JCT Research, Vol. 2, No. 7, July 2005

*Second Place 2004 Roon Award Competition Paper*

Latex Produced with Carboxylic Acid Comonomer For Waterborne Coatings: Particle Morphology Variations with Changing pH Daisuke Fukuhara† and Donald Sundberg**— University of New Hampshire*

Carboxylic acid comonomers are often added to latex formulations to improve colloidal stability and to help control the physical structure of composite (core-shell type) particles. We have performed a systematic study of the incorporation of methacrylic acid (MAA) within styrene/butyl acrylate copolymer seed latices, and determined the eventual effect on the morphology of the composite particles when using a second-stage monomer that is either polar (MMA) or nonpolar (BMA). These latices have been produced in batch and semibatch reactions in the pH range of 3–7. At low pH, the MAA groups are not ionized, but at the higher pH they may be nearly completely ionized. Here, we report that for batch reactions carried out within the above pH range, the latex particle morphologies of the PMMA second-stage systems change dramatically with increasing pH, while those for the PBMA system do not change at all. These results show that one cannot easily generalize the effect of acid comonomers on the morphology of composite latex particles, as this depends upon the choice of the copolymers in the latex and the process characteristics of the polymerization reaction. Keywords: Transmission electron microscopy, acrylics; latices, colloids, emulsions, latex, water-based, molecular modeling, simulation, carboxylic acid

C

ontrol of the morphology of latex particles has been a well-practiced art within industry for some time now, given its great importance in determining the physical properties of composite polymer systems. These structured latex particles find applications in modern coatings and adhesives, impact modifiers, medical diagnostics, etc. Among the significant number of variables used to produce structured latex particles, two of them stand out due to the fact that in small amounts they make large differences in the latex properties. These two variables are crosslinking and the incorporation of carboxylic acid comonomers. This article deals with the latter. Acid comonomers, particularly acrylic and methacrylic acids but also fumaric and itaconic acids, are very commonly added to latex recipes. While this is not common when preparing impact modifiers, it is nearly always done for latices that are intended for the coatings markets. Often the acids are added to improve the mechanical, freeze-thaw, and pigment-mixing stability of the latices. There have been many studies done to determine the location of the acid polymer in the latex particles and in the aqueous phase surrounding them.1-4 Another area of study has been the partitioning of the acid monomers between the aqueous and particle phases during polymerization. Acrylic, itaconic, and fumaric acids are highly partitioned

to the water phase even at pH levels below the pKas of these acids, while methacrylic acid (MAA) is more reasonably balanced between the phases.5 The use of acrylic acid (AA) tends to produce significant amounts of water-soluble polymer, and that portion of the acid copolymer that is in the particles is usually located near the outer surface of the latex particles. With MAA, there is significantly less water-soluble polymer formed and the MAA copolymer in the particle is somewhat more evenly dispersed within the particle. These dramatic differences between acrylic and methacrylic acids were reported in the study by Dos Santos et al.,3 where they contrasted the results for the two comonomers during the production of poly(styreneco-n-butyl acrylate) latices. At a pH of 2.2, they reported that 88% of the MAA is located in the interior portions of the particles, with 10% in the outer regions of the particles and only 2% in the water phase. For acrylic acid, the corresponding results showed nearly an even distribution of the carboxyl groups among all three locations. As the pH level during the reaction was adjusted upwards to pH = 4, the results for the MAA remained about the same while those for AA showed nearly 80% of the carboxyl groups in the water phase and only about 5% in the interior sections of the particles. As the pH was further increased to 6.0, both MAA and AA copolymers were found

Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, on October 27-29, 2004, in Chicago, IL. * Nanostructured Polymers Research Center, Materials Science Program, Durham, NH 03824. † Asahi Chemical Industry Company Ltd, Yako, Kawasaki-ku, Japan. ** Author to whom all correspondence should be addressed. Email: [email protected].

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D. Fukuhara and D. Sundberg Table 1—Typical Recipe for a Seed Latex

Table 3—Citrate-Phosphate Buffer Solutions

System

Ingredients

Weight (g)

Initial

DI-water Pre-seed latex (20.1% solids) Citric acid Di-sodium hydrogen phosphate

A: 0.1 M solution of citric acid B: 0.2 M solution of di-sodium hydrogen phosphate x ml of A + y ml of B, diluted to a total of 100 ml

231.0 34.1 0.36 0.07

Monomer

Styrene n-Butyl acrylate Methacrylic acid

96.6 77.3 19.3

Initiator Solution

DI-water Potassium persulfate

50.0 0.20

Surfactant Solution

DI-water Citric acid Di-sodium hydrogen phosphate Sodium dodecyl sulfate

x

39.8............................... 30.7............................... 24.3............................... 17.9............................... 6.5.................................

496.0 0.93 0.17 2.0

y

pH

10.2 19.3 25.7 32.1 43.6

3.0 4.0 5.0 6.0 7.0

reactions. This is a sequel to our studies at low pH levels, as reported earlier.6

EXPERIMENTAL to be highly partitioned to the water phase with almost no incorporation in the particles.

In order to prepare seed latex particles with a narrow particle size distribution and with the MAA uniformly distributed within the polymer particles, we started by creating “pre-seed” latices at the same overall composition as the target for the larger seed latex particles. These “preseed” latices were prepared by batch reactions at 70°C and at a solids content of 20 wt%. Sodium dodecyl sulfate (SDS) was used as a stabilizing surfactant and potassium persulfate (KPS) as an initiator. All of the “pre-seed” latices had particle sizes in the 50–70 nm range, and the pH during the reactions was controlled at about 3.0.

Our general interest in acid-containing latex particles is related to potential changes in reaction kinetics compared to otherwise similar nonacid-containing systems; and, more importantly, is also related to the effect of the acid comonomers on the morphology development within composite latex particles. We have chosen to work with MAA to ensure that the acid groups are predominately located within the particles. A systematic study was conducted about the effects of incorporating various levels of MAA in a seed latex polymer, analyzing the resultant changes of the reaction rates and particle morphology for a number of different polymer systems and reaction conditions. As done by Dos Santos,3 it is often interesting to work with a poly(styrene-co-butyl acrylate) [P(S-BA)] system that is popular in waterborne coatings research. We then added acrylic second-stage polymers to construct composite particles. In our case, we modified the hydrophobic P(S-BA) seed copolymer with MAA at levels up to 10 wt% and then used (separately) methyl methacrylate (MMA) and n-butyl methacrylate (BMA) monomers in the second-stage polymerizations without any acid comonomers. The reactions were done in both batch and semibatch modes to control the level of monomer present in the latex particles during the reactions, leading at times to the so-called “starve-fed” monomer conditions. These reactions were carried out at 70°C. In this particular study, the goal was to investigate the reaction and morphology changes that may be caused by varying the pH level of the aqueous phase from acidic to more basic conditions (thus potentially ionizing the carboxylic acid groups in the seed copolymer) during the

Three different seed latices were prepared at MAA levels of 0, 5, and 10 wt% in styrene-butyl acrylate copolymers. These reactions were carried out in a one-liter glass reactor at 70°C and under starve-fed monomer conditions by feeding the three monomers over a period of seven hours. SDS and KPS were added periodically during the reaction to maintain colloidal stability and to assure the continued production of free radicals during the long reaction times. The “pre-seed” latex particles were all grown to about 150 nm in diameter (measured by Coulter Nanosizer) and the final polymer particles had the same 54°C glass transition temperatures (Tg) when measured by differential scanning calorimetry (DSC, Perkin Elmer Pyris 1 at a scan rate of 10°C/min) in the dry polymer state. The polymer molecular weights were measured with a Waters GPC using polystyrene standards. Importantly, the incorporation of MAA in the styrene-butyl acrylate copolymers was measured by H1 NMR as a function of reaction time. In all cases, the MAA formed statistical terpolymers with the styrene-butyl acrylate monomers. The adsorption of SDS on the surface of the various seed polymer particles was determined by titrating cleaned latices7 with concentrated SDS solutions,

Table 2—Seed Latex Property Data Latex

St wt%

DF54 . . . . . . . . 70 DF50 . . . . . . . . 60 DF63 . . . . . . . . 50

BA wt%

MAA wt%

Diam. nm

Mn x10-3

Mw/Mn —

As Å2/mol

pHa —-

Dry Tg °C

Wet Tg °C

30 35 40

0 5 10

153 151 153

353 338 298

3.2 4.4 5.3

58.8 87.1 92.6

4.0 4.1 4.1

54 54 53

53 47 41

(a) pH at which As and Tgs were measured.

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Latex Produced with Carboxylic Acid Comonomer Table 4—Typical Recipe for a Second-Stage Polymerization System

Ingredients

Weight (g)

Initial

DI-water Seed latex (DF63, 17.9% solids) SDS Citric acid Di-sodium hydrogen phosphate MMA monomer

92.3 83.7 0.094 0.130 0.049 15.00

Initiator solution 1a

DI-water KPS

10.00 0.0085

Initiator solution 2b

DI-water KPS

2.00 0.0012

(a) Added all at once at beginning of reaction. (b) Added evenly over the first two hours of the reaction.

and measuring the latex conductivity. Here, our interest was to obtain the adsorption area of SDS on the polymer surfaces at 100% saturation of surfactant (i.e., at the CMC condition). Table 1 shows a typical recipe for the seed latex production, this one at 10% MAA. Table 2 displays latex property data for all three seed latices. All of the seed latices were passed through columns containing mixed ion-exchange resins in order to remove any residual initiator and buffers, as well as to remove water-soluble oligomers that might have been produced in the seed latex reactions. This provided us with very clean seed latices with which to conduct our second-stage polymerization reactions. These reactions, as reported in this article, were conducted at 70°C under batch process conditions in a 250-ml glass reactor under nitrogen with continuous stirring. The MMA (and, separately, the BMA) second-stage monomer was allowed to swell the seed latex particles for two hours before starting the reaction and the final solids content of the latices was 15 wt%. In all cases, the ratio of monomer to seed polymer was 1:1. The pH was controlled by adding a citrate-phosphate buffer system (see Table 3) in order to conduct the second-stage polymerizations in the pH range of 3–7, spanning the pKas of the MAA seed terpolymers. All second-stage latices contained small amounts of SDS to assure colloidal stability (final surface coverage was 10–15% of saturation) and the reactions were allowed to continue for three hours. Periodic samples were withdrawn from the reactor to determine the rates of monomer conversion (via gravimetry). A typical recipe is shown in Table 4. The composite latex particles were subsequently analyzed by transmission electron microscopy (TEM), DSC and GPC (subtracting the seed polymer signal from the total to obtain the second-stage polymer molecular weight distributions). The electron microscopy was performed on a Hitachi H600 TEM with microtomed sections of polymer (dried latex powder was dispersed in epoxy resin [Semedain High-Super 30] and cured at room temperature for one hour) that were stained with ruthenium tetraoxide vapor.

RESULTS AND DISCUSSION Our overall experimental study was designed to create a landscape in which the polarity of a nonpolar, styrene-

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Figure 1—TEM photo of microtomed sections of 0% MAA seed polymer/ PMMA particles formed at low pH.

butyl acrylate seed polymer latex was modified by methacrylic acid, and subsequently a polar (MMA) or a nonpolar (BMA) monomer polymerized in the presence of these seed polymers. Under batch reaction conditions, where the latex particles have a reasonable chance of

(a)

(b) Figure 2—TEM photos of microtomed sections of (a) 5% and (b) 10% MAA seed polymer/PMMA particles formed at low pH.

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D. Fukuhara and D. Sundberg tigate the potential effects of ionizing (by increasing the pH of the reaction medium) some of the carboxylic acid groups in the seed polymer on the reaction rates and particle structure. This, we surmised, might alter the interfacial tensions at the polymer/water and polymer/polymer interfaces and lead to different particle morphologies than those achieved otherwise. We will first discuss the effects seen when the second-stage monomer is methyl methacrylate and then turn to the butyl methacrylate system.

PMMA Second-Stage Polymer Figure 3—Monomer conversion vs. time plot for 0% MAA seed polymer/PMMA reactions at pH = 2.9 (Exp 54 and 97) and 7.1 (Exp 81).

Figure 4—TEM photo of microtomed sections of 0% MAA seed polymer/PMMA particles formed at pH = 7.1.

achieving their thermodynamic equilibrium morphology, we were interested in seeing whether or not the influence of the carboxylic acid in the seed polymer would be strong enough to alter the reaction rates and the final morphology of the particles, especially as we changed the polarity of the second-stage monomer from polar to nonpolar. The first portion of the overall study was conducted at a pH of about 3 and reported recently,6 but some of the highlights will be repeated here for context. A second part of the study, and the subject of this article, was designed to inves-

It is interesting to note the effects of increasing the MAA content in P(S-BA) latex particles on the morphology of composite particles with PMMA as the second stage. With no acid in the seed polymer, there is a large difference in the polarities of the two polymers (with the PMMA being the more polar), and, at pH = 3, a core-shell structure is likely to be the equilibrium morphology. This is confirmed experimentally6 in the TEM photo for this system as shown in Figure 1. Here, the dark phase is the styrene-containing seed polymer, as stained by the ruthenium. As the MAA content of the seed polymer was increased to 5%, the particle structure remained a core-shell (Figure 2a); but, when the level reached 10% there was very little structure seen in the particle (Figure 2b). Quantitative DSC analysis8 of the 10% sample showed the existence of two Tgs but also showed there to be more than 60% of the total polymer in the interfacial, or mixed state, region. Thus, it might appear that with this much MAA in the P(S-BA), this polymer is relatively compatible with the second-stage PMMA and little noticeable phase separation may take place. Given that this latex was produced by a batch process in which the monomer concentration was relatively high until the end of the reaction and, thus, providing an environment in which polymer chains can diffuse relatively easily, we would have expected phase separation to happen easily if it was going to happen at all. Here we have seen a remarkable effect of the addition of MAA, even at low pH levels.

The new work utilized the 0 and 10% acid level seed latices and analyzed the results for PMMA second-stage experiments in which the pH was raised in increments from 3 to 7. For the 0% MAA seed latex, Figure 3 shows that conversion versus time curves for two experiments conducted at pH levels of 2.9 and 7.1. These batch reactions are quite rapid and there is no difference between the data for the two experiments. Regarding the possible morphology differences with pH, Figure 4 shows that at pH = 7.1 the particles still have a well-defined core-shell structure, as they did for the same system at low pH (see Figure 1). This result is not surprising since there is nothing in the recipes that is likely to be significantly affected by the change in pH. The same cannot be said when the seed polymer contains 10% methacrylic acid. Figure 5 displays the conversion-time curves for five experiments conducted at various pH levels in the 3–7 range. Although there are some slight difFigure 5—Monomer conversion vs. time plots for 10% MAA seed polymer/PMMA reactions ferences in the data sets, there is no disat pH levels between 3 and 7. 512

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Latex Produced with Carboxylic Acid Comonomer cernable trend with pH and we can say nothing specific about the dependency of the reaction rates on pH. On the other hand, there is a rather strong dependency of the particle structure on the pH. Figures 6a, 6b, and 6c show TEM results for reactions conducted at pH levels of 4.9, 6.1, and 6.9, respectively. Also, utilizing Figure 1 (pH = 2.9) as the low pH reference, it is clear that with increasing pH the latex particles change from no apparent structure to significant structure; at pH = 6.9, we interpret the TEM photo to show a more or less hemispherical structure. The change in morphology appears to happen between pH 4.9 and 6.1, as best we can tell from the TEM photos. Complementary DSC data for these same samples showed that the amount of interfacial (mixed) polymer remained at over 60% as the pH changed from 2.9 to 4.9, but then decreased significantly to under 50% (yet this is still a lot of interfacial polymer) at pH levels of 6.1 and 6.9. This is further evidence of greater phase separation. Combined with the reaction kinetic data displayed in Figure 5, we have to conclude that the polymerization rates were not significantly sensitive to the differences in morphology as the pH changed. This suggests that the free radical populations in the particles were nearly the same under these batch reaction conditions. This is not an intuitive result, given the considerations of two-phase emulsion polymerization kinetics that we have put forth on theoretical grounds.9 In order to understand the previously mentioned results more fully, we prepared P(BA-co-MAA) at 10% MAA by solution polymerization. Then we determined its interfacial tension against water with pendant drop experiments10 at different pH levels and at different temperatures. These data are plotted in Figure 7 and from them we have determined that the pKa of this copolymer is close to 6, and not very sensitive to temperature in the range of interest. Using a value of the pKa of 5.8 and the data from Figure 7, we computed the probable degree of ionization of the 10% MAA containing seed polymer as a function of pH, as shown in Table 5. Here we see that for the pH = 4.9 reaction condition we expect α to be only 0.1, but at pH = 6.1 the degree of ionization is high at 0.67. At the highest pH employed, α is over 0.9 and the ionization is nearly complete. We think this means that the effective polarity of the seed polymer continually increases as the pH is raised, creating a very polar material at the 6–7 pH level. The data in Figure 7 certainly confirm that P(BA-co-MAA) at 10% MAA has very low interfacial tension against the water in this pH range. We know from previous work11 that the interfacial tension of PMMA against pure water is about 18 mN/m and that is very much higher than the values for P(BA-co-MAA) at pH = 6-7, as shown in Figure 7. With this dramatic shift in polarities one would think that the particle structure might be an inverted core-shell, with the seed polymer forming the shell. However, just as the pH has affected the interfacial tension at the water interface, it also has an effect on the interfacial tensions between the two polymers. As the pH increases above the pKa of the MAA containing seed polymer, the relative polarities of the seed and second-stage polymers invert. At pH = 7 and nearly complete ionization of the MAA groups, we estimate that the interfacial tension between the seed polymer and the PMMA may be as high as 6 mN/m. Calculations of the free energy of various particle mor-

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phologies12 show that at a value of 4 mN/m for the polymer/polymer interfacial tension, one would expect an inverted core-shell arrangement. Changing the value to 6 mN/m results in a hemispherical equilibrium morphol-

(a)

(b)

(c) Figure 6—TEM photos of microtomed sections of 10% MAA seed polymer/ PMMA particles formed at (a) pH = 4.9, (b) pH = 6.1, and (c) pH = 6.8.

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D. Fukuhara and D. Sundberg BMA system6 and its TEM is shown in Figure 8a. The reason for the hemispherical morphology is because the two polymers are not very different in polarity, yet they are different enough to phase separate under batch processing conditions. At the same low pH level, increasing the MAA content in the seed polymer to 5% causes the morphology to change to an inverted core-shell with the nonpolar PBMA as the core, as shown by the TEM photo in Figure 8b. The same result is obtained when the MAA level is increased to 10%.6 These results very nicely show the effect of making polarity changes to the seed polymer when both the seed and second-stage polymers are relatively nonpolar without any acid copolymer present. Figure 7—Interfacial tensions of 90% BA: 10% MAA (wt%) copolymer against water as a function of pH and temperature.

Table 5—Degree of Ionization as a Function of pH Fractional Ionization, α

pH

2.9 .............................................................................0.00 4.0 .............................................................................0.02 4.9 .............................................................................0.11 6.1 .............................................................................0.67 6.9 .............................................................................0.91

ogy, in reasonable agreement with the experiment. What is clear from these considerations is that the pH changes have dramatically altered the polarity of the seed polymer but not the second-stage polymer. The TEM and DSC data appear to be the result of large changes in the interfacial properties of the seed polymer containing the MAA, that being due to large changes in the compatibility between the two polymers.

As the pH of the aqueous phase is increased from low levels, we would expect the seed polymer that contains MAA to respond in the same way as previously described for the PMMA second-stage system. In turn, the effective polarity of the seed polymer should increase and become more contrasted with the PBMA second-stage polymer at higher pH levels. In a complementary set of experiments to those for the PMMA system, we found the following results. For the 0% MAA seed polymer system, increasing the pH from 3 to 7 showed no effect on the reaction kinetics (Figure 9), consistent with the PMMA system at 0% MAA. Figure 10, in contrast to Figure 8a, shows that there is also no change with pH for the morphology, both resulting in hemispherical structures. DSC data for this system are unobtainable as the Tgs of the seed and secondstage polymers are too close together to distinguish the separate polymers in the temperature scans. This is unfortunate because it forces us to rely upon a single analytical technique to make judgments about particle morphology.

In contrast to the MMA case, when we add BMA to a 0% seed polymer at pH = 3, we obtain hemispherical particles rather than the core-shell structures seen for the MMA system.6 This is the equilibrium structure for the

As we now move to pH levels above the pKa of the MAA terpolymer (ca. 5.8), the seed polymer containing 10% MAA becomes ionized, and with it comes a dramatic increase in polarity. Figure 11 shows that there is no change in the batch reaction kinetics that we believe to be meaningful as the pH is changed from 3.1 to 7.1. The TEM photo for the particles created at pH = 7.1 is shown in Figure 12, where one sees very clear evidence of an inverted core-shell structure (dark phase is the seed

(a)

(b)

PBMA Second-Stage Polymer

Figure 8—TEM photos of microtomed sections of (a) 0% and (b) 5% MAA seed polymer/PBMA particles formed at low pH.

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Latex Produced with Carboxylic Acid Comonomer

Figure 9—Monomer conversion vs. time plots for 0% MAA seed polymer/PBMA reactions at pH = 3.1 (Exp 71) and pH = 7.1 (Exp 93).

polymer). Thus, in comparison to the structure of the particles at 5% MAA (and also for the 10% MAA sample6) and pH = 3.1, there is no difference between them. We think that because this seed polymer was already much more polar than the second-stage PBMA at low pH, the polarity difference was just enhanced as the pH levels increased. This resulted in the preferred inverted core-shell structure. Our calculations, via a software program called UNHLATEXTM_Eqmorph,12 of the free energy of various alternative equilibrium structures (very low interfacial tension between seed polymer and water [ca. 2mN/m], high value for PBMA and water [ca. 28], and high value for the polymer/polymer interface [ca. 7]) show that one should expect a structure that is very close to an inverted core-shell morphology under these experimental conditions.

Figure 11—Monomer conversion vs. time plots for 10% MAA seed polymer/PBMA reactions at (a) pH = 3.1 and (b) pH = 7.1.

base represent the experimental results for reactions conducted at pH ~ 3.0, with the results towards the front representing MMA in the second stage and results towards the back representing the BMA system. This makes it easy to see the effect of adding MAA to the seed polymer by moving from the left side of the base (0% MAA) to the right side of the base (10% MAA). The black phase represents the seed polymer in all cases (as in the TEM photos). Note that we have drawn the structure of the 10% MAA seed polymer/PMMA second-stage system with gray tones to signify that this particle contains mostly interfacial, or mixed, polymers.

The above discussion points out that the effect of incorporating acid comonomers in a hydrophobic seed latex and the simultaneous effect of carrying out polymerizations at different pH levels have different kinds of results for different systems. We find it useful to capture the morphology variations in a three-dimensional diagram, as shown in Figure 13. Here, the four corners that form the

The morphologies resulting from changes in pH levels are seen in the top four corners of the diagram. These are the results for pH ~ 7.0. The overall perspective provided from this diagram is that at any pH level, changing the methacrylic acid level in the seed polymer and/or changing the polar nature of the second-stage polymer results in different structures, as long as the reactions are carried out in the batch processing mode. Conversely, changing the pH level makes a difference in only a limited number of cases (far right hand corners only) and that depends critically upon the chosen system.

Figure 10—TEM photo of microtomed sections of 0% MAA seed polymer/PBMA particles formed at pH = 7.1.

Figure 12—TEM photo of microtomed sections of 10% MAA seed polymer/PBMA particles formed at pH = 7.1.

Both Systems Considered Together

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Figure 13—Three-dimensional diagram showing particle morphology results for different MAA levels, second-stage polymers, and pH levels.

CONCLUSIONS

References

The experiments conducted here have shown that although the second-stage polymerization reactions were very rapid, the morphologies achieved for the various systems were all at or near thermodynamic equilibrium (usually marked by the characteristic that each polymer phase is contiguous and each resides in one portion of the latex particle). The overall reaction kinetics were all essentially unaffected by changes in the pH of the aqueous phase even though the particle morphologies were sometimes affected. Increasing the aqueous phase pH above 6.0 has no effect on morphology unless at least one of the polymers (in our case, the seed polymer) contains carboxylic acid comonomer. With acid, the effective polarity of the polymer increases dramatically as the pH is raised above the pKa of that polymer and marked changes in morphology may be seen. This study has shown that when the acid is in the relatively nonpolar polymer, raising the pH to around 7.0 transforms that polymer into the more polar component and thereby causes a change in the particle morphology. If the acid is in the already more polar polymer (or if the polymers are very close in polarity), then it is not expected that increases in pH levels will affect the morphology results.

(1) Vijayendrin, B.R., “Effect of Carboxylic Monomers on Acid Distribution in Carboxylated Polystyrene Latices,” J. Appl. Polym. Sci., 23, 893-901 (1979). (2) Wang, S.T. and Poehlein, G.W., “Characterization of WaterSoluble Oligomer in Acrylic Acid-Styrene Emulsion Copolymerization,” J. Appl. Polym. Sci., 50, 2173-2183 (1993). (3) Dos Santos, H.M., McKenna, T.F., and Guillot, J., “Emulsion Copolymerization of Styrene and n-Butyl Acrylate in Presence of Acrylic and Methacrylic Acids: Effect of pH on Kinetics and Carboxyl Group Distribution,” J. Appl. Polym. Sci., 65, 2343-2355 (1997). (4) Hsu, S-C., Liao, Y-L., Lee, C-F., and Chiu, W-Y., “Polymer Latex Containing Carboxylic Acid Functional Groups: 1. Synthesis of Polymer Latex from MMA and AA,” J. Appl. Polym. Sci., 74, 31113120 (1999). (5) Moroi, S. and Hosoi, K., Kobunshi Kagaku, 26, 424 (1969). (6) Fukuhara, D. and Sundberg, D.C., “The Role of Carboxylic Acid Comonomers in Morphology Control of Synthetic Latex Particles—Batch Reactions,” Prog. Colloid Polym. Sci., 124, 18-21 (2003). (7) Stubbs, J.M., Durant, Y.G., and Sundberg, D.C., “Competitive Adsorption of Sodium Dodecyl Sulfate on Two Polymer Surfaces within Latex Blends,” Langmuir, 15, 3250-3257 (1999). (8) Hourston, D.J. and Song, M., “Quantitative Characterization of Interfaces in Rubber-Rubber Blends by Means of ModulatedTemperature DSC,” J. Appl. Polym. Sci., 76, 1791-1798 (2000). (9) Durant, Y.G., Carrier, R., and Sundberg, D.C., “Mathematical Model for the Emulsion Polymerization Reaction Kinetics of Two Phase Latex Particles,” Polym. React. Eng., 11, 433-455 (2003). (10) Anastasiadis, S.H., Gancarz, I., and Koberstein, J.T., “Interfacial Tension of Immiscible Polymer Blends: Temperature and Molecular Dependence,” Macromolecules, 21, 2980-2987 (1988). (11) Winzor, C.L. and Sundberg, D.C., “Conversion Dependent Morphology Predictions for Composite Emulsion Polymers: 1. Synthetic Latices,” Polymer, 33, 3797-3810 (1992). (12) Durant, Y.G. and Sundberg, D.C., “An Advanced Computer Algorithm for Determining Morphology Development in Latex Particles,” J. Appl. Polym. Sci., 58, 1607-1618 (1995).

ACKOWLEDGMENTS We thank the Asahi Chemical Industry Company, Ltd. for its personnel (DF) and partial financial support of this work. We also thank Ola Karlsson and Jeffrey Stubbs for critical discussions, and the UNH Latex Morphology Industrial Consortium (AtoFina, Mitsubishi Chemicals, NeoResins, and Surface Specialties, UCB) for partial financial support.

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