Environmental Engineering Reference
In-Depth Information
light-responsive materials are typically triggered by conformational changes of certain dye molecules in the polymer backbones
or as pendent groups [29, 46-50], or by the local generation of heat due to dyes linked either to a thermally responsive polymer
or to a hydrogel [27, 30]. However, dyes are usually chemically reactive, are susceptible to bleaching, and their responses are
generally very slow [14, 25, 35, 51]. Therefore, research has been focused on developing alternative materials, such as nano-
composites that couple two or more different materials and synergize their individual physical properties.
22.1.3
nanocomposite materials (thermally responsive polymers and metallic nanoparticles)
Developing hybrid or nanocomposite materials that couple metallic nanoparticles to responsive polymers is a field of research
that has received a lot of attention due to the extensive research done in the synthesis and design of each of the individual com-
ponents (responsive polymers and metallic nanoparticles). specifically, coupling thermally responsive polymers to metallic
nanoparticles has allowed the development of materials that can be triggered both optically and thermally since the incorporated
metallic nanoparticles act as amplifiers and converters of light of a specific wavelength into heat [52]. There are two major
advantages of these materials over the traditional optical responsive polymers (using dyes). The first is the higher chemical sta-
bility of both the nanoparticles and the polymer, and the second is the fast optical response of the nanocomposite, since it is
triggered by a coupled mechanism between the fast thermal response of the polymer and the capability of the metallic nanopar-
ticles to convert absorbed light into heat, taking advantage of the rapid heat conduction at small scales. These optothermally
responsive nanocomposites have been synthesized as temperature-responsive PNIPAM hydrogels that have entrapped metallic
particles within their structure [13, 53-55] and have found applications to control flow in microfluidic devices. synthesis of
these systems presents an advantage since it is simple to physically entrap nanoparticles within the hydrogel; however, the
downside is the long time taken to trigger the response of these systems and their requirement for high-power lasers. It is there-
fore the aim of this work to develop a synthetic route that couples polymer chains to metallic nanoparticles, and thus obtain
stable nanocomposites. These can be further used as “on-off” valves to control flow in microfluidic systems using low-power
lasers and faster switching times.
As a result, in the remainder of this chapter, we will focus on the characteristic of each of the components that play a role in
the synthesis and response of these optothermally responsive materials.
22.2
thermally responsIve polymers (pnIpam)
PNIPAM is one of the most well-studied thermally responsive materials in the literature. PNIPAM is a polymer that contains
both hydrophobic and hydrophilic chemical groups as illustrated in Figure 22.2. Due to these chemical characteristics, it has
been determined to be a temperature-responsive polymer with a lower critical solubility temperature (lCsT) of around 32°C in
water. such behavior of solubility in water can be explained thermodynamically with the gibbs free energy equation of the
PNIPAM/water system: Δ
G
mix
= Δ
H
mix
−
T
Δ
S
mix
. since a negative gibbs free energy determines spontaneous processes, and
PNIPAM has been determined to have an lCsT, we must assume that both Δ
H
mix
and Δ
S
mix
must be negative. In this case, as
the
T
increases, the entropy term becomes more positive and eventually reaches the enthalpy term to switch the gibbs energy
from a negative value that determines a miscible system to a positive value that determines an immiscible system.
Carbon
Nitrogen
Oxygen
Hydrophilic
Hydrophobic
fIgure 22.2
Chemical structure of poly-
N
-isopropylacrylamide. The hydrophilic amide groups are capable of forming hydrogen bonds
at temperatures below 32°C. The hydrophobic isopropyl group becomes dominant at 32°C and causes the polymer to phase separately from
man's aqueous environment. This gives the polymer its responsive characteristics.
