Langmuir 2008, 24, 3801-3806
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Synthesis of Amphoteric Polystyrene Particles Using Mixed Initiators Aaron Olsen, Huai-Chin Lee, Marios Hatzopoulos, Jeroen S. van Duijneveldt, and Brian Vincent* School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K. ReceiVed NoVember 19, 2007. In Final Form: January 8, 2008 The synthesis of amphoteric polystyrene latex particles, using a mixture of cationic (amidinium based) and anionic (carboxylic acid based) initiators in a surfactant-free emulsion polymerization reaction is investigated; this extends work described in an earlier paper by Bolt et al. Electrophoretic mobility measurements show the effect of the initiator concentration ratio on the isoelectric point (IEP) of the particles. A good correlation with theoretical predictions is obtained. Particle size and polydispersity are determined as a function of the lag time between the addition of each initiator. An increase in particle size and polydispersity is observed at short lag times. It is shown that this is due to the ratio of the cationic to anionic surface charge approaching unity during the reaction. At long lag times an increase in polydispersity may occur due to late-stage, secondary nucleation upon addition of the second initiator. Increasing the reaction pH to reduce the degree of ionization of the cationic initiator greatly reduces the polydispersity and has a significant effect on the IEP of the particles. This effect is ascribed to the burial of a fraction of the neutral amidine groups below the particle surface due to their increased solubility in the monomer. Slow addition of the second initiator was found to reduce the polydispersity of the particles, while maintaining an IEP value consistent with that expected for the ratio of initiators added.
Introduction The widespread use of polymer latex particles as model colloidal particles is largely due to the high degree of monodispersity that may be obtained and the ease with which physical characteristics, such as their size and surface charge (anionic, cationic or amphoteric), may be controlled. This paper is concerned with the synthesis of amphoteric (zwitterionic) latex particles and the control of their average size, monodispersity, and isoelectric points (IEP) using a mixed initiator system and is an extension of previous work reported by Bolt et al.1 Homola and James, some time ago,2 proposed using surfactantfree, emulsion polymerization of mixtures of ionizable monomers as a route to creating amphoteric latex particles. The methacrylic acid (MAA) and dimethylaminoethylmethacrylate (DMAEM) they employed imparted -COOH and tertiary N groups, respectively, to the particle surface. A significant number of investigations then followed on from this initial work.3-14 A variation of this copolymerisation method was investigated by Chern et al.15,16 In that work (anionic) MAA was copolymerized with (neutral) methyl methacrylate, the positive * To whom correspondence should be addressed. E-mail: brian.
[email protected]. (1) Bolt, P. S.; Goodwin, J. W.; Ottewill, R. H. Langmuir 2005, 21, 99119916. (2) Homola, A.; James, R.O. J. Colloid Interface Sci. 1977, 59(1), 123-134. (3) James, R. O.; Homola, A.; Healy, T. W. J. Chem. Soc. Faraday I 1977, 73, 1436-1445. (4) Healy, T. W.; Homola, A.; James, R. O. Faraday Discuss. Chem. Soc. 1978, 65, 156-163. (5) Harding, I.H.; Healy, T.W. J. Colloid Interface Sci. 1982, 89(1), 185-201. (6) Harding, I. H. Colloid Polym. Sci. 1985, 263, 58-66. (7) Harding, I. H.; Healy, T. W. J. Colloid Interface Sci. 1985, 107(2), 382397. (8) Furusawa, K.; Anzai, C. Colloid Polym. Sci. 1987, 265, 882-888. (9) Myers, D. F.; Saville, D. A. J. Colloid Interface Sci. 1989, 131(2), 461470. (10) Furusawa, K.; Chen, Q.; Tobori, N. J. Colloid Interface Sci. 1990, 137(2), 456-461. (11) Rosen, L. A.; Saville, D. A. J. Colloid Interface Sci. 1990, 140(1), 82-92. (12) Rosen, L. A.; Saville, D. A. Langmuir 1991, 7, 36-42. (13) Ha, H.-J.; Park, Y.-J.; An, J. H.; Kim, J.-H. J. Appl. Polym. Sci. 1997, 66, 1899-1909. (14) Rugge, A.; Tolbert, S. H. Langmuir 2002, 18, 7057-7065.
charge in this case being imparted by the initiator, 2,2′-azobis(2-methylpropionamide)dihydrochloride (V50). More recently, Taira et al.17 proposed a further co-polymerization route for the preparation of amphoteric particles, combining L-lysine with ethylene glycol diglycidyl ether (EGDGE). In this case the L-lysine is itself amphoteric while the EGDGE does not contribute to the surface charge of the particles. In yet another variation, Fang et al.18 proposed a method that employed an inherently amphoteric species. Co-polymer particles, based on styrene and acrylamide monomers, were prepared using an amphoteric initiator, 2,2′-azobis[N-(2-carboxyethyl)-2-2methylpropionamidine]hydrate. The surface charge in this case is imparted solely by the initiator. This method has been employed to prepare several other types of copolymer particles,19,20 as well as homopolymer particles.21-25 Post-synthesis treatment of homo-ionic or non-ionic particles has been another avenue explored for the preparation of amphoteric particles. Kawaguchi et al.26 synthesized styreneacrylamide co-polymer particles and then treated them with NaOCl and NaOH, converting the surface amide groups to amino and carboxyl groups, by the Hoffmann reaction and competitive hydrolysis, respectively. This method has also generated a series (15) Chern, C.-S.; Lee, C.-K.; Chang, C.-J. Colloid Polym. Sci. 2003, 281, 1092-1098. (16) Chern, C.-S.; Lee, C.-K.; Chang, C.-J. Colloid Polym. Sci. 2004, 283, 257-264. (17) Taira, S.; Du, Y.-Z.; Kodaka, M. Biotechnol. Bioeng. 2006, 93(2), 396400. (18) Fang, S.-J.; Fujimoto, K.; Kondo, S.; Shiraki, K.; Kawaguchi, H. Colloid Polym. Sci. 2000, 278, 864-871. (19) Fang, S.-J.; Kawaguchi, H. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 211, 79-84. (20) Fang, S.-J.; Kawaguchi, H. Colloid Polymer Sci. 2002, 280, 984-989. (21) Gu, S.; Inukai, S.; Konno, M. J. Chem. Eng. Jpn 2002, 35, 977-981. (22) Gu, S.; Inukai, S.; Konno, M. J. Chem. Eng. Jpn 2003, 36, 1231-1235. (23) Yamada, Y.; Sakamoto, T.; Gu, S.; Konno, M. J. Colloid Interface Sci. 2005, 281, 249. (24) Lee, J.; Ha, J. U.; Choe, S.; Lee, C.-S.; Shim, S. E. J. Colloid Interface Sci. 2006, 298, 663-671. (25) Gu, S.; Akama, H.; Nagao, D.; Kobayashi, Y.; Konno, M. Langmuir 2004, 20, 7948-7951. (26) Kawaguchi, H.; Hoshino, H.; Ohtsuka, Y. J. Appl. Polym. Sci. 1981, 26(6), 2015-2022.
10.1021/la703613p CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008
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of subsequent investigations.27-29 Another method, proposed by Musyanovych et al.,30 used carboxyl-functionalized polystyrene latex particles that had been prepared using a mixture of hydroperoxide monomer, acrylic acid, and styrene. The reaction of residual, un-unreacted hydroperoxide groups present on the surface with an amino functional monomer rendered the surface amphoteric. There are a number of problems associated with the methods referred to so far for the synthesis of amphoteric polymer latex particles. Incorporation of ionic monomer into particles inevitably produces a gel-like surface in an aqueous medium so the particles are not ideal as models for solid spheres. Moreover, if the isoelectric point (IEP) of the resultant particles is determined by the relative amounts of each monomer used in the reaction, it is impossible to change the IEP without altering the actual composition of the particles and so, potentially, their physical properties. The drawback of using an inherently amphoteric monomer or initiator is that the IEP of the particles cannot be altered without employing an entirely different species. Posttreatment, while being more flexible than the other methods described, does imply additional preparatory steps. As a potential solution to the problems described above, Bolt et al.1 investigated the possibility of using a single non-ionic monomer (styrene), but with two different initiators, in the same surfactant-free emulsion polymerization reaction. The surface charge is thus imparted solely by the anionic and cationic initiators used, in this case: 4,4′-azobis(4-cyanopentanoic acid) (ABCPA) and V50, respectively. The use of a non-ionic monomer ensured a hydrophobic surface, unlike the gel-like surface of ionic copolymer particles in aqueous media. The IEP is determined by the relative concentration of the initiator fragments on the particle surface. It was noted by Bolt et al. that a delay (or ‘lag-time’) was required between addition of the anionic and cationic initiators in order to produce monodisperse particles. It was suggested that this is due to the need to avoid balancing of opposite charges on the particle surface and, hence, particle coagulation. A basic theoretical analysis of the relative concentrations of the respective initiator fragments present as a function of time, for different lag times, confirmed this suggestion. The current investigation sought to extend the work of Bolt et al.,1 in order to elucidate more thoroughly the relationship between the particle properties and the initiator lag time for a similar dual-initiator synthesis of amphoteric polystyrene latex particles. A systematic approach was taken to observing the effect of lag time on the physical properties (size, polydispersity, and isoelectric point) of the particles produced. Experimental Section 1. Materials. All water used was purified by reverse-osmosis (Milli-Q) and had a resistance of 18.2 MΩ cm (at 25 °C). The styrene monomer (Sigma-Aldrich, 99%) was passed through a glass column packed with activated Al2O3 (Sigma-Aldrich) prior to use to remove the polymerization inhibitor and was subsequently used within 1 week. 4,4′-Azobis(4-cyanopentanoic acid) (ABCPA, Fluka), 2,2′-azobis(2-methylpropionamide)dihydrochloride (V50, Wako), NaOH, and HCl were used as received. 2. Latex Synthesis. The latex particles were prepared using the method described by Bolt et al.1 The reaction recipe was the same for all reactions. The total reaction mixture volume was 400 mL, and the concentration of styrene used was 0.3 mol L-1. ABCPA was (27) Kawaguchi, H.; Hoshino, H.; Ohtsuka, Y. Colloids Surf. 1983, 6, 271281. (28) Kawaguchi, H.; Hoshino, H.; Amagasa, H.; Ohtsuka, Y. J. Colloid Interface Sci. 1984, 97(2), 465-475. (29) Shirahama, H.; Ohno, H.; Suzawa, T. Colloids Surf. 1991, 60, 1-17. (30) Musyanovych, A.; Adler, H.-J. P. Langmuir 2005, 21, 2209-2217.
Olsen et al. added at a concentration of 1 × 10-3 mol L-1, while the concentration of V50 was determined by the desired molar ratio of anionic to cationic initiator. Reactions were carried out in a 500 mL round-bottom flask, fitted with a condenser, a thermometer, a nitrogen inlet, a stirrer with paddle-type blade, and a spare inlet for addition of reagents. The vessel was immersed in a temperature-controlled oil bath and the reaction temperature maintained at 80 ( 1 °C. The contents of the reaction vessel were stirred at 300 rpm. A period of 1 h was allowed for equilibration of the water temperature and purging with nitrogen. After addition of the styrene, a low nitrogen flow was passed above the reaction mixture to maintain positive pressure in the reaction vessel. The ABCPA was dissolved in 20 mL of NaOH solution, the concentration of which depended upon the desired reaction pH: 0.04 mol L-1 for pH 8.5 ( 0.2 and 0.06 mol L-1 for pH 9.3 ( 0.1. The final pH of each particle suspension was checked in each case. The required amount of V50 was dissolved in 10 mL water, and both initiators were washed in with a further 10 mL of water. The first initiator (ABCPA for “positive” lag times and V50 for “negative” lag times) was added 10 min after addition of the styrene, followed by the second initiator after the desired lag time for the given reaction. Total reaction times of 21 h were allowed in all cases. After synthesis, the latex dispersions were cooled to room temperature and samples were taken for pH and solids concentration analysis. The particles were then filtered through glass wool to remove coagulum and dialyzed against purified water in 12 000-14 000 Da pore-size dialysis tubing (Visking), which had been well boiled prior to use. 3. Particle Characterization. The electrophoretic mobility of the particles was measured, as a function of pH, using a Brookhaven Zeta Plus apparatus. Particle suspensions were prepared at a solids concentration of 0.01 wt %. The pH of each suspension was first lowered to 3, using a dilute solution of HCl, then raised to the desired pH using a dilute solution of NaOH. In this regard, Bolt et al.1 had shown that the IEP of their (carboxyl plus amidine functionalized) polystyrene latex particles was independent of the concentration of Na+ and Cl- ions. The electrophoretic mobility of each sample was determined from the average of 30 measurements. Scanning electron microscopy (SEM, Jeol JSM 5600LV) was used to obtain images of each of the synthesized particle systems. A sample of each was pipetted onto freshly cleaved mica sheets mounted on an SEM stub. The samples were coated with 15 nm of Pt/Pd using an Agar high-resolution sputter coater. The number average size and polydispersity were determined from these images using the ImageJ software package. Approximately 1000 particles were analyzed for each system. Care was taken to avoid, as far as possible, errors introduced by particle size fractionation upon drying on the SEM stub. The percentage of monomer converted to particles (i.e., the yield) of each reaction was determined by gravimetric analysis of a known mass of the particle dispersion, after drying in an oven at 60 °C.
Results and Discussion 1. Dependence of the IEP on the Initiator Concentration Ratio. Figure 1a shows the electrophoretic mobility, as a function of pH, for the amphoteric particles synthesized using relative initiator concentrations ([ABCPA]/[V50]) of 2 and 10. It is apparent that, as this ratio increases (i.e., less V50 for a fixed ABCPA concentration), so, at low pH values, the positive electrophoretic mobility decreases, while, at high pH values, the negative electrophoretic mobility increases, as expected. The IEP of the particles, i.e., the pH at which the electrophoretic mobility is equal to zero, is thus observed to decrease accordingly. The IEP is plotted against the relative initiator concentration in Figure 1b. The IEP of amphoteric particles, synthesized by the method of Bolt et al.,1 can be predicted for any initiator ratio by invoking the Henderson-Hasselbalch equation. For the case at hand, where
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Figure 1. (a) Electrophoretic mobility as a function of pH, in 10-3 mol L-1 KCl solution, for particles synthesized using initiator ratios ([ABCPA]/[V50]) of (B) 10 and (O) 2 (the lag time was 120 min and the reaction was carried out at pH 8.5); (b) the IEP as a function of relative initiator concentration (error bars are estimates). The solid line indicates the predicted IEP from eq 4 assuming a pKa value for carboxyl groups of 4.6.32
there is an excess of carboxyl groups, it may be assumed that the IEP occurs at a pH value where all the amidine groups, but only a certain fraction of the carboxyl groups, are charged. So, for the region of interest, the Henderson-Hasselbalch equation can be expressed as
pH ) pKa - log
( ) 1 - RC RC
(1)
where RC is the degree of ionization (the charged fraction) of the surface carboxyl groups. Assuming negligible specific adsorption of ions, there must be an equivalent number of anionic and cationic sites on each particle at the IEP, so
RCNC ) RANA
(2)
where RA is the degree of ionization of the surface amidine groups and NC and NA are the total number of surface carboxyl and amidine groups, respectively. It may be reasonably assumed that the final ratio of the two functional groups (i.e., ionized plus neutral) on the surface (NC/ NA) attained by the particles is the same as the total initiator concentration ratio (R ) [ABCPA]/[V50]) employed (provided no functional groups are “buried”). At the IEP, where it is assumed RA is equal to 1 (see earlier), then from eq 2 it follows that
RC )
1 R
(3)
Substitution of eq 3 into eq 1 leads to
IEP ) pKa - log(R - 1)
Figure 2. Number average particle diameter and percentage variance as a function of initiator lag time at (a) pH 8.5 ( 0.2 and (b) 9.3 ( 0.1.
shows a good correlation. Thus, the IEP of particles synthesized by this method may be controlled and predicted, with some degree of accuracy, by varying the relative concentrations of the anionic and cationic initiators. 2. Particle Size and Polydispersity. Figure 2 shows the number average diameter and polydispersity (variance ) standard deviation/average diameter, expressed as a percentage) of the particles synthesized with R ) 2 at pH 8.5 ( 0.2 (Figure 2a) or pH 9.3 ( 0.1 (Figure 2b), plotted as a function of the lag time between the addition of the anionic and the cationic initiators. Time zero corresponds to the addition of the anionic initiator; hence, negative lag times imply addition of the cationic initiator first. Goodwin et al.31 have proposed the following empirical equation for estimating the diameter (D) in nm of anionic polystyrene latex particles:
[
log D ) 0.238 log
]
[I][M]1.723 4929 - 0.827 + T [P]
where [I] is the ionic strength, [M] is the monomer concentration, [P] is the initiator concentration, and T is the absolute temperature. In making use of eq 5 in the present case some thought has to be given to evaluating [I] and [P]. The ionic strength was taken to be that associated with the addition of NaOH to adjust the reaction pH (see Section 2.2). So this amounts to [I] ) 2 × 10-3 M in the case of pH 8.5. Regarding the effective initiator concentration, [P], it is assumed that the particle size is determined by the net charge on the growing particles, which is related to the difference in concentration of the two initiators. Hence,
[P]eff ) RC[ABCPA] - RA[V50] (4)
Comparison of eq 4, assuming a surface carboxyl pKa of 4.6,31 with the experimentally determined IEP values in Figure 1b, (31) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sci. 1974, 252, 464-471.
(5)
(6)
The pKa values for the carboxyl and amidine groups are ∼4.632 and 10.2,33 respectively. Hence, at least at pH 8.5 the fraction (32) Ottewill, R. H.; Shaw, J. N. J. Electroanal. Chem. Interfacial Electrochem. 1972, 37, 133-142. (33) Goodwin, J. W.; Ottewill, R. H.; Pelton, R. Colloid Polymer Sci. 1979, 257, 61-69.
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Figure 3. Charged free radical ratio (cationic/anionic) as a function of time for an R value of 2, RC and RA equal to 1, V50 and ABCPA half lives taken as 28 min and 2 h,1 respectively, and for lag times between -30 and 120 min. Time zero corresponds to addition of the first initiator.
of carboxyl and amidine groups that are charged (RC and RA, respectively) may both be taken as equal to 1. At pH 9.3 this is no longer true for the amidine groups. Use of eq 5, based on the above assumptions, leads to a value of 265 nm for the particle diameter. This is close to the values observed at long lag times at pH 8.5 in Figure 2a and at all lag times at pH 9.3 in Figure 2b. We consider below possible reasons for the presence of larger particles at lower lag times at pH 8.5, the better agreement at pH 9.3, and the observed trends in polydispersity at both pH values. 2.1. Charge-Balance Coagulation and Coalescence. At pH 8.5, for lag times less than ∼60 min, the average particle diameter strongly exceeds the predicted value (Figure 2a); in addition, there is an increase in the polydispersity of the particles. This indicates the possibility of a growth mechanism which is not present for single initiator, surfactant-free emulsion polymerization. One such possibility is discussed below. Bolt et al.1 assumed that, at any given point in the polymerization reaction, the surface charge ratio on the particles was proportional to the ratio of free radicals in solution. This is based on various provisions, namely, that (i) there is 100% radical efficiency, (ii) there is rapid inclusion of the radicals into the particles after decomposition, (iii) all end groups are at the particle surface, and (iv) all the free radicals are in the charged state at the pH of the reaction. Hence, the particles could attain net zero charge (ratio of surface charge groups equal to one) if at some point in the reaction the concentration ratio of free radicals reaches a value of one. This could lead to particle coagulation, followed by coalescence since the particles will contain unreacted monomer and, hence, be “soft”, at this point in the reaction. Figure 3 shows the calculated ratio of cationic to anionic charged free radicals present at a given time after addition of the first initiator, at various lag times. For these calculations the half-lives of V50 and ABCPA at 80 °C were taken to be 28 min and 2 h,1 R ) 2, and both RC and RA were assumed to be 1 at pH 8.5. It is evident from Figure 3 that if the faster decaying cationic initiator is added too soon after the anionic initiator the relative charge density of each charge type on the surface of the growing particles will be close or equal to unity for some period of time. Similarly, if the cationic initiator is added before the anionic initiator (i.e., negatiVe lag times), then the relative charge density must necessarily pass through unity at some time during the reaction as the particles change from being predominantly cationic to anionic. In both cases, as the relative charge ratio approaches unity, then as discussed earlier, particle coalescence can occur, which would not occur in a single initiator synthesis. During the early stages of all surfactant-free, emulsion polymerization reactions particle coalescence occurs because
Figure 4. Particle size distributions from SEM analysis for polystyrene particles synthesized with an initiator lag time of (a) 45, (b) 90, and (c) 120 min at pH 8.5 ( 0.2.
the surface charge density of the initial, small particles is not high enough. However, at some stage in the polymerization reaction, a critical particle size is reached, where the surface charge density is sufficiently high for the particles to become kinetically stabilized against further coalescence. After this point, particle growth continues by addition of oligomers formed in the aqueous phase and conversion of monomer present in the particle. Since, thereafter, particles grow more-or-less at the same rate, monodispersity is favored. However, in a mixed initiator system, as studied in this work, if at some point during the reaction the surface charge ratio of particles approaches unity then further coalescence can occur, as described above, leading to a greater average final particle size, and greater polydispersity. For the polymerization reactions carried out at pH 8.5 (Figure 2a), the percentage variance is considerably reduced at long lag times where this later-stage particle coalescence is largely avoided, as predicted from the calculations shown in Figure 3. However, the actual percentage variance is still ∼10%, which is higher than that produced, in general, for single-initiator polymerizations. Figure 4 shows the final particle size distributions for these longer lag time systems. For a lag time of 45 min, the particle size range is shifted to higher values than at 90 or 120 min, suggesting that the later-stage coalescence is still occurring, with virtually no final particles having the predicted average diameter of 265 nm. However, at the two longer lag times the average diameter is in this region. We will return later to a discussion of what may be happening with the systems shown in Figure 4. Figure 5 shows some theoretical calculations for R ) 2, at lag times of +15 (Figure 5a) and -15 min (Figure 5b). By reducing the degree of ionization of the less concentrated initiator, in this
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Figure 6. Number average particle diameter and percentage variance as a function of initiator addition time at pH 8.5 ( 0.2 and an initiator lag time of 30 min.
Figure 5. Calculated values of the charged free radical ratio (cationic/ anionic) as a function of time, for R ) 2, RC ) 1 and RA between 0.5 and 1 (at increments of 0.1), for a lag time of (a) +15 and (b) -15 min.
case the cationic initiator, the maximum charge ratio for any given lag time should be reduced, and, as such, the minimum lag time required to avoid excessive coalescence should be reduced. This is demonstrated theoretically in Figure 5a, which shows the calculated charged free radical ratio as a function of time for a constant lag time of 15 min and RA between 1 and 0.5. Decreasing RA reduces the charge ratio at all times during synthesis, reducing the chance of coalescence, even at low positive lag times. However, when the cationic initiator is added first (i.e., negative lag times), as in Figure 5b, then the chargeneutralization region cannot be avoided; the charged free radical ratio always passes through a value of 1, whatever the value of RA. The degree of ionization of the amidine groups (RA) can be reduced during polymerization by increasing the pH closer to its pKa value (10.2). Figure 2b shows the effect of increasing the pH to 9.3, i.e., from 8.5 in Figure 2a. At all the lag times studied the final average particle diameter is now much closer to the predicted value of 265 nm. Also, the percentage variance is 10% or less, except at zero lag time. It is not clear why the polydispersity is so large, at zero lag time, given that the average particle size is still close to the expected value; it could be that a charge neutralization period occurs at a very early stage in the polymerization. Although increasing the reaction pH significantly reduces the particle polydispersity, electrophoretic mobility measurements, as a function of pH, revealed that the IEP is also affected. For the particles synthesized at R ) 2 and pH 8.5, lag times of 60, 90, and 120 min produced particles with an IEP of 4.5. In comparison, the IEP of the particles synthesized at pH 9.3 were observed to be around 3.5, indicating a greater ratio of the carboxyl groups to amidine groups on the particle surfaces. Uncharged amidine groups are less hydrophilic than the charged form, so are more likely to become “buried” below the surface of a particle. Since a greater fraction of the (entirely charged) carboxyl groups remain at the surface, the final surface charge ratio does not reflect the initiator feed ratio. The outcome is equivalent to adding a lower ratio of V50 to ABCPA (i.e., a higher R value), at a pH where all the surface groups are charged.
Figure 7. SEM of particles synthesized at pH 8.5 ( 0.2 with a lag time of 30 min and addition time of 120 min.
Figure 8. Percentage yield (monomer conversion) as a function of the mean particle diameter.
2.2. Secondary Nucleation. Regarding the particle size distributions shown in Figure 4, while for lag times of 60 min and above, at pH 8.5, the particles produced are comparatively monodisperse, it is apparent that, at all lag times, the systems are bimodal. However, unlike the 45 min lag time, at 60 min and above the upper mode is closer to the predicted diameter, so the bimodality is not likely to be due to excessive particle coalescence. Also, the lower particle size distribution is very broad, indicating no convergence to a stable size. A more plausible explanation for the size distributions observed in Figure 4b and c is the formation of new particles upon addition of the second initiator, i.e., secondary nucleation. In single initiator systems it is suggested that once nucleation has occurred the concentration of oligomers in solution remains below the critical micelle concentration (cmc), thus preventing further nucleation, but if a second dose of initiator is added, the oligomer concentration may increase above the cmc, leading to secondary nucleation. Presumably, if this occurs after some critical time in the reaction, there will be insufficient remaining time for the newly formed particles to coalesce to a common size. Thus, the type of polydispersity described above will be observed. Note
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that the size of the secondary peak for the 120 min lag time is greater than that of the 90 min lag time; there is less time between secondary nucleation and the end of the reaction for coalescence to the characteristic stable size in the latter case. 3. Metered Initiator Addition. It is apparent from the discussion in the previous section that the overlap of the lag-time regions that results in charge-balance coalescence and residual secondary nucleated particles occurs at pH 8.5. This prevents the synthesis of monodisperse particles at this pH. Reduction of the upper boundary of the lag-time region for charge-balance coalescence by increasing the pH to 9.3 (decreasing the degree of ionization of the amidine groups) eliminates this overlap, such that for lag times of 7 and 15 min monodisperse particles are produced. However, while reducing the polydispersity, increasing the pH results in a significant fraction of the amidine groups becoming buried below the particle surface, thus reducing the IEP. Slow addition of the second initiator (in this case using a syringe pump at a constant rate) reduces the increase in total initiator concentration after initial nucleation. Therefore, this should also reduce the number of particles formed by secondary nucleation. Also, by adding the more rapidly dissociating V50 (compared to ABCPA) slowly, the ratio of positive to negative sites on the particle surfaces can be prevented from approaching one. The lag-time region over which charge balance coalescence occurs is thus reduced. These effects combine to open up a region of lag times where neither charge balance coalescence nor secondary nucleation occur, therefore allowing the synthesis of monodisperse particles. Figure 6 shows the effect of increasing the second initiator addition time from 0 to 180 min, at a constant initiator lag time of 30 min, on the resulting particle size and polydispersity. As expected, both decrease with increasing addition time and tend to a diameter in reasonable agreement with eq 5 and a lower polydispersity. An SEM picture of the particles synthesized using a second initiator addition time of 120 min is shown in Figure 7, illustrating the low polydispersity of these particles. The IEP values of the particles synthesized using addition times of 120 and 180 min were found to be 4.5 and 4.4, respectively, consistent with the ratio of initiator added, i.e., R ) 2. It can be concluded that, unlike increasing the pH, using a slower addition time improves the particle monodispersity without affecting the IEP. 4. Reaction Yield. Figure 8 shows the percentage yield for all the polymerization reactions studied, plotted as a function of the final number average particle diameter. As can be seen, a high yield (>90%) is obtained when the conditions were such that the expected particle diameter was achieved (∼250 nm). Where coalescence occurred during the reaction, leading to
Olsen et al.
significantly larger diameters than predicted, much lower monomer conversions result, as is apparent from Figure 8. It may well be that if particle coalescence takes place, then termination of any living chains in the individual particles occurs, preventing further polymerization in the coalesced particles.
Conclusions Variation of the ratio of initiator concentrations added during particle synthesis is shown to change the IEP of the final particles. As the concentration of the cationic initiator is increased, relative to a constant anionic initiator concentration, the IEP is observed to also increase. The measured IEP values correlate well with predictions based on an extension of the Henderson-Hasselbalch equation. The lag time between the addition of two, oppositely charged initiators during the synthesis of amphoteric polystyrene latex particles, is shown to affect their size, polydispersity, and IEP. Large-scale coalescence due to balancing of opposite charges on the particle surface is thought to be responsible for the increase in particle size and polydispersity at shorter positive and negative lag times for synthesis at pH 8.5. Theoretical estimations of the surface charge ratio support this assertion. For longer positive lag times, secondary nucleated particles (formed upon addition of the second initiator) are still present at the end of the reaction due to insufficient coalescence time, again contributing to the polydispersity of the final particle suspension. Synthesis at pH 9.3, where there is a lower degree of ionization of the amidine initiator, results in monodisperse particles at short lag times, confirming that the large particle sizes and high polydispersity observed at pH 8.5 at these same short lag times is indeed due to charge-balance coalescence. However, while avoiding polydispersity due to both charge-balance coalescence and secondary nucleation, the uncharged groups have a tendency to become buried during synthesis. Metered addition of the second initiator enabled the synthesis of monodisperse particles to be achieved without affecting the IEP. By reducing the increase in total initiator concentration after initial nucleation, secondary nucleation is limited and, by preventing the ratio of positive to negative sites on the particle surfaces from approaching 1, the inclusion rate of carboxyl groups on the surface from the faster dissociating ABCPA initiator is also limited. Acknowledgment. The authors would like to acknowledge the support of the Marie Curie Research Training Network on Dynamical Arrest (Contract No. MRTN-CT-2003-504712) in carrying out this work. LA703613P
