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Articles Polymerization of Miniemulsions Containing Predissolved Polystyrene and Using Hexadecane as Costabilizer P. J. Blythe,† B. R. Morrison,‡ K. A. Mathauer,‡ E. D. Sudol,† and M. S. El-Aasser*,† Emulsion Polymers Institute and Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015-4732, and Kunststofflaboratorium, BASF Aktiengesellschaft, ZKD-B1, D67056 Ludwigshafen, Germany Received February 8, 1999. In Final Form: September 23, 1999 The effect on the polymerization kinetics of predissolving polystyrene polymers that have various molecular weights and end groups in styrene miniemulsions prepared using hexadecane as costabilizer was studied. It was noted that the rate of polymerization and number of droplets nucleated were affected by the molecular weight of the polymer that was predissolved. This dependence was attributed to a change in the number of droplets formed during homogenization as the molecular weight of the polymer was varied. However, the end group of the predissolved polymer did not have an affect on the kinetics. In all cases, predissolving polymer into a miniemulsion stabilized using hexadecane as costabilizer resulted in a much lower degree of enhancement in the kinetics compared to similar miniemulsion systems using cetyl alcohol as costabilizer. Since droplet sizes in miniemulsions containing hexadecane have been shown to be stable with time, unlike miniemulsions containing cetyl alcohol, these experiments were taken as evidence that the dominant cause for “enhanced droplet nucleation” is the preservation of droplet number prior to the addition of initiator.
Introduction Miniemulsions have droplet sizes between 50 and 500 nm and appear opaque and milky. In addition to an ionic surfactant, a secondary material termed a costabilizer (previous work has referred to this material as a cosurfactant1-5) is added to the system to reduce droplet degradation.1-3 Typical costabilizers that have been employed in miniemulsion systems are fatty alcohols and long-chain alkanes. Particle nucleation in miniemulsions occurs predominantly in the monomer droplets due to the relatively high surface area of the discontinuous phase that allows the droplets to compete effectively for free radicals. However, the kinetic behavior of miniemulsions stabilized using an alkane as costabilizer, (i.e., hexadecane) is significantly different from miniemulsions using a fatty alcohol, (i.e., cetyl alcohol). Tang et al.4 noted by dilatometry that the initial rates of polymerization are slower when cetyl alcohol is used as the costabilizer compared to hexadecane. Miller et al.5 observed the same behavior using calorimetry. He noted that miniemulsions stabilized using hexadecane have a significant jump in the rate of polymerization directly after the injection of the initiator followed by a small rise in the rate. However, * To whom correspondence should be addressed. † Lehigh University. ‡ BASF Aktiengesellschaft. (1) Ugelstad J.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Lett. Ed. 1973, 111, 503. (2) Choi, Y. T.; El-Aasser, M. S.; Sudol, E. D.; Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2973. (3) Delgado, J.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. Ed. 1986, 24, 861. (4) Tang, P. L.; Sudol, E. D.; Silebi, C. A.; El-Aasser, M. S. J. Appl. Polym. Sci. 1991, 43, 1059. (5) Miller, C. M.; Sudol, E. D.; Silebi, C. A.; El-Aasser, M. S. Macromolecules 1995, 28, 2754.
in miniemulsions stabilized using cetyl alcohol, there was no initial sharp rise in the rate of polymerization. This system also undergoes a long, steady rise in the rate curve that has been attributed to a long nucleation period. Overall, reaction times in miniemulsions stabilized using cetyl alcohol tend to be longer than miniemulsions stabilized using hexadecane at identical surfactant (SLS) concentrations. Measurement of droplet size and droplet stability has revealed a difference in miniemulsions stabilized using either cetyl alcohol or hexadecane. Ugelstad et al.6 measured the level of free surfactant versus time in miniemulsions stabilized using various costabilizers. The amount of free surfactant measured in the aqueous phase of different systems was used to determine the relative sizes of droplets between systems and an idea of the droplet stability provided by each costabilizer. It was noted that hexadecane produced a much finer droplet size than cetyl alcohol. However, a more interesting phenomenon is the stability of the droplets. It was noted that the average droplet size does not change as a function time when hexadecane was used as the costabilizer. This is in stark contrast to the miniemulsion droplets stabilized using cetyl alcohol that were shown to degrade significantly as a function of time. Miller et al.7 and Chern and Chen8 have observed similar behavior using capillary hydrodynamic fractionation and dynamic light scattering, respectively. The differences in the polymerization kinetics and average droplet size in miniemulsions stabilized using (6) Ugelstad, J.; Mørk, P. C.; Kaggerud, K.; Ellingsen, T.; Berge, A. Adv. Colloids Interface Sci. 1980, 13, 101. (7) Miller, C. M.; Venkatesan, J.; Sudol, E. D.; El-Aasser, M. S. J. Colloid Interface Sci. 1994, 162, 11. (8) Chern, C. S.; Chen, T. J. Colloid Polym. Sci. 1997, 275, 546.
10.1021/la990126d CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/1999
Miniemulsions Containing Predissolved Polystyrene
either cetyl alcohol or hexadecane are attributed to the differences in the physical properties of the costabilizer. Ugelstad et al.6 suggested that the effectiveness of the costabilizer against diffusion of monomer is directly related to its water solubility. The water solubility of hexadecane (5.9 × 10-6 g/dm3) is approximately a factor of 10 less than cetyl alcohol (4.1 × 10-5 g/dm3).6 Thus, when hexadecane is used as the costabilizer, it is expected to result in a smaller droplet size than cetyl alcohol. When cetyl alcohol is used as the costabilizer, an interfacial complex of the fatty alcohol and surfactant may form at the droplet-water interface that is an additional mechanism against droplet degradation.9-12 Since hexadecane is not a surface active ingredient, this material will not create an interfacial complex. This interfacial complex has also been suggested to cause a reduction in radical entry into the monomer droplets.13,14 This could account for the relatively low rates of polymerization observed at early times in the reaction in miniemulsions stabilized using cetyl alcohol. It has been shown in styrene miniemulsion systems using cetyl alcohol as the costabilizer that the addition of a small amount of polymer into the miniemulsion droplets before polymerization results in a large enhancement in the rate of polymerization and the number of droplets nucleated5,15,16 (nucleation here refers to polymerization initiated in monomer droplets by entry of a free radical from the aqueous phase). Three possible explanations have been proposed to explain this phenomenon: (1) the polymer chains disrupt the order of the condensed phase residing at the oil-water interface resulting in “openings”, which facilitate radical entry; (2) as polymer chains are introduced, the viscosity of the droplets increases which, in turn, could provide a longer residence time for entering radicals to propagate rather than desorb from the modified droplet; and (3) the polymer may provide extra stability to the small, uninitiated monomer droplets, allowing them to compete for radicals.13,17 Recent work by Blythe et al.18 has suggested that the most likely explanation correctly describing enhanced droplet nucleation in miniemulsions stabilized using cetyl alcohol is that predissolving polymer into the monomer droplets prior to the polymerization preserves the droplet number not only during the reaction but prior to the addition of the initiator. Thus, it is expected that if the costabilizer provides excellent droplet stability, the effect of predissolving polymer into the monomer droplets prior to homogenization should be minimal. For this reason, polymerizations were conducted to determine the effect of predissolving polymer into the monomer prior to homogenization of miniemulsions stabilized using hexadecane. The effect on miniemulsion kinetics of varying (9) Hallworth, G. W.; Carless, J. E. J. Pharm. Pharmacol. 1972, 24, 71. (10) Lack, C. D.; El-Aasser, M. S.; Silebi, C. A.; Vanderhoff, J. W.; Fowkes, F. M. Langmuir 1987, 3, 1155. (11) Cockbain, E. G.; McRoberts, T. S. J. Colloid Interface Sci. 1953, 8, 440. (12) Pithayanukul, P.; Pilpel, N. J. Colloid Interface Sci. 1982, 89, 494. (13) Miller, C. M.; Sudol, E. D.; Silebi, C. A.; El-Aasser, M. S. Macromolecules 1995, 28, 2772. (14) Tang, P. L. Kinetic Factors in Miniemulsion Polymerization. Ph.D. Dissertation, Lehigh University, 1991. (15) Miller, C. M.; Blythe, P. J.; Sudol, E. D.; El-Aasser, M. S. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2365. (16) Miller, C. M.; Sudol, E. D.; Silebi, C. A.; El-Aasser, M. S. Macromolecules 1995, 28, 2765. (17) Miller, C. M.; El-Aasser, M. S. In Polymeric Dispersions: Principles and Applications; Asua, J. M., Ed.; NATO ASI Series, Series E: Applied Sciences; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997; Vol. 335, p 109. (18) Blythe, P. J.; Morrison, B. R.; Mathauer, K. A.; Sudol, E. D.; El-Aasser, M. S. Macromolecules 1999, 32, 6944.
Langmuir, Vol. 16, No. 3, 2000 899 Table 1. Properties of the Predissolved Polymers Used in the Miniemulsion Polymerizations sample BASF ZK751/34a BASF R2/910 BASF R2/939 BASF R2/913
Mn (g/mol) Mw (g/mol) PDI (Mw/Mn) 40 400 39 000 94 200 206 000
42 300 45 100 105 000 263 000
chain end SO3H H H
1.05 1.16 1.11 1.28
Table 2. Basic Recipe for the Polymerizations ingredient
amount
grams
styrene polystyrenea deionized water sodium lauryl sulfateb hexadecaneb sodium bicarbonateb potassium persulfateb
19.8-20.0 parts 0.0-0.2 parts 80 parts 10 mM 40 mM 0.67-5.34 mM 0.67-5.34 mM
300.5-303.5 0.0-3.04 1207.7 3.440 12.71 0.067-0.533 0.213-1.704
a
Not present in all recipes. b Based on the aqueous phase. Table 3. Experimental Variables Studied
experiment identifier
polym chain end
polym Mn (g/mol)
[KPS] (mM)
LE31 LE54 LE34 LE33 LE32 LE40 LE35 LE36 LE37 LE43 LE41 LE42 LE45 LE70 LE61 LE63 LE60
none SO3H H H none H H H none H H H none H H H
none 40 400 39 000 94 200 206 000 none 39 000 94 200 206 000 none 39 000 94 200 206 000 none 39 000 94 200 206 000
0.67 0.67 0.67 0.67 0.67 1.33 1.33 1.33 1.33 2.66 2.66 2.66 2.66 5.33 5.33 5.33 5.33
the molecular weight and the end group of the predissolved polymer will also be reported. Experimental Section Materials. Styrene (BASF) was distilled under a reduced pressure of 15 mmHg and stored at -5 °C for later use. Sodium lauryl sulfate, SLS (Fluka, 99% pure), was used as received. Potassium persulfate, KPS (Fluka), was recrystallized from deionized water and dried under vacuum. The following chemicals were used as received: hexadecane (Fluka) and sodium bicarbonate (Fluka). Deionized water was used in the experiments. Table 1 lists the different polymers that were dissolved in the monomer prior to homogenization to form the miniemulsions. The polystyrene samples were prepared by anionic polymerization to provide narrow molecular weight distributions and the desired end group. Recipe and Procedures. The recipe used in the polymerizations is listed in Table 2. The aqueous phase was created by mixing the SLS, deionized water (less 1% of the water which was saved to dissolve the initiator), and NaHCO3. When polystyrene was used, it was predissolved in the monomer until all visible traces of the polymer disappeared. The hexadecane was then mixed with the oil phase for approximately 2 min. The oil and aqueous phases were then mixed in a beaker for 20 min using a stir bar. The resulting emulsion was then sonified (Branson Sonic Power Co.) for 60 s at 50% duty, power 7, and pulsed. Finally, the resulting emulsion was passed through the Microfluidizer (Model 110T, Microfluidics Corp.) for 10 passes at 80 psi inlet pressure. The resulting miniemulsion was then added to the reactor, where the KPS water solution was added after heating the miniemulsion to 70 °C. Table 3 lists the experiments and the variables studied in this paper.
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Calorimetry. Experiments were conducted in a calorimeter designed by BASF researchers. The heat of reaction (Qr) (J/s), measured by a calorimeter, can be related to the rate of polymerization by the following expression:
Rp )
Qr ∆HpVH2O
(1)
where ∆Hp is the total heat of polymerization (J/mol), Rp is the rate of polymerization ((mol/dm3 of water)/s), and VH2O is the volume of water in the reactor (dm3). The following expression can be used to compute the apparent fractional conversion determined by calorimetry at any time during the reaction:
∫Q t
x(t) )
0
r
dt (2)
∆HpM0
where M0 is the initial moles of monomer and x(t) is the fractional conversion. Particle Size Measurements. Samples were withdrawn during the reaction for particle size analysis. These were short stopped with a 1% aqueous hydroquinone solution and placed in an ice bath. Samples were stripped of monomer under vacuum in a Buchler Instruments Flash-Evaporator. Capillary hydrodynamic fractionation (CHDF) was used to determine the particle size distributions (Matec CHDF-1100). The number of particles per dm3 of water was computed from the following relationship:
Np )
6Mmwx
(3)
πFpDv3
where Mmw is the initial monomer-to-water ratio (g/dm3), x is the fractional conversion, Fp is the polymer density (g/cm3), and Dv is volume average diameter (cm). Droplet Size Measurements. Miniemulsion droplet size measurements were made using an analytical ultracentrifuge. The analytical ultracentrifuge has been reported to accurately measure particle diameters between 10 and 3000 nm and to be an effective tool for measuring the droplet size distribution in miniemulsion systems formed using hexadecane.19,20 In this method, one part of the miniemulsion is diluted by approximately five parts of a water/surfactant mixture. The diluted sample is then placed in a cell that is inserted into a rotor. The sample is then rotated at high speed (600-60 000 rpm) and is thus subjected to significant centrifugal forces that fractionates the droplets according to the following equation:
D)
[
]
18η ln(rs/rm) 2
(Fo - Fw)ω t
1/2
(4)
where D is the diameter of the droplet (cm), η is the viscosity of the medium (g/cm/s), rm is the radial position of the initial turbidity front (mm), rs is the radial position at which the turbidity is measured (mm), Fo and Fw are the densities of the oil and water phases, respectively (g/cm3), ω is the angular velocity of the rotor (1/s), and t is the time it takes for a change in turbidity to be measured at radius rs (s). A known set of standards can be used to create a calibration curve that relates the diameter of the particles passing the detector versus the time the particles pass the detector. An assumption used in the analysis of the ultracentrifuge data was the calibration curve used for polymer particles can be used for the monomer droplets. A similar assumption has been made when using other sizing techniques.7 In reality, for polydisperse samples, the change in measured turbidity will be gradual as the different size droplets pass by rs as a function of time. By superimposing many monodisperse particle fractions over the measured continuous transition in turbidity, an integral mass distribution of diameters is computed. (19) Maechtle, W.; Klodwig, U. Makromol. Chem. 1979, 180, 2507. (20) Blythe, P. J.; Sudol, E. D.; El-Aasser, M. S.; Mathauer, K.; Morrison, B. R.; Maechtle, W. Congress Proceedings: Second World Congress on Emulsion, Bordeaux, France, Vol. 2; 1997; p 2-1-087/05.
Figure 1. Effect of predissolving polystyrene polymers of different molecular weights that are terminated with hydrogen on the rate of polymerization versus time in styrene miniemulsions formed with 10 mM SLS/40 mM HD: (a) Mn ) 39 000 g/mol, (b) Mn ) 94 200 g/mol, (c) Mn ) 206 000 g/mol, and (d) no polymer. [KPS] ) 0.67 mM; Tr ) 70 °C.
Results and Discussion Effect of Molecular Weight of the Predissolved Polymer. Figure 1 shows the effect of predissolving polystyrene polymers having different molecular weights on the rate of polymerization versus time in miniemulsions stabilized using 10 mM SLS and 40 mM hexadecane. It is obvious that the rate of polymerization versus time in these miniemulsions is a function of the molecular weight of the polymer that has been predissolved in the monomer prior to homogenization. The miniemulsion containing the low molecular weight polymer exhibits the highest initial rate of polymerization while the miniemulsion containing the high molecular weight polymer has the lowest initial rate of polymerization. The miniemulsion containing the middle molecular weight predissolved polymer and the miniemulsion not containing any polymer have similar initial rates of polymerization. The dependence of the initial rate of polymerization on the predissolved polymer property will be addressed later in this paper. Perhaps the most striking feature of Figure 1 is the lack of the “enhanced droplet nucleation” phenomenon observed in miniemulsions stabilized with cetyl alcohol and homogenized emulsions.18,21 In miniemulsions stabilized using hexadecane, there is little enhancement in the rate of polymerization caused by predissolving polymer in the initial miniemulsion droplets (although there is a slight enhancement in the rate of polymerization, there is no dramatic jump in the rate of polymerization). It is important to remember, miniemulsions produced with hexadecane as the costabilizer have been shown to have an average droplet size that is stable with time. However, homogenized emulsions and miniemulsions prepared with cetyl alcohol as the costabilizer are comprised of droplets that degrade with time. The fact that the enhanced droplet nucleation phenomenon is only seen in systems which are documented to have relatively unstable droplet sizes strongly suggests that the dominating mechanism causing the enhanced rates of polymerization when polymer is predissolved into the monomer is the preservation of droplet number. It is suggested that since hexadecane can effectively stabilize against diffusive degradation of the droplets (Ostwald ripening), the added stabilizing mechanism provided by predissolved polymer does not significantly change the droplet number. However, when a less efficient costabilizer such as cetyl alcohol is used, (21) Blythe, P. J.; Klein, A.; Sudol, E. D.; El-Aasser, M. S. Macromolecules 1999, 32, 4225.
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Figure 2. Analytical ultracentrifugation droplet size analysis (mass fraction and cumulative mass fraction) of styrene miniemulsions prepared using 10 mM SLS/40 mM HD: (a) containing 1% 39 000 g/mol polystyrene, hydrogen terminated; (b) containing no polystyrene; and (c) containing 1% 206 000 g/mol polystyrene, hydrogen terminated. The droplet distribution fractograms are normalized to 1. Table 4. Effect of the Molecular Weight of the Predissolved Polymer on Droplet Size, Radius of Gyration of Polymer Chains, and Intrinsic Viscosity predissolved polymer (g/mol)
droplet radius-wt av (nm)
radius of gyration of predissolved polym (nm)
intrinsic viscosity (dL/g)
none 39 000 206 000l
31 19 34
5.5 12.5
0.2412 0.8929
the polymer adds an extra stabilizing mechanism to the initial droplets to preserve the number of nucleation sites available for polymerization. Initial Droplet Size. Table 4 lists the weight average droplet radii measured using an analytical ultracentrifuge for miniemulsions created using hexadecane as the costabilizer. Also included are the computed radii of gyration of the polymer chains in solution and the dilute solution viscosities of the polymer solutions. The radius of gyration of the polymer chains was computed from the following equation:
Rg ) AMw1/2
(5)
where Rg is the radius of gyration (nm), A is a constant (0.0275 nm/(g/mol)1/2 for these experiments22), and Mw is the molecular weight (g/mol). Figure 2 shows the droplet size distributions and the cumulative percentage of droplets versus droplet size for the various miniemulsions as measured by the analytical ultracentrifuge. It is apparent that predissolving polymer in the monomer prior to homogenization has an effect on the droplet size in miniemulsions stabilized using hexadecane. Furthermore, the molecular weight of the predissolved polymer appears to have a significant effect on the droplet size produced during homogenization when hexadecane is used as costabilizer. Figure 2 shows a significant tail of off-size larger droplets when the 206 000 g/mol polymer is predissolved prior to the formation of the miniemulsion that is not present in the other samples studied. From the cumulative fraction of droplets versus droplet size, it is apparent that approximately 40% of the mass of the monomer is located in droplets greater than 100 nm in (22) Cotton, J. P.; Decker, D.; Benoit, H.; Farnoux, B.; Higgins, J.; Jannink, G.; Oder, R.; Picot, C.; desCloizeaux, J. Macromolecules 1974, 10, 861.
diameter when the 206 000 g/mol polymer is used. However, when the miniemulsion does not contain any predissolved polymer, only approximately 15% of the mass of the monomer is located in droplets greater than 100 nm. Comparison of the initial rates of polymerization (Figure 1) with the initial droplet size is revealing. The miniemulsion containing the 39 000 g/mol polystyrene has the smallest initial droplet size (corresponding to the highest initial droplet number) and has the highest initial rate of polymerization. The miniemulsion containing the 206 000 g/mol polystyrene has the largest initial droplet size (corresponding to the lowest initial droplet number) and exhibits the slowest initial rate of polymerization. Thus, it becomes apparent that the initial rate of polymerization is proportional to the number of nucleation sites (droplets) that are available at early times in the polymerization. The dependence of the rate of polymerization on the number of initial droplets is reproducible and is presented elsewhere.23 The effect of polymer on the formation of miniemulsion droplets when hexadecane is used as the costabilizer is proposed to be 2-fold. First, the monomer in the small droplets can be maintained at the equilibrium swelling potential of the polymer/monomer/costabilizer mixture. Thus, this influence of polymer will promote generation of smaller size droplets in 1% polymer miniemulsion systems compared to miniemulsion systems that contain no polymer. Ugelstad et al.6 were able to show that there is a minimum droplet size predicted by thermodynamics when a costabilizer is employed in formation of the miniemulsion. Thus, droplets that are produced by the Microfluidizer that are below this critical size will not be stabilized by the presence of hexadecane. However, the addition of polymer in the droplets will act to preserve each of the initial droplets formed during the homogenization step due to the equilibrium swelling of the polymer with monomer coupled with the complete lack of water solubility of the polymer. As polymer is added to the monomer phase, its viscosity increases. This results in less efficient homogenization of this oil phase into the aqueous phase and accordingly a population of larger size droplets. Thus, as the molecular weight of the polymer increases, the viscosity of the oil phase is increased and droplets produced by homogenization are larger. As seen in Table 4, the dilute solution viscosity of the system containing 1% 206 000 g/mol polystyrene is nearly 330% greater than that of the 1% 39 000 g/mol polystyrene solution. It is feasible that this increase in the viscosity is resulting in less efficient homogenization. Another effect that may oppose small droplet formation is the possibility that the radius of gyration of the predissolved polymer chains will be greater than the droplet diameter. The polymer chain will avoid being confined in a space smaller than the radius of gyration and thus will create a force that will prevent the formation of miniemulsion droplets with a radius less than the radius of gyration. As seen in Table 4, polymer chains with a molecular weight of 39 000 g/mol have a radius of gyration of 5.5 nm that is much less than the average droplet radius measured by ultracentrifugation. However, polymer chains with a molecular weight of 206 000 g/mol have a radius of gyration of 15.5 nm, and this is in the size range measured by ultracentrifugation of the miniemulsion droplets prepared using hexadecane. Thus, it is possible that the polymer chains (23) Blythe, P. J. Enhanced Droplet Nucleation in Miniemulsion Polymerization: A Kinetic and Mechanistic Study. Ph.D. Dissertation, Lehigh University, 1998.
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Figure 3. Effect of predissolving polymers of different molecular weights that are terminated with hydrogen on the number of particles formed versus time in miniemulsions created with 10 mM SLS/40 mM HD: (a) Mn ) 39 000 g/mol [b], (b) Mn ) 94 200 g/mol [2], (c) Mn ) 206 000 g/mol [(], and (d) no polymer [9]. [KPS] ) 0.67 mM; Tr ) 70 °C.
Figure 4. Effect of predissolving polymers of different molecular weights and terminated with hydrogen on the rate of polymerization and number of particles formed versus conversion in miniemulsions created with 10 mM SLS/40 mM HD: (a) Mn ) 39 000 g/mol [2], (b) Mn ) 206 000 g/mol [9], and (c) no polymer [b]. [KPS] ) 0.67 mM; Tr ) 70 °C. (Here, “7E+17”, for example, represents 7 × 1017.)
are providing a force against the formation of very small droplets in systems containing 1% predissolved polymer, which has a molecular weight in the range of 200 000 g/mol. Effect of Molecular Weight of the Predissolved Polymer on Evolution of Particle Number. Figure 3 shows the evolution of the number of particles (based on CHDF measurements) as a function of the reaction time in polymerizations where polystyrene polymers of different molecular weights were predissolved in styrene miniemulsions stabilized using hexadecane. Data for a miniemulsion stabilized using hexadecane but not containing any predissolved polymer is included for reference. It is apparent that predissolving any of the polymers studied resulted in a significant enhancement in the length of the nucleation period when compared to the miniemulsion that did not contain predissolved polymer. Figure 4 compares the rate of polymerization and number of particles versus fractional conversion. It shows that miniemulsions containing 1% polymer undergo a more significant rise in the polymerization rate curves after the initial rapid increase after the initiator addition than the miniemulsion containing no predissolved polymer. The reason for the enhanced rise in the heat curves in the 1% polymer systems is the greater continued nucleation in the miniemulsions containing 1% polymer compared to the miniemulsion containing no polymer. The continued
Blythe et al.
Figure 5. Effect of the initiator concentration on the final number of particles produced from miniemulsions formed using 10 mM SLS/40 mM HD that contain (a) no predissolved polymer (×), (b) Mn ) 206 000 g/mol (b), (c) Mn ) 94 200 g/mol (2), and (d) Mn ) 39,000 g/mol (9). [KPS] ) 0.67-5.34 mM; Tr ) 70 °C.
nucleation is most likely due to the preservation of the droplet number for longer time periods into the reaction by the presence of the 1% polymer. As particles grow during the polymerization, they demand monomer at the expense of the smallest uninitiated droplets. Hexadecane alone is unable to slow this thermodynamic drive for monomer redistribution. It is suggested the droplets may disappear because the droplet size decreases as monomer is donated to the growing polymer particles. It is possible the droplet size decreases below the minimum droplet size predicted thermodynamically by Ugelstad et al., and thus the monomer in the droplet disappears to either the droplets or the growing particles. However, by predissolving 1% polymer into all the initial monomer droplets, there is a thermodynamic force in each droplet requiring the retention of a fraction of the initial monomer to satisfy polymer to monomer equilibrium requirements. It is this added force of the polymer that preserves the droplet number for longer time periods into the reaction. The effect of various predissolved molecular weight polymers on the final number of particles nucleated versus initiator concentration for miniemulsion polymerizations is shown in Figure 5. The data for miniemulsions not containing any predissolved polymer is included for reference. It is apparent in miniemulsion systems containing 1% polymer, regardless of the molecular weight, there is virtually no dependence of the final number of particles nucleated on the initiator concentration. It has been reported by TEM that the final particle size distributions for latexes produced from miniemulsions stabilized using hexadecane are not bimodal (although the system used in this work used both vinyl acetate and vinyl 2-ethylhexanoate monomers).24 These latexes are different from those produced by polymerization of styrene miniemulsions stabilized using cetyl alcohol that have been noted to give bimodal particle size distributions.16 In miniemulsion latexes prepared using cetyl alcohol as costabilizer, the smaller size particles were considered to be created from the droplets that never captured an aqueous phase radical during the polymerization. Thus, these small particles were comprised of a combination of the cetyl alcohol and the predissolved polymer (3/1 weight ratio) initially present in the droplets (monomer lost by diffusion to growing particles). However, during the drying of the sample for TEM, hexadecane is a volatile component and will not be present in the final latex that is studied. Therefore, droplets that are never entered by a radical would only appear as polymer chains that are not detected (24) Kitzmiller, E. L. Private communication.
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Figure 6. Effect of predissolving polymers terminated with different end groups on the rate of polymerization versus time in miniemulsions formed with 10 mM SLS/40 mM HD: (a) Mn ) 39 000 g/mol, hydrogen terminated, and (b) Mn ) 40 400 g/mol, sulfonate terminated. [KPS] ) 0.67 mM; Tr ) 70 °C.
by TEM. The CHDF measurements indicate a constant final particle number with increasing initiator concentration for miniemulsions stabilized using hexadecane. Since the CHDF should not be able to resolve any particles resulting from the original droplets that were never entered by a free radical, the lack of dependence of the particle number on initiator concentration is a strong indication that virtually 100% of the droplets capture radicals when polymer is predissolved in the initial miniemulsion droplets. If more droplets were nucleated as the initiator concentration increased, the particle number measured by the CHDF would increase with increasing initiator concentration. The dependence of the final number of particles nucleated versus initiator concentration for the miniemulsions formed with hexadecane as the costabilizer and not containing any predissolved polymer is given by eq 6. Obviously, less than
Nfinal ∝ [I]0.11 p
(6)
100% of the droplets become polymer particles when polymer is not present in the system. The most likely explanation is that a fraction of the droplets disappear by diffusion to the growing polymer particles before they have a chance to capture an aqueous phase radical. Thus, even in miniemulsions that utilize hexadecane, there is a significant advantage of predissolving polymer into the miniemulsion droplets prior to polymerization. Effect of Predissolved Polymer End Group. Miniemulsions formed using hexadecane as the costabilizer do not exhibit condensed structures of costabilizer and surfactant at the droplet/water interface (as in the CA/ SLS system). Therefore, it would be expected that the presence of the predissolved polymer at or near the droplet/ water interface would not have any effect on the polymerization kinetics. Figure 6 shows the effect of varying the chain end of the predissolved polymer from hydrogen to a sulfonate group on the rate of polymerization versus time curves in miniemulsions formed using hexadecane. It is expected that the polymer chains terminated with hydrogen will entropically avoid the surface of the droplets while the chains containing a sulfonate end group will be anchored at the droplet/water interface by the hydrophilic sulfonate. This behavior was substantiated by interfacial tension measurements.23 From Figure 6, it is apparent that there is no significant difference in the kinetics from predissolving polymer chains possessing different end groups. The slight difference in the kinetics is most likely due to either (1) experimental error or (2) the differences
in the molecular weight and molecular weight distributions of the predissolved polymer. The hydrogen-terminated polymer has a Mw of 45 100 g/mol while the sulfonate-terminated polymer has a Mw of 42 300 g/mol indicating a greater fraction of the hydrogen-terminated polymer is in the form of longer chains. Analytical ultracentrifugation work presented earlier in the paper suggests that the longer the chain length of the predissolved polymer, the greater the droplet size produced during homogenization. Thus, it is suggested that a possible contributing reason that the rate of polymerization in the system containing hydrogen-terminated polymer is lower than the system containing sulfonateterminated polymer is that the hydrogen-terminated polymer has a higher fraction of longer chains which reduces the initial droplet number. Ramifications for “Enhanced Droplet Nucleation”. Blythe et al. have presented results obtained by predissolving polymers into miniemulsions prepared using cetyl alcohol as costabilizer and compared these to homogenized emulsions (no costabilizer) containing the same predissolved polymer.18,21 In both cases, predissolving polymer resulted in significant enhancement in the rate of polymerization and the number of droplets nucleated. Literature data were also cited for these systems without the predissolved polymer noting that the droplets in these emulsions tend to degrade with time (Ostwald ripening). It was suggested that the main cause for the enhancement in the rate of polymerization and number of droplets nucleated is the polymer preserving the droplets not only during the polymerization but also prior to the addition of initiator. Thus, the polymer is acting to preserve the initial number of droplets that subsequently become polymer particles during the polymerization. In this paper, a system (miniemulsion using hexadecane as the costabilizer) was chosen in which the droplet size has been reported to be constant as a function of time. Thus, if preservation of droplet number prior to the addition of initiator was a dominating effect causing “enhanced droplet nucleation”, this system should not exhibit similar enhanced behavior. Indeed, although there were changes in the rate of polymerization and number of droplets nucleated in miniemulsions formed using hexadecane as the costabilizer, the magnitude of these changes was significantly less than those seen in homogenized emulsions and miniemulsions costabilized with cetyl alcohol. Thus, the results of this paper further support the idea that the dominating factor causing enhanced droplet nucleation in miniemulsions formed using cetyl alcohol as costabilizer is the preservation of droplet number by the presence of the polymer in the miniemulsion droplets. Conclusions This paper reports the effects of predissolving polystyrene of various molecular weights and end groups on the polymerization kinetics of styrene miniemulsions that are formed using hexadecane as the costabilizer. It was noted that predissolving polymer has an effect on the rate of polymerization and number of droplets nucleated, albeit a much smaller effect than seen for homogenized emulsions and miniemulsions formed using cetyl alcohol as the costabilizer. The molecular weight of the predissolved polymer also had an effect on the kinetics of polymerization. It was shown that as the molecular weight of the predissolved polymer increased, the droplet number formed during homogenization decreased. Also, it was shown that the presence of polymer in the initial miniemulsion droplets extends the lifetime of the droplets during the polymerization, allowing for a greater number
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of droplets to be nucleated. It was noted that varying the chain end of the predissolved polymer chain from hydrogen (hydrophobic) to sulfonate (hydrophilic) has little effect on the kinetics. The significantly lower degree of enhancement in the rate of polymerization by the addition of polymer in the miniemulsions stabilized using hexadecane as compared to cetyl alcohol is taken as further evidence that the dominating cause for enhanced droplet nucleation in miniemulsion systems using cetyl alcohol as the costabilizer is the preservation of the droplet number by the presence of the polymer.
Blythe et al.
Acknowledgment. Financial support from BASF AG and the National Science Foundation under Grant CTS9628783 is greatly appreciated. Anionic polymerizations conducted by Dr. Konrad Knoll and analytical ultracentrifugation work by Dr. Walter Maechtle at BASF are highly appreciated. The laboratory assistance of Mr. Ruediger Janotte and Mr. Stephan Wahl, both of BASF, is also acknowledged. LA990126D
