Synthesis of surface-functionalized polystyrene sub-micron spheres using novel amphiphilic comonomer

Article abstract of DOI:10.1016/j.polymer.2012.05.058

A facile one-pot emulsion polymerization is reported to prepare surface-functionalized sub-micron spheres using novel amphiphilic comonomer as the reactive emulsifier. Specifically, amphiphilic 3-[2-(acryloyloxy)ethoxy]-3- oxopropyl(phenyl) phosphinic sodium (AEOPS) was used not only as a comonomer, but also as a good emulsifier for the polymerization due to the relatively long chain of amphiphilic structure. Nearly monodisperse polystyrene sub-micron spheres with tunable sizes and zeta potentials were obtained. The phosphinic group-functionalized surfaces were demonstrated by the use of these particles to remove heavy metal ions (such as Cd²⁺, Pb²⁺) from water with a high efficiency.

Full text of DOI:10.1016/j.polymer.2012.05.058

Polymer 53 (2012) 3508e3513
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of surface-functionalized polystyrene sub-micron spheres using novel
amphiphilic comonomer
,
a
b
Xiaodi Pan a, Jinxin Cao a, Yingjie Wang a, Wei Huang a, Daoben Hua a , Xiulin Zhu , Hongjun Liang
*
a Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science, and School of Radiation
Medicine and Protection, Medical College, Soochow University, Suzhou 215123, PR China
b Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile “one-pot” emulsion polymerization is reported to prepare surface-functionalized sub-micron
spheres using novel amphiphilic comonomer as the reactive emulsifier. Specifically, amphiphilic 3-[2-
(acryloyloxy)ethoxy]-3-oxopropyl(phenyl) phosphinic sodium (AEOPS) was used not only as a como-
nomer, but also as a good emulsifier for the polymerization due to the relatively long chain of amphi-
philic structure. Nearly monodisperse polystyrene sub-micron spheres with tunable sizes and zeta
potentials were obtained. The phosphinic group-functionalized surfaces were demonstrated by the use
of these particles to remove heavy metal ions (such as Cd2þ, Pb2þ) from water with a high efficiency.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 5 March 2012
Received in revised form
26 May 2012
Accepted 27 May 2012
Available online 4 June 2012
Keywords:
Emulsion polymerization
Amphiphilic comonomer
Surface functionalization
1. Introduction
polymerization using reactive surfactants [15e17]. Recently there
are some literatures about one pot synthesis of amphiphilic block
Surface-functionalized polymeric colloidal particles from
nanoscopic [1] to micron [2] sizes represent an important class of
materials that have attracted widespread interests in various fields,
such as drug delivery [3], photonic crystals [4,5], colloidal assembly
[6], templated synthesis of functional materials [7], and many
more. The particle surfaces are normally functionalized by grafting
[8e11] or derivatization reactions [12e14]. For example, Lime et al.
[11] performed hydrobromination of residual vinyl groups on
divinylbenzene polymer particles, and then grafted the polymers
by atom transfer radical surface graft polymerization of glycidyl
methacrylate; Bayramoglu and Arica [14] obtained poly(2-
hydroxyethyl methacrylate/ethylenglycol dimethacrylate) micros-
phers containing surface-anchored sulfonic acid via the successive
reactions:the microspheres were first prepared via suspension
polymerization; after activation of the surface hydroxyl by bromi-
nation, surface-initiated atom transfer radical polymerization of
glycidyl methacrylate was conducted; and then the epoxy groups of
comb polymer was converted into sulfonic acid groups with reac-
tion of sodium sulfite.
copolymer latex [18e27]. Lefay et al. [27] prepared polystyrene or
poly(methyl methacrylate/n-butyl acrylate) microspheres using
amphiphilic gradient poly(styrene-co-acrylic acid) copolymer as
emulsion polymerization stabilizer, and Mohanty et al. [22] studied
electrosterically stabilized colloidal particles of different diameters
between 70 nm and 400 nm by amphiphilic diblock copolymers
poly(styrene)-block-poly(styrene sulfonate) as an emulsifier. In our
previous work, a “one-pot” emulsion polymerization was used to
prepare functionalized poly(styrene/divinylbenzene) (St/DVB) with
poly(3-[2-(acryloyloxy)ethoxy]-3-oxopropyl (phenyl)phosphinic
acid)-b-polystyrene as an amphiphilic emulsifier [24]. However, it
always requires complex reactions to synthesize the amphiphilic
block copolymer for the “one-pot” emulsion polymerization.
We also notice that some polymeric microspheres were
prepared by emulsion polymerization of styrene with other vinyl
monomers [28e31], such as methacrylate acid [32,33], sodium
styrene sulfonate [34], and etc. For example, Blom et al. [28] per-
formed the emulsion polymerization of n-butyl acrylate in the
presence of a polymerizable sulfobetaine monomer; Ramos et al.
[29] investigated 2-(methacryloyloxy)ethyl] trimethyl ammonium
chloride and vinylbenzyl trimethyl ammonium chloride as ionic
comonomers in the emulsion polymerization of styrene. However,
the colloid stability was relatively bad from these kinds of emulsion
polymerizations, as shown by the high amounts of coagulum
produced (4.2e38.1%) [29,35].
In order to eliminate the tedious and complicated reaction steps
often involved with such surface functionalization approaches,
there have been great interests in search of “one-pot” emulsion
* Corresponding author. Tel.: þ86 512 65882050; fax: þ86 512 65880089.
E-mail addresses: dbhua@ustc.edu, dbhua_lab@suda.edu.cn (D. Hua).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.05.058

X. Pan et al. / Polymer 53 (2012) 3508e3513
3509
In this paper, we design and synthesize a new amphiphilic func-
tional monomer, 3-[2-(acryloyloxy)ethoxy]-3-oxopropyl(phenyl)
phosphinic sodium (AEOPS), which has a relatively long chain of
amphiphilic structure that may be expected to stabilize the poly-
meric colloid better. Then we perform the emulsion polymerization
with AEOPS to investigate its efficacy as emulsifier, while phosphinic
group of AEOPS is covalently anchored onto PSt sub-micron spheres
during the polymerization process, and endows the PSt sub-micron
spheres with a good ionic coordination property [36e41]. As
a result, nearly monodisperse polystyrene sub-micron spheres with
tunable sizes and zeta potentials could be obtained, and the phos-
phinic group-functionalized surfaces were demonstrated by the use
of these particles to remove toxic heavy metal cations from water
with a high efficiency.
(PHw7.0). 0.50 mL of the solution was removed to record the
atomic absorption spectra at some pre-set interval times. Before the
atomic absorption spectra of the samples were recorded, calibra-
tion curves of Pb2þ and Cd2þ were constructed.
2.4. Characterization
1H nuclear magnetic resonance (1H NMR) spectra were obtained
on a Varian INVOA-400 instrument. Fourier transform infrared (FT-
IR) spectra were recorded on a Varian-1000 spectrometer. The Z-
average size and the polydispersity index of the particles were
measured by a Malvern HPP5001 high-performance particle sizer
(HPPS). Transmission electron microscopy (TEM) images were
taken with a FEI Tecnai G20 electron microscope, using an accel-
erating voltage of 160 kV. The nanocarrier solution (w5 ml) was put
2. Experimental section
on 400 mesh Ultrathin Type-A TEM Grid (Ted Pella, Redding, CA)
and immediately wicked dry by kimwipes, and then imaged
directly under TEM. Energy-dispersive X-ray (EDX) analysis was
carried out by a Hitachi S570 scanning electron microscope
equipped with an EDAX-PV 9100 energy dispersion X-ray fluores-
cence analyzer. The zeta potential measurements were conducted
on a Malvern Zetasizer NanoZS system with irradiation from
a 632.8 nm HeeNe laser. The critical micelle concentration (CMC)
was determined using pyrene as a fluorescence probe, and the
fluorescence spectra were recorded using FLS920 fluorescence
spectrometer with the excitation wavelength of 335 nm while the
emission fluorescence at 373 and 384 nm were monitored. The
sorption of Pb2þ or Cd2þ was measured for PSt sub-micron spheres
using atomic absorption spectrometry (DUO-220, Varian of Amer-
ica). Surface tension measurements of AEOPS aqueous solution
were carried out by du Nouy Ring method using a JK99C automatic
surface tensiometer and a platinum ring (Shanghai Zhongchen Co.,
China). The stock solutions were prepared by dissolving the AEOPS
into deionized water and then a series of AEOPS aqueous solutions
with different concentrations were obtained by diluting the stock
solution with deionized water. The obtained values were checked
through measurement of the surface tension of deionized water.
2.1. Materials and reagents
3-[hydroxy(phenyl)phosphoryl]propanoic acid, kindly donated
by Sinopec Yizheng Chemical fibers, Co. Ltd., was dried for 24 h at
80 ꢀC before use. 2-Hydroxyethyl acrylate (Alfa Aesar China (Tian-
jin) Co., Ltd., 97.0%) was distilled under reduced pressure prior to
use. AEOPS was synthesized according to a modified literature
method [42] (see Supporting Information). St (Sinopharm Chemical
Reagent Co., Ltd., CP) was washed with a 5% sodium hydroxide
aqueous solution and then with deionized water until neutraliza-
tion, and after being dried with anhydrous sodium sulfate over-
night, it was distilled under reduced pressure, stored under N2
at ꢁ20 ꢀC prior to use. All other chemical agents were used as
received.
2.2. Synthesis of functional PSt sub-micron spheres by emulsion
polymerization
The sub-micron spheres were prepared via emulsion polymer-
ization with AEOPS as the emulsifier and comonomer, styrene as
the monomer, DVB as the cross-linker and ammonium persulfate
(APS) as the initiator. A typical procedure for the emulsion poly-
merization can be described as follows: Polymerization was per-
formed in a 150 mL one-necked round-bottomed flask. Initially,
0.1 g of AEOPS was dissolved in 80 mL of distilled water, then, 1 g of
St and 0.02 g of DVB were added under stirring. The mixture was
dispersed under ultrasonic irradiation (75 W, 40 kHz) for w2 min at
room temperature (w22.0 ꢀC), and purged with Argon gas for
20 min to get rid of the oxygen. After it was heated to 70 ꢀC, an
aqueous solution of the initiator (APS, 0.04 g dissolved in 2 mL of
distilled water) was added to the mixture. After the polymerization
was conducted for 24 h to ensure complete conversion, and the
reaction mixture was slowly cooled to room temperature. Finally,
the latex particles diluted in water by 1:40 (v/v) for the measure-
ments of Z-average size and zeta potentials.
3. Results and discussion
In this study, the amphiphilic monomer AEOPS was synthesized
by the esterification of 3-[hydroxy(phenyl)phosphoryl]propanoic
acid and 2-hydroxyethyl acrylate in the presence of DCC and DMAP.
The structure of AEOPS was confirmed by 1H NMR spectrum
(Fig. 1a). The characteristic peaks at
d
¼ 7.4e7.8 ppm (eC6H5) and
5.8e6.4 ppm (eCH¼CH2) indicate the successful synthesis of
AEOPS. The amphiphilicity of the AEOPS monomer was determined
by measuring its critical micelle concentration (CMC) using pyrene
as a fluorescence probe: the concentration of monomer was varied
from 1.0 ꢂ 10ꢁ4 to 1.0 ꢂ 10ꢁ2 g/L while the concentration of pyrene
was fixed at 0.6 mM, and the CMC was estimated as the cross-point
when extrapolating the intensity ratio I373/I384 at low and high
concentration regions. The CMC of AEOPS is determined to be
1.211 ꢂ 10ꢁ3 g/L (Fig. 1b). In addition, surface tension measure-
ments were also carried out in AEOPS aqueous solutions (Fig. S1, see
Supporting Information). The surface tension shows a decreasing
tendency with increasing concentration of AEOPS, especially at
w10ꢁ3 g/L of the concentration, which is very close to the CMC
value from the method of fluorescence spectra.
In addition, the emulsion polymerizations with the different
recipe were also carried out: the concentration of AEOPS was
changed from 0.30 g/L to 7.32 g/L to investigate the influence on the
size of microspheres at 12.5 g/L of St concentration; and the
concentrations of AEOPS (12.5 g/L) and monomer (250 g/L) were
successfully used to obtain the high solid content (w24.6%).
2.3. Coordination of PSt sub-micron spheres with metal ions
The emulsion polymerization of St/DVB was performed with
AEOPS as the emulsifier and comonomer. After the polymerization,
the solid content was collected by centrifugation. The morphology
of the PSt sub-micron spheres was revealed by TEM. As shown in
Fig. 2A: the as-produced PSt particles are spherical and mono-
disperse with a diameter of about 200 nm, and the outer shell and
0.145 g of PSt sub-micron spheres were dispersed in distilled
water (10 ml) under ultrasonic irradiation, and then dialyzed
(MWCO ¼ 3500) in a dilute ionic aqueous solutions, i.e. 4.00 mmol/
L Pb(NO3)2 aqueous solution (PHw7.0) or CdCl2 aqueous solution

3510
X. Pan et al. / Polymer 53 (2012) 3508e3513
Fig. 1. (a) 1H NMR spectrum (400 MHz, D2O) of AEOPS at a low concentration and (b) The fluorescence intensity ratio I373/I384 of pyrene as a function of AEOPS concentration.
core are clearly observed. The particle sizes and size distributions
further demonstrate that the sub-micron spheres have a symmetric
and very narrow size distribution. The single peak is positioned at
247 nm, which agrees very well with TEM observations; the small
polydispersity index further suggests that there is no aggregation in
the dispersion of the sub-micron spheres (Fig. 2B). For PSt micro-
spheres from emulsion polymerization with high solid content
(24.6%), there are still a single peak and a small polydispersity index
for particle size distributions (Fig. S2, see Supporting Information),
which suggest the successful emulsion polymerization with high
solid content. All the results indicate that AEOPS can be used as an
emulsifier to stabilize the emulsions in substitution of surfactants,
which may be associated with the amphiphilic structure of AEOPS.
The surface structures of the sub-micron spheres were charac-
terized by FT-IR spectra (Fig. 2C). In comparison with AEOPS
(Fig. 2C, trace a), the characteristic peaks at 1192 cmꢁ1 (eP¼O),
1026 cmꢁ1 (ePeO) and 1620 (eC¼O) occur for the functional PSt
sub-micron spheres, while the peak at 998 cmꢁ1 (eC¼C) disap-
pears (Fig. 2C, trace b). The zeta potential measurement was also
implemented for the characterization of the surface function of PSt
sub-micron spheres. Fig. 2D shows the PSt sub-micron spheres
have a zeta potential of ꢁ26.0 mV which should be ascribed to the
anchored phosphinic group. These results unambiguously show
that the PSt sub-micron spheres are functionalized by phosphinic
groups of AEOPS, which may be covalently anchored onto PSt sub-
micron spheres by the copolymerization process.
Fig. 2. (A) TEM image and (B) particle size distribution of PSt sub-micron spheres obtained from emulsion polymerization with AEOPS as emulsifier with the concentration of
1.22 g/L (i.e. [AEOPS] ¼ 1.22 g/L): Z-average size ¼ 247 nm, polydispersity index ¼ 0.042; (C) IR spectra of (a) AEOPS and (b) functional PSt sub-micron spheres; and (D) The zeta
potential distribution curve for PSt sub-micron spheres ([AEOPS] ¼ 1.22 g/L).

X. Pan et al. / Polymer 53 (2012) 3508e3513
3511
Fig. 3. The sorption of (A) Cd2þ and (B) Pb2þas a function of the time for the PSt sub-micron spheres with the zeta potential: (a) ꢁ26.0 mV, (b) ꢁ51.3 mV, and (c) ꢁ70.7 mV.
Expectedly, the size and the zeta potential of the sub-micron
spheres are tunable by the concentration of AEOPS used in the
emulsion polymerizations. The more AEOPS in the polymerization
will lead to the smaller size like conventional emulsion polymeri-
zation (Fig. S3, see Supporting Information); whereas, at relatively
high concentration of AEOPS, the size of the final particles would
become larger (Fig. S4, see Supporting Information), which may be
attributed that some emulsifiers might be entrapped into the
micelle at relatively high concentration, thereby combining some
water into the micelle to enlarge the sizes. In addition, the more
negative zeta potential of the sub-micron spheres could be
obtained with larger concentration of AEOPS in the polymerization
(Fig. S5, see Supporting Information). To quantitatively examine the
degree of surface functionalization, we tested the ionic coordina-
tion property of the as-prepared PSt sub-micron spheres. Specifi-
cally, the PSt sub-micron spheres were treated with dilute solution
of heavy metal ions, such as Cd2þ and Pb2þ. Those heavy metal ions
are known as environmental hazards that cause long-term risk to
the ecosystem and humans. The sorption of the metal ions as
a function of the time was depicted in Fig. 3 for PSt sub-micron
Fig. 4. TEM images and size distributions of PSt sub-micron sphereeCd2þ complex (A, B) and PSt sub-micron sphereePb2þ complex (C, D). All the PSt sub-micron spheres before ion
coordination have a zeta potential of ꢁ26.0 mV.

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X. Pan et al. / Polymer 53 (2012) 3508e3513
bonds. Our results indicate that the surface-functionalized PSt sub-
micron spheres prepared by the “one-pot” emulsion polymeriza-
tion process with the reactive amphiphilic comonomer not only
have a tunable surface function density, but also are significantly
more efficient in removing heavy metal ions pollution from water.
The presences of Pb2þ and Cd2þ on the surface of the PSt sub-
micron spheres are further confirmed by EDX spectra (Fig. S6, see
Supporting Information). The morphologies and size distributions
of PSt sub-micron sphereePb2þ and Cd2þ complexes were charac-
terized by TEM and HPPS, respectively. In comparison with PSt sub-
micron spheres before coordination with the metal ions (Fig. 2A),
the complexes have the similar coreeshell structure (Fig. 4A and C).
At the same time, it should be noticed that the complexes have the
multi-peaks in the size distribution (Fig. 4B and D), which suggests
that some inter-particle aggregates occurred when the PSt sub-
micron spheres are coordinated with Pb2þ and Cd2þ, respectively.
In order to further understand the difference of ion sorption
capacities, we studied the zeta potential of the PSt sub-micron
sphereeion complexes. The zeta potential is a measure of the
number of charges per particle surface, and it can be used to assign
the change of the surface charge created by anions and cations. The
PSt sub-micron spheres ([AEOPS] ¼ 1.22 g/L) has a zeta potential
of ꢁ26.0 mV (Fig. 5A, trace a) due to the anchored phosphinic
group, and after it coordinated with Cd2þ and Pb2þ, the zeta
potentials is expectedly increased to ꢁ16.2 mV (Fig. 5A, trace b)
and ꢁ16.4 mV (Fig. 5A, trace c), respectively. Surprisingly, we
obtained essentially the same zeta potential for ion-bound PSt sub-
micron spheres even though their initial zeta potentials vary
greatly before ion coordination, as an example shown in Fig. 5. The
results further showed the PSt sub-micron spheres with different
initial zeta potential would have a different cation sorption
capacity: the more negative zeta potential, the higher capacity.
4. Conclusion
Fig. 5. The zeta potential distribution curves for the original PSt sub-micron
spheres : ꢁ26.0 mV (A, trace a), ꢁ51.3 mV (B, trace a), and ꢁ70.7 mV (C, trace a);
PSt sub-micron sphereeCd2þ complex : ꢁ16.2 mV (A, trace b), ꢁ16.5 mV (B, trace b),
and ꢁ19.2 mV (C, trace b); and PSt sub-micron sphereePb2þ complex : ꢁ16.8 mV
(A, trace c), ꢁ16.6 mV (B, trace c), and ꢁ19.5 mV (C, trace c).
In summary, we report a facile approach to prepare surface-
functionalized polystyrene sub-micron spheres by “one-pot”
emulsion polymerization using novel amphiphilic comonomer as
an emulsifier. The functional comonomer has a long chain of
amphiphilic structure. The resultants were characterized by 1H
NMR, FT-IR, TEM, HPPS, EDX, zeta potential and fluorescence
spectra. The results show that AEOPS could be used not only as
a comonomer due to the acrylate structure, but also as a good
emulsifier with the CMC of 1.211 ꢂ 10ꢁ3 g/L. The coreeshell
structured, surface-functionalized PSt sub-micron spheres were
shown to have a monodisperse diameter with tunable sizes
(dw102 nm) and zeta potentials by changing the concentration of
AEOPS used in the emulsion polymerizations, and the phosphinic
groups are covalently anchored onto PSt sub-micron spheres
during the polymerization process. The surface-functionalized
polymer particles coordinate well with Pb2þ or Cd2þ to form the
aggregates with the larger sizes and polydispersities, and the
sorption capacity of the sub-micron spheres is increased with the
more negative zeta potentials. This property may be used for the
removal of heavy metal ions from wastewater. We expect that
a similar concept should be generically applicable to the emulsion
polymerizations using other similar functional amphiphilic
monomers. As such, this surfactant-free approach enables the
control of both the particle sizes and particle surface functions with
unlimited choice of particle compositions.
spheres with the different zeta potentials. The sorption of Cd2þ and
Pb2þ can reach the maximum capacity of 0.612 mmol/g and
0.346 mmol/g within 600 min for PSt sub-micron spheres with the
zeta potential of ꢁ26.0 mV, respectively (Fig. 3A, trace a and 3B,
trace a). The difference of sorption capacity between Cd2þ and Pb2þ
may be ascribed to the different charge density because the elec-
trostatic interaction is essential for the ion sorption in this study:
for the smaller volume of Cd2þ, there is a relatively larger charge
density, which may lead to the larger sorption capacity. Interest-
ingly, the maximum ion sorption capacity of the PSt sub-micron
spheres is tunable when their zeta potentials vary (Fig. 3). For
instance, when the zeta potential is changed to ꢁ70.7 mV, the
maximum ion sorption capacity of the PSt microsphere reaches
2.255 mmol/g for Cd2þ, and 0.982 mmol/g for Pb2þ ions, respec-
tively (Fig. 3A, trace c and 3B, trace c). These values are markedly
larger than the reported numbers of functionalized PSt sub-micron
spheres prepared by other surface functionalization approaches:
0.054 mmol/g for Cd2þ reported by Trochimczuk [40]; or
0.152 mmol/g for Pb2þ and 0.274 mmol/g for Cd2þ, respectively,
reported by Liu et al. [43]. The difference between the sorption
capacities should be ascribed to the different surface-
functionalized methods: Trochimczuk [40] obtained the function-
alized sub-micron spheres with phenylphosphinic acids by reacting
styrene/divinylbenzene (St/DVB) resin with PCl3 in the presence of
FriedeleCrafts catalyst; and Liu et al. [43] grafted chitosan onto
amine-functionalized magnetite nanoparticles via Schiff base
Acknowledgments
We thank Natural Science Foundation of China (50903060,
21174100), Specialized Research Fund for the Doctoral Program of

X. Pan et al. / Polymer 53 (2012) 3508e3513
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3513
Higher Education of China (20093201120004), A Project Funded by
the Priority Academic Program Development of Jiangsu Higher
Education Institutions, Program of Innovative Research Team of
Soochow University, and Colorado School of Mines Foundation for
their financial support.
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