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urnal of Colloid and Interface Science 230, 210–212 (2000)

oi:10.1006/jcis.2000.7088, available online at http://www.idealibrary.com on

NOTE

A Simple Method for Producing Micrometer-Sized MonodispersePolystyrene Particles in Aqueous Media

A method was developed for producing micrometer-sized, highly

onodisperse polystyrene particles in single stage polymerization

n aqueous media. The method employs an amphoteric surfac-

nt, dimethyldodecylbetaine, that is added to the reaction sys-

m of styrene/K 2S2O8/water in early stage of reaction. Polymer-

ations were carried out over a range of initiator concentrations

–8 mmol/dm3 H2O). The polymerization procedure could pro-uce particles with an average diameter of 3.4 m with a coefficient

f variation of particle size distribution of 4.0% . C 2000 Academic Press

KeyWords: chemical reaction; single stage polymerization; parti-

e size distribution; monodisperse polystyrene particles; soap-free

mulsion p olymerization.

INTRODUCTION

Methods for producing micrometer-sized polymer particles have been stud-

d by many researchers. Nonaqueous dispersion polymerization (1, 2), seeded

owth (3, 4), and two-step swelling techniques (5, 6) are typical methods thatavebeen proposed. Although various aspects of these techniques are successful,

ey have apparent drawbacks of being multistage polymerizations or requiring

azardous solvents.

To overcome these problems, the authors proposed a technique for single-

age polymerization in aqueous media, which was based on soap-free emulsion

olymerization (7, 8). In the previous work, monodisperse micrometer-sized

articles could be produced by adding cationic and anionic surfactants to the

stem in two steps, according to the surface charge of the particles. Recently,

e authors proposed a method, which employed an amphoteric surfactant in-

ead of cationic and anionic surfactants. In the polymerization of styrene, an

mphoteric surfactant wasadded at different reaction times and it wasfound that

is procedure could produce highly monodisperse particles with an average di-

meter of 2 m (9). The present work examines single-stage polymerizations of

yrene at different initiator concentrations in the presence and in the absence of n electrolyte (KCl), with the objective of producing micrometer-sized particles

at are highly monodisperse.

EXPERIMENTAL

All chemicalswere obtained from Wako Pure Chemical Industry, Ltd.(Osaka,

pan). Styrene (99%) was used after inhibitor removal and purification by dis-

lation at reduced pressure under a nitrogen atmosphere. Potassium persul-

te (KPS) (98%) and potassium chloride (KCl) (98%) were used as received.

imethyldodecylbetaine (DMDB) was synthesized by the following reaction of

methyldodecylamine with sodium chloroacetate in water at 70◦C according

an available literature procedure (10, 11):

12H25N(CH3)2 +ClCH2COONa → C12H25-N+(CH3)2-CH2COO− +NaCl.

The resulting aqueous solution contained equal moles of DMDB and NaCl a

was used in the polymerization experiments.

Polymerizations were carried out in a batch reactor (11 cm i.d., 12 cm

height) equipped with four baffles and an agitator one-third of the reactor

ameter. The reactor was charged with 700 cm3 water and 100 cm3 styre

and nitrogen gas was bubbled through the solution to remove oxygen from

system. Polymerization was initiated by the addition of 50 cm3 of an aque

solution having a KPS concentration of 3.2 or 6.4 mmol. At a reaction time10 min, 50 cm3 of the resulting aqueous solution containing 0.40 mmol DM

was added to thesystem. Theentire reactionwas performed at 70◦C ina nitro

atmosphere under agitation at 300 rpm.

At appropriate intervals during the reaction, small samples (less than 20 cm

were withdrawn from the reactor to determine monomer-to-polymer conv

sion and particle size distributions. Hydroquinone was added to the sample

terminate the polymerization. Conversions were determined from the wei

of polymer contained in reaction mixtures from the withdrawn samples. E

particle size distribution was determined by measuring more than 200 parti

diameters with transmission electron microscopy (Hitachi H-800). From the

croscopy measurements, the average diameter, d V, and coefficient of variati

C V, were calculated from the following relationships:

d V =

i

ni d 3i

i

ni

1/3

C V =

i

d i −

i n i d i

i n i

21/2

i n i d i

i n i

,

where n i is the number of particles having diameter d i .

RES ULTS AND DISCUSSION

Figure 1 shows the dependence of monomer-to-polymer conversion on

action time at two different initiator concentrations. The polymerization rincreased with an increase in initiator concentration as expected.For each exp

iment, the polymerization rate gradually accelerated until reaction terminati

This is probably due to the gel effect and an increase in volume of reaction l

with time.

Figure 2 shows the evolution of particle size distributions with convers

for the experiments of Fig. 1. The size distribution at initiator concentration

8 mmol/dm3 H2O was unimodal throughout the reaction and attained a h

monodispersity of C V = 3.9% with d V = 1.6 m. For the polymerization

initiator concentration of 4 mmol/dm3 H2O, the size distribution was initia

unimodal but became bimodal as the reaction progressed. The system attai

a coefficient of variation of 2.9% for an average particle size of 2.6 m di

garding the secondary particles.

In ordinary soap-free emulsion polymerization, an increase in initiator c

centration decreases particle size. It can be considered that the higher iostrength caused by the increase in initiator concentration reduces electrosta

210021-9797/00 $35.00



NOTE 2

FIG. 1. Conversion versus reaction time during polymerization at initiator

oncentrations of 4 mmol/dm3 H2O () and 8 mmol/dm3 H2O (); monomer

oncentrations: 1.1 mol/dm3 H2O; stirring speed: 300 rpm; DMDB concentra-

on: 0.5 mmol/dm3 H2O; DMDB addition time: 10 min.

pulsion between particles and promotes particle aggregation. A similar ef-

ct can be expected in the present polymerizations and is possibly the rea-

on for the appearance of the larger particles at the lower initiator concentra-

ons.

The appearance of the secondary particles in Fig. 2a can be attributed to an

crease in electric surface potential of particles during the reaction. As the

action proceeds, ionic groups, originating from ionic radicals, accumulate on

e particle surface. From the measurements, the number of particles formed

the early stage of the reaction in Fig. 2a is less than one-fourth of that in

g. 2b. Therefore, even though the generation rate of ionic radicals for Fig. 2a

half of that for Fig. 2b, the particles in the reaction of Fig. 2a can have

gher surface density of ionic groups. The higher surface charge density ande lower ionicstrength induce stronger electrostaticrepulsion between particles

nd ionic radicals. If the stronger electrostatic repulsion is an important factor

FIG. 2. Evolution of the particle size distributions with conversion for the

xperiments of Fig. 1 at initiator concentrations of 4 mmol/dm 3 H2O (a) andmmol/dm3 H2O (b).

FIG. 3. Particle size distributions and TEM photograph of final parti

at a KCl concentration of 20 mmol/dm3 H2O and an initiator concentratio

4 mmol/dm3 H2O before the decantation (a) and after decantation (b), (c).

for the generation of the secondary particles, the generation may be suppres

by the addition of electrolytes to the solution.

Figure 3a shows final size distribution in a polymerization in which K

was added to the system after 1 h reaction time. The concentration of KCl w

20 mmol/dm3 H2O. In this reaction, only a few secondary particles were g

erated, as can be seen in Fig. 2a. Thus, the addition of the electrolyte seeme

be effective for suppressing the generation of the secondary particles. A tr

of the secondary particles was removed from the reactant mixture by decation. As shown in Figs. 3b and 3c, the secondary particles were complet

removed. The d V and C V values of these particles were 3.4 m and 4.0

respectively.

The d V value for the case of electrolyte addition was more than six tim

larger than that in ordinary soap-free emulsion polymerization in correspond

reaction conditions without any surfactants. It is difficult to explain the str

effect that the addition of the amphoteric surfactant has on the particle si

One reason may be the incorporation of the amphoteric surfactant into partic

which would increase the osmotic pressure of monomer in the particles.

expansion of particle volume due to the osmotic pressure might give rise

the rapid capture of radicals, and reduce the precipitation of new particles. T

total volume of particles in early reaction stage is so small that the formation

particlesis strongly affectedby theadsorption or incorporation of small amouof surfactant.

CONCLUSIONS

A method that used amphoteric surfactant for polymerization of styrene o

a range of initiator concentrations was examined. At low initiator concen

tion, secondary particles were generated in an intermediate stage of the re

tion, which resulted in a bimodal particle size distribution. Generation of s

ondary particles was greatly suppressed by the addition of KCl to the react

system.

R EF ER ENCES

1. Almog, Y., Reich, S., and Levy, M., Br. Polym. J. 14, 131 (1982).



12 NOTE

2. Horak, D., J. Polym. Sci. A 37, 3785 (1999).

3. Vanderhoff, J. W., Vitkuske, J. F., Bradford, E. B., and Alfrey, T., Jr.,

J. Polym. Sci. 20, 225 (1956).

4. Kim, J. H., Sudol, E. D., El-Aaser, M. S., Vanderhoff, J. W., and Kornfelt,

D. M., Chem. Eng. Sci. 43, 2025 (1988).

5. Ugelstad, J., and Mork, P. C., Adv. Colloid Interface Sci. 13, 101

(1980).

6. Ugelstad, J., Muftakhamba, H. R., and Mork, P. C., J. Polym. Sci. Polym.

Symp. 22, 225 (1985).

7. Gu, S., and Konno, M., J. Chem. Eng. Japan 30, 742 (1997).

8. Gu, S., Mogi, T., and Konno, M., J. Colloid Interface Sci. 207, 113

(1998).

9. Konno, M., and Orihara, S., J. Chem. Eng. Japan 33, (2000) in press.

0. Schwartz, A. M., and Perry, J. W., in “Surface Active Agents,” p. 162.

Interscience, New York, 1949.

11. Fujimoto, T., in “Shin Kaimenkasseizai Nyumori,” p. 87. Sanyo Ka

Kogyo, Tokyo, 1992.

Shinya Orih

Mikio Konn

Department of Chemical Engineering

Tohoku University

Sendai 980-8579, Japan

Received March 20, 2000; accepted July 5, 2000

1 To whom correspondence should be addressed. E-mail: Konno@mick

che.tohoku.ac.jp.

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