a simple method for producing micrometer-sized monodisperse particles in water
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8/17/2019 A Simple Method for Producing Micrometer-Sized Monodisperse Particles in Water
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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
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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).
8/17/2019 A Simple Method for Producing Micrometer-Sized Monodisperse Particles in Water
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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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