Study of particle growth by seed emulsion polymerization with ...

Colloids and Surfaces
A: Physiochemical and Engineering Aspects 153 Ž 1999.
165172
Study of particle growth by seed emulsion
polymerization with counter-charged monomer and
initiator system
Shinzo Omi a, , Kouji Fujiwara a , Masatoshi Nagai a , Guang-Hui Ma a ,
Akira. Nakano b
a Graduate School of Bio-Applications and Systems Engineering, Tokyo Uniersity of Agriculture and Technology, 2-24-16
Nakamachi, Koganei, Tokyo 1848588, Japan
b Nippon Zeon Co. Ltd., Takaoka, Toyama 933, Japan
Abstract
A two-step polymerization process was proposed to obtain composite latices with an average size ranging from
submicrons to 1 m. Negatively charged PMMA Seed latex particles were prepared by using anionic sodium lauryl
sulfate Ž SLS ., and ammonium persulfate Ž APS .. After the removal of unreacted monomer and initiator by dialysis,
seeded emulsion polymerization was carried out by swelling the PMMA seeds with monomers consisting of
hydrophilic dimethyl aminoethyl methacrylate Ž DMAEMA.
and hydrophobic styrene. A cationic initiator, 2,2azobisŽ
2-amidinopropane. 2HCl Ž V-50 ., was used. A small amount of non-ionic polyoxyethylene Ž 23.
nonylphenyl
ether Ž POE23.
solution was added. The weight percent of DMAEMA in co-monomer, weight ratio of monomer to
polymer Ž MP ., and pH of the latex were changed to obtain controlled coagulation and further growth of the seed
particles. Incorporation of DMAEMA was investigated by TEM observation of the particles stained with
iodomethane, and by measuring the rate of quarternization of DMAEMA when iodomethane gradually diffused into
the particles from the continuous phase. A maximum growth of the particles, from the initial average diameter of
0.21 m to a final 0.56 m, was attained when pH was set at 10.0 with a lower POE23 concentration. The final
particles were stable aggregates of the several seed particles with DMAEMA incorporated between the coagulated
seeds as well as deposited on the surface of the particles. The nucleation of DMAEMA-rich secondary particles was
enhanced when the pH was set at 9.0. Decreasing the amount of POE23 normally promoted the growth unless the
stability of the latex was affected. 1999 Elsevier Science B.V. All rights reserved.
Keywords: Emulsion polymerization; PMMA seed latex; Controlled coagulation; Styrene; DMAEMA
Corresponding author.
0927-775799$ - see front matter 1999 Elsevier Science B.V. All rights reserved.
Ž .
PII: S 0 9 2 7 - 7 7 5 7 9 8 0 0 4 3 9 - 7

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1. Introduction
Composite latices, modified with hydrophilic
components, have been applied to outdoor paints
which resist the deposition of oily contaminants
in precipitations, cardboard paints, and so forth.
Also, a new designing concept, a sophisticated
coagulation process leading to a controlled size
distribution, successfully delayed the shear thinning
at the higher shear region, resisting paints to
drip down when brushed on a vertical surface. A
controlled coagulation of polymer particles by
incorporating an appropriate amount of watersoluble
polymers in the particles was tried for a
commercial product which provided a deft relieflike
pattern as a finishing paint while retaining
the above rheological profile 1 . For a thick coating,
only one coating is necessary compared to a
repeated brushing by using a conventional polyvinylacetate
Ž VAc-Veova.
paint. Each primary
polymer particle has a diameter between 0.1 and
1.0 m, finally growing up to a broad size distribution
from 1.0 to 4.0 m by a controlled
coagulation process during the polymerization.
The unique water-soluble polymer is partly composed
of non-electrolyte acryl amide, weak electrolytes
such as acrylic or methacrylic acid, and
the rest composed of strong electrolytes such as
styrene sulfonic acid, sodium salt. The SEM photographs
revealed two-dimensional, cluster-like,
and non-spherical coagulums with the size distribution
described above.
In this study, we will propose a different procedure
which yield spherical coagulated particles by
employing a seeded emulsion polymerization in
which negatively charged seed particles are
swollen with a co-monomer composed of hydrophobic
styrene Ž ST.
and 520% of hydrophilic
dimethylaminoethyl methacrylate Ž DMAEMA .,
and then polymerized by a cationic initiator of
2,2-azobisŽ 2-amizinopropane. 2HCl Ž V-50 .. A
hydrophilic portion can be incorporated either by
coagulation or during the seed polymerization. By
changing the composition and the size of the seed
polymer particles, DMAEMA content in the comonomer,
and pH of the reaction mixture, the
final particle size and distribution can be controlled.
In this article, the results obtained from
polymethyl methacrylate Ž PMMA.
seed latex will
be reported as an introduction of our procedure.
Further results obtained by using modified seed
latices, PŽ MMA-co-styrene ., PŽMMA-co-butyl
acrylate ., and PŽ MMA-co-2-ethylhexyl acrylate .,
will follow including an effect of agitation during
the seed polymerization.
2. Experimental
2.1. Materials
Styrene Ž ST. and methyl methacrylate Ž MMA.
Ž Kishida Chemical Co. . were commercial grade,
distilled under reduced pressure, and stored in a
refrigerator prior to use. Dimethyl aminoethyl
methacrylate Ž DMAEMA, Tokyo Chemical Co. .
was reagent grade and used as received.
Ammonium persulfate ŽAPS, Wako Pure
Chemical Co. . was used for the emulsion polymerizations
to prepare PMMA seed latices.
2,2-AzobisŽ 2-amidinopropane. 2HCl ŽV-50,
Wako Pure Chemical Co. . was used for the seeded
emulsion polymerizations. They were reagent
grade and used as received.
Sodium lauryl sulfate Ž SLS, Merck.
was used
for the emulsion polymerizations to prepare
PMMA seed latices. Polyoxyethylene nonylphenylether
with 23 units of ethylene oxide
Ž POE23, Kao Co. . was used for the seeded emulsion
polymerizations to stabilize swollen particles.
They were reagent grade and used as received.
A chain transfer agent, t-dodecyl mercaptan
Ž TDM, Tokyo Chemical Co. ., was used as received.
Methanol, ethanol, and tetrahydrofuran Ž THF.
Ž Kishida Chemical Co. . were all commercial grade
and used as received.
Iodomethane Ž Tokyo Chemical Co. . was
reagent grade and used as received for the quarternization
of amino groups in DMAEMA.
2.2. Polymerization apparatus
A flat-bottom glass separator flask, either 500-
or 1000-ml capacity, was used as a polymerization
reactor. A nitrogen inlet, a condenser, a thermocouple,
and a feeder for initiator solution were

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S. Omi et al. Colloids Surfaces A: Physiochem. Eng. Aspects 153 1999 165172 167
equipped on the top of the flask. The nitrogen
was released from the top of the condenser. A
semi-circular flat blade was equipped as an impeller.
The agitation rate was kept at 350 min 1
for the preparation of PMMA seed latices, and
240 min 1 for the seeded polymerizations. The
reactor was immersed in a thermostat controlled
at 333 K during the polymerization.
2.3. Preparation of PMMA seed latex
Four seed latices of different particle size were
prepared for all the investigations. MMA Ž 300 g.
dissolving 0.5 g of TDM was dispersed in 700 ml
of 0.3 wt.% SLS aqueous solution in a 1000-ml
separator flask. Nitrogen gas was gently bubbled
through the dispersion for 1 h whilst the temperature
was gradually raised to 333 K. APS solution
Ž 20 ml ., after a thorough bubbling by nitrogen,
was added in the reactor. The polymerization was
carried out for 3 h at 333 K under a nitrogen
atmosphere. The average diameter of the seeds
were changed by changing the amount of APS.
The details of data are shown in Table 1.
2.4. Seeded polymerization
The seed latex was filled in Visking membranes,
dialyzed overnight under the tap water,
and then immersed in distilled water for 10 h. A
weighed amount of the seed latex containing 40 g
of the solid was mixed with an aqueous solution
of POE23 so that the total weight will be 340, 320
or 300 ml depending on a selected monomer to
seed polymer ratio Ž MP .. Under agitation by a
magnet bar, 40, 60 or 80 g of the co-monomer
consisting of ST and DMAEMA was added, and
the agitation was continued at room temperature
for 30 min. A 380-g sample of the mixture was
transferred in a 500-ml separator flask. During
the nitrogen bubbling for 1 h and the subsequent
heating to the reaction temperature for 30 min,
the seed polymer particles were allowed to swell
by absorbing the monomers. After 20 ml of the
initiator solution was added, the polymerization
was carried out for 24 h at 333 K. The total
weight was fixed as 400 g. After the polymerization,
the final monomer conversion was measured
gravimetrically. Polymers in a weighed latex sample
were precipitated by using methanol, centrifuged,
washed several times, and dried in a
vacuum oven.
2.5. Quarternization of DMAEMA
Ten microliters of the dialyzed latex was added
in a 100-ml triangle flask and diluted with a
known amount of water. As the stoppered flask
was stirred continuously with a magnet tip, an
excess amount of methyl iodide dissolved in
methanol was added in a certain interval. Moles
of iodide ion liberated from the quarternization
of DMAEMA was measured by an ion meter
Ž Custany LAB, Horiba ., and the rate of quarternization
of DMAEMA in the polymer particles
was estimated 2 .
2.6. Analysis
Average diameters of the polymer particles
were measured by counting 200 particles in SEM
Ž SM-35CF II-A, JEOL.
photographs. Morphologies
of the polymer particles were observed from
TEM photographs of stained ultra-thin microtomed
samples. The polymer particles were sealed
in epoxy resins. The michrotomed samples were
Table 1
Properties of seed PMMA latex
Run no. Average Coefficient of Number of polymer Zeta- Tg
diameter Ž m. variation Ž %. 16 3
particles Ž 10 dm . potential Ž mV. Ž K.
1502 0.21 5.74 7.22 No data No data
1503 0.26 4.84 3.20 No data 394
1504 0.18 4.86 9.15 60.0 394
1508 0.21 6.45 6.03 88.7 394

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exposed to the vapor of methyl iodide, and then
observed by a TEM ŽH-700H and H-7000, Hitachi
..
Average molecular weights Ž M and M .
w
n were
calculated from MWD curves obtained by GPC
Ž HLC-8020, Toso .. Tetrahydrofuran was used as
an eluant.
The latex sample was diluted to 10 5 10 6 -fold,
and the -potential of the polymer particles was
measured by a zeta potential analyzer ŽZeecom
IP-140A, Michrotech Nichion Co. Ltd. ..
3. Results and discussion
3.1. PMMA seed latex
The properties of the PMMA seed latex are
shown in Table 1. The average diameters are 0.18,
0.21 and 0.26 m. Corresponding to the use of
anionic APS and SLS, -potential revealed fairly
high negative values.
3.2. Deelopment of coagulation
A typical course of the coagulative process is
shown in Fig. 1 where the average diameter and
the number of polymer particles are plotted
against the polymer yield. A marked increase of
the average diameter was observed in the initial
stage of polymerization, followed by a plateau,
and a dramatic increase in the final stage. The
number of polymer particles was calculated from
the average diameter and the polymer yield, and
its decrease in the initial stage implied that the
increase in the average diameter was not only due
to the growth of each seed polymer particle but
mainly due to the considerable coagulation
between the particles. The subsequent plateau
may indicate that the polymerization proceeded
by consuming the swollen monomers in the polymer
particles. The final particle size is definitely
controlled by the intensive coagulation, which
started approx. 50% monomer conversion, and
seemed to correspond to an onset of the Trommsdorff
effect. A simple calculation implies each
final particle is an aggregate of 3.3 seed particles.
It can be said that the coagulation between the
particles will progress affected by the intensity of
Fig. 1. Progress of coagulation.
agitation as well as the electrostatic attraction
force. In this article, the investigation was focused
on the latter effect, keeping the agitation rate at
a constant 240 min 1 . The effect of agitation
during the polymerization will be discussed in the
next article by using a low Tg seed particles,
PŽ MMA-co-2EHA ., in which a more intensive
Ž yet well controlled. coagulation was observed 3 .
A coagulation mechanism will be proposed after
all the experimental data were shown.
3.3. Effect of DMAEMA content in monomer
SEM photographs of the PMMA seed Ž 1503 .,
and the final polymer particles with different content
of DMAEMA in monomer are shown in Fig.
2. The monomer to polymer ratio Ž MP.
was 2.0.
The final particle size increased with increasing
content of DMAEMA, and attained a maximum
of 0.60 m at 15% DMAEMA content. As shown
in the photographs, the coefficient of variation
became higher, from 4.84% of the seed to 10.3%
Ž 511, 5% DMAEMA ., 11.8% Ž 509, 10% ., and
12.4% Ž 510, 15% ., indicating uneven coagulation
and growth of the polymer particles.
3.4. Effect of monomer to polymer weight ratio
( ) MP
Effect of the monomer to polymer ratio to the
volumetric growth of the polymer particles, and to

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S. Omi et al. Colloids Surfaces A: Physiochem. Eng. Aspects 153 1999 165172 169
Fig. 2. SEM photographs of the final particles with different DMAEMA content in monomer. Seed PMMA latex is shown as a
reference.
Fig. 3. Final growth of polymer particles as a function of DMAEMA content in monomer.

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S. Omi et al. Colloids Surfaces A: Physiochem. Eng. Aspects 153 1999 165172
the coagulation number of the seed particles are
shown in Fig. 3 as a function of the DMAEMA
content in monomer. The relative growth in Fig.
3a is defined as the volumetric ratio of the final
particles to the seeds Ž d .
pd s
3 , overlapping the
two effects; the growth by the polymerization of
monomer and the coagulation between the seed
particles. On the other hand, the coagulation
number, NN s, solely estimates the intensity of
coagulation. dp
and ds
are the average diameters
of the final particles and the seeds, respectively,
and N and Ns
are the number of polymer parti-
cles. Clearly, the extent of coagulation depends
strongly on MP Žalthough only one point was
shown for MP1.5 ., and also on the DMAEMA
content when MP was 2.0. Non-swollen, excess
amount of the monomer may have promoted the
coagulation. The effect of the particle size is not
so significant except at the lower DMAEMA content.
3.5. TEM photograph of ultra-thin section of polymer
particles stained with methyl iodide
An ultra-thin section of the polymer particles
embedded in epoxy resin was placed on a copper
mesh, and exposed to the atmosphere of methyl
iodide for 3 days. TEM photographs of Run 509
and 510 are shown in Fig. 4. Although the contrast
is not so clear, shadowed domains indicate
rich in DMAEMA as the iodide atoms deflect
transmission beam. Small globules Žapprox. 0.1
m in diameter.
can be seen in Fig. 4a attached
on the surface of the seed particles. The other
DMAEMA rich domains are visible in Fig. 4b in
the center of polymer particles. The tiny globules
were probably nucleated in the aqueous phase
due to the high solubility of DMAEMA whilst the
domains visible in the center of the particles were
formed by the co-polymerization of DMAEMA
swollen with styrene and also by the globules
entrapped when the swollen particles coagulated
together.
3.6. Quarternization of DMAEMA distributed in the
final polymer particles
The quarternized DMAEMA expressed as the
weight per unit surface area of the final particles
Fig. 4. TEM photograph of ultra-thin layer of PMMAST-
DMAEMA particles stained with methyl iodide.
is plotted against the day elapsed, and shown in
Fig. 5. Because methyl iodide is sparsely soluble
in water, it was added as a solution in methanol.
A few microliters of the solution was added once
per day in order to maintain the iodide concentration
in the aqueous phase in excess. Although
the volume of the added solution was kept at a
minimum, the stability of the latex became deteriorating,
and the measurement was forced to shut
down after 5 days due to the deposition of agglomerates.
Normally, 4050% of DMAEMA in a
recipe was detected after 5 days. As the
DMAEMA content or MP becomes lower, the
percentage reached as high as 80%. An attempt
to measure the total incorporated DMAEMA in
a dispersion of the dissolved polymer solution
failed to yield any consistent result.
Fig. 5a shows that the higher DMAEMA con-

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S. Omi et al. Colloids Surfaces A: Physiochem. Eng. Aspects 153 1999 165172 171
Fig. 5. Quarternized amount of DMAEMA plotted against elapsed day.
tent in monomer Ž MP2.
resulted in a higher
incorporation in the polymer particles. All the
profiles can be expressed as two straight lines; the
initial, higher rate corresponds to the quarternization
of DMAEMA distributed near the surface
Ž see Fig. 4a .. The later straight lines indicates
the contribution of DMAEMA trapped inside
the particles. The fact that the rate of quarternization
remained nearly constant can be explained
by the unreacted-core model proposed by
Yagi and Kunii 4 . According to their model, this
case can be regarded as the case where the mass
transfer of methyl iodide Ž and iodide ion.
at the
waterparticle interface controls the whole reaction.
Once entering the particle, methyl iodide
diffuses through the quarternized shell, and can
reach to the unreacted core. The quarternization
takes place rapidly. The diffusion of iodide ion
through the interface may play a rate controlling
step. Once the DMAEMA in the interface is
quarternized, they will form a charged layer which
plays as a diffusion barrier across the interface.
In Fig. 5b, the quarternization between the two
latices of nearly same particle size but different
coagulation number is compared. The specific
surface area of the particles can be regarded as
nearly the same. Apparently, the latex with a
higher number of coagulation shows a higher rate
of the quarternization, and may be predicted with
a higher incorporation of DMAEMA.
3.7. Effect of pH alue
The pH of the reaction mixture before an addition
of the V-50 solution was always 10.0 when no
attempt was done to adjust the pH value. This
implies the liberation of OH ions due to the
formation of quarternized ammonium ion,
CH CH N HŽ CH .
2 2 3 2. The -potential of the
final latex particles remained negative, although
the absolute value became smaller. When the pH
of the reaction mixture was preadjusted to 9.0 by
adding a hydrogen chloride solution, and 25%
DMAEMA in the co-monomer, a nucleation of
the secondary particles was observed Ž MP2.0 ..
The resulting -potential of the final latex particles
was 24.4 mV compared to 50 mV Žaver-
age.
at pH10.0. These observations suggest that
the coagulation process at pH10.0 proceeded
well under control regarding the balance between
the counter charges. On the other hand at pH
9.0, excess protons promoted the cationic charge
of V-50 which encouraged the precursor chains
nucleated in the aqueous phase growing to the
stable particles.
3.8. Proposed coagulation mechanism
By considering the experimental results, the
photographs and the correlations in the figures, a
mechanism of the coagulation process throughout

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and III unless the pH of the reaction mixture was
preset at 9.0. The addition of POE23 should be
decided on the balance between the controlled
coagulation of the particles and the uncontrollable
loss of the latex stability.
4. Conclusion
Fig. 6. Proposed mechanism of particle growth.
the reaction period is proposed in Fig. 6. When
the seed polymerization is started, unstable precursor
chains are formed in the aqueous phase,
and precipitate. The composition of these precursors
is rich in DMAEMA because of its higher
solubility in water. These chains are adsorbed on
the surface of polymer particles, and gradually
forms DMAEMA rich domains. Meanwhile, mild
coagulation between the seed particles takes place
which are viscous due to the swelling of monomers
Ž Stage II .. The conversion of the swollen
monomers to polymers is mainly responsible for
the slow growth of the particles in Stage III.
Gradually, the DMAEMA rich domains on the
surface grow to such an extent that a part of
negative surface charges are neutralized, leading
the particles in an electrostatically unstable state.
Finally, the particles undergo an extensive coagulation
process until a new stability is regained
Ž Stage IV .. No noticeable formation of the secondary
particles were observed during Stage II
An effective and controlled growth of seed
polymer particles was observed when PMMA
seeds were swollen with a mixture of styrene and
DMAEMA, and the polymerization was carried
out with a counter-charged initiator Ž V-50 .. An
extensive coagulation seemed to take place in the
later stage of the polymerization when a part of
the negative surface charge was neutralized with
the counter ions, and the original stability was
lost.
Further investigations by using other seed particles,
with different hydrophilicity, glass transition
temperature and so forth, will be reported in
the next article together with the effect of agitation
intensity.
References
1 Y. Hasegawa, Technologies for Productions of Functional
and Composite Polymeric Microspheres, Assessment of
Polymer Manufacturing Process No.17, The Soc. of Polym.
Sci., Japan Ž 1995. 185 Ž in Japanese ..
2 G.-H. Ma, T. Fukutomi, J. Appl. Polym. Sci. 43 Ž 1991.
1451.
3 K. Fujiwara, MS thesis, Tokyo University of Agriculture
and Technology, Tokyo, Japan Ž 1998. Ž in Japanese ..
4 S. Yagi, D. Kunii, Chem. Eng. Sci., 16 Ž 1961.
364, 372,
380.