Two methods 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 (M_n) increases linearly with monomer conversion. The polydispersity (M_w/M_n) 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.

Poly (styrene-block-butyl 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 of Materials.

A different 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 presented above

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).		Figure 4. Molecular weights of the polystyrene in the nanocomposite made by NMP increase linearly and polydispersities (Mw/Mn) are low.

For more information about Professor Shipp and his research, please call him at 315-268-2393 or send email to dshipp@clarkson.edu.