have been investigated, as outlined in Scheme 1. The first
is ATRP from an alkyl halide initiator intercalated into the clay.
The second is NMP using clay modified with a polymerizable surfactant.
A general feature of LRPs is that the number average molecular weight
(Mn) increases linearly with monomer conversion. The
polydispersity (Mw/Mn) is usually less than
1.5, and often around 1.2 and lower. In the case of the ATRP of
styrene in the presence of the initiator modified clay and copper
(I) bromide catalyst, the polymerization exhibited excellent control
of the molecular weights (Figure 1). The clay platelets were significantly
(although not completely) dispersed in the polystyrene (Figure 2).
These materials can also be thought of as polymer brushes since
the chain ends are anchored to the silicate layers.
acrylate) block copolymer-silicate nanocomposites were synthesized
in a similar manner (Scheme 2). At room temperature, the polystyrene
segment is hard while the poly (butyl acrylate) is soft. By combining
the two polymer segments into the one polymer chain, thermo-reversible
phase separation can be achieved. These materials underwent phase
separation in a similar manner to pristine block copolymers, except
that the domain size of the phases were approximately 10 times smaller
in the nanocomposite compared to the pristine block copolymer (~
4 nm vs. ~ 40 nm). This is due to the chain ends being immobilized
on the silicate platelets. A TEM image of the nanocomposite is shown
in Figure 3. These materials have potential applications as reinforced
elastomers and adhesives. This work will soon appear in Chemistry
approach for controlling the polymer architecture within PLSNs is
to incorporate a monomer entity within the clay layers before polymerization,
and then perform LRP of a chosen monomer with the modified clay
dispersed in the reactor. This approach has been done using an intercalated
polymerizable surfactant (molecularly similar to styrene) and the
NMP of styrene. As shown in Figure 4, the molecular weights in this
system are also well controlled. This methodology has the advantage
over ATRP in that no metal catalyst is present. Furthermore, the
polymer chains made in this manner may have several anchor points
to attach to the silicate layers, and so potentially offer different
morphology and properties to those obtained using the ATRP method
One of the
biggest challenges in this area is how to correlate structure/morphology
of the nanocomposite with preparation conditions. This is a complex
issue because of the competing thermodynamic and kinetic phenomena
occurring, and the many variables that may be altered. The work
being carried out by Professor Shipp and his group will advance
the understanding of these relationships through the structural
study of nanocomposites made with well-defined polymers, in addition
to the development of novel materials for a wide variety of applications.
Figure 3. A TEM image of poly(styrene-block-butyl acrylate)
block copolymer-silicate nanocomposites (exfoliated region)
stained by hydrazine and osmium tetraoxide to increase contrast
between the phases (butyl acrylate segments are the dark spots).
4. Molecular weights of the polystyrene in the nanocomposite
made by NMP increase linearly and polydispersities (Mw/Mn)
more information about Professor Shipp and his research, please
call him at 315-268-2393 or send email to email@example.com.