Langmuir 2001, 17, 6865-6870
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Emulsion Polymerization of Styrene Using the Homopolymer of a Reactive Surfactant X. Wang, E. D. Sudol, and M. S. El-Aasser* Emulsion Polymers Institute and Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 Received April 30, 2001. In Final Form: July 27, 2001 The emulsion polymerization of styrene using the homopolymer of sodium dodecyl allyl sulfosuccinate as a polymeric surfactant (poly(TREM)) was studied in terms of the polymerization kinetics, the nucleation mechanisms, and the properties of the final latexes. It was found that the relationship between the maximum rate of polymerization and the final number of polymer particles was one-to-one under all experimental conditions (Rp ∝ Np1.0). The dependencies of these on the initiator and polymeric surfactant concentrations varied, depending on experimental conditions (Rp ∝ Np1.0 ∝ [E]0.2-0.4; Rp ∝ Np1.0 ∝ [I]0.6-0.8). These variations were attributed to the increased ionic strength effects with increasing concentration of poly(TREM), a polyelectrolyte, and its properties as a polymeric surfactant. It was inferred from the polymerization kinetics (Rp vs time) that homogeneous nucleation was dominant using poly(TREM) even with concentrations exceeding its critical micelle concentration. This differs from the monomeric TREM LF-40 surfactant. Characterization of the amount of poly(TREM) bound to the polymer particles was carried out by serum replacement studies and ion exchange/conductometric titration measurements. Evidence is given that more than half of the poly(TREM) was bound to the polymer particles, either by grafting of the poly(TREM) and/or irreversible adsorption. The amount of bound poly(TREM) increased with increasing surfactant concentration and increasing initiator concentration (i.e., decreasing particle diameter). Contact angles of water measured on films formed from the latexes showed that the poly(TREM) does not migrate significantly to the surface of the films, which is consistent with the latex surface characterization results.
Introduction Surfactants play major roles in particle nucleation and particle growth during emulsion polymerization. Polymerization rates, particle size, and size distribution are all determined by the surfactant among other process variables. During postpolymerization processes, such as stripping, storage, shipping, and formulation, surfactants are important as well. Polymeric surfactants act as steric stabilizers and have received considerable attention in industrial application. They have been widely used and studied in emulsion polymerization.1,2 A general review has been published by Piirma.3 Polymeric surfactants typically have a low critical micelle concentration (cmc). They provide the steric repulsion between interacting particles, which gives the latex excellent stability against high electrolyte concentrations, freeze-thaw cycling, and high shear rates. When the polymeric surfactant also has ionic groups in the molecular structure, electrostatic and steric effects are combined. This type of polymeric surfactant is particularly important and interesting. Poly(TREM), which is the homopolymer of TREM LF-40 (sodium dodecyl allyl sulfosuccinate),4 has both these features. The structure of TREM LF-40 is presented below; poly(TREM) in this case has a molecular weight of about 6000, or ∼14 units of TREM LF-40 per molecule. The behavior of poly(TREM) in the emulsion polymerization of styrene is reported here.
(1) Kusters, J. M. H.; Napper, D. H.; Gilbert, R. G.; German, A. L. Macromolecules 1982, 25, 7043.
The general rate of reaction in an emulsion polymerization5-8 is described by
Rp ) kp[M]pNpn j /NA
(1)
where Rp is the rate of polymerization (mol/(dm3 s)), kp is the propagation rate coefficient (dm3/(mol s)), [M]p is the concentration of monomer in the latex particles (which can be thermodynamically determined and is assumed to be the same in each particle) (mol/dm3), Np is the number j is of particles per total aqueous phase volume (dm-3), n the average number of free radicals per latex particle, and NA is Avogadro’s number (mol-1). The number of particles produced, as predicted by Smith-Ewart theory, is as follows:
Np ) K′[E]ne[I]ni
(2)
where K′ is a constant, [E] is the surfactant concentration, and [I] is the initiator concentration. For emulsion polymerizations following Smith-Ewart case 2 kinetics (e.g., emulsion polymerizations of styrene with an anionic surfactant), the values of (ne) and (ni) are equal to 0.6 and 0.4, respectively. Varela de la Rosa et al.9,10 recently (2) Coen, E. M.; Lyons, R. A.; Gilbert, R. G. Macromolecules 1996, 29, 5128. (3) Piirma, I. Polymeric Surfactants; Surfactant Science Series No. 42; Marcel Dekker: New York, 1993. (4) Wang, X.; Sudol, E. D.; El-Aasser, M. S. Emulsion Polymerization of Styrene Using a Reactive Surfactant and Its Polymeric Counterpart: Kinetic Studies. Macromolecules, in press. (5) Smith, W. V.; Ewart, R. W. J. Chem. Phys. 1948, 16, 592. (6) Smith, W. V. J. Am. Chem. Soc. 1948, 70, 3695. (7) Smith, W. V. J. Am. Chem. Soc. 1949, 71, 4077. (8) Harkins, W. D. J. Am. Chem. Soc. 1947, 69, 1428. (9) Varela de la Rosa, L.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 461. (10) Varela de la Rosa, L.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 4073.
10.1021/la010641n CCC: $20.00 © 2001 American Chemical Society Published on Web 09/20/2001
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performed a detailed kinetics study employing the Mettler RC1 reaction calorimeter and have shown that the emulsion polymerization of styrene carried out using the anionic surfactant SLS (sodium lauryl sulfate) follows Smith-Ewart theory in terms of the dependencies of Rp and Np on [E] and [I]. The values of n j in these reactions were in the vicinity of 0.5 at the rate maximum (Rp max, where droplets disappear), and [M]p was constant (5.4 mM) (i.e., the rate maximum occurred at the same conversion independent of the experimental conditions). It was expected that at the rate maximum Rp ∝ Np1.0. This is what was found. On the other hand, Riess and his group11,12 have studied emulsion polymerizations using polystyrene-poly(ethylene oxide) (PS-PEO) block copolymers, poly(methyl methacrylate)-poly(ethylene oxide) diblock copolymers, and poly(methyl methacrylate)poly(acrylic acid) (PMMA-PAA) diblock copolymers as surfactants that form “hairy latexes”. The results have shown that the efficiency of the polymeric surfactant decreased with increasing molecular weight of the copolymer and increasing polystyrene content for PS-PEO copolymers. The kinetics of the emulsion polymerizations of styrene using these polymeric surfactants seemed to indicate a short or nonexistent micellar nucleation period, and much (40-80 wt %) of the copolymer surfactant was found entrapped in the latex particles. The dependencies were reported to be Rp ∝ [E]1.33 and Np ∝ [E]1.37 (i.e., Rp ∝ Np1.0). In emulsion polymerizations of methyl methacrylate (MMA) using PMMA-PAA block copolymers, it was found that Np ∝ [E]0.97.11,12 More recently, functionalized block copolymers, such as PS-PEO-X, where X is a sulfonate or sulfate group, have been of interest. Ayoub et al.13,14 have studied emulsion polymerizations of vinyl acetate and styrene using a polymeric surfactant prepared from polyoxyethylene methyl ether and styrene. The results have shown that Rp ∝ [E]0.66-0.77 and Rp ∝ [I]0.76-0.9. Also, their studies of the emulsion polymerization of styrene with amphiphatic copolymers [of vinyl acetate and methoxy polyoxyethylene (PVAc-b-MPOE) (35:65, 27:73, 19:81 wt/ wt) prepared with a macroradical initiator in the presence of benzoyl peroxide] as surfactant found that Rp ∝ [E]0.33-0.39 and Rp ∝ [I]0.67-0.8. Although the particle size decreased with increasing [E], it increased with increasing [I]. This differs from the classical Smith-Ewart theory. Cochin and Laschewsky15 have investigated the emulsion polymerization of styrene using the conventional surfactant N,N-dimethyl-N-dodecylammonium bromide, its reference reactive surfactant containing a methacryloyl moiety, and its polymeric surfactant. The reactive surfactant and the polymeric surfactant yielded latexes with high surface tensions, which was not found using conventional surfactants. The reactive surfactant was efficiently fixed to the latex by copolymerization, while small amounts of the polymeric surfactants were grafted onto the latexes during the reaction. All three surfactants gave high polymerization rates and yields, along with stable and small-size monodisperse particles. Most of the polymeric surfactants in the studies cited above were nonionic surfactants. Poly(TREM), however, is an anionic polymeric surfactant that can provide both steric and electrostatic stability. The kinetics and mech(11) Hurtrez, G.; Dumas, P.; Riess, G. Polym. Bull. 1998, 40, 203. (12) Calderara, F.; Riess, G. Macromolecules 1994, 27, 1210. (13) Ayoub, M. M. H.; Nasr, H. E.; Rozik, N. N. J. Macromol. Sci., Pure Appl. Chem. 1998, A35 (7&8), 1415. (14) Ayoub, M. M. H. J. Elastomers Plast. 1998, 30 (3), 207. (15) Cochin, D.; Laschewsky, A. Macromolecules 1997, 30, 2278.
Wang et al.
anism of the emulsion polymerization of styrene using this kind of surfactant have not been reported and are the subject of this paper. Experimental Section Materials. Styrene monomer (Aldrich) was washed with 10 wt % aqueous NaOH (Fisher Scientific) and DI (deionized) water and distilled under vacuum prior to use. Buffer (NaHCO3, Aldrich) and initiator (Na2S2O8, FMC Corp.) were analytical grade. Sodium dodecyl allyl sulfosuccinate (TREM LF-40; MW ) 428 g/mol; 40% solution in water and isopropyl alcohol) was used as received (Cognis Corp., formerly Henkel Corp.). This material was hydrogenated (H-TREM) (courtesy Air Products and Chemicals) for use in control studies. A monodisperse polystyrene seed latex (92 nm; Dow Chemical Co.) was used after cleaning by ion exchange. Hydroquinone (Aldrich) was used as received. Deionized water was used throughout. The polymeric surfactant, poly(TREM), was prepared by solution polymerization of TREM LF-40 (28.08 g), with NaHCO3 (10 mM) as the buffer, Na2S2O8 (16 mM) as the initiator, and DI water (360 g) as the solvent. The reaction was carried out at 60 °C for 12 h and 90 °C for an additional 9 h to obtain the highest reaction conversion and to decompose the initiator completely, since this material would be used as is without further purification. Emulsion Polymerization Procedures. Emulsion polymerizations were carried out in the Mettler RC1 reaction calorimeter (MP 10 1-L reactor) equipped with a pitched blade impeller and baffle. The reactor was first charged with the aqueous phase containing the buffer and the surfactant, followed by the styrene monomer. The reactor was flushed with nitrogen (Zero grade, Airgas/JWS Technologies Inc.) for about 20 min. The temperature was set at 25 °C and a calibration was performed, and then the temperature was ramped to 60 °C over 10 min and a second calibration was performed. The initiator solution was then added to start the reaction. When the reaction was completed, an aqueous solution (1 wt %) of hydroquinone was added to the latex, and then two more calibrations were made at the reaction temperature and 25 °C. The calibrations were necessary for determining the heat transfer coefficients and heat capacities needed for evaluating the raw data. Continuous heat of reaction versus time curves were obtained directly from the RC1 evaluation software, and the polymerization rate was calculated using eq 3:
Rp )
Qr ) KQr VH2O∆H
∫Q t
∆H )
0
f
r
(3)
dt
xfMA0
(4)
where the heat of the reaction Qr (J/s) can be used directly to obtain the reaction rate, Rp (mol/dm3/s). xf is the fractional conversion of the reaction, MA0 is the initial amount of monomer charged into the reactor (mol), t is the reaction time (s), VH2O is the volume of DI water charged (dm3), and ∆H is the heat of polymerization of styrene (J/mol). K is a constant, and since there is only a constant difference between Rp and Qr, Rp can be represented by Qr. Analytical Methods. Monomer conversion was determined independently by both gas chromatography (GC, HewlettPackard 5890) and gravimetry. The reaction rates were obtained from the Mettler RC1. The continuous conversion versus time curves were obtained by integration of the Rp versus time curves. The particle size and particle size distribution of the latexes were measured by capillary hydrodynamic fractionation (CHDF model 1100, Matec Applied Sciences) and transmission electron microscopy (TEM, Philips EM 400). The characterization of the latex particle surfaces for the presence of the poly(TREM) was carried out by desorption and conductometric titration. Desorption via serum replacement was used to measure the amount of poly(TREM) not bound to the particles and, by difference, the total amount of incorporated
Emulsion Polymerization of Styrene surfactant, being either on the particle surface or buried inside the polymer particles. DI water was continuously flushed through a serum replacement cell containing a known amount of latex until the water flowing out of the cell had the same conductivity as the DI water. The amount of surfactant in the effluent was determined by gravimetry. The amount of incorporated surfactant was estimated by subtracting the amount of surfactant in the effluent from the original amount of surfactant in the latex (based on the recipe). Conductometric titration was used to determine the amount of chemically bound surfactant on the polymer particle surfaces. Ion-exchange resins (both cationic and anionic, in their OH- and H+ forms, respectively; AG 1-X4, AG 50W-X4, 20-50 mesh, BioRad Laboratories) were washed with DI water until the wash water had the same conductivity as fresh DI water. The resins were then mixed and washed again. The latexes were diluted to 5 wt % solids and then cleaned by successive ion-exchange cycles using the mixed bed resin and employing a 1:1 weight ratio of polymer to resin. The latex and mixed bed resin were mixed together for 2 h and filtered. Ion exchange continued until the latex exhibited the same conductivity after two successive cycles. The cleaned latexes were then titrated with 0.02 N NaOH, and the conductivity was monitored continuously during titration. The amount of strong acid groups (SO4-) was calculated from the end point, which represents the amount of chemically bound surfactant on the polymer particle surface, plus a small amount due to initiator fragments (1 year). The particle size distributions of the latexes were measured by CHDF and TEM. Somewhat larger particles were formed using the polymeric surfactant poly(TREM) compared to TREM LF-40 (based on equal weights), as might be expected. Nonetheless, the particles are all smaller than 100 nm. The results are given in Table 2. Polymer Particle Surface Characterization. It is of interest to characterize the latexes prepared with poly(TREM) in order to determine where the polymeric surfactant ends up after the polymerization. To do this, various methods or combinations of methods can be used, such as serum replacement, ultracentrifugation, or ion exchange/conductometric titration. Ultracentrifugation was found to be an inefficient method to separate the serum phase from the polymer particle phase for these polystyrene latexes because of the presence of small particles in the serum phase as determined by dynamic light scattering (Nicomp, model 370). Serum replacement was successfully used to perform this separation. Control Experiments. To quantitatively measure the amount of chemically bound surfactant, the latexes were first cleaned by serum replacement. This was followed by an ion-exchange cycle and conductometric titration. To test the efficiency and accuracy of these methods, the same amount of each surfactant, TREM LF-40 and poly(TREM), as used to synthesize the latexes, was blended with two portions of a cleaned PS latex (92 nm diameter). After 24 h of mixing, these latexes were washed via serum replacement or cleaned by mixed-bed ion-exchange resins followed by conductometric titrations. In addition, for comparison, two latexes were synthesized and characterized using a nonreactive version of TREM LF-40. This surfactant (H-TREM) is the hydrogenated version of TREM LF-40 containing no carbon-carbon double bonds. It should only be physically adsorbed on the polystyrene particles and, therefore, should be removable. Results are given in Table 3. These show that for the clean PS latex with added TREM LF-40, greater than 98 wt % of the surfactant could be removed. In addition, more than 98 wt % of the H-TREM could be removed from the latexes synthesized using it as the surfactant. However, for the PS latex with added poly(TREM), only 84 and 81 wt % of the poly(TREM) could be removed by ion exchange and serum replacement, respectively. Therefore, part of the poly(TREM) was irreversibly adsorbed on the polymer particles. This could be attributed to anchoring of the polymer at more than one site along the chain, or perhaps
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Table 3. Results of the Control Experiments Characterizing the Polystyrene Latexes
latex
fraction of surfactant not desorbed by serum replacement
fraction of surfactant not desorbed by ion exchange
cleaned PS + TREM LF-40a cleaned PS + poly(TREM)a PS with H-TREM (30 mM)b PS with H-TREM (40 mM)b
0.018 0.187 0.028 0.033
0.013 0.160 0.015 0.017
a
Surfactant added to cleaned latex. b Latex synthesized using H-TREM as surfactant.
Figure 3. Amount of bound poly(TREM) based on total polymer formed (g/g) and the fraction of bound poly(TREM) as a function of the initial surfactant concentration; [I] ) 8 mM. The poly(TREM) concentration is based on the molecular weight of TREM LF-40.
Figure 4. Amount of bound poly(TREM) based on total polymer formed (g/g) and the fraction of bound poly(TREM) versus the polymer particle surface area; [I] ) 8 mM.
this adsorbed polymer represents the higher molecular weight fraction of the poly(TREM) (the molecular weight distribution was not determined). Effect of the Poly(TREM) Concentration. To determine the optimal reaction conditions for emulsion polymerizations using the polymeric surfactant, that is, the conditions that result in the maximum amount of surfactant bound (grafted or irreversibly adsorbed) to the surface of the particles, several parameters affecting the incorporation were investigated. First, the polystyrene latexes synthesized using various poly(TREM) concentrations were characterized by ion exchange/conductometric titration. The results are shown in Figures 3 and 4. Figure 3 shows the amount of surfactant bound to the particles based on the polymer (PS) formed (g/g) and the fraction of the initial surfactant that is bound to the polymer particles as a
Table 4. Results of the Characterization of the Polystyrene Latex Prepared Using Poly(TREM) as the Surfactant (30 mM [E], 8 mM [I]) surfactant chemically bound (ion exchange/ titration)
surfactant in aqueous phase (B), 6% solids
surfactant desorbed by washing (C)
surfactant chemically bound (100% (B + C))
76 wt %
8 wt %
9 wt %
83 wt %
function of the poly(TREM) concentration. Two scales are presented for the x-axis, one being the true poly(TREM) concentration (top) while the other is based on the molecular weight of TREM LF-40 for reference. The results indicate that with increasing surfactant concentration, that is, decreasing particle size of the final latexes, the amount of chemically bound surfactant (g/g) increases, while the fraction bound decreases. If the amount of bound surfactant (g/g) is plotted against the polymer particle surface area, A (estimated from the particle volume average diameter Dv and Da; A ) Np(πDa2) ) Mox/(Fp(1/ 6)πDv3)(πDa2)), as in Figure 4, it can be seen that the surface area is one factor determining the amount of bound surfactant. With increasing surface area, the amount of surfactant incorporated increases. This could indicate that an important locus for grafting onto the poly(TREM) during the emulsion polymerization is the particle/water interface. Since the surface area (A) is proportional to the surfactant concentration to the 0.6-0.8 power (calculated by the least linear square fit for the experimental range in this paper) (i.e., A ∝ [E]0.6-0.8), A increases with increasing [E], and the incorporation of the surfactant also increases. However, the fraction of the surfactant incorporated decreases since the exponent is less than 1.0 (with changing [E]). These results indicate that poly(TREM) as a surfactant has a significantly higher degree of incorporation compared to TREM LF-40.16 However, the exact mechanism of the incorporation of poly(TREM) has yet to be determined. It is expected that much of it is by grafting of the monomer to poly(TREM) with some irreversible adsorption of poly(TREM) on the polymer particles, as shown earlier in the control experiments. The characterization results in Table 4 also show that both the bound surfactant and the fraction incorporated are larger as measured by serum replacement (83 wt %) compared to ion exchange/conductometric titration (76 wt %). This implies that among the bound surfactant, only a small fraction of the poly(TREM) is entrapped inside the particles with the majority being on the particle surface. Effect of Initiator Concentration. The initiator concentration is another parameter that can affect the chemically bound surfactant. Figure 5 shows the amount of bound poly(TREM) (g) per gram of polymer and the fraction incorporated as a function of the initiator concentration. These results indicate that with increasing initiator concentration, that is, decreasing particle size, both the amount and the fraction of bound poly(TREM) increase significantly. Again, these are plotted versus the surface area in Figure 6. The dependence of the surface area (A) on the initiator concentration varied from 0.7 to 0.9 power depending on the poly(TREM) concentration. Therefore, with increasing [I] at a constant stabilizer concentration, A increases and the fraction of permanently bound surfactant increases. These characterization results show that the fraction of the poly(TREM) bound to the polymer particles can be increased by increasing the initiator concentration and
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Wang et al. Table 5. Contact Angle (deg) Measurements on Polystyrene Latex Films film formed at 180 °C latex cleaned PS latex PS + TREM LF-40 (blend, 30 mM) PS (poly(TREM)a (30 mMb) poly(TREM) PS + poly(TREM)a (blend, 30 mMb)
Figure 5. Amount of bound poly(TREM) based on total polymer formed (g/g) and the fraction of bound poly(TREM) as a function of the initial initiator concentration; [E] ) 30 mM.
film formed from toluene-swollen latex at 25 °C
film/ air
film/ substrate
film/ air
film/ substrate
68°
