Emulsion Polymerization of Styrene and Methyl Methacrylate Using a

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Emulsion Polymerization of Styrene and Methyl Methacrylate Using a Hydrophobically Modified Inulin and Comparison with Other Surfactants Je´re´mie Nestor,† Jordi Esquena,*,† Conxita Solans,† Bart Levecke,‡ Karl Booten,‡ and Tharwat F. Tadros§ Departament de Tecnologia de Tensioactius, Institut d’Investigacions Quı´miques i Ambientals de Barcelona (IIQAB), CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain, Orafti Bio Based Chemicals, Aandorenstraat 1, B-3300 Tienen, Belgium, and 89 Nash Grove Lane, Wokingham, Berkshire RG40 4HE, U.K. Received December 3, 2004. In Final Form: February 17, 2005 The use of a new class of graft polymer surfactants, based on inulin, in emulsion polymerization of poly(methyl methacrylate) (PMMA) and polystyrene (PS) particles is described. PS and PMMA were synthesized by emulsion polymerization, and stable particles with a high monomer content (50 wt %) were obtained with a very small amount of polymeric surfactant ([surfactant]/[monomer] ) 0.0033). The latex dispersions were characterized by dynamic light scattering and by transmission electron microscopy to obtain the average particle size and the polydispersity index, and the stability was determined by turbidimetry measurements and expressed in terms of critical coagulation concentration. The last section gives a comparison of PMMA particles prepared by emulsion polymerization using classical surfactants from different types as emulsifiers with that obtained using the copolymer surfactant. It shows the superiority of INUTEC SP1 as it is the only one that allows stable particles at 20 wt % monomer content, with a smaller ratio [surfactant]/[monomer] ) 0.002.

1. Introduction A hydrophobically modified inulin surfactant (INUTEC SP1) has been recently synthesized.1-2 This molecule is a graft copolymer consisting of an inulin (polyfructose) backbone on which several alkyl groups are grafted. The molecule was used for stabilization of oil-in-water (O/W) emulsions. It proved to be very effective for steric stabilization3,4 of emulsions both at high temperature and in the presence of high electrolyte concentration.5 Measurement of the cloud point of the polyfructose backbone showed absence of any cloudiness up to 100 °C and at electrolyte concentrations reaching 4 mol‚dm-3 NaCl and 1 mol‚dm-3 MgSO4. This clearly demonstrated the superiority of this polyfructose-based surfactant as an emulsion stabilizer when compared with surfactants based on poly(ethylene oxide). The reason for this high stability was attributed to the structure and the conformation of the polymeric surfactant which can provide enhanced steric stabilization.6 The multipoint anchor of the chain by several alkyl groups on the oil droplet, as well as the strong hydratation of the polyfructose loops, provide very effective stabilization. * Corresponding author. E-mail: [email protected]. † Institut d’Investigacions Quı´miques i Ambientals de Barcelona, CSIC. ‡ Orafti. § 89 Nash Grove Lane. (1) Stevens, C. V.; Meriggi, A.; Booten, K. Biomacromolecules 2001, 2, 1-16. (2) Stevens C. V.; Meriggi, A.; Peristeropoulou, M.; Christov, P. P.; Booten, K.; Levecke, B.; Vandamme, A.; Pittevils, N.; Tadros, Th. F. Biomacromolecules 2001, 2, 1256-1259. (3) Napper, D. H. Polymeric Stabilization of Dispersions; Academic Press: London, 1983. (4) Einarson, M. B.; Berg, J. C. J. Colloid Interface Sci. 1993, 155, 165-172. (5) Tadros Th. F.; Vandamme, A.; Levecke, B.; Booten, K.; Stevens, C. V. Adv. Colloid Interface Sci. 2004, 108, 207-226. (6) Tadros Th. F. Adv. Colloid Interface Sci. 2003, 104, 191-226.

The main aim of the present investigation is to extend the use of the polymeric surfactant in emulsion polymerization of polystyrene (PS) and poly(methyl methacrylate) (PMMA) particles. Some preliminary results were reported by Esquena et al.7 Particular attention was given to the surfactant concentration and the monomer concentration. In this work we will show that high solid content can be produced using a small amount of polymeric surfactant, which is very important in applications for production of latex dispersions. In many industrial applications, high latex concentrations up to 40-55%8 are useful for preparation of good coatings. Another important aspect in our research is to produce stable latex dispersions in the presence of high electrolyte concentrations. As will be described in this paper, the critical coagulation concentration (CCCCaCl2) of latex particles prepared using the polymeric surfactant and postaddition of more molecules is quite high ensuring absence of flocculation. These results confirmed the previous study on emulsions which showed that the fructose chains remain hydrated even at high temperature. The latex particles obtained using INUTEC SP1 as emulsifier were compared with particles obtained using other emulsifiers commonly used in the industry of emulsion polymerization. 2. Experimental Section 2.1. Materials. The main surfactant used in this study was INUTEC SP1, supplied by ORAFTI Bio Based Chemicals (Tienen, Belgium), and it was synthesized as described before.1,2 This is essentially a graft copolymer made of a polyfructose backbone on which some alkyl groups are grafted. Its average molecular weight is approximately 5000 g‚mol-1. The purity of such a surfactant was higher than 97%, and it makes a clear solution (7) Esquena, J.; Dominguez, F. J.; Solans, C.; Levecke, B.; Booten, K.; Tadros, Th. F. Langmuir 2003, 25, 10463-10467. (8) Guyot, A.; Chu, F.; Schneider, M.; Graillat, C.; McKenna, T. F. Prog. Polym. Sci. 2002, 27, 1573-1615.

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at concentrations less than 0.1 wt %, above which a turbid solution appears which could be due to some association of the polymeric chains. For comparison, other surfactants were used: (a) Surfonic N-800, which is a nonylphenol ethoxylate with 80 ethylene oxide units, supplied by Huntsman (Belgium); (b) two alcohol ethoxylate surfactants, namely, C12(EO)6 and C13-15(EO)7 both supplied by UNIQEMA; (c) sodium dodecyl sulfate (SDS) supplied by SIGMA, which was purified by recrystallization from ethanol solution; (d) dodecyldimethyl(3-sulfopropyl)ammonium hydroxide (LSB) supplied by SIGMA with a purity g98%; (e) dodecyltrimethylammonium bromide with a purity g99% (LTAB) supplied by SIGMA and used as received; (f) a triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) surfactant, (EO)20(PO)70(EO)20 (Pluronic P123), supplied by BASF. Styrene (Merck) or methyl methacrylate (Aldrich) were used as monomers. These were purified by passing them through basic chromatographic aluminum oxide in order to remove the hydroquinone inhibitor. As initiator, K2S2O8 from Fluka with a purity higher than 99% was used. Deionized water was further purified by filtration through a milli-Q system. The electrolyte used was CaCl2‚2H2O (purity >99%) supplied by SIGMA. It was dried at 200 °C during 5 h in order to eliminate the residual water. 2.2. Methods. Emulsion Polymerization. All latexes were prepared by emulsion polymerization in a round bottle batch reactor (volume 500 mL), and dispersions were agitated with a U-shaped Teflon stirrer (300 rpm) located 1 cm over the bottom of the vessel. The reactions were carried out for 24 h for polystyrene (PS) particles and 6 h for poly(methyl methacrylate) (PMMA) particles, at a constant temperature of 80 °C under a controlled nitrogen atmosphere. The conversion rates were determined at the end of the reaction, for each latex synthesized, by a gravimetric method after evaporation of all volatile compounds of the dispersion, at 50 °C during 12 h. Critical Association Concentration (CAC). The CAC for the polymeric surfactant was determined by two methods: (a) Static light scattering by measuring the scattered light intensity at 90°, at different concentrations. The CAC can be located at the break point of intensity vs concentration plot. (b) Surface tensiometry, measuring the surface tension, γ, as a function of surfactant concentration. Again, the CAC was located at the break point of γ vs concentration curve. All measurements have been carried out at 25 °C. Particle Size Determination. The mean particle size and polydispersity index of the latex dispersions were determined by dynamic light scattering (DLS). A Malvern 4700 instrument (Malvern Instruments, Malvern, U.K.) was used for this purpose. This instrument was equipped with an argon laser (λ ) 488 nm) with variable intensity to cover the wide size range involved. Measurements were carried out at different scattering angles at a constant temperature of 25 °C. The PCS data were analyzed by the CONTIN method and a constrained regularization calculation algorithm known as REPES incorporated in the analysis package GENDIST (interactive program for the analysis of homodyne PCS data). Transmission electron micrographs (Hitachi H-800) were also obtained to assess particle size and shape. The latex suspensions were diluted, placed on a copper-carbon grid, and then dried. Observations were carried out in an electric field of 200 kV. INUTEC Aggregate Morphology. The geometry of aggregates was investigated by static light scattering, SLS, by measuring the scattered light intensity of surfactant aqueous solutions above the CAC, at different angles between 35° and 135°. The Rayleigh scattering theory, valid when the scattering centers are small compared to the wavelength of the radiation,9 was applied in our study. CCC and Diffuse Double Layer Potential (φd) Determinations. The stability of the dispersion in the presence of electrolytes was assessed by determining the CCC, by measuring the turbidity as a function of time for different electrolyte concentrations using a spectrophotometer (Varian, Cary 300 Bio UV-Visible). The (9) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1977.

Nestor et al.

Figure 1. Critical association concentration (CAC) measurements by surface tension and dynamic light scattering. initial slopes of these curves are directly proportional to the initial coagulation rate. These slopes increased with increasing electrolyte concentration until a maximum was reached at the critical coagulation concentration (CCC). Above the CCC there was no further increase in the slope, which reaches a constant value, kr. Therefore, the stability can be expressed in terms of the stability factor W,10-11 obtained as the ratio of the rate constants for rapid, kr, and slow coagulation, ks

Wexp ) kr/ks

(1)

In rapid coagulation Wexp is approximately equal to 1, while higher values of Wexp are obtained for aggregations under conditions of slow coagulation. The CCC can be obtained from a plot of the log of the stability factor, W, vs the log of the electrolyte concentration; the CCC corresponds to the intersection of the curve with the X axis. From the same plot, φd, which is related to the electrical double layer repulsion, can be calculated using Overbeek 12 approximations. The slope of the stability curve, -d log W/d log Ce, where Ce is the electrolyte concentration, is related to the particle radius, a, to the diffuse potential associated with the charged particle surface, φd, and to the electrolyte valence, z, by the expression

-d log W/d log Ce ) (2.15 × 109)aγ2/z2

(2)

γ ) tanh(zeψd/4kT)

(3)

where

where e is the electronic charge, k the Boltzmann constant, and T the absolute temperature. All CCC measurements were carried out using CaCl2 as electrolyte. For a 2:1 electrolyte the valency, z, is 31/2 as described by Israelachvili.13

3. Results and Discussion 3.1. Association Behavior of the Hydrophobically Modified Inulin Surfactant (INUTEC SP1). Figure 1 shows the variation of surface tension (error bars indicate the standard deviation) and light scattering intensity with INUTEC SP1 concentration. The γ vs log C results show a break in the curve at 7 × 10-7 mol‚dm-3, which may indicate the formation of polymer aggregates. The light scattering results show another break at higher INUTEC SP1 concentration (5 × 10-6 mol‚dm-3) when comparing with the surface tension results. The light scattering results are more sensitive for detection of polymer aggregation in solution. The intensity (10) de las Nieves, F. J.; Daniels, E. S.; El-Aasser, M. S. Colloids Surf., A 1991, 60, 107-126. (11) Fuchs, N. Z. Phys. 1934, 89, 736. (12) Overbeek, J. Th. G. In Colloid Science; Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1952; Vol. 1. (13) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1991.

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Figure 2. Variation of particle size, obtained by emulsion polymerization using INUTEC SP1 as emulsifier, as a function of monomer concentration and initiator concentration: filled squares, [initiator] ) 0.0125 wt %; open circles, [initiator]/[monomer] ) 0.00125. The plots refer to (a) polystyrene and (b) poly(methyl methacrylate) particles.

of the scattered light increases proportionally with the number of aggregates.14 In a plot of the scattered intensity versus the surfactant concentration, a sharp change of the slope is observed at the CAC. The CAC is in the range 7 × 10-7 to 5 × 10-6 mol‚dm-3, which is typical for a polymeric surfactant that may associate with a limited monomer unit (low association number). Other measurements of laser light scattering were carried out between 35° and 135° to investigate the morphology of aggregates. The results were fitted to the Guinier equation15

(

I(q) ) I(0) exp -

)

q2Rg2 3

(4)

where q is the scattering vector, Rg the radius of gyration of the polymer, and I(0) the intensity at 0° angle. It was concluded that the INUTEC SP1 surfactant aggregates were neither spherical nor cylindrical as no lineal relation was obtained between the scattering vector q and the scattered light intensity. Furthermore, measurements of dynamic surface tension have shown that the polymeric surfactant diffuses very slowly to the water/air interface as it was found that 12 h was necessary to reach an equilibrium state, as expected for a high molecular weight surfactant.16 3.2. PS and PMMA Particles Prepared with INUTEC SP1 as Emulsifier. In this study, PMMA and PS particles were synthesized by the emulsion polymerization process described above using INUTEC SP1 as emulsifier. A preliminary study of the preparation of the different latexes showed that the optimum [INUTEC]/[monomer] weight ratio is 0.0033 on the monomer fraction for PS particles and 0.001 for PMMA particles. This preliminary study demonstrated that the emulsion polymerization of latex particles has to be carried out in a narrow range of INUTEC concentration, where particles are stable. Higher or lower concentrations of INUTEC SP1 cause flocculation or phase separation, respectively. The initiator concentration was kept constant at 0.0125 wt % with respect to the total composition, in the first experiment, and then the ratio of initiator to monomer was fixed at 0.00125. The monomer conversion rate was measured for all samples synthesized at the end of the reaction, and all have a monomer conversion higher than 85%. Figure 2 shows the results of the diameter, d, as a (14) Finsy, R. Adv. Colloid Interface Sci. 1994, 52, 79-143. (15) Svergon, D. In Neutron, X-ray and Light Scattering; Linder, P., Zemb, Th., Eds.; North-Holland, Delta Series: Amsterdam, 1991; p 88. (16) Tadros, Th. F. In Novel Surfactants; Marcel Dekker: New York, 2003; pp 543-583.

Table 1. Main Features of Latex Particles Obtained by Emulsion Polymerization Using INUTEC SP1 as Emulsifier, [Initiator] ) 0.0125 wt % monomer styrene

methyl methacrylate

[monomer]/ wt %

diameter/ nm

polydispersity index

5 10 20 30 40 50 10 20 30

193 197 230 238 306 373 266 305 478

0.02 0.03 0.04 0.02 0.15 0.33 0.02 0.01 0.08

function of monomer concentration at two initiator concentrations. The PS and PMMA particle sizes increase with increasing monomer concentration. With the initiator concentration constant at 0.0125 wt %, stable particles could be obtained up to 50 wt % for PS particles and up to 30 wt % for PMMA particles. Increasing the total concentration of initiator increases the particle size and allows one to obtain stable particles up to 40 wt % for PMMA particles. The concentrations of INUTEC SP1 used were very low, and stable particles were obtained at high solid content. The transmission electron microscopy (TEM) micrographs presented in Figure 3 show the particle size distribution of the PS and PMMA particles, for different monomer concentrations. The particle sizes observed in the TEM micrographs were in accordance with those determined by DLS, shown in Table 1. Polydispersity of the latexes was also obtained by light scattering determinations. It can be observed in Table 1 that the polydispersity index of the latexes is low, indicating narrow size distribution, below 40 wt % styrene and 30 wt % MMA. The results of the stability of PS and PMMA particles are shown in Figure 4. The stability is expressed in terms of CaCl2 CCC and φd obtained from a plot of the stability factor, W, vs the electrolyte concentration, as described in the Experimental Section. Figure 4 shows the stability results obtained for PS particles with a monomer content between 5 and 40 wt %, keeping the same concentration of [K2S2O8] ) 0.0125 wt % for all particles in the first experiment (Figure 4a) and then keeping a constant ratio of [K2S2O8]/[monomer] ) 0.00125 (Figure 4b). Reproducibility has been previously studied, and the standard deviation is indicated by the error bars. It can be observed that the CCC value remains low for all the samples, between 0.0175 and 0.05 mol‚dm-3, and that the calculated potentials are very low for all particles and decrease when increasing the monomer content at

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Figure 3. Influence of monomer content on the particle morphology: (a) as a function of styrene concentration, the ratio [INUTEC]/ [styrene] was 0.0033; (b) as a function of methyl methacrylate concentration, the ratio [INUTEC]/[MMA] was 0.001. The concentration of initiator was 0.0125 wt %.

Figure 4. Influence of the styrene content on CaCl2 CCC and φd, obtained by emulsion polymerization using INUTEC SP1 as emulsifier: (a) constant initiator concentration ) 0.0125 wt %; (b) constant ratio [initiator]/[styrene] ) 0.00125. Open circles refer to diffuse potential, and filled squares refer to CaCl2 CCC.

constant initiator concentration. As the monomer content increases, the particle size increases too. In all of these experiments the initiator concentration is the same, so the charge density per particle decreases when increasing the monomer content of the latexes. Figure 4 also shows that the diffuse potential increases when keeping the [initiator]/[monomer] ratio constant at 0.00125. The low value of potential can be easily explained because INUTEC surfactant is nonionic and does not provide charges to the particle. Probably, most of the negative particle surface charges are due to the sulfate groups produced by the initiator K2S2O8.17 The influence of the initiator concentration on the stability is shown in Figure 5. It presents the results of φd for the PMMA particles synthesized with a monomer content between 10 and 40 wt % for a constant concentration of initiator of 0.0125 wt % and for a constant ratio [initiator]/[monomer] of 0.00125. The CaCl2 CCC values for all these samples were low, between 0.02 and 0.03 mol‚dm-3, and independent of the

monomer content. It can be observed that for the PMMA particles, when the initiator concentration constant is kept at 0.0125 wt %, the potential decreases for the same reasons as discussed before, whereas for PMMA particles with a constant ratio [initiator]/[monomer] of 0.00125 the potential increases. These results prove once again that most of the negative surface charges are due to the initiator. 3.3. Postaddition of INUTEC SP1. To obtain particles with high CCC, experiments where INUTEC SP1 has been added after polymerization (postaddition) were carried out. PS particles with a 5 wt % monomer and 0.0125 wt % K2S2O8 were prepared using the surfactant-free polymerization process.18 The mean diameter of these particles was 328.6 nm with a polydispersity index of 0.027. CCC was determined with CaCl2 as electrolyte. Plots of the stability factor, W as a function of electrolyte concentration, are shown in Figure 6. These results clearly indicated that INUTEC SP1 produces a different effect used during the polymerization

(17) Richards, J. R.; Congalidis, J. P.; Gilbert, R. G. J. Appl. Polym. Sci. 1989, 37, 2727-2756.

(18) Kotera, A.; Furusawa, K.; Takeda, Y. Kolloid Z. Z. Polym. 1970, 227, 677.

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Langmuir, Vol. 21, No. 11, 2005 4841 Table 3. Emulsion Polymerization Comparison of Sample Prepared with INUTEC SP1 or with Other Classical Surfactants, [Surfactant]/[MMA] ) 0.001 surfactant INUTEC SP1 INUTEC SP1 SDS SDS C12(EO)6 C13-15(EO)7 LSB LTAB NP-800 P123

Figure 5. Influence of the methyl methacrylate content on the particle diffuse potential, obtained by emulsion polymerization using INUTEC SP1 as emulsifier: filled squares, [initiator] ) 0.0125 wt %; open circles, [initiator]/[MMA] ) 0.00125.

Figure 6. Variation of the stability of surfactant-free PS particles, as a function of the INUTEC SP1 concentration postadded and comparison with PS particles obtained by emulsion polymerization using INUTEC SP1 as emulsifier. Table 2. Influence of the INUTEC SP1 Postaddition on CaCl2 and φda sample

CCCCaCl2/M

φd/mV

PS (surfactant-free) PS synthesized with 0.01 wt % INUTEC SP1 PS + postadded 0.01 wt % INUTEC SP1 PS + postadded 0.05 wt % INUTEC SP1

0.028 0.052

-5.4 -6.2

0.199 0.398

-5.7 -5.4

a

The monomer concentration is 5 wt %.

or added after polymerization. For postadded INUTEC, there was a sharp increase in the stability of PS particles at an [INUTEC SP1]/[PS] weigh ratio of 0.002. In absence of the polymeric surfactant, the CCC value for CaCl2 was 0.028. At [INUTEC SP1]/[PS] weight ratio of 0.002 the CCC was of 0.199 M, and increasing this ratio to 0.01, the CCC was 0.398 M. However, using 0.01 wt % of INUTEC SP1 as emulsifier in the emulsion polymerization, the CaCl2 CCC remains low with a value of 0.052 M. Therefore, particle stability is greatly enhanced by surfactant postaddition. This high stability in the presence of high electrolyte concentration is due to the polyfructose stabilizing chain, which remains highly hydrated at such high electrolyte concentration. From the slope of the logarithm plots of stability factor, W as a function of electrolyte concentration, the diffuse layer potential, φd, was calculated and results are summarized in Table 2. As expected for a nonionic surfactant,19,20 the value of the potential remains constant when adding INUTEC SP1

[surfactant]/ [MMA]/ wt % wt % 0.02 0.04 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.02

20 40 20 30 20 20 20 20 20 30

appearance stable stable slight flocculation flocculation + separation flocculation + separation flocculation + separation flocculation + separation flocculation + separation flocculation + separation flocculation + separation

to the surfactant-free PS particles with a value of φd equal to 5.7 ( 0.5 mV. As described before, the CCC increases when adding more INUTEC SP1 to the particles. These results clearly indicate that the polymeric surfactant stabilizes the particles by a steric mechanism. 3.4. Comparison of Stability Using INUTEC SP1 with Other Surfactants. Stable latex dispersions were obtained at high monomer content using a small amount of INUTEC SP1 ([INUTEC SP1]/[MMA] ) 0.001). PMMA particles were synthesized by the same method using other surfactants at the same concentration. The results are shown in Table 3. INUTEC SP1 is the only surfactant that allows the formation of stable PMMA particles up to 40 wt % monomer at a ratio [INUTEC SP1]/[MMA] ) 0.001. With other C12 carbon chain surfactants from different types at the same concentration, stable particles could not be obtained at the same [INUTEC]/[MMA] ratio. Other experiments have shown that stable PMMA particles could be obtained up to 40 wt % of MMA using SDS and LTAB at a higher concentration (2.4 wt %). Furthermore for the other surfactants tested, even at high concentration, stable particles could not been obtained at 40 wt % of monomer. These results prove once again the superiority of the INUTEC SP1 used as emulsifier in the emulsion polymerization because of the strongly hydrated polyfructose backbone. 4. Conclusions PS and PMMA particle dispersions, with a monomer content up to 50 wt %, are well stabilized using INUTEC SP1 at very low concentrations as emulsifier in the emulsion polymerization process. CCCCaCl2 of such dispersions remains low but can be enhanced significantly by postaddition of small quantities of INUTEC SP1. A sharp increase in the stability of particles is achieved at [INUTEC]/[PS] weight ratio of 0.002 despite that the diffuse potential does not increase. Particle stabilization is due mainly to steric mechanisms, as this polymeric surfactant does not affect the diffuse potential. Stable particles at high monomer content could not be obtained using such a small concentration of other surfactant tested. Acknowledgment. The authors gratefully acknowledge financial support from ORAFTI Bio Based Chemicals and from the Spanish Ministry of Science and Education (PPQ2002-04514-C03-03 grant). LA047018Y (19) Vincent B.; Edwards, J.; Emmett, S.; Jones, A. Colloids Surf., A 1986, 18, 261. (20) Oterwill, R. H.; Walker, T. J. Chem. Soc., Faraday Trans. 1974, 70, 917.