overlap. The salt ions cause conformational changes of the polyelectrolytes
by shielding the charges on the polyelectrolytes, enabling a polymer to
“extend.” As a result, although it becomes more difficult for the polyelectrolyte to diffuse deeper into the assembly, the presence of salt ions opens
up more polymer segments to competitive ion exchange with counterions.
Polyelectrolyte charge—whether a polyelectrolyte is strong or weak—also
contributes to the extent of overlap observed. With a weaker polyelectrolyte
or a polyelectrolyte with lower charge density, a flat/extended conformation
of the polymer cannot be achieved. As a result, the polyelectrolyte cannot
penetrate the outer film layer to as great a degree. Overall, ionic concentration plays a significant role (in addition to the charge density of the
polyelectrolyte) in contributing to the interpenetration of the polyelectrolyte within the film (Radeva and Petkanchin, 1997).
Interpenetration results in the bulk of the film appearing to be more
homogeneous, while the surface of the terminal layer has an overall
charge. Interfacial overlap also results in a lack of crystalline structuring.
This limits the applications of ESA films in producing materials that
require ordering for functionality purposes, such as chromophore
ordering for nonlinear optical responses. However, the organizational
structure of ESA films has been found to be useful for the construction of
conductive polymeric materials. The overlap from interpenetration allows
electrons to flow easily through the film, which would not be the case if
the layers were stratified into organized layers.
10.2.3 Multilayer formation
Charge reversal, interpenetration, and complexation all contribute to the
structure and formation of a multilayer. Broader factors to consider for
successful multilayer formation are film stability and film thickness. The
first, film stability, requires that a completed multilayer assembly be
stable in structure when removed from solution so that the film does not
change or degrade following assembly. The second, film thickness, can be
tailored based on ESA conditions and number of multilayers.
Film stability requires that the polyelectrolyte assembly be irreversible. As
a result, spontaneous desorption of material from the solid substrate film
cannot occur in a well-formed assembly. Desorption due to small ion
competitors or desorption due to exposure to polyelectrolyte solution all
cannot occur in order to maintain the irreversibility of the film. It has been
determined that desorption of material occurs with such slow kinetics
that the ESA film can be considered irreversible.
POLYELECTROLYTES 395
by shielding the charges on the polyelectrolytes, enabling a polymer to
“extend.” As a result, although it becomes more difficult for the polyelectrolyte to diffuse deeper into the assembly, the presence of salt ions opens
up more polymer segments to competitive ion exchange with counterions.
Polyelectrolyte charge—whether a polyelectrolyte is strong or weak—also
contributes to the extent of overlap observed. With a weaker polyelectrolyte
or a polyelectrolyte with lower charge density, a flat/extended conformation
of the polymer cannot be achieved. As a result, the polyelectrolyte cannot
penetrate the outer film layer to as great a degree. Overall, ionic concentration plays a significant role (in addition to the charge density of the
polyelectrolyte) in contributing to the interpenetration of the polyelectrolyte within the film (Radeva and Petkanchin, 1997).
Interpenetration results in the bulk of the film appearing to be more
homogeneous, while the surface of the terminal layer has an overall
charge. Interfacial overlap also results in a lack of crystalline structuring.
This limits the applications of ESA films in producing materials that
require ordering for functionality purposes, such as chromophore
ordering for nonlinear optical responses. However, the organizational
structure of ESA films has been found to be useful for the construction of
conductive polymeric materials. The overlap from interpenetration allows
electrons to flow easily through the film, which would not be the case if
the layers were stratified into organized layers.
10.2.3 Multilayer formation
Charge reversal, interpenetration, and complexation all contribute to the
structure and formation of a multilayer. Broader factors to consider for
successful multilayer formation are film stability and film thickness. The
first, film stability, requires that a completed multilayer assembly be
stable in structure when removed from solution so that the film does not
change or degrade following assembly. The second, film thickness, can be
tailored based on ESA conditions and number of multilayers.
Film stability requires that the polyelectrolyte assembly be irreversible. As
a result, spontaneous desorption of material from the solid substrate film
cannot occur in a well-formed assembly. Desorption due to small ion
competitors or desorption due to exposure to polyelectrolyte solution all
cannot occur in order to maintain the irreversibility of the film. It has been
determined that desorption of material occurs with such slow kinetics
that the ESA film can be considered irreversible.
POLYELECTROLYTES 395