Strong polyelectrolytes are fully ionizable, whereas weak polyelectrolytes,
with fractional charge, are only partially ionizable. Typically, polyelectrolyte conformation is “flat” because the charges on a linear polymer
repel each other due to Coulombic repulsion. With the addition of salt
ions, however, this conformation can be altered to a more coiled or collapsed state due to screening of the charges on the polymer by the other
ions in the solution. This ability to change polyelectrolyte conformation
can change their function and is very useful in materials chemistry.
Synthetic organic chemists have been able to control the characteristics of
polyelectrolyte assemblies in many different manners, most simply by
altering monomeric units, electrolyte group, and polymer length. As a
result, polyelectrolytes make a very tunable building block for complex
materials. They have been used in a number of applications ranging from
drug delivery agents, to components in conducting films, to facilitating
colloidal suspensions, and to creating mimics of biological molecules.
Recently, polyelectrolyte microcapsules have found use as microreactors
for catalyst activity, precipitation reactions, crystallization reactions, and
polymerization reactions.
10.2.1 Electrostatic self-assembly
One useful feature of polyelectrolytes is that many of them can interact
with each other in a process known as electrostatic self-assembly (ESA).
As a simple example, Figure 10.5 illustrates the complexation of two
oppositely charged polyelectrolytes in the bulk phase. Polyelectrolytes
can also adhere to oppositely charged surfaces, forming nanofilms.
Because ESA relies on attractive interactions between materials of alternating charge, a number of different kinds of molecules can be used, such
as small inorganic molecules, proteins, dendrimers, and polyelectrolytes.
Our exploration of ESA films will focus on polyelectrolytes. The flexibility
of the polyelectrolytes used for ESA, as well as the ease of the selfassembly process, gives it advantages over other methods of creating thin
films, such as the LB technique. In contrast to ESA, the LB technique
requires far more instrumentation, time, and cost to create films that are
limited in long-term stability.
Films formed through ESA techniques can be composed of many adjacent layers and are generally well ordered on the supramolecular scale
depending on the reaction conditions in which they are made. Both the
amount of polyelectrolyte as well as the order in which they are deposited
can be easily controlled. In the formation of the first layer, a polyion
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
390
with fractional charge, are only partially ionizable. Typically, polyelectrolyte conformation is “flat” because the charges on a linear polymer
repel each other due to Coulombic repulsion. With the addition of salt
ions, however, this conformation can be altered to a more coiled or collapsed state due to screening of the charges on the polymer by the other
ions in the solution. This ability to change polyelectrolyte conformation
can change their function and is very useful in materials chemistry.
Synthetic organic chemists have been able to control the characteristics of
polyelectrolyte assemblies in many different manners, most simply by
altering monomeric units, electrolyte group, and polymer length. As a
result, polyelectrolytes make a very tunable building block for complex
materials. They have been used in a number of applications ranging from
drug delivery agents, to components in conducting films, to facilitating
colloidal suspensions, and to creating mimics of biological molecules.
Recently, polyelectrolyte microcapsules have found use as microreactors
for catalyst activity, precipitation reactions, crystallization reactions, and
polymerization reactions.
10.2.1 Electrostatic self-assembly
One useful feature of polyelectrolytes is that many of them can interact
with each other in a process known as electrostatic self-assembly (ESA).
As a simple example, Figure 10.5 illustrates the complexation of two
oppositely charged polyelectrolytes in the bulk phase. Polyelectrolytes
can also adhere to oppositely charged surfaces, forming nanofilms.
Because ESA relies on attractive interactions between materials of alternating charge, a number of different kinds of molecules can be used, such
as small inorganic molecules, proteins, dendrimers, and polyelectrolytes.
Our exploration of ESA films will focus on polyelectrolytes. The flexibility
of the polyelectrolytes used for ESA, as well as the ease of the selfassembly process, gives it advantages over other methods of creating thin
films, such as the LB technique. In contrast to ESA, the LB technique
requires far more instrumentation, time, and cost to create films that are
limited in long-term stability.
Films formed through ESA techniques can be composed of many adjacent layers and are generally well ordered on the supramolecular scale
depending on the reaction conditions in which they are made. Both the
amount of polyelectrolyte as well as the order in which they are deposited
can be easily controlled. In the formation of the first layer, a polyion
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
390