Study on CaCO 3 /PMMA nanocomposite microspheres by soapless emulsion polymerization

Transcription

1 Available online at Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) Study on CaCO 3 /PMMA nanocomposite microspheres by soapless emulsion polymerization Xiaokun Ma, Bing Zhou, Yanhui Deng, Ye Sheng, Chengyu Wang, Yan Pan, Zichen Wang Institute of Chemistry, Jilin University, Changchun , PR China Received 13 December 2006; received in revised form 21 June 2007; accepted 27 June 2007 Available online 1 July 2007 Abstract A soapless emulsion polymerization method was applied to synthesize CaCO 3 /PMMA spherical composite with different loading of CaCO 3. CaCO 3 nanoparticles were pretreated with oleic acid after the carbonation process of Ca(OH) 2 slurry by CO 2, in order to improve the compatibility between the CaCO 3 particles and MMA monomer in emulsion system. The results of photon correlation spectroscopy (PCS) showed the particles size of composites were bigger than the pure PMMA. And the size increased with the increase of the content of CaCO 3 nanoparticles. TEM images showed that the morphology of the composite microspheres was uniform and CaCO 3 nanoparticles can be well encapsulated in the polymeric microsphere, and were located at the edge of the spheres. The results of DTG and TG indicated that the CaCO 3 nanoparticles could improve the thermal stability of PMMA. Moreover, capsulation of CaCO 3 by PMMA can increase the acid-resistant of CaCO 3 nanofillers Elsevier B.V. All rights reserved. Keywords: Methyl methacrylate (MMA); Calcium carbonate; Composite microspheres; Thermal stability; Acid-resistant 1. Introduction It is a new challenge in nanotechnology that the possibility of combining properties of organic and inorganic components in a unique composite material. In the past years, many kinds of composites have been considered as innovative advanced materials, and promising applications have been expected in many fields, including optics, electronics, ionics, mechanics, membranes, protective coatings, catalysis, sensors, biology, and others [1 7]. Several methods have been used to produce polymer composite microspheres, such as miniemulsion polymerization [8], intercalative polymerization [9], emulsion polymerization [10], hybrid latex polymerization [11] and so on. However, more versatile synthetic approaches are needed to find an effective solution to the advantage of dispersing of inorganic particle in polymer matrix. In other words, the key factors for the preparation of functional composite microspheres are to promote a strong interface adhesion between the matrix and nanofillers. Poly (methyl methacrylate), PMMA, is an important commercial plastic and it is odorless, tasteless, and nontoxic, so it can be used in many fields, Such as in aircraft glazing, signs, Corresponding author. Tel.: ; fax: address: wangzc@mail.jlu.edu.cn (Z. Wang). lighting, dentures, food-handling equipment, contact lenses and so on [12,13]. Unfortunately, PMMA has the poor abrasion resistance and thermal stability [14,15], which is one of the reasons for its limited use in some fields. In this work, calcium carbonate (CaCO 3 ) nanoparticles were pretreated with oleic acid, in order to improve the compatibility between nanofillers and MMA monomer. The result of TEM characterization can provide sufficient details about the compatibility between interface of the CaCO 3 nanoparticles and PMMA matrix. CaCO 3 /PMMA composite microspheres were obtained in soapless emulsion polymerization process because this method can provide advantages for the synthesis of monodisperse latex. The morphology of composite microspheres is uniform and the encapsulating ratios of CaCO 3 in the composites can be modulated. The composite microspheres not only improved the thermal stability of PMMA but also increased the acid-resistant of CaCO 3 nanofillers. 2. Experimental 2.1. Materials Calcium oxide (CaO) was of reagent grade, provided by Changzhou Menghe Chemicals factory. Methyl methacrylate /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.colsurfa

2 X. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) (MMA) was analytical reagent and not distilled. Ammonium persulfate (APS) were used without further purification as an initiator. Oleic acid (OA) was analytical reagent and conserved at 4 C. Distilled and deionized water was used throughout the work Preparation of CaCO 3 latex Calcium oxide (CaO) was put into boiling deionized water, and then saturated Ca(OH) 2 slurry was diluted to 0.4 M after impurity deposition was removed. A mixed gas of carbon dioxide (CO 2 ) and nitrogen (N 2 ) with a molar ratio 1:3 was introduced into the slurry until ph of the solution was reached to 7 to prepare nanometer CaCO 3 latex. And the temperature was an important factor for the preparation of CaCO 3 nanoparticles. In this work, the temperature was controlled under 15 C which made the distribution of CaCO 3 nanoparticles narrower and the mean size of CaCO 3 particles was around 40 nm. Some ethanol was added in oleic acid (OA) in order to improve the solubility of oleic acid in water. The volume ratio of oleic acid to ethanol was 1:1. Then oleic acid ethanol solution was added dropwise into the flask at 60 C under continuous stirring for 1 h [16 18]. In addition, the amount of oleic acid was 2 wt% to CaCO 3 nanoparticles. The modified CaCO 3 nanoparticles were hydrophobic and the active ratio could reach to 99 wt% Preparation of CaCO 3 /PMMA nanocomposite microspheres A 500 ml four-necked flask equipped with thermometer, mechanical stirrer, reflux condenser and N 2 inlet, was charged with deionized water and OA-modified CaCO 3 latex. APS, as an initiator, and MMA were added dropwise into the flask at 60 C, the amounts of reactants were listed in Table 1. The mixture had been maintained at 60 C for 0.5 h and then at 80 C for 4 h. The product was collected by suction filtration and then dried at 90 C for 6 h. Those CaCO 3 nanoparticles unencapsulated in the PMMA can be eliminated from the composites after dipping into 0.01 M HCl for 1 h, and in contrast with the quantities of composites, the encapsulated ratio of CaCO 3 nanofiller can be calculated by TGA Characterization FT-IR was recorded by a Shimadzu FTIR-8400S that employs a KBr pellet method. A Melttler Toledo 825 e instrument was Fig. 1. IR spectrum of: (a) CaCO 3 before emulsion polymerization; (b) CaCO 3 /PMMA composite microspheres; (c) pure PMMA; (d) the composites after calcined. applied to analyze the filler content and the thermal stability of the composites. The samples were heated from 50 to 650 Cat20 C/min in an air atmosphere. The composite particles size distribution was analyzed by 3000 HSA analyzer (Malvern) photon correlation spectroscopy (PCS). The morphology of CaCO 3 /PMMA composite microspheres was investigated through TEM (transmission electron microscopy) obtained from HITACHI H-8100 electron microscope. 3. Results and discussion 3.1. FT-IR spectra of the CaCO 3 /PMMA composites The components of CaCO 3 /PMMA composite microspheres can be determined by the investigation of FT-IR. Curves (a d) in Fig. 1 showed FT-IR spectra of OA-modified CaCO 3 nanoparticles, CaCO 3 /PMMA composites, pure PMMA and composites calcined at 550 C for 4 h, respectively. In Fig. 1, curve (a) showed that the stretching vibration of the C H at 2847, 2914 cm 1 came from the CH 2 and CH 3 in the oleic acid, respectively. Here, the curve (a) was obtained after the redundant oleic acid was eliminated by rinsing three times with hot ethanol. So the result indicated that there exists the interaction between oleic acid and CaCO 3 nanoparticles, not the single physical absorption. Compared with pure PMMA (c) and OAmodified CaCO 3 (a), the typical bands of PMMA are found at 1728 and 1140, 1189, 1237, and 1274 cm 1 in the IR spectrum Table 1 The different quantities of CaCO 3 in soapless emulsion process Sample MMA (ml) CaCO 3 content (g) APS (g) H 2 O (ml) Yield (%) Encapsulating ratio (%)

3 192 X. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) Fig. 2. The particle size distribution of: (a) pure PMMA; (b) CaCO 3 /PMMA composite 2; (c) CaCO 3 /PMMA composite 3; (d) CaCO 3 /PMMA composite 4; (e) CaCO 3 /PMMA composite 5. of the composites (b). In addition, the specific adsorption peak of pure CaCO 3 at 1442 cm 1 was broadened due to the interaction between PMMA and CaCO 3 [19]. From curve (d) in Fig. 1, IR spectrum of calcined composites was well coincident with that of CaCO 3, which has provided the proof of the existence of CaCO 3 in PMMA Morphology and size distribution of the CaCO 3 /PMMA composites PCS method is based on the Brownian motion of particle in a fluid and fit the data with Stokes Einstein equation. Fig. 2 showed the particle size distribution of pure PMMA and some CaCO 3 /PMMA composites list in Table 1. All of the samples were dispersed evenly into deionized water to prepare suspension after sonification for 3 min. The average particle size of pure PMMA prepared under the same condition was nm as shown in Fig. 2(a). Curves (b e) in Fig. 2 showed the particle size distributions of CaCO 3 /PMMA composites 2 5, and the average particle size were 300.2, 323.3, 339.4, nm, respectively. The narrow size distribution indicated that all the samples prepared using this method could obtain the uniform polymer microspheres. However, the size increased with the increase of the content of CaCO 3 nanoparticles. This change showed the more CaCO 3 nanoparticles added the more could be encapsulated in the PMMA matrix. The morphology of OA-modified CaCO 3 nanopaticles and CaCO 3 /PMMA composite microspheres (sample 5) were characterized by TEM. As shown in Fig. 3(a), the morphology of CaCO 3 nanopaticles was cubic, and the mean size of CaCO 3 nanopaticles was around 40 nm. Fig. 3(b) showed the TEM pictures of the CaCO 3 /PMMA composite microspheres (sample 5) taken from the emulsion system. The size of the polymer spheres was about 350 nm and CaCO 3 nanoparticles can be well encapsulated in the polymeric matrix, and most of the CaCO 3 nanoparticles were located at the edges of the spheres. The size of composite microsphere could well coincided with the date came from the PCS. So the possible mechanism for the formation of the CaCO 3 /PMMA composite spheres in a soapless emulsion polymerization process can be proposed as follows. For the soapless emulsion polymerization of MMA with APS as initiator, the generation of the latex particles was certified to follow the mechanism of homogeneous nucleation. Due to the interfacial compatibility of OA-modified CaCO 3 nanopaticles with the monomers, CaCO 3 nanopaticles can be adsorbed around the oligomers during the stage of MMA growing from monomers to MMA oligomers. When the oligomers reached their critical chain length, the composites precipitated from the aqueous phase and then formed as primary particles, and the CaCO 3 nanopaticles can be encapsulated in the primary particles TGA of the composite microspheres Fig. 4 showed TGA plots of CaCO 3, pure PMMA and CaCO 3 /PMMA composites with the various loadings of CaCO 3. Fig. 3. TEM photographs of: (a) CaCO 3 nanopaticles; (b) CaCO 3 /PMMA composite microspheres.

4 X. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) Fig. 4. TGA plots of CaCO 3 pretreated with oleic acid, pure PMMA and CaCO 3 /PMMA composites. (a) OA-modified CaCO 3 ; (b) composite 5; (C) composite 4; (d) composite 3; (e) composite 2; (f) pure PMMA. The unencapsulated CaCO 3 was removed by 0.01 M HCl solution before testing, so the amounts of CaCO 3 in the composites can be regarded as the encapsulating ratios. Fig. 4(a) showed that the decomposition temperature of CaCO 3 is up to 600 C, and the weight loss was 1.8 wt%, which was derived from the removal of the decomposition of oleic acid on the particles surface. The value is in good agreement with the ratio of the initial reactant. The pure PMMA synthesized under the same polymerization condition decomposed from 260 to 400 C, as can be seen in Fig. 4(f). The weight loss between 260 and 600 Cin the composites can be used to calculate the amount of CaCO 3 in the composites. The curves (b e) in Fig. 4 showed the amounts of CaCO 3 in the composites were 29.1, 23.2, 18.2 and 7.2 wt%, respectively. It was worth pointing out that the further increase of filler decreases both yield and encapsulating ratio as the sample 6 listed in Table 1. This is likely due to the more coagulation occurred when the more CaCO 3 nanoparticles were participated in the polymerization process, and the coagulation decreased the stability of latex. Therefore, such a soapless emulsion polymerization process was effective to increase the filling amount of CaCO 3 in the composites below 30 wt%. Fig. 5. DTG curves of PMMA and composite microspheres (a) DTG of pure PMMA; (b) DTG of composite 2; (c) DTG of composite 4; (d) DTG of composite 5. the pure PMMA, which indicated that the thermal stability of the polymer matrix was improved by the addition of CaCO 3.In addition, the result could be proved by the TG analysis as mentioned above. The curves of the composites had the obvious shift to the higher temperature comparing with the curve of pure PMMA Acid-resistant of nanocaco 3 in CaCO 3 /PMMA composites The acid-resistant of CaCO 3 nanoparticles in the CaCO 3 /PMMA composite microspheres can be analyzed by TGA, when the same sample was dipped in different concentration HCl solution for 12 h. The composite 5 was dipped in 10 5, 10 4, M HCl, respectively, as shown the curve in Fig. 6(b e). Clearly, the weight loss of CaCO 3 nanoparticles was not obvious till the ph value down 3.4. Thermal stability of PMMA in composite microspheres Thermo-gravimetric analysis of the CaCO 3 /PMMA composite microspheres can give out the information of the content of CaCO 3 nanoparticles, while the curve of DTG can clearly exhibit the variation ratio of weight to the time (dw/dt) as a function of temperature. The peaks of DTA can be exactly indicated the temperature at the maximal reactive velocity. The DTG curves of pure PMMA, composites 2, 4 and 5 listed in Table 1 were showed in Fig. 5(a d), respectively. The pure PMMA reach the maximal reactive velocity at 278 C and the value was the lowest in all the curves. The temperature at the maximal reactive velocity increased with the increasing of the CaCO 3 nanoparticles, as shown in Fig. 5, the temperature increased to 349, 354 and 362 C, respectively. It was worth mentioning that the temperature of the composite 5 increased by about 84 C compared to Fig. 6. TGA of sample 5 and the composite dipped in different HCl solution for12 h. (a) Composite 5; (b) composite 5 dipped in 10 5 M HCl for 12 h; (c) composite 5 dipped in 10 4 M HCl for 12 h; (d) composite 5 dipped in 10 3 M HCl for 12 h; (e) composite 5 dipped in 10 2 M HCl for 12 h.

5 194 X. Ma et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) to 2. Therefore, it was indicated that the nanoparticles could be protected against decomposition in the subacidity solution because of the encapsulation by the PMMA matrix. 4. Conclusion In this work, CaCO 3 /PMMA composites microspheres were synthesized by soapless emulsion polymerization. The CaCO 3 pretreated with oleic acid were encapsulated in the PMMA organic matrix due to the good compatibility between the nanofillers and the polymeric matrix. And this method was effective to increase the loadings of CaCO 3 in the composites, which can improve the thermal stability of PMMA and the acid-resistant of CaCO 3 nanoparticles in composites. References [1] W. He, C. Pan, T. Lu, Soapless emulsion polymerization of butyl methacrylate through microwave heating, J. Appl. Polym. Sci. 80 (2001) [2] Z. Huang, Z. Lin, Z. Cai, K. Mai, Physical and mechanical properties of nano-caco 3 /PP composites modified with acrylic acid, Plast. Rubbers Comp. 33 (2004) 343. [3] Y. Lu, J. McLellan, Y. Xia, Synthesis and crystallization of hybrid spherical colloids composed of polystyrene cores and silica shells, Langmuir 20 (2004) [4] M. Chen, S. Zhou, B. You, L. Wu, A novel preparation method of raspberrylike PMMA/SiO 2 hybrid microspheres, Macromolecules 38 (2005) [5] Z. Li, Y. Zhu, Surface-modification of SiO 2 nanoparticles with oleic acid, Appl. Surf. Sci. 211 (2003) 315. [6] M. Avella, M.E. Errico, S. Martelli, E. Martuscelli, Preparation methodologies of polymer matrix nanocomposites, Appl. Organomet. Chem. 15 (2001) 435. [7] W. Wu, T. He, J. Chen, X. Zhang, Y. Chen, Study on in situ preparation of nano calcium carbonate/pmma composite particles, Mater. Lett. 60 (2006) [8] F. Tiarks, K. Landfester, M. Antonietti, Silica nanoparticles as surfactants and fillers for latexes made by miniemulsion polymerization, Langmuir 17 (2001) [9] Y. Yang, X. Kong, C. Kan, C. Sun, Encapsulation of calcium carbonate by styrene polymerization, Polym. Adv. Technol. 10 (1999) 54. [10] P. Liu, W.M. Liu, Q.J. Xue, In situ radical transfer addition polymerization of styrene from silica nanoparticles, Eur. Polym. J. 40 (2004) 267. [11] I. Tissot, C. Novat, F. Lefebvre, E. Bourgeat-Lami, Hybrid latex particles coated with silica, Macromolecules 34 (2001) [12] M. Avella, M. Errico, E. Martuscelli, Novel PMMA/CaCO 3 nanocomposites abrasion resistant prepared by an in situ polymerization process, Nano Lett. 1 (2001) 213. [13] J. Shim, J. Kim, S. Han, I. Chang, H. Kim, H. Kang, O. Lee, K. Suh, Zinc oxide/polymethylmethacrylate composite microspheres by in situ suspension polymerization and their morphological study, Colloid Surf. A 207 (2002) 105. [14] X. Xie, Q. Liu, R. Li, X. Zhou, Q. Zhang, Z. Yu, Y. Mai, Rheological and mechanical properties of PVC/CaCO 3 nanocomposites prepared by in situ polymerization, Polymer 45 (2004) [15] C.J. Renger, S.J. Burrows, R.A. Shanks, Processing, crystallization, and dynamic mechanical analysis of high molar mass polysiloxane-modified PP/CaCO 3 composites, J. Appl. Polym. Sci. 82 (2001) [16] Y. Sh, B. Zhou, C. Wang, X. Zhao, Y. Deng, Z. Wang, In situ preparation of hydrophobic CaCO 3 in the presence of sodium oleate, Appl. Surf. Sci. 253 (2006) [17] C. Wang, Y. Sheng, X. Zhao, Y. Pan, H. Bala, Z. Wang, Synthesis of hydrophobic CaCO 3 nanoparticles, Mater. Lett. 60 (2006) 854. [18] C. Wang, Y. Sheng, H. Bala, X. Zhao, J. Zhao, X. Ma, Z. Wang, A novel aqueous-phase route to synthesize hydrophobic CaCO 3 particles in situ, Mater. Sci. Eng. C 27 (2007) 42. [19] Z. Li, J. Zhang, J. Du, T. Mu, Z. Liu, J. Chen, B. Han, Preparation of cadmium sulfide/poly(methyl methacrylate) composites by precipitation with compressed CO 2, J. Appl. Polym. Sci. 94 (2004) 1643.