Full text of DOI:10.1002/1521-3927(20000601)21:9<574::AID-MARC574>3.0.CO;2-O

574
Macromol. Rapid Commun. 21, 574–578 (2000)
Communication: Micrometer sized multi-hollow spheres
of epoxy resin were prepared by a physical method so-
called phase inversion emulsification technique. The for-
mation mechanism of the titled spheres was studied by
incomplete phase inversion. The requisite for the forma-
tion of multi-hollow spheres was that irreversible coales-
cence among the water droplets under shear action before
the phase inversion point existed. This process could be
facilitated by a lower emulsifier concentration and a
higher emulsification temperature. Moreover, a theoretical
explanation of the formation of the titled spheres was pre-
sented.
Mechanistic investigation on the formation of epoxy
resin multi-hollow spheres prepared by a phase
inversion emulsification technique
Zhenzhong Yang*, Delu Zhao, Mao Xu, Yuanze Xu
Polymer Physics Laboratory, Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences,
Beijing 100080, China
Yangz@pplas.icas.ac.cn
(Received: December 1, 1999; revised: February 16, 2000)
mation of copolymer containing acrylic acid as emulsifier
for the second emulsion polymerization in the presence
of organic solvents. At the end of the experiments, the
residual solvents were removed by distillation and multi-
hollow polymer particles were yielded. This method
involves residual organic solvents, which obviously
Introduction
Micrometer sized multi-hollow polymer spheres have
found many potential applications such as in the indus-
trial areas of separation and adsorption, catalysis, electro-
nic, printing and information. Recently, hard latex parti-
cles with interior cavities have been found to have poten-
tial uses in coatings as opacifiers. The pacification effect
originates from multiple scattering and interference con-
tributed to the scattering ability of the internal pores
rather than light absorption of such as kaolin or calcium
carbonate1–2). The study on the opacification mechanism
of porous spheres indicates that visible light is scattered
more efficiently by voids having diameters of 200–
800 nm3).
brings in many drawbacks9, 10)
.
According to Garti, double emulsions are commonly
prepared by two-step mode: hydrophobic emulsifier for
the W/O inner droplets, and hydrophilic emulsifiers for
the external O/W emulsion. However, this process also
involves too many procedures11, 12)
.
Recently, we have found an easy method to prepare
multi-hollow spheres of epoxy resins by incomplete
phase inversion progress13, 14) with the added advantage
that no organic solvents are involved in this method.
Therefore, from the viewpoint of a practical application,
this method is very attractive to achieve multi-hollow
polymer spheres owing to its low cost and simplicity. In
this paper, we attempt to understand the formation
mechanism of the multi-hollow spheres prepared by
phase inversion emulsification.
Some approaches have been reported to prepare multi-
hollow polymer spheres. According to Okubo et al.4, 5, 6)
,
sub-micrometer sized multi-hollow polymer structures
could be obtained via stepwise treatment with alkali fol-
lowed by acid of emulsion copolymers containing poly-
styrene and acrylate. Moreover, Okubo has successfully
prepared micrometer sized polymer spheres via dynamic
swelling seeded polymerization7, 8). In order to achieve
cavities within the spheres, the post-treatment is similar
to the aforementioned procedure. Generally speaking,
this method involves many tedious treatment procedures,
producing lots of waste. This makes the production more
time intensive and thus the products more expensive.
Instead, Schlarb et al. developed a new method to pre-
pare multi-hollow polymer particles, which involved for-
Experimental part
Materials
Bisphenol A epoxy resin E-20 (epoxide equivalent val—ue
0.20 mol/100 g resin) with an average molecular weight Mw
= 1000, was purchased from Chinese East Tianjin Chemicals
Macromol. Rapid Commun. 21, No. 9
i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000
1022-1336/2000/0905–0574$17.50+.50/0

Mechanistic investigation on the formation of epoxy resin multi-hollow ...
575
Co. and used as received. The polymeric e—mulsifier E325
with an number average molecular weight Mn = 4.66104,
measured by GPC15), is a multiblock copolymer composed of
10 wt.-% of epoxy resin E-20 as a hydrophobic component
and 90 wt.-% poly (ethylene glycol) PEG10000 as the hydro-
philic component. Both of the epoxy resin E-20 and the
emulsifier E325 are powdery at ambient temperatures.
Description of phase inversion progress
After the powdery bisphenol A epoxy resin E-20 and some
amount of emulsifier E325 were charged into a glass emulsi-
fication device at ambient temperatures, the mixture was
progressively heated until melted. Then, the mixture was
kept homogeneous by stirring and keeping the temperature
constant, and deionized water was added to the mixture con-
tinuously to drive the phase inversion. Meanwhile, the elec-
trical properties were monitored by directly reading the cur-
rent impedance value measured with a HIOKI 3520 LCR Hi
TESTER, which was connected with two well designed par-
allel rod electrodes fully immersed in the mixture. Once the
impedance of the system suddenly dropped accompanied by
an acceleration of the motor during the experiment, a contin-
uous phase inversion from the epoxy resin to the water phase
had occurred and the system could be dispersed in water.
This critical water content is defined as the phase inversion
point (PIP). Next, a large amount of water was added in
order to cool and dilute the inverted system, and the products
were obtained after drying under vacuum.
Characterization
Very diluted dispersions were spread onto a metal plate and
Fig. 1. Representative scanning electron micrographs of the
multi-hollow particles prepared at 5% emulsifier E325 and
858C (scale bar is 5 lm)
A: Contour of the particles (top), B: Internal structure of the par-
ticles (botton)
dried at ambient temperature under vacuum. The dried pow-
ders were sputtered with Au and observed with an Hitachi
scanning electron microscopy (S-530).
During the phase inversion process, samples of representa-
tive water content were taken off the emulsification device
and cooled immediately in an ice/water mixture. The cooled
samples were fractured in liquid nitrogen, and the fractured
surface was dried at ambient temperature under vacuum. The
dried fracture was sputtered with Au in vacuum and then
observed under a scanning electron microscopy (S-4200).
The water content of the systems, which is defined as the
weight ratio of water to epoxy resin, was determined by dry-
ing the systems at 1408C for 30 min and weighing the mass
loss under ambient conditions. The emulsifier concentration
refers to the weight ratio of the emulsifier to epoxy resin.
about 5 lm diameter with the hollows about 1 lm dia-
meter. Fig. 1B shows the internal structure of the parti-
cles. It can be seen that the cavities are interconnected
within the particles.
Some variables to control formation of the multi-
hollow spheres
It is found that the effects of emulsifier concentration and
emulsification temperature on the structural features of
the waterborne particles prepared by the phase inversion
emulsification technique are very pronounced. Fig. 2
shows the dependence of water content at PIP and the
structural features of the waterborne particles on the
Results and discussions
Representative morphology of the multi-hollow spheres
Fig. 1 shows two representative scanning electron micro- emulsifier concentration. Below the curve, the amount of
graphs of the multi-hollow particles of epoxy resin pre- water is not sufficient to drive phase inversion from
pared by the incomplete phase inversion emulsification epoxy resin into the aqueous phase. Therefore, the system
technique at 858C when the emulsifier concentration is in region 3 is a water dispersed in oil (W/O) system. On
about 5%. The contour of the particles is given in the other hand above the curve, phase inversion occurs
Fig. 1A. It can be seen that the particles are multi-hollow and the continuous phase of the system is water. There is

576
Z. Yang, D. Zhao, M. Xu, Y. Xu
higher, a W/O/W complex dispersion dominates. This
means that the perfection of phase inversion becomes less
with increasing temperature. This is likely to be corre-
lated with the weakening interfacial film with tempera-
ture.
Formation process of the multi-hollow spheres by
incomplete phase inversion emulsification
In order to understand the formation process of the multi-
hollow particles, the morphological evolution was char-
acterized by observing the representative samples with
SEM during the phase inversion process, as shown in
Fig. 4. It is noted that the emulsifier E325 concentration
is fixed at 2.33% and emulsification temperature is 858C.
In this case, the critical water content at PIP is 24.65%.
As can be seen in Fig. 4A, when the water content is
21.72% before PIP, it is found that some bigger deformed
water drops coexist with smaller water droplets. On
increasing water content, for example to 24.64%, a local
phase inversion occurs and the interconnected structure is
formed by the coalescence among the bigger water drops
(Fig. 4B). When the water content reaches 24.65%
(Fig. 4C), the continuous phase is suddenly inverted from
epoxy resin into the aqueous phase, as shown by the abrupt
decrease in the electrical resistance. It can be seen that lots
of smaller water droplets are trapped in the bigger water-
borne structure. Therefore, a W/O/W complex dispersion
is obtained in this case. As shown in Fig. 4D, the structure
becomes spherical owing to the decrease in surface energy
on further addition of water. Some interconnected network
within the spheres is also seen in Fig. 4D. This is consis-
tent with the structure shown in Fig. 1.
Fig. 2. The dependence of water content at PIP and the struc-
tural features of the waterborne particles on the emulsifier con-
centration
Fig. 3. The dependence of water content at PIP and the struc-
tural features of the waterborne particles on emulsification tem-
perature
a transition region simply indicated by a vertical line
from region 1 to region 2. In region 1, emulsifier concen-
tration is relatively high and discrete smaller waterborne
particles (O/W dispersion) dominate. In region 2, porous
particles (W/O/W dispersion) dominate at lower emulsi-
fier concentration. The water content at PIP asymptotes a
constant when emulsifier concentration exceeds 5%. This
infers that the emulsifier is sufficient to almost saturate
the W/O interface before PIP at this critical emulsifier
concentration.
Fig. 3 shows the dependence of water content at PIP
and the structural features of the waterborne particles on
the emulsification temperature. It can be seen that the
water content at PIP increases monotonically with tem-
perature. Below the curve, the water content is not suffi-
cient to drive phase inversion and the system in region 3
is a water dispersed in oil (W/O) system. Above the
curve, the continuous phase is aqueous. In region 1, the
emulsification temperature is low, and discrete smaller
particles dominate resulting from the complete phase
inversion process. In region 2, when the temperature is
Theoretical analysis of the multi-hollow spheres
formation
Before PIP, the added water is dispersed in the viscose
epoxy resin and broken-up into smaller droplets under
shear field16, 17, 18, 19). Meanwhile, the emulsifier molecules
diffuse onto the water/resin interfaces, and interfacial
films are formed to stabilize the water droplets. The
dynamic coalescence rate R among the water droplets
could be impeded owing to the interfacial film, which is
in agreement with the Smoluchowski20, 21, 22) theory devel-
oped by Davis and Rideal23):
ꢀ
ꢁ
dn
dt
2
3
r
E
R ˆ
ˆ
kTn2
exp
ꢀ1†
ga
kT
where k is the Boltzman constant, T the absolute tempera-
ture, n the number fraction of dispersed phase, r the
radius of dispersed phase, g the viscosity of the system, a
the collision radius, and E is the energy barrier mainly
determined by the strength of the interfacial film.

Mechanistic investigation on the formation of epoxy resin multi-hollow ...
577
Fig. 4. Morphological evolution during an incomplete phase inversion process
(Water content A: 21.72%, B: 24.64%, C: 24.65%, D: 28.85%)
The formation process of the multi-hollow particles by is satisfied that the attraction among the water droplets is
incomplete phase inversion is shown in Fig. 5. At low comparable with repulsion, larger water droplets will be
emulsifier concentrations, the emulsifier molecules are forced into a continuous phase at PIP under shear field,
not sufficient to diffuse onto the newly formed water/ and lots of smaller water droplets are trapped within the
resin interface and smaller water droplets are poorly sta- larger waterborne structure. This corresponds to structure
bilized. Therefore, the smaller water droplets coalesce to 3. Therefore, multi-hollow waterborne particles of epoxy
form larger ones. In this case, the interfacial film is less resin (W/O/W dispersions) are prepared by an incomplete
saturated with the emulsifier molecules. This analysis phase inversion emulsification technique as illustrated by
explains the dependence of phase inversion perfection on structure 4 in Fig. 5.
emulsifier concentration. Even the interfacial film is satu-
To summarize, when coalescence among the water dro-
rated, the strength of the interfacial film will also plets occurs prior to PIP, larger water droplets will be
decrease with temperature by weakening the hydrogen formed via the irreversible coalescence among smaller
bond between PEO units in the emulsifier and water. water droplets under shear field. They are randomly dis-
Therefore, the coalescence among the smaller droplets persed in the epoxy resin continuous phase. This process
becomes much more marked with increasing temperature. results in broad size distribution of the water droplets.
In these two cases, an irreversible coalescence among the The water/resin interfacial film will become further wea-
dispersed water droplets to form some larger ones exists. kened when the water droplets become larger. In this
This process corresponds to the change from structure 1 case, the phase inversion will take place via the forced
to 2 in Fig. 5. For phase inversion before the requirement coalescence among the deformed larger water drops sub-

578
Z. Yang, D. Zhao, M. Xu, Y. Xu
fore, both decreasing the emulsifier concentration and
increasing the emulsification temperature could facilitate
the formation of epoxy resin multi-hollow particles by an
incomplete phase inversion emulsification technique.
Acknowledgement: This work was supported by the National
Key Project for Fundamental Research “Macromolecular Con-
densed State” of the Chinese Science and Technology Ministry,
and National Natural Science Foundation of China under Grant
No. 29774038
1)
C. Rennel, M. Rigdhal, Colloid Polym. Sci. 272, 1111 (1994)
D. M. Fasano, J. Coat. Technol. 752, 109 (1987)
W. D. Ross, Ind. Eng. Chem. Prod. Res. Dev. 13, 45 (1974)
M. Okubo, H. Mori, Colloid Polym. Sci. 275, 634 (1997)
2)
3)
4)
Fig. 5. Illustration of incomplete phase inversion evolution
5)
M. Okubo, K. Ichikawa, Fujimura, Colloid Polym. Sci. 269,
1257 (1991)
6)
M. Okubo, A. Ito, A. Hashiba, Colloid Polym. Sci. 274, 801
jected to a shear field, and the smaller water droplets are
trapped within the waterborne structure. Therefore, multi-
hollow particles of epoxy resin are achieved by an incom-
plete phase inversion emulsification technique.
(1996)
7)
M. Okubo, M. Shiozaki, M. Tsujihiro, Y. Tsukuda, Colloid
Polym. Sci. 269, 222 (1991)
M. Okubo, M. Shiozaki, Polym. Int. 30, 469 (1993)
B. Schlarb, M. G. Rau, S. Haremz, Prog. Org. Coat. 26, 207
8)
9)
(1995)
10)
B. Sclarb, S. Haremza, W. Heckmann, B. Morrison, R. Mul-
ler-Mall, M. G. Rau, Prog. Org. Coat. 29, 201 (1996)
S. Matsumoto, Y. Kita, D. Yonezawa, J. Colloid Interface
Conclusion
11)
Sci. 57, 353 (1976)
N. Garti, Acta Polym. 49, 606 (1998)
Z. Z. Yang, Y. Z. Xu, H. Yu, D. L. Zhao, Acta Polym. Sinica
1, 119 (1997)
Z. Z. Yang, D. L. Zhao, M. Xu, Y. Z. Xu, Chem. J. Chin.
Univ. 18, 1568 (1997)
Z. Z. Yang, Y. Z. Xu, S. J. Wang, H. Yu, W. Z. Cai, Chin. J.
Polym. Sci. 15, 92 (1997)
G. I. Taylor, Proc. R. Soc. London A 146, 501 (1934)
L. Rayleigh, Proc. R. Soc. London A 29, 71 (1879)
S. Tomotika, Proc. R. Soc. London A 150, 332 (1935)
S. Tomotika, Proc. R. Soc. London A 153, 302 (1936)
M. V. Smoluchowski, Phys. Z. 17, 557 (1916)
M. V. Smoluchowski, Phys. Z. 17, 585 (1916)
M. V. Smoluchowski, Z. Phys. Chem (Leipzig) 92, 129
The formation mechanism of epoxy resin multi-hollow
particles by incomplete phase inversion is investigated. It
is found that the formation of larger water droplets by
irreversible coalescence among the smaller water droplets
before PIP is a requisite condition. This condition is
achieved by lowering the strength of the interfacial film
by decreasing the emulsifier concentration and increasing
the emulsification temperature. At low emulsifier concen-
tration, there are insufficient emulsifier molecules to dif-
fuse onto the freshly formed surfaces, and stabilization of
the dispersed water droplets becomes low. In this case,
the interfacial film is unsaturated by the emulsifier mole-
cules. With increasing temperature, the strength of the
interfacial film becomes less and the irreversible coales-
cence among the water droplets are remarkable. There-
12)
13)
14)
15)
16)
17)
18)
19)
20)
21)
22)
(1917)
J. T. Davis, E. K. Rideal, “Interfacial Phenomena”, 2nd Ed.,
23)
Academic, New York 1963, p. 344