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Na-MMT nanocomposite obtained by seeded emulsion polymerization
European Polymer Journal 48 (2012) 1683–1695
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Macromolecular Nanotechnology
On the stability and properties of the polyacrylate/Na-MMT nanocomposite obtained by seeded emulsion polymerization Onur Yılmaz a,⇑, Catalina N. Cheaburu b, Gürbüz Gülümser a, Cornelia Vasile b a
Ege University, Faculty of Engineering, Leather Engineering Department, 35100 Bornova- Izmir, Turkey Petru Poni Institute of Macromolecular Chemistry of the Romanian Academy, Laboratory of Physical Chemistry of Polymers, 41A Grigore Ghica Voda Alley, 700487 Iasi, Romania
a r t i c l e
i n f o
Article history: Received 29 February 2012 Received in revised form 3 July 2012 Accepted 18 July 2012 Available online 27 July 2012 Keywords: Nanocomposites Seeded emulsion polymerization Sodium montmorillonite Stabilization Clay Latex
a b s t r a c t For high performance waterborne coatings usually polymer latexes with low emulsifier content are more preferred. Although polymer/clay nanocomposites offer improved properties, it is difficult to produce clay based nanocomposite latexes containing low emulsifier due to the stabilization problems especially caused by organoclays. Present study deals with the preparation of a tBA/BA/MAA ternary copolymer/clay nanocomposite containing 3 wt.% sodium montmorillonite (Na+-MMT) via seeded emulsion polymerization. Experimentally it was observed that even the usage of hydrophilic clay caused stabilization problem and a certain amount of emulsifier (>1 wt.%) was necessary to obtain stable latexes. In addition, the usage of a low molecular weight water soluble polymer as steric barrier was found to increase the stability of system. Obtained nanocomposite latex showed fine particle size diameter (127 nm) and very narrow size distribution (PDI = 0.06). The WAXD and TEM investigations indicated that a mostly exfoliated nanocomposite was obtained. Thermal analyses (DSC, DMTA and TGA) showed that there was no change at Tg of the copolymer while very high improvement was obtained for elastic modulus and a slight increase in thermal stability. According to the rheological measurements, the nanocomposite latex showed a higher low shear viscosity, a stronger shear thinning behavior and an improved physical stability in comparison to the reference latex. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Emulsion polymerization is a powerful technique for preparing polymer/clay nanocomposites [1]. Due to the ecological and environmental care issues waterborne polymers gained more importance especially for coating applications in many fields such as: automotive, textile, leather, wood, paints, etc. [2]. The emulsion polymers used for coating purposes should fulfill some criteria like resistance to abrasion, heat, water, UV light, chemicals, etc. according to their end use. In addition, some of the latex properties are also of importance such as; emulsion stability, particle size and size distribution, total emulsifier content, physical ⇑ Corresponding author. Tel.: +90 232 3112644; fax: +90 232 3425376. E-mail address: [email protected] (O. Yılmaz). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2012.07.010
stability and flow behavior as well as suitable mechanical and thermal properties of the coating film. All these factors affect the application of latex and the performance of coating as well. The field of polymer/clay nanocomposites as a branch of nanotechnology field showed a new route in the last decade for preparation of polymers with enhanced properties such as increased modulus, thermal stability, nonflammability, good barrier properties, chemical resistance and electrical conductivity, etc. [3]. This achievement is usually explained by the nanometric dispersion of clay layers within the polymer matrix which provides a better interaction of polymer chains with the layers having increased surface area. Therefore, the dispersion degree of clay in polymer matrix is the key factor for preparing nanocomposites. These are known as intercalated and exfoliated
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b
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structures of the nanocomposites [4–9]. In both of these cases the nanocomposites exhibit better mechanical and thermal properties in comparison to the pure polymer, however, exfoliation is usually more preferred since it gives superior improvements. In nanocomposite preparations usually the surface of clay layers is modified by cationic surfactants including primary, secondary, tertiary, and quaternary alkyl ammonium or alkyl phosphonium compounds to change the nature of the clay from hydrophilic to hydrophobic and to make it more compatible with organic polymers or monomers [10,11]. In this way it is easier to obtain successful intercalated and/or exfoliated structures whether in situ polymerization or direct mixing techniques are used in the preparation of nanocomposites. Considering the polymer/clay nanocomposite preparations using emulsion polymerization both organically modified or natural sodium montmorillonites are used by researchers. Due to the hydrophilic nature of Na+-montmorillonite (Na-MMT) it is easy to disperse in water, however, many studies resulted in intercalated structures because of the insufficient interaction of clay and hydrophobic polymers [12–16]. The other approach including the use of organically modified-montmorillonites (OMMT) seems to be more proper to obtain exfoliated nanocomposites by emulsion polymerization, as it was shown in several studies [17–19]. The usage of organoclays in emulsion polymerization for nanocomposite preparations can be useful especially when the stability of the latex is not an important issue for the final application of the polymer. In most of these cases the nanocomposites are obtained by centrifuging or precipitating after the completion of the reaction [13,19– 22]. However, when the stability of latex is important as for many coating applications which include direct usage of polymer dispersions, organoclays may not be the best choice for preparing nanocomposite latexes. The main difficulty of organoclays in emulsion polymerization is their hydrophobicity as a result of the type of alkyl ammonium compounds. These agents such as dimethyl dehydrogenated tallow ammonium salt which are widely used as modifiers are too hydrophobic for aqueous systems and can cause coagulation problems. To overcome this problem the emulsifier content can be increased, however, too much emulsifier usage will lead to variations in particle sizes of the final latex and cause poor water resistance and mechanical properties of the polymer coating. According to the stability problem caused by organoclays in emulsion polymerization some other researchers suggested to use lightly modified clays [23] or different approaches such as mini-emulsion techniques [24–27] to overcome this problem and to decrease the amount of emulsifier usage. In one of our previous studies [28] a detailed description of the stability problem caused by organoclays in conventional batch emulsion polymerization also in terms of reaction mechanisms was presented. The findings of this study revealed the importance of the clay type and showed some approaches to obtain long term stable nanocomposite latexes including a polymeric stabilizer which helped to decrease the total emulsifier content. Indeed, moderately hydrophobic clays seem to be more proper for
conventional emulsion polymerization providing an exfoliation and better stabilization, while hydrophilic clays result in rather intercalated structures. However, in every case a relatively important amount of emulsifiers are still needed for stabilization. Moreover, the usage of Na-MMT based clays in batch emulsion polymerization does not prevent the formation of coagulum in latexes without adding certain amount of emulsifier. Along with the hydrophobicity of the clay, the usage of more hydrophobic monomers also decreases the stability of the latexes. Therefore, in these cases it is more difficult to obtain stable nanocomposite latexes using batch emulsion polymerization whether organically modified or unmodified clays are used. To increase the stability of the latex, seeded or continuous emulsion polymerization systems may be a better alternative for nanocomposite preparations since they provide a better control for the system. However, there are limited numbers of studies about preparing stable nanocomposite latexes using seeded emulsion polymerization. Diaconu et al. synthesized stable and partially exfoliated poly (MMA-co-BA)/Na-MMT nanocomposites by seeded semi batch emulsion polymerization by using 4 wt.% emulsifier [29]. Similarly, Sedláková et al. also reported a way for preparing stable nanocomposite latexes by seeded emulsion polymerization of polystyrene and poly (butyl methacrylate) using Na-MMT which was modified with a cationic reactive agent (2-(acryloyloxy) ethyl trimethylammonium chloride) [30]. They also used an emulsifier ratio of 1.3–2.5 wt.% (SDS) on monomer mass basis. The present study deals with the synthesis of Poly (tBA-co-BA-co-MAA)/Na-MMT nanocomposite via seeded emulsion polymerization providing stable latex with low emulsifier content and very uniform particle size distribution as well as enhanced mechanical and thermal properties of the film.
2. Experimental 2.1. Materials As montmorillonite clay Cloisite Na+ (Na-MMT) and Cloisite 30B (OMMT) (Southern Clay Products Inc.) were used in the study for nanocomposite synthesis. Cloisite Na+ is a natural sodium montmorillonite clay that usually has 1 nm thickness and 70–150 nm length leading to high aspect ratios and a high surface area of 750 m2/g. Cloisite 30B was used as an organomontmorillonite that is produced from Na-MMT clay by using organic modifier of methyl tallow bis-2-hydroxyethyl quaternary ammonium (modifier conc. 90 meq/100 g, weight loss of ignition: 30%). Butyl acrylate (BA,P99%), tert-Butyl acrylate (tBA, 98%) and methacrylic acid (MAA, 99%) were supplied from Aldrich and used as monomers. The other necessary chemicals; dodecyl benzene sulfonic acid sodium salt as emulsifier (SDBS, C18H29NaO3S, tech. grade), potassium persulfate (KPS, K2S2O8, P 99%) as initiator, sodium bicarbonate (NaHCO3, P 99.5%) as buffer, poly(methacrylic acid) sodium salt (PMAA–Na+, Mw 4000–6000, 40 wt.% in H2O) as water soluble polymeric stabilizer were also
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2.2. Preparation of the acrylate copolymer/Na-MMT nanocomposite by seeded emulsion polymerization Na-MMT clay (3 wt.% based on monomer weight) was first dispersed in water and mixed overnight, and ultrasonicated for 30 min prior to the polymerization. Selected amount of SDBS emulsifier, PMAA–Na+ polymeric stabilizer, NaHCO3, half of KPS initiator and MAA monomer were first dissolved in water and mixed in the reactor for 15 min. Subsequently, the clay dispersion was added to the reactor and the system mixed for another 15 min. The pH of the medium was adjusted to seven by dropping 1 M NaOH solution. After putting 1/7 portion of tBA/BA monomer mixture to form a seed latex, the system was purged with nitrogen gas for 15 min to remove the dissolved oxygen from the reaction medium. The pre-emulsion was first mixed at 700 rpm to maintain a homogenous reaction medium. When the temperature was reached 75 °C the system was maintained for 60 min at 350 rpm to prepare the seed latex. The rest of the monomer mixture (6/7 portion of tBA/BA) was dropped into the reaction medium within 90 min by injection through the rubber septum. After 30 min the other half of KPS solution was dropped and the system was reacted for another 90 min. At the end of reaction the system was cooled down to room temperature and nanocomposite latex was obtained with a total solid content of 25% and a conversion ratio over 97%. The reference latex was also prepared under the same conditions without using Na-MMT. The experiments were done in triplicate and average values were presented. The nanocomposite films necessary for analysis were obtained by casting emulsions on glass substrate and drying at room temperature for 3 days and in oven at 70 °C for 1 day. 2.3. Characterization 2.3.1. Particle size and zeta potential analysis A nanoZS model (Malvern Instruments, UK) zetasizer instrument was used for the measurement of particle size and zeta potential of the latexes using techniques of dynamic light scattering and laser doppler microelectrophoresis, respectively.
clamped at the same pressure by adjusting the micrometer torque and scanned in the range of 600–4000 cm1. 2.3.3. Wide-angle X-ray diffraction (WAXD) The average interlayer spacing of Na-MMT before and after polymerization was investigated by a Philips PW 3710 Wide Angle X-ray Diffractometer (WAXD) (Cu–Ni radiation, k = 0.154 nm). The distances between clay layers were calculated with Bragg’s law: 2d sin h = nk; where k is the wavelength of the X-ray, d is the interspacing distance and h is the angle of incident radiation. 2.3.4. Transmission electron microscopy (TEM) For TEM investigation of the nanocomposite film ultrathin (60 nm) sections were cut, under liquid N2, from a stained (RuO4 vapour for 90 min) sample using an Ultracut UCT (Leica) ultramicrotome. Subsequently, the sample was examined by using a JEM 200CX (JEOL, Japan) microscope. The TEM micrographs were taken at acceleration voltage 100 kV, recorded on a photographic film and digitized with a PC-controlled digital camera DXM1200 (Nikon, Japan). 2.3.5. Mechanical and thermal properties Elastic moduli of the copolymer films were measured with a Rheometer Anton Paar MCR301 instrument equipped with dynamic mechanical analysis accessories at a constant frequency of 1 Hz within 50 to 100 °C temperature range. Differential scanning calorimetry (DSC) thermographs were recorded with a Perkin Elmer Diamond DSC instrument at a heating rate of 10 °C/min under N2 atmosphere from 70 to 100 °C. The thermogravimetric behavior of the copolymer films was also investigated by a Perkin–Elmer Thermogravimetric Analyzer (TGA) with a heating rate of 20 °C/min under O2 atmosphere from 30 to 500 °C. 2.3.6. Rheological measurements The rheological investigations of the nanocomposite latexes were performed at 25 °C by means of an Anton Paar 301 Rheometer (USA) device using a cone/plate geometry measuring system with an angle of 1.003°. The flow behavior was tested by rotational controlled shear rate condition (CSR) where the viscosity (g) of the samples were measured as a function of increasing shear rate (c = 0.01– 200 s1). Storage modulus (G0 ) and loss modulus (G00 ) of the nanocomposite latexes were measured as a function of angular frequency (x = 0.1–200 s1) by using oscillatory tests. To perform the frequency sweep tests, the linear viscoelastic range of the samples (LVE) was obtained from amplitude sweep tests (with a strain amplitude between 0.01 and 500%) using a constant angular frequency x = 10 s1.
3. Results and discussion 2.3.2. Fourier transform infrared spectroscopy (FTIR) The information on the chemical composition of copolymer films was obtained by means of a Perkin–Elmer Spectrum-100 ATR–FTIR. The instrument was equipped with solid cell accessories and a 45o ZnSe crystal. The samples covered the whole crystal surface (25 10 mm) and were
3.1. Polymerization conditions For the experiments the monomer ratios of tBA/BA/ MAA was selected as 55/43/2 based on weight fraction. This ratio was selected according to obtain a Tg value below
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supplied from Aldrich. NaOH (Merck, P 98.0%) was used as a common base. All chemicals were used as received without any further purification. The polymerization reactions took place in a three necked 500 mL glass reactor equipped with a condenser, nitrogen pipe and a rubber septum for feeding. The reactor was put in a water bath which was placed on a magnetic heater with a digital temperature control and the mixing of the reaction medium was carried out with a magnetic stirrer.
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Table 1 Emulsion polymerization trials for nanocomposite synthesis. Trial no
Composition
Stabilizers
1 2 3 4 5 6 7 8
tBA/BA/MAA tBA/BA/MAA–Na+MMT tBA/BA/MAA–Na+MMT tBA/BA/MAA–Na+MMT tBA/BA/MAA tBA/BA/MAA–Na+MMT tBA/BA/MAA–Na+MMT tBA/BA/MAA–OMMT
0.5% 0.5% 1.0% 1.0% 1.0% 1.0% 1.0% 1.0%
Polymerization system
Observation of latex aspect
Seeded Seeded Seeded Seeded Seeded Seeded Batch Seeded
Stable Coagulated Partially coagulated Stable (nanocomposite latex) Stable (reference) Coagulated Coagulated Coagulated
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SDBS; 0.5% PMAA–Na+ SDBS; 0.5% PMAA–Na+ SDBS; 0.5% PMAA–Na+ SDBS 1.0% PMAA–Na+ SDBS 1.0% PMAA–Na+ SDBS SDBS 1.0% PMAA–Na+ SDBS 1.0% PMAA–Na+
Fig. 1. The affect of polymeric stabilizer on the stability of low emulsifier containing Na-MMT/polymer nanocomposite latexes.
0 °C which allows the final polymer to be used in coating applications of soft and flexible substrates. The amount of Na-MMT was not varied in the study and used at a fixed ratio of 3 wt.% based on total monomer mass. According to our results from previous studies [28,31] which was also reported by some other researchers [16,21,32] the ratio of 3 wt.% may be an optimal amount for clay materials to be used in emulsion polymerizations. Fewer amounts of clay usage might lead to insufficient improvements of final polymer whereas a higher percentage would lead more difficulties for latex stabilization.
For nanocomposite synthesis an experimental setup was designed including polymerization trials having variations on stabilizer amounts, polymerization system and clay type (Table 1). In the experiments, a low molecular weight water soluble polymer poly (methacrylic acid sodium salt) (Mw 3000) was used as a steric stabilizer in combination with the emulsifier (SDBS). In comparison to conventional surfactants, water soluble polymers have lower mobility at particle/water interfaces and thus decreasing the risk of coagulation [33,34]. The reason of MAA monomer usage at a low portion in hydrophobic
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Fig. 2. Particle size distributions of the pure latex and nanocomposite latex.
Table 2 Z-average diameter, polydispersity index (PDI) and zeta potential (ZP) values of the reference and nanocomposite latex. Sample
Z-average diameter (nm)
Polydispersity index (PDI)
Zeta potential (mV)
Reference Nanocomposite
113 127
0.02 0.06
50.6 49.4
monomer composition of t-BA/BA was to possess more hydrogen bonding sites in the final polymer chain for coating purposes. As it is known small portions of carboxylic acid monomers helps to improve the stabilization of the latexes [35], the freeze–thaw and mechanical stability [36,37], enhances adhesion and increases the critical pigment volume concentration value [38]. In addition, the seeded emulsion polymerization, which was used for the nanocomposite synthesis, usually give a better control for particle growth stage and decreases the risk of coagulation [30,39,40]. During the experiments the pH of the initial emulsion was adjusted to a neutral value (pH = 7) by using NaOH solution to remove the acidity caused by MAA addition. This neutralization provides to increase the anionic repulsions between the particles and increase the stabilization of latex. This was also shown in a detailed study by Nunes et al. where NaOH addition decreased the zeta potential value and increased the stability of latex whereas acid addition caused latex coagulation [41]. The trials indicated that the addition of hydrophilic clay (Na-MMT) at lower levels emulsifier and stabilizer amounts (<1.0 wt.%) resulted in coagulation of the latex (Table 1, no. 2 and 3). The stability problem caused by organoclays is usually due to the transportation of layers from monomer droplets to the aqueous phase and to the
growing particles according to the polymerization mechanism. However, it’s also interesting to observe coagulation for hydrophilic clay which should behave different in terms of polymerization mechanism due to their existence in aqueous phase. Choi et al. prepared exfoliated and stable nanocomposite latexes using two types of clay (sodium montmorillonite and laponite) [17]. In their results they indicated that laponite layers separated sodium montmorillonite layers and they were adsorbed on latex particles acted like steric barriers that prevented the latex from coagulation. In addition, when they used only sodium montmorillonite, the latex showed tendency to coagulate. It seems that sodium montmorillonite clay (Cloisite Na) having a high aspect ratio (length/thickness) together with its increased surface area after dispersion, might be acting as an alternative reaction loci for growing polymer particles due to its hydrophilic layers having negative surface charges. This might lead to stabilization problems when the polymerization reaches to a critical chain length. Chern et al. studied the kinetics of styrene emulsion polymerization in the presence of sodium montmorillonite [42]. They reported that the critical micelle concentration (CMC) of SDS emulsifier was decreased to lower values in the presence of Na-MMT and they showed that Na-MMT platelets could act as extra reaction loci at relatively low levels of emulsifier (9 mM). For these reasons the minimum emulsifier amount necessary to obtain stable latexes containing clays can be higher than the blank latex without clay. In our experiments a stable nanocomposite latex including 3 wt.% Na-MMT, 1 wt.% SDBS and 1 wt.% PMAA–Na+ was obtained successfully (Table 1, no. 4). The usage of water soluble polymeric stabilizer in combination with the emulsifier helped to increase the stability of nanocomposite latex. It seemed that the existence of stabilizer units in aqueous phase prevented the clay layers and latex particles from collision which might lead to coagulation. An illustrative image was given in Fig. 1 showing the
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Fig. 3. FTIR spectra of: (a) the copolymer films and Cloisite Na+; (b) expanded peak area at 1030–1065 cm1.
behavior of polymeric stabilizer during nanocomposite synthesis. The trials also showed that without the polymeric stabilizer 1 wt.% of emulsifier (SDBS) was not sufficient to give stable latex (Table 1, no. 6). Moreover, when a batch technique was used instead of seeded emulsion polymerization (Table 1, no. 7), where all other parameters were the same a coagulated latex was obtained. Similarly, when Na-MMT was replaced with an organomontmorillonite (Cloisite 30B) again the nanocomposite latex was not stable (Table 1, no. 8). 3.2. Particle size and zeta potential analysis The results of dynamic light scattering analysis were given in Fig. 2 and Table 2. The average particle size
diameter of the reference latex was around 113 nm. In the case of nanocomposite latex having 3 wt.% sodium montmorillonite, the average particle diameter was found to be 127 nm which was slightly increased in comparison to the reference. The particle size distributions of the samples showed only one narrow peak for both latexes (Fig. 2). Accordingly, the polydispersity index (PDI) values of the latexes were very low as 0.02 for the reference latex and 0.06 for the nanocomposite latex which was slightly increased. However, all differences were negligible and latexes with fine particle sizes and very homogenous distributions were obtained by seeded emulsion polymerization. The zeta potential value of the reference sample was found to be 50.6 mV while the zeta potential of the nanocomposite latex was measured as 49.4 mV. Both latexes
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Fig. 4. WAXD patterns of: (a) the reference and nanocomposite copolymer together with the corresponding clay; (b) the reference and nanocomposite copolymer within extended 2h range.
had very similar zeta potential values which can be attributed to latexes with very good stability. 3.3. FTIR spectra of the samples The FTIR spectra of copolymer films were given in Fig. 3. From the spectra the main absorbance bands of poly (tBA-co-BA-co-MAA) with or without clay were found at 1065, 1143, 1450, 1725 cm1 which were attributed to stretching of C–O–C, OR–C–C, C–H bending (d) of CH2, C=O stretching, respectively. The bands observed between 2790 and 3000 cm1 were assigned to sym and asym C–H stretching of CH2 and CH3 stretching of tBA, BA and MAA. The broad band appeared in the range of
3100–3700 cm1 with low intensity could be due the stretching of C–OH groups of MAA. In Fig. 3a the IR spectrum of sodium montmorillonite (Cloisite Na+) was also given. The characteristic absorption band of the clay due to the stretching of Si–O–Si groups forming the crystalline structure of the clay layers was observed at around 1030 cm1 in the spectrum. However, it was difficult to observe the clay peak in the spectrum of nanocomposite due to its low content and a possible overlapping on the absorption band of 1035 and 1065 cm1 of poly (tBA-coBA-co-MAA). On the other hand, an increase in the intensity and widening of these bands was also observed due to the aforementioned Si–O–Si absorption band (Fig. 3b) and clay presence.
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Fig. 5. TEM images of the nanocomposite film, (a) lower magnification (b) higher magnification.
Fig. 6. DSC curves of the reference and nanocomposite films.
3.4. WAXD analysis The WAXD patterns of the clay and the copolymer films were shown in Fig. 4. According to the data, the main reflection of Cloisite Na+ appeared at around 6.9° 2h degrees which corresponded to a d-value of 1.28 nm. However, this reflection of clay seemed to be diminished for the nanocomposite sample exhibiting an exfoliated structure (Fig. 4a). The comparison of the pure copolymer and nanocomposite was given in Fig. 4b within a higher 2h degree range. The intensity range given in ordinate-Y was lowered for better observation of the peaks. At 2h degrees of around 3.9–4.2° a very small peak was observed for the
nanocomposite sample, however, a similar peak with smaller intensity was also detected for the pure copolymer. Therefore, it was difficult to say if this peak corresponded to an intercalation or not. In addition some semi-crystalline regions were also observed in the diffractogram of the pure polymer that gave some peaks at higher 2h degrees (17–20°). However, in the diffractogram of the nanocomposite the intensity of these peaks was lowered indicating a decrease in (semi)crystallinity. This phenomenon could be due to the exfoliation of the clay where the layers were distributed unevenly between polymer chains that interrupted the formation of crystalline regions. Consequently, the nanocomposite was formed successfully
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and a well distribution of clay layers in the polymer matrix was achieved. 3.5. TEM observations of the nanocomposite Fig. 5 shows the TEM micrographs of the nanocomposite film at different magnifications. In the micrographs the dark structures represent the clay layers and lighter regions the polymer. At lower magnification of the nanocomposite film (Fig. 5a), an irregular distribution of the clay layers could be seen where individual layers located towards different directions. This showed the good dispersion of the clay layers in polymer matrix where its ordered structure was disturbed. At higher magnification the disordered structure of clay layers was more clear (Fig. 5b). However, some darker regions were still observed in the micrographs showing the existence of some intercalated parts or possible edge to edge interactions of some clay layers. For emulsion based nanocomposites including especially hydrophilic clays these interactions are possible. Because the hydrophilic clay layers are located mostly on the surface of latex particles and/or in aqueous phase, the interactions are unavoidable upon the film formation by direct casting from dispersion. However, as it was shown in XRD the intensity of these ordered parts are very low, thus it can be said that a mostly exfoliated nanocomposite was obtained. 3.6. Thermal and mechanical properties of the nanocomposite The DSC curves of the copolymers with and without clay showed clear phase transitions (Fig. 6). According to the calculations the Tg of pure copolymer was found at around 2.0 °C while the Tg of nanocomposite was found as 2.8 °C. Normally clay layers play a role of increasing the Tg of polymers as they confine the movement of polymer chains especially in the case of intercalated nanocomposites. However, for rubbery and soft polymers, the
existence of clay in polymer matrix might not have an important effect on Tg, especially for exfoliated nanocomposites. Similar results were also reported by some other authors [15,29,43–45]. The obtained nanocomposite had even very slightly lowered Tg value which could be probably due to the exfoliation state of nanocomposite that having relatively decreased (semi)crystallinity as assessed by XRD. Infect this is a more desirable situation for obtaining nanocomposites without changing Tg where elastic and flexible polymers are desired. The elastic moduli (E0 ) of copolymer films as function of temperature were given in Fig. 7. With the usage of 3 wt.% Na-MMT, the increase in elastic modulus of the nanocomposite in respect to the pure polymer was very drastic and very high values were obtained (higher with 103 order of magnitude). The thermogravimetric analysis results were presented in Fig. 8. TG curves showed that the thermogravimetric behavior of the samples were very similar. Only a slight increase in thermal stability with 2–4 °C for peak temperature was obtained (Fig. 8b) and after 40% mass loss further thermo-oxidative decomposition temperatures were shifted to slightly higher values. This kind of situation sometimes can be observed especially for exfoliated nanocomposites. The enhancement of thermo-oxidative stability of nanocomposites is usually due to the restricted thermal motion of polymer chains localized inside the clay galleries as in the case of intercalated nanocomposites [46]. The thermal stabilities are also dependent on the clay loading ratio. At higher clay ratios (i.e. 5–20 wt.%) higher thermal stabilities are usually obtained due to the inorganic nature of clay. However, these clay ratios are very problematic for emulsion polymerization system to obtain stable latexes. In our case the Tg of nanocomposite did not show any shift and even a small decrease was observed which indicated that the polymer chains were not confined. This may explain the slight improvement of thermal stability for the nanocomposite.
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Fig. 7. Storage (elastic) modulus vs. temperature curves of the copolymer films obtained from DMTA.
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Fig. 8. Thermogravimetric behavior of the pure copolymer and nanocomposite: (a) TGA curves; (b) derivative of TGA curves.
3.7. Rheological behavior of the latexes The flow behavior of the nanocomposite and blank latexes were given in Fig. 9 where the viscosity changes as a function of different shear rates were plotted. At low shear rates the viscosity of the nanocomposite latex was higher in comparison to the reference latex. This was an expected situation since well-distributed clay layers increased significantly the internal surface areas which led to increased viscosity values. With the increase of shear rates both latexes showed decrease in viscosity exhibiting a shear thinning behavior. However, the shear thinning effect was found to be stronger for the nanocomposite latex. Usually the polymers having higher molecular weights have a tendency to entangle with their neighboring macromolecules in their three dimensional network at ‘‘rest’’ state (low shear rates). But during the shear process, the
molecules are usually oriented in the shear direction by entangling to a certain extend which lower their flow resistance. This behavior is attributed to the physical jamming or percolation of randomly distributed silicate layers [47]. The results obtained from oscillatory test were shown in Fig. 10 where storage modulus (G0 ) and loss modulus (G0 0 ) were plotted as function of the angular frequency (x) under a constant amplitude strain (LVE range). The storage modulus (G0 ) of the nanocomposite latex was much higher than the reference latex showing that the stability was increased by clay incorporation. In addition, for all frequency ranges, G0 had higher values than G00 (G0 > G00 ) for the nanocomposite latex sample which indicated that the elastic behavior dominated the viscous one. Generally, within the stable dispersions, emulsions and gels, intermolecular interaction forces form three-dimensional net-
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Fig. 9. Shear viscosities of the reference and nanocomposite latexes.
Fig. 10. Viscoelastic behavior of the latexes at variable frequencies.
works and thus they are showing G0 > G00 in whole frequency ranges with a slight increase in the slope at higher frequencies [48]. However, for the reference latex a cross over point (G0 0 > G0 ) was observed which showed that the latex behaved more like a liquid at higher frequencies. According to these results it can be said that by clay addition the viscosity and physical stability of latex were increased with an enhanced shear thinning behavior. 4. Conclusions Nanocomposite based on tBA–BA–MAA/Na+MMT was successfully synthesized by seeded emulsion polymerization. During the polymerizations it was observed that the
usage of hydrophilic clay caused coagulation problems and a certain amount of emulsifier was necessary to obtain stable latex. Besides, the addition of a water soluble polymer in reaction medium increased the stabilization of latex. In addition, seeded emulsion polymerization also provided a better reaction control than batch system and decreased the risk of particle coagulation. The obtained nanocomposite latex showed fine particle size and very narrow particle size distributions with high zeta potential value exhibiting very good latex stability. The nanocomposite film showed mostly exfoliated structure which was proved by XRD and TEM observations. The thermal analyses indicated that Tg of the copolymer was not affected by clay addition; however, storage
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modulus was increased significantly. The rheological investigations evidenced that the viscosity and physical stability of the latex was increased by clay loading. The nanocomposite latex also showed a stronger shear thinning behavior which is a desired property especially for spraying solutions. The obtained nanocomposite latex could be proper for various coating applications of soft surfaces such as textile and leather. Acknowledgements The authors acknowledge the support of the EOL through the FP7-NMP-2007-CSA-1 project NaPolyNet: Setting up research-intensive clusters across the EU on characterization of polymer nanostructure and Cost Action FA090. They also thank to the project of ‘‘Industrial Doctorate Program of Textile and Leather – 2007 DPT 001’’ supported by T.R. Ministry of Development.
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References [1] Nobel ML, Mendes E, Picken SJ. Acrylic-based nanocomposite resins for coating applications. J Appl Polym Sci 2007;104:2146–56. [2] Chern CS. Emulsion polymerization mechanisms and kinetics. Prog Polym Sci 2006;31:443–86. [3] Zhu J, Wilkie CA. Intercalation compounds and clay nanocomposites. In: Kickelbick G, editor. Weinheim: Verlag GmbH & Co.; 2007. p. 151–75. [4] Fukushima Y, Inagaki S. Synthesis of an intercalated compound of montmorillonite and 6-polyamide. J Inclusion Phenom Macrocyclic Chem 1987;5:473–82. [5] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon 6-clay hybrid. J Mater Res 1993;8:1185–9. [6] Kojima Y, Usuki A, Kawasumi M, Okada A, Kurauchi T, Kamigaito O. Sorption of water in nylon 6-clay hybrid. J Appl Polym Sci 1993;49:1259–64. [7] Kojima Y, Usuki A, Kawasumi M, Okada A, Karauchi T, Kamigaito O. One-pot synthesis of nylon 6-clay hybrid. J Polym Sci. 1993;31:1755–8. [8] Yano K, Usuki A, Karauchi T, Kamigaito O. Synthesis and properties of polyimide-clay hybrid. J Polym Sci. 1993;31:2493–8. [9] Giese RF, van Oss CJ. Colloid and Surface Properties of Clays and Related Minerals. New York: Marcel Dekker, Inc.; 2002. [10] Blumstein A. Polymerization of adsorbed monolayers II thermal degradation of the inserted polymers. J Polym Sci A 1965;3:65–73. [11] Krishnamoorti R, Vaia RA, Giannelis EP. Structure and dynamics of polymer-layer silicate nanocomposites. Chem Mater 1996;8: 1728–34. [12] Lee DC, Jang LW. Preparation and characterization of PMMA-clay hybrid composite by emulsion polymerization. J Appl Polym Sci 1996;61:1117–22. [13] Noh MW, Lee DC. Synthesis and characterization of PS-clay nanocomposite by emulsion polymerization. Polym Bull 1999;42: 619–26. [14] Noh MH, Jang LW, Lee DC. Intercalation of styrene–acrylonitrile copolymer in layered silicate by emulsion polymerization. J Appl Polym Sci 1999;74:179–88. [15] Tong X, Zhao H, Tang T, Feng Z, Huang B. Preparation and characterization of poly(ethyl acrylate)/bentonite nanocomposites by in situ emulsion polymerization. J Polym Sci. 2002;40:1706–11. [16] Lai MC, Chang KC, Yeh JM, Liou SJ, Hsieh MF, Chang HS. Advanced environmentally friendly anticorrosive materials prepared from water-based polyacrylate/Na+-MMT clay nanocomposite latexes. Euro Polym J. 2007;43:4219–28. [17] Choi YS, Xu M, Chung IJ. Synthesis of exfoliated poly(styrene-coacrylonitrile) copolymer/silicate nanocomposite by emulsion polymerization; monomer composition effect on morphology. Polymer 2003;44:6989–94. [18] Xu M, Choi YS, Kim YK, Wang KH, Chung IJ. Synthesis and characterization of exfoliated poly(styrene-co-methyl methacrylate)/ clay nanocomposites via emulsion polymerization with AMPS. Polymer 2003;44:6387–95.
[19] Zheng JP, Wang JX, Gao S, Yao KD. Synthesis and characterization of PMMA/montmorillonite nanocomposites by emulsion polymerization. J Mater Sci 2005;40(17):4687–9. [20] Huang X, Brittain WJ. Synthesis and characterization of PMMA nanocomposites by suspension and emulsion polymerization. Macromolecules 2001;34:3255–60. [21] Yeh JM, Liou SJ, Lai MC, Chang YW, Huang CY, Chen CP, et al. Comparative studies of the properties of poly(methyl methacrylate)– clay nanocomposite materials prepared by in situ emulsion polymerization and solution dispersion. J Appl Polym Sci 2004;94:1936–46. [22] Wang T, Wang M, Zhang Z, Ge X, Fang Y. Preparation of core (PBAlayered silicate)–shell (PS) structured complex via c-ray radiation seeded emulsion polymerization. Mater Lett 2006;60:2544–8. [23] Lorah DP, Slone RV. Emulsion polymerization methods involving lightly modified clay and compositions comprising same. US Patent 6759463; 2004. [24] Tong Z, Deng Y. Synthesis of water-based polystyrene-nanoclay composite suspension via miniemulsion polymerization. Ind Eng Chem Res 2006;45:2641–5. [25] Sun Q, Deng Y, Wang ZL. Synthesis and characterization of polystyrene-encapsulated laponite composites via miniemulsion polymerization. Macromol Mater Eng 2004;289:288–95. [26] Diaconu G, Paulis M, Leiza JR. High solids content waterborne acrylic/montmorillonite nanocomposites by miniemulsion polymerization. Macromol React Eng 2008;2:80–9. [27] Diaconu G, Micˇušík M, Bonnefond A, Paulis M, Leiza JR. Macroinitiator and macromonomer modified montmorillonite for the synthesis of acrylic/MMT nanocomposite latexes. Macromolecules 2009;42: 3316–25. [28] Yilmaz O, Cheaburu CN, Durraccio D, Gulumser G, Vasile C. Preparation of stable acrylate/montmorillonite nanocomposite latex via in situ batch emulsion polymerization: Effect of clay types. Appl Clay Sci 2010;49:288–97. [29] Diaconu G, Paulis M, Leiza JR. Towards the synthesis of high solids content waterborne poly(methyl methacrylate-co-butyl acrylate)/ montmorillonite nanocomposites. Polymer 2008;49:2444–54. [30] Sedláková Z, Pleštil J, Baldrian J, Šlouf M, Holub P. Polymer-clay nanocomposites prepared via in situ emulsion polymerization. Polym Bull 2009;63:365–84. [31] Yılmaz O, Chiriac AP, Cheaburu CN, Nita LE, Gülümser G, Duraccio D, et al. Nanocomposites based on montmorillonite/acrylic copolymer for aqueous coating of soft surfaces. Solid State Phenom: Nanocompos. Mater. 2009;151:129–34. [32] Yu T, Lin J, Xu J, Chen T, Lin S. Novel polyacrylonitrile nanocomposites containing Na-montmorillonite and nano SiO2 particle. Polymer 2005;46:5695–7. [33] Ball LE. Anionic polymeric stabilizers for oil-in-water emulsions. US Patent 5021526; 1991. [34] Feustel D, Stritzke KH. Polymeric stabilizers for emulsion polymerization. US Patent 5277978; 1994. [35] Amalvy JI. Semicontinuous emulsion polymerization of methyl methacrylate, ethyl acrylate, and methacrylic acid. J Appl Polym Sci 1996;59:339–44. [36] Quadrat O, Snuparek J. Structure and flow properties of lattices containing carboxylic groups. Prog Org Coat 1990;18:207–28. [37] Karunasena A, Glass JE. Interactions in associative thickener/220 nm methyl methacrylate latex dispersions. Prog Org Coat 1989;17: 301–20. [38] Koh AYC, Mange S, Bothe M, Leyrer RJ, Gilbert RG. The influence of copolymerization with methacrylic acid on poly(butyl acrylate) film properties. Polymer 2006;47:1159–65. [39] Kang K, Kan C, Du Y, Liu D. Study on soap-free P(MMA–EA–AA or MAA) latex particles with narrow size distribution. Polym Adv Technol 2006;17:193–8. [40] van Herk A, Gilbert B. Emulsion Polymerisation. In: van Herk A, editor. Editor. UK: Blackwell Publishing Ltd; 2005. p. 46–79. [41] Nunes JS, Vasconcelos CL, Dantas TNC, Pereira MR, Fonseca JLC. Electrokinetic behavior of a poly(butyl acrylate-co-methacrylic acid) latex. Colloids Surf A 2006;275:148–52. [42] Chern CS, Lin JJ, Lin YL, Lai SZ. Kinetics of styrene emulsion polymerization in the presence of montmorillonite. Europ Polym J 2006;42:1033–42. [43] Becker O, Varley R, Simon G. Morphology, thermal relaxations and mechanical properties of layered silicate nanocomposites based upon high-functionality epoxy resins. Polymer 2002;43:4365–73. [44] Lee HT, Lin LH. Waterborne polyurethane/clay nanocomposites: novel effects of the clay and its interlayer ions on the morphology and physical and electrical properties. Macromolecules 2006;39: 6133–41.
O. Yılmaz et al. / European Polymer Journal 48 (2012) 1683–1695
[47] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Progr Polym Sci 2008;33:1119–98. [48] Brummer R. Rheology essentials of cosmetic and food emulsions. Berlin: Springer; 2006.
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[45] Xu Y, Brittain WJ. Preparation of polyacrylate-clay nanocomposites by in-situ polymerization and heterocoagulation. J Appl Polym Sci 2006;101:3850–5. [46] Blumstein A. Polymerization of adsorbed monolayers: I, Preparation of the clay–polymer complex.. J Polym Sci. 1965;3:2653–61.
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Contents lists available at SciVerse ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
On the stability and properties of the polyacrylate/Na-MMT nanocomposite obtained by seeded emulsion polymerization Onur Yılmaz a,⇑, Catalina N. Cheaburu b, Gürbüz Gülümser a, Cornelia Vasile b a
Ege University, Faculty of Engineering, Leather Engineering Department, 35100 Bornova- Izmir, Turkey Petru Poni Institute of Macromolecular Chemistry of the Romanian Academy, Laboratory of Physical Chemistry of Polymers, 41A Grigore Ghica Voda Alley, 700487 Iasi, Romania
a r t i c l e
i n f o
Article history: Received 29 February 2012 Received in revised form 3 July 2012 Accepted 18 July 2012 Available online 27 July 2012 Keywords: Nanocomposites Seeded emulsion polymerization Sodium montmorillonite Stabilization Clay Latex
a b s t r a c t For high performance waterborne coatings usually polymer latexes with low emulsifier content are more preferred. Although polymer/clay nanocomposites offer improved properties, it is difficult to produce clay based nanocomposite latexes containing low emulsifier due to the stabilization problems especially caused by organoclays. Present study deals with the preparation of a tBA/BA/MAA ternary copolymer/clay nanocomposite containing 3 wt.% sodium montmorillonite (Na+-MMT) via seeded emulsion polymerization. Experimentally it was observed that even the usage of hydrophilic clay caused stabilization problem and a certain amount of emulsifier (>1 wt.%) was necessary to obtain stable latexes. In addition, the usage of a low molecular weight water soluble polymer as steric barrier was found to increase the stability of system. Obtained nanocomposite latex showed fine particle size diameter (127 nm) and very narrow size distribution (PDI = 0.06). The WAXD and TEM investigations indicated that a mostly exfoliated nanocomposite was obtained. Thermal analyses (DSC, DMTA and TGA) showed that there was no change at Tg of the copolymer while very high improvement was obtained for elastic modulus and a slight increase in thermal stability. According to the rheological measurements, the nanocomposite latex showed a higher low shear viscosity, a stronger shear thinning behavior and an improved physical stability in comparison to the reference latex. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Emulsion polymerization is a powerful technique for preparing polymer/clay nanocomposites [1]. Due to the ecological and environmental care issues waterborne polymers gained more importance especially for coating applications in many fields such as: automotive, textile, leather, wood, paints, etc. [2]. The emulsion polymers used for coating purposes should fulfill some criteria like resistance to abrasion, heat, water, UV light, chemicals, etc. according to their end use. In addition, some of the latex properties are also of importance such as; emulsion stability, particle size and size distribution, total emulsifier content, physical ⇑ Corresponding author. Tel.: +90 232 3112644; fax: +90 232 3425376. E-mail address: [email protected] (O. Yılmaz). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2012.07.010
stability and flow behavior as well as suitable mechanical and thermal properties of the coating film. All these factors affect the application of latex and the performance of coating as well. The field of polymer/clay nanocomposites as a branch of nanotechnology field showed a new route in the last decade for preparation of polymers with enhanced properties such as increased modulus, thermal stability, nonflammability, good barrier properties, chemical resistance and electrical conductivity, etc. [3]. This achievement is usually explained by the nanometric dispersion of clay layers within the polymer matrix which provides a better interaction of polymer chains with the layers having increased surface area. Therefore, the dispersion degree of clay in polymer matrix is the key factor for preparing nanocomposites. These are known as intercalated and exfoliated
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b
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structures of the nanocomposites [4–9]. In both of these cases the nanocomposites exhibit better mechanical and thermal properties in comparison to the pure polymer, however, exfoliation is usually more preferred since it gives superior improvements. In nanocomposite preparations usually the surface of clay layers is modified by cationic surfactants including primary, secondary, tertiary, and quaternary alkyl ammonium or alkyl phosphonium compounds to change the nature of the clay from hydrophilic to hydrophobic and to make it more compatible with organic polymers or monomers [10,11]. In this way it is easier to obtain successful intercalated and/or exfoliated structures whether in situ polymerization or direct mixing techniques are used in the preparation of nanocomposites. Considering the polymer/clay nanocomposite preparations using emulsion polymerization both organically modified or natural sodium montmorillonites are used by researchers. Due to the hydrophilic nature of Na+-montmorillonite (Na-MMT) it is easy to disperse in water, however, many studies resulted in intercalated structures because of the insufficient interaction of clay and hydrophobic polymers [12–16]. The other approach including the use of organically modified-montmorillonites (OMMT) seems to be more proper to obtain exfoliated nanocomposites by emulsion polymerization, as it was shown in several studies [17–19]. The usage of organoclays in emulsion polymerization for nanocomposite preparations can be useful especially when the stability of the latex is not an important issue for the final application of the polymer. In most of these cases the nanocomposites are obtained by centrifuging or precipitating after the completion of the reaction [13,19– 22]. However, when the stability of latex is important as for many coating applications which include direct usage of polymer dispersions, organoclays may not be the best choice for preparing nanocomposite latexes. The main difficulty of organoclays in emulsion polymerization is their hydrophobicity as a result of the type of alkyl ammonium compounds. These agents such as dimethyl dehydrogenated tallow ammonium salt which are widely used as modifiers are too hydrophobic for aqueous systems and can cause coagulation problems. To overcome this problem the emulsifier content can be increased, however, too much emulsifier usage will lead to variations in particle sizes of the final latex and cause poor water resistance and mechanical properties of the polymer coating. According to the stability problem caused by organoclays in emulsion polymerization some other researchers suggested to use lightly modified clays [23] or different approaches such as mini-emulsion techniques [24–27] to overcome this problem and to decrease the amount of emulsifier usage. In one of our previous studies [28] a detailed description of the stability problem caused by organoclays in conventional batch emulsion polymerization also in terms of reaction mechanisms was presented. The findings of this study revealed the importance of the clay type and showed some approaches to obtain long term stable nanocomposite latexes including a polymeric stabilizer which helped to decrease the total emulsifier content. Indeed, moderately hydrophobic clays seem to be more proper for
conventional emulsion polymerization providing an exfoliation and better stabilization, while hydrophilic clays result in rather intercalated structures. However, in every case a relatively important amount of emulsifiers are still needed for stabilization. Moreover, the usage of Na-MMT based clays in batch emulsion polymerization does not prevent the formation of coagulum in latexes without adding certain amount of emulsifier. Along with the hydrophobicity of the clay, the usage of more hydrophobic monomers also decreases the stability of the latexes. Therefore, in these cases it is more difficult to obtain stable nanocomposite latexes using batch emulsion polymerization whether organically modified or unmodified clays are used. To increase the stability of the latex, seeded or continuous emulsion polymerization systems may be a better alternative for nanocomposite preparations since they provide a better control for the system. However, there are limited numbers of studies about preparing stable nanocomposite latexes using seeded emulsion polymerization. Diaconu et al. synthesized stable and partially exfoliated poly (MMA-co-BA)/Na-MMT nanocomposites by seeded semi batch emulsion polymerization by using 4 wt.% emulsifier [29]. Similarly, Sedláková et al. also reported a way for preparing stable nanocomposite latexes by seeded emulsion polymerization of polystyrene and poly (butyl methacrylate) using Na-MMT which was modified with a cationic reactive agent (2-(acryloyloxy) ethyl trimethylammonium chloride) [30]. They also used an emulsifier ratio of 1.3–2.5 wt.% (SDS) on monomer mass basis. The present study deals with the synthesis of Poly (tBA-co-BA-co-MAA)/Na-MMT nanocomposite via seeded emulsion polymerization providing stable latex with low emulsifier content and very uniform particle size distribution as well as enhanced mechanical and thermal properties of the film.
2. Experimental 2.1. Materials As montmorillonite clay Cloisite Na+ (Na-MMT) and Cloisite 30B (OMMT) (Southern Clay Products Inc.) were used in the study for nanocomposite synthesis. Cloisite Na+ is a natural sodium montmorillonite clay that usually has 1 nm thickness and 70–150 nm length leading to high aspect ratios and a high surface area of 750 m2/g. Cloisite 30B was used as an organomontmorillonite that is produced from Na-MMT clay by using organic modifier of methyl tallow bis-2-hydroxyethyl quaternary ammonium (modifier conc. 90 meq/100 g, weight loss of ignition: 30%). Butyl acrylate (BA,P99%), tert-Butyl acrylate (tBA, 98%) and methacrylic acid (MAA, 99%) were supplied from Aldrich and used as monomers. The other necessary chemicals; dodecyl benzene sulfonic acid sodium salt as emulsifier (SDBS, C18H29NaO3S, tech. grade), potassium persulfate (KPS, K2S2O8, P 99%) as initiator, sodium bicarbonate (NaHCO3, P 99.5%) as buffer, poly(methacrylic acid) sodium salt (PMAA–Na+, Mw 4000–6000, 40 wt.% in H2O) as water soluble polymeric stabilizer were also
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2.2. Preparation of the acrylate copolymer/Na-MMT nanocomposite by seeded emulsion polymerization Na-MMT clay (3 wt.% based on monomer weight) was first dispersed in water and mixed overnight, and ultrasonicated for 30 min prior to the polymerization. Selected amount of SDBS emulsifier, PMAA–Na+ polymeric stabilizer, NaHCO3, half of KPS initiator and MAA monomer were first dissolved in water and mixed in the reactor for 15 min. Subsequently, the clay dispersion was added to the reactor and the system mixed for another 15 min. The pH of the medium was adjusted to seven by dropping 1 M NaOH solution. After putting 1/7 portion of tBA/BA monomer mixture to form a seed latex, the system was purged with nitrogen gas for 15 min to remove the dissolved oxygen from the reaction medium. The pre-emulsion was first mixed at 700 rpm to maintain a homogenous reaction medium. When the temperature was reached 75 °C the system was maintained for 60 min at 350 rpm to prepare the seed latex. The rest of the monomer mixture (6/7 portion of tBA/BA) was dropped into the reaction medium within 90 min by injection through the rubber septum. After 30 min the other half of KPS solution was dropped and the system was reacted for another 90 min. At the end of reaction the system was cooled down to room temperature and nanocomposite latex was obtained with a total solid content of 25% and a conversion ratio over 97%. The reference latex was also prepared under the same conditions without using Na-MMT. The experiments were done in triplicate and average values were presented. The nanocomposite films necessary for analysis were obtained by casting emulsions on glass substrate and drying at room temperature for 3 days and in oven at 70 °C for 1 day. 2.3. Characterization 2.3.1. Particle size and zeta potential analysis A nanoZS model (Malvern Instruments, UK) zetasizer instrument was used for the measurement of particle size and zeta potential of the latexes using techniques of dynamic light scattering and laser doppler microelectrophoresis, respectively.
clamped at the same pressure by adjusting the micrometer torque and scanned in the range of 600–4000 cm1. 2.3.3. Wide-angle X-ray diffraction (WAXD) The average interlayer spacing of Na-MMT before and after polymerization was investigated by a Philips PW 3710 Wide Angle X-ray Diffractometer (WAXD) (Cu–Ni radiation, k = 0.154 nm). The distances between clay layers were calculated with Bragg’s law: 2d sin h = nk; where k is the wavelength of the X-ray, d is the interspacing distance and h is the angle of incident radiation. 2.3.4. Transmission electron microscopy (TEM) For TEM investigation of the nanocomposite film ultrathin (60 nm) sections were cut, under liquid N2, from a stained (RuO4 vapour for 90 min) sample using an Ultracut UCT (Leica) ultramicrotome. Subsequently, the sample was examined by using a JEM 200CX (JEOL, Japan) microscope. The TEM micrographs were taken at acceleration voltage 100 kV, recorded on a photographic film and digitized with a PC-controlled digital camera DXM1200 (Nikon, Japan). 2.3.5. Mechanical and thermal properties Elastic moduli of the copolymer films were measured with a Rheometer Anton Paar MCR301 instrument equipped with dynamic mechanical analysis accessories at a constant frequency of 1 Hz within 50 to 100 °C temperature range. Differential scanning calorimetry (DSC) thermographs were recorded with a Perkin Elmer Diamond DSC instrument at a heating rate of 10 °C/min under N2 atmosphere from 70 to 100 °C. The thermogravimetric behavior of the copolymer films was also investigated by a Perkin–Elmer Thermogravimetric Analyzer (TGA) with a heating rate of 20 °C/min under O2 atmosphere from 30 to 500 °C. 2.3.6. Rheological measurements The rheological investigations of the nanocomposite latexes were performed at 25 °C by means of an Anton Paar 301 Rheometer (USA) device using a cone/plate geometry measuring system with an angle of 1.003°. The flow behavior was tested by rotational controlled shear rate condition (CSR) where the viscosity (g) of the samples were measured as a function of increasing shear rate (c = 0.01– 200 s1). Storage modulus (G0 ) and loss modulus (G00 ) of the nanocomposite latexes were measured as a function of angular frequency (x = 0.1–200 s1) by using oscillatory tests. To perform the frequency sweep tests, the linear viscoelastic range of the samples (LVE) was obtained from amplitude sweep tests (with a strain amplitude between 0.01 and 500%) using a constant angular frequency x = 10 s1.
3. Results and discussion 2.3.2. Fourier transform infrared spectroscopy (FTIR) The information on the chemical composition of copolymer films was obtained by means of a Perkin–Elmer Spectrum-100 ATR–FTIR. The instrument was equipped with solid cell accessories and a 45o ZnSe crystal. The samples covered the whole crystal surface (25 10 mm) and were
3.1. Polymerization conditions For the experiments the monomer ratios of tBA/BA/ MAA was selected as 55/43/2 based on weight fraction. This ratio was selected according to obtain a Tg value below
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supplied from Aldrich. NaOH (Merck, P 98.0%) was used as a common base. All chemicals were used as received without any further purification. The polymerization reactions took place in a three necked 500 mL glass reactor equipped with a condenser, nitrogen pipe and a rubber septum for feeding. The reactor was put in a water bath which was placed on a magnetic heater with a digital temperature control and the mixing of the reaction medium was carried out with a magnetic stirrer.
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Table 1 Emulsion polymerization trials for nanocomposite synthesis. Trial no
Composition
Stabilizers
1 2 3 4 5 6 7 8
tBA/BA/MAA tBA/BA/MAA–Na+MMT tBA/BA/MAA–Na+MMT tBA/BA/MAA–Na+MMT tBA/BA/MAA tBA/BA/MAA–Na+MMT tBA/BA/MAA–Na+MMT tBA/BA/MAA–OMMT
0.5% 0.5% 1.0% 1.0% 1.0% 1.0% 1.0% 1.0%
Polymerization system
Observation of latex aspect
Seeded Seeded Seeded Seeded Seeded Seeded Batch Seeded
Stable Coagulated Partially coagulated Stable (nanocomposite latex) Stable (reference) Coagulated Coagulated Coagulated
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SDBS; 0.5% PMAA–Na+ SDBS; 0.5% PMAA–Na+ SDBS; 0.5% PMAA–Na+ SDBS 1.0% PMAA–Na+ SDBS 1.0% PMAA–Na+ SDBS SDBS 1.0% PMAA–Na+ SDBS 1.0% PMAA–Na+
Fig. 1. The affect of polymeric stabilizer on the stability of low emulsifier containing Na-MMT/polymer nanocomposite latexes.
0 °C which allows the final polymer to be used in coating applications of soft and flexible substrates. The amount of Na-MMT was not varied in the study and used at a fixed ratio of 3 wt.% based on total monomer mass. According to our results from previous studies [28,31] which was also reported by some other researchers [16,21,32] the ratio of 3 wt.% may be an optimal amount for clay materials to be used in emulsion polymerizations. Fewer amounts of clay usage might lead to insufficient improvements of final polymer whereas a higher percentage would lead more difficulties for latex stabilization.
For nanocomposite synthesis an experimental setup was designed including polymerization trials having variations on stabilizer amounts, polymerization system and clay type (Table 1). In the experiments, a low molecular weight water soluble polymer poly (methacrylic acid sodium salt) (Mw 3000) was used as a steric stabilizer in combination with the emulsifier (SDBS). In comparison to conventional surfactants, water soluble polymers have lower mobility at particle/water interfaces and thus decreasing the risk of coagulation [33,34]. The reason of MAA monomer usage at a low portion in hydrophobic
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Fig. 2. Particle size distributions of the pure latex and nanocomposite latex.
Table 2 Z-average diameter, polydispersity index (PDI) and zeta potential (ZP) values of the reference and nanocomposite latex. Sample
Z-average diameter (nm)
Polydispersity index (PDI)
Zeta potential (mV)
Reference Nanocomposite
113 127
0.02 0.06
50.6 49.4
monomer composition of t-BA/BA was to possess more hydrogen bonding sites in the final polymer chain for coating purposes. As it is known small portions of carboxylic acid monomers helps to improve the stabilization of the latexes [35], the freeze–thaw and mechanical stability [36,37], enhances adhesion and increases the critical pigment volume concentration value [38]. In addition, the seeded emulsion polymerization, which was used for the nanocomposite synthesis, usually give a better control for particle growth stage and decreases the risk of coagulation [30,39,40]. During the experiments the pH of the initial emulsion was adjusted to a neutral value (pH = 7) by using NaOH solution to remove the acidity caused by MAA addition. This neutralization provides to increase the anionic repulsions between the particles and increase the stabilization of latex. This was also shown in a detailed study by Nunes et al. where NaOH addition decreased the zeta potential value and increased the stability of latex whereas acid addition caused latex coagulation [41]. The trials indicated that the addition of hydrophilic clay (Na-MMT) at lower levels emulsifier and stabilizer amounts (<1.0 wt.%) resulted in coagulation of the latex (Table 1, no. 2 and 3). The stability problem caused by organoclays is usually due to the transportation of layers from monomer droplets to the aqueous phase and to the
growing particles according to the polymerization mechanism. However, it’s also interesting to observe coagulation for hydrophilic clay which should behave different in terms of polymerization mechanism due to their existence in aqueous phase. Choi et al. prepared exfoliated and stable nanocomposite latexes using two types of clay (sodium montmorillonite and laponite) [17]. In their results they indicated that laponite layers separated sodium montmorillonite layers and they were adsorbed on latex particles acted like steric barriers that prevented the latex from coagulation. In addition, when they used only sodium montmorillonite, the latex showed tendency to coagulate. It seems that sodium montmorillonite clay (Cloisite Na) having a high aspect ratio (length/thickness) together with its increased surface area after dispersion, might be acting as an alternative reaction loci for growing polymer particles due to its hydrophilic layers having negative surface charges. This might lead to stabilization problems when the polymerization reaches to a critical chain length. Chern et al. studied the kinetics of styrene emulsion polymerization in the presence of sodium montmorillonite [42]. They reported that the critical micelle concentration (CMC) of SDS emulsifier was decreased to lower values in the presence of Na-MMT and they showed that Na-MMT platelets could act as extra reaction loci at relatively low levels of emulsifier (9 mM). For these reasons the minimum emulsifier amount necessary to obtain stable latexes containing clays can be higher than the blank latex without clay. In our experiments a stable nanocomposite latex including 3 wt.% Na-MMT, 1 wt.% SDBS and 1 wt.% PMAA–Na+ was obtained successfully (Table 1, no. 4). The usage of water soluble polymeric stabilizer in combination with the emulsifier helped to increase the stability of nanocomposite latex. It seemed that the existence of stabilizer units in aqueous phase prevented the clay layers and latex particles from collision which might lead to coagulation. An illustrative image was given in Fig. 1 showing the
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Fig. 3. FTIR spectra of: (a) the copolymer films and Cloisite Na+; (b) expanded peak area at 1030–1065 cm1.
behavior of polymeric stabilizer during nanocomposite synthesis. The trials also showed that without the polymeric stabilizer 1 wt.% of emulsifier (SDBS) was not sufficient to give stable latex (Table 1, no. 6). Moreover, when a batch technique was used instead of seeded emulsion polymerization (Table 1, no. 7), where all other parameters were the same a coagulated latex was obtained. Similarly, when Na-MMT was replaced with an organomontmorillonite (Cloisite 30B) again the nanocomposite latex was not stable (Table 1, no. 8). 3.2. Particle size and zeta potential analysis The results of dynamic light scattering analysis were given in Fig. 2 and Table 2. The average particle size
diameter of the reference latex was around 113 nm. In the case of nanocomposite latex having 3 wt.% sodium montmorillonite, the average particle diameter was found to be 127 nm which was slightly increased in comparison to the reference. The particle size distributions of the samples showed only one narrow peak for both latexes (Fig. 2). Accordingly, the polydispersity index (PDI) values of the latexes were very low as 0.02 for the reference latex and 0.06 for the nanocomposite latex which was slightly increased. However, all differences were negligible and latexes with fine particle sizes and very homogenous distributions were obtained by seeded emulsion polymerization. The zeta potential value of the reference sample was found to be 50.6 mV while the zeta potential of the nanocomposite latex was measured as 49.4 mV. Both latexes
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Fig. 4. WAXD patterns of: (a) the reference and nanocomposite copolymer together with the corresponding clay; (b) the reference and nanocomposite copolymer within extended 2h range.
had very similar zeta potential values which can be attributed to latexes with very good stability. 3.3. FTIR spectra of the samples The FTIR spectra of copolymer films were given in Fig. 3. From the spectra the main absorbance bands of poly (tBA-co-BA-co-MAA) with or without clay were found at 1065, 1143, 1450, 1725 cm1 which were attributed to stretching of C–O–C, OR–C–C, C–H bending (d) of CH2, C=O stretching, respectively. The bands observed between 2790 and 3000 cm1 were assigned to sym and asym C–H stretching of CH2 and CH3 stretching of tBA, BA and MAA. The broad band appeared in the range of
3100–3700 cm1 with low intensity could be due the stretching of C–OH groups of MAA. In Fig. 3a the IR spectrum of sodium montmorillonite (Cloisite Na+) was also given. The characteristic absorption band of the clay due to the stretching of Si–O–Si groups forming the crystalline structure of the clay layers was observed at around 1030 cm1 in the spectrum. However, it was difficult to observe the clay peak in the spectrum of nanocomposite due to its low content and a possible overlapping on the absorption band of 1035 and 1065 cm1 of poly (tBA-coBA-co-MAA). On the other hand, an increase in the intensity and widening of these bands was also observed due to the aforementioned Si–O–Si absorption band (Fig. 3b) and clay presence.
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Fig. 5. TEM images of the nanocomposite film, (a) lower magnification (b) higher magnification.
Fig. 6. DSC curves of the reference and nanocomposite films.
3.4. WAXD analysis The WAXD patterns of the clay and the copolymer films were shown in Fig. 4. According to the data, the main reflection of Cloisite Na+ appeared at around 6.9° 2h degrees which corresponded to a d-value of 1.28 nm. However, this reflection of clay seemed to be diminished for the nanocomposite sample exhibiting an exfoliated structure (Fig. 4a). The comparison of the pure copolymer and nanocomposite was given in Fig. 4b within a higher 2h degree range. The intensity range given in ordinate-Y was lowered for better observation of the peaks. At 2h degrees of around 3.9–4.2° a very small peak was observed for the
nanocomposite sample, however, a similar peak with smaller intensity was also detected for the pure copolymer. Therefore, it was difficult to say if this peak corresponded to an intercalation or not. In addition some semi-crystalline regions were also observed in the diffractogram of the pure polymer that gave some peaks at higher 2h degrees (17–20°). However, in the diffractogram of the nanocomposite the intensity of these peaks was lowered indicating a decrease in (semi)crystallinity. This phenomenon could be due to the exfoliation of the clay where the layers were distributed unevenly between polymer chains that interrupted the formation of crystalline regions. Consequently, the nanocomposite was formed successfully
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and a well distribution of clay layers in the polymer matrix was achieved. 3.5. TEM observations of the nanocomposite Fig. 5 shows the TEM micrographs of the nanocomposite film at different magnifications. In the micrographs the dark structures represent the clay layers and lighter regions the polymer. At lower magnification of the nanocomposite film (Fig. 5a), an irregular distribution of the clay layers could be seen where individual layers located towards different directions. This showed the good dispersion of the clay layers in polymer matrix where its ordered structure was disturbed. At higher magnification the disordered structure of clay layers was more clear (Fig. 5b). However, some darker regions were still observed in the micrographs showing the existence of some intercalated parts or possible edge to edge interactions of some clay layers. For emulsion based nanocomposites including especially hydrophilic clays these interactions are possible. Because the hydrophilic clay layers are located mostly on the surface of latex particles and/or in aqueous phase, the interactions are unavoidable upon the film formation by direct casting from dispersion. However, as it was shown in XRD the intensity of these ordered parts are very low, thus it can be said that a mostly exfoliated nanocomposite was obtained. 3.6. Thermal and mechanical properties of the nanocomposite The DSC curves of the copolymers with and without clay showed clear phase transitions (Fig. 6). According to the calculations the Tg of pure copolymer was found at around 2.0 °C while the Tg of nanocomposite was found as 2.8 °C. Normally clay layers play a role of increasing the Tg of polymers as they confine the movement of polymer chains especially in the case of intercalated nanocomposites. However, for rubbery and soft polymers, the
existence of clay in polymer matrix might not have an important effect on Tg, especially for exfoliated nanocomposites. Similar results were also reported by some other authors [15,29,43–45]. The obtained nanocomposite had even very slightly lowered Tg value which could be probably due to the exfoliation state of nanocomposite that having relatively decreased (semi)crystallinity as assessed by XRD. Infect this is a more desirable situation for obtaining nanocomposites without changing Tg where elastic and flexible polymers are desired. The elastic moduli (E0 ) of copolymer films as function of temperature were given in Fig. 7. With the usage of 3 wt.% Na-MMT, the increase in elastic modulus of the nanocomposite in respect to the pure polymer was very drastic and very high values were obtained (higher with 103 order of magnitude). The thermogravimetric analysis results were presented in Fig. 8. TG curves showed that the thermogravimetric behavior of the samples were very similar. Only a slight increase in thermal stability with 2–4 °C for peak temperature was obtained (Fig. 8b) and after 40% mass loss further thermo-oxidative decomposition temperatures were shifted to slightly higher values. This kind of situation sometimes can be observed especially for exfoliated nanocomposites. The enhancement of thermo-oxidative stability of nanocomposites is usually due to the restricted thermal motion of polymer chains localized inside the clay galleries as in the case of intercalated nanocomposites [46]. The thermal stabilities are also dependent on the clay loading ratio. At higher clay ratios (i.e. 5–20 wt.%) higher thermal stabilities are usually obtained due to the inorganic nature of clay. However, these clay ratios are very problematic for emulsion polymerization system to obtain stable latexes. In our case the Tg of nanocomposite did not show any shift and even a small decrease was observed which indicated that the polymer chains were not confined. This may explain the slight improvement of thermal stability for the nanocomposite.
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Fig. 7. Storage (elastic) modulus vs. temperature curves of the copolymer films obtained from DMTA.
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Fig. 8. Thermogravimetric behavior of the pure copolymer and nanocomposite: (a) TGA curves; (b) derivative of TGA curves.
3.7. Rheological behavior of the latexes The flow behavior of the nanocomposite and blank latexes were given in Fig. 9 where the viscosity changes as a function of different shear rates were plotted. At low shear rates the viscosity of the nanocomposite latex was higher in comparison to the reference latex. This was an expected situation since well-distributed clay layers increased significantly the internal surface areas which led to increased viscosity values. With the increase of shear rates both latexes showed decrease in viscosity exhibiting a shear thinning behavior. However, the shear thinning effect was found to be stronger for the nanocomposite latex. Usually the polymers having higher molecular weights have a tendency to entangle with their neighboring macromolecules in their three dimensional network at ‘‘rest’’ state (low shear rates). But during the shear process, the
molecules are usually oriented in the shear direction by entangling to a certain extend which lower their flow resistance. This behavior is attributed to the physical jamming or percolation of randomly distributed silicate layers [47]. The results obtained from oscillatory test were shown in Fig. 10 where storage modulus (G0 ) and loss modulus (G0 0 ) were plotted as function of the angular frequency (x) under a constant amplitude strain (LVE range). The storage modulus (G0 ) of the nanocomposite latex was much higher than the reference latex showing that the stability was increased by clay incorporation. In addition, for all frequency ranges, G0 had higher values than G00 (G0 > G00 ) for the nanocomposite latex sample which indicated that the elastic behavior dominated the viscous one. Generally, within the stable dispersions, emulsions and gels, intermolecular interaction forces form three-dimensional net-
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Fig. 9. Shear viscosities of the reference and nanocomposite latexes.
Fig. 10. Viscoelastic behavior of the latexes at variable frequencies.
works and thus they are showing G0 > G00 in whole frequency ranges with a slight increase in the slope at higher frequencies [48]. However, for the reference latex a cross over point (G0 0 > G0 ) was observed which showed that the latex behaved more like a liquid at higher frequencies. According to these results it can be said that by clay addition the viscosity and physical stability of latex were increased with an enhanced shear thinning behavior. 4. Conclusions Nanocomposite based on tBA–BA–MAA/Na+MMT was successfully synthesized by seeded emulsion polymerization. During the polymerizations it was observed that the
usage of hydrophilic clay caused coagulation problems and a certain amount of emulsifier was necessary to obtain stable latex. Besides, the addition of a water soluble polymer in reaction medium increased the stabilization of latex. In addition, seeded emulsion polymerization also provided a better reaction control than batch system and decreased the risk of particle coagulation. The obtained nanocomposite latex showed fine particle size and very narrow particle size distributions with high zeta potential value exhibiting very good latex stability. The nanocomposite film showed mostly exfoliated structure which was proved by XRD and TEM observations. The thermal analyses indicated that Tg of the copolymer was not affected by clay addition; however, storage
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modulus was increased significantly. The rheological investigations evidenced that the viscosity and physical stability of the latex was increased by clay loading. The nanocomposite latex also showed a stronger shear thinning behavior which is a desired property especially for spraying solutions. The obtained nanocomposite latex could be proper for various coating applications of soft surfaces such as textile and leather. Acknowledgements The authors acknowledge the support of the EOL through the FP7-NMP-2007-CSA-1 project NaPolyNet: Setting up research-intensive clusters across the EU on characterization of polymer nanostructure and Cost Action FA090. They also thank to the project of ‘‘Industrial Doctorate Program of Textile and Leather – 2007 DPT 001’’ supported by T.R. Ministry of Development.
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References [1] Nobel ML, Mendes E, Picken SJ. Acrylic-based nanocomposite resins for coating applications. J Appl Polym Sci 2007;104:2146–56. [2] Chern CS. Emulsion polymerization mechanisms and kinetics. Prog Polym Sci 2006;31:443–86. [3] Zhu J, Wilkie CA. Intercalation compounds and clay nanocomposites. In: Kickelbick G, editor. Weinheim: Verlag GmbH & Co.; 2007. p. 151–75. [4] Fukushima Y, Inagaki S. Synthesis of an intercalated compound of montmorillonite and 6-polyamide. J Inclusion Phenom Macrocyclic Chem 1987;5:473–82. [5] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon 6-clay hybrid. J Mater Res 1993;8:1185–9. [6] Kojima Y, Usuki A, Kawasumi M, Okada A, Kurauchi T, Kamigaito O. Sorption of water in nylon 6-clay hybrid. J Appl Polym Sci 1993;49:1259–64. [7] Kojima Y, Usuki A, Kawasumi M, Okada A, Karauchi T, Kamigaito O. One-pot synthesis of nylon 6-clay hybrid. J Polym Sci. 1993;31:1755–8. [8] Yano K, Usuki A, Karauchi T, Kamigaito O. Synthesis and properties of polyimide-clay hybrid. J Polym Sci. 1993;31:2493–8. [9] Giese RF, van Oss CJ. Colloid and Surface Properties of Clays and Related Minerals. New York: Marcel Dekker, Inc.; 2002. [10] Blumstein A. Polymerization of adsorbed monolayers II thermal degradation of the inserted polymers. J Polym Sci A 1965;3:65–73. [11] Krishnamoorti R, Vaia RA, Giannelis EP. Structure and dynamics of polymer-layer silicate nanocomposites. Chem Mater 1996;8: 1728–34. [12] Lee DC, Jang LW. Preparation and characterization of PMMA-clay hybrid composite by emulsion polymerization. J Appl Polym Sci 1996;61:1117–22. [13] Noh MW, Lee DC. Synthesis and characterization of PS-clay nanocomposite by emulsion polymerization. Polym Bull 1999;42: 619–26. [14] Noh MH, Jang LW, Lee DC. Intercalation of styrene–acrylonitrile copolymer in layered silicate by emulsion polymerization. J Appl Polym Sci 1999;74:179–88. [15] Tong X, Zhao H, Tang T, Feng Z, Huang B. Preparation and characterization of poly(ethyl acrylate)/bentonite nanocomposites by in situ emulsion polymerization. J Polym Sci. 2002;40:1706–11. [16] Lai MC, Chang KC, Yeh JM, Liou SJ, Hsieh MF, Chang HS. Advanced environmentally friendly anticorrosive materials prepared from water-based polyacrylate/Na+-MMT clay nanocomposite latexes. Euro Polym J. 2007;43:4219–28. [17] Choi YS, Xu M, Chung IJ. Synthesis of exfoliated poly(styrene-coacrylonitrile) copolymer/silicate nanocomposite by emulsion polymerization; monomer composition effect on morphology. Polymer 2003;44:6989–94. [18] Xu M, Choi YS, Kim YK, Wang KH, Chung IJ. Synthesis and characterization of exfoliated poly(styrene-co-methyl methacrylate)/ clay nanocomposites via emulsion polymerization with AMPS. Polymer 2003;44:6387–95.
[19] Zheng JP, Wang JX, Gao S, Yao KD. Synthesis and characterization of PMMA/montmorillonite nanocomposites by emulsion polymerization. J Mater Sci 2005;40(17):4687–9. [20] Huang X, Brittain WJ. Synthesis and characterization of PMMA nanocomposites by suspension and emulsion polymerization. Macromolecules 2001;34:3255–60. [21] Yeh JM, Liou SJ, Lai MC, Chang YW, Huang CY, Chen CP, et al. Comparative studies of the properties of poly(methyl methacrylate)– clay nanocomposite materials prepared by in situ emulsion polymerization and solution dispersion. J Appl Polym Sci 2004;94:1936–46. [22] Wang T, Wang M, Zhang Z, Ge X, Fang Y. Preparation of core (PBAlayered silicate)–shell (PS) structured complex via c-ray radiation seeded emulsion polymerization. Mater Lett 2006;60:2544–8. [23] Lorah DP, Slone RV. Emulsion polymerization methods involving lightly modified clay and compositions comprising same. US Patent 6759463; 2004. [24] Tong Z, Deng Y. Synthesis of water-based polystyrene-nanoclay composite suspension via miniemulsion polymerization. Ind Eng Chem Res 2006;45:2641–5. [25] Sun Q, Deng Y, Wang ZL. Synthesis and characterization of polystyrene-encapsulated laponite composites via miniemulsion polymerization. Macromol Mater Eng 2004;289:288–95. [26] Diaconu G, Paulis M, Leiza JR. High solids content waterborne acrylic/montmorillonite nanocomposites by miniemulsion polymerization. Macromol React Eng 2008;2:80–9. [27] Diaconu G, Micˇušík M, Bonnefond A, Paulis M, Leiza JR. Macroinitiator and macromonomer modified montmorillonite for the synthesis of acrylic/MMT nanocomposite latexes. Macromolecules 2009;42: 3316–25. [28] Yilmaz O, Cheaburu CN, Durraccio D, Gulumser G, Vasile C. Preparation of stable acrylate/montmorillonite nanocomposite latex via in situ batch emulsion polymerization: Effect of clay types. Appl Clay Sci 2010;49:288–97. [29] Diaconu G, Paulis M, Leiza JR. Towards the synthesis of high solids content waterborne poly(methyl methacrylate-co-butyl acrylate)/ montmorillonite nanocomposites. Polymer 2008;49:2444–54. [30] Sedláková Z, Pleštil J, Baldrian J, Šlouf M, Holub P. Polymer-clay nanocomposites prepared via in situ emulsion polymerization. Polym Bull 2009;63:365–84. [31] Yılmaz O, Chiriac AP, Cheaburu CN, Nita LE, Gülümser G, Duraccio D, et al. Nanocomposites based on montmorillonite/acrylic copolymer for aqueous coating of soft surfaces. Solid State Phenom: Nanocompos. Mater. 2009;151:129–34. [32] Yu T, Lin J, Xu J, Chen T, Lin S. Novel polyacrylonitrile nanocomposites containing Na-montmorillonite and nano SiO2 particle. Polymer 2005;46:5695–7. [33] Ball LE. Anionic polymeric stabilizers for oil-in-water emulsions. US Patent 5021526; 1991. [34] Feustel D, Stritzke KH. Polymeric stabilizers for emulsion polymerization. US Patent 5277978; 1994. [35] Amalvy JI. Semicontinuous emulsion polymerization of methyl methacrylate, ethyl acrylate, and methacrylic acid. J Appl Polym Sci 1996;59:339–44. [36] Quadrat O, Snuparek J. Structure and flow properties of lattices containing carboxylic groups. Prog Org Coat 1990;18:207–28. [37] Karunasena A, Glass JE. Interactions in associative thickener/220 nm methyl methacrylate latex dispersions. Prog Org Coat 1989;17: 301–20. [38] Koh AYC, Mange S, Bothe M, Leyrer RJ, Gilbert RG. The influence of copolymerization with methacrylic acid on poly(butyl acrylate) film properties. Polymer 2006;47:1159–65. [39] Kang K, Kan C, Du Y, Liu D. Study on soap-free P(MMA–EA–AA or MAA) latex particles with narrow size distribution. Polym Adv Technol 2006;17:193–8. [40] van Herk A, Gilbert B. Emulsion Polymerisation. In: van Herk A, editor. Editor. UK: Blackwell Publishing Ltd; 2005. p. 46–79. [41] Nunes JS, Vasconcelos CL, Dantas TNC, Pereira MR, Fonseca JLC. Electrokinetic behavior of a poly(butyl acrylate-co-methacrylic acid) latex. Colloids Surf A 2006;275:148–52. [42] Chern CS, Lin JJ, Lin YL, Lai SZ. Kinetics of styrene emulsion polymerization in the presence of montmorillonite. Europ Polym J 2006;42:1033–42. [43] Becker O, Varley R, Simon G. Morphology, thermal relaxations and mechanical properties of layered silicate nanocomposites based upon high-functionality epoxy resins. Polymer 2002;43:4365–73. [44] Lee HT, Lin LH. Waterborne polyurethane/clay nanocomposites: novel effects of the clay and its interlayer ions on the morphology and physical and electrical properties. Macromolecules 2006;39: 6133–41.
O. Yılmaz et al. / European Polymer Journal 48 (2012) 1683–1695
[47] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Progr Polym Sci 2008;33:1119–98. [48] Brummer R. Rheology essentials of cosmetic and food emulsions. Berlin: Springer; 2006.
MACROMOLECULAR NANOTECHNOLOGY
[45] Xu Y, Brittain WJ. Preparation of polyacrylate-clay nanocomposites by in-situ polymerization and heterocoagulation. J Appl Polym Sci 2006;101:3850–5. [46] Blumstein A. Polymerization of adsorbed monolayers: I, Preparation of the clay–polymer complex.. J Polym Sci. 1965;3:2653–61.
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