Mono-disperse polystyrene (PS) spheres were synthesized. The styrene polymerization was carried out using 4,4´-azobis 4-cyanovaleric acid (ACVA) or potassium persulfate (KPS) as initiators in the presence of polyvinyl pyrrolidone (PVP) as surfactant or stabilizing agent. Size measurement and its distribution were followed by scanning electron microscopy. A very narrow size distribution was obtained by controlling the synthesis temperature. For both initiators, it was observed that smaller PS particles are obtained when PVP amount increases. Besides, the ssphere size decreased when KPS amount increased at a fixed PVP concentration. Similar results were obtained for the polystyrene spheres synthesized using ACVA as initiator. On the other hand, the influence of a co-monomer addition in the sphere size was also analyzed. The suitable selection of the initiator type and its concentration, as well as the surfactant agent amount, led to a controlled obtainment of the polystyrene microspheres, appropriate to be used as template in the synthesis of hollow spherical materials.Centro de Investigación y Desarrollo en Ciencias Aplicada

Blanco, Mirta Noemí

Gorsd, Marina Noelia

Pizzio, Luis René

Procedia Materials Science

English

SEDICI - Repositorio de la UNLP

 Procedia Materials Science  1 ( 2012 )  432 – 438 
2211-8128 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SAM/CONAMET 2011, Rosario, Argentina. 
doi: 10.1016/j.mspro.2012.06.058 
11th International Congress on Metallurgy & Materials SAM/CONAMET 2011. 
Synthesis of polystyrene microspheres to be used as template in 
the preparation of hollow spherical materials: study of the 
operative variables 
M. N. Gorsd* , M. N. Blanco, L. R. Pizzio 
de Química, Facultad de 
Ciencias Exactas, UNLP-CCT La Plata, CONICET, 47 N° 257, 1900 La Plata, Argentina 
 
Abstract 
Mono-disperse polystyrene (PS) spheres were synthesized. The styrene polymerization was carried out using 
4,4´-azobis 4-cyanovaleric acid (ACVA) or potassium persulfate (KPS) as initiators in the presence of 
polyvinyl pyrrolidone (PVP) as surfactant or stabilizing agent. Size measurement and its distribution were 
followed by scanning electron microscopy. A very narrow size distribution was obtained by controlling the 
synthesis temperature. For both initiators, it was observed that smaller PS particles are obtained when PVP 
amount increases. Besides, the ssphere size decreased when KPS amount increased at a fixed PVP 
concentration. Similar results were obtained for the polystyrene spheres synthesized using ACVA as initiator. 
On the other hand, the influence of a co-monomer addition in the sphere size was also analyzed. The suitable 
selection of the initiator type and its concentration, as well as the surfactant agent amount, led to a controlled 
obtainment of the polystyrene microspheres, appropriate to be used as template in the synthesis of hollow 
spherical materials. 
 
© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of 11th 
International Congress on Metallurgy & Materials  SAM/CONAMET 2011. 
Keywords: Polystyrene; operative variables; template; hollow spheres 
 
 
* Corresponding author. Tel.:+54-221-421-1353; fax:+54-221-421-1353. 
E-mail address: marinagorsd@conicet.gov.ar. 
Available online at www.sciencedirect.com
 2012 Published by Elsevier Ltd. Selection and/or pe r-review under esponsibility of SAM/
CONAMET 2011, Rosario, Argentina.
433 M. N. Gorsd et al. /  Procedia Materials Science  1 ( 2012 )  432 – 438 
1. Introduction 
A variety of hollow spherical materials have been prepared by using polystyrene (PS) spheres as hard 
templates (Zhang et al., 2009). These materials present interesting characteristics such as low density, high 
surface area, excellent charge capacity, high permeability and, in addition, they have potential application in 
different fields such as storage and delivery of drugs, catalysis, separations, chromatography, and 
immobilization systems (Yoon et al., 2006). 
Various synthetic methods have been proposed for the fabrication of hollow materials, particularly for 
silica hollow spheres. Few studies have employed PS spheres as a polymeric template, though it has been 
reported as the most suitable method for the synthesis of mono-disperse hollow silica spheres (Suárez et al., 
2007). 
To obtain the polystyrene (latex) template, styrene polymerization by means of the radicalary 
polymerization mechanism has been used. The macromolecule formation by free radicals comprises the 
following steps: initiation, propagation and termination of chain growth by either termination or chain 
transfer.These radicals are generated employing polymerization initiators, organic or inorganic, which give 
different characteristics to the synthesized polymeric materials. 
The water-soluble initiators (inorganic) produce almost all of the free radicals in the aqueous phase, while 
the initiators soluble in organic solvents distribute their radicals among four phases, monomer-micelle, 
monomer-polymer particles, monomer droplets and aqueous phase. Thus, water-soluble and oil-soluble 
initiators bring about differences in the mechanism of polymerization (Nomura et al., 2005). However, 
theoretical and/or experimental studies conducted in polymerizations using these initiators have concluded 
that the kinetics of polymerization is basically similar for both types of initiators (Nomura et al., 1993). 
Also, a surfactant agent is added, whose role is to produce stable particles, preventing coagulation during 
the polymerization (Hong et al., 2010). Both the surfactant agent and the polymerization initiator and their 
concentrations play an important role in the obtained particle size and yield to final product (Paine, 1990). 
The addition of a co-monomer in the synthesis of the PS spheres is an alternative strategy for modifying 
the surface of the spheres, and a factor to be considered during the preparation of materials as templates for 
the synthesis of hollow spheres. 
We found that the most commonly polymerization initiator reported in the literature is 
2,2´azobisisobutyronitrile (AIBN). However, this compound is classified as highly inflammable and its 
exportation and importation is restricted in several countries. Taking this into account, we decided to replace 
AIBN by 4,4´ azobis 4-cyanovaleric acid (ACVA), which according to the literature has not been previously 
reported as polymerization initiator.   
 Since many of the results reported in the literature were not reproducible in the laboratory and, in some 
cases, were not suitable for the desired purpose, in this work polystyrene templates were synthesized using 
different polymerization initiators (ACVA or KPS) and a surfactant agent (PVP). Their influence on the size 
and distribution of the spheres was studied. Also 3-amino-1-propene as co-monomer was added, and its effect 
on the characteristics of the latex spheres was observed. 
2. Experimental 
2.1. Synthesis of polystyrene spheres 
The polystyrene spheres were prepared from styrene (Aldrich, > 99%) using potassium persulfate (KPS) 
(Mallinckrodt, > 99%) or 4,4´ azobis 4-cyanovaleric acid (ACVA) (Aldrich, > 98%) as polymerization 
initiators, in the presence of polyvinyl pyrrolidone (PVP) (Aldrich, PM = 29000) as surfactant agent. Different 
434   M. N. Gorsd et al. /  Procedia Materials Science  1 ( 2012 )  432 – 438 
PVP to styrene ratios were employed: 1, 2, 3, 6, 8 and 10% (w/w). A fixed KPS or ACVA to styrene ratio 
(1% (w/w)) was used, and ethanol-water as solvent for the inorganic initiator (10:4 (v/v)) or ethanol for the 
organic initiator. 
The mixture was poured into a 250 cm3 reactor, fitted with a reflux condenser, a temperature controller and 
a magnetic stirrer. N2 was bubbled for 15 min in order to remove oxygen from the reactor.  
The temperature was increased up to 70 ° C, and then the initiator dissolved in an aliquot of ethanol (or 
ethanol-water) was added to the reactor. The polymerization was carried out at 300 rpm for 24 h. After 
completion of this time, the latex was repeatedly washed with ethanol and stored in the form of dispersion in 
ethanol. 
To study the influence of the initiator concentration on the size of the spheres and its distribution, the same 
test was carried out, but this time the amount of initiator was varied between 0.5% and 1% w/w respect to 
styrene, and the PVP to styrene ratio was fixed in 3% (w/w), for both KPS and ACVA initiators. 
Similarly as described above, polystyrene spheres were synthesized by adding 3-amino-1-propene 
(allylamine) as co-monomer to 10 g of styrene, in a ratio of 10 and 20% respect to styrene, using in this case 
fixed amounts of ACVA as initiator and surfactant agent (PVP) of 1% and 3% (w/w), respectively. 
2.2. Characterization 
The morphological study of polystyrene spheres was performed by scanning electron microscopy (SEM), 
using Philips model 505 equipment. The determination of the sphere diameter, its distribution and the average 
diameter was carried out from the SEM images of the samples, measuring 150 spheres in each image. 
3. Results and discussion 
The polymerization starts in homogeneous phase by action of the initiator, generating nuclei from which 
colloidal particles are produced, which grow and end up separating of the reaction medium. PVP could be 
considered as a stabilizing agent in the polymerization reaction, which acts as a surfactant. PVP also plays an 
important role in obtaining stable particles with spherical shape, and prevents the aggregation of colloidal 
particles during the polymerization (Zou et al., 2008). 
For both initiators, it was observed that the addition of PVP allows to obtain spheres in suspension without 
them are grouped in clusters. It was also found that the particle size decreased when the amount of PVP is 
increased. At higher concentrations smaller particles are produced, so the PVP can stabilize a large surface 
area (Kim et al., 2006). 
Figure 1 shows the morphology of the polystyrene microspheres prepared with 1, 3 and 10% of PVP to 
styrene ratio, using ACVA as polymerization initiator. The average diameter of the spheres according to the 
different PVP proportions used is presented in Figure 2; as example it can be mentioned that the average 
diameter of the spheres obtained with 3% of PVP respect to styrene was 2.5 μm. 
 
435 M. N. Gorsd et al. /  Procedia Materials Science  1 ( 2012 )  432 – 438 
 
Figure 1. SEM micrographs (5000X) of polystyrene spheres prepared using ACVA as polymerization initiator and different PVP to 
styrene ratios: a) 1, b) 3, c) 10% (w/w). Bar: 5 μm. 
 
0 2 4 6 8 10
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
3,0
3,2
A
ve
ra
ge
 d
ia
m
et
er
 (μ
m
)
PVP to styrene ratio (%)
 
Figure 2. Mean diameter of polystyrene spheres obtained using ACVA as polymerization initiator and different PVP:styrene ratios.  
Regardless the PVP amount used in the synthesis of the latex spheres, the diameter distribution was very 
narrow, which was achieved by carefully controlling of temperature during the synthesis. For example, Figure 
3 shows the distribution obtained for spheres synthesized with 3% PVP (w/w), and ACVA as polymerization 
initiator. 
436   M. N. Gorsd et al. /  Procedia Materials Science  1 ( 2012 )  432 – 438 
2200 2250 2300 2350 2400 2450
0
5
10
15
20
25
Fe
cu
en
cy
Average diameter (μm)
 
Figure 3. Size distribution of PS spheres synthesized using ACVA as polymerization initiator and 3% (w/w) PVP: styrene ratio. 
The same dependence on the amount of surfactant agent was observed for polystyrene spheres synthesized 
with 1, 3 and 10% (w/w) of PVP, using KPS as polymerization initiator. In this case, the average diameter of 
the spheres was 2 μm for the minimum proportion of PVP, 1 μm for the intermediate quantity and an average 
diameter of 0.5 μm when 10% (w/w) is used (Figure 4).  
The difference in the size of the PS particles obtained with different initiators has been studied in detail by 
Nomura et al., 2005, who attributed this size difference to different diffusion rates of the initiators. The KPS, 
soluble in aqueous phase, diffuses at a fast rate to the monomer drop where polymerization takes place, 
resulting in particles of smaller diameter. In contrast, the diffusion rate is lower in aqueous phase for the 
organic initiator (ACVA), slowing down the polymerization process and, therefore, obtaining larger size 
particles. Similar results were obtained by Capek, 2001, who considers that the diffusion rate of the initiator 
molecules is the controlling step of the polymerization rate. 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4. SEM micrographs of polystyrene spheres prepared with KPS as polymerization initiator and different PVP:styrene ratios: a) 1% 
(w/w) (5000X); b) 3% (w/w)(20000X), c) 10% (w/w). (10000X). 
To study the influence of the initiator amount in the sphere size and its distribution, the same test was 
performed, but this time the initiator concentration was varied between 0.5 and 1% (w/w) respect to styrene, 
437 M. N. Gorsd et al. /  Procedia Materials Science  1 ( 2012 )  432 – 438 
and left the PVP: styrene ratio fixed in 3% (w/w), for both the initiator KPS and for the ACVA. It was 
observed that, when the concentration of the initiator increased, the sphere size decreased. The average 
diameter was 2 m for a KPS concentration of 0.5% (w/w), 1.4 m in diameter for 0.8% (w/w) of initiator, 
and 0.5 m for the maximum used concentration (1% (w/w)). It can be concluded that, as the initiator 
concentration increases, the polymerization process rate is higher and, therefore, smaller PS particles are 
obtained. This is consistent with studies by Miller et al., 1995, who reported that the decrease in the particle 
size is a result of the formation of a high fraction of small monomer droplets for a short period of nucleation. 
Furthermore, they attributed the increase in the particle diameter, when the amount of initiator is decreased, to 
the presence of large droplets of monomer for a longer nucleation time, and to the entry of free radicals at an 
early stage of polymerization. 
  
Two different volumes of 3-amino-1-propene were used for the synthesis carried out with the co-monomer, 
and fixed amounts of initiator (ACVA) and surfactant agent. It was observed that the sphere diameter varied 
according to the amount of co-monomer. For the addition of 1 ml of 3-amino-1-propene, spheres of 1 μm in 
diameter were obtained, whereas when 2 ml of co-monomer were added, the particle size decreased obtaining 
PS spheres of 0.5 μm in diameter. The results obtained by SEM are presented in the following micrographs 
(Figure 5). 
 
 
Figure 5. SEM micrographs (10000X) of polystyrene spheres prepared using ACVA as polymerization initiator and different allylamine 
amount as co-monomer: a) 1 ml; b) 2 ml. Bar: 2 μm. 
It is clear that the addition of co-monomer influences the size of the synthesized latex spheres. Indeed, the 
main objective of this addition is to modify the surface of the spheres, since the amino groups of the co-
monomer would cover the surface of the PS spheres with positive charge. It can be considered that this effect 
of the 3-amino-1-propene addition is an important factor in optimizing the coating of the spheres to obtain 
certain hollow spherical materials (Zou et al., 2008). In the particular case to which the templates studied in 
this work are aimed, corresponding to silica-based materials, an acid-base interaction with the hydroxyl 
groups of silica can be generated. 
4. Conclusions 
In this work we report for the first time the use of 4,4´ azobis 4-cyanovaleric acid (ACVA) as organic 
polymerization initiator and its comparison with potassium persulfate (KPS) inorganic one  in the synthesis of 
polystyrene microspheres. PS particles were obtained in a controlled manner with a very narrow size 
distribution for the different conditions employed during the synthesis, being changed the type of 
438   M. N. Gorsd et al. /  Procedia Materials Science  1 ( 2012 )  432 – 438 
polymerization initiator and its concentration, as well as the concentration of surfactant agent, under a careful 
control of the reaction temperature. Thus, polymeric materials suitable to be used as template in the synthesis 
of hollow spheres were achieved.  
The results showed that the variation of the concentration of both the surfactant agent and the initiators can 
modify the average diameter of the obtained spheres. For a fixed concentration of initiator, a decrease in the 
size of the spheres was observed as the surfactant concentration increased. In the case that the PVP 
concentration is held fixed and the initiator concentration varied, a similar trend was obtained. In all cases, the 
PVP acts as an excellent stabilizer of the materials obtained. 
The use of ACVA as polymerization initiator let us to synthesize latex spheres with controlled diameter 
and very narrow size distribution, so we can consider ACVA as a suitable replacement of the most employed 
but highly inflammable AIBN initiator. 
The addition of 3-amino-1-propene as a co-monomer is an important factor of study and currently we are 
more deeply working on this synthesis parameter, which will allow obtaining results concerning the surface 
characteristics of the material and its future application in the synthesis of hollow spherical materials. 
Acknowledgements 
The authors thank M. Theiller for her experimental contribution, and UNLP and CONICET for the 
financial support.  
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Synthesis of polystyrene microspheres to be used as template in the preparation of hollow spherical materials: study of the operative variables

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Synthesis of polystyrene microspheres to be used as template in the preparation of hollow spherical materials: study of the operative variables

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