Preparation of Calcium Carbonate/Poly(methylmethacrylate) Composite Microspheres by SoaplessEmulsion Polymerization

Xiaokun Ma, Bing Zhou, Ye Sheng, Chengyu Wang, Yan Pan, Shanshan Ma, Ye Gao,Zichen Wang

College of Chemistry, Jilin University, Changchun, 130023, China

Received 31 March 2006; accepted 26 January 2007DOI 10.1002/app.26486Published online 16 May 2007 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Calcium carbonate (CaCO3)/poly(methylmethacrylate) (PMMA) composite microspheres were syn-thesized in a soapless emulsion polymerization process.CaCO3 nanoparticles were pretreated with oleic acid toimprove the compatibility between CaCO3 and the mono-mer methyl methacrylate, and this effectively enhanced themass fraction of CaCO3 in the composite. The results ofthe thermogravimetric analysis showed that the thermalstability of the composite microspheres was better thanthat of PMMA. X-ray diffraction and Fourier transforminfrared were applied to investigate the chemical andphysical properties of the composite microspheres, and the

results proved that the composites were composed ofCaCO3 and PMMA. Scanning electron microscopy andtransmission electron microscopy pictures showed that themorphology of the composite microspheres was uniform,and the CaCO3 nanoparticles could be well dispersed inthe polymeric matrix. The soapless emulsion polymeriza-tion process effectively increased the filling amount ofCaCO3 in the composite microspheres. � 2007 Wiley Period-icals, Inc. J Appl Polym Sci 105: 2925–2929, 2007

Key words: compatibility; composites; emulsion polymer-ization; fillers; matrix

INTRODUCTION

As nanotechnology has developed, composite micro-spheres have attracted increasing interest for bothfundamental research and industrial applications.1,2

Composite microspheres often exhibit propertiescompletely different from those of polymeric systemsin which an inorganic component has been added toa polymeric matrix at a micrometer level.3 Some un-expected properties of composite microspheres, suchas superconductivity, magnetism, nonlinear optics,thermal stability,4–6 and dynamic mechanical proper-ties, have been observed because of the enormousinterfacial adhesion area of nanoparticles.

Several methods have been used to produce poly-mer composite microspheres, such as miniemulsionpolymerization,7 intercalative polymerization,8 emul-sion polymerization,9 and hybrid latex polymeriza-tion.10 Soapless emulsion polymerization has beenfrequently used to synthesize nanoparticle/poly-meric composite microspheres. Few surfactants or nosurfactants have been used in such systems, whichprovide advantages for the synthesis of monodis-perse latex. In soapless emulsion polymerization sys-tems, polymer particles have been stabilized by ion-

ized initiators.12 When inorganic fillers without par-ticipation in the polymerization process [e.g.,calcium carbonate (CaCO3)] have been added, thelocus and velocity of the polymerization have beenchanged to stabilize the emulsion system.

Yang et al.13 obtained CaCO3/polystyrene (PS)composites by soapless polymerization. The encapsu-lating ratio was about 60%, and the morphology of thecomposites was not uniform. Yu et al.14 pretreatedCaCO3 with g-methacryloxypropyltrimethoxysilaneto introduce polymerizable groups onto its surfaceand prepared CaCO3/PS composites through anemulsion polymerization process. Avella et al.15 pre-pared CaCO3/poly(methyl methacrylate) (PMMA)nanocomposites and studied the abrasion-resistantbehavior of the materials. However, the content ofCaCO3 in the nanocomposites was reasonably low(<6%).

CaCO3 is the cheapest commercially availableinorganic material and is extensively used as a par-ticulate filler in the manufacture of paint, paper, rub-ber, plastics, and so forth.16 CaCO3 can be modifiedwith different surfactants such as fatty acids, phos-phonate, and titanate through the reaction of ad-sorbed hydroxy on the particle surface. In this study,CaCO3 nanoparticles were pretreated with oleic acid(OA), which provided them with high compatibilitybetween the nanofillers and methyl methacrylate(MMA) monomer.

Correspondence to: Z. Wang (wangzc@mail.jlu.edu.cn).

Journal of Applied Polymer Science, Vol. 105, 2925–2929 (2007)VVC 2007 Wiley Periodicals, Inc.

In this work, we obtained CaCO3/PMMA compos-ite microspheres through a soapless emulsion poly-merization process. The scanning electron micros-copy (SEM) photographs showed that the morphol-ogy of the composite microspheres was uniform andthat the size of the composite microspheres wasabout 350 nm. The encapsulating ratios of CaCO3 inthe composite microspheres could be controlled inthis soapless emulsion polymerization process.

EXPERIMENTAL

Materials

The MMA monomer, with a density of 0.943 g/cm3,was obtained from Tianjin Chemicals Factory(China) and was not distilled. The ammonium per-sulfate (APS) initiator and OA surfactant were ana-lytical-grade and were used without further purifica-tion. Distilled and deionized water was usedthroughout the work.

Preparation of the CaCO3 latex

Calcium oxide (CaO) was put into boiling deionizedwater, and then a saturated Ca(OH)2 slurry wasdiluted after the impurity deposit was removed. Thegas of CO2 was aerated into the slurry to prepare ananometer CaCO3 latex through the control of theflux of CO2 until the pH of the solution was downto 7. OA was dissolved in ethanol to be well dis-persed in water and was added dropwise into theflask at 608C for 1 h. The CaCO3 latex was self-pre-pared with particle sizes of 40–100 nm.

Preparation of the CaCO3/PMMAcomposite microspheres

A 500-mL, four-necked flask equipped with a ther-mometer, mechanical stirrer, reflux condenser, andN2 inlet was charged with deionized water (100 g)and an OA-modified nanometer CaCO3 latex. APS(0.100 g), as an initiator, and MMA (5 mL) wereadded dropwise to the flask at 608C, and theamounts of the CaCO3 nanoparticles are listed inTable I. The mixture was maintained at this tempera-

ture for 0.5 h and then at 808C for 4 h. The productwas collected by suction filtration and then dried at708C for 6 h. CaCO3 nanoparticles unencapsulated inPMMA were eliminated from the composite micro-spheres after 1 h of dipping in 0.01M HCl. In con-trast to the quantities of the composite microspheres,the encapsulated ratio of the CaCO3 nanofiller couldbe calculated.

Characterization

Fourier transform infrared (FTIR) was recorded withan Ominic System 2000 (USA), which employed aKBr pellet method. X-ray diffraction (XRD) patternsof the products were taken on a Shimadzu (Japan)model XRD-6000 that filtered Cu Ka radiation. Ther-mogravimetric analysis (TGA) was conducted with aMettler–Toledo (USA) 825e instrument at a heatingrate of 208C/min in an air atmosphere. The mor-phology of the CaCO3/PMMA composite micro-spheres was investigated by SEM with a ShimadzuSSX-55 electron microscope and by transmissionelectron microscopy (TEM) with a Hitachi (Japan) H-8100 electron microscope.

RESULTS AND DISCUSSION

Synthesis of the CaCO3/PMMAcomposite microspheres

The components of the CaCO3/PMMA compositemicrospheres were determined through FTIR and

TABLE IDifferent Quantities of CaCO3 in the Soapless

Emulsion Process

SampleCaCO3

content (g)Yield(%)

Encapsulatingratio (%)

1 0.47 78.9 6.72 0.75 77.6 7.23 1.10 85.4 18.24 1.45 83.1 23.35 2.00 84.7 28.16 2.35 64.2 17.6

Figure 1 IR spectra of (a) CaCO3 before emulsion poly-merization, (b) pure PMMA, (c) CaCO3/PMMA compositemicrospheres, and (d) the composites after calcination.[Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

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XRD investigations. Figure 1(a–d) shows FTIR spec-tra of CaCO3, pure PMMA, CaCO3/PMMA compo-sites, and composites calcined at 5508C for 4 h,respectively. As shown in Figure 1(b), typical bandsof PMMA were found at 1728, 1145, 1192, 1272, and1240 cm21 and were assigned to the stretching vibra-tions of C¼¼O and C��O (symmetric and asymmet-ric). Compared with pure PMMA and CaCO3, thecomposites showed an adsorption band at 1435cm21 that was broadened by the interaction betweenPMMA and CaCO3.

17 Through a comparison withthe curve of CaCO3, as shown in Figure 1(a), anadsorption band at 1728 cm21 was found in the IRspectrum of the composites, indicating the presenceof an ester group that came from PMMA. As shownin Figure 1(d), the IR spectrum of the calcined com-posites well agreed with that of CaCO3, and thisprovided proof of the existence of CaCO3 in PMMA.

The presence of CaCO3 nanoparticles in theCaCO3/PMMA composite microspheres was con-firmed by XRD analysis, and the spectra are shownin Figure 2. In the pattern, the diffraction peaks wereobserved at 2u values of 29.35, 48.62, 39.42, and47.508, which corresponded to CaCO3 crystals ascalcite. Figure 2(b–e) shows the XRD peaks of com-posite samples 5–2 and were according to the orderof samples listed in Table I (5–2). All the character-istic XRD peaks of the fillers could be observed,and this indicated that the CaCO3 nanoparticleswere encapsulated in the PMMA matrix. On theother hand, the intensity of the characteristic XRDpeaks of the fillers became weaker with a decreas-ing amount of CaCO3. The difference in the inten-

sity was ascribed to the thickness of the PMMAlayer on the surface of CaCO3 particles.

Encapsulating ratio of the CaCO3/PMMAcomposite microspheres

Table I lists the yields and encapsulating ratios ofthe different composite microspheres. The yields andencapsulating ratios of the CaCO3/PMMA compositemicrospheres gradually increased with an increasingamount of CaCO3 when the filler mass fraction wasbelow 2.00 g. A further increase in the fillerdecreased both the yield and encapsulating ratio.More CaCO3 could cause more coagulation, decreas-ing the stability of the latex. As a result, the appro-priate concentration of CaCO3 was below 30 wt % inthis case.

Figure 3 shows TGA plots of CaCO3, pure PMMA,and CaCO3/PMMA composite microspheres withdifferent amounts of CaCO3. Redundant CaCO3 wasremoved with a 0.01M HCl solution before testing,so the amounts of CaCO3 in the composites could beregarded as encapsulating ratios. Figure 3(a) showsthat CaCO3 did not decompose below 6008C, andthe weight loss was 8.6%, which was derived fromthe removal of water absorbed onto the particles sur-face and the decomposition of OA. The pure PMMAsynthesized under the same polymerization condi-tions decomposed from 260 to 4008C, as shown inFigure 3(f). The weight loss between 260 and 6008Cin the composites was used to calculate the amountof CaCO3 in the composite microspheres. Figure 3(b–e) shows that the concentrations of CaCO3 in thecomposites were 28.1, 23.3, 18.2, 7.2, and 6.7 wt %,

Figure 2 XRD plots of (a) CaCO3 pretreated with OA, (b)CaCO3/PMMA composites with 28.1 wt % CaCO3, (c)CaCO3/PMMA composites with 23.2 wt % CaCO3, (d)CaCO3/PMMA composites with 18.2 wt % CaCO3, and (e)CaCO3/PMMA composites with 7.1 wt % CaCO3. [Colorfigure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]

Figure 3 TGA plots of CaCO3 pretreated with OA, purePMMA, and CaCO3/PMMA composite microspheres.[Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

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respectively. In this work, such a soapless emulsionpolymerization process was effective for increasingthe filling amount of CaCO3 in the composite mic-rospheres. The decomposition temperature of thecomposite microspheres was higher than that ofPMMA, and the decomposition temperature increasedwith increasing filler content. As a result, it wassuggested that the nanoparticles could improve thethermal stability of the PMMA matrix.

Morphology of the CaCO3/PMMAcomposite microspheres

Figure 4(a,b) shows the SEM pictures of purePMMA and CaCO3/PMMA composite microspheres.Both the pure PMMA and CaCO3/PMMA compositemicrosphere particles were uniform. The diameterswere about 200 and 350 nm, respectively. The mor-phology of CaCO3/PMMA was not smooth as thatof PMMA. In addition, CaCO3 nanoparticles couldnot be observed in the CaCO3/PMMA compositesamples anywhere. The result allowed us to assumethat CaCO3 nanoparticles were located in the PMMApolymer microspheres and made the polymer par-ticles become larger. Shim et al.18 prepared zincoxide/PMMA composite microspheres by in situ sus-pension polymerization. In their work, they studiedthe influence of the amount of ZnO on the morphol-ogy of the composites and proved that ZnO nano-particles were in the PMMA polymer spheres. Asimilar conclusion also could have been obtained forthe CaCO3/PMMA composite microspheres; how-ever, the morphology of the CaCO3/PMMA wasuniform and was unchanged when the concentrationof CaCO3 nanoparticles increased up to 30 wt %.

Figure 4(c) shows the TEM pictures of the CaCO3/PMMA composite microspheres (sample 5) takenfrom the emulsion system. The size of the polymerspheres was in good agreement with the results ofthe SEM pictures. The CaCO3 nanoparticles wereencapsulated or embedded in the PMMA matrix. Inaddition, the CaCO3 nanoparticles were apt to dis-perse at the edge of the PMMA polymer spheres.Therefore, a possible mechanism for the formation ofthe CaCO3/PMMA composite microspheres in asoapless emulsion polymerization process could beproposed as follows.

For the soapless emulsion polymerization of MMAwith APS as the initiator, the particle generation wascertified to follow the mechanism of homogeneousnucleation. In the aqueous phase, the APS initiatordecomposed and propagated with the monomer toform oligomers.

Because of the interfacial compatibility of the OA-modified CaCO3 nanoparticles and the monomers,CaCO3 nanoparticles could be adsorbed around theoligomers during the stage of MMA growing from

the monomers to the MMA oligomers. When theoligomers reached their critical chain length, thecomposites precipitated from the aqueous phase andthen formed as primary particles, and the CaCO3

nanoparticles could be encapsulated in the primaryparticles. These primary particles were unstable dur-ing the reaction; the coagulation between the pri-mary particles proceeded strenuously at the begin-

Figure 4 (a) SEM micrograph of pure PMMA, (b) SEMmicrograph of CaCO3/PMMA composite microspheres,and (c) TEM photograph of CaCO3/PMMA compositemicrospheres.

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ning of the reaction until the polymer particlesreceived enough surface charge, which led the com-posites to become uniform spheres, as shown inScheme 1.

CONCLUSIONS

In this work, CaCO3/PMMA composite micro-spheres were synthesized by soapless emulsion poly-merization. This method was effective in increasingthe filling amount of CaCO3 in the composite micro-spheres. CaCO3 pretreated with OA was encapsu-lated in the PMMA organic matrix because of thegood compatibility between the nanofillers and the

polymeric matrix. The encapsulating structures arepossibly interesting for the generation of water-bornehybrid coatings, which show the ability of the poly-mer for high mechanical scratch resistance providedby inorganic nanoparticles.

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Scheme 1 Schematic illustration of the formation processof CaCO3/PMMA composite spheres. [Color scheme canbe viewed in the online issue, which is available atwww.interscience.wiley.com.]

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