiments were done at the same polymerization condition shown in Table 1. Corresponding kinetic data, i.e. light transmittance, conversion, particle size, etc., are almost identical. It means that the reproducibility of the experiment is perfect. Among them, only the results from one of the experiments are listed and discussed in Section 3.2 and Section 3.3 due to limited space.
recorded as the functions of the reaction time in order to further testify the microemulsion polymerization and the result is shown in Fig. 6. As shown in Fig. 4, the light transmittance increases quickly from about 5% to about 80% following the reaction proceeding in the period of 0–120 min. The light transmittance reaches its maximum at 120 min and decreases slowly with reaction proceeding. From Fig. 5, one can know that the conversion increases quickly from 0 to the maximum in the period of 0–120 min and remains nearly unchanged after that. According to Fig. 6, one can know that the oil–water interfacial tension decreases firstly and then increases following the reaction proceeding. The change value in the process of the whole polymerization is very low and the interfacial tension is also very low. It proves that the polymerization takes place in the microemulsion system along with the appearance of polymer particles [18]. For the purpose of manifesting the kinetics directly, the polymerization rate was obtained by differentiating the conversion with the reaction time (Fig. 5). The polymerization rate of the microemulsions under different reaction time and conversion is shown in Fig. 6 and Fig. 7, respectively. As shown in Fig. 7, the polymerization rate increases quickly to the maximum following the reaction proceeding. From Fig. 8, one can find that the polymerization rate reaches its maximum at about 20 min corresponding to a conversion of about 25%, and then decreases quickly with reaction proceeding in the period of about 20–120 min. The polymerization rate keeps constant after that. The continuous increase of the polymerization rate in the first interval can be attributed to the rapid and continuous increase of the amount of the microlatex particles from 0, which leads to nucleation. In the nucleation period, the living ions can migrate into the micelles and swell the micelles to form the microlatex particles/the new particles. In addition, there is enough emulsifier to stabilize the new particles and the micelles do not vanish in the microemulsion polymerization. Therefore, there does not exist constant reaction rate period in the microemulsion polymerization process as shown in Fig. 7, which is different from that in the conventional emulsion polymerization [12,16,20–22]. Moreover, with the
3.2. Microemulsion polymerization kinetics In order to demonstrate the process kinetics, the light transmittance of the emulsion and the conversion are measured as the functions of the reaction time. The results are shown in Figs. 4 and 5. In addition, the oil–water interfacial tension of the emulsion is also
Table 1 Recipe of O/W microemulsion polymerization of D4.
Fig. 2. A type TEM micrograph of the PDMS microemulsion obtained by the microemulsion polymerization of D4 (recipe for the polymerization is the same as that in Table 1; T = 353 K; t = 360 min).
Water DBDA n-Pentane NaOH D4
106.0 g 16.0 g 6.0 g 0.2 g 24.0 g
69.65 wt.% 10.51 wt.% 3.94 wt.% 0.13 wt.% 15.77 wt.%
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Fig. 4. The light transmittance of the microemulsion as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
Fig. 6. The oil–water interfacial tension of the emulsion as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
polymerization proceeding, the difference of the surfactant concentration between the micelle and the microlatex particle decreases to a constant, and the polymerization rate also changes as shown in Fig. 7. The detailed physical mechanism will be described and discussed in Section 3.3. However, here we also point out that we cannot explain the above kinetic data according to the corresponding physical mechanism described in Section 3.3.
3.3.1. Dispersion period (I) (t = 0 min) The polymerization system exists at a dispersion state due to the absence of NaOH, and the substances, including D4, DBDA, n-pentane and H2O may be found in this period. It can be seen from Fig. 10I that
three kinds of particles may exist in the system. According to Ref. [23], they seem to be the micelles, swollen-micelles and monomer droplets with the sizes of 0–10 nm, 0–10 nm and 145–770 nm, respectively. It is well known that the micelles can be obtained by the assembly of emulsifier molecules and the micelles can exist in the microemulsion system due to the enough emulsifier. Meanwhile, along with the addition of monomer (D4) into the reactor and due to the very limited solution ability of D4 in the aqueous phase, the monomer molecules can be dispersed to form monomer droplets, whose surfaces cover emulsifier molecules. This means that the monomer droplets are the reservoir of monomer molecules. However, there are still few monomer molecules dissolving in the aqueous phase, namely dissociative monomers. Moreover, the micelles can improve the solution ability of D4 in the aqueous phase. Therefore, the limited dissociative monomers can be adsorbed into some micelle particles to form monomer swollen-micelles, which leads to the increase of micelle particle in volume. Some monomer molecules within the monomer droplets can diffuse into the aqueous phase to become dissociative monomers. Therefore, there are dynamic equilibriums of the monomers within the monomer droplets, the aqueous phase and the micelles. It is possible for the comexistence of the micelles and swollen-micelles in the presence of gigantic monomer droplets. In practice, the swollen-micelles with the size about 145–770 nm besides the micelle due to the very high emulsifier concentration (about
Fig. 5. The polymerization conversion as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
Fig. 7. The polymerization rate as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
3.3. Physical mechanism of microemulsion polymerization In order to demonstrate the nucleation mechanism (here nucleation denotes the formation of microlatex particles, and will be explained later) of this polymerization, the particle size and its distribution were measured as the functions of time during the microemulsion polymerization. The experimental results are shown in Table 2 and Figs. 9 and 10. According to Table 2, the whole polymerization process of the microemulsion system can be divided into four steps:
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Fig. 8. The polymerization rate as a function of the conversion (recipe for the polymerization is the same as that in Table 1; T = 353 K).
14.45%, wt./wt.) in the system. Indeed, the concentration is much higher than the critical micelle concentration (CMC) (about 0.1%, wt./wt.). Therefore, the monomer molecules mainly lie in the micelles and the swollen-micelles in the dispersion period and there is a dynamic balance of the monomer concentration between micelle and swollenmicelle. However, from Fig. 9I, it seems that mainly monomer droplets and bulky micelles in the system exist. Because the size of the micelle or the swollen-micelle is much less than that of the monomer droplet, they could not be observed in the system by DLS.
Table 2 Average particle sizes and particle size distributions during the microemulsion polymerization of D4. Time (min)
Peak number
Particle size (nm)
Distribution width (nm)
Weight of particles (%)
t=0 t=5 t = 15 t = 20 t = 25 t = 55
1 1 1 1 1 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 3 1 2 3 1 2 3
533.0 387.5 403.9 438.2 442.6 4.7 21.8 4.1 22.9 4.8 26.0 3.6 24.5 4.4 26.6 3.5 26.9 4.2 29.7 3.8 31.3 3.7 32.3 3.8 32.7 3.4 31.2 276.3 3.3 28.7 286.6 2.8 28.9 284.9
614.8 115.7 112.7 111.6 109.7 2.9 5.6 1.2 10.7 4.6 19.1 4.4 16.6 0.9 7.2 1.7 6.8 3.8 19.4 3.8 21.7 4.6 22.8 4.8 22.7 3.3 33.9 63.8 2.6 30.5 68.3 1.9 30.3 69.5
100 100 100 100 100 63.8 36.2 56.4 43.6 51.5 48.5 48.8 51.2 45.4 54.6 41.7 58.3 39.8 60.2 35.4 64.6 30.7 69.3 28.5 71.5 16.9 66.7 16.4 13.0 71.7 15.3 9.6 76.3 14.1
t = 85 t = 115 t = 135 t = 155 t = 185 t = 215 t = 245 t = 275 t = 305 t = 365
t = 425
t = 485
3.3.2. Colloid formation and reaction period (II) (0 b t ≤ 25 min) In this period, the polymerization is initiated by the addition of NaOH into the system. The particle size distribution curves measured at different times (t = 5, 20, 25 min) are shown in Fig. 9II-a–II-c. In Table 2 and Fig. 9II-a–II-c, both the average particle size and its distribution width decrease. The peak number of the particle distribution curves is found to be unique observed by DLS. Following the addition of NaOH, the living ions formed in the aqueous phase can migrate into the swollen-micelles and the micelles. Hence the polymerization takes place in the corresponding swollenmicelles and the micelles, turning them into the microlatex particles. The process described above is referred to as the nucleation process. It is insomuch of the continual formation of the new kind of particles (the microlatex particles) and the continual of arising polymerization in the period, therefore it can be called the colloid formation and reaction period. Certainly, the living ions can also migrate into the monomer droplets to initiate polymerization in the monomer droplets. However, comparing with the total number of the swollen-micelles and the micelles, that of the monomer droplets can be ignored. Therefore, for the microemulsion polymerization of D4, the nucleation occurs mostly in the swollen-micelles, while the polymerization takes place in the microlatex particles. As the polymerization continues in this period, the monomer in the monomer droplets will also migrate into the microlatex particles, which results in the decrease of the peak intensity of the monomer droplet size distributions and the increase of the peak intensity of the microlatex particle size distributions. In addition, with the increase of the total amount of the microlatex particles, DBDA in the bulky micelles may also migrate to the microlatex particles, which leads to the disappearance of the bulky micelles. Therefore, the particle size distribution becomes narrow. Comparing the size of the monomer droplets with that of the microlatex particles in the system, we can see that the latter could not be observed by using DLS. Therefore, only one peak is observed in Fig. 9II-a–II-c. From the above discussion, it comes to the conclusion that, during the colloid formation and reaction, four kinds of particles exist in the system. They are micelles, swollen-micelles, monomer droplets and new microlatex particles. A typical particle size distribution is shown in Fig. 10II. 3.3.3. Colloid reaction period (III) (25 b t ≤ 305 min) As soon as the monomer droplets disappear at the end of the last period, the particles in the system begin to react with each other, meaning that the polymerization goes into step (III). In comparison with step (II), there is no formation of new kinds of particles in the period while the polymerization is proceeding. In the period, it can be seen from Fig. 10III that there exist two kinds of particles in the system wherein the particles are shown as the micelle and the microlatex with sizes of 0–10 and 10–50 nm, respectively, which can be observed by using DLS, as shown in Fig. 9IIIa–III-c. As shown in Table 2 and Fig. 9IIIa–III-c, the micelle size has no much change but its number apparently decreases with the polymerization proceeding. The micelle number can be easily calculated based on the multi-mode particle distribution peaks, for instance, when the average micelle particle size is 3.8 nm, the number are 63.8%, 51.5% and 28.5% at a time of 55 min, 11.5 min, and 305 min, respectively. It can also be found that the microlatex has a slightly bigger size and the particle number also increases as the polymerization proceeds. Similarly, the microlatex particle number can also be obtained, for example, the number is 36.2% when the average particle size is 21.8 nm and the time is 55 min (see Table 2). Due to the absence of the monomer droplets as the storage of monomer, the monomers which polymerize in the microlatex particles are supplied by the microlatex particle. Simultaneously, the micelle in the system does not polymerize. Hence, with the polymerization proceeding in the period, the amount of the monomer in the microlatex
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Fig. 9. Particle kinetics of the microemulsion polymerization of D4 (I: t = 0 min in the dispersion period; II-a: t = 5 min in the colloid formation and reaction period; II-b: t = 20 min in the colloid formation and reaction period; II-c: t = 25 min in the colloid formation and reaction period; III-a: t = 55 min in the colloid reaction period; III-b: t = 115 min in the colloid reaction period; III-c: t = 305 min in the colloid reaction period; IV: t = 485 min in the agglomeration period) (recipe for the polymerization is the same as that in Table 1; T = 353 K).
particle decreases. Meanwhile, both the size of the microlatex particles and their numbers increase. However, the size of the micelles keeps unchanged while their number decreases. It must be noted that, in the periods of step (I) and step (II), there exist two peaks which represent the micelles and the microlatex particles, respectively, however, they are ignored due to the large amount of monomer droplets in the system.
micelle size (0–10 nm) and the microlatex particle size (10–50 nm) are much smaller than that of the agglomerate particles, they can still be detected by DLS owing to the absolutely high particle number produced, which can be seen from Table 2 and Fig. 9IV. For example, the agglomerate particle number in the system is 16.4%, but the total number of micelles and microlatex particles is 83.6% at t = 365 min. Fig. 10IV shows a typical particle distribution at this step.
3.3.4. Agglomeration period (IV) (t N 305 min) Although the conversion and the polymerization rate are constant due to lack of the monomer, a few chain-condensation reactions may still proceed and some unstable microlatex particles may agglomerate to form much large particles because of the mechanical agitating or other factors. It is the agglomeration that leads to a third peak as shown in Fig. 9IV, they represent the micelle, the microlatex particle and the microlatex agglomeration particle with the sizes of 0–10, 10– 50 and 250–310 nm, respectively. Table 2 also shows that there is no big number and size percentage changes of these three kinds of particles. But the agglomerate particle sizes increase slightly and the particle number decreases with the polymerization proceeding. The particle number was obtained as 16.4% (average particle size = 276.3 nm, t = 365 min), 15.3% (average particle size = 286.6 nm, t = 425 min) and 14.1% (average particle size = 284.9 nm, t = 485 min). Although the
4. Conclusions Using DBDA as the surfactant and n-pentane as the cosurfactant, the relatively stable PDMS microemulsions were prepared via the polymerization of D4 in aqueous microemulsion. Microemulsion polymerization experiments were carried out at ambient operating conditions. The light transmittance of microemulsion, the polymerization conversion, the particle size and the particle size distribution as the functions of the reaction time were measured to evaluate the polymerization behaviours including the polymerization kinetics and the nucleation location. The detailed conclusions include: (1) The content of the emulsifier in a microemulsion polymerization system is very high. In our experiments, it manifests that the emulsifier (DBDA + n-pentane) concentration should be no
Fig. 10. Physical model of the microemulsion polymerization of D4 (I: the dispersion period; II: the colloid formation and reaction period; III: the colloid reaction period; IV: the agglomeration period) (recipe for the polymerization is the same as that in Table 1; T = 353 K).
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less than 12.5%. In addition, according to the experimentally measured conversion, it is found that there is no constant reaction rate period in the whole microemulsion polymerization process. (2) According to the particle sizes and their distributions in the studied system, the whole process in the microemulsion polymerization of D4 can be divided into four steps, and the physical model for each step was then developed. They are described as follows: (I) the dispersion period, (II) the colloid formation and reaction period, (III) the colloids reaction period and (IV) the agglomeration period. The polymerization rate increases at the step (II) due to the continuous formation of new colloids. In the period (III) the monomer particles diminish till completely disappear, so the monomers in the colloid are consumed for the polymerization, which decreases the polymerization rate. (3) For the microemulsion polymerization of D4, the nucleation occurs mostly in the swollen-micelles, while the polymerization takes place in the microlatex particles. Finally, there we also point out that we cannot associate the kinetics of the microemulsion polymerization of D4 with its physical mechanism entirely in this paper. Further studies on the microemulsion polymerization of D4 are in progress in our group. Acknowledgments
[5]
[6] [7]
[8]
[9] [10]
[11] [12] [13]
[14] [15]
[16] [17]
The authors thank National Natural Science Foundation of China (No. 20406016), Nation Defense Key Laboratory of Ocean Corrosion and Anti-corrosion of China (No. 51449020205QT8703), and Fujian Province Science and Technology Office of China (No. 2005H040) for joint financial support.
[18] [19]
[20]
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Particle kinetics and physical mechanism of microemulsion polymerization of octamethylcyclotetrasiloxane Ya-Qing Zhuang a, Xiang Ke a, Xiao-Li Zhan b,⁎, Zheng-Hong Luo a,⁎ a b
Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Department of Chemical and Biochemical engineering, Zhejang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 21 November 2009 Received in revised form 11 March 2010 Accepted 12 March 2010 Available online 20 March 2010 Keywords: Octamethylcyclotetrasiloxane Microemulsion polymerization Kinetics Physical model Nucleation mechanism
a b s t r a c t In the present study, the particle kinetics and physical mechanism of microemulsion polymerization of octamethylcyclotetrasiloxane (D4) were investigated by using dodecyldimethylbenzyl ammonium bromide (DBDA) as a surfactant and n-pentane as a cosurfactant. The light transparence of the emulsion, oil–water interfacial tension and the polymerization conversion as functions of the polymerization time were recorded. Furthermore, the particle sizes and their distributions in the process were measured by using dynamic light scattering technique (DLS). The results show that there does not exist constant-rate reaction period during the polymerization. The results of DLS show that the microemulsion polymerization can be distinguished as four steps, namely (I) the dispersion period, (II) the colloid formation and reaction period, (III) the colloid reaction period, and (IV) the agglomeration period. Corresponding physical models for each period were discussed. It has been found that the nucleation occurs mostly in the swollen-micelles and the polymerization occurs mostly in the new microlatex particles for the microemulsion polymerization of D4. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The ring-opening polymerization of octamethylcyclotetrasiloxane (D4) and/or the copolymerization of D4 with functional siloxane monomers can be used to prepare a wide variety of silicones. Silicones with various microstructures and architectures are useful in industry mainly due to their absence of toxicity, high chain flexibility and low surface energy [1–5]. Furthermore, emulsions based on silicones have been prepared for industrial applications [1–3]. A very simple way to synthesize silicones is the direct polymerization of the monomer in emulsion, namely emulsion polymerization. For instance, the emulsion polymerization of D4 can be used to prepare polydimethylsiloxane (PDMS) directly. In addition, PDMS, one kind of silicone, can be used to manufacture a series of industrial and civil products, such as lubricant, leather glossy agent, paint film polishing agent, etc. [6]. For the preparation of the emulsions based on PDMS, there are mainly three emulsion-polymerization methods, namely, conventional emulsion polymerization, miniemulsion polymerization and microemulsion polymerization [1,2,7–12]. PDMS latexes prepared by the conventional emulsion polymerization are usually unstable due to their large particle sizes in the order of 1000 nm [2]. However, by imposing an external power on the emulsion system prior to the polymerization, the latexes with particle sizes in the order of 50–500 nm prepared by the miniemulsion polymerization are stable [9,10]. Compared with the
⁎ Corresponding author. Tel.: + 86 592 2187190; fax: + 86 592 2187231. E-mail address: [email protected] (Z.-H. Luo). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.03.017
above two latexes, the latexes in microemulsion are the most transparent and their particle sizes are in the order of 0–100 nm [11,12]. In practice, the microemulsion polymerization needs a higher emulsifier concentration instead of the external power used in the miniemulsion polymerization. In short, the microemulsions are transparent and thermodynamically stable mixtures of oil and water, and are stabilized by surfactant and usually with a cosurfactant [12,13]. Accordingly, the microemulsion polymerization has attracted more and more concerns and been widely applied in industry. Up to now, there were many reports on the microemulsion polymerization [8,11–15]. Full et al. [14] succeeded in preparing stable microemulsion via the microemulsion polymerization of styrene (St) by the addition of cosurfactant and investigated the effects of the addition of cosurfactant on the microemulsion polymerization behaviour. Xu et al. [11] reported the copolymerization of St with butyl acrylates (BA) in emulsion and microemulsion. It was found that the copolymerization behaviour of St/BA in microemulsion is obviously different from that in emulsion copolymerization. The result of St/BA copolymerization strongly suggests that the real reaction position of the microemulsion polymerization is the inter-side of the emulsifier layer. Xu et al. [15] also investigated the microemulsion polymerization of methyl methacrylate (MMA) initiated with Benzoyl Peroxide (BPO) in 1999. In their research, the growth of monomer-swollen polymer particles was studied by using photo correlation spectroscopy, and the polymerization kinetics was studied with a dilatometer. Shi et al. [13] investigated the kinetics of and the mechanism of microemulsion copolymerization of St and acrylonitrile in oil-in-water microemulsions by using n-butanol as a cosurfactant. Recently, Yu and Zhan [8] studied the microemulsion
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polymerization of D4. However, in their work, the reaction recipe of the microemulsion polymerization was not reported and the microemulsion polymerization system was only confirmed via measuring the light transmittance of the system, corresponding polymerization kinetics was ignored. Based on the above discussions, it is clear that previous studies on the microemulsion polymerization concentrate on the issues of the emulsification systems, the nucleation mechanisms, and the process kinetics. Furthermore, it is also clear that past studies are limited to the microemulsion ho/co-polymerizations of unsaturated monomers and the resultant products are mainly the polymers with flexible hydrocarbon bond [16]. Less attention was paid to organosilicone polymers in aqueous solution because their microemulsions are usually difficult to be prepared. Moreover, the theory of microemulsion on organosilicone polymer was rarely reported. On the other hand, there were also reports regarding the ring-opening polymerization of D4 in bulk or miniemulsion. Recently, Hémery et al. [1,7] investigated the anionic polymerization kinetics and mechanism of D4 in miniemulsion. Chen et al. [17] investigated the cationic polymerization of D4 initiated by acid-treated bentonite in bulk. However, the above reports are not regarding the microemulsion polymerization of D4, corresponding studies can still throw some light on this work. In this work, we investigate the kinetics and physical mechanism of microemulsion polymerization of D4 by using dodecyldimethylbenzyl ammonium bromide (DBDA) as a surfactant and n-pentane as a cosurfactant. The light transparence of the emulsion and the polymerization conversion as the functions of the polymerization time are recorded. Furthermore, the particle sizes and their distributions in the process are also measured by dynamic light scattering technique (DLS). 2. Experimental details 2.1. Materials D4 (Union Carbide Co., USA) was used as the polymerization monomer. Chromatographic analysis indicated that no other cyclic or linear matter was present. DBDA (Xuanguang Science and Technology Company of Xiamen, China) was used as the surfactant, and n-pentane (Chemical Reagent Company of Shanghai, China) was used as the cosurfactant. The polymerization catalyst was sodium hydroxide (NaOH, Chemical Reagent Company of Xiamen, China). Acetic acid (Chemical Reagent Company of Shanghai, China) was selected as the pH regulator, and ethanol and acetone (Chemical Reagent Company of Xiamen, China) were chosen as the separation solvents. All reagents were used without further purification. Deionized water as the aqueous phase was used as the aqueous phase throughout this experiment. 2.2. Polymerization At first, DBDA, n-pentane and deionized water were successively added into a 250 ml reactor equipped with a four-blade agitator, a thermometer and a reflux condenser. The mixture was stirred at 400 rpm to be well dispersed; at the same time the mixture temperature was raised from room temperature to 353 K. After the emulsifier (DBDA + n-pentane) was dissolved in water entirely, D4 was added to the reactor and emulsified for 10 min. Then NaOH was added into the reactor, meanwhile, the reaction was designated as t = 0 min. This reaction would last about 650 min wherein the sampling was withdrawn from the reactor in a regular interval. Finally, the reaction was stopped by adding acetic acid. 2.3. FTIR analysis The product, polymer latex, obtained in Section 2.2 was precipitated with a large amount of ethanol, and the polymer was then dissolved in acetone and precipitated again with ethanol so as to completely
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remove the emulsifier. After that, the obtained polymer was dried under vacuum. The polymer thus obtained was hot-pressed under 170 °C into a piece of thin film and then analyzed by FTIR (Nicolet FTIR-8201). 2.4. Determination of light transmittance and conversion The clarity of the emulsions diluted to 70 g/l was determined by measuring the light transmittance with a visible light spectrophotometer (Model 721, Precision & Scientific Instrument of Shanghai, China) at a wavelength of 500 nm. The polymer conversion was recorded as a function of time and the polymerization rate was calculated by further differentiation. The conversion of monomer was measured by gravimetry by drying the sampled emulsion to constant weight in vacuum at 120 °C [13,17]. 2.5. Transmission electron microscopy (TEM) analysis The obtained latexes were stained by using phosphotungstic acid as a staining agent and then examined by using a transmission electron microscope (Tecnai F30) at an accelerating voltage of 300 kV. 2.6. Interfacial tension analysis Interfacial tensions were measured according to the spinning drop method [18] using a SITE LP-10 Interfacial Tensiometer (Institute of Petroleum Engineering, Clausthal, Germany). 2.7. Determination of particle size and size distribution The emulsion particle size and size distribution were measured by dynamic light scattering technique (DLS technique) from the Malvern Zeta-sizer 3000 (Malvern Instruments GmbH, Herrenberg, Germany) at ambient temperature. 3. Results and discussion 3.1. Microemulsion polymerization conditions In order to obtain the typical microemulsion polymerization recipe, the effect of the emulsifier (DBDA + n-pentane) concentration on the light transmittance and average particle size of the emulsion were firstly investigated. The results show that the average particle size of the emulsion decreases with the increase of the emulsifier concentration [3,8,19]. When the ratio of (DBDA + n-pentane)/mixture (wt./wt.) is no less than 12.5%, the light transmittance of the emulsion is close to 80% and the appearance of the emulsion is transparent. TEM results of latexes prove that the emulsion is microemulsion. It shows that the emulsion exists exactly as the microemulsion state. The experimental results on the light transmittance of the microemulsions at different emulsifier concentrations are shown in Fig. 1. A typical TEM result of the latex is given in Fig. 2. Furthermore, a typical IR-spectrum of the polymer produced is given in Fig. 3. According to Figs. 1 and 2, the emulsifier concentration in the aqueous should be larger than 12.5% in order to form a microemulsion to perform a microemulsion polymerization of D4. This emulsifier concentration required is much higher than that of a conventional emulsion polymerization system, which is consistent with what was observed in Refs. [8,12–14,16,20–22]. In addition, Fig. 2 shows that the particles in the microemulsion polymerization system are at the nanoscale, which is different from that reported in Refs. [8,19]. Furthermore, as shown in Fig. 3, the bands at 1021–1090, 800–870, and 1260, 2926 cm− 1 are the characteristic bands of Si–O, Si–C and –CH3, respectively, which confirms that PDMS latex has been produced in the microemulsion polymerization.
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Fig. 1. The light transmittance histories of the microemulsion at different emulsifier (DBDA + n-pentane) concentrations. (DBDA:n-pentane= 8:3(wt./wt.); H2O:D4:NaOH = 530:120:1 (wt./wt./wt.); T = 353 K).
Fig. 3. A typical FTIR spectrum of the PDMS latex obtained by the microemulsion polymerization of D4 (recipe for the polymerization is the same as that in Table 1; T = 353 K; t = 360 min).
Based on the above experimental results, one can obtain that an approximately transparent PDMS microemulsion can be prepared by the microemulsion polymerization of D4 at a high emulsifier (DBDA + npentane) concentration (N12.5%). In the following experiments, the mass concentration of emulsifier is about 14.45% and corresponding recipe is listed in Table 1. In addition, three microemulsion polymerization experiments were done at the same polymerization condition shown in Table 1. Corresponding kinetic data, i.e. light transmittance, conversion, particle size, etc., are almost identical. It means that the reproducibility of the experiment is perfect. Among them, only the results from one of the experiments are listed and discussed in Section 3.2 and Section 3.3 due to limited space.
recorded as the functions of the reaction time in order to further testify the microemulsion polymerization and the result is shown in Fig. 6. As shown in Fig. 4, the light transmittance increases quickly from about 5% to about 80% following the reaction proceeding in the period of 0–120 min. The light transmittance reaches its maximum at 120 min and decreases slowly with reaction proceeding. From Fig. 5, one can know that the conversion increases quickly from 0 to the maximum in the period of 0–120 min and remains nearly unchanged after that. According to Fig. 6, one can know that the oil–water interfacial tension decreases firstly and then increases following the reaction proceeding. The change value in the process of the whole polymerization is very low and the interfacial tension is also very low. It proves that the polymerization takes place in the microemulsion system along with the appearance of polymer particles [18]. For the purpose of manifesting the kinetics directly, the polymerization rate was obtained by differentiating the conversion with the reaction time (Fig. 5). The polymerization rate of the microemulsions under different reaction time and conversion is shown in Fig. 6 and Fig. 7, respectively. As shown in Fig. 7, the polymerization rate increases quickly to the maximum following the reaction proceeding. From Fig. 8, one can find that the polymerization rate reaches its maximum at about 20 min corresponding to a conversion of about 25%, and then decreases quickly with reaction proceeding in the period of about 20–120 min. The polymerization rate keeps constant after that. The continuous increase of the polymerization rate in the first interval can be attributed to the rapid and continuous increase of the amount of the microlatex particles from 0, which leads to nucleation. In the nucleation period, the living ions can migrate into the micelles and swell the micelles to form the microlatex particles/the new particles. In addition, there is enough emulsifier to stabilize the new particles and the micelles do not vanish in the microemulsion polymerization. Therefore, there does not exist constant reaction rate period in the microemulsion polymerization process as shown in Fig. 7, which is different from that in the conventional emulsion polymerization [12,16,20–22]. Moreover, with the
3.2. Microemulsion polymerization kinetics In order to demonstrate the process kinetics, the light transmittance of the emulsion and the conversion are measured as the functions of the reaction time. The results are shown in Figs. 4 and 5. In addition, the oil–water interfacial tension of the emulsion is also
Table 1 Recipe of O/W microemulsion polymerization of D4.
Fig. 2. A type TEM micrograph of the PDMS microemulsion obtained by the microemulsion polymerization of D4 (recipe for the polymerization is the same as that in Table 1; T = 353 K; t = 360 min).
Water DBDA n-Pentane NaOH D4
106.0 g 16.0 g 6.0 g 0.2 g 24.0 g
69.65 wt.% 10.51 wt.% 3.94 wt.% 0.13 wt.% 15.77 wt.%
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Fig. 4. The light transmittance of the microemulsion as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
Fig. 6. The oil–water interfacial tension of the emulsion as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
polymerization proceeding, the difference of the surfactant concentration between the micelle and the microlatex particle decreases to a constant, and the polymerization rate also changes as shown in Fig. 7. The detailed physical mechanism will be described and discussed in Section 3.3. However, here we also point out that we cannot explain the above kinetic data according to the corresponding physical mechanism described in Section 3.3.
3.3.1. Dispersion period (I) (t = 0 min) The polymerization system exists at a dispersion state due to the absence of NaOH, and the substances, including D4, DBDA, n-pentane and H2O may be found in this period. It can be seen from Fig. 10I that
three kinds of particles may exist in the system. According to Ref. [23], they seem to be the micelles, swollen-micelles and monomer droplets with the sizes of 0–10 nm, 0–10 nm and 145–770 nm, respectively. It is well known that the micelles can be obtained by the assembly of emulsifier molecules and the micelles can exist in the microemulsion system due to the enough emulsifier. Meanwhile, along with the addition of monomer (D4) into the reactor and due to the very limited solution ability of D4 in the aqueous phase, the monomer molecules can be dispersed to form monomer droplets, whose surfaces cover emulsifier molecules. This means that the monomer droplets are the reservoir of monomer molecules. However, there are still few monomer molecules dissolving in the aqueous phase, namely dissociative monomers. Moreover, the micelles can improve the solution ability of D4 in the aqueous phase. Therefore, the limited dissociative monomers can be adsorbed into some micelle particles to form monomer swollen-micelles, which leads to the increase of micelle particle in volume. Some monomer molecules within the monomer droplets can diffuse into the aqueous phase to become dissociative monomers. Therefore, there are dynamic equilibriums of the monomers within the monomer droplets, the aqueous phase and the micelles. It is possible for the comexistence of the micelles and swollen-micelles in the presence of gigantic monomer droplets. In practice, the swollen-micelles with the size about 145–770 nm besides the micelle due to the very high emulsifier concentration (about
Fig. 5. The polymerization conversion as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
Fig. 7. The polymerization rate as a function of the reaction time (recipe for the polymerization is the same as that in Table 1; T = 353 K).
3.3. Physical mechanism of microemulsion polymerization In order to demonstrate the nucleation mechanism (here nucleation denotes the formation of microlatex particles, and will be explained later) of this polymerization, the particle size and its distribution were measured as the functions of time during the microemulsion polymerization. The experimental results are shown in Table 2 and Figs. 9 and 10. According to Table 2, the whole polymerization process of the microemulsion system can be divided into four steps:
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Fig. 8. The polymerization rate as a function of the conversion (recipe for the polymerization is the same as that in Table 1; T = 353 K).
14.45%, wt./wt.) in the system. Indeed, the concentration is much higher than the critical micelle concentration (CMC) (about 0.1%, wt./wt.). Therefore, the monomer molecules mainly lie in the micelles and the swollen-micelles in the dispersion period and there is a dynamic balance of the monomer concentration between micelle and swollenmicelle. However, from Fig. 9I, it seems that mainly monomer droplets and bulky micelles in the system exist. Because the size of the micelle or the swollen-micelle is much less than that of the monomer droplet, they could not be observed in the system by DLS.
Table 2 Average particle sizes and particle size distributions during the microemulsion polymerization of D4. Time (min)
Peak number
Particle size (nm)
Distribution width (nm)
Weight of particles (%)
t=0 t=5 t = 15 t = 20 t = 25 t = 55
1 1 1 1 1 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 3 1 2 3 1 2 3
533.0 387.5 403.9 438.2 442.6 4.7 21.8 4.1 22.9 4.8 26.0 3.6 24.5 4.4 26.6 3.5 26.9 4.2 29.7 3.8 31.3 3.7 32.3 3.8 32.7 3.4 31.2 276.3 3.3 28.7 286.6 2.8 28.9 284.9
614.8 115.7 112.7 111.6 109.7 2.9 5.6 1.2 10.7 4.6 19.1 4.4 16.6 0.9 7.2 1.7 6.8 3.8 19.4 3.8 21.7 4.6 22.8 4.8 22.7 3.3 33.9 63.8 2.6 30.5 68.3 1.9 30.3 69.5
100 100 100 100 100 63.8 36.2 56.4 43.6 51.5 48.5 48.8 51.2 45.4 54.6 41.7 58.3 39.8 60.2 35.4 64.6 30.7 69.3 28.5 71.5 16.9 66.7 16.4 13.0 71.7 15.3 9.6 76.3 14.1
t = 85 t = 115 t = 135 t = 155 t = 185 t = 215 t = 245 t = 275 t = 305 t = 365
t = 425
t = 485
3.3.2. Colloid formation and reaction period (II) (0 b t ≤ 25 min) In this period, the polymerization is initiated by the addition of NaOH into the system. The particle size distribution curves measured at different times (t = 5, 20, 25 min) are shown in Fig. 9II-a–II-c. In Table 2 and Fig. 9II-a–II-c, both the average particle size and its distribution width decrease. The peak number of the particle distribution curves is found to be unique observed by DLS. Following the addition of NaOH, the living ions formed in the aqueous phase can migrate into the swollen-micelles and the micelles. Hence the polymerization takes place in the corresponding swollenmicelles and the micelles, turning them into the microlatex particles. The process described above is referred to as the nucleation process. It is insomuch of the continual formation of the new kind of particles (the microlatex particles) and the continual of arising polymerization in the period, therefore it can be called the colloid formation and reaction period. Certainly, the living ions can also migrate into the monomer droplets to initiate polymerization in the monomer droplets. However, comparing with the total number of the swollen-micelles and the micelles, that of the monomer droplets can be ignored. Therefore, for the microemulsion polymerization of D4, the nucleation occurs mostly in the swollen-micelles, while the polymerization takes place in the microlatex particles. As the polymerization continues in this period, the monomer in the monomer droplets will also migrate into the microlatex particles, which results in the decrease of the peak intensity of the monomer droplet size distributions and the increase of the peak intensity of the microlatex particle size distributions. In addition, with the increase of the total amount of the microlatex particles, DBDA in the bulky micelles may also migrate to the microlatex particles, which leads to the disappearance of the bulky micelles. Therefore, the particle size distribution becomes narrow. Comparing the size of the monomer droplets with that of the microlatex particles in the system, we can see that the latter could not be observed by using DLS. Therefore, only one peak is observed in Fig. 9II-a–II-c. From the above discussion, it comes to the conclusion that, during the colloid formation and reaction, four kinds of particles exist in the system. They are micelles, swollen-micelles, monomer droplets and new microlatex particles. A typical particle size distribution is shown in Fig. 10II. 3.3.3. Colloid reaction period (III) (25 b t ≤ 305 min) As soon as the monomer droplets disappear at the end of the last period, the particles in the system begin to react with each other, meaning that the polymerization goes into step (III). In comparison with step (II), there is no formation of new kinds of particles in the period while the polymerization is proceeding. In the period, it can be seen from Fig. 10III that there exist two kinds of particles in the system wherein the particles are shown as the micelle and the microlatex with sizes of 0–10 and 10–50 nm, respectively, which can be observed by using DLS, as shown in Fig. 9IIIa–III-c. As shown in Table 2 and Fig. 9IIIa–III-c, the micelle size has no much change but its number apparently decreases with the polymerization proceeding. The micelle number can be easily calculated based on the multi-mode particle distribution peaks, for instance, when the average micelle particle size is 3.8 nm, the number are 63.8%, 51.5% and 28.5% at a time of 55 min, 11.5 min, and 305 min, respectively. It can also be found that the microlatex has a slightly bigger size and the particle number also increases as the polymerization proceeds. Similarly, the microlatex particle number can also be obtained, for example, the number is 36.2% when the average particle size is 21.8 nm and the time is 55 min (see Table 2). Due to the absence of the monomer droplets as the storage of monomer, the monomers which polymerize in the microlatex particles are supplied by the microlatex particle. Simultaneously, the micelle in the system does not polymerize. Hence, with the polymerization proceeding in the period, the amount of the monomer in the microlatex
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Fig. 9. Particle kinetics of the microemulsion polymerization of D4 (I: t = 0 min in the dispersion period; II-a: t = 5 min in the colloid formation and reaction period; II-b: t = 20 min in the colloid formation and reaction period; II-c: t = 25 min in the colloid formation and reaction period; III-a: t = 55 min in the colloid reaction period; III-b: t = 115 min in the colloid reaction period; III-c: t = 305 min in the colloid reaction period; IV: t = 485 min in the agglomeration period) (recipe for the polymerization is the same as that in Table 1; T = 353 K).
particle decreases. Meanwhile, both the size of the microlatex particles and their numbers increase. However, the size of the micelles keeps unchanged while their number decreases. It must be noted that, in the periods of step (I) and step (II), there exist two peaks which represent the micelles and the microlatex particles, respectively, however, they are ignored due to the large amount of monomer droplets in the system.
micelle size (0–10 nm) and the microlatex particle size (10–50 nm) are much smaller than that of the agglomerate particles, they can still be detected by DLS owing to the absolutely high particle number produced, which can be seen from Table 2 and Fig. 9IV. For example, the agglomerate particle number in the system is 16.4%, but the total number of micelles and microlatex particles is 83.6% at t = 365 min. Fig. 10IV shows a typical particle distribution at this step.
3.3.4. Agglomeration period (IV) (t N 305 min) Although the conversion and the polymerization rate are constant due to lack of the monomer, a few chain-condensation reactions may still proceed and some unstable microlatex particles may agglomerate to form much large particles because of the mechanical agitating or other factors. It is the agglomeration that leads to a third peak as shown in Fig. 9IV, they represent the micelle, the microlatex particle and the microlatex agglomeration particle with the sizes of 0–10, 10– 50 and 250–310 nm, respectively. Table 2 also shows that there is no big number and size percentage changes of these three kinds of particles. But the agglomerate particle sizes increase slightly and the particle number decreases with the polymerization proceeding. The particle number was obtained as 16.4% (average particle size = 276.3 nm, t = 365 min), 15.3% (average particle size = 286.6 nm, t = 425 min) and 14.1% (average particle size = 284.9 nm, t = 485 min). Although the
4. Conclusions Using DBDA as the surfactant and n-pentane as the cosurfactant, the relatively stable PDMS microemulsions were prepared via the polymerization of D4 in aqueous microemulsion. Microemulsion polymerization experiments were carried out at ambient operating conditions. The light transmittance of microemulsion, the polymerization conversion, the particle size and the particle size distribution as the functions of the reaction time were measured to evaluate the polymerization behaviours including the polymerization kinetics and the nucleation location. The detailed conclusions include: (1) The content of the emulsifier in a microemulsion polymerization system is very high. In our experiments, it manifests that the emulsifier (DBDA + n-pentane) concentration should be no
Fig. 10. Physical model of the microemulsion polymerization of D4 (I: the dispersion period; II: the colloid formation and reaction period; III: the colloid reaction period; IV: the agglomeration period) (recipe for the polymerization is the same as that in Table 1; T = 353 K).
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less than 12.5%. In addition, according to the experimentally measured conversion, it is found that there is no constant reaction rate period in the whole microemulsion polymerization process. (2) According to the particle sizes and their distributions in the studied system, the whole process in the microemulsion polymerization of D4 can be divided into four steps, and the physical model for each step was then developed. They are described as follows: (I) the dispersion period, (II) the colloid formation and reaction period, (III) the colloids reaction period and (IV) the agglomeration period. The polymerization rate increases at the step (II) due to the continuous formation of new colloids. In the period (III) the monomer particles diminish till completely disappear, so the monomers in the colloid are consumed for the polymerization, which decreases the polymerization rate. (3) For the microemulsion polymerization of D4, the nucleation occurs mostly in the swollen-micelles, while the polymerization takes place in the microlatex particles. Finally, there we also point out that we cannot associate the kinetics of the microemulsion polymerization of D4 with its physical mechanism entirely in this paper. Further studies on the microemulsion polymerization of D4 are in progress in our group. Acknowledgments
[5]
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[8]
[9] [10]
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[14] [15]
[16] [17]
The authors thank National Natural Science Foundation of China (No. 20406016), Nation Defense Key Laboratory of Ocean Corrosion and Anti-corrosion of China (No. 51449020205QT8703), and Fujian Province Science and Technology Office of China (No. 2005H040) for joint financial support.
[18] [19]
[20]
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