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Ultrasound-assisted emulsion polymerization of poly(methyl methacrylate-co-butyl acrylate): effect of initiator content and temperature.

INTRODUCTION

Emulsion polymerization is a scientifically, technologically, and commercially important polymerization method. High molecular weight polymers can be produced at fast polymerization rates. The continuous water phase is an excellent conductor of heat, enabling fast polymerization rates without loss of temperature control, moreover the viscosity of the reaction medium remains close to that of water during the polymerization depending on polymer content of final latex. Today, emulsion polymerization is a considerable part of a massive global industry [1-3]. Polymers such as acrylonitrile butadiene styrene (ABS), polystyrene (PS), poly-methyl methacrylate (PMMA), etc. can be easily prepared via emulsion polymerization processes. The conventional emulsion polymerization besides all of its benefits has some disadvantages, such as high polydispersity of produced particles, instability of colloidal particles, application of initiator in high levels which may affect the purity and properties of the final products [2, 4-6].

Recently, ultrasound-assisted emulsion polymerization was introduced as an alternate solution to more enhanced conventional emulsion polymerization without fundamental changes in the process [7-9]. Ultrasound-assisted polymerization refers to the application of ultrasound (US) waves with frequencies between 20 kHz and 1 MHz in polymer synthesis [9]. According to cavitation's theory formation, growth and implosive collapse of sonication bulbs in liquid media generate extremely high local temperature (up to 5000 K) and pressure (up to 1000 atm) which could affect the polymerization reactions significantly [9, 10]. In recent years, application of ultrasound waves in several polymerization processes has been reported [8, 11-13]. In some cases acceptable improvements have been achieved, while some disadvantages such as more complicated polymerization mechanisms and production of unwanted byproducts have also been reported [9, 14, 15].

Ultrasound-assisted emulsion polymerization, due to the nature of the method, has several advantages over the conventional emulsion polymerization such as enhanced polymerization rate, narrow particles size distribution, higher monomer conversion, and so on; however the precise control over the condition depends on many experimental parameters [16-20].

In the present work, low power ultrasound pulses (20-40 watts) were applied on conventional emulsion polymerization and the effect of initiator concentration and polymerization temperature on monomer conversion, glass transition temperature, molecular weight, particle size, and particle size distribution were investigated. The aim of this study mainly concentrated on industrial improvements in conventional emulsion polymerization without fundamental changes in process.

EXPERIMENTAL

Material and Equipments

Methyl methacrylate (MMA) and butyl acrylate (BA) monomers were purchased from Aldrich and used after the removal of inhibitors. The inhibitor was removed by washing the monomer with 5% aqueous NaOH and water, followed by drying over [Na.sub.2]S[O.sub.4]. Monomers were then distilled under reduced pressure. Tetrahydrofuran (THF) was supplied by Lancaster Co. Potassium persulfate (KPS) and sodium bicarbonate (NaHC[O.sub.3]) were purchased from Merck Chemical Co. Sodium dodecyl sulfate (SDS), acrylic acid (AA), and Triton X-100 were supplied by Aldrich.

Characterization

The ultrasound generator instrument was a Sonopuls HD 3000 (Germany) that was equipped with a standard titanium probe (22 mm diameter). Thermogravimetric properties were studied by General TA2100 instrument thermal analyzer (USA) with a heating rate of 10[degrees]C/min from initial temperature to 700[degrees]C under dynamic Argon flow (50 [cm.sup.3]/min) according to the manufacturer's instruction. Particle size was measured by a SEMATECH light scattering (France) at 633 nm wave length. The analysis was carried out at 25[degrees]C with scattering angle of 90[degrees]. Philips EM 208S transmittance electron microscopy (TEM) with 120 kV accelerating voltage (The Netherland) was applied to investigate the morphology and particle size too, samples were freeze dried on carbon grids at -30[degrees]C. Dynamic mechanical thermal analysis (DMTA) was taken by DMA 8000 Perkin Elmer instrument (USA). Gel permeation chromatography (GPC) was done by GPC Agilent 1100 (CA, USA), equipped with refractometer index detector (RID) to determine average molecular weights of the obtained polymers due to ASTM D6579. [P.sub.L] Gel Lopem ([10.sup.3] [Angstrom], [10.sup.4] [Angstrom], [10.sup.5] [Angstrom] Agilent) columns were applied in series at 30[degrees]C and average mobile phase flow rate of 1 mL [min.sup.-1]. They were calibrated and analytical grade tetrahydrofuran (THF) was used as solvent in all experiments. A Titraplus potentiometer with a specific electrode for lead and Calomel electrode as reference (Tacussel, model XS 300) was used for the potentiometric analysis. H-NMR spectra were taken by 500 MHz Bruker (Germany) instrument, the copolymers were freeze-dried and the organic component dissolved in CDC13 prior to NMR analysis.

Methodology

Decomposition of KPS. Two 500-mL glass reactors were loaded with 5.49 mM KPS aqueous solution. To remove dissolved oxygen, the systems were purged with argon for 30 mm. The KPS decompositions were conducted in an inert atmosphere at 75[degrees]C, with and without sonication. To maintain the pH during the reactions at constant values ([+ or -]5%) small amounts of buffer were added to reactors. Samples of the reaction mixture were taken using a sampler pipette (from middle of the reactor) at different reaction times. The samples were cooled in an ice bath and quickly analyzed.

The concentration of the sulfate ions was determined with a standard solution of lead(II)perchlorate trihydrate. First determined amounts of Pb[(CI[O.sub.4]).sub.2]. 3[H.sub.2]O was dissolved in 100 mL of distilled water, then the obtained solution (titrant) was placed in a burette. Samples of KPS solution (5 mL) were taken every 30 min and titrated by titrant in a potentiometer under constant mechanical mixing (300 rpm).

Polymerization

All polymerizations were performed in a 500-mL reactor constructed of stainless steel. Reactor was equipped with sonication probe, mechanical stirrer, digital thermometer, stopper, dropping funnel, and argon gas inlet. The temperature was controlled [+ or -]2 by a heating band and a conventional heat exchanger coil. A water-cooled reflux condenser prevents loss of volatile components (monomers). Figure 1 is a schematic representation of polymerization set up.

The conventional emulsion samples were prepared according to the instruction given in Table 1. The ingredients ratio and polymerization procedure were optimized according to our previous studies [21]. The initial charge of reactor containing initiator, surfactants, and buffer was dissolved in water and the obtained solution was added into the reactor, to remove dissolved oxygen, it was mechanically stirred at 400 rpm under argon purge. After 10 min when the reactor had reached the desired temperature, the rest of the water, initiator, and monomers were added to the reactor.

In the case of ultrasound-assisted emulsion polymerization, the agitation rate is turned down low enough (200 rpm) to prevent vortex formation. The horn is inserted into the reactor and sonication is started.

The optimum condition for sonication parameters was obtained experimentally 20-40% power and 0.3-0.5 second of cycle. The total time of each polymerization (conventional and ultrasound-assisted) was 120 min. Theoretical [T.sub.g] of final copolymer (methyl methacrylate to butyl acrylate ratio) was calculated from Fox equation around 30[degrees]C [22]. During the polymerization, a small amount of sample was drawn every 15 min to determine monomers conversion by gravimetric analysis. The conventional emulsion polymerization found a wide variety of industrial applications. Most of these polymerizations take place in elevated temperatures; high initiator concentration and efficient mechanical agitation; indeed, polymerization temperature and initiator concentration are the most important and determining industrial factors which affect the efficiency of methods as well as the costs of the final product. The effect of initiator concentration on monomer conversion, [T.sub.g] of final polymer, molecular weight, particle size, and particle size distribution were investigated as shown in the procedure given in Table 1.

The effect of polymerization temperature on conventional and ultrasound-assisted emulsion polymerization was investigated as in the following procedure (Table 2).

RESULTS AND DISCUSSION

The conventional emulsion polymerization besides all its benefits also has some disadvantages such as high polymerization temperature, application of high amounts of the initiator and its separation problems, high polydispersity of obtained polymeric particles. From here, the application of ultrasound could be an alternative solution to some of these problems without fundamental changes in the process. In this section, some of the results obtained from this study were presented. The effect of ultrasound pulses on KPS decomposition rate was studied by potentiometry method and then the polymerization under ultrasound irradiation was investigated. It is proved that, ultrasound pulses facilitate the polymerization process by two main mechanisms. First, by reducing the particle size, the total surface area of the phase boundary increases. The second reason refers to the direct energy transfer to the dissolved chemical reagents. Short-lived high temperature and pressure cavitations contribute to molecular decomposition and increase the reactivity of many chemical species.

Effect of Ultrasound Pulses on KPS Degradation

Potassium persulfate is one of the most common initiators of emulsion polymerization systems. For modeling, control, and product improvement, it is useful to know its decomposition rate coefficient and how it is influenced by ultrasound pulses.

Potassium persulfate decomposition rate was determined by titration based on reactions 1 and 2. Figure 2 illustrates a typical titration curve of potentiometric analysis of the sulfate ions obtained on the thermal KPS decomposition after 30 min of decomposition reaction.

Reaction 1. Sulfate ions formation by the thermal decomposition of persulfate

[S.sub.2][O.sub.8.sup.2-] + 2[H.sub.2]O [right arrow] 1/2 [O.sub.2] + 2[H.sup.+] + + 2S[O.sub.4.sup.2-]

Reaction 2: Analysis of the sulfate ions by precipitation with the Pb[(CI[O.sub.4]).sub.2]

S[O.sub.4.sup.2-] + Pb[(Cl[O.sub.4]).sub.2] [right arrow] PbS[O.sub.4] [down arrow] +2Cl[O.sub.4.sup.2-]

Results for thermal decomposition and ultrasound-assisted decomposition of KPS are summarized in Fig. 3. The comparison of these two series indicated that the decomposition rate of KPS was significantly affected by ultrasound pulses. The two main reasons were acceptable; first, according to cavitation theory the ultrasound irradiation provides high local temperature and pressures which facilitate the KPS decomposition. The second reason refers to the confining effect of solvents molecules (cage effect). The cage effect causes secondary wastage reactions including recombination of radicals to regenerate the initiator. On the other hand, this effect decreases the decomposition rate of initiator. The cage effect mainly depends on solvent nature as well as its viscosity and agitation rate of solution. By application of ultrasound irradiation in polymerization media, the agitation rate deeply increases which leads to decreased cage effect [23, 24].

Effect of Initiator Content on Monomers Conversion

The effect of initiator concentration on monomers conversion was studied by several teams, it was found that by increasing the initiator concentration, conversion increases and final average polymer particle size decreases which leads to a broader final particle size distribution (PSD). Moreover, the initiator residual to some extent affects the final products purity and properties [25]. So increasing monomer conversion without further increase in initiator concentration could be valuable for industrial applications. Monomer conversion in various initiator concentrations for conventional and ultrasound-assisted emulsion polymerization is shown in Figs. 4 and 5, respectively. From the plots it can be seen that by decreasing initiator concentration, monomer conversion in both systems decreases.

As seen in Fig. 5, in the ultrasound-assisted emulsion polymerization monomer conversion is much higher than in conventional system. These results also revealed that most of the ultrasound-assisted emulsion polymerization (dissociation of initiator to free radicals) occurs in the first 15 min of reaction. This means that the main initiator that was added to the reactor was consumed in the first 15 min and then the generation of radicals takes place due to the extreme environment created by acoustic cavitation.

The comparison of these results also illustrated that for USEM series at first 15 min of the reaction, the polymerization rate ([R.sub.p]) is 1.91 time faster than CEM series ([R.sub.p]USEM series/[R.sub.p]CEM series=1.91). However, after 30 min polymerization rate ratio decreased to 1.41 and after 150 min it is almost 1. On the other hand, when the polymerization rate is our priority ultrasound-assisted emulsion polymerization is an alternative solution.

Effect of Polymerization Temperature on Monomer Conversion

The monomer conversion profiles at 75, 60, 45, and 30[degrees]C for various conventional emulsion polymerization and ultrasound-assisted polymerization are shown in Figs. 6 and 7, respectively. From the plots it can be seen that the reaction temperature in conventional emulsion polymerization affects the monomer conversion much more than initiator content. While in ultrasound-assisted emulsion polymerization monomer conversion is influenced mainly by initiator concentration and reaction time. It can be explained by different radical formation mechanisms. In a conventional emulsion polymerization, radicals are generated from chemical initiators by heat decomposition, while in ultrasound-assisted emulsion polymerization free radicals could be generated by various routes such as initiator dissociation or water molecules scission by ultrasound waves, direct exiting of monomer or polymer chains, and so on.

Particle Size Investigation

Particle size and particle size distribution were measured by dynamic light scattering (DLS) (Fig. 8), results also were confirmed by TEM. Figures 9 and 10 represent typical TEM images of [CEMI.sub.1] and [USEMI.sub.1] samples which are in good agreement with DLS results.

Effect of Initiator concentration on particle size in both polymerization systems is summarized in Table 3.

It was found that in both polymerization systems, increasing initiator concentration led to slight decreases in particle size, while for ultrasound-assisted emulsion polymerization it was more than conventional polymerization. It seems that ultrasound pulses in the first minutes of polymerization increase instant free radical to monomer ratio as well as mixing efficiency, which results in smaller and more uniform particles. Effect of reaction temperature on particle size was also investigated, results indicated that by increasing reaction temperature, particle size slightly decreased in both polymerization systems.

Glass Transition Temperature

Typical dynamic mechanical analysis for the copolymers obtained by conventional and ultrasound-assisted emulsion polymerization is shown in Fig. 11. Tan ([delta]) refers to the loss factor and is the ratio of viscous to elastic response and could be used as an indication of glass transition temperature ([T.sub.g]) and the level of structural homogeneity of the sample.

USEM series with symmetrical and unimodal curves characterizes very homogeneous chains, whereas CEMI series show asymmetrical and semi-bimodal curves which describe a material with uneven distribution of monomers in chains. The effect of initiator concentration and the polymerization temperature on [T.sub.g] is summarized in Fig. 12.

As shown in Fig. 12a, [T.sub.g] for ultrasound-assisted emulsion polymerization shows good agreement with theoretical predictions (30[degrees]C calculated from Fox equation) while in conventional polymerization experimental [T.sub.g] is on average 10[degrees]C higher than Fox equation. The first reason for this pattern refers to more volatility of butyl acrylate, at higher polymerization rates more butyl acrylate monomers are added to polymer chains which decrease the [T.sub.g] of the obtained copolymer. The second reason arises from the radical formation mechanism. In conventional emulsion polymerization, thermal dissociations of initiator is the only mechanism to initiation, while in ultrasound-assisted emulsion polymerization, especially when initiator concentration is lower than usual, other radical formation mechanisms also appear and, to some extent affect the final polymers properties. Generation of radicals on main polymer chains and secondary chain growth on activated sites seem to be one of the most affective causes on [T.sub.g] of the obtained polymers.

The effect of polymerization temperature on [T.sub.g] of the obtained copolymers (Fig. 12b) also indicated that in ultrasound-assisted emulsion polymerization, by increasing polymerization temperature [T.sub.g] decreased. These results are in good agreement with previous results and the formation of branched polymer chains are the main reason for this pattern.

Molecular Weight and Molecular Weight Distribution

GPC was employed for determination of average molecular weights of polymeric particles and their molecular weight distribution (MWD). GPC results for conventional and ultrasound-assisted emulsion polymerization under different initiator concentration were taken and results have been summarized in Table 4.

It can be seen that in both polymerization systems decreasing initiator concentration increases molecular weight (Fig. 13a), but molecular weight distribution shows a different pattern. In ultrasound-assisted emulsion polymerization by decreasing initiator concentration, molecular weight distribution shows significant decreases while in conventional one the decrease is slight (Fig. 13b).

These results indicated that the effect of ultrasound pulses on molecular weight and its distribution is related to initiator concentration. Application of ultrasound pulses in conventional emulsion polymerization enhances dissociation of initiator to free radicals and moreover, leads to the uniform and smaller polymeric particles. At high initiator concentration, molecular weight decreases could be attributed to increase in average surface area and increase in the possibility of free radical entrance into the individual particles [26-28]. Therefore, the probability of termination reactions and lowering of molecular weights increases. On the other hand, the molecular weights of the polymer formed by ultrasound initiation ([USEMI.sub.0]) are considerably larger than those prepared by the conventional method, which can be attributed to the numbers of individual particles polymerizing independently.

Mechanism of Polymerization

In a conventional polymerization, radicals are generated from chemical initiators that readily decompose by heating. We propose in ultrasound-assisted emulsion polymerization a conventional initiator is the main source of radicals, also they may come from the degradation of water molecules, monomers, and formed polymer chains. This view was also confirmed by FTIR (Fig. 14a) and H-NMR results. As can be seen in Fig. 14a, the carbonyl group in USEMI(1) sample shows a bimodal peak and C--O stretch (1020 [cm.sup.-1]) to C--H stretch (2920 [cm.sup.-1]) ratio decreased significantly, which refers to the partially un-uniform or crosslinked structure [29].

The detailed information on polymerization mechanism could be achieved from H-NMR spectroscopy (Fig. 14b). These results indicated that by application of ultrasound pulses in emulsion polymerization OC[H.sub.2] signal of butyl acrylate at 3.78-4.17 ppm increased and relatively OC[H.sub.3] signal of MMA at 3.5-3.75 ppm decreased slightly. On the other hand, the butyl acrylate concentration on copolymer chain partially increased [30]. These results also are in good agreement with DMTA results and glass transition temperature changes.

CONCLUSIONS

In this study, ultrasound pulses (20-40 watts) were applied on conventional emulsion polymerization and the effect of initiator concentration and polymerization temperature on monomer conversion and final properties of poly(methyl methacrylate-co-butyl acrylate) were investigated. Results indicated that with the use of ultrasound pulses on conventional emulsion polymerization higher monomer conversion was obtained, polymerization rate (especially in the first 15 min of the reaction) improved significantly, molecular weight distribution decreased remarkably and large number of smaller and more uniform particles formed. Comparison of monomer conversion in both conventional and ultrasound-assisted polymerization at various initiator concentration and temperature also revealed that ultrasound pulses cause fast dissociation of initiator to free radicals, however when the initiator concentration decreases less than critical concentration, relatively small radicals are generated on their own. Indeed, application of ultrasound enhances some aspects of conventional emulsion polymerization, initiator/monomer ratio can be decreased and polymerization at lower temperatures is possible. We hope that these results could be useful for production of large scale, monodisperse acrylic copolymers.

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Iraj Hasanzadeh, Mehdi Barikani, Ali Reza Mahdavian

Polyurethane Department, Iran Polymer and Petrochemical Institute, Tehran, Iran

Correspondence to: M. Barikani; e-mail: m.barikani@ippi.ac.ir

DOI 10.1002/pen.24249

TABLE 1. The procedure for conventional (CEM) and
ultrasound-assisted emulsion polymerization (USEM) with
various initiator contents.

sample             Monomer  SDS   Triton X-100  NaHC[O.sub.3]

[CEMI.sub.(1)]      93.82   0.56       0.14           0.4
[CEMI.sub.(1/2)]    93.82   0.56       0.14           0.4
[CEMI.sub.(1/4)]    93.82   0.56       0.14           0.4
[CEMI.sub.(1/8)]    93.82   0.56       0.14           0.4
[USEMI.sub.(1)]     93.82   0.56       0.14           0.4
[USEMI.sub.(1/2)]   93.82   0.56       0.14           0.4
[USEMI.sub.(1/4)]   93.82   0.56       0.14           0.4
[USEMI.sub.(1/8)]   93.82   0.56       0.14           0.4
[USEMI.sub.(0]      93.82   0.56       0.14           0.4

                                        Temperature
sample             Water    KPS   AA    ([degrees]C)

[CEMI.sub.(1)]      316   0.469  0.028       75
[CEMI.sub.(1/2)]    316   0.235  0.028       75
[CEMI.sub.(1/4)]    316   0.117  0.028       75
[CEMI.sub.(1/8)]    316   0.059  0.028       75
[USEMI.sub.(1)]     316   0.469  0.028       75
[USEMI.sub.(1/2)]   316   0.235  0.028       75
[USEMI.sub.(1/4)]   316   0.117  0.028       75
[USEMI.sub.(1/8)]   316   0.059  0.028       75
[USEMI.sub.(0]      316     0    0.028       75

All amounts are in gram and total solid content was set at
around 30% for all the samples.

TABLE 2. Procedure to investigation the effect of
polymerization temperature on conventional and
ultrasound-assisted emulsion polymerization.

sample             Monomer   SDS    Triton X-100   NaHC[O.sub.3]

[CEMT.sub.(75)]     93.82    0.56       0.14            0.4
[CEMT.sub.(60)]     93.82    0.56       0.14            0.4
[CEMT.sub.(45)]     93.82    0.56       0.14            0.4
[CEMT.sub.(30)]     93.82    0.56       0.14            0.4
[USEMT.sub.(75)]    93.82    0.56       0.14            0.4
[USEMT.sub.(60)]    93.82    0.56       0.14            0.4
[USEMT.sub.(45)]    93.82    0.56       0.14            0.4
[USEMT.sub.(30]     93.82    0.56       0.14            0.4

                                           Temperature
sample             Water    KPS      AA    ([degrees]C)

[CEMT.sub.(75)]     316    0.469   0.028        75
[CEMT.sub.(60)]     316    0.469   0.028        60
[CEMT.sub.(45)]     316    0.469   0.028        45
[CEMT.sub.(30)]     316    0.469   0.028        30
[USEMT.sub.(75)]    316    0.469   0.028        75
[USEMT.sub.(60)]    316    0.469   0.028        60
[USEMT.sub.(45)]    316    0.469   0.028        45
[USEMT.sub.(30]     316    0.469   0.028        30

All amounts are in gram and total solid content
was set at around 30% for all the samples.

TABLE 3. Particle size and particle size distribution for
polymer obtained by conventional and ultrasound-assisted
emulsion polymerization in different initiator
concentration.

                    Particle
Sample              size (nm)        SD

[CEMI.sub.(1)]         164      [+ or -] 37
[CEMI.sub.(1/2)]       198      [+ or -] 24
[CEMI.sub.(1/4)]       234      [+ or -] 25
[CEMI.sub.(1/8)]       256      [+ or -] 28
[USEMI.sub.(1)]         96      [+ or -] 8
[USEMI.sub.(1/2)]      121      [+ or -] 11
[USEMI.sub.(1/4)]      149      [+ or -] 13
[USEMI.sub.(1/8)]      161      [+ or -] 12
[USEMI.sub.(0)]        198      [+ or -] 19

TABLE 4. Molecular weight analysis for conventional and
ultrasound assisted emulsion polymerization at different
initiator concentration.

                        [W.sub.w]            [M.sub.w]
Sample              [10.sup.4] (g/mol)   [10.sup.4] (g/mol)   MWD

[CEMI.sub.(1)]             6.32                 3.65          1.73
[CEMI.sub.(1/2)]           6.44                 3.28          1.96
[CEMI.sub.(1/4)]           6.92                 3.70          1.87
[CEMI.sub.(1/8)]           7.74                 4.71          1.64
[USEMI.sub.(1)]            5.86                 2.15          2.72
[USEMI.sub.(1/2)]          6.32                 3.61          1.75
[USEMI.sub.(1/4)]          7.36                 5.01          1.47
[USEMI.sub.(1/8)]          8.52                 6.60          1.29
[USEMI.sub.(0)]            9.87                 8.15          1.21
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Author:Hasanzadeh, Iraj; Barikani, Mehdi; Mahdavian, Ali Reza
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Date:Feb 1, 2016
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