Synthesis of Poly(Methyl Methacrylate) Nanoparticles Via Differential Microemulsion Polymerization ⇑ ⇑ Liang Yuan A, Yun Wang B, Maozhi Pan B, Garry L

European Polymer Journal 49 (2013) 41–48

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European Polymer Journal

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Macromolecular Nanotechnology Synthesis of poly(methyl methacrylate) nanoparticles via differential microemulsion polymerization ⇑ ⇑ Liang Yuan a, Yun Wang b, Maozhi Pan b, Garry L. Rempel a,c, , Qinmin Pan a, a College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu, PR China b Zhejiang Fangyuan Reﬂective Materials Co., Taizhou 318000, Zhejiang, PR China c Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1 article info abstract

Article history: Poly(methyl methacrylate) nanoparticles are synthesized via differential microemulsion Received 12 June 2012 polymerization, where the monomers in the reaction system are kept at very low concen- Received in revised form 28 September 2012 trations by continuous differential addition. The effects of various reaction conditions, such Accepted 11 October 2012 as initiator, surfactants, reaction temperature and monomer loadings, on the particle size Available online 22 October 2012 and molecular weight have been investigated. The effects of initiator and reaction temperature on latex particle numbers and polymer chain numbers within the particles were also Keywords: investigated. It was observed that the dependence of particle size on reaction temperature Particle size and initiator concentration was non-monotonic. The optimized conditions were screened Polymer nanoparticles Poly(methyl methacrylate) to prepare nanoparticles with the smallest size. Differential microemulsion polymerization Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction merization are the most widely used methods. Microemul- sion polymerization was first reported in the early 1980s Polymer nanomaterials have received much attention [21]. This process can produce transparent or translucent due to their special properties arising from their small size. polymer microlattices with particle diameters as small as Owing to the high surface/volume ratio of the small parti- 10 nm, while the particle diameters by conventional emul- cles, the percentage of molecules or atoms on the surface sion polymerization are generally 50–300 nm [22]. How- is greatly increased. These properties are very desirable ever, two major drawbacks have limited the applications for applications in various fields, such as drug delivery [1– of microemulsion polymerization: (1) high surfactant/ MACROMOLECULAR NANOTECHNOLOGY 4], sensors [5,6], molecular gels [7,8], semiconductors [9], monomer weight ratios, usually larger than 1, are required catalysts [10,11], environmental applications [12] and to produce small particles; (2) only a low solid content, usu- industrial applications [13]. A variety of methods have been ally less than 10 wt.%, could be made. Thus, expensive puri- reported to prepare polymer nanoparticles, such as solvent fication procedures would be required to remove surplus evaporation [14], molecular assembly [15], template surfactant and to concentrate the lattices. Miniemulsion chemistry [16], dendrimer polymerization [17], mechanical polymerization is a method that uses water-insoluble pulverization [18], microemulsion polymerization [19] and monomers or water-insoluble reagents to make fine parti- miniemulsion polymerization [20]. In these methods, cles, by which the obtained particle sizes are usually in microemulsion polymerization and miniemulsion poly- the range of 50–500 nm [23], with a wider particle size distribution (PSD) than that obtained by microemulsion polymerization. In addition, for a miniemulsion polymerization ⇑ Corresponding authors at: College of Chemistry, Chemical Engineer- system, a high shear force is usually required to produce ing and Materials Science, Soochow University, Suzhou 215123, Jiangsu, small particles, which is usually not preferred in applica- PR China. tions. Hence, it is imperative to find a more practical meth- E-mail addresses: [email protected] (G.L. Rempel), qpan@suda. edu.cn (Q. Pan). od for nanoparticles preparation. Gan et al. [24] reported

the polymerization of methyl methacrylate in ternary Win- Sinopharm Chemical Reagent Co., Ltd., China), calcium sor I-like systems, in which cetyltrimethylammonium bro- chloride anhydrous (AR grade, P96% purity, Sinopharm mide (CTAB) was used as surfactant. Although relatively Chemical Reagent Co., Ltd., China), cetyltrimethylammo- high polymer/surfactant weight ratios (around 8:1) were nium bromide (CTAB) (AR grade, P99% purity, Sinopharm achieved in their systems, the produced particle sizes were Chemical Reagent Co., Ltd., China) and TritonX-100 (emul- still quite large (30–60 nm), and the particle size distribu- sifier OP) (CP grade, Sinopharm Chemical Reagent Co., Ltd., tion was rather broad. Ming et al. [25] reported a modified China) were used as received. Deionized water was used as microemulsion polymerization process to prepare nanosize obtained from the College of Chemistry, Chemical Engi- PMMA latex with a high solid content, in which the diame- neering and Materials Science of Soochow University, ters were lower than 20 nm and relatively concentrated Suzhou, China. (10–30 wt.%) latexes were attained; however, the weight ratio of surfactant/polymer (1/10) was still relatively high. Chen and Zhang [26] used a c-ray-induced method for the 2.2. Preparation of PMMA nanoparticles polymerization of methyl methacrylate in a microemulsion system, which is stabilized by a mixture of sodium 12- PMMA nanoparticles were prepared by differential acryloxy-9-octadecenoic acid (AOA) and sodium dodecyl microemulsion polymerization, which means only a differ- sulfate (SDS) at room temperature. The transparent micro- ential amount of the monomer existed in the reaction sys- lattices produced had higher monomer contents (up to tem during the reaction period. The operational procedures 38.6 wt.%); however they still required a relatively high for the reaction were similar to those presented elsewhere amount of surfactant (5.5 wt.%), and the particle size of [28]. Simply, the initiator and surfactant were mixed in a 3 the lattices was larger than 30 nm. Chen et al. [27] reported certain amount of water in a 250 cm four-necked Pyrex a new monomer feeding system for microemulsion poly- glass reactor (SAMDUK, China), equipped with a magnetic merization and PMMA nanoparticles were produced with stirrer, a reflux condenser, a thermometer and a dropping a number-average diameter of 18.4 nm and the surfac- funnel. When the temperature in the system reached a tant/monomer weight ratio was at about 0.15, which was designated level, MMA was continuously added as small 1 high as well. To explore a practical technical route in which droplets (at a given rate of 0.15 ± 0.02 mL min via a drop- the surfactant amount required could be significantly de- ping funnel) for a specified period of time to achieve the creased and the nanoparticle size could be controlled, a dif- differential level of the monomer content in the reaction ferential microemulsion polymerization was proposed [28], system. After completion of the MMA addition, the reac- in which, the monomer was continuously added at such a tion system was maintained at the reaction temperature low speed that the monomer content in the reaction system for a certain aging time before cooling. is at a differential level, but not a starved level. With this strategy, it was confirmed that the reaction conditions 2.3. Separation of polymer samples for characterization could be milder than those required in regular emulsion polymerization processes [28–32], and the particle size The resultant polymer was precipitated using a satu- realized was similar to or even smaller than that produced rated calcium chloride solution and was separated by a by microemulsion polymerization. vacuum filtration technique. The surfactant, calcium chlo- Although a few papers [28–30,32] have reported the ride and initiator were washed off the particles three times investigations on the synthesis of nanosized PMMA via dif- with a sufficient amount of warm de-ionized water. ferential microemulsion polymerization, the emphasis of the previous studies was mainly focused on the develop- ment of the methodology and the minimization of the 2.4. Characterization of the synthesized products

MACROMOLECULAR NANOTECHNOLOGY surfactant amount needed. The objective of this study is to further simplify the recipes by eliminating the use of a The compositions of the resultant polymers were ana- co-surfactant and to investigate the effects of various lyzed by Fourier transform infrared spectroscopy (Thermo operational parameters, such as initiators, surfactants, Nicolet iS10, USA). monomer amount, and reaction temperature, involved in The Z-average particle size and the distribution of the the differential microemulsion polymerization, on the particle size were measured by a laser particle size ana- performance of the polymerization reaction, and to reveal lyzer (Malvern Zetasizer Nano-ZS, UK).

the optimal conditions for synthesizing the smallest The number-average-molecular weight (Mn) and nanoparticles. weight-average molecular weight (Mw) as well as the molecular weight polydispersity index (PDI) were determined using a Waters 1515 gel permeation chro- 2. Experimental matograph (GPC, Waters, USA) equipped with a refractive index detector (Waters 2414), using HR1, HR2, and HR4 2.1. Materials (7.8 300 mm2, 5 mm beads size) columns which allows the measurement of molecular weights over the range of Methyl methacrylate (MMA) (CP grade, Sinopharm 102–5 105 g mol1. THF was used as the eluent at 30 °C Chemical Reagent Co., Ltd., China), sodium dodecyl sulfate with a ﬂow rate of 1.0 mL min1. The GPC samples were in- (SDS) powder (purity P 86%, Sinopharm Chemical Reagent jected using a Waters 717 plus autosampler and calibrated Co., Ltd., China), ammonium persulfate (APS) (AR grade, with PMMA standards from Waters. L. Yuan et al. / European Polymer Journal 49 (2013) 41–48 43

2.5. Calculation of latex particle number, polymer chain number per particle, and conversion

The total number of latex particles in the system (NP) and the number of polymer chains per particle (N) as well as the conversion (Xm) are calculated according to the following equations:

6q0VXm NP ¼ ð1Þ qpD3

4 qpðD=2Þ3N N ¼ A ð2Þ 3 Mn

W1 Xm ð%Þ¼ 100 ð3Þ Fig. 2. Effect of monomer amount on the particle size of poly(methyl W2 methacrylate). 3 where q0 is the density of MMA (0.94 g cm at 25 °C), V is the total volume of MMA, X is polymerization conversion, m to 3750 cm1). Thus, it was conﬁrmed that our product is q is the density of PMMA (1.17 g cm3 at 25 °C), D is the 23 1 the PMMA. diameter of the particle, NA is 6.02 10 mol , Mn is the number-average molecular weight, and W1 and W2 are the weights of the polymer and MMA. 3.2. Effect of monomer amount on PMMA particle size

The MMA amount in the reaction system affects the 3. Results and discussion particle size of the resultant polymer. The experimental results are shown in Fig. 2, which indicate that an increase in 3.1. Conﬁrmation of the product composition the MMA consumption results in an increase in the particle size. When the monomer amount is 10 mL, the particle size The FTIR spectrum of PMMA nanoparticles is shown in is at about 14.5 nm. With increasing monomer amount, the A Fig. 1. The stretching vibration frequency of ( CH3) particle size also increases. Moreover, the growth rate of 1 1 appears at wave numbers of 2946 cm and 2846 cm , particle size becomes faster with a further monomer load- A and the bending vibration frequency of ( CH3) is at a wave ing increase. For example, when 30 mL of MMA is added, 1 1 number of 1384 cm . The wave number 1731 cm corre- the particle size increases to 28.5 nm, and the particle size sponds to the stretching vibration frequency of (AC@O). is about twice of the size when 10 mL of MMA is used. The 1 The wave number 1147 cm represents the stretching particle size distribution also becomes large with increas- vibration frequency of (ACAO). Compared with the spec- ing the monomer amount. To validate the reliability of trum of standard PMMA, a match of 94.85% was obtained; the experiments, all the experiments were repeated three the only difference being that our samples contain water, times and the results are also shown in Fig. 2, which indi- which is shown in the marked area in Fig. 1 (from 3200 cate that good reproducibility of the experiments was achieved.

3.3. Effect of initiator amount on PMMA particle size MACROMOLECULAR NANOTECHNOLOGY

The effects of the amount of APS on particle size were investigated and the results are shown in Fig. 3, which indicates that as the initiator concentration increases, the particle size in the resulting latex decreases at first and then increases gradually. Such a phenomenon was first dis- covered by Gan [33] for modified microemulsion polymerization of methyl acrylate and Gan explained that this phenomenon was caused by the co-existence of two processes: (1) oligomeric radicals could transfer between the aqueous phase and the polymer particles and (2) oligomeric radicals could continue to undergo the nucleation process and form new particles, and these two processes could dominate the reaction in the different stages. The authors thought that this phenomenon could be understood in a simpler manner. At a low initiator concentration, the active sites which could grow into particles would be Fig. 1. FTIR spectrum of PMMA nanoparticles. less. Then with the same monomer amount, the particle 44 L. Yuan et al. / European Polymer Journal 49 (2013) 41–48

surfactant: when SDS concentration reached the critical micelle concentration (CMC), the more the surfactant amount, the more the micelles generated. Consequently, more latex particles were produced. Therefore, if the monomer amount is not changed, the more surfactant used, the smaller the particle size obtained.

3.5. Effects of surfactant type on particle size

The surfactant is one of the key components in microemulsion polymerization; the effects of three types of surfactant, SDS (anionic), CTAB (cationic) and TritonX-100 (non-ionic), on the particle size of the resultant polymers were investigated in this work. The experimental data indi- cated that using the three types of surfactant, the particle Fig. 3. Effect of initiator amount on the particle size and PSD of poly size of ﬁnal product is different: for SDS it is 15.1 nm, for (methyl methacrylate). CTAB it is 42 nm and for TritonX-100 it is 413 nm with the conditions of APS/water = 0.83 g L1, MMA/water = 14/84 (volume ratio), surfactant/water = 9.5 g L1, and size would be larger; with the increase of initiator content, 80 °C, which indicate SDS is the best in terms of reducing more active sites could be formed. Therefore, the monomer the particle size of PMMA. This is possibly because: (1) amount that each active site could share becomes less and the SDS molecular weight is much smaller than the other small particles would result; when initiator concentration two surfactants, and when adding the same surfactant is high enough, the number of free radicals one particle amount, SDS may generate more micelles than other could share would increase and then some dead polymer two; (2) the cationic surfactants contain amine compounds particles could be initiated again resulting in a larger par- which have an inhibitory effect to monomers; also they ticle size. Thus, there is possibly an optimal concentration can be easily oxidized by the peroxide initiator, and the an- for producing the smallest particles, and in our experi- ionic surfactants, which can bring electrostatic charge onto ments {APS/water = 0.83 g L1, MMA/water = 14/84 (vol- the latex particle surface, can prevent ion aggregation and ume ratio), SDS/water = 4.8 g L1 and 9.5 g L1, and result in good mechanical stability of the emulsion. 80 °C}, the optimal concentration of initiator is 0.83 g L1. Fig. 3 also shows that with an increase in the amount of 3.6. Effects of surfactant concentration on particle size initiator, the particle size gradually tends to a narrower distribution. The effect of the APS amount on the charac- The dependence of the particle size on the SDS concen- teristics of PMMA emulsions also could be observed as tration is illustrated in Fig. 6. The data obtained by He and shown in Fig. 4, which indicates that with a decrease in Pan [29], Norakankorn et al. [30] and Wang et al. [32] are the particle size the emulsion becomes more transparent, also shown in Fig. 5, for the purpose of comparison. The as observed earlier in a study of Wang et al. [32]. system investigated by He and Pan [29] is quite similar to the system investigated in this work, except that an 3.4. Effect of monomer/surfactant ratio on PMMA particle size additional co-surfactant was used in their work whereas no co-surfactant was used in the present work. The exper- From Fig. 3, the effect of monomer/surfactant ratio on imental data indicate that the particle size in both cases

MACROMOLECULAR NANOTECHNOLOGY the particle size also could be observed; the weight ratio decreases with an increase in the surfactant concentration. of monomer/surfactant has a great effect on the particle The particle size shown in curve 5 decreased rapidly with size of PMMA in this system. The weight ratio of mono- increasing SDS concentration, and did not significantly mer/surfactant of curve 1 is 32.9:1, and the weight ratio change when it reached a level of 10.3 g L1 SDS concentra- of monomer/surfactant of curve 2 is 16.45:1, which is half tion. The particle size shown in curve 4 also decreased rap- of that of curve 1. The particle size of curve 1 is signifi- idly with increasing SDS concentration, but it did not cantly higher than the particle size of curve 2. This significantly change when it reached a level of 9.5 g L1 phenomenon can be explained by the property of the SDS concentration. The turning point in the current work

Fig. 4. Effect of APS amount on character of the PMMA microemulsion. L. Yuan et al. / European Polymer Journal 49 (2013) 41–48 45

Fig. 5. Effect of SDS amount and monomer/water ratio on the particle size Fig. 6. Effect of mixed surfactants on the particle size of poly(methyl of poly(methyl methacrylate). methacrylate). happens much earlier than that in the system investigated by He and Pan [29]. These findings imply that if the particle size of PMMA in the systems investigated by Norakankorn size located at the turning point is acceptable, the system et al. [30] and Wang et al. [32], in which hydrophobic investigated in the present work not only would be much initiators were used, whereas, there is a certain effect on more economical than the system investigated by He and the particle size in the present work, in which a water sol- Pan [29], but also provides particles of a smaller size. uble initiator was used. The experimental data in Fig. 6 also The data obtained by Norakankorn et al. [30] and Wang indicate that the particle size at a lower monomer/water et al. [32] in Fig. 5 are also similar to the system studied in ratio is smaller than that at a higher monomer/water ratio the present work, except oil-soluble initiator AIBN and BPO used in the present work. This phenomenon may be ex- were used in their work, while a water-soluble initiator plained by the particle nucleation mechanism. For a hydro- (APS) was used in the present work. The experimental data phobic initiator, the nucleation happens mainly in micelles indicate that when using the differential microemulsion for which the amount of the water in the system may not polymerization method, the particle size shown in all of significantly affect the nucleation. However, for our system the seven curves decreases with an increase of surfactant in which a water soluble initiator was used, the nucleation content. However, on comparison of the particle size of mainly occurred in the water phase. More water would curve 1 [30] and curve 7 [32] or curve 2 [30] and curve 6 provide a higher probability of homogeneous nucleation [32], the particle size that was initiated by AIBN and BPO in the water phase and then each particle would share less shows a decrease with increasing SDS concentration, and monomer amount when the total monomer amount is did not significantly change when it reached a level of fixed; thus a smaller particle size would result at a higher 2.67 g L1 SDS concentration. However, the particle size monomer/water ratio. obtained by Wang et al. [32] (curves 6 and 7) is significantly smaller than that obtained by Norakankorn et al. 3.8. Effects of mixed surfactant concentration on particle size [30] (curves 1 and 2). This implies that the more hydropho- and its distribution bic initiator BPO is better than the less hydrophobic initiator AIBN in reducing the particle size in the differential

The dependence of the particle size and its distribution MACROMOLECULAR NANOTECHNOLOGY microemulsion polymerization system. The particle size on the mixed surfactant (TritonX-100 + SDS) is illustrated in the present work (for the case when 14 mL MMA and in Fig. 6. The experimental data indicate that the particle 84 mL water were used) is smaller than the results ob- size decreases from 21 nm to 18 nm as the molar ratio of tained by Norakankorn et al. [30], while it is larger than TritonX-100/SDS increases from 0.065 to 0.15, and then that obtained by Wang et al. [32]. Compared with curve the particle size increases from 18 nm to 37.5 nm as the 5, curve 4 of the current work did not use any co-surfactant molar ratio of TritonX-100/SDS increases from 0.15 to while all other conditions were the same. The results dem- 3.14. This phenomenon can be explained by the properties onstrated by curve 5 were obtained by He and Pan [29],in of the two surfactants. The effect of the anionic surfactant which 1-pentanol was used as a co-surfactant. The use of on the colloid dispersion stabilization is achieved through the co-surfactant does not show any beneﬁt, which means the role of the repulsive force between two similarly that the co-surfactant can be eliminated in this reaction charged electric double layers to the latex particles, non- system to simplify the recipe. ionic surfactants are adsorbed on the latex particle surface which increases the steric hindrance effect [34] and then 3.7. Effects of monomer/water ratio on particle size increases the stability of the latex particles, and when the molar ratio of TritonX-100/SDS is 0.15, the non-ionic From Fig. 5, the effect of monomer/water ratio on the and anionic surfactants produced a synergistic effect that particle size could also be observed. It indicates that the results in the decrease of the particle size. Because the monomer/water ratio does not greatly affect the particle emulsifying effect of the non-ionic surfactant is less than 46 L. Yuan et al. / European Polymer Journal 49 (2013) 41–48

the anionic surfactant, in the range of the molar ratio of temperature is beneﬁcial in generating particles with a TritonX-100/SDS (0.15–3.14), with increasing the amount narrow size distribution. The effect of the reaction temper- of the non-ionic surfactant, the particle size increases, ature on the characteristics of the PMMA emulsion is and Fig. 6 also indicates that with increasing the non-ionic shown in Fig. 8, which also indicates that with an increase surfactant amount, the particle size distribution gradually in the reaction temperature, the number of latex particles narrows. showed a trend of decreasing at ﬁrst and then increasing.

3.10. Effect of APS amount on molecular weight, conversion, 3.9. Effect of temperature on PMMA particle size NP and N The experimental results for the effect of the reaction The dependences of the particle size, molecular weight temperature on the particle size are shown in Fig. 7. The (M and M ) and M =M on the initiator amount over the experimental data indicate that the particle size decreases w n w n range of 0.01–0.13 g are presented in Table 1. The experi- with an increase in the reaction temperature from 70 to mental data indicate that at a low initiator concentration, 80 °C; at the condition when 0.8 g SDS is used, the particle both the particle size and molecular weight are relatively size decreases from 20.6 to 15.8 nm; and at 1.6 g SDS, the larger; with an increase in the initiator amount, the M particle size decreases from 19.8 to 12.8 nm. This result w decreases from 6.97 105 to 0.89 105, and M decreases is similar with that reported by Liu and Pan [31]. However, n from 2.57 105 to 0.47 105, however, the trend in the if the reaction temperature is further increased beyond particle size is different. When a low amount of the initia- 80 °C, the particle size shows an increase, and when the tor was used, with an increase in the amount of initiator reaction temperature reaches 95 °C, the particle size is as used, the particle size decreased and the molecular weight large as 27.2 nm for the case that 0.8 g of SDS is used, decreased as well, suggesting that the size of the reaction and the particle size is 22.5 nm when 1.6 g of SDS is used. sites could affect the molecular weights; however, when This phenomenon can be explained by the effect of the amount of the initiator used is higher (such as higher temperature on the initiator-decomposition, chain-propa- than 0.07 g), the particle size increased as explained in gation, chain-termination reactions and properties of the the previous sections, while the molecular weight did not latex particles. Over the low temperature range, with an show a signiﬁcant change. The reason is probably that increase in the reaction temperature, the decomposition when the particle size is large enough, the molecular rate of the initiator is accelerated, and more free radicals weight would not be affected by the size of the reaction are generated, which promote the formation of more sites but controlled by the reaction kinetics. particles; in turn, the average particle size is decreased. The dependences of the conversion, N and N on initia- However, with a further increase in the number of free rad- P tor amount are also illustrated in Table 1. The experimental icals caused by the increase in temperature, the phenome- data indicate that with an increase in the initiator amount non observed is similar to that obtained when the effect of from 0.01 to 0.13 g, the polymerization conversion the initiator amount was investigated. More free radicals increases from 93.4% to 95.2%. The total number of latex could increase the probability of the dead polymer parti- particles in the system (N ) increases signiﬁcantly with cles being re-initiated and then larger particles would be P an increase in the initiator amount from 0.01 to 0.07 g, produced. and then N decreases to a certain extent with an increase The dependence of the particle size distribution on the P in the initiator amount of 0.07–0.13 g. The phenomenon reaction temperature is also illustrated in Fig. 7. The observed when the initiator amount used is less than experimental data indicate that the particle size decreases 0.07, can be explained by the effect of initiator amount with an increase in the reaction temperature from 70 to on the particle formation: when the initiator concentration 95 °C; this phenomenon indicates that a high reaction

MACROMOLECULAR NANOTECHNOLOGY is low, increasing the initiator concentration results in more nucleation in the water phase and then more particles could be formed. Although it is still not very clear why with the further increase in the initiator amount, the particle numbers would decrease, it may be associated with a synergistic effect of a faster consumption rate of the monomer, a higher radical termination rate, and a higher probability of the surplus free radicals entering the growing particles or dead particles. The number of polymer chains per particle (N) is also affected by the initiator amount used. The average number of polymer chains per particle increases from 29 to 56 as the initiator amount is increased from 0.01 to 0.13 g. When the initiator amount is increased, it leads to an increase in the free-radical number in the aqueous phase, and the probability of free radical diffusion into the latex particles is increased. Therefore, it is understandable that with an Fig. 7. Effect of temperature and monomer/surfactant on the particle size increase in the initiator amount, the number of polymer of poly(methyl methacrylate). chains per particle increases. However, over the range of L. Yuan et al. / European Polymer Journal 49 (2013) 41–48 47

Fig. 8. Effect of temperature on characters of PMMA microemulsion.

Table 1

Effect of APS amount on particle size, molecular weight, conversion, NP and N.

5 5 18 Run APS (g) Particle size (nm) Mw 10 Mn 10 Mw=Mn NP 10 NXm (%) 1 0.01 27.3 6.97 2.57 2.71 0.98 29 93.4 2 0.03 16.0 2.47 1.18 2.10 4.89 13 93.9 3 0.05 14.9 1.43 0.66 2.17 6.13 18 93.9 4 0.07 13.9 1.26 0.54 2.33 7.54 18 94.5 5 0.09 16.7 1.15 0.41 2.80 4.35 42 95.1 6 0.11 18.4 0.95 0.37 2.57 3.28 62 95.0 7 0.13 19.2 0.89 0.47 1.90 2.87 56 95.2

Table 2

Effect of reaction temperature on particle size, molecular weight, conversion, NP and N.

5 5 18 Run Temperature (°C) Particle size (nm) Mw 10 Mn 10 Mw=Mn NP 10 NXm (%) 8 70 20.6 2.99 0.88 3.40 2.3 37 93.9 9 75 16.2 2.06 1.02 2.02 4.8 15 94.6 10 80 15.9 1.34 0.71 1.89 5.1 21 95.2 11 85 16.5 1.01 0.54 1.87 4.5 31 94.5 12 90 20.7 1.37 0.60 2.28 2.2 54 92.9 13 95 27.2 1.58 0.64 2.47 1.0 116 93.6

0.03–0.07 g of the initiator used, the number of polymer indicate that the polymerization conversion increases with chains in each particle is relatively small, which is possibly an increase in the reaction temperature from 70 to 95 °C, a result of the small particle size produced. however, an over high reaction temperature does not provide any improvement in the conversion. On one hand, 3.11. Effect of reaction temperature on molecular weight, a high reaction temperature may shorten the living time of the radicals; an over high reaction temperature may cause conversion, NP and N volatilization of a small amount of monomer. Therefore, an over high reaction temperature may negatively affect the

The dependences of the particle size, molecular weight MACROMOLECULAR NANOTECHNOLOGY yield. The dependence of N and N on the temperature is (Mw and Mn) and Mw=Mn on the reaction temperature (70– P 95 °C) are presented in Table 2. It is seen from Table 2 that similar to the dependence on the initiator amount used 5 5 as shown in Table 1. the Mw decreases from 2.99 10 to 1.01 10 with an increase in the reaction temperature from 70 to 85 °C which can be understood from classical free radical polymeriza- 5 5 tion theory. Mw increases from 1.01 10 to 1.58 10 4. Conclusions with an increase in the reaction temperature from 85 to 95 °C, which is possibly a result of some synergistic effects Using differential microemulsion polymerization, caused by the larger particle size under this condition, and PMMA particles can be controllably synthesized with then the radicals diffuse much slower into the particles and respect to their nanosize and molecular weight. This meth- radical termination is retarded. Similar phenomenon has od is suitable for obtaining poly(methyl methacrylate) also been observed for Mn and Mw=Mn. With a rise in the with a particle size of less than 20 nm. It was found that reaction temperature, the molecular weight gradually the reaction temperature and initiator amount have a great tends to a narrow size distribution, while if the reaction effect on the particle size, molecular weight, the total num- temperature further rises, the molecular weight tends to ber of latex particles in the system (NP) and the number of a broad distribution. polymer chains per particle (N). There is an optimal

The dependence of the conversion, NP and N on initiator concentration of the initiator and temperature in terms amount is also shown in Table 2. The experimental data of synthesizing the smallest nanoparticles of PMMA. 48 L. Yuan et al. / European Polymer Journal 49 (2013) 41–48

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