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Jan 21, 2013 - The use of sodium montmorillonite clay as a stabilizer in the surfactant-free emulsion polymerization of n-butyl acrylate/styrene (n-BA...
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Surfactant-Free Miniemulsion Polymerization of n‑BA/S Stabilized by NaMMT: Films with Improved Water Resistance Audrey Bonnefond,† Maria Paulis,† Stefan A. F. Bon,‡ and José R. Leiza*,† †

POLYMAT and Grupo de Ingeniería Química, Departamento de Química Aplicada, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Hiribidea 72, 20018 Donostia-San Sebastián, Spain ‡ Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K. ABSTRACT: The use of sodium montmorillonite clay as a stabilizer in the surfactant-free emulsion polymerization of n-butyl acrylate/styrene (n-BA/S) was assessed. It was shown that the use of the clay alone did not yield the desired armored latex particles. A functional comonomer, that is, a phosphate ester of poly(ethylene glycol) monomethacrylate, was used to improve the interaction between the polymer and clay, thus allowing for the clay platelets to adhere to the surface of the polymer particles. The morphology of the films obtained for these two different scenarios was similar and resembled a honeycomb structure. However, their waterresistance properties differed drastically. The water absorption and water vapor permeation rate were much lower in the hybrid n-BA/ S/clay films in the presence of the functional monomer than in the films obtained without the functional monomer.

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INTRODUCTION During the last years, it has been shown that the presence of clay minerals in polymer materials provides a great opportunity to improve the properties of the final product. Clay minerals are chosen for their high aspect ratio, which upon intercalation or exfoliation of the platelets in the polymer matrix provide a huge interfacial interaction that is responsible for the enhanced properties. Furthermore, the low cost of the nanoclays as compared to that of other nanofillers (carbon nanotubes, graphene, etc.) makes them very attractive from an industrial perspective. Among the techniques employed for the synthesis of waterborne polymer/clay nanocomposites, emulsion and miniemulsion polymerization are the most used in the domain of coatings and adhesives1 and many authors took advantage of the ability of the clay to swell spontaneously in water to obtain intercalated or exfoliated clay platelets in the film. Most authors employed conventional emulsion polymerization techniques to do so.1−11 Even if the presence of clay allowed an increase in the mechanical properties of the material,2−4 the amount of surfactant needed to stabilize hybrid latexes containing clay is considerably higher.12 Unfortunately, it is known that the presence of surfactants worsens the water resistance of the films.13 To circumvent the inconvenience that the presence of surfactant can induce, emulsion polymerization has been carried out in the absence of surfactant and the clay has been employed as a Pickering stabilizer.14−23 On the basis of the results obtained by Pickering,14 much work has been carried out using different clays as polymer particle stabilizers. In 2005, Cauvin and colleagues17 reported the first surfactant-free Pickering miniemulsion polymerization © 2013 American Chemical Society

of styrene using LaponiteRD as a stabilizer. Afterward, they showed that it was possible to obtain stable Pickering miniemulsion latexes with a variety of hydrophobic monomers19 and surfactant-free emulsion polymerization with Laponite XLS.23 They showed that the stability of the clayarmored latexes depended on the affinity between the clay and the latex particles. Other authors took advantage of the fact that the clays can be easily modified to improve the assembly of clay particles at fluid−fluid interfaces.18,20−23 Voorn and co-workers18 managed for the first time to synthesize montmorillonite-based nanocomposites by surfactant-free inverse Pickering miniemulsion polymerization of aqueous acrylamide or 2-hydroxyethyl methacrylate in cyclohexane using organically modified clay platelets as stabilizers. They obtained final particles in the range of 700 nm to 1 μm diameter stabilized by Cloisite 20A platelets on their surfaces. Bourgeat-Lami and colleagues20 reported the synthesis of Pickering latexes using styrene as a monomer and Laponite RD modified with a macromonomer (PEG terminated with a methacrylate) as stabilizer. Later, they reported the first high-solids-content Pickering emulsion polymerization of poly(styrene-co-butyl acrylate) in the presence of modified LaponiteRD.22 They claimed that the use of a poly(ethylene glycol) methyl ether acrylate that adsorbed onto the clay surface enhanced the wetting of the clay on the polymer particles and therefore a stable Pickering Received: November 27, 2012 Revised: January 15, 2013 Published: January 21, 2013 2397

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monomers) were mixed for 20 min at 500 rpm and ultrasonically dispersed at 80% amplitude, 80% duty cycle for 15 min. The size of the platelets (determined by dynamic light scattering) and the pH of the dispersion were measured. Then, it was added to the reactor and stirred at 300 rpm under nitrogen. After 5 min, the monomers were added (n-butyl acrylate/styrene, n-BA/S 60/40 wt %), and the temperature was increased to 80 °C. Once the temperature was reached, the initiator (potassium persulfate, KPS, 1 wbm % dissolved in 5 g of water) was added in a shot. For the preparation of the miniemulsion, sodium montmorillonite clay (NaMMT) and SipomerPAM100 (1.65 wbm %) were mixed in water for 20 min at 500 rpm and ultrasonically dispersed at 80% amplitude, 80% duty cycle for 15 min. The organic phase containing the monomer (n-BA/S 60/40 wt %) and stearyl acrylate (SA) was also mixed for 15 min. Then, the organic phase was added to the clay aqueous dispersion, and this mixture was stirred for 15 min and subsequently sonicated for 15 min as above. The miniemulsion was then added to the reactor, and when the temperature reached 80 °C, the initiator (KPS) was added in a shot. Characterization Techniques. Polymer particle and monomer droplet sizes and size distributions were measured by dynamic light scattering (DLS) using a Zetasizer Nano Series (Malvern Instruments Ltd.). The conversions were measured gravimetrically. The clay, surfactants, salts, and SipomerPAM100 (when present) were considered not to be polymerizable fractions. The gel contents were measured gravimetrically after 24 h of Soxhlet extraction with tetrahydrofuran (THF). The sol molecular weights and sol molecular weight distributions were measured by size exclusion chromatography (SEC). The equipment was calibrated with polystyrene standards, and for sample analysis, the Mark−Houwink constants of the copolymers were calculated from the values for the homopolymers, taking into account the copolymer composition (BA/ S 60/40). The values of the Mark−Houwink constants employed were Kn‑BA = 12.2 × 10−3 mL/g, KS = 15.8 × 10−3 mL/g, αn‑BA = 0.7, and αS = 0.704.24 CryoTEM was carried out in the Electron Microscopy Facility, School of Life Sciences, at the University of Warwick. Samples were prepared in a Gatan CP3 cryoplunger. Eight microliters of sample (diluted to 0.5% solids content) was placed on a glow-discharged (in air) lacey carbon grid and blotted for 8 s. The sample was plunged into liquid ethane at −170 °C. The chamber had a relative humidity of 80%. The equipment was a Jeol 2011 microscope operated at 200 kV with a LaB6 filament, and data collection was done on a Gatan Ultrascan 1000 camera. The cryo holders were Gatan 626 holders. The dried films were cryosectioned with a Leica EMUC6 cryoultramicrotome at a temperature of 30 °C below the Tg of the sample with a Diatome 45° diamond knife, and the observations were made in a Tecnai G2 20 Twin device at 200 kV (FEI Electron Microscopes). Wide-angle X-ray diffraction (WAXD) analyses were performed on a D8 Advance (Bruker) (Cu Kα radiation with λ = 0.154056 nm) at room temperature. The range of the diffraction angles was 2θ = 2−12° at a scanning rate of 0.01° × (5 s)−1. To determine the hydrophilicity of the films, latexes were cast on silicone molds and dried in a controlled environment (23 °C and 55% humidity) for 1 week. The contact angle of a droplet of water on the films was followed with time, using an OCA 20 goniometer (DataPhysics Instruments GmbH) in air in a controlled environment (23 °C and 55% humidity). Water uptake and water vapor transmission rate measurements of the films were also performed on the dried films. For the water uptake measurements, the films were first weighed and placed into closed cups full of water. At given times, the films were taken out of the water, dried with paper, and weighed to determine the amount of water they absorbed. Afterward, they were immersed again in the same cup of water in order to follow the measurement. The water vapor transmission rate measurements (WVTR) were carried out in the following way: the film was placed onto a gravimetric cell that contained water in the bottom part in such a way that the film

emulsion could be obtained. In addition, they reported that the clay content was a factor that affected the colloidal stability of the latex; at 30% solids content, 5 wt % LaponiteRD with respect to the monomers was not sufficient to stabilize the particles, and an increase in the clay content (12 wt % with respect to the monomers) was necessary to obtain a stable latex. Zhang and co-workers21 reported the synthesis of poly(methyl methacrylate) colloidal particles stabilized and initiated by montmorillonite clay layers with poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMA) polymer brushes. Nevertheless, the TEM micrographs presented did not clearly show the location of the montmorillonite clay platelets in the latex. Furthermore, a two-pot synthesis was needed in this work. Most of the works presented above employed a clay having a small aspect ratio (Laponite, for instance) to stabilize the polymer particles effectively. A small number of publications dealt with polymer particles stabilized with a clay of higher aspect ratio such as montmorillonite by direct surfactant-free emulsion polymerization.16,21 Furthermore, the study of the properties of the films obtained from the clay-armored latexes is scarce. In this work, the synthesis of stable surfactant-free emulsion polymerization latexes using sodium montmorillonite as the only stabilizer for polymer particles is reported. The effect of the clay loading, the addition of a functional monomer (SipomerPAM100), and the polymerization strategy used (emulsion vs miniemulsion) on the morphology of the latex and the film are studied. In addition, the water uptake and water vapor transmission rate measured for the hybrid latexes were proven to be substantially better than in pure latexes.

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EXPERIMENTAL PART

Materials. The natural clay, NaMMT, was kindly supplied by Southern Clay Products Inc. (Texas). Monomers n-butyl acrylate (nBA) and styrene (S) were provided by Sigma-Aldrich. Sodium pyrophosphate (Na4P2O7, Sigma-Aldrich) was employed as a peptizing agent in order to stabilize the clay platelets. SipomerPAM100 was kindly supplied by Rhodia, and potassium persulfate (KPS, SigmaAldrich) was used as the initiator. All materials were used as received. Emulsion and Miniemulsion Polymerizations. The emulsion and miniemulsion polymerizations were carried out in a 250 mL jacketed reactor equipped with a stainless steel anchor rotating at 300 rpm under a nitrogen atmosphere. The recipes used are given in Table 1. For the emulsion polymerization procedure, the aqueous phase containing water, clay (NaMMT), and sodium pyrophosphate (5 wt % based on clay) or SipomerPAM100 (varied amounts with respect to

Table 1. Formulation of the Surfactant-Free Emulsion and Miniemulsion Polymerizations Carried out at 5% SC and 80 °C ingredients NaMMT Na4P2O7 SipomerPAM100 n-BA S SA KPS

wbm %

wt % (with respect to water)

Aqueous Phase 0−28 0−1.5 0−1.65 Organic Phase 60 40 0−4 Initiator 1

0−1.5 0−0.08 0−0.09 3.35 2.1 0−0.22 0.05 2398

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Figure 1. Evolution of (a) conversion and (b) particle size for the reactions carried out by surfactant-free emulsion polymerization (−●−) without clay (run 1), (−○−) with 14 wbm% NaMMT (run 2), (−■−) with 18.7 wbm% NaMMT (run 3), and (−□−) with 28 wbm% NaMMT (run 4) with respect to monomer in the presence of pyrophosphate. in the upper part of the cell did not touch the water. The cell was placed onto a balance in order to measure the weight loss of water during time at constant temperature. From the value of the water vapor transmission rate, the water permeability of the film was obtained.

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RESULTS AND DISCUSSION

Emulsion Polymerization. Surfactant-free emulsion polymerizations of n-BA/S (60/40 w/w) were carried out using NaMMT as stabilizer and in the presence or absence of the functional monomer SipomerPAM100. This monomer contains a phosphate group that is able to interact with the hydroxyl groups on the edges of the clay25 and ethylene oxide units that form ion−dipole interactions with the clay ions on the surface of the platelets.26 In addition, it contains a methacrylate functionality that is able to react with the monomers. Because the monomer has a low molecular weight (around 500 g·mol−1) and it is soluble in water at concentrations below 48 wt %, it was mixed in the aqueous phase with the clay prior to polymerization. Surfactant-free emulsion polymerizations were carried out by varying the amount of clay between 0 and 28 wt % with respect to the monomer (0−1.5 wt % with respect to water). From all of the latexes (including those synthesized in the absence of NaMMT), only the latex synthesized with 9.3 wbm % NaMMT (the lowest amount used) and pyrophosphate yielded coagulum (38%). Similar results were obtained by BourgeatLami and colleagues,22 who observed that below a certain amount of clay with respect to the monomers coagulation occurred and a greater amount of clay was necessary to obtain stable latexes. Figures 1 and 2 present the evolution of conversion and particle size for reactions carried out with and without clay (with pyrophosphate) and with SipomerPAM100, respectively, and Table 2 presents the characterization results of the stable latexes synthesized with NaMMT and the latexes synthesized in the absence of clay. For the formulation that did not contain SipomerPAM100, the reactions carried out in the presence of NaMMT were faster. The first 50 min of reaction was almost the same for all concentrations of NaMMT (the same was also observed for the evolution of the particle size). After this time, differences showed up in a manner that cannot be rationalized as a function of the concentration of NaMMT. The polymerization was fastest for the experiment with 14 wbm%. Nevertheless, the latexes containing clay ended with a similar particle size (∼210

Figure 2. Time evolution of (−●−) conversion and (−○−) particle size for the reaction carried out by surfactant-free emulsion polymerization with 1.65 wbm% SipomerPAM100 and with 14 wbm % NaMMT (run 6).

nm) that was substantially smaller than that of the latex synthesized in the absence of NaMMT (∼300 nm). Thus, the amount of clay did not influence the final particle size because similar diameters were measured for the three reactions containing clay, but experiments with clay increased the number of stable particles (Table 2) and hence the polymerization rate. In the absence of clay, the particles are stabilized only by the initiator fragment charges (SO4− groups). Consequently, in the early stages of particle formation, because the polymer particles are small, their surface area is high and their surface charge density is low. Therefore, newly formed polymer particles coalesce to form polymer particles with a lower surface area and a higher surface charge density. When the charges on the polymer particle surfaces are enough to provide stability by electrostatic repulsion, the system is stable and the number of particles becomes constant. In the presence of clay, some colloidally unstable precursor particles may flocculate onto clay platelets that stabilize them because of their high charge density. As a result more particles of a smaller size can be made stable during the polymerization.27,28 This explains why occasionally armored particles were found in the cryo-TEM micrographs (Figure 3). For the latexes containing SipomerPAM100, it was observed that the kinetics of reaction with 14 wbm% NaMMT was slower than the corresponding one with pyrophosphate (cf. Figures 2 and 1). Furthermore, the evolution of the particle size was completely different for run 2 and 6, and the final particle sizes were half the size of those obtained in the absence of the 2399

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Table 2. Characterization Results of the Surfactant-Free Latexes Synthesized by Emulsion and Miniemulsion Polymerization with Different Amounts of Clay and without or with SipomerPAM100

a

wbm% NaMMT, run

Dp/nm

X/%

none, run 1 14, run 2 18.7, run 3 28, run 4

301 207 212 209

88.2 94 93 81

none, run 5 14, run 6a

107 129

64 91.5

14, run 7a

181

90.5

pHi

pHf

gel/%

Mw/10−3 g·mol−1

Emulsion without SipomerPAM100 4.4 0 7 0 6.5 0 7.4 0 Emulsion with SipomerPAM100 3 2.5 0 6.2 4.9 12 Miniemulsion with SipomerPAM100 and SA 6.2 4.6 45 8 8 8 8

Np 10−15/L

Nplat 10−17/L

90 146 85 127

3 12 11 8

1 1.2 1.7

163 156

47 38

0.9

200

14

0.9

0.26 meq of SipomerPAM100/g of clay.

Figure 3. CryoTEM pictures obtained from the final latex synthesized by surfactant-free emulsion polymerization with 14 wbm% NaMMT with respect to the monomer (run 2). The bar is 200 nm.

functional monomer (runs 1−4). This is a clear indication of a change in the nucleation mechanism of polymer particles. In runs 1−4, polymer particles were likely nucleated by homogeneous nucleation with the additional stabilization provided by clay platelets exfoliated in the aqueous phase (Figure 3). Adsorption isotherms of SipomerPAM100 in clay aqueous dispersions revealed that around 60% of SipomerPAM100 was adsorbed on the clay and the rest was in the aqueous phase. Thus, the free functional monomer might have reacted in the aqueous phase with n-BA/S oligoradicals and created amphiphilic moieties that might have aggregated to form particles that adsorbed monomer and served as polymerization nuclei. The evolution of the particle size distribution (PSD) in Figure 4 supports this hypothesis. Initially, the PSD is due to the nanoclay dispersed in the aqueous phase. After some time, a bimodal distribution was observed with a population of particles smaller than 100 nm and the larger population due to the clay platelets. The polymer particles grew, and the platelets adsorbed at the surface of the particles or the small particles containing SipomerPAM100 species at the surface adhered to the exfoliated clay platelets (Figure 5), creating a stable dispersion with significantly smaller polymer particle sizes than when the functional monomer was not used. The micrographs displayed in Figures 3 and 5 (obtained by cryoTEM analysis) highlight the significant difference between

Figure 4. Particle size distribution of the reaction carried out by surfactant-free emulsion polymerization with 1.65 wbm% SipomerPAM100 and with 14 wbm% NaMMT (run 6) at (−●−) 0 min, (−○−) 10 min, and (−■−) the end of the reaction.

the morphology of the latex obtained without SipomerPAM100 (run 2, Figure 3) and the one obtained in the presence of the functional monomer (run 6, Figure 5). In run 2, even though some clay platelets were located at the surface of the polymer particles, many free platelets were also observed in the aqueous phase and most of the polymer particles were spherical and were not attached to clay platelets. In the presence of the functional monomer (run 6), the 2400

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Figure 5. Micrographs obtained by cryoTEM analyses of the latex synthesized by surfactant-free emulsion polymerization and containing SipomerPAM100 and 14 wbm% NaMMT with respect to monomer (run 6). The bar is 100 nm.

characterization of the final latex (run 7) is presented in Table 2.

morphology of the latex changed significantly, showing a larger number of exfoliated clay platelets located at the surface of the polymer particles. The driving force for the adsorption of platelets onto polymer particles is its energy of attachment that depends on the contact angle between the platelet and the monomer/water interface.15 Because sodium montmorillonite is hydrophilic, it does not have good compatibility with the monomers. Consequently, in the absence of the functional monomer, most of the clay platelets remained dispersed in the aqueous phase and very few acted as polymer particle stabilizers. However, when the functional monomer was adsorbed into the clay platelets, it conveyed hydrophobic character to them and facilitated the adsorption of clay onto the polymer particles. The gel contents were measured by Soxhlet extraction using THF as the solvent. Runs 1 to 5 did not present any gel. This result was expected because other authors proved that by the addition of 10 wt % styrene to the polymerization of n-butyl acrylate practically no gel was formed during the polymerization as a result of the reduced chain transfer to the polymer reaction and the higher reactivity ratio of the styrene monomer.29,30 Surprisingly, gel was obtained in the presence of clay and SipomerPAM100 (run 6). This result was not expected and could be explained by the presence of the clay with SipomerPAM100. Actually, we believe that these values for the gel did not account for a cross-linked polymer network or large polymer chains. SipomerPAM100 adsorbed at the surface and edges of the clay platelets contained a methacrylic group that was able to react with the comonomers during the polymerization. Therefore, the reaction between SipomerPAM100 (attached to the clay) and active growing chains might lead to polymer/clay structures not soluble in THF. These polymer chains would remain in the filter. Therefore, a higher gel content was measured in the presence of clay and SipomerPAM100. Note that in the absence of SipomerPAM100 the polymerizations with clay alone (run 2−4) did not yield gel, and in the absence of clay, the polymerization with Sipomer alone did not yield gel either (Table 2). Miniemulsion Polymerization. As described previously, a surfactant-free miniemulsion polymerization (run 7) was also carried out. Figure 6 presents the evolution of the conversion, and the particle size observed in this reaction and the

Figure 6. Time evolution of (−●−) conversion and (−○−) particle size for the latex synthesized by surfactant-free miniemulsion polymerization with 14 wbm% NaMMT in the presence of SipomerPAM100 and stearyl acrylate (run 7).

The polymerization rate of the miniemulsion polymerization (run 7) was much faster than the corresponding emulsion polymerization (run 6) and slightly faster than run 2 carried out without SipomerPAM100 (Figure 7). The final number of polymer particles calculated from the particle sizes (Table 2) indicated that run 6 had the largest number and that runs 7 and 2 had similar values (slightly more in run 7 than in run 2). At first glance, this is in disagreement with the polymerization rates discussed above. Therefore, this result cannot be explained by the mechanisms that describe a true emulsion polymerization process. In run 6, the SipomerPAM100 participated in the polymer particle nucleation (creating amphiphilic species by reaction with the hydrophobic oligoradicals of S and BA) and stabilization, as explained previously. This nucleation mechanism mimics, to a certain extent, the nucleation of particles in dispersion polymerization, which proceeds at a much slower rate.31 This led to the slower formation of smaller particles and likely to a higher termination of oligoradicals in the aqueous phase, which reduced the polymerization rate in run 6. In the surfactant-free miniemulsion polymerization, the initial droplets formed by 2401

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(run 2), with SipomerPAM100 and 14 wbm% NaMMT (run 6), and by miniemulsion polymerization with SipomerPAM100 and 14 wbm% clay (run 7), respectively). The micrographs of the three films show an arrangement of the clay platelets with a structure that resembles the films made of latex particles with clay platelets at the interface of the particle and aqueous phases.33 However, as depicted in Figure 6, this was not true in the case of run 2. Therefore, during film formation (evaporation of water and coalescence of polymer particles), the exfoliated clay platelets got trapped leading to structures that are similar to honeycomb morphologies. In addition, the micrographs obtained at larger magnification showed that predominantly intercalated morphologies (darker regions) were obtained as a consequence of the high loading of clay with respect to the polymer (14 wbm% NaMMT). On the contrary, in the presence of SipomerPAM100, the micrographs evidenced the good dispersion of the clay platelets in the polymer matrix. The number of darker regions (representative of clay stacks or aggregates) is smaller for runs 6 and 7. The higher-magnification micrographs also displayed a higher degree of exfoliation although the large loading of clay (14 wt % with respect to the monomers), led to a predominantly intercalated morphology. The intercalated morphologies observed in the three cases were confirmed by the WAXD analysis presented in Figure 12. The interlayer space of the clay was larger for runs 6 and 7 because of the presence of SipomerPAM100 that was adsorbed on the clay surface and edges and that reacted with the polymer. Thus, in the presence of SipomerPAM100, the clay had a better compatibility with the polymer matrix, and this resulted in a better intercalation of the polymer in the clay platelets. Film Properties. One of the main advantages of latexes stabilized by clay platelets is that no surfactants are used and therefore films would likely be less sensitive to water. To measure the water resistance of the films, water uptake and water vapor transmission rate (WVTR) measurements were carried out on the films. Figure 13 presents the water uptake results obtained for runs 2, 6, and 7 (with clay and with and without SipomerPAM100, obtained by surfactant-free emulsion polymerizations and with

Figure 7. Evolution of conversion for the reactions carried out by surfactant-free emulsion polymerization (−●−) with 14 wbm% NaMMT and pyrophosphate (run 2), (−○−) with 14 wbm% NaMMT and SipomerPAM100 (run 6), and (−■−) with 14 wbm% NaMMT and SipomerPAM100 by surfactant-free miniemulsion polymerization (run 7).

sonication and stabilized by SipomerPAM100 and clay platelets were rapidly nucleated and a large number of polymer particles (Figure 8) were formed, which led to a much higher polymerization rate than in run 6. Run 2 was also faster than run 6, likely because in the absence of SipomerPAM100 polymer particles were nucleated by homogeneous nucleation, quickly leading to a compartmentalized system and to a faster polymerization rate. Interestingly, the gel content for run 7 was larger than for run 6. As previously explained, the formation of gel should be related to the presence of SipomerPAM100. The size of the polymer particles in run 7 was larger than in run 6, but according to the kinetics, the radical flux should be higher in run 7 because the polymerization rate is significantly faster. This might explain a higher concentration of radicals in the polymer particles and hence a higher likelihood of bimolecular termination of branched chains and hence gel formation.30,32 Morphology of the Films. Figures 9−11 display TEM micrographs of the films cast with latex from runs 2, 6, and 7 (i.e., the final latexes synthesized by surfactant-free emulsion polymerization with pyrophosphate and 14 wbm% NaMMT

Figure 8. Micrographs obtained by TEM analysis of the latex synthesized with SipomerPAM100 and with 14 wbm% NaMMT (run 7) by surfactantfree miniemulsion polymerization. 2402

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Figure 9. TEM micrographs of the cryo-sectioned film obtained from run 2 (14 wbm% NaMMT and pyrophosphate).

Figure 10. TEM micrographs of the cryo-sectioned film obtained from run 6 (14 wbm% NaMMT and Sipomer PAM100).

Figure 11. TEM micrographs of the cryo-sectioned film obtained from run 7 (14 wbm% NaMMT and Sipomer PAM100 by miniemulsion polymerization).

It can be observed that the film cast from run 2 absorbed more water and absorbed it much faster than the two other

clay and SipomerPAM100 obtained by surfactant-free miniemulsion polymerization, respectively). 2403

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Table 3. Contact Angle Measurements for Runs 2, 6, and 7

Table 4 presents the results of water vapor transmission rate (WVTR) measurements obtained from the permeability to water measurements of the different hybrid films.

Figure 12. WAXD spectra of (−●−) NaMMT, (−○−) run 2, (−□−) run 6, and (−■−) run 7.

Table 4. Water Vapor Transmission Rate (WVTR) for Films Cast from Latexes in Runs 2 and 6 run

WVTR/Barrer

without SipomerPAM100 (run 2) with SipomerPAM100 (run 6)

84 ± 8 19 ± 2

In the absence of the functional monomer, the water vapor permeability was higher than that with SipomerPAM100. The presence of SipomerPAM100 helped to reduce the water vapor transmission rate resulting from the higher hydrophobicity of the film as explained before in the presence of the functional monomer.

Figure 13. Water uptake measurement for runs 2, 6, and 7. Legend: (−●−) run 2, (−○−) run 6, (−■−) run 7, synthesized with clay by surfactant-free emulsion polymerization without SipomerPAM100, with SipomerPAM100, and by surfactant-free miniemulsion polymerization, respectively.

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CONCLUSIONS In this work, the use of NaMMT clay as a stabilizer for the polymerization of n-BA/S by a one-pot surfactant-free emulsion and miniemulsion polymerization was studied. It was proven that when pristine clay was used, the NaMMT clay platelets were dispersed in the latex aqueous phase and did not preferentially interact with the polymer particles. However, when a monomer able to attach to the clay surface (Sipomer PAM100) was added to the recipe, the wettability of the clay with the polymer mixture was improved and a Pickering morphology was obtained. Furthermore, the use of miniemulsion polymerization with the clay as a droplet stabilizer in the presence of Sipomer PAM100 led to a faster polymerization rate and a higher gel content. In all cases, when 14 wbm% clay was employed in the recipe, structures that resemble the films made of latex particles with clay platelets at the interface of the polymer particles and aqueous phase were obtained after film formation. Nevertheless, even if the film morphology was a honeycomb type in all cases, the compatibility between the clay and the polymer was not the same, which resulted evident after the evaluation of the final properties. Thus, the water absorption of the film obtained from the latexes containing NaMMT alone was fast. Also, the WVTR was higher. In contrast, the addition of SipomerPAM100 to a latex containing clay helped to decrease the water uptake of the film compared to that of the respective film obtained from the latex without clay. As a conclusion, it can be said that an efficient way to produce more water-resistant polymer films has been obtained by the use of montmorillonite clay and a functional monomer such as SipomerPAM100.

films did. In the presence of the functional monomer, because this one absorbed on the surface and edges of the clay and reacted with the polymer, the clay was attached to the polymer and became more hydrophobic, absorbing less water. In addition, the honeycomb morphology that the clay platelets adopted in the film prevented the polymer present inside the clay shell from swelling. Thus, the water uptake decreased. Without SipomerPAM100, the presence of hydrophilic sodium montmorillonite clay conveyed a hydrophilic character to the film and thus the water uptake was higher. The lower water uptake observed for run 7 compared to that for run 6 could be attributed to the higher gel observed in this case that could be ascribed to a higher degree of attachment of the polymer on the clay platelets, owing to the presence of SipomerPAM100 adsorbed on the clay surface. Thus, the clay was more hydrophobic, and the film absorbed less water. Table 3 presents the contact angle measurements of a droplet of water on the films with clay and without SipomerPAM100 (run 2), with clay and SipomerPAM100 (run 6) obtained by surfactant-free emulsion polymerization, and with clay and SipomerPAM100 obtained by surfactant-free miniemulsion polymerization (run 7) as a function of time. The results confirm those previously obtained because the film obtained from run 2 was the most hydrophilic and presented the lowest contact angles with water as a function of time whereas the most hydrophobic films were obtained when SipomerPAM100 was used in the synthesis of the film containing clay. 2404

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 943015329. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the European Union (Woodlife project FP7-NMP-2009-SMALL-246434), Basque Government (GVIT-303-10), and UPV/EHU (UFI 11/56) is gratefully acknowledged. We thank the Electron Microscopy Facility, School of Life Sciences, University of Warwick (Welcome Trust grant reference 055663/Z/98/Z) for instrument use and technical support and especially Ian Portman for his help with the preparation of the samples and the cryoTEM pictures. The sGIKer UPV/EHU for the electron microscopy facilities of the Gipuzkoa unit and SGI/IZO-sGIker UPV/EHU (supported by the National Program for the Promotion of Human Ressources within the National Plan of Scientific Research, Development and Innovation-Fondo Social Europeo, Gobierno Vasco and MCyT) are also gratefully acknowledged.

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dx.doi.org/10.1021/la3047033 | Langmuir 2013, 29, 2397−2405