Journal of Colloid and Interface Science 368 (2012) 400–405

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Journal of Colloid and Interface Science

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Synthesis of poly(vinyl acetate–methyl methacrylate) copolymer microspheresusing suspension polymerization

Md. Shahidul Islam a, Jeong Hyun Yeum b, Ajoy Kumar Das a,⇑a Department of Applied Chemistry and Chemical Engineering, University of Dhaka, Dhaka 1000, Bangladeshb Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702-701, South Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 September 2011Accepted 1 November 2011Available online 10 November 2011

Keywords:Suspension polymerizationPoly(vinyl acetate–methyl methacrylate)copolymerMicrospheres

0021-9797/$ - see front matter Crown Copyright � 2doi:10.1016/j.jcis.2011.11.002

⇑ Corresponding author. Fax: +880 2 8615583.E-mail address: ajoykddu@gmail.com (A.K. Das).

Poly(vinyl acetate–methyl methacrylate) (VAc–MMA) copolymer microspheres were prepared using sus-pension polymerization at low temperature initiated with 2,20-azobis(2,4-dimethyl valeronitrile)(ADMVN). The poly(VAc–MMA) copolymer microspheres can be used over a large area where homopoly-mers, polyvinyl acetate (PVAc) and methyl methacrylate (PMMA) microspheres are capable of being putto use. The prepared microspheres were characterized by scanning electron microscopy (SEM), X-ray dif-fraction (XRD), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Obtainedcopolymer microspheres have 200 lm average diameter and higher thermal stability than those ofhomopolymer.

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1. Introduction

As the polymer industry becomes more competitive, polymermanufacturers face increasing pressures for the production costreductions and more stringent polymer quality requirements [1].Therefore, the development of comprehensive methods to controlpolymer quality during a polymerization is the key to the efficientproduction of tailored, high-quality polymers and the improve-ment of plant operability and economics.

Polymeric microspheres have been arisen great interest in thefields of biology and medicine [2–6]. These microspheres can beprepared via monomer polymerization including conventionalemulsion polymerization [7,8], dispersion polymerization [9], sus-pension polymerization [10,11] and seed polymerization [12], andactivated swelling method [13]. Among these methods, suspensionpolymerization is simple and more suitable for massive productionof microspheres [14].

In suspension polymerization, the monomer is dispersed in a li-quid (usually water) by vigorous stirring and by the addition of sta-bilizers [15]. A monomer-soluble initiator is added in order toinitiate chain-growth polymerization. The polymer is obtained inthe form of microspheres at a high degree of conversion. The mainadvantage of this process is that the heat of polymerization caneasily be removed via aqueous phase. Polymerization proceeds inthe droplet phase and in most cases, occurs by a free radicalmechanism. In order to prevent settling or creaming, agitation is

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normally continued throughout the course of the reaction. Suspen-sion polymerization usually requires the addition of small amountsof a stabilizer to hinder coalescence and break-up of dropletsduring polymerization [16]. The size distribution of the initialemulsion droplets and, hence, also of the polymer particles thatare formed is dependent upon the balance between dropletbreak-up and droplet coalescence. This is in turn controlled bythe type and speed of agitator used, the volume fraction of themonomer phase, and the type and concentration of stabilizer used.

Suspension polymerization can provide a suitable method forthe production of copolymers at high conversions. If the copolymeris insoluble in its monomer or is cross-linked, then solution poly-merization may be unsuitable due to low conversions and poorcontrol of the reaction. The use of suspension polymerization canovercome such problems. The nature of the copolymer formed willdepend upon the solubilities and reactivity ratios of the compositemonomers. However, the reactivity ratios in suspension copoly-merization often vary from those in solution (or emulsion)polymerization. This may be due to various factors including differ-ent mixing of monomers in the two methods [17] and a slight sol-ubility of one (or more than one) of the monomers in thecontinuous phase [18]. A simulation procedure has been developedfor the copolymerization of styrene and acrylonitrile by Hagberg[19], which predicts the molecular weight of the polymer formed,while allowing for the effect the water solubility of acrylonitrilewill have on the reactivity ratios. Moreover, various types ofcopolymers such as vinyl chloride/methyl methacrylate [20], sty-rene/methyl methacrylate [21], vinyl pyrrolidone/ethylene dim-ethylacrylate [22] and vinyl chloride/divinyls [23] were producedusing suspension polymerization.

ights reserved.

Table 1Suspension copolymerization conditions of VAc/MMA system.

Conditions Name/value

Type of initiator ADMVNType of suspending agent PVAInitiator concentration 0.0001, 0.0005, 0.001 mol/mol of monomerSuspending agent

concentration1.5 g/dl of water

Monomer/water 0.5 l/lRpm 500Temperature 30, 40, 50 �C

Fig. 2. VAc/MMA suspension copolymerization using ADMVN concentration of0.0001 mol/mol of monomer at 30 �C with monomer ratio, VAc/MMA = 1/1(bymole).

Md.S. Islam et al. / Journal of Colloid and Interface Science 368 (2012) 400–405 401

Homopolymer of vinyl acetate (VAc) has been widely studied inthe literature because of its excellent optical properties, biocom-patibility and few foreign-body reactions in vivo [24]. Polymethylmethacrylate (PMMA) that is obtained from polymerization ofmethyl methacrylate is an important polymeric material with highlight transmittance, colorlessness, chemical resistance and weath-ering corrosion resistance. Due to these superior characteristics,PMMA is used over a large area in coating, optical fiber, outdoorelectrical applications, etc. [25–27]. Therefore, the copolymer ofvinyl acetate and methyl methacrylate can be used in the afore-mentioned fields, which are agreeable for polyvinyl acetate andpolymethyl methacrylate. Besides, vinyl acetate/methyl methacry-late copolymer and their corresponding homopolymers are com-mercially important components in many paints, adhesives andbinders [28].

In the previous reports, PVAc, PVAc/Silver, and PVAc/montmo-rillonite [29,30] and PMMA and PMMA/silver [31] microsphereswere prepared using the technique of suspension polymerization.In this paper, we report the results of suspension copolymeriza-tions of VAc/MMA initiated with 2,20-azobis(2,4-dimethyl valero-nitrile) (ADMVN) at low temperature.

Fig. 3. VAc/MMA suspension copolymerization using ADMVN concentration of0.0001 mol/mol of monomer in various temperature with monomer ratio, VAc/MMA = 1/1(by mole).

2. Experimental

2.1. Materials

Methyl methacrylate and vinyl acetate purchased from Aldrichwere sequentially washed with NaHSO3 aqueous solution andwater and then dried with anhydrous CaCl2, followed by distilla-tion in a nitrogen atmosphere under reduced pressure [32]. Themonomer-soluble initiator, ADMVN (Wako), was recrystallizedtwice in methanol before use. Poly(vinyl alcohol) (PVA) with

Fig. 1. VAc/MMA suspension copolymerization using ADMVN concentration of0.0001 mol/mol of monomer at 30 �C with monomer ratio, VAc/MMA = 3/2(bymole).

Fig. 4. VAc/MMA suspension copolymerization at 30 �C temperature using differentADMVN concentration with monomer ratio, VAc/MMA = 1/1(by mole).

number-average molecular weight of 127,000 and degree of sapon-ification of 88% (Aldrich Co.) was used as a suspending agent. De-ionized water was put to profitable use for all the experiments.

Fig. 5. SEM images of microspheres of poly(VAc–MMA) copolymers (a) General view, (b) Close-up view showing individual particle.

Fig. 6. SEM images of microspheres of (a) PVAc, (b) PMMA.

Fig. 7. Size distribution of poly(VAc–MMA) copolymer microspheres formed bysuspension polymerization.

Fig. 8. XRD patterns of (a) Homopolymer, PVAc, (b) Homopolymer, PMMA and (c)poly(VAc–MMA) copolymer microspheres.

402 Md.S. Islam et al. / Journal of Colloid and Interface Science 368 (2012) 400–405

2.2. Methods

2.2.1. Synthesis of poly(VAc–MMA) copolymer microspheresFor synthesizing poly(VAc–MMA) copolymer microspheres, sus-

pension polymerization of those both monomers was conductedsimultaneously in a reactor. Suspending agent was dissolved inwater under a nitrogen atmosphere with constant stirring in a250-ml reactor fitted with a condenser. The ADMVN was added ata fixed polymerization temperature. After predetermined times,

the reaction mixture was cooled and kept for 1 day to allow the pre-cipitation of the poly(VAc–MMA) copolymer microspheres [29–31].The final poly(VAc–MMA) copolymer microspheres were filteredand washed with warm water and finally dried at 105 �C for 3 h toremove monomer residue.

Eqs. (1)–(5), reported by Gomes et al. [33], were used to calcu-late the conversion of individual monomers and the overall conver-sion of the copolymerization.

Md.S. Islam et al. / Journal of Colloid and Interface Science 368 (2012) 400–405 403

dNfed A=dt ¼ FA;in ð1Þ

dNfed B=dt ¼ FB;in ð2Þ

where Nfed A and Nfed B are the number of moles fed for monomer Aand B and FA,in and FB,in are the molar flow rates of monomer A andB, respectively.

XA ¼ 1� NA=Nfed A ð3Þ

XB ¼ 1� NB=Nfed B ð4Þ

Fig. 9. FTIR spectra of (a) Poly(VAc–MMA) copolymer.

X ¼ ðXAMA þ XBMBÞ=ðMA þMBÞ ð5Þ

where MA and MB are molecular weight of monomer A and B,respectively, and XA and XB are the conversions of monomers Aand B, while X is the overall conversion.

The detailed polymerization conditions are listed in Table 1.

2.3. Characterizations

The morphology and properties characterization of poly(VAc–MMA) copolymer microsphere were observed with a field-emissionscanning electron microscope (FE-SEM) (JEOL, model JSM-6380)after gold coating and an X-ray diffraction (XRD) (Philips modelX’Pert APD) with the CuKa radiation with wavelength of0.154 nm. The scanning rate was 2�/min ranging 5–40� (2h). Thethermal behavior of poly(VAc–MMA) copolymer microsphere wasstudied with DSC (model Q-10) and TGA techniques (model Q-50)from TA instruments, USA at the rate of 10 �C/min from room tem-perature to 500 and 600 �C, respectively, under the nitrogen gasatmosphere. To obtain the size distribution, five SEM photographsand more than 300 spheres are collected by computer, which linkedwith the SEM, followed by statistical analysis of data by computer.

Fig. 10. DSC data of (a) Homopolymer, PVAc, (b) Homopolymer, PMMA and (c)poly(VAc–MMA) copolymer microspheres.

3. Results and discussion

3.1. Suspension copolymerization deportment of poly(VAc–MMA)copolymer microspheres

In this article, ADMVN was used to prepare poly(VAc–MMA)copolymer microspheres at room temperature. The individual con-versions of both VAc and MMA monomers during the reactionswere estimated using Eqs. (3) and (4), and the overall conversionwas subsequently calculated according to Eq. (5) based on homo-polymerization. The monomer conversion vs. time data obtainedat 30 �C temperature with initiator concentration of 0.0001 mol/mol of monomer are plotted as shown in Figs. 1 and 2.

Final conversions ranging from 85% to 90% were achieved in theexperiments. They all exhibited a two-stage rate effect as describedin Dube and Penlidis [34]. Since MMA is much more reactive thanVAc according to the reactivity ratios (rMMA = 24.025 andrVAc = 0.0261) [35], MMA dominated the beginning of the reactionto form polymer mostly composed of MMA, while VAc dominatedthe polymerization after the MMA was depleted. As shown in Figs.1 and 2, VAc was not reacting much during the early stages of thecopolymerizations and behaved much like a solvent so that theMMA homopolymerization gel effect was dampened by the VAc.During the latter stages of reaction, the copolymerization ratewas very high, owing to the fact that the VAc homopolymerizationexhibited an autoacceleration in the high viscosity environment ofthe polymer chains. Comparison between Figs. 1 and 2 indicatesthat when the MMA percentage in the monomer feed was in-creased from 40% to 50%, the two-stage rate effect was delayedto higher conversions. Higher polymerization rates were also

observed during the first stage of the polymerization, while theoverall polymerization rates did not change very much.

Fig. 3 presents the conversion–time relationship at differentpolymerization temperatures with an initiator concentration of0.0001 mol/mol of monomer used, while the monomer ratio is 1.Although low initiator concentration was used, the conversion in-creased steadily with the reaction time at a reaction temperatureof 30–50 �C until 90–95% of conversion. The high conversion sug-gests that the chain transfer and termination reactions were notsignificant under the conditions used in this study. Conversionsat 30 �C temperature with different initiator concentrations areshown in Fig. 4 [30,31]. The polymerization rate was increasedwith increasing the initiator concentration, which coincided wellwith the theoretical predictions [36].

3.2. Morphology of poly(VAc–MMA) copolymer microspheres

The SEM image of prepared poly(VAc–MMA) copolymer parti-cles is shown in Fig. 5. It is clearly seen that the prepared copoly-mer particles are spherical shape with roughness outside due tofracture nature of surfaces, whereas PVAc and PMMA microspheres(prepared by suspension polymerization) surface is smooth (Fig. 6).It is also found that the copolymer microspheres have different size(Fig. 5a) with an average diameter of 200 lm (Fig. 7), which issmaller to some extent in size than homopolymers (average diam-eter 225 lm) [29].

Fig. 11. TGA data of (a) poly(VAc–MMA) copolymer, (b) Homopolymer, PVAc and(c) Homopolymer, PMMA microspheres.

404 Md.S. Islam et al. / Journal of Colloid and Interface Science 368 (2012) 400–405

3.3. XRD data

The X-ray diffraction scans of PVAc, PMMA and poly(VAc–MMA) copolymer as a function of Bragg angle (2h) are displayedin Fig. 8. The broadened background scattering areas of PVAc andPMMA suggest the presence of amorphous nature.

Fig. 8 illustrates the diffract grams of homopolymers PVAc,PMMA and poly(VAc–MMA) copolymer in the 2h range between5� and 40�, which are similar and without any sharp diffractionpeaks confirming their non-crystalline nature. The interlayer

spacing of the system was determined by the diffraction peak inthe X-ray method, using the Bragg equation k = 2d sinh, where dis the spacing between diffractional lattice planes, h is the diffrac-tion position, while k is the wavelength of X-ray. PVAc is known tobe an amorphous polymer and shows two broad peaks at 2h valuesof 13.5� and 22.5� [37]. PMMA is also an amorphous polymer andexhibits two board peaks at 2h values of 15� and 30� with theirintensities decreasing systematically [38,39]. The copolymer thatis obtained by happening copolymerization of vinyl acetate (VAc)and methyl metacrylate (MMA) displayed two broad peaks at 2hvalues of 12.60� and 28.70�, which indicate that the copolymeriza-tion is successful.

3.4. FTIR spectra

Fig. 9 shows the FTIR spectrum revealing a strong band at1738 cm�1, which can be attributed to the carbonyl group of PVAcand PMMA segments [40].

3.5. Thermal behaviors

The DSC melting curves of poly(VAc–MMA) copolymer and twohomopolymers, PVAc and PMMA, are displayed in Fig. 10.

Prepared PVAc and PMMA microspheres show large thermo-gram peaks of degradation temperature at 317 �C and 368 �C,which fall on the same point with pure PVAc [41] and PMMA[42], respectively. However, the poly(VAc–MMA) copolymerexhibits larger degradation temperature (381 �C) to a greatextent than both homopolymers, which can be explained onthe effect of copolymerization. The copolymerization results inthe formation of copolymers containing different types of unitin a chain. The presence of units differing in structure often pre-sents difficulties in the detachment of molecules during thermaldegradation [43].

The thermal stability of three polymers was evaluated by TGA(Fig. 11). TGA is a technique used to accurately track the in situweight changes of a sample during a heating process, thereby pro-viding information on thermal degradation.

Fig. 11 shows the TGA curves of PVAc, PMMA homopolymersand poly(VAc–MMA) copolymer. From the Fig. 11, it is evident thatthe prepared PMMA homopolymer and poly(VAc–MMA) copoly-mer show two different degradation steps. The first, at lower tem-peratures (i.e. at a temperature < 300 �C), is monomer evolutioninitiated at the unstable terminal double bonds present in someof the macromolecules as a consequence of the disproportionatetermination reaction, while the second follows the random bondscission of the polymer chains. These reactions have different ener-gies of activation depending on the chain length and percentage ofdouble-bonded chain ends in the polymer. The pure PMMA andpoly(VAc–MMA) copolymer (Fig. 10) had 5% and 3% weight lossat 188 �C, respectively. PMMA lost 96% of its weight at 421 �Cand completely degraded at 426 �C, while poly(VAc–MMA) copoly-mer lost 90% of its weight at 410 �C and completely degraded at420 �C. On the other hand, PVAc homopolymer had 1% weight lossat 138 �C, and it lost 97% of its weight at 395 �C and fully degradedat 410 �C. However, the thermal decomposition temperature incase of poly(VAc–MMA) copolymer (i.e. 331 �C) is larger to a greatextent than both homopolymers, PVAc and PMMA (243 �C and303 �C, respectively). Degradation activation energies (E) of PVAc,PMMA and poly(VAc–MMA) copolymers were calculated by usingFreeman–Carroll equation and the data obtained from derivativesof dynamic thermograms. In Fig. 11, we have presented DlogU(along the y axis) versus DlogW (along the x axis) plots of few typ-ical decompositions, where U(=dW/dt) and W are the rate ofdecomposition and fractional weight loss, respectively. It is foundthat the variation is quite linear, suggesting that Freeman and

Fig. 12. TGA kinetic plots of poly(VAc–MMA) copolymer and homopolymers PMMAand PVAc.

Md.S. Islam et al. / Journal of Colloid and Interface Science 368 (2012) 400–405 405

Carroll [44,45] type of analysis is applicable for the kinetics ofthese decompositions. It is assumed in such kinetics that the soliddecomposes to give another solid and the rate (U) is related toinstantaneous weight fraction of the material W by the Eq. (6) [46].

U ¼ �dW=dt ¼ A expð�E=RTÞWn=Uh ð6Þ

where Uh is the heating rate = 10 �C/min, A = frequency factor/min,E is the activation energy in J/gmol, T is the temperature in K, R isthe gas constant = 8.315 J/kmol and n is the order of reaction. Theweight fraction W is taken as the ratio of the weight of the materialpresent at time t to the initial weight of the sample (at t = 0). Theslope of this plot (Fig. 12) gives the value for E, and the intersectionof ordinate indicates the order of reaction.

4. Conclusions

Poly(VAc–MMA) copolymer microspheres were prepared withsuccess making use of suspension polymerization and character-ized by SEM, XRD, DSC and TGA. The surface of spherical structurecopolymer is rough, whereas same shape homopolymer surface issmooth. Thermal stability of copolymer microspheres is enhancedmore than that of homopolymers.

Acknowledgments

The support of this research by the Kyungpook National Univer-sity, South Korea is gratefully appreciated. A.K. Das gratefullyacknowledges supports from the University of Dhaka, Bangladesh.

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