Preparation of Micrometer-Sized Polymer Particles with Control of

A previously proposed method of soap-free emulsion polymerization employing an amphoteric initiator, 2,2'-azobis [N-(2-carboxyethyl)-2-methylpropionam...
0 downloads 0 Views 201KB Size
Download PDF
10958

Langmuir 2006, 22, 10958-10962

Preparation of Micrometer-Sized Polymer Particles with Control of Initiator Dissociation during Soap-Free Emulsion Polymerization Daisuke Nagao, Tatsuro Sakamoto, Hiroyuki Konno, Shunchao Gu, and Mikio Konno* Department of Chemical Engineering, Graduate School of Engineering, Tohoku UniVersity, 6-6-07, Aoba, Aramaki-aza, Aoba-ku, Sendai, 980-8579, Japan ReceiVed May 23, 2006. In Final Form: September 25, 2006 A previously proposed method of soap-free emulsion polymerization employing an amphoteric initiator, 2,2′-azobis [N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate (VA-057), was extended to synthesize micrometer-sized polystyrene particles with low polydispersity in an acidic region of pH from 3.3 to 4.6. A buffer system of CH3COOH/CH3COONa was used for the adjustment of pH, which was aimed at effective promotion of particle coagulation in early stage of the polymerization. In these experiments, CH3COOH concentration was varied from 20 to 360 mM at a CH3COONa concentration of 10 mM. Polymer particles with an average size of 1.8 µm and low polydispersity were obtained at the CH3COOH concentration of 40 mM for the concentrations of 1.1 M styrene monomer and 10 mM initiator. To more precisely control dispersion stability of particles, experiments in which pH was stepwisely changed during the polymerization were also carried out. This polymerization method could enhance the average size of particles to 2.2 µm while retaining the monodispersity of particles. Furthermore, combination of pH stepwise change and monomer addition during the polymerization could produce particles with an average size of 3.0 µm and low polydispersity.

Introduction Typical methods developed in the past for producing micrometer-sized polymer particles with low polydispersity were nonaqueous dispersion polymerization,1,2 seeded growth,3,4 and two-step swelling techniques.5 However, those methods have apparent drawbacks of requiring multistage polymerization or the use of hazardous solvents. In addition, a common problem with the conventional methods is the use of a considerable amount of surfactant. A surfactant-free heterogeneous polymerization in water solvent is soap-free emulsion polymerization, which is usually applied to the production of sub-micrometer-sized polymer particles. Dominant factors for particle sizes in the polymerization were well studied in the literature.6-8 The application of this polymerization method to the synthesis of micrometer-sized particles can overcome the problems of the conventional methods. The authors have recently developed a new technique based on soap-free emulsion polymerization6,7 and prepared micrometersized polymer particles with low polydispersity using a weakelectrolyte initiator in aqueous buffer solution.9-11 Since dissociation of the weak-electrolyte initiator depends on the * To whom correspondence should be addressed. Tel: +81-22-795-7239. Fax: +81-22-795-7241. E-mail: [email protected]. (1) Ober, C. K.; Lok, K. P.; Hair, M. L. J. Polym. Sci., Polym. Lett. Ed. 1985, 23, 103-108. (2) Okubo, M.; Konishi, Y.; Takebe, M.; Minami, H. Colloid Polym. Sci. 2000, 278, 919-926. (3) Vanderhoff, J. W.; Vitkuske, J. F.; Bradford, E. B.; Alfrey, T., Jr. J. Polym. Sci. 1956, 10, 225-234. (4) Okubo, M.; Okada, M.; Miya, T.; Takekohm, R. Colloid Polym. Sci. 2001, 279, 807-812. (5) Ugelstad, J.; Mork, P. C. AdV. Colloid Interface Sci. 1980, 13, 101-140. (6) Goodall, A. R.; Wilkinson, M. C.; Hearn, J. J. Polym. Sci. 1977, 15, 21932218. (7) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Br. Polym. J. 1973, 5, 347-362. (8) Konno, M.; Terunuma, Y.; Saito, S. J. Chem. Eng. Jpn. 1991, 24, 429437. (9) Gu, S.; Inukai, S.; Konno, M. J. Chem. Eng. Jpn. 2002, 35, 977-981. (10) Gu, S.; Inukai, S.; Konno, M. J. Chem. Eng. Jpn. 2003, 36, 1231-1235. (11) Gu, S.; Akama, H.; Nagao, D.; Kobayashi, Y.; Konno, M. Langmuir 2004, 20, 7948-7951.

solution pH, the use of weak-electrolyte initiator enables control of particle surface charge density through the adjustment of solution pH. In the previous study, we synthesized polystyrene particles with low polydispersity in the polymerization using an amphoteric initiator, 2,2′-azobis [N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate (VA-057, see the chemical structure in Supporting Information (1)), at pH values in basic regions.9,10,12 Since net charge of ionizable groups in the initiator molecule can also be controlled by pH in an acidic region, the polymerization method may be applicable to the synthesis of micrometersized polymer particles under acidic conditions. A purpose of the present work is to examine this applicability. Furthermore, change in pH during the polymerization can be more effective for the control of the coagulation and dispersion stability of particles than the preservation of steady pH. Another purpose of this work is to demonstrate the availability of the method of pH change. Experimental Section Materials. All the chemicals used were special grades of Wako Pure Chemical Industries (Osaka, Japan). Styrene (99%) was distilled at a reduced pressure under a nitrogen atmosphere after inhibitor removal. VA-057, NaOH (1 M), CH3COOH (99.7%), and hydroquinone were used as received. Water was deionized and distilled to have an electric resistance higher than 180 kΩ‚m. Polymerization. Polymerization was conducted in a batch reactor (11 cm I.D. and 12 cm height) equipped with four baffles and a six-blade pitched paddle impeller. The width of the baffles was 1 cm, and the diameter and width of the impeller were 6 and 1 cm, respectively. Styrene and aqueous buffer solution were charged into the reactor and then deoxygenated by bubbling with nitrogen for 20 min under stirring. After the reactant mixture was heated up to 65 °C, the bubbling was stopped, and initiator solution was added to the reaction mixture to initiate polymerization. The initial concentrations of styrene and VA-057 were 1.1 M and 10 mM, respectively. The concentration of CH3COONa was 10 mM in all experiments in (12) Yamada, Y.; Sakamoto, T.; Gu, S.; Konno, M. J. Colloid Interface Sci. 2005, 281, 249-252.

10.1021/la061451l CCC: $33.50 © 2006 American Chemical Society Published on Web 11/04/2006

Micrometer-Sized Polymer Particles

Langmuir, Vol. 22, No. 26, 2006 10959

Table 1. pH Value of Buffer Solution Used for Polymerization (65 °C)

a

CH3COOH concentration [mM]a

pH

20 40 80 360

4.6 4.3 4.0 3.3

[CH3COONa] ) 10 mM.

which CH3COOH concentration was varied. All the concentrations are based on the volume of the aqueous phase. In certain experiments, aqueous solution of CH3COOH and/or styrene monomer were added to the reaction system during the polymerization. All runs were conducted under nitrogen atmosphere at an impeller speed of 300 rpm. Characterization of the Particles. In the polymerization, small samples of reaction mixture (less than 20 cm3) were withdrawn to measure particle size distributions and polymer content in the mixture. Hydroquinone was added to the samples to terminate the polymerization. More than two hundred particle diameters were measured for each reaction mixture with a transmission electron microscope (Zeiss, LEO 912 OMEGA) operated at 100 kV to determine the number-averaged diameter, dn, the volume-averaged diameter, dV, the standard deviation, σ, and the coefficient of variation of particle size distribution, CV, defined as follows:

∑ n d /∑ n ) d ) ( ∑ n d /∑ n ) σ ) (∑ (d - d ) /∑ n ) dn ) (

i

i

i

3

V

i

1/3

i

i

2

i

CV )

n

1/2

i

σ × 100 dn

Figure 1. Polymer yields during polymerizations at CH3COOH concentrations of 20 (4), 40 (b), 80 (O), and 360 (2) mM.

(1) (2) (3) (4)

where ni is the number of particles with diameter di. The polymer weights in the samples were measured after removal of volatile components with freeze-drying. Polymer yield was defined as the percentage of the polymer weight relative to the weight of monomer added. Therefore, if polymer is completely dispersed in the reactant solution (no adhesion to the reactor), the percentage can be regarded as monomer-to-polymer conversion. For the measurement of zeta potential of polymer particles, a small amount of reaction mixture was withdrawn and diluted with the buffer solution that has the same composition as the reactant solution and no VA-057 initiator. Electrophoretic mobility of the particles was measured with electrophoresis light scattering (ELS8000, Otsuka Electronics) at ambient temperature. The Smoluchowski equation was used to convert the electrophoretic mobility into the zeta potential.

Results and Discussion Effect of Initial CH3COOH Concentration. Table 1 shows pH values of buffer solutions used for the polymerizations, in which CH3COOH concentration ranged from 20 to 360 mM at a fixed CH3COONa concentration of 10 mM. The variations of pH values were measured during the polymerizations. Although the pH values slightly increased during the polymerizations, the increases in pH were found to be less than 0.2. Since high ionic strength in the reactant solution causes instability of particle dispersion, concentrations of the buffer components are set at low values in comparison with conventional applications. The initiator of VA-057 has two amidino groups and two carboxy groups in a molecule, of which pKa values are 3.6 and 9.8.9,13 Under the pH conditions of Table 1, almost 100% (13) Fang, S.; Fujimoto, K.; Kondo, S.; Shiraki, K.; Kawaguchi, H. Colloid Polym. Sci. 2000, 278, 864-871.

Figure 2. Volume-averaged diameters of polymer particles at the CH3COOH concentrations of 20 (4), 40 (b), 80 (O), and 360 (2) mM.

of the amidino groups are cationized. On the other hand, 33% and 94% of the carboxy groups are anionized at pH equal to 3.3 and 4.6, respectively. Therefore, net charge of ionizable groups in an initiator molecule increases with a decrease in pH. Since the pKa value of acetic acid at 65 °C is 4.82,13 estimated concentration of CH3COO- is 10.9 mM for pH ) 3.3 and 11.3 mM for pH ) 4.6. As a consequence, total ionic strength of the reactant solution increases with a decrease in pH. Figure 1 shows polymer yields in the polymerizations at the CH3COOH concentrations listed in Table 1. More than 85% of the polymer yield was attained in the polymerizations at the CH3COOH concentrations of 80 and 360 mM. No adhesion of polymer to the reactor was observed at these concentrations. Since the polymerization temperature was well below the glass transition temperature of polystyrene (ca. 100 °C), polymerization terminated before complete conversion of monomer to polymer. At 40 mM, the adhesion of small amount of polymer was observed and the polymer yield was lower than those at the higher CH3COOH concentrations. At 20 mM, the sudden decrease in the polymer yield observed after 5.5 h suggested a sharp increase in the adhesion amount of polymer with time. Figure 2 shows volume-averaged diameters during the polymerizations of Figure 1. The average diameter obtained was larger at lower CH3COOH concentration at each reaction time. The net charge of ionizable groups in the initiator molecule increases with a decrease in pH while decomposition rate of the initiator is almost constant in the pH range in Table 1.13 Therefore, the pH decrease probably brought about an increase in surface charge density of polymer particles, resulting in the appearance of the small particles. Figure 3 shows TEM images of polymer particles obtained in the polymerization of Figure 1. The reaction times of the TEM observation were 6.5 h for the CH3COOH concentration of 20 mM (Figure 3A) and 9 h for the concentrations of 40, 80, and

10960 Langmuir, Vol. 22, No. 26, 2006

Nagao et al.

Figure 5. Time-variation of zeta potentials of polymer particles in the polymerizations without the addition of CH3COOH (O), with the addition of CH3COOH at 3 h (0) and the addition of both CH3COOH and styrene at 4 h (9). The first polymerization was the experiment of Figure 3A and the second was Figure 6C. Figure 3. TEM images of polymer particles formed at the CH3COOH concentrations of 20 (A), 40 (B), 80 (C), and 360 (D) mM.

Figure 4. Coefficient of variation of diameters for the particles formed at the CH3COOH concentrations of 20 (4), 40 (b), 80 (O), and 360 (2) mM.

360 mM (Figure 3B-D). In the image of A, a large number of particles with diameters less than 200 nm were observed together with micrometer-sized particles. The small particles appeared after the reaction time of 5.5 h. On the other hand, polymer particles with low polydispersity were observed in the TEM images of B, C, and D. Figure 4 shows CV values of diameters for the particles formed in the polymerizations of Figure 1. The CV value at 6.5 h for the CH3COOH concentration of 20 mM was not plotted in Figure 4 because of bimodal shape of the distribution. The decrease in CV in each experiment indicated that self-sharpening of size distributions appeared in the growth of particles. Among the experiments, the self-sharpening effect most weakly appeared at the CH3COOH concentration of 20 mM. Namely, the CV at this CH3COOH concentration was close to CV values at the CH3COOH concentrations of 40 and 80 mM in the early reaction of 2 h and approached with time to the CV at 360 mM CH3COOH that brought about the smallest dV among the experiments. The slow decrease in CV at 20 mM might be caused by instability of particle dispersion. Zeta potentials of polymer particles in this polymerization are presented in Figure 5 (see open circles). A sharp decrease in the zeta potential after 4 h is an evidence for the instability, which causes particle coagulation and particle adhesion to the reactor. The particle coagulation usually widens the particle size distribution. The instability causes reduction in the number of particles dispersed in the reactant solution, which reduces absorption rate of newly generated radicals into polymer particles, eventually leading to the formation of the small particles observed in Figure 3A. If the zeta potential can be raised in the intermediate stage of the polymerization, the CV may change along the curves of CV

Figure 6. Size distributions of polymer particles formed at the reaction time of 9 h in polymerizations in which CH3COOH concentration was raised from 20 to 40 mM at the reaction times of 0.5 h (A), 1 h (B), 3 h (C), and 5 h (D).

for the CH3COOH concentrations of 40 and 80 mM. In such a case, we may be able to obtain larger particles with lower polydispersity. In experiments in the following section, the present work has examined this possibility. Effect of CH3COOH Addition in Polymerization. Figure 6 shows size distributions of polymer particles in polymerizations in which CH3COOH was added at different reaction times to raise the CH3COOH concentration from 20 to 40 mM. The twin peaks located around 1.4 µm and 1.8 µm were obtained for the addition time of 0.5 h (A). The bimodal distribution was possibly caused by an increase in particle surface potential due to the CH3COOH addition in early stage of 0.5 h. In Figure 6B, the CH3COOH addition at 1 h could drastically suppress the generation of smaller particles and increase the average particle size to 2.0 µm, although a small amount of particles around 1.3 µm was observed. On the other hand, small particles were scarcely observed in the distributions of C and D. The distribution of Figure 6C revealed the lowest polydispersity among the four experiments examined. Zeta potentials of polymer particles in the polymerization of Figure 6C are presented in Figure 5 (see open squares). The potential drop observed in the polymerization without the CH3COOH addition (open circles) did not appear during the polymerization of Figure 6C. In the latter polymerization the

Micrometer-Sized Polymer Particles

Figure 7. Averaged diameter (b), coefficient of variation of particle size distribution (O), and polymer yield (0) in the polymerization of Figure 6C.

potential increased after the addition of CH3COOH, which improved colloidal stability of particles. Figure 7 shows dV, CV, and polymer yield during the polymerization of Figure 6C. No polymer adhesion to the reactor was observed in the experiment. The final polymer yield was 82%. The dV and CV values at the reaction time of 10 h attained to 2.2 µm and 2.2%, respectively. The CV in the experiment changed very similarly to those at the CH3COOH concentrations of 40 and 80 mM in Figure 4. Disregarding the experimental data at the CH3COOH concentrations of 20 and 40 mM in Figure 2 where the polymer adhesion to the reactor was observed, comparison of the polymer yield in Figures 1 and 7 shows the following tendencies (see Supporting Information (3)). Final polymer yield was slightly small in the case where large polymer particles were produced. In the intermediate reaction stage, the polymer yield curves had similar slopes, indicating almost the same polymerization rates. In the early reaction stage, the polymerization rate was a little slow in the case where large particles were obtained. It can be considered that in the early stage where particles are very small the polymerization rate is dominated by the number of particles in the reaction system.13 On the other hand, in the intermediate stage where a number of radicals coexist in a polymer particle the polymerization rate does not depend on the particle number but on total reaction volume.17 However, this situation cannot be held in the late stage where reactant diffusion rate in the particles extremely drops. This diffusion drop might be responsible for the termination of polymerization at the slightly lower polymer yield. To confirm an amphoteric property of the polymer particles produced in the present polymerization, zeta potential of the particles was measured in the reaction solvent at different pH. Figure 8 shows zeta potentials of the particles. The zeta potential was positive in a strongly acidic region and negative in weakly acidic and basic regions, which revealed that polymer particles prepared with VA-057 in an acidic region have an amphoteric property. The isoelectric point of the particles was not neutral pH, which might be caused by the hydrolysis of amidino groups.18 To determine the optimum time of CH3COOH addition for producing large particles, the dV and CV values of the final particles are plotted against the CH3COOH addition time in Figure 9. It seemed that the lowest CV could be obtained around an addition (14) Harned, H. S.; Ehlers, R. W.; J. Am. Chem. Soc. 1933, 55, 652-656. (15) Bradford, E. B.; Vanderhoff, J. W.; Alfrey, Jr., T. J. Colloid Sci. 1956, 11, 135-149. (16) Orihara, S.; Konno, M. J. Colloid Interface Sci. 2000, 230, 210-212. (17) Poehlein, G. W.; Vanderhoff, J. W. J. Polym. Sci. 1973, 11, 447-452. (18) Fang, S.; Fujimoto, K.; Kondo, S.; Shiraki, K.; Kawaguchi, H. Colloid Polym. Sci. 2001, 279, 589-596.

Langmuir, Vol. 22, No. 26, 2006 10961

Figure 8. Zeta potentials of polymer particles suspended in solutions at different pH. The polymer particles were sampled at 7 h in the polymerization of Figure 6C.

Figure 9. The final values of dv (b) and Cv (O) in the polymerizations where CH3COOH concentration was raised from 20 to 40 mM at different reaction times.

time of 3-4 h, while the dV increased with the time of CH3COOH addition. In the next section the CH3COOH addition time of 4 h is chosen to further enlarge the size of particles with low polydispersity. Application of Monomer Addition. A simple method for enlarging the size of particles is an increase in initial monomer concentration. However, the initial increase often instabilizes particle dispersion, especially at low particle surface potential. In such a case, it is desirable to increase monomer concentration after the raise of the surface potential. Figure 10 shows TEM images of particles obtained with the additions of different volumes of monomer at the reaction time of 4 h. In each polymerization, CH3COOH concentration was raised from 20 to 40 mM by the CH3COOH addition just before the monomer addition. The volumes of monomer added to the polymerizations in Figure 10A and 10B were half of and equal to the initial monomer volume, respectively. Micrometer-sized particles with low polydispersity were obtained for both additions of monomer volume, although a small number of submicronsized particles were observed for the polymerization of Figure 10B. However, the small particles can easily be removed with a simple separation such as decantation.16 Size distributions of polymer particles were measured during the polymerization of Figure 10B. The distributions obtained at 2 and 10 h had single peaks, while the distribution at 12 h had double peaks consisting of micron-sized large particles and submicron-sized particles (see Supporting Information (4)). The transient distribution indicates that the small particles were generated in the final stage of polymerization after 10 h. Electric surface potential in this polymerization is presented in Figure 5 (see filled squares). The zeta potential of particles after the monomer addition increased after the CH3COOH addition, finally

10962 Langmuir, Vol. 22, No. 26, 2006

Nagao et al.

acceleration was due to the gel effect after the disapperence of monomer drops. The decrease in CV observed in each polymerization in Figure 11 indicated the appearance of the selfsharpening. According to Bradford et al.,15 the self-sharpening effect can be evaluated with the following power-law equation:

dV d πD3 ) ) kDn dt dt 6

(5)

where V and D are the volume and diameter of polymer particles. n is the power-law index that represents the dependence of particle growth rate on D. k is a constant that depends on the nature of the environment. Although k during the polymerization might vary with time, relative growth of particles is only determined by the power-law index. In a case where particles with different diameters of Di and Dj stay in the same reaction field, the relative growth is expressed by

dDi n-2

Di

Figure 10. TEM images of polymer particles prepared with the additions of different volumes of St monomer to the system of the polymerization in which CH3COOH was added at 4 h. The total concentrations of St after the additions were 1.65 (A) and 2.2 (B) M, respectively. The other reaction conditions were the same as the polymerizations in Figure 9.

)

dDj Djn-2

(6)

Therefore, if a certain value is assumed as the index n, one can calculate relation between dv and Cv of distributions sequent to a given starting distribution. The CV values that were calculated for each dv with n ) 2.4 from the distribution at 2 h are presented as the three sets of curves in Figure 11. The calculated curves well fit the experimental data. The plot of CV against dV presented in Supporting Information (5) indicates that the experimental data fall on the same curve and are expressed with the power index. The index value of 2.4 is similar to the value of 2.5 reported by Bradford et al. for the seeded growth of submicron-sized polystyrene particles.15 Therefore, it could be considered that the growth mechanism of micrometer-sized particles is similar to the one of submicron-sized particles.

Conclusion

Figure 11. The dv and CV values of polymer particles during the polymerizations with and without the addition of monomer. No monomer was added during the polymerization (O, b); monomer was added at 4 h to the total monomer concentrations 1.65 (4, 2) and 2.2 (0, 9) M. The three guide lines were drawn for dV. The CV lines drawn with (s), (--), and (‚‚‚) were calculated results obtain with a power-law index of 2.4 for the three polymerizations (b, 2, 9).

attaining the highest value among the three polymerizations shown in Figure 5. This attainment of the zeta potential is considered to be responsible for the appearance of the small particles in Figure 10B. Disregarding the small particles, the CV value of size distribution in Figure 10B was as low as 2.3%. To discuss the mechanism of particle growth, Figure 11 shows the dV and CV values obtained in a series of the polymerizations where CH3COOH was added at 4 h and total monomer concentration was varied with and without the monomer addition. Since the small particles appeared after 10 h, they were disregarded to calculate the dV and CV values of the distributions of main peak. In the early reaction stage before 6 h, the dV values lay on the same curve. However, separation of the dV curves took place in the intermediate stage where the particle growth was accelerated earlier at lower total monomer concentration. This growth

The polymerizations conducted with an amphoteric initiator of VA-057 at the different initial concentrations of CH3COOH revealed that the size distributions of polymer particles were strongly affected by pH that was adjusted by CH3COOH concentration. Particles with an average size of 2.0 µm and low polydispersity could be prepared with the addition of CH3COOH. The polymer particles prepared with VA-057 initiator in an acidic region were found to have an amphoteric property. Combination of pH stepwise change and monomer addition during the polymerization could also be applied to the synthesis of larger polymer particles. The combination could produce particles with an average size of 3.0 µm and low polydispersity. Comparison of experimental and calculated CV values plotted against dV indicated that the growth mechanism of micrometer-sized particles is similar to the one of submicron-sized particles. Acknowledgment. This research was partially supported by a Grant-in-Aid for Science Research (No. 17760551) and for the COE project, Giant Molecules and Complex Systems from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: TEM images of polymer particles in the size distributions of Figure 6A-D. Comparison of the polymer yield in Figures 1 and 7. The plot of CV against dV shown in Figure 11. These material are available free of charge via the Internet at http://pubs.acs.org. LA061451L