Reinforcement of Polymeric Latexes by In Situ Polymerization

Copyright © 2009 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 9, 1–7, 2009

Reinforcement of Polymeric Latexes by In Situ Polymerization Andres F. Vargas1 2 ∗ , Witold Brostow2 , Haley E. Hagg Lobland2 , Betty L. López1 , and Oscar Olea-Mejia2 3 1

Grupo Ciencia de los Materiales, Instituto de Química, Universidad de Antioquia, Calle 62, 52 59 Medellín, Antioquia, Colombia 2 Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science and Engineering, University of North Texas, 1150 Union Circle # 305310, Denton, TX 76203-5017, USA 3 Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA), Facultad de Química, Universidad Autónoma del Estado de México, Km. 12 de la carretera Toluca-Atlacomulco, San Cayetano 50200, Mexico

Keywords: Emulsion Polymerization, Silica Particles, Hybrid Latexes, Scratch Resistance.

1. INTRODUCTION Polymeric nanocomposites show unusual properties— often related to the important role of interfacial forces and to chemistry of surface molecular layers as the size of the dispersed phase decreases and its surface area increases. Thus, better mechanical,1–3 electrical,4 thermal5–7 or tribological8–10 properties can be obtained by modifying polymers with inorganic nanoparticles. In the field of coatings it is important to reduce the use of organic solvents in the formulations or replace them by waterborne coatings; this suggests improvement3 of properties of polymeric latexes used in the coatings. Nanoparticles have been used to improve several properties of polymers, for instance to increase the Young modulus and yield stress of poly(ethyl acrylate) latex encapsulating functionalized silica particles.1 Modified nanoparticles of TiO2 were introduced into a polyacrylate resin; films obtained from the modified resins exhibit an increase of hardness and also a reduction in water adsorption and ∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2009, Vol. 9, No. xx

permeability.11 A paint containing silica nanoparticles in an acrylic latex obtained by emulsion polymerization has a better flame resistance than a similar paint without silica.12 There are several ways to prepare polymer nanocomposites such as blending, solvent mixing, in situ polymerization, etc. A disadvantage of blending is the large amount of energy needed to achieve a homogeneous phase. On the other hand, an advantage of in situ polymerization is the number of parameters of the polymer and the filler that can be controlled simultaneously to optimize the properties of the final material. Polymeric latexes have been modified with inorganic particles by means of miniemulsion,13 emulsion14 polymerization among others; emulsion polymerization is the most frequently used. Thus nanocomposites with different morphologies such as core–shell13 raspberry-like,15 daisy-shape16 were obtained. The structures are dependent on the surface and the size of the inorganic particles and also on matrix–filler interactions. The interactions can be varied by using surfactants,17 or functionalization agents.5 13 As discussed by Kopczynska and Ehrenstein18

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doi:10.1166/jnn.2009.1329

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Two silicas with different particle sizes have been synthesized by the Stöber method. The particles have been functionalized with methacryloyl groups. In situ emulsion polymerization of butyl acrylate and methyl methacrylate in the presence of functionalized silica particles was performed. The ratio of butyl acrylate to methyl methacrylate was varied in order to optimize the composition for improvement of tribological and thermophysical properties. The silica particles morphology and functionalization have been determined respectively by scanning electronic microscopy and infrared spectroscopy. The composites were characterized also by thermogravimetric analysis, differential scanning calorimetry, microscratch testing and static light scattering. The latex reinforced with the smallest functionalized silica exhibits higher thermal stability than the non reinforced latex, along with lower penetration depth and higher residual depth in progressive load scratch testing. Thus, the resistance to penetration is increased while viscoelastic healing is hampered by silica particles.

Reinforcement of Polymeric Latexes by In Situ Polymerization

interface energies are decisive for properties of multiphase materials. In this work, two nanosilicas with different particle size have been functionalized with methacryloyl groups and used to reinforce butyl acrylate/methyl methacrylate copolymers by in situ emulsion polymerization. Our goal was to study the effect of copolymer composition, silica particles incorporation as well as silica particle size on the thermal and tribological properties of the synthesized latexes. A pertinent review shows that much remains to be done to improve polymer tribology,19 and we are advancing our knowledge through this work.

2. EXPERIMENTAL DETAILS 2.1. Materials Tetraethoxysilane (TEOS) and butyl acrylate (BA) were purchased from Fluka. Ammonium hydroxide (28–30% in water), ethanol, methanol, ammonium persulfate, sodium dodecyl sulphate, methacryloylpropyltrimethoxisilane (MPS) and methyl methacrylate (MMA) were purchased from Sigma-Aldrich. All of them were reagent grade and used without further purification.

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2.2. Silica Particle Synthesis and Functionalization Two silica particles of different size were synthesized according to the well known procedure of Stöber.20 Ethanol, ammonium hydroxide and water were introduced in a three neck round flask equipped with a refrigerating system. Then the mixture was stirred at 300 rpm to homogenize at room temperature. After that TEOS was added into the solution and the reaction proceeded for 4 hours under stirring. The reagents ratio was varied to obtain two different silica particles sizes. An alcoholic suspension of silica was centrifugated at 7000 rpm or 8000 rpm, depending of the particles size, and washed with deionizated water several times to remove the ammonium hydroxide and ethanol. After that the particles were washed once more with ethanol and allowed to dry overnight at room temperature. Functionalization of silica particles was achieved as follows. First the silica powder was crushed, then water and methanol were added to the powder, and then the suspension was ultrasonicated for 1 hour to re-disperse the particles. In a two neck round flask the silica dispersion was mixed with metacryloylpropyltrimethoxisilane (MPS) in a molar ratio of 1:10 and a few drops of ammonium hydroxide were added. After that, it was left to react for 4 h at 70 C.21 2.3. Polymerization Emulsion copolymerization of BA and MMA was carried out by using seeded semibatch emulsion polymerization 2

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in a reactor equipped with a condenser, mechanical stirrer, addition funnel, nitrogen inlet and outlet. First, the initiator was dissolved in a part of water and the rest of water was charged into the reactor. The monomers were mixed in a separate beaker. Then the surfactant and a part of the monomer mixture were added to the reactor. The other portion of the monomer mixture was poured into the addition funnel. The addition of the monomer was performed in two steps. First, the initiator solution was added, then a half of the monomer was added dropwise for 30 minutes through a funnel and allowed to react for 30 minutes. Then the second half of the monomer was poured dropwise for 30 min, and the reaction lasted for another 30 min. The total time of the polymerization reaction was two hours since the addition of the initiator. The same procedure described above was followed for the synthesis of the reinforced latex, except that the functionalized silica was pre-emulsified with the monomer and the surfactant. The silica concentration used was 4% weight nominal respect to monomers. 2.4. Scanning Electron Microscopy (SEM) A FEI dual beam FESEM/FIB microscope, where FIB means focused ion beam, was used to analyze the morphology of the silica particles. The samples were dispersed in ethanol, and then a few drops were poured into the SEM holder and allowed to dry. Finally the samples were put in a high vacuum chamber and a high voltage was applied to coat the sample with a thin layer of metallic gold. In order to see the nanoparticles inside the composites the combination of the techniques SEM-FIB was performed. This method has recently been successfully used by our group in low density polyethylene with aluminum particles added.22 2.5. Fourier Transform Infrared Analysis (FTIR) IR spectra were obtained in a Perkin-Elmer Spectrum One machine. KBr pellets were prepared with functionalized and non-functionalized silica particles. For the polymeric latex, a dilution of 1:10 was made, and a film was formed by evaporation of the emulsion on a ZnSe disc. 2.6. Thermogravimetrical Analysis (TGA) Thermogravimetric analysis was performed in a TA Instruments Q500 equipment. Films were obtained from the latex through evaporation. The thermal stability of the films was evaluated heating from room temperature to 800 C at 20 C/min in nitrogen atmosphere at the flow rate of 100 mL · min−1 . J. Nanosci. Nanotechnol. 9, 1–7, 2009

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2.7. Differential Scanning Calorimetry (DSC)

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TA Instruments Q100 was used to determine glass transition Tg temperatures. Thermal history was erased at the heating rate of 30 C/min to 200 C, the sample cooled to −80 C, and then heated at 20 C/min to 200 C. 2.8. Progressive Load Scratch Test CSEM microscratch tester was used. First films were deposited on polycarbonate substrates and then analyzed in progressive mode using a 200 m conical diamond indenter. Instantaneous penetration depth Rp and residual depth Rh values in the scratch grooves after viscoelastic recovery were recorded. 2.9. Particle Size

(b)

The particle size analysis was carried out from the SEM images using the open source software ImageJ® . In addition, the emulsions were tested using light scattering in a Microtac S3000 particle size analyzer. The emulsions were put directly in the sampler and then analyzed. 2.10. Haze

Two kinds of silica particles with different sizes were synthesized by changing ammonium hydroxide concentration while maintaining constant TEOS and water concentration using the well known Stöber method.20 The molar ratios were: for silica S1 NH4 OH 0.3/TEOS 0.2/Water 6.8; and for silica S2 NH4 OH 0.4/TEOS 0.2/Water 6.8. Figure 1 shows the SEM micrographs for S1 and S2 silica samples. It can be observed that the particles have spherical shape, smooth surface and the particle size is quite uniform. Moreover, no coalescence is observed either in the S1 or S2 samples. S1 silica sample presents an average size of 291±38 nm and S2 silica sample 350 ± 33 nm. This indicates that the particles size can be controlled by changing the synthesis parameters. Figure 2 shows the infrared spectra for the silica particles. A band at 1100 cm−1 corresponds to Si–O–Si and at 945 cm−1 to Si–OH groups. A small band at 1720 cm−1 observed for the functionalized silica can be attributed to carbonyl groups of the MPS methacryloyl moiety. The presence of this band indicates that functionalization was successfully achieved. J. Nanosci. Nanotechnol. 9, 1–7, 2009

Fig. 1. SEM of silica particles S1 (a) and S2 (b).

4. COPOLYMERS In order to obtain copolymers with different glass transitions, three copolymers were synthesized by varying the proportions of BA/MMA in the following weight ratios: 50:50, 60:40 and 70:30.

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3. CHARACTERISTICS OF SILICA PARTICLES

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The haze of the samples was measured in a Colorquest II spherical spectrophotometer. Films were obtained after evaporation of the emulsion over polycarbonate substrates and then the samples were analyzed.


5. LIGHT SCATTERING (LS) RESULTS

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IR spectrum for (a) 60BA/40MMA and (b) 60BA/40MMA/S1.

By following the conversion of 60BA/40MMA over time, an optimized time of reaction of 120 minutes was obtained. This time was used for all the reactions.

Figure 4(a, b, c) shows the particle size distribution versus light intensity. All samples have an average particle size around 100 nm with a narrow distribution. A slight distribution broadening is seen when BA content increases. The sample 70BA/30MMA present two more peaks around 500 and 1600 nm. BA and MMA were copolymerized randomly. BA is more hydrophobic than MMA; therefore, BA particles need more surfactant adsorbed on their surfaces to achieve colloidal stability. Since surfactant concentration was kept constant while BA concentration was increased, particles with poor surfactant coverage could form by heterogeneous nucleation, thus changing the particle size distribution. 5.2. Latex with Functionalized Silica S1

4.1. Latex Without Silica

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All polymeric latexes without silica gave stable emulsions, still unchanged after 6 months. Figure 3(a) there is a typical IR spectrum for 60BA/40MMA latex. The spectrum shows vibrations at 2960 cm−1 (stretching saturated C–H), 1456 cm−1 and 1385 cm−1 (symmetrical and asymmetrical extension –CH3 , –CH2 –) 1735 cm−1 , 1245 cm−1 , 1165 cm−1 (carbonyl C O, ester –C–O–), all characteristic of polyacrylate. No signal for double bonds is observed, indicating complete polymerization. 4.2. Latex with Functionalized Silica Latexes synthesized with functionalized silica were also stable after 6 months. Figure 3(b) shows a typical IR spectrum obtained for the 60BA/40MMA/S1 hybrid polymer. The spectrum shows the same bands seen for the latex without silica. However, in the present case there is a small shoulder around 1100 cm−1 , which is assigned to the Si–O–Si extension of the silica. This is an indication that the silica was successfully incorporated into the polymer.

Figure 4(d, e, f) shows the size distribution versus intensity for the latex with functionalized S1 silica. The incorporation of S1 into the emulsion causes changes in the size distribution of all samples as follow: 50BA/50MMA/S1 shows a sharp distribution centered around 100 nm, 60BA/40MMA/S1 shows maximum size distribution at 115 nm and 70BA/30MMAS1 shows a broader distribution at 120 nm. A broadening with increasing BA contents is seen here also. 5.3. Latex with Silica S2 From Figure 4(g, h, i) it is seen that the silica particle size increases and considerable changes in the morphology of the latexes take place. The samples 50BA/50MMA/S2 and 60BA/40MMA/S2 show maxima in their size distributions at 127 nm and 136 nm, and shoulders at 270 nm and 550 nm, respectively. The sample 70BA/30MMA/S2 presents a multimodal distribution with maxima at 167 nm, 395 nm and 1670 nm. The size distribution is wider for this latex than those for hybrid latexes with functionalized silica S1.

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Fig. 4. Particle size distribution of latex without and with silica (a) 50BA/50MMA, (b) 60BA/40MMA (c) 70BA/30MMA (d) 50BA/50MMA/S1, (e) 60BA/40MMA/S1 (f) 70BA/30MMA/S1 (g) 50BA/50MMA/S2, (h) 60BA/40MMA/S2 (i) 70BA/30MMA/S2.

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6. MORPHOLOGY AND PHYSICAL PROPERTIES

7. THERMOPHYSICAL PROPERTIES 7.1. Thermogravimetric Analysis (TGA) The TGA technique has been described by Menard23 and also by Lucas and her colleagues.24 Figure 6 shows the

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thermal decomposition curve for 60BA/40MMA; only one step is seen. The maximum decomposition temperature (Tdmax ) is obtained from the derivative of weigh loss versus temperature. Table I shows maximum decomposition temperatures for all the samples. An increase in Tdmax is observed for all samples with S1 silica. However, samples containing S2 do not show this behavior; actually, a reduction in decomposition temperature is observed for 60BA/40MMA/S2. Since silica has a thermal stability significantly higher than the polymer, it is plausible to expect a positive effect on the thermal stability of the composites. This was observed for S1 but not for S2. S1 particles are small, the total interfacial area is large and the area of interaction between the polymer matrix and the particles is also large, facilitating a stronger influence of the silica particles on properties of the composites. Thus, for 50BA/50MMA the addition of silica has resulted in an increase of the decomposition temperature by almost 16 K. For 60BA/40MMA the effect is smaller but clear, 7 K or so. For 70BA/30MMA we have an 18 K increase. By contrast, in the case of S2 samples a positive effect on thermal stability is not found due to the larger particle sizes and therefore smaller overall interface area. For 50BA/50MMA the addition of S2 results in a change Thermal stability results.

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Image of a film from 50BA/50MMA/S1 after FIB milling.

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Tdmax / C 4380 4536 4366 4473 4542 4331 4363 4545 4381

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Figure 5 shows a SEM micrograph of a 50BA/50MMA/S1 film after the FIB milling. After removal by FIB of some layers of the polymer, the particles underneath can be seen. We note that functionalized silica particles are well dispersed in polymer matrix. For coating industry properties of resins such as high hardness and also flexibility are essential. Therefore, synthesized emulsions were air-dried at 80 C, and films prepared by deposition of small quantities of latex on glass plates. From the macroscopic appearance of the films it could be seen that the 50BA/50MMA sample was the hardest, retaining its transparency. 60BA/40MMA was softer than the former and again also transparent. Finally 70BA/30MMA was the softest while sticky. All samples show good adhesion to glass. While 50BA/50MMA exhibits high hardness, other samples adhere to glass substrate better. Harder materials exhibit less viscoelastic recovery, see Section 9 below.

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Changes in morphology with addition of functionalized silica can be explained by absorption of surfactant. The functionalized silicas (S1 and S2) have hydrophobic character, which allows them to absorb surfactant during the polymerization. Consequently some surfactant is not available to stabilize the monomer droplets. As a result, some monomer droplets may increase in size, reducing their surface/volume ratio. This in turn changes the progress of emulsion polymerization and as a final result the size distribution can vary for the latexes with silica.


Weigth / %

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Table II. Glass transition of polymeric films.

restricted due to the interaction between silica and polymer chains-through covalent bonds or interactions between chains attached to the silica and surrounding chains. On the other hand, 50BA/50MMA/S2 shows a lower Tg value than the sample without silica. The particle diameter has an important role in silica particles interactions with polymeric chains; as above when discussing thermal stability, lower surface area and interruptions in the matrix structure are important. The 70BA/30MMA sample shows a reduction in Tg with both silica particles size (S1, S2). Apparently, when BA content is less than 60 wt%, the main factor that affects the Tg is the silica particle size; at higher BA contents the Tg is affected mainly by the amount of BA. Due to the better thermal properties obtained with silica S1, these samples were subjected to the remaining tests.

Sample

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176 262 80 −70 −12 −68 −108 −181 −198

Table III. Haze results for latexes with S1. Haze 50BA/50MMA 60BA/40MMA 70BA/30MMA 50BA/50MMA/S1 60BA/40MMA/S1 70BA/30MMA/S1

134 202 171 264 236 238

8. TRANSPARENCY Haze is a measure of lack of transparency caused by light scattering—including scattering by particles of the dispersed phase. For potential optical applications it is important to measure the haze. Our films were deposited on polycarbonate substrates. Haze results for uncoated polycarbonate (highly transparent) were used a reference. Table III lists the haze results. As more and more particles are incorporated into polymeric matrices, an increase in haze occurs as expected. This agrees with the results obtained by LS and FIB.

of Tdmax of ≈1 K, downwards for that matter. For 60BA/40MMA there is a decrease of Tdmax by 13 K, apparently an effect of interruption of cohesion of the polymer matrix structure by the additive. For 70BA/30MMA we have an increase of Tdmax by 2 K.

DSC was used to locate glass transition temperatures Tg . This technique has also been described by Menard23 and by Lucas and coworkers.24 The importance of Tg has been discussed before.25 A so-called midpoint method was used, namely the temperature at which a line connecting the extrapolated baselines below and above the glass transition region intersects with the DSC curve was taken as the Tg . Only one Tg is observed in each of the samples; presumably random copolymers are obtained. Table II lists the results. 50BA/50MMA/S1 and 60BA/40MMA/S1 show each an increase in Tg value with respect to the same sample without silica. We infer that the polymer chains mobility is (b)

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9. SCRATCH RESISTANCE Earlier work has shown how incorporation of nanopowders or micropowders can improve tribological properties of Polyamide 6,26 an epoxy10 or polyethylene or a thermoplastic elastomer.27 Earlier experience also shows how useful are scratch resistance determinations.19 28 Our latex films on polycarbonate substrates were studied in progressive load scratch testing up to failure. Figure 7 contains penetration depth Rp versus force diagrams for all samples.

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7.2. Differential Scanning Calorimetry (DSC)

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Fig. 8. Viscoelastic recovery versus applied force for all samples: (a) 50BA/50MMA/S1 (b) 50BA/50MMA (c) 60BA/40MMA/S1 (d) 60BA/40MMA (e) 70BA/30MMA/S1 and (f) 70BA/30MMA.

For all the film types, both Rp and Rh (not shown) increase with the force until the film failure point. Rp is lower for samples containing silica S1, indicating a reinforcement by the functionalized silica. Viscoelastic recovery has been calculated19 28 as Rh = 1− · 100% (1) Rp

Acknowledgments: We are grateful to COLCIENCIAS, Bogota, Colombia for financial support under the program “Apoyo a la comunidad científica nacional a través de los doctorados nacionales 2005.” Partial support by the Robert A. Welch Foundation, Houston (Grant # B-1203) is acknowledged also.

References and Notes 1. P. Espiard, A. Guyot, J. Perez, G. Vigier, and L. David, Polymer 36, 4397 (1995). 2. S. Vitry, A. Mezzino, C. Gauthier, J.-Y. Cavaille, F. Lefebvre, and E. Bourgeat-Lami, Comptes Rendus Chimie 6, 1285 (2003). 3. L. D. Perez, L. F. Giraldo, W. Brostow, and B. L. Lopez, e-Polymers 29 (2007). 4. Y. R. Hernandez, A. Gryson, F. M. Blighe, M. Cadek, V. Nicolosi, W. J. Blau, Y. K. Gun’ko, and J. N. Coleman, Scripta Mater. 58, 69 (2008). 5. Y. L. Liu, C. Y. Hsu, and K. Y. Hsu, Polymer 46, 1851 (2005).

Received: 11 July 2008. Accepted: 30 October 2008.


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Figure 8 shows versus force. Samples with higher contents of BA show higher recovery; we recall that BA imparts elastomeric character. The nature of elastomeric behavior has been analyzed in detail by Mark and Erman.29 We see that the recovery values for samples with high BA content are not far from 100%. For samples with silica the viscoelastic recovery is lower than for those without silica, suggesting that viscoelastic healing is hampered by silica particles. However, from the end-user standpoint, this small loss is minor and is compensated by improved resistance to scratching and increased thermal stability.

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