Supporting information
Impact of non-ionic surfactant chemical structure on
morphology and stability of polystyrene nanocomposite latex
Nagi Greesha and Suprakas Sinha Raya, b *
a
DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial
Research, Pretoria 0001, South Africa.
b
Department of Applied Chemistry, University of Johannesburg, Doornforntein 2028,
Johannesburg, South Africa
*Corresponding author. Fax: +27 12 841 2388; E-mail: rsuprakas@csir.co.za
Table 1. Physical properties of EFD clay
Physical characteristics
Value
Density / g.cc-1 (from Pycnometer)
2.36
Total pore volume / cc.g-1 (from Pycnometer)
0.58
Specific surface area / m2.g-1 (from BET)
34.53
Pore volumea / cc.g-1 (from BET)
0.09
Pore sizeb / nm (from BET)
18.73
Modulus (E′) at 30 °C / GPac
10.08
Length of each platelet /nm
~50
Thickness of each platelet /nm
~0.9
Average particle size measured using TEM
~208.8 nm
a
BJH adsorption cumulative volume of pores between 1.7 nm and 3.0 x 106 nm in diameter
b
BJH adsorption average pore diameter
c
Dynamic mechanical analyzer using pocket material sample holder
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Emulsion polymerization using non-ionic surfactants
The morphology of the PS particles prepared with a low surfactant concentration was observed
to be undefined, with large particle sizes and a broad particle size distribution. The use of these
surfactants provides steric stability only and can result in latexes with large particles and broad
particle size distributions. The dispersion obtained after polymerization was unstable, and phase
separation occurred upon centrifugation at 5000 rpm for 30 min (Figure 1(a’ and c’) inset). This
instability is caused by coalescence when two droplets combine after they have collided to form
an aggregate. The rate of coalescence is dependent on the droplet encounter rate and the
properties of the droplet surface.
The average particle size varied within the range of 0.31.8 µm, depending on the concentration
of the surfactants. As the concentration of the surfactants increased (from 0.7 to 2 wt.% relative
to the monomer concentration), the diameter of the PS particles decreased from 1.8 to 0.3 µm.
This is an expected result because the amount of surfactants is a determining factor in the
stabilization of the newly nucleated particles, reducing their aggregation during the early stages
of the growth process. Therefore, the number of formed nuclei increased with increasing
surfactant concentration, which resulted in a large number of small particles. The tendency for
particle size to decrease with increasing stabilizer concentration has been observed by other
researchers [1,2,3]. However, as mentioned previously, the use of a large amount of surfactant is
highly undesirable.
The TEM images in Figure 1 (main text) also show that the morphology and the particles sizes
of PS were not affected by the type of surfactant used, although the two surfactants have
different chemical structures. On the other hand, Igepal®CO-630 behaves in a completely
different manner during emulsion polymerization. Figure 2 (main text) show TEM images of the
PS latex prepared by emulsion polymerization at two different concentrations (1.2 and 2 wt.%
relative to monomer) of Igepal®CO-630.
At a low surfactant concentration (Figure 2 (a&a’)), an unstable polymer latex (inset, Figure
2(a’)) was obtained due to molecular diffusion degradation, which results from the growth of
larger monomer droplets at the expense of smaller droplets. This growth mechanism occurs due
to the difference in the chemical potential between droplets of different radii [4,5]. The growth of
droplets occurs by the molecular diffusion of monomers through the continuous phase. In other
words, Ostwald ripening is a growth mechanism by which small particles effectively are
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consumed by the larger particles [5-7]. As the concentration of Igepal®CO-630 increased from
1.2 to 2 wt.% relative to the monomer, the diameter of the PS particles decreased, as shown in
Figure 2(b); however, Ostwald ripening persisted.
During polymerization, the particle formation of the PS latexes was monitored by TEM. Figure
2(c & c’) shows TEM images of the PS latex formed after 3 h of polymerization; it is clear that
the PS latex underwent Ostwald ripening during the early stages of polymerization. In oil-inwater emulsion polymerization, Ostwald ripening causes the diffusion of monomers from smaller
to larger droplets due to the greater solubility of the single monomer molecules in the larger
monomer droplets [7]. The rate of this diffusion process is linked to the solubility of the
monomer in the continuous (water) phase of the emulsion, which can lead to the destabilization
of emulsions (for example, by creaming and sedimentation) [5]. Ostwald ripening was observed
to depend on the type of surfactants used. In the present study, the Ostwald ripening effect was
not observed when Span®40 and Brij®S10 were used; only the use of Igepal®CO-630 caused
Ostwald ripening. The reason for this behavior is not yet clear, but looks like it links with the
chemical structure of the surfactants, which can affect the chemical potential of the droplets
formed. For example, when Span®40 and Brij®S10 were used, the droplet encounter rate was
faster than that when Igepal®CO-630 was used. Therefore, small particles aggregated to form
large particles.
Based on our previous discussions, coalescence and Ostwald repining cause monomer droplets to
deform to form new, larger droplets, usually at the expense of smaller droplets. Therefore, to
form a stable emulsion system, coalescence and Ostwald ripening must be suppressed, which can
be achieved by an efficient emulsification procedure and by using a suitable type and amount of
surfactant [6,8]. Recently, the literature has witnessed a dramatic increase in the number of
studies investigating, in detail, the effect of Ostwald ripening and possible ways to suppress it [913]. For example, Davis et al [9] and Webster and Cates [10] reported that the addition of a lowvapor-pressure component that is completely soluble in the dispersed phase will significantly
enhance the stability of emulsion systems. The presence of this soluble compound effectively
creates an osmotic pressure inside droplets, which will prevent the diffusion of monomer from
smaller to larger droplets. This work attempts to use organically modified clay to prevent or
reduce the Ostwald ripening effect by suppressing monomer diffusion from smaller to larger
droplets.
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