"These controlled nano-structures are manufactured using atom transfer radical polymerization (ATRP), which is an exceptionally robust way to uniformly control the growth of every polymer chain, while employing a broad range on monomers. ATRP allows one to design materials with specific chemical and architectural features," says Matyjaszewski. Using ATRP, his laboratory has created polymeric brushes with gradient compositional densities that force a material to alter its response to the changing environment, such as temperature or pressure, in highly selective ways.
Bottle-brush structures are created in the Matyjaszewski lab by growing the bristle-like strands of one polymer type (for instance, polyacrylates or polyacrylamides) from a backbone of another polymer (such as a polymethacrylate) (see Figure 1). They are very similar in structure to proteoglycans found in the cartilage protecting our joints. Some of these novel materials are considered supersoft elastomeric materials because they are 100 times softer than rubber.
"The only materials as soft as these supersoft brushes are hydrogels, but unlike existing hydrogels, which are used in materials like contact lenses, these materials won't 'dry out' when their environment changes, since every part of the structure is covalently linked. These structures would be ideal for developing materials for artificial skin or wound healing, or for modifying the tactile response of surfaces," notes Matyjaszewski.
Other brush structures can be grown from nano-particles of glass or gold, giving rise to corona-shaped structures that combine organic and inorganic materials at the nano-scale through extremely strong chemical covalent bonds (see Figure 2). Many macro- or micro-scale synthetic hybrid materials combining organic and inorganic compounds are strong, but they break under stress. By creating these nano-scale molecular hybrids using ATRP, the Matyjaszewski lab has created extremely durable materials that won't crack. These materials could work well as a coating for cars, where you want to combine a hard surface to avoid scratches with soft bulk properties to avoid cracking, according to Matyjaszewski.
Other applications for polymer brushes include coatings that would provide a barrier to prevent corrosive substances from penetrating and damaging a substrate. Matyjaszewski also has shown that these brushes can move in concert across surfaces, suggesting that they could make new lubricants in industrial settings. Organic brush polymers with inorganic cores could be used to reinforce car tires much more effectively than current technology. Other kinds of nano-size brush polymers could be used as carriers that penetrate the tiniest blood vessels to reach human tissues, or seep through dense soil and slowly deliver toxin-neutralizing compounds from their encapsulated core.
ATRP is a controlled living polymerization process that differs from conventional methods to make polymers, in which it's difficult to control the composition and architecture of the resulting compound. With ATRP, a complex polymer structure is made by adding one monomer at a time to a slowly growing polymer chain by interaction of the growing chain end with a special catalyst, including hybrid catalysts with low levels of metals in solution. As Matyjaszewski explains, monomers are added to the chain end one unit at time. The process can also be shut down or re-started at will, depending on how the temperature and other conditions are varied. This methodology allows precise control over the composition and architecture of the many structures they can create.
Matyjaszewski's groundbreaking paper on ATRP, first published in 1995, has spawned considerable industrial and academic research in this field of controlled polymerization and has been cited more than 700 times. He heads a research consortium that interacts with 21 industrial companies from around the world interested in creating novel polymeric materials for their markets. Some corporations have licensed ATRP technology and started commercial production. The present consortium, built on the first ATRP consortium founded in 1995 with 11 international industrial organizations, aims to explore the polymer science underlying their targeted activities, and to train both university and industrial scientists in procedures for responsive polymeric material development. The Center for Macromolecular Engineering (www.chem.cmu.edu/groups/maty/) is funded both by the consortium and government agencies, including the National Science Foundation and the Environmental Protection Agency.
Dr. Matyjaszewski carries out research on molecular brushes in collaboration with Professor Martin Moeller, University of Aachen; Professor Tadeusz Pakula, Max Planck Institute, Mainz; Professor Sergei Sheiko, University of North Carolina, Chapel Hill; and Professor Tomasz Kowalewski, Carnegie Mellon.