sodium styrenesulfonate copolymer latex particles

Surface Characterization of Styrene/Sodium Styrenesulfonate Copolymer Latex Particles HISASHI TAMAI,

1

KAZUHIDE NIINO, AND TOSHIRO SUZAWA

Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, ShitamL SaLio-cho, Higashihiroshima-shi, Hiroshima 724, Japan Received September 28, 1987; accepted September 12, 1988 The surface characteristics of styrene/sodium styrenesulfonate copolymer latex particles prepared by seed polymerization in the absence of emulsifier were investigated. The surface charge densities, determined by conductometric and potentiometric titrations, increase with increasing sodium styrenesulfonate content and are higher than those o f the seed latices. Also, the ~'-potentials were found to decrease with increasing sodium styrenesulfonate content, which is suggestive of an outward shift of the slipping plane, due to increased water solubility of the polymer. It was suggested that in addition to the increase in surface charge density, water-soluble polyelectrolyte at the particle surface enhances the colloidal stability of styrene/sodium styrenesulfonate copolymer latex. © 1989AcademicPress,Inc.

INTRODUCTION

Polystyrene (PS) latex prepared in the absence of emulsifier has been widely used as a model colloid, since it is highly spherical, monodisperse in particle size, and relatively easy to obtain. In addition to these particle properties, characteristics such as surface charge, ~'-potential, etc., have been investigated in detail by many authors (1-5). According to some authors, PS latex particles prepared using potassium persulfate as initiator have only strong acidic (sulfate) groups which come from the initiator (6-8). On the other hand, some authors have observed both strong acidic groups and weak acidic groups by conductometric titration (9-11 ). It has been thought that sulfate groups from potassium persulfate are spontaneously hydrolyzed and oxidized in the course of preparation and storage of the latex (12). Weak acidic groups observed by conductometric titrations would be due to carboxyl groups from sulfate groups. Styrene/sodium styrenesulfonate copolymer To whom correspondence should be addressed.

JournalofColloidandInterfaceScience,Vo]. 13i, No. 1, August1989

(P(St/NaSS)) (13) and styrene/sodium sulfodecylstyryl ether copolymer (P (St / SSDSE)) (14), latices which have strong acidic groups in much larger quantifies, have been prepared. Sulfonate groups arising from comonomer with sulfonate groups are stable for hydrolysis, and these latices are expected to be colloidally stable due to the high surface charge density from the sulfonate groups. However, the particle size of P(St/NaSS) latex prepared by the usual batch polymerization technique is generally much smaller than that of PS latex even when the amount of added sodium styrenesulfonate is small (13). Thus, a seed copolymerization technique was investigated. Tsaur et al. reported that for the preparation of P(St/SSDSE) latex, control of the surface charge density and the particle diameter was possible (14). In this work, P (St/NaSS) latices which have particle diameters of 400-500 nm were prepared by seed polymerization in the absence of emulsifier and the surface characteristics of the particles were investigated by measuring the surface charge densities, ~'-potentials, and colloidal stability against electrolyte. 0021-9797/89 $3.00 Copyright© 1989by AcademicPress.Inc. All rightsof reproductionin any formreserved.

2

TAMAI, NIINO, AND SUZAWA EXPERIMENTAL

Materials. Styrene (Wako Pure Chemical Co., Ltd.) was distilled twice under reduced pressure. Sodium styrenesulfonate and potassium persulfate (Wako Pure Chemical Co., Ltd.) were recrystallized from a water-ethanol mixture and water, respectively. Sodium chloride (NaC1), hydrochloric acid (HC1), and sodium hydroxide (NaOH, Wako Pure Chemical Co., Ltd.) were all analytical grade materials and were used without further purification. Distilled and deionized water were used throughout the experiments. Preparation of latices. P (St/NaSS) latices were prepared in the absence of emulsifier, using potassium persulfate as initiator (13). As the particle sizes of P(St/NaSS) latices prepared at higher fractions of sodium styrenesulfonate to styrene decreased very strongly with increasing amounts of added sodium styrenesulfonate, seed polymerizations were carried out. Surface charge density. Surface charge densities of P (St/NaSS) latex particles were measured by conductometric and potentiometric titration. Conductometric titrations were performed immediately after purification of the latices. The titrations were done with 5 × 10--4 mole dm-3 NaOH solution in a stirred vessel under N2 atmosphere. The conductivity was measured using a TOA Model CM-30ET conductivity meter. Potentiometric titrations of the cleaned latices were carried out using a Hitachi-Horiba Model F-7SS pH meter. The titrations were done on the latices containing 10-3 mole dm-3 HCI with 5 × 10 -3 mole dm -3 NaOH solution. Surface charge density (a) was calculated from the conductometric endpoint by Fnpa 3W' where Fis the Faraday constant, n is the number of moles of NaOH taken up by the particles, p is the density of latex particles, a is the radius of latex particles, and W is the total solid content of latex. Journal of Colloid and Interface S'cience, Vol. 131,No. 1, August1989

f-potentiaL f-potentials of latex particles were obtained by microelectrophoresis methods. Electrophoretic mobilities were measured using a Rank Brothers Mark II microelectrophoresis apparatus and ~'-potentials were calculated according to the Henry equation (15). Colloidal stability. Rapid-mixing coagulation experiments were performed using a JASCO UVIDEC 610 spectrophotorneter with a SFC-333 flow-cell device (light-path length 10 mm) connected with a Union MX-7 sample mixing device. In this apparatus, equal volumes of latex dispersion and NaC1 solution were introduced into the two syringes and then rapidly mixed by depressing the plunger. The wavelength used, provided by a tungsten lamp, was 680 nm. The particle concentration in the stopped-flow apparatus was 1.5 × 1012 din-3 after mixing with the NaC1 solution. RESULTS AND DISCUSSION

Preparation of Latices P(St/NaSS) latices were prepared by the usual batch emulsion polymerization technique without emulsifier. The polymerization recipes of the latices are shown in Table I. P (St / NaSSo) latex (prepared without sodium styrenesulfonate) is PS latex, and the fraction of sodium styrenesulfonate was increased in the order of P(St/NaSS2)b P(St/NaSS3)I, P (St / NaSS4) ~, and P (St / NaSS5 )i. The seed polymerization was carried out under the same conditions as those for the preparation of the seed latices, using P(St/NaSS3)I, P(St/ NaSS4)I, and P(St/NaSSs)I latices as seed latices. The conditions of seed polymerization are given in Table II. Figure 1 gives electron microphotographs of P (St / NaSS) latex particles prepared. Although the latex particles with low sodium styrenesulfonate content are spherical, P(St/NaSSs)II latex particles with high sodium styrenesulfonate content are slightly egg-shaped. Neither the coagulant nor the new small particles were observed in the latex dispersions after seed polymerization. In addition, the particle sizes of the latices obtained by seed polymerization, viz., P(St/

STYRENE/SODIUM STYRENESULFONATE LATICES TABLE I Preparation of P(St/NaSS) Latices Latex

P(St/NaSSo)

P(St/NaSS2)

P(St/NaSS3)I

P(St/NaSS4)I

P(St/NaSSs)I

Styrene (M) NaSS" (M) KPSb (M) NaC1 (M) Temperature (°C) Time (h) Particle diameter (nm) Uc

0.6 -4.5 X 10-4 -70 24 471 1.0005

1 2 × 10-3 7.5 X 10-4 2 × 10-2 60 14 325 1.0006

2 6 × 10-3 1.5 × 10-3 4 × 10-2 60 12 256

2 8 × 10.3 1.5 × 10-3 4 × 10-2 60 12 208

2 1 X 10 .2 1.5 X 10-3 4 × 10.2 60 12 180

a Sodium styrenesulfonate. b Potassium persulfate. c Uniformity ratio.

NaSS3)n, P ( S t / N a S S 4 ) n , a n d P ( S t / N a S S s ) I b are a l m o s t consistent with those calculated from the a m o u n t s o f seed latices a n d their diameters, a s s u m i n g that styrene m o n o m e r freshly charged i n seed latex is completely incorporated i n t o the particles. Accordingly, styrene m o n o m e r freshly charged i n seed polymerizati0n is thought to be incorporated into the final latex particles. A small a m o u n t o f water-soluble oligomers or polymers generated TABLE II Preparation of P(St/NaSS) Latices by Seed Polymerization Latex

P(St/NaSS3N

P(St/NaSS4N

P(St/NaSSs)II

Styrene (M) NaSS~ (M) KPSb (M) NaC1 (M) Seed latex (m2-dm-3)c Temperature (°C) Time (h) Particle diameter (nm)

2 6 × 10-3 1.5 × 10-4 4 X 10.2

2 8 × 10-3 1.5 × 10-4 4 X 10.2

2 1 × 10-~ 1.5 X 10-3 4 X 10-2

7 × 10 2

8 × 10 2

9

60 12

60 12

60 12

Uu

501 1.0023

394 1.0029

× 10 2

from s o d i u m styrenesulfonate m a y exist i n latex dispersions. T h e number-average diameter, Dn, for each latex was calculated from particle sizes determ i n e d from the electron microphotographs. A p p r o x i m a t e l y 30 particles were c o u n t e d for each latex. It is k n o w n that with mixtures of styrene a n d a hydrophilic m o n o m e r such as acrylic acid c o p o l y m e r latex, a n increase i n the fraction of hydrophilic m o n o m e r usually leads to smaller particle sizes ( 1 6 - 1 8 ) . As shown i n T a b l e I, although the a m o u n t o f sod i u m styrenesulfonate was very small, the particle sizes decreased very sharply. This t r e n d is similar to the result reported b y J u a n g et al. ( 1 3 ) . O n the other h a n d , a n increase i n the a m o u n t o f styrene m o n o m e r was effective in the e n l a r g e m e n t o f particle size. As s h o w n i n T a b l e II, although the particle sizes o f the

~o , .o_

°aJ

I 2 ~i

365 1.0043

a Sodium styrenesulfonate. bPotassium persulfate. cTotal surface area of seed latex particles added. d Uniformity ratio.

1710.1. Electron microphotographs of P(St/NaSS) latices. (1) P(St/NaSSo), (2) P(St/NaSS2), (3) P(St/ NaSS3)n, (4) P(St/NaSS4)m (5) P(St/NaSSs)n. Journal of Colloid and Interface Science, VoL 131, No. 1, August 1989

4

TAMAI, NIINO, A N D S U Z A W A

latices prepared by seed polymerization also decreased with increasing amounts of sodium styrenesulfonate added, P (St/NaSS) latices having particle sizes of 400-500 nm were obtained. The weight-average diameter, /)w, was calculated using the formula /gw = niD4/niD 3, where ni is the number of particles with the diameter Di. The uniformity ratios, U, defined by U = Dw/D, were less than 1.05. The latices obtained are considered monodisperse. In earlier papers ( 14, 19), it was shown that centrifugation and ultracentrifugation are effective techniques for removing water-soluble polyelectrolyte and impurities on the particle surface and in the serum. After all the latices obtained were dialyzed using well-boiled Visking dialysis tubing for over 1 week, centrifugation and redispersion of the latices were repeated three times. The latices were finally ion-exchanged with cationic and anionic mixed resins (Diaion PK 212-sulfonic acid type and PA 312-trimethylammonium type from Mitsubishi Kasei Co., Ltd.).

Surface Charge Density In Figs. 2 and 3, the results of conductometric titrations are shown. Tsaur et al. (14)

// " - . . ....

Volume of 5 x 10-3N-NaOH(ml

FIG. 3. Conductometric titration curves. O, P ( S t / NaSS3); O, P(St/NaSS4); O, P(St/NaSSs).

detected the weak acidic groups for P(St/ SSDSE) latices prepared using K 2 S 2 O 8 a s initiator. In our work, as shown in Figs. 2 and 3, the distinction between strong acidic groups and weak acidic groups seems ambiguous. This might be attributed to the very low surface density of weak acidic groups compared with that of strong acidic groups on the basis of sodium styrenesulfonate. Therefore, one titration end point was determined from the intercept of the two straight lines in Figs. 2 and 3. The calculated charge densities of P(St/ NaSS) latices are shown in Table III. Similarly,

TABLE III Surface Charge Densities (a), Critical Coagulation Concentrations (CCC), and Thickness of Water-Soluble Polymer Layer (A) o

~,

Latex

i

1

I

i

i

i

,

2 /* $ 6 7 Volume of 5 x 10-3N-NaOH(m0

i

8

FIG, 2. Conductometric titration curves. D, P ( S t / NaSSo); ©, P(St/NaSS2); O, P(St/NaSS3)u; I), P ( S t / NaSS4)n; O, P(St/NaSSs)11. Journal of Colloid andlnterface Science, Vol. 131, No. 1, August 1989

P(St]NaSS0) P(St/NaSSz) P(St/NaSS3)I P(St/NaSSa)n P(St/NaSS4h

P(St]NaSS4)n P(St/NaSSs)I P(St/NaSSs)n

u (tLC/m2) )< 104

CCC (mole dm-3)

A (nm)

2.7 7.5 10.7 15.1 12.0 17.0 12.4 20.7

2.1 × 10-I 3.6 × 10-1 -4.5 × 10-1 -8.6 × 10-1 -12.3 × 10-1

-1,1 -2.0 -2.7 -3.6

STYRENE/SODIUM STYRENESULFONATE LATICES

5

100 surface charge densities as a function of pH, calculated from potentiometric titrations, are t3--cr-----~ ~ D shown in Fig. 4. The surface charge densities of P (St / NaSS) latices are independent of pH. As shown in Table III and Fig. 4, the surface charge densities dramatically increase with increasing sodium styrenesulfonate content. The sodium styrenesulfonate moieties are concenoH trated at the particle surface. However, the FIG. 5. ~'-Potentials of P(St/NaSS) laticesas a function surface charge densities of the seed latices, viz., of pH at 10 -3 mole dm -3 ionic strength and 25°C. [3, P(St/NaSS3)~, P(St/NaSS4)I, and P(St/ P(St/NaSS0); ©, P(St/NaSS2); o, P(St/NaSS3)II; I0, NaSSs)b are slightly lower than those calcu- P(St/NaSS4)II; O, P(St/NaSSs)ll. lated assuming that all sodium styrenesulfonate added exists at the particle surface and is effective to charge. A part of the sodium sty- tent despite the increase in surface charge renesulfonate added is not supposed to con- density of the latex particles. Tsaur et al. (20) tribute to the surface charge. Interestingly, the reported that regarding the P(St/SSDSE) laP(St/NaSS) latices prepared by seed poly- tex, the Stern potential increased with increasmerization show higher surface charge densi- ing surface charge density. On the other hand, ties than those of their seed latices. Sodium it is generally known that the ~'-potential may styrenesulfonate freshly added in the seed po- decrease as a result of an outward shift of the lymerization would be mainly incorporated slipping plane. This happens, for example, into the surface of new P(St/NaSS) latex par- upon adsorption of polymers and surfactants. In earlier work (18), we observed that alticles. though the ~'-potentials of styrene/acrylamide copolymer latex particles decreased with an ~-Potential increasing fraction of acrylamide, their colloidal stability against electrolyte increased. Figure 5 shows the ~'-potentials of P(St/ These results were explained by the presence NaSS) latices as a function of pH at ionic of a water-soluble polymer layer arising from strength 10-3. As shown in Fig. 5, the negative acrylamide on the particle surface. Similarly, ~--potentials of P(St/NaSS) latices decrease in this work, the dispersed size of P (St/NaSS ) with increasing sodium styrenesulfonate conlatex particles was measured by viscosimetry (21). The viscosities of the latex dispersions were measured with an Ostwald viscosimeter at various volume fractions of latex particles. The thickness of the water-soluble polymer layer on the particle surface of P(St/NaSS) latex was approximately estimated from the relations between the reduced viscosity of latex 710 dispersion and the volume fraction of latex particles (Fig. 6). The method has been described in an earlier paper (22). The results 0 4 5 6 7 8 9 are shown in Table 1II. The estimated thickpH ness of the water-soluble polymer layer slightly F~G.4. Surfacechargedensitiesas a functionof pH. D, increased with increasing sodium styrenesulP(St/NaSS0); ©, P(St/NaSS2); A, P(St/NaSS3)fi Z~, fonate content. Consequently, one possible P(St/NaSS4)t; A, P(St/NaSSs)~; O, P(St/NaSS3)u; ~, interpretation for the lowering of ~'-potentials P(St/NaSS4),; O, P(St/NaSSs)n.

Journal of Colloid and Interface Science, Vol. 131, No. 1, August 1989

6

TAMAI, NIINO, AND SUZAWA

4£

3.c

2.~

~

§

'

2,

~

~

~

~

~

~o

(*td

FIG. 6. Relation between the reduced viscosities OlsP/ 4~) and the volume fraction (40 at 25°C and 10 -2 MNaC1. [:3, P(St/NaSS0); ©, P(St/NaSS2); ~, P(St/NaSS3)n; ~, P(St/NaSS4)I~; I , P(St/NaSSs)n.

of P(St/NaSS) latex with an increase in sodium styrenesulfonate content might be given by the presence of water-soluble polymeric electrolyte at the particle surface. The effect of the outward shift of the slipping plane on the ~'-potential appears to be higher than that of the increase in surface charge density. Figure 7 shows the ~'-potentials of P (St/NaSS) latex particles as a function of NaCI concentration. The negative ~'-potentials of P (St/NaSS) latices pass through a maximum. In earlier work (23), it was found that ~'-potentials of PS latex prepared in the absence of emulsifier (using potassium persulfate as initiator) also showed a maximum as a function of electrolyte concentration, and the presence of an "invisible" hairy layer at the particle surface was invoked to account for that maximum. The idea is that ~'-potential depends on the stretching of the hair. On the same grounds we conclude that 10C

Log C Nac~(M)

FIG. 7. ~'-Potentials of P (St / NaSS) latices as a function of NaC1 concentration at 25°C. [], P(St/NaSSo); C3, P(St/ NaSS2); ~, P(St/NaSS3)x~; I), P(St/NaSS4)H; o, P(St/ NaSSs)n. Journal of Colloid and Interface Science, Vol. 131, No. 1, August 1989

10-1

enact(I,t)

i

lo

FIG. 8. Rate constants of coagulation (kjl) as a function of NaCI concentration, vq, P(St/NaSSo); ©, P(St/NaSS2); O, P(St/NaSS3)n; ©, P(St/NaSS4)n; O, P(St/NaSSsh~.

the surface of P(St/NaSS) latex particles is also hairy.

Colloidal Stability Stability was studied by measuring the change in turbidity (At) with time upon addition of electrolyte. Ar increased linearly with time during the initial stage of coagulation (-100 s) and the rate constant of coagulation (k~l) was calculated from the slope of AT versus time (24). Figure 8 shows kl~ as a function of NaC1 concentration, k~~of P (St / NaSS) latex increases with the increasing concentration of NaCI and reaches a constant value at high NaC1 concentration. The values of the critical coagulation concentrations (CCC) for NaC1 are given in Table III. The CCC tends to shift to high NaC1 concentrations with increasing sodium styrenesulfonate content. Sodium styrenesulfonate enhances the colloidal stability of P (St/NaSS) latex against electrolyte. Although the conductometric and potentiometric titrations indicated that the surface charge densities increased with increasing amounts of sodium styrenesulfonate, the ~'potentials decreased. We therefore suppose that the increase in stability is due not only to enhanced electrostatic repulsion but also to a layer of water-soluble polymer enriched in sodium styrenesulfonate. ACKNOWLEDGMENT The authors greatly thank Dr. Cohen Stuart of Agricultural University for critically reading the manuscript.

STYRENE/SODIUM STYRENESULFONATE LATICES REFERENCES 1. Kotera, A., Furusawa, K., and Takeda, Y., KolloidZ. Z. Polym. 239, 677 (1970). 2. Goodall, A. R., Wilkinson, M. C., and Hearn, J., J. Polym. Chem. Ed. 15, 2193 (1977). 3. Stone-Masui, J., and Watillon, A., J. Colloid Interface Sci. 52, 429 (1975). 4. Bijsterbosch, B. H., Colloid Polym. Sci. 256, 343 (1978). 5. Brouwer, W. H., and Zsom, R. L. J., Colloids Surf 24, 195 (1987). 6. Furusawa, K., Norde, W., and Lyklema, J., Kolloid Z. Z. Polym. 250, 908 (1972). 7. Ono, H., and Saeki, H., Brit. Polym. J. 7, 21 (1975). 8. Goodall, A. R., Hearn, J., and Wilkinson, M. C., Brit. Polym. J. 10, 141 (1978). 9. Bijsterbosch, B. H., Colloid Polym. Sci. 256, 343 (1978). 10. Ono, H., Jidai, E., and Shibayama, K., Brit. Polym. J. 7, 109 (1975). 11. Goodall, A. R., Hearn, J., and Wilkinson, M. C., J. Polym. Sci. Chem. Ed. 15, 2193 (1977).

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12. Goodwin, J. W., Hearn, J., Ho, C. C., and Ottewill,. R. H., Brit. Polym. J. 5, 347 (1973). 13. Juang, M. S.-D., and K.rieger, I. M., Z Polym. Sci. 14, 2089 (1976). 14. Tsaur, S-L., and Fitch, R. M., J. ColloidlnterfaceSei. 115, 450 (1987). 15. Henry, D. C., Proc. R. Soc. London A 133, 105 ( 1931 ). 16. Ceska, G. W., J. Appl. Polym. Sci. 18, 427 (1974). 17. Ceska, G. W., J. Appl. Polym. Sci. 18, 2493 (1974). 18. Tamai, H., Murakami, T., and Suzawa, T., J. Appl. Polym. Sci. 30, 3857 (1985). 19. Chonde, Y., and Krieger, I. M., J. Colloid Interface Sci. 77, 138 (1980). 20. Tsaur, S-L., and Fitch, R. M., J. Colloid Interface Sci. 115, 463 (1987). 21. Fleer, G. J., Koopal, L. K., and Lyldema, J., Kolloid Z. Z. Polym. 250, 689 (1972). 22. Tamai, H., Iida, A., and Suzawa, T., Colloid Polym. SoL 262, 77 (1984). 23. Vandenhoven, Th. J. J., Ph.D. thesis, Agricultural University, Wageningen, The Netherlands, 1984. 24. Lichtenbelt, J. W. Th., Ras, H. J. M. C., and Wiersema, P. H., J. Colloid Interface ScL 46, 522 (1974).

Journal of Colloid and Interface Science, Vol. 131, No, 1, August 1989