C hapter 10 nanomaterial product Forms and Functions
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marily dependent on the surface area. In conventional porous polymers, for example, the smallest pore size commonly used is around
1 um, whereas the intent of developing nanoporous polymers is to
have pore sizes 100 nm or less; this in turn means big increases in
relative surface pore areas. Actual pore diameters may be important
in applications such as filtering. The amounts of solid material and
lengths of travel along solid walls surrounding pores are important
in thermal applications.
These various geometric and other attributes of porous materials can
be literally designed into many materials via choices in the primary
matrix materials and in the processing method(s) used to create
the pore structure. Processing methods generally fall into three
primary categories: methods that employ thermal or mechanical
means only, methods that use some type of pore-generating agent,
and methods that use some form of template. In the first category,
various approaches such as melt extrusions, cold stretching, or sintering have been used to make conventional bulk porous materials
but are not largely not relevant to making nanoporous materials. (A
form of radiation etching, however, is possible.) More commonly,
some type of pore-generating agent is mixed into a host material
and literally causes foaming or some other kind of pore development process. Processes of this type are widely used with conventional porous materials and remain useful for making nanoporous
materials. Polymer foaming techniques using blowing agents, often
based on carbon dioxide, have long been used to produce closed
cells for structures with micro-sized pores, and a similar approach
can be used to produce nanoporous polymers.
Various solvents can be used to produce a variety of porous morphologies. The last category uses “templates,” which can include
some type of molecule or phase around which a polymer is
formed. These templates are in turn leached out. Templates can
also be derived from materials inherently containing nanopores.
Terms such as molecular imprinting or micellar imprinting are used to
describe some of these processes. Self-assembly processes are also
possible. Several of these approaches are suitable for making nanoporous materials. The choice of which of these several approaches
is ultimately used to create a nanoporous material depends on the
porous morphology desired, which in turn depends on the application environment.
Porous materials are commonly used for thermal insulation purposes. In insulation materials with small pores, heat transfer occurs
in several ways. Conductive heat transfer occurs in the intercon-
448
marily dependent on the surface area. In conventional porous polymers, for example, the smallest pore size commonly used is around
1 um, whereas the intent of developing nanoporous polymers is to
have pore sizes 100 nm or less; this in turn means big increases in
relative surface pore areas. Actual pore diameters may be important
in applications such as filtering. The amounts of solid material and
lengths of travel along solid walls surrounding pores are important
in thermal applications.
These various geometric and other attributes of porous materials can
be literally designed into many materials via choices in the primary
matrix materials and in the processing method(s) used to create
the pore structure. Processing methods generally fall into three
primary categories: methods that employ thermal or mechanical
means only, methods that use some type of pore-generating agent,
and methods that use some form of template. In the first category,
various approaches such as melt extrusions, cold stretching, or sintering have been used to make conventional bulk porous materials
but are not largely not relevant to making nanoporous materials. (A
form of radiation etching, however, is possible.) More commonly,
some type of pore-generating agent is mixed into a host material
and literally causes foaming or some other kind of pore development process. Processes of this type are widely used with conventional porous materials and remain useful for making nanoporous
materials. Polymer foaming techniques using blowing agents, often
based on carbon dioxide, have long been used to produce closed
cells for structures with micro-sized pores, and a similar approach
can be used to produce nanoporous polymers.
Various solvents can be used to produce a variety of porous morphologies. The last category uses “templates,” which can include
some type of molecule or phase around which a polymer is
formed. These templates are in turn leached out. Templates can
also be derived from materials inherently containing nanopores.
Terms such as molecular imprinting or micellar imprinting are used to
describe some of these processes. Self-assembly processes are also
possible. Several of these approaches are suitable for making nanoporous materials. The choice of which of these several approaches
is ultimately used to create a nanoporous material depends on the
porous morphology desired, which in turn depends on the application environment.
Porous materials are commonly used for thermal insulation purposes. In insulation materials with small pores, heat transfer occurs
in several ways. Conductive heat transfer occurs in the intercon-