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poly(methyl methacrylate) core–shell nanoparticles
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Novel synthesis with an atomized microemulsion technique and characterization of nano-calcium carbonate (CaCO3 )/poly(methyl methacrylate) core–shell nanoparticles Aniruddha Chatterjee, Satyendra Mishra ∗ University Institute of Chemical Technology, North Maharashtra University, Jalgaon 425 001, Maharashtra, India
a r t i c l e
i n f o
Article history: Received 1 September 2012 Received in revised form 3 November 2012 Accepted 22 November 2012 Keywords: Atomized microemulsion Core–shell nanoparticles Thermal properties Compatibility of core–shell nanoparticles with polymer matrix
a b s t r a c t The synthesis of hard-core/soft-shell calcium carbonate (CaCO3 )/poly(methyl methacrylate) (PMMA) hybrid structured nanoparticles (<100 nm) by an atomized microemulsion polymerization process is reported. The polymer chains were anchored onto the surface of nano-CaCO3 through use of a coupling agent, triethoxyvinyl silane (TEVS). Ammonium persulfate (APS), sodium dodecyl sulfate (SDS) and n-pentanol were used as the initiator, surfactant and cosurfactant, respectively. The polymerization mechanism of the core–shell latex particles is discussed. The encapsulation of nano-CaCO3 by PMMA was confirmed using a transmission electron microscope (TEM). The grafting percentage of the core–shell particles was investigated by thermogravimetric analysis (TGA). The nano-CaCO3 /PMMA core–shell particles were characterized by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The FTIR results revealed the existence of a strong interaction at the interface of the nano-CaCO3 particle and the PMMA, which implies that the polymer chains were successfully grafted onto the surface of the nano-CaCO3 particles through the link of the coupling agent. In addition, the TGA and DSC results indicated an enhancement of the thermal stability of the core–shell materials compared with that of the pure nano-PMMA. The nano-CaCO3 /PMMA particles were blended into a polypropylene (PP) matrix by melt processing. It can also be observed using scanning electron microscopy (SEM) that the PMMA chains grafted onto the CaCO3 nanoparticles interfere with the aggregation of CaCO3 in the polymer matrix (PP matrix) and thus improve the compatibility of the CaCO3 nanoparticles with the PP matrix. © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, considerable effort has been expended on the elaboration of core–shell nanoparticles (CSNPs). Microemulsion polymerization is a unique process for producing CSNPs of nanosize scale (Aguiar, Gonzalez-Villegas, Rabelero, Mendizábal, & Puig, 1999). CSNP core/shell composite materials consist of a core structural domain covered by a shell domain (Fleming, Mandal, & Walt, 2001), which may be composed of a variety of materials including polymers, inorganic solids, and metals (Wang, Liu, Ohnuma, Kainuma, & Ishida, 2002; Wang et al., 2007). Such materials have been widely used in some fields, such as paints (Li, Jones, Dunlap, Hua, & Zhao, 2006; Nishizawa, Nishimura,
∗ Corresponding author. Tel.: +91 257 2258420; fax: +91 257 2258403. E-mail address: [email protected] (S. Mishra).
Saitoh, Fujiki, & Tsubokawa, 2005), cosmetics (Chen, Rajh, Wang, & Thurnauer, 1997; Hong, Li, & Pan, 2007; Ringward & Pemberton, 2000), coatings (Nishizawa et al., 2005; Rajh, Tiede, & Thurnauer, 1996), adhesives (Dong et al., 2008), industrial electronic materials (Bawden & Turner, 1988), reinforcement of rubber/plastics (Kato, Uchida, Kang, Uyama, & Zkada, 2003) and biochemistry (Galperin & Margel, 2007), because the integrated properties of such materials are better than those of their single component counterparts (Hall, Davis, & Mann, 2000). The method generally used to synthesize core–shell particles is stepwise emulsion polymerization (Chen, Qian, & Zhang, 2008; Erdem, Sudol, Dimonie, & El-Aasser, 2000; Liu, Ye, Lin, & Zhou, 2008; Liu, 2006; Yang & Lu, 2005), in which latex core particles are produced in the first step, and the shell polymer is produced in the second step. In this method, the core particles act as “seed particles”, on the surface of which the shell polymers are supposed to be dispersed. During emulsion polymerization, the surfactant plays a major role by stabilizing
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http://dx.doi.org/10.1016/j.partic.2012.11.005
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the dispersion medium. In the conventional emulsion polymerization process (Ni et al., 2005; Zhu, Shi, Cai, Zhao, & Liao, 2008), the in situ emulsion polymerization process (Chen, Liu, Zhang, & Wang, 2007; Ding et al., 2004; Li et al., 2007; Liu et al., 2008), and the miniemulsion process (Antonietti & Landfester, 2002; Asua, 2002; Crespy & Landfester, 2009; Erdem et al., 2000; Rabelero et al., 2005; Topfer & Schmidt-Naake, 2007; Zyl, de Wet-Roos, Sanderson, & Klumperman, 2004), the percentage of surfactant is low, resulting in an insufficient number of micelles to provide sufficient protection for single inorganic particles. Thus, during polymerization, there is a high possibility of agglomeration of and instability in the dispersion, resulting in less encapsulation of the inorganic particles by the polymer matrix. Recently, we have synthesized polystyrene nanoparticles (nPS) and poly(methyl methacrylate) (nPMMA) particles by a modified microemulsion polymerization process (Mishra & Chatterjee, 2011a) as well as a novel atomized microemulsion process (Mishra & Chatterjee, 2011b), and the isolated nPS and nPMMA particles were blended with polypropylene (PP) (Chatterjee & Mishra, 2012; Mishra & Chatterjee, 2011b) and linear low-density polyethylene (LLDPE) (Mishra, Chatterjee, & Rana, 2011) to study their rheological, thermal and mechanical properties. In our previous work (Mishra, Chatterjee, & Singh, 2011), we have successfully prepared nano-CaCO3 /PS core–shell particles, with CaCO3 as a core and PS as a shell, by an atomized polymerization technique, and the isolated CSNPs were incorporated in a PP matrix. This present work is an extension of our previous work to develop an efficient process for the production of nanoCaCO3 /PMMA core–shell nanoparticles by the atomized microemulsion process. Furthermore, we have also focused on improving the performance of the (nano-CaCO3 /PMMA)/PP nanocomposites by enhancing the interfacial adhesion by adding a small amount (1 wt.%) of encapsulated nano-CaCO3 . Thus, this approach will improve the compatibility of the nano-CaCO3 with the polymer matrix through the lipophilic polymer layer (PMMA
layer) grafted onto the surface of the nano-CaCO3 . The polymer shell coatings not only prevent the aggregation of nanoparticles but also produce excellent compatibility between the filler particles and the polymer matrix (Shen, Lin, Li, & Nan, 2007). 2. Experimental 2.1. Materials Nano-CaCO3 particles with diameters in the range of 10–50 nm were synthesized by the solution spray process reported elsewhere (Kulkarni, Patil, Ghosh, & Mishra, 2009; Mishra, Kulkarni, Patil, & Ghosh, 2009). The nanoparticles were modified by the silane coupling agent, triethoxyvinyl silane (TEVS). The monomer, methyl methacrylate (MMA); the initiator, ammonium persulfate (APS); the surfactant, sodium dodecyl sulfate (SDS); and the cosurfactant, n-pentanol, were purchased from S.D. Fine Chemicals Ltd., Mumbai, India. MMA was treated with a 5% NaOH aqueous solution to remove the inhibitor and was distilled under reduced pressure in a nitrogen atmosphere prior to polymerization. All other materials were of analytical grade and were used without further purification. Water was double distilled and deionized. 2.2. Surface modification of the nano-CaCO3 TEVS was used for the modification of the nano-CaCO3 particles. First, the desired amount of silane coupling agent, i.e., TEVS (5 phr with respect to weight of CaCO3 ) was dissolved in acetone, and 20 g of nano-CaCO3 particles was dispersed in the silane-acetone mixture under mechanical stirring at 500 rpm. The mixture was then sonicated for 1 h and stirred at 500 rpm at room temperature for 2 h. The TEVS and CaCO3 reacted in the dispersion overnight. Afterwards, the surface modified nano-CaCO3 particles were collected by filtration and rinsed four times with acetone, and the filter cake was dried at 100 ◦ C under low vacuum for 12 h.
Fig. 1. Atomized reaction vessel for synthesis of nano-CaCO3 /PMMA core–shell particles via the atomized microemulsion technique.
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2.3. Preparation of the nano-CaCO3 /PMMA core–shell nanoparticles Stable dispersions of the spherical nano-CaCO3 /PMMA core–shell nanoparticles (CSNPs) with diameters < 100 nm were synthesized by an oil/water (o/w) atomized microemulsion process (Fig. 1). The detailed procedure is as follows: 4 g of SDS was dissolved in 290 g of deionized water to form a clear solution, and 8 g of surface modified CaCO3 nanoparticles was dispersed in one part (40 g) of the clear SDS/water solution and sonicated for 30 min. The remaining 250 g of clear solution was poured into a 1 L stainless steel reactor with an agitator under constant stirring at 200 rpm, and 0.45 g of APS dissolved in 10 g of water was added to the reactor. The reactor was sealed, and the temperature was raised to 60 ◦ C to form free radicals. Then, 20 g of stabilizer-free MMA and 8 g of the dispersed solution of CaCO3 nanoparticles were sprayed into the reactor at the rate of 0.02 and 0.09 g/s, respectively, in the form of fine mist through the atomizers (nozzles) by a small, low pressure, metering pump under a nitrogen atmosphere purged through a nitrogen inlet. The streams of monomer and inorganic particles were sprayed at a specific angle so that they made contact prior to dispersing over the entire surface of the clear solution, where a unique microemulsion environment proceeds. The rate at which the liquid is atomized depends, within limits, solely on the volume that is being delivered on to the atomizing surface and the pump frequency. Typically, the higher the frequency was, the lower the processing capability was. Baffles were maintained at the top of the reactor to bounce the MMA and nano-CaCO3 streams back from the outgoing air. The exhaust then passed through a distillation column to recover the MMA and the nano-CaCO3 . Throughout the reaction, a temperature of 60 ◦ C and constant agitation were maintained. After the complete atomization of both particulates, the reaction continued for 1.5 h, and the polymerization reaction was stopped by cooling the dispersion to room temperature. A translucent dispersion was formed after the complete polymerization reaction. A butterfly valve, located at the bottom of the reactor dish, was fitted to discharge the reaction mixture. The process was regulated through control of the orifice size, the low pressure, the speed, the reaction temperature and the distance between the atomizer and the reaction zone. 2.4. Analysis of the grafting percentage of CSNPs The microemulsion reaction gave rise to the formation of CSNPs as well as free nano-CaCO3 and PMMA nanoparticles. To determine the grafting percentage, it is necessary to separate the CSNPs from the free nano-CaCO3 and PMMA nanoparticles (Tang, Liu, Sun, Zheng, & Cheng, 2007). In a typical analysis, the original latex-containing CSNPs, the free nano-CaCO3 and the PMMA nanoparticles were dried in an air oven at 120 ◦ C for 2 h. The same amount of latex was isolated in acetone under constant stirring. The core–shell and PMMA nanoparticles settled to form a residue, and the free nano-CaCO3 particles remained dispersed. The supernatant solution with the free nano-CaCO3 particles was carefully separated from the residue and centrifuged at 10,000 rpm to obtain the free nano-CaCO3 particles. Wt. of dry latex sample or Wt. of free nano-CaCO3 + PMMA nanoparticles + CSNPs = P, Wt. of centrifuged free nano-CaCO3 = Q, % free nano-CaCO3 = Q/P × 100%.
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The core–shell and polymer nanoparticle residue was washed 3 times with a deionized water/acetone mixture (50/50), vacuum filtered and vacuum dried at 50 ◦ C to obtain a powder. The CSNPs were then separated from the free PMMA nanoparticles by Soxhlet extraction with acetone for 12 h (Sheng et al., 2006). Wt. of CSNPs + free PMMA nanoparticles = R, Wt. of CSNPs after Soxhlet extraction = S, % free PMMA nanoparticles = R − S/R × 100%, % CSNPs = S/R × 100%. The content of the shell polymer on the nano-CaCO3 core was determined by thermogravimetric analysis (TGA). First, the free nano-CaCO3 and PMMA nanoparticles were separated from the CSNPs by the above method. Then, 10–12 mg of sample was heated from room temperature to 600 ◦ C at the rate of 10 ◦ C/min under a nitrogen atmosphere at a flow rate of 50 mL/min. The PMMA shells totally decomposed at 600 ◦ C, and the nano-CaCO3 cores remained as a residue. Wt. of CSNPs before TGA = T, Wt. of residue after TGA = U, % grafted nano-CaCO3 = U/T × 100%, % shell polymer = T − U/T × 100%. 2.5. Stability of the nano-CaCO3 /PMMA core–shell nano-suspensions The stability of CSNPs was compared to that of bare CaCO3 by the sedimentation test (Mishra, Chatterjee, & Singh, 2011; Tang, Cheng, & Ma, 2006). The method is as follows: the CSNPs were separated from the sample by the Soxhlet extraction method (see Section 2.4). After separation, 2 g of CSNPs dispersed in 100 mL acetone was poured into a test tube with a ruler and a cover. The test tube was allowed to stand at room temperature. After a definite time interval, the depth of the sediment from the surface of the suspension was recorded. The smaller the depth from the surface of suspension, the greater the stability of the CSNPs in acetone. The sedimentation percentage of the CSNPs dispersed in acetone was determined by the following equation: X=
H × 100 H0
where X is the sedimentation percentage, H (cm) is the depth of the sediment from the surface of the suspension, and H0 (cm) is the total depth of the suspension (Mishra, Chatterjee, & Singh, 2011). 2.6. Other characterization and measurements The particle size and morphology of the CSNPs were measured and confirmed using a transmission electron microscope (TEM), PHILIPS CM200, Eindhoven, The Netherlands, with 75 A of filament current and 200 kV of accelerating voltage. The particle diameters were measured directly from the TEM. FTIR spectra were obtained using a Fourier transform infrared spectrophotometer (FTIR-8000, Shimadzu, Tokyo, Japan). FTIR was used to characterize the functional groups of the nano-CaCO3 , the surface modified CaCO3 and the nano-CaCO3 /PMMA core–shell nanoparticles. The number of scans per sample was 25, and the resolution of the measurements was 4 cm−1 . The recorded wavenumber range was 4000–500 cm−1 . All samples were in powdered form and were measured at room temperature. DSC was carried out to investigate the glass transition temperature (Tg ) of the CSNPs using a differential scanning calorimeter
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Fig. 2. TEM micrographs of the nanoparticles sampled from the atomized microemulsion process: (a) bare nano-CaCO3 , (b) modified nano-CaCO3 /PMMA (CaCO3 /MMA = 8/20 g), and (c) pure nPMMA (MMA = 22.5 g).
(DSC-60, Shimadzu, Tokyo, Japan). The temperature was programmed from 30 ◦ C to 300 ◦ C at a heating rate of 10 ◦ C/min under a nitrogen atmosphere (50 mL/min). The thermal stabilities of the nano-CaCO3 , the nano-PMMA and the CSNPs were determined by a thermogravimetric analyzer (TGA50, Shimadzu, Tokyo, Japan); 10 mg of sample was placed on a Pt pan for TGA measurement. The temperature was programmed from 30 ◦ C to 600 ◦ C at a heating rate of 10 ◦ C/min under nitrogen atmosphere to avoid thermoxidative degradation.
3. Results and discussion 3.1. Morphology of the nano-CaCO3 /PMMA core–shell nanoparticles The TEM images of bare nano-CaCO3 , modified nanoCaCO3 /PMMA and pure nPMMA are shown in Fig. 2. It is clear from Fig. 2(a) that the bare nano-CaCO3 particles were suspended without encapsulation and that the grafting percentage was much lower. This effect was due to the hydrophilic nature of the bare nano-CaCO3 . The hydrophilicity did not allow monomer absorption during atomization (see Section 3.2 for details) and led to
incompatibility with the hydrophobic polymer shell. Therefore, in this case, the percentage of free polymer particles as well as free nano-CaCO3 was higher. In contrast, the core–shell particles shown in Fig. 2(b) have a spherical morphology with good monodispersity. The dark phases and translucent phases of single spheres in the micrographs represent the nano-CaCO3 and the PMMA polymer, respectively (Tang et al., 2006). It was obvious that the nano-CaCO3 particles were encapsulated by the polymer shell. In addition, due to the atomization of MMA, it was also very difficult to prevent the formation of free polymer particles during the grafting process. If Fig. 2(b) is compared with Fig. 2(c) (pure nPMMA), then some particles are observed without the CaCO3 core; this lack of CaCO3 core implied that some MMA were used for the nucleation of nPMMA to form free polymer particles.
3.2. Mechanism of nano-CaCO3 /PMMA core–shell nanoparticle synthesis by atomized microemulsion polymerization Generally, nano-CaCO3 has the tendency to agglomerate because of its high surface area and energy. Thus, it is difficult to achieve uniform dispersion of nano-CaCO3 in the polymer matrix. Additionally, due to the high density of nano-CaCO3 , they either
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Scheme 1. Reaction mechanism for the adsorption of TEVS onto nano-CaCO3 particles and the formation of nano-CaCO3 /PMMA core–shell particles via the atomized microemulsion technique.
settle or form aggregates during polymerization. Surface modification and ultrasonication are the best ways to reduce the surface energy of nano-CaCO3 , to increase its compatibility with the polymer matrix and to increase the dispersion homogeneity. For perfect encapsulation of the nano-CaCO3 particles, the surface of the nanoCaCO3 must be hydrophobic enough to have a close affinity for MMA (Tang et al., 2006, 2007). This depends on the composition of the functional groups on the nano-CaCO3 surface. Therefore, the CaCO3 nanoparticles were treated by TEVS, a reactive silane coupling agent, to functionalize the nano-CaCO3 surface. The reaction can proceed between each Si OC2 H5 group and the hydroxyl groups on the surface of the nanoparticles (Scheme 1 shows only one Si OC2 H5 group hydrolyzed by one hydroxyl group). As a result, the TEVS molecules were grafted onto the nano-CaCO3 surface. In addition, TEVS contains an unsaturated double bond, which can copolymerize with the unsaturated groups of MMA to covalently graft to at least part of the polymer during the polymerization. Consequently, TEVS links the polymer and the nano-CaCO3 particles through chemical bonds, and the link results in enhancement of the interfacial adhesion and compatibility between the inorganic nano-CaCO3 and the polymer matrix (Tang et al., 2007; Demjen, Pukanszky, Foldes, & Nagy, 1997). The organic polymer chains of TEVS induce steric hindrance to the associations between nano-CaCO3 particles and prevent aggregation of the particles. The organic molecules of TEVS on the nano-CaCO3 particle surface have an affinity to MMA. Therefore, during atomization, the nano-CaCO3 makes contact with the MMA stream, and a monomer layer is formed on the surface of the nanoCaCO3 prior to dispersion on the surface of the solution. Because of the TEVS modification, the MMA shell was formed at the first, or primary, stage of the reaction, and the reaction time was reduced. Next, the MMA-layered nano-CaCO3 particles were dispersed uniformly in the water solution, where the maximum concentration of micelles [above the ‘critical micelle concentration’ (CMC)] was present to stabilize the dispersion at a controlled temperature. The radicals in the water phase were then adsorbed onto the surface layer formed by the MMA. As a result, the polymerization locus was transferred from the aqueous phase to the surface of the nanoCaCO3 . This reaction site was favored because of the larger surface area of the nano-CaCO3 particles and the easier adsorption of MMA on the particle surface. The main chain propagating reaction was carried out at the surface of the nano-CaCO3 particle on which the encapsulating PMMA shell was formed (Scheme 1).
peak at 1642 cm−1 indicated the presence of C C unsaturation in the TEVS attached to the nano-CaCO3 surface. In general, the broad absorption bands in the range between 1100 and 1050 cm−1 were the characteristic bands of Si O C in TEVS (Posthumus, Magusin, Brokken-Zijp, Tinnemans, & van der Linde, 2004); whereas in Fig. 3(b), the peaks at 1100 and 1087 cm−1 were indicative of the formation of Si O CaCO3 bonds on nano-CaCO3 . In Fig. 3(c), the nano-CaCO3 /PMMA core–shell nanoparticles were characterized by the C H stretching bands of the CH2 group at 2850 and 2992 cm−1 , indicating the presence of the CH2 group in the backbone. The formation of nPMMA was confirmed by the IR spectrum, shown in Fig. 6(c), in which the absorptions at 1148 cm−1 ( C O) and 1731 cm−1 ( C O) indicated the existence of nPMMA (Mishra, Chatterjee, & Rana, 2011; Shi, Bao, Huang, & Weng, 2004). Two distinct peaks at 2949 and 2997 cm−1 indicated the asymmetrical and symmetrical stretching modes of C H in the methyl ( CH3 ) group. The symmetrical bending vibration (ıs-CH3 ) occurs near 1383 cm−1 , whereas the asymmetrical bending vibration (ıas-CH3 ) appears near 1450 cm−1 (Marimuthu & Madras, 2007; Tsai, Lin, Guo, & Chu, 2008). The disappearance of the peak at 1642 cm−1 implied that the silane coupling agent had covalently bonded with MMA through the unsaturated double bond
3.3. FTIR analysis of nano-CaCO3 /PMMA core–shell nanoparticles The FTIR absorption spectra of bare nano-CaCO3 particles, modified nano-CaCO3 particles and CSNPs are shown in Fig. 3. In the spectrum of the bare nano-CaCO3 particles, the broad absorption band at 3336 cm−1 (Fig. 3(a)) indicated the presence of OH on the CaCO3 surface (Hong, Qian, & Cao, 2006; Sheng et al., 2006). In the case of the modified nano-CaCO3 particles (Fig. 3(b)), the sharp
Fig. 3. FTIR spectra of (a) bare nano-CaCO3 , (b) modified nano-CaCO3 , and (c) nanoCaCO3 /PMMA core–shell particles.
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Fig. 5. TGA curves of (a) modified nano-CaCO3 , (b) nano-CaCO3 /PMMA core–shell particles, and (c) pure nPMMA.
Fig. 4. DSC curves of (a) pure nPMMA and (b) nano-CaCO3 /PMMA core–shell particles.
during polymerization. It can be inferred from the above results that the silane links the polymer and the nano-CaCO3 nanoparticles through chemical bonds. Consequently, the silane leads to the enhanced adhesion and compatibility between nano-CaCO3 and the polymer chains of PMMA. In addition, the downward shift of the absorption peaks from 1100–1087 cm−1 (Fig. 3(b)) to 1092–1080 cm−1 (Fig. 3(c)) could be attributed to the formation of (nano-CaCO3 ) O Si PMMA bonds resulting from the reaction with TEVS (Mishra, Chatterjee, & Singh, 2011). 3.4. Thermal analysis of nano-CaCO3 /PMMA core–shell nanoparticles The DSC thermograms of pure nPMMA and the extracted CSNPs are shown in Fig. 4. The DSC curve of the nPMMA particles (Fig. 4(a)) shows a lower glass transition temperature peak (Tg = 128 ◦ C) than that of the CSNPs (Tg = 136 ◦ C) (Fig. 4(b)) (Bershtein et al., 2009; Mishra, Chatterjee, & Rana, 2011). One possible reason is the strong interfacial interaction between the nano-CaCO3 core and the PMMA shell; the silane coupling cross linked the core and shell (Demjen et al., 1997; Hong et al., 2006; Tang et al., 2007) and grafted the PMMA chains onto the surface of the nanoparticles. Thus, the
mobility of the PMMA chains on the particle surface was restricted, and more energy was needed to transition from a glassy state to a rubbery state. Therefore, the glass transition temperature increased for the PMMA-grafted nano-CaCO3 (Laruelle, Parvole, Francois, & Billon, 2004). In comparison, Fig. 5 shows the TGA thermograms of the modified nano-CaCO3 , CSNPs and the pure nPMMA particles. Fig. 5(a) showed no change in the onset decomposition temperature (don ) of the modified nano-CaCO3 , and the weight loss percentage (WL ) was 1.2%, which was due to the decomposition of the silane coupling agent and the adsorbed water on the particles’ surfaces (Mishra, Chatterjee, & Singh, 2011; Sheng et al., 2006; Tang et al., 2007). In addition, a remarkable difference in the thermal behaviors of the nano-CaCO3 /PMMA particles and the pure nPMMA particles was observed (Fig. 5(b) and (c)). The nano-CaCO3 /PMMA (Fig. 5(b)) particles showed a higher thermal stability (don = 386 ◦ C) with lower weight loss (WL = 62%) than the pure nPMMA particles (don = 369 ◦ C with 100% WL ) (Mishra, Chatterjee, & Rana, 2011) (Fig. 5(c)); the different behaviors further supported the existence of a strong interaction between the nano-CaCO3 core and the PMMA shell. The weight loss of the CSNPs between 386 ◦ C and 600 ◦ C (Fig. 5(b)) was used to calculate the amount of grafted nano-CaCO3 in the CSNPs; thus, the extracted CSNPs showed 58% grafted nano-CaCO3 with 32% free nano-CaCO3 and 10% free nPMMA.
Fig. 6. SEM images of the PP nanocomposites with (a) 1 wt.% of CSNPs and (b) bare nano-CaCO3 .
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3.5. Dispersion uniformity of CSNPs in a PP matrix To investigate the dispersion of CSNPs in a polymer matrix, the CSNPs (1 wt.%) as well as the bare nano-CaCO3 particles were added to a PP matrix by melt processing in a Brabender Plastograph at 190 ◦ C for 10 min with a rotor speed of 60 rpm. The compounded composites were obtained in the form of lumps. These lumps were then crushed to obtain coarse particles (approximately 3–4 mm in diameter) of the composites and subjected to injection molding at a melt temperature of 210 ◦ C and a molding temperature of 30 ◦ C for further analysis (Mishra & Chatterjee, 2011a; Mishra, Chatterjee, & Rana, 2011; Mishra, Chatterjee, & Singh, 2011). The morphologies of the CSNPs and the nano-CaCO3 particles in the PP matrices were observed using SEM, as shown in Fig. 6. The white spots in the figure correspond to nano-CaCO3 particles. The encapsulating polymer merged with the PP matrix (Fig. 6(a)) during the thermal processing because of the compatibility of the encapsulating polymer and the matrix; thus, only the spots from the nano-CaCO3 particles can be observed. The nano-CaCO3 were found to be homogeneously dispersed in the matrix. Consequently, the nano-CaCO3 particles were integrated with the PP matrix by the grafting copolymer chains (Mishra, Chatterjee, & Singh, 2011; Tang et al., 2006), whereas particle aggregates were formed by the bare nano-CaCO3 , and these aggregates imply that the particles debonded from the matrix, as shown in Fig. 6(b) (Mishra, Chatterjee, & Singh, 2011). 4. Conclusions Nano-CaCO3 /PMMA core–shell particles with nano-CaCO3 as a core and PMMA as a shell were successfully synthesized by an atomized polymerization technique. The nanoparticles obtained have a perfect core–shell structure. The core–shell was formed via covalent bonding of PMMA to the nano-CaCO3 particles through the OC2 H5 group of TEVS at the core surface. The appearance of new peaks that exhibit shifts in the FTIR spectra confirms the interaction of PMMA and nano-CaCO3 in the core–shell particles. The increased thermal stability and the better Tg also suggest that a definite interaction must have occurred during polymerization. The reinforcement of CSNPs in the PP matrix results in a combination of the ‘stiffening’ effect provided by the better dispersion of the rigid nano-CaCO3 core and the ‘softening’ effect provided by the soft PMMA shell. Thus, the preparation of nanocomposites by reinforcement of CSNPs is an excellent method for obtaining well-dispersed nano-CaCO3 . This preparation opens the way to wider industrial utilization of CSNPs. Further studies on the relationship between the melt processing conditions and the dispersion of CSNPs are underway in our laboratory, as are detailed investigations of the rheological, thermal and mechanical properties. Acknowledgment The authors are thankful to the University Grants Commission (UGC), New Delhi for providing the financial support [project file No: 40-10/2011(SR), dated-July 14, 2011] to conduct this research. References Aguiar, A., Gonzalez-Villegas, S., Rabelero, M., Mendizábal, E., & Puig, J. E. (1999). Core–shell polymers with improved mechanical properties prepared by microemulsion polymerization. Macromolecules, 32, 6767–6771. Antonietti, M., & Landfester, K. (2002). Polyreactions in miniemulsions. Progress in Polymer Science, 27, 689–757. Asua, J. M. (2002). Miniemulsion polymerization. Progress in Polymer Science, 27, 1283–1346. Bawden, M. J., & Turner, S. R. (Eds.). (1988). Electronic and photonic applications of polymers. In Advances in chemistry series. Washington, DC: ACS.
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Novel synthesis with an atomized microemulsion technique and characterization of nano-calcium carbonate (CaCO3 )/poly(methyl methacrylate) core–shell nanoparticles Aniruddha Chatterjee, Satyendra Mishra ∗ University Institute of Chemical Technology, North Maharashtra University, Jalgaon 425 001, Maharashtra, India
a r t i c l e
i n f o
Article history: Received 1 September 2012 Received in revised form 3 November 2012 Accepted 22 November 2012 Keywords: Atomized microemulsion Core–shell nanoparticles Thermal properties Compatibility of core–shell nanoparticles with polymer matrix
a b s t r a c t The synthesis of hard-core/soft-shell calcium carbonate (CaCO3 )/poly(methyl methacrylate) (PMMA) hybrid structured nanoparticles (<100 nm) by an atomized microemulsion polymerization process is reported. The polymer chains were anchored onto the surface of nano-CaCO3 through use of a coupling agent, triethoxyvinyl silane (TEVS). Ammonium persulfate (APS), sodium dodecyl sulfate (SDS) and n-pentanol were used as the initiator, surfactant and cosurfactant, respectively. The polymerization mechanism of the core–shell latex particles is discussed. The encapsulation of nano-CaCO3 by PMMA was confirmed using a transmission electron microscope (TEM). The grafting percentage of the core–shell particles was investigated by thermogravimetric analysis (TGA). The nano-CaCO3 /PMMA core–shell particles were characterized by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The FTIR results revealed the existence of a strong interaction at the interface of the nano-CaCO3 particle and the PMMA, which implies that the polymer chains were successfully grafted onto the surface of the nano-CaCO3 particles through the link of the coupling agent. In addition, the TGA and DSC results indicated an enhancement of the thermal stability of the core–shell materials compared with that of the pure nano-PMMA. The nano-CaCO3 /PMMA particles were blended into a polypropylene (PP) matrix by melt processing. It can also be observed using scanning electron microscopy (SEM) that the PMMA chains grafted onto the CaCO3 nanoparticles interfere with the aggregation of CaCO3 in the polymer matrix (PP matrix) and thus improve the compatibility of the CaCO3 nanoparticles with the PP matrix. © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, considerable effort has been expended on the elaboration of core–shell nanoparticles (CSNPs). Microemulsion polymerization is a unique process for producing CSNPs of nanosize scale (Aguiar, Gonzalez-Villegas, Rabelero, Mendizábal, & Puig, 1999). CSNP core/shell composite materials consist of a core structural domain covered by a shell domain (Fleming, Mandal, & Walt, 2001), which may be composed of a variety of materials including polymers, inorganic solids, and metals (Wang, Liu, Ohnuma, Kainuma, & Ishida, 2002; Wang et al., 2007). Such materials have been widely used in some fields, such as paints (Li, Jones, Dunlap, Hua, & Zhao, 2006; Nishizawa, Nishimura,
∗ Corresponding author. Tel.: +91 257 2258420; fax: +91 257 2258403. E-mail address: [email protected] (S. Mishra).
Saitoh, Fujiki, & Tsubokawa, 2005), cosmetics (Chen, Rajh, Wang, & Thurnauer, 1997; Hong, Li, & Pan, 2007; Ringward & Pemberton, 2000), coatings (Nishizawa et al., 2005; Rajh, Tiede, & Thurnauer, 1996), adhesives (Dong et al., 2008), industrial electronic materials (Bawden & Turner, 1988), reinforcement of rubber/plastics (Kato, Uchida, Kang, Uyama, & Zkada, 2003) and biochemistry (Galperin & Margel, 2007), because the integrated properties of such materials are better than those of their single component counterparts (Hall, Davis, & Mann, 2000). The method generally used to synthesize core–shell particles is stepwise emulsion polymerization (Chen, Qian, & Zhang, 2008; Erdem, Sudol, Dimonie, & El-Aasser, 2000; Liu, Ye, Lin, & Zhou, 2008; Liu, 2006; Yang & Lu, 2005), in which latex core particles are produced in the first step, and the shell polymer is produced in the second step. In this method, the core particles act as “seed particles”, on the surface of which the shell polymers are supposed to be dispersed. During emulsion polymerization, the surfactant plays a major role by stabilizing
1674-2001/$ – see front matter © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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the dispersion medium. In the conventional emulsion polymerization process (Ni et al., 2005; Zhu, Shi, Cai, Zhao, & Liao, 2008), the in situ emulsion polymerization process (Chen, Liu, Zhang, & Wang, 2007; Ding et al., 2004; Li et al., 2007; Liu et al., 2008), and the miniemulsion process (Antonietti & Landfester, 2002; Asua, 2002; Crespy & Landfester, 2009; Erdem et al., 2000; Rabelero et al., 2005; Topfer & Schmidt-Naake, 2007; Zyl, de Wet-Roos, Sanderson, & Klumperman, 2004), the percentage of surfactant is low, resulting in an insufficient number of micelles to provide sufficient protection for single inorganic particles. Thus, during polymerization, there is a high possibility of agglomeration of and instability in the dispersion, resulting in less encapsulation of the inorganic particles by the polymer matrix. Recently, we have synthesized polystyrene nanoparticles (nPS) and poly(methyl methacrylate) (nPMMA) particles by a modified microemulsion polymerization process (Mishra & Chatterjee, 2011a) as well as a novel atomized microemulsion process (Mishra & Chatterjee, 2011b), and the isolated nPS and nPMMA particles were blended with polypropylene (PP) (Chatterjee & Mishra, 2012; Mishra & Chatterjee, 2011b) and linear low-density polyethylene (LLDPE) (Mishra, Chatterjee, & Rana, 2011) to study their rheological, thermal and mechanical properties. In our previous work (Mishra, Chatterjee, & Singh, 2011), we have successfully prepared nano-CaCO3 /PS core–shell particles, with CaCO3 as a core and PS as a shell, by an atomized polymerization technique, and the isolated CSNPs were incorporated in a PP matrix. This present work is an extension of our previous work to develop an efficient process for the production of nanoCaCO3 /PMMA core–shell nanoparticles by the atomized microemulsion process. Furthermore, we have also focused on improving the performance of the (nano-CaCO3 /PMMA)/PP nanocomposites by enhancing the interfacial adhesion by adding a small amount (1 wt.%) of encapsulated nano-CaCO3 . Thus, this approach will improve the compatibility of the nano-CaCO3 with the polymer matrix through the lipophilic polymer layer (PMMA
layer) grafted onto the surface of the nano-CaCO3 . The polymer shell coatings not only prevent the aggregation of nanoparticles but also produce excellent compatibility between the filler particles and the polymer matrix (Shen, Lin, Li, & Nan, 2007). 2. Experimental 2.1. Materials Nano-CaCO3 particles with diameters in the range of 10–50 nm were synthesized by the solution spray process reported elsewhere (Kulkarni, Patil, Ghosh, & Mishra, 2009; Mishra, Kulkarni, Patil, & Ghosh, 2009). The nanoparticles were modified by the silane coupling agent, triethoxyvinyl silane (TEVS). The monomer, methyl methacrylate (MMA); the initiator, ammonium persulfate (APS); the surfactant, sodium dodecyl sulfate (SDS); and the cosurfactant, n-pentanol, were purchased from S.D. Fine Chemicals Ltd., Mumbai, India. MMA was treated with a 5% NaOH aqueous solution to remove the inhibitor and was distilled under reduced pressure in a nitrogen atmosphere prior to polymerization. All other materials were of analytical grade and were used without further purification. Water was double distilled and deionized. 2.2. Surface modification of the nano-CaCO3 TEVS was used for the modification of the nano-CaCO3 particles. First, the desired amount of silane coupling agent, i.e., TEVS (5 phr with respect to weight of CaCO3 ) was dissolved in acetone, and 20 g of nano-CaCO3 particles was dispersed in the silane-acetone mixture under mechanical stirring at 500 rpm. The mixture was then sonicated for 1 h and stirred at 500 rpm at room temperature for 2 h. The TEVS and CaCO3 reacted in the dispersion overnight. Afterwards, the surface modified nano-CaCO3 particles were collected by filtration and rinsed four times with acetone, and the filter cake was dried at 100 ◦ C under low vacuum for 12 h.
Fig. 1. Atomized reaction vessel for synthesis of nano-CaCO3 /PMMA core–shell particles via the atomized microemulsion technique.
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2.3. Preparation of the nano-CaCO3 /PMMA core–shell nanoparticles Stable dispersions of the spherical nano-CaCO3 /PMMA core–shell nanoparticles (CSNPs) with diameters < 100 nm were synthesized by an oil/water (o/w) atomized microemulsion process (Fig. 1). The detailed procedure is as follows: 4 g of SDS was dissolved in 290 g of deionized water to form a clear solution, and 8 g of surface modified CaCO3 nanoparticles was dispersed in one part (40 g) of the clear SDS/water solution and sonicated for 30 min. The remaining 250 g of clear solution was poured into a 1 L stainless steel reactor with an agitator under constant stirring at 200 rpm, and 0.45 g of APS dissolved in 10 g of water was added to the reactor. The reactor was sealed, and the temperature was raised to 60 ◦ C to form free radicals. Then, 20 g of stabilizer-free MMA and 8 g of the dispersed solution of CaCO3 nanoparticles were sprayed into the reactor at the rate of 0.02 and 0.09 g/s, respectively, in the form of fine mist through the atomizers (nozzles) by a small, low pressure, metering pump under a nitrogen atmosphere purged through a nitrogen inlet. The streams of monomer and inorganic particles were sprayed at a specific angle so that they made contact prior to dispersing over the entire surface of the clear solution, where a unique microemulsion environment proceeds. The rate at which the liquid is atomized depends, within limits, solely on the volume that is being delivered on to the atomizing surface and the pump frequency. Typically, the higher the frequency was, the lower the processing capability was. Baffles were maintained at the top of the reactor to bounce the MMA and nano-CaCO3 streams back from the outgoing air. The exhaust then passed through a distillation column to recover the MMA and the nano-CaCO3 . Throughout the reaction, a temperature of 60 ◦ C and constant agitation were maintained. After the complete atomization of both particulates, the reaction continued for 1.5 h, and the polymerization reaction was stopped by cooling the dispersion to room temperature. A translucent dispersion was formed after the complete polymerization reaction. A butterfly valve, located at the bottom of the reactor dish, was fitted to discharge the reaction mixture. The process was regulated through control of the orifice size, the low pressure, the speed, the reaction temperature and the distance between the atomizer and the reaction zone. 2.4. Analysis of the grafting percentage of CSNPs The microemulsion reaction gave rise to the formation of CSNPs as well as free nano-CaCO3 and PMMA nanoparticles. To determine the grafting percentage, it is necessary to separate the CSNPs from the free nano-CaCO3 and PMMA nanoparticles (Tang, Liu, Sun, Zheng, & Cheng, 2007). In a typical analysis, the original latex-containing CSNPs, the free nano-CaCO3 and the PMMA nanoparticles were dried in an air oven at 120 ◦ C for 2 h. The same amount of latex was isolated in acetone under constant stirring. The core–shell and PMMA nanoparticles settled to form a residue, and the free nano-CaCO3 particles remained dispersed. The supernatant solution with the free nano-CaCO3 particles was carefully separated from the residue and centrifuged at 10,000 rpm to obtain the free nano-CaCO3 particles. Wt. of dry latex sample or Wt. of free nano-CaCO3 + PMMA nanoparticles + CSNPs = P, Wt. of centrifuged free nano-CaCO3 = Q, % free nano-CaCO3 = Q/P × 100%.
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The core–shell and polymer nanoparticle residue was washed 3 times with a deionized water/acetone mixture (50/50), vacuum filtered and vacuum dried at 50 ◦ C to obtain a powder. The CSNPs were then separated from the free PMMA nanoparticles by Soxhlet extraction with acetone for 12 h (Sheng et al., 2006). Wt. of CSNPs + free PMMA nanoparticles = R, Wt. of CSNPs after Soxhlet extraction = S, % free PMMA nanoparticles = R − S/R × 100%, % CSNPs = S/R × 100%. The content of the shell polymer on the nano-CaCO3 core was determined by thermogravimetric analysis (TGA). First, the free nano-CaCO3 and PMMA nanoparticles were separated from the CSNPs by the above method. Then, 10–12 mg of sample was heated from room temperature to 600 ◦ C at the rate of 10 ◦ C/min under a nitrogen atmosphere at a flow rate of 50 mL/min. The PMMA shells totally decomposed at 600 ◦ C, and the nano-CaCO3 cores remained as a residue. Wt. of CSNPs before TGA = T, Wt. of residue after TGA = U, % grafted nano-CaCO3 = U/T × 100%, % shell polymer = T − U/T × 100%. 2.5. Stability of the nano-CaCO3 /PMMA core–shell nano-suspensions The stability of CSNPs was compared to that of bare CaCO3 by the sedimentation test (Mishra, Chatterjee, & Singh, 2011; Tang, Cheng, & Ma, 2006). The method is as follows: the CSNPs were separated from the sample by the Soxhlet extraction method (see Section 2.4). After separation, 2 g of CSNPs dispersed in 100 mL acetone was poured into a test tube with a ruler and a cover. The test tube was allowed to stand at room temperature. After a definite time interval, the depth of the sediment from the surface of the suspension was recorded. The smaller the depth from the surface of suspension, the greater the stability of the CSNPs in acetone. The sedimentation percentage of the CSNPs dispersed in acetone was determined by the following equation: X=
H × 100 H0
where X is the sedimentation percentage, H (cm) is the depth of the sediment from the surface of the suspension, and H0 (cm) is the total depth of the suspension (Mishra, Chatterjee, & Singh, 2011). 2.6. Other characterization and measurements The particle size and morphology of the CSNPs were measured and confirmed using a transmission electron microscope (TEM), PHILIPS CM200, Eindhoven, The Netherlands, with 75 A of filament current and 200 kV of accelerating voltage. The particle diameters were measured directly from the TEM. FTIR spectra were obtained using a Fourier transform infrared spectrophotometer (FTIR-8000, Shimadzu, Tokyo, Japan). FTIR was used to characterize the functional groups of the nano-CaCO3 , the surface modified CaCO3 and the nano-CaCO3 /PMMA core–shell nanoparticles. The number of scans per sample was 25, and the resolution of the measurements was 4 cm−1 . The recorded wavenumber range was 4000–500 cm−1 . All samples were in powdered form and were measured at room temperature. DSC was carried out to investigate the glass transition temperature (Tg ) of the CSNPs using a differential scanning calorimeter
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Fig. 2. TEM micrographs of the nanoparticles sampled from the atomized microemulsion process: (a) bare nano-CaCO3 , (b) modified nano-CaCO3 /PMMA (CaCO3 /MMA = 8/20 g), and (c) pure nPMMA (MMA = 22.5 g).
(DSC-60, Shimadzu, Tokyo, Japan). The temperature was programmed from 30 ◦ C to 300 ◦ C at a heating rate of 10 ◦ C/min under a nitrogen atmosphere (50 mL/min). The thermal stabilities of the nano-CaCO3 , the nano-PMMA and the CSNPs were determined by a thermogravimetric analyzer (TGA50, Shimadzu, Tokyo, Japan); 10 mg of sample was placed on a Pt pan for TGA measurement. The temperature was programmed from 30 ◦ C to 600 ◦ C at a heating rate of 10 ◦ C/min under nitrogen atmosphere to avoid thermoxidative degradation.
3. Results and discussion 3.1. Morphology of the nano-CaCO3 /PMMA core–shell nanoparticles The TEM images of bare nano-CaCO3 , modified nanoCaCO3 /PMMA and pure nPMMA are shown in Fig. 2. It is clear from Fig. 2(a) that the bare nano-CaCO3 particles were suspended without encapsulation and that the grafting percentage was much lower. This effect was due to the hydrophilic nature of the bare nano-CaCO3 . The hydrophilicity did not allow monomer absorption during atomization (see Section 3.2 for details) and led to
incompatibility with the hydrophobic polymer shell. Therefore, in this case, the percentage of free polymer particles as well as free nano-CaCO3 was higher. In contrast, the core–shell particles shown in Fig. 2(b) have a spherical morphology with good monodispersity. The dark phases and translucent phases of single spheres in the micrographs represent the nano-CaCO3 and the PMMA polymer, respectively (Tang et al., 2006). It was obvious that the nano-CaCO3 particles were encapsulated by the polymer shell. In addition, due to the atomization of MMA, it was also very difficult to prevent the formation of free polymer particles during the grafting process. If Fig. 2(b) is compared with Fig. 2(c) (pure nPMMA), then some particles are observed without the CaCO3 core; this lack of CaCO3 core implied that some MMA were used for the nucleation of nPMMA to form free polymer particles.
3.2. Mechanism of nano-CaCO3 /PMMA core–shell nanoparticle synthesis by atomized microemulsion polymerization Generally, nano-CaCO3 has the tendency to agglomerate because of its high surface area and energy. Thus, it is difficult to achieve uniform dispersion of nano-CaCO3 in the polymer matrix. Additionally, due to the high density of nano-CaCO3 , they either
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Scheme 1. Reaction mechanism for the adsorption of TEVS onto nano-CaCO3 particles and the formation of nano-CaCO3 /PMMA core–shell particles via the atomized microemulsion technique.
settle or form aggregates during polymerization. Surface modification and ultrasonication are the best ways to reduce the surface energy of nano-CaCO3 , to increase its compatibility with the polymer matrix and to increase the dispersion homogeneity. For perfect encapsulation of the nano-CaCO3 particles, the surface of the nanoCaCO3 must be hydrophobic enough to have a close affinity for MMA (Tang et al., 2006, 2007). This depends on the composition of the functional groups on the nano-CaCO3 surface. Therefore, the CaCO3 nanoparticles were treated by TEVS, a reactive silane coupling agent, to functionalize the nano-CaCO3 surface. The reaction can proceed between each Si OC2 H5 group and the hydroxyl groups on the surface of the nanoparticles (Scheme 1 shows only one Si OC2 H5 group hydrolyzed by one hydroxyl group). As a result, the TEVS molecules were grafted onto the nano-CaCO3 surface. In addition, TEVS contains an unsaturated double bond, which can copolymerize with the unsaturated groups of MMA to covalently graft to at least part of the polymer during the polymerization. Consequently, TEVS links the polymer and the nano-CaCO3 particles through chemical bonds, and the link results in enhancement of the interfacial adhesion and compatibility between the inorganic nano-CaCO3 and the polymer matrix (Tang et al., 2007; Demjen, Pukanszky, Foldes, & Nagy, 1997). The organic polymer chains of TEVS induce steric hindrance to the associations between nano-CaCO3 particles and prevent aggregation of the particles. The organic molecules of TEVS on the nano-CaCO3 particle surface have an affinity to MMA. Therefore, during atomization, the nano-CaCO3 makes contact with the MMA stream, and a monomer layer is formed on the surface of the nanoCaCO3 prior to dispersion on the surface of the solution. Because of the TEVS modification, the MMA shell was formed at the first, or primary, stage of the reaction, and the reaction time was reduced. Next, the MMA-layered nano-CaCO3 particles were dispersed uniformly in the water solution, where the maximum concentration of micelles [above the ‘critical micelle concentration’ (CMC)] was present to stabilize the dispersion at a controlled temperature. The radicals in the water phase were then adsorbed onto the surface layer formed by the MMA. As a result, the polymerization locus was transferred from the aqueous phase to the surface of the nanoCaCO3 . This reaction site was favored because of the larger surface area of the nano-CaCO3 particles and the easier adsorption of MMA on the particle surface. The main chain propagating reaction was carried out at the surface of the nano-CaCO3 particle on which the encapsulating PMMA shell was formed (Scheme 1).
peak at 1642 cm−1 indicated the presence of C C unsaturation in the TEVS attached to the nano-CaCO3 surface. In general, the broad absorption bands in the range between 1100 and 1050 cm−1 were the characteristic bands of Si O C in TEVS (Posthumus, Magusin, Brokken-Zijp, Tinnemans, & van der Linde, 2004); whereas in Fig. 3(b), the peaks at 1100 and 1087 cm−1 were indicative of the formation of Si O CaCO3 bonds on nano-CaCO3 . In Fig. 3(c), the nano-CaCO3 /PMMA core–shell nanoparticles were characterized by the C H stretching bands of the CH2 group at 2850 and 2992 cm−1 , indicating the presence of the CH2 group in the backbone. The formation of nPMMA was confirmed by the IR spectrum, shown in Fig. 6(c), in which the absorptions at 1148 cm−1 ( C O) and 1731 cm−1 ( C O) indicated the existence of nPMMA (Mishra, Chatterjee, & Rana, 2011; Shi, Bao, Huang, & Weng, 2004). Two distinct peaks at 2949 and 2997 cm−1 indicated the asymmetrical and symmetrical stretching modes of C H in the methyl ( CH3 ) group. The symmetrical bending vibration (ıs-CH3 ) occurs near 1383 cm−1 , whereas the asymmetrical bending vibration (ıas-CH3 ) appears near 1450 cm−1 (Marimuthu & Madras, 2007; Tsai, Lin, Guo, & Chu, 2008). The disappearance of the peak at 1642 cm−1 implied that the silane coupling agent had covalently bonded with MMA through the unsaturated double bond
3.3. FTIR analysis of nano-CaCO3 /PMMA core–shell nanoparticles The FTIR absorption spectra of bare nano-CaCO3 particles, modified nano-CaCO3 particles and CSNPs are shown in Fig. 3. In the spectrum of the bare nano-CaCO3 particles, the broad absorption band at 3336 cm−1 (Fig. 3(a)) indicated the presence of OH on the CaCO3 surface (Hong, Qian, & Cao, 2006; Sheng et al., 2006). In the case of the modified nano-CaCO3 particles (Fig. 3(b)), the sharp
Fig. 3. FTIR spectra of (a) bare nano-CaCO3 , (b) modified nano-CaCO3 , and (c) nanoCaCO3 /PMMA core–shell particles.
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Fig. 5. TGA curves of (a) modified nano-CaCO3 , (b) nano-CaCO3 /PMMA core–shell particles, and (c) pure nPMMA.
Fig. 4. DSC curves of (a) pure nPMMA and (b) nano-CaCO3 /PMMA core–shell particles.
during polymerization. It can be inferred from the above results that the silane links the polymer and the nano-CaCO3 nanoparticles through chemical bonds. Consequently, the silane leads to the enhanced adhesion and compatibility between nano-CaCO3 and the polymer chains of PMMA. In addition, the downward shift of the absorption peaks from 1100–1087 cm−1 (Fig. 3(b)) to 1092–1080 cm−1 (Fig. 3(c)) could be attributed to the formation of (nano-CaCO3 ) O Si PMMA bonds resulting from the reaction with TEVS (Mishra, Chatterjee, & Singh, 2011). 3.4. Thermal analysis of nano-CaCO3 /PMMA core–shell nanoparticles The DSC thermograms of pure nPMMA and the extracted CSNPs are shown in Fig. 4. The DSC curve of the nPMMA particles (Fig. 4(a)) shows a lower glass transition temperature peak (Tg = 128 ◦ C) than that of the CSNPs (Tg = 136 ◦ C) (Fig. 4(b)) (Bershtein et al., 2009; Mishra, Chatterjee, & Rana, 2011). One possible reason is the strong interfacial interaction between the nano-CaCO3 core and the PMMA shell; the silane coupling cross linked the core and shell (Demjen et al., 1997; Hong et al., 2006; Tang et al., 2007) and grafted the PMMA chains onto the surface of the nanoparticles. Thus, the
mobility of the PMMA chains on the particle surface was restricted, and more energy was needed to transition from a glassy state to a rubbery state. Therefore, the glass transition temperature increased for the PMMA-grafted nano-CaCO3 (Laruelle, Parvole, Francois, & Billon, 2004). In comparison, Fig. 5 shows the TGA thermograms of the modified nano-CaCO3 , CSNPs and the pure nPMMA particles. Fig. 5(a) showed no change in the onset decomposition temperature (don ) of the modified nano-CaCO3 , and the weight loss percentage (WL ) was 1.2%, which was due to the decomposition of the silane coupling agent and the adsorbed water on the particles’ surfaces (Mishra, Chatterjee, & Singh, 2011; Sheng et al., 2006; Tang et al., 2007). In addition, a remarkable difference in the thermal behaviors of the nano-CaCO3 /PMMA particles and the pure nPMMA particles was observed (Fig. 5(b) and (c)). The nano-CaCO3 /PMMA (Fig. 5(b)) particles showed a higher thermal stability (don = 386 ◦ C) with lower weight loss (WL = 62%) than the pure nPMMA particles (don = 369 ◦ C with 100% WL ) (Mishra, Chatterjee, & Rana, 2011) (Fig. 5(c)); the different behaviors further supported the existence of a strong interaction between the nano-CaCO3 core and the PMMA shell. The weight loss of the CSNPs between 386 ◦ C and 600 ◦ C (Fig. 5(b)) was used to calculate the amount of grafted nano-CaCO3 in the CSNPs; thus, the extracted CSNPs showed 58% grafted nano-CaCO3 with 32% free nano-CaCO3 and 10% free nPMMA.
Fig. 6. SEM images of the PP nanocomposites with (a) 1 wt.% of CSNPs and (b) bare nano-CaCO3 .
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3.5. Dispersion uniformity of CSNPs in a PP matrix To investigate the dispersion of CSNPs in a polymer matrix, the CSNPs (1 wt.%) as well as the bare nano-CaCO3 particles were added to a PP matrix by melt processing in a Brabender Plastograph at 190 ◦ C for 10 min with a rotor speed of 60 rpm. The compounded composites were obtained in the form of lumps. These lumps were then crushed to obtain coarse particles (approximately 3–4 mm in diameter) of the composites and subjected to injection molding at a melt temperature of 210 ◦ C and a molding temperature of 30 ◦ C for further analysis (Mishra & Chatterjee, 2011a; Mishra, Chatterjee, & Rana, 2011; Mishra, Chatterjee, & Singh, 2011). The morphologies of the CSNPs and the nano-CaCO3 particles in the PP matrices were observed using SEM, as shown in Fig. 6. The white spots in the figure correspond to nano-CaCO3 particles. The encapsulating polymer merged with the PP matrix (Fig. 6(a)) during the thermal processing because of the compatibility of the encapsulating polymer and the matrix; thus, only the spots from the nano-CaCO3 particles can be observed. The nano-CaCO3 were found to be homogeneously dispersed in the matrix. Consequently, the nano-CaCO3 particles were integrated with the PP matrix by the grafting copolymer chains (Mishra, Chatterjee, & Singh, 2011; Tang et al., 2006), whereas particle aggregates were formed by the bare nano-CaCO3 , and these aggregates imply that the particles debonded from the matrix, as shown in Fig. 6(b) (Mishra, Chatterjee, & Singh, 2011). 4. Conclusions Nano-CaCO3 /PMMA core–shell particles with nano-CaCO3 as a core and PMMA as a shell were successfully synthesized by an atomized polymerization technique. The nanoparticles obtained have a perfect core–shell structure. The core–shell was formed via covalent bonding of PMMA to the nano-CaCO3 particles through the OC2 H5 group of TEVS at the core surface. The appearance of new peaks that exhibit shifts in the FTIR spectra confirms the interaction of PMMA and nano-CaCO3 in the core–shell particles. The increased thermal stability and the better Tg also suggest that a definite interaction must have occurred during polymerization. The reinforcement of CSNPs in the PP matrix results in a combination of the ‘stiffening’ effect provided by the better dispersion of the rigid nano-CaCO3 core and the ‘softening’ effect provided by the soft PMMA shell. Thus, the preparation of nanocomposites by reinforcement of CSNPs is an excellent method for obtaining well-dispersed nano-CaCO3 . This preparation opens the way to wider industrial utilization of CSNPs. Further studies on the relationship between the melt processing conditions and the dispersion of CSNPs are underway in our laboratory, as are detailed investigations of the rheological, thermal and mechanical properties. Acknowledgment The authors are thankful to the University Grants Commission (UGC), New Delhi for providing the financial support [project file No: 40-10/2011(SR), dated-July 14, 2011] to conduct this research. References Aguiar, A., Gonzalez-Villegas, S., Rabelero, M., Mendizábal, E., & Puig, J. E. (1999). Core–shell polymers with improved mechanical properties prepared by microemulsion polymerization. Macromolecules, 32, 6767–6771. Antonietti, M., & Landfester, K. (2002). Polyreactions in miniemulsions. Progress in Polymer Science, 27, 689–757. Asua, J. M. (2002). Miniemulsion polymerization. Progress in Polymer Science, 27, 1283–1346. Bawden, M. J., & Turner, S. R. (Eds.). (1988). Electronic and photonic applications of polymers. In Advances in chemistry series. Washington, DC: ACS.
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