Environmental Engineering Reference
In-Depth Information
Therefore, we called the process step by step (SbS) instead of LbL. While the thickness of
the layer cannot be controlled by the SbS process, the relative amount of covered area can
be inely controlled. In that way, the amount of material is controlled.
2.2.2 Nanoporous Hydrogels
A cross-linked polymer is made of linear polymer chains linked by short chains that act as
a cross-link. Since the secondary structure is an extended three-dimensional network, all
cross-linked polymers are nanoporous [115]. However, to be useful in aquananotechnology,
the nanoporosity should be completely wetted by water. Hydrogels are cross-linked polymers
where the polymer chains interact strongly with water (through hydrogen bonding, ion-dipole
interaction, etc.). Therefore, the polymer swells in presence of water to an expanded state.
Cross-linked polyacrylamides are widely used hydrogels for different applications,
including water remediation [116]. One advantage of polyacrylamides (and related poly-
mers) is the possibility to produce different materials for copolymerization of monomeric
acrylamides bearing different functional groups [117]. We have synthesized copolymer
polyacrylamides including sulfonic acid
(
−
−
SO
3
groups, acrylic acid [118], and
N
-isopropyl
groups. The materials can be synthesized as bulk materials or in the form of microparticles,
nanoparticles, or thin ilms [119]. Both microparticles and nanoparticles can be produced
by similar methods to carbon materials. Microparticles (Figure 2.7a) are synthesized by
radical polymerization of vinylic monomers (e.g., acrylamide) inside the water pools of an
inverse water-in-oil miniemulsion. Nanoparticles (Figure 2.7b) can be made by controlled
nucleation of the radical polymerization of soluble acrylamides.
The advantage of having at least one dimension of the solid in the microscale-nanoscale
range being small is to achieve fast mass transport. For a diffusion process obeying Fick's
law [120], the time (
t
) necessary to penetrate a material is given by
)
2
x
D
t
=
(2.1)
2
12
(a)
(b)
10
8
6
4
2
0
1
0 µm
75
100 25
150 75
200 25
250 75
Diameter (nm)
FIGURE 2.7
(a) Fluorescence microscopy of poly(acrylamide) hydrogel microparticles synthesized by radical polymeriza-
tion inside water pools of inverse miniemulsion. The gels are dyed with a luorescent dye. (b) Dynamic light
scattering measurement of hydrogel nanoparticles made by controlled nucleation.
