Research Coordinator, National Institute of Advanced Industrial Science and Technology (AIST)
Nanostructures fabricated with polymers
—Designing high-order structures with block copolymer—
Nylon, polyester and other ordinary polymers are made up of polymer chains of various molecular weights. However, the molecular weight of the polymers can be made uniform through “anionic living polymerization”. “Block copolymers” composed of different types of polymer chains and “telechelic polymers” containing terminal functional groups can also be synthesized using this living polymerization. Prof. Nakahama has realized his idea -- designing high-order structures of polymers at the nanoscale level -- through accurate control of primary structures using anionic living polymerization.
A block copolymer, in which the constituent polymer chains of each block do not mix each other, has various nanoscale-level phase separated structures. If monomers, from which polymers are made, contain functional groups, anionic initiators and propagating active-end anions react with the functional groups to terminate the polymerization. Thus, block copolymers were for many years synthesized only from styrene, butadiene and other hydrocarbon monomers without functional groups.
In the mid 80’s, Prof. Nakahama found that functional groups of the monomer could be protected by replacing them with more stable moieties and that the protective groups could be removed after polymerization. He says, “This method is difficult to carry out. If the functional groups are not protected tightly, polymerization will be blocked. But if the protection is too tight, it will be very difficult to remove the protection after polymerization. I examined many combinations of polymerization conditions and protecting groups to be used. High-order nanostructures can be designed with primary structures such as polymer chain lengths and block sequences. If functional groups are introduced to specific parts of a nanostructure, these parts will have special functions.” He developed a method to synthesize block copolymers from polymers with functional groups and has been designing the configurations of many high-order nanostructures and their functions.
Prof. Nakahama has also developed porous membranes with nanoscale holes from triblock copolymers (with a structure of -(AAA···AAA)-(BBB···BBB)-(AAA···AAA)-) made of polyisoprene and polystyrene with Si-OR groups. When a high-order structure of this copolymer is treated with acid, the Si-OR groups are hydrolyzed and the polystyrene micro domain is fixed firmly through silicon’s cross-linking reaction. Then, the structure is treated with ozone, and polyisoprene chain is decomposed and resolved resulting in porous polystyrene membranes containing silicon. The size of the holes can be changed by altering the molecular weights of polystyrene and polyisoprene segments in the copolymer. These membranes may be used as materials for sensors with suitable holes for specific enzymes.
Prof. Nakahama has also studied peculiar properties of a high-order structure of polymers. The block copolymer of hydrophilic PHEME (poly hydroxyethyl methacrylate) and hydrophobic polystyrene has a high-order structure composed of cylinder-like arrays of polystyrene and PHEMA filling spaces among the polystyrene arrays. Film of this block copolymer may be used as an anti-thrombogenic material because it is highly biocompatible and does not activate blood platelets. Prof. Nakahama became very interested in the surface of this film, which gradually becomes hydrophilic when wet, although dry film is usually hydrophobic because its surface is covered with the polystyrene domain. Researchers were confused about this phenomenon at the time. They were unable to study the wet surface of the film with electron microscopes, whose insides should be kept in a vacuum state when used to check samples. Prof. Nakahama discovered that the structure of the wet surface is maintained even after it is stained and dried. He found out how the hydrophobic surface becomes hydrophilic by observing the process closely. There are very tiny defects on the polystyrene surface and water gradually enters these defects, making the inside PHEMA expand. The expanded PHEMA domain comes out to the surface, changing it to hydrophilic. He also discovered that the film’s surface returns to a hydrophobic state when dried.
Prof. Nakahama wants young researchers to thoroughly study basic research. “I hope they will conduct basic studies with courage and enthusiasm for developing new fields on their own,” he says. He also advises them to learn the engineering approach in addition to the scientific one. He says, “Science and engineering have been different historically but with some overlapping. I think young researchers should make it clear the difference in science and engineering to find out where they stand when starting a new research.”
