Mar 2, 2005
Nanoparticles direct assembly of copolymers
Researchers at the University of Massachusetts, Amherst, Argonne National Laboratory, University of South Carolina, the University of Pittsburgh, all in the US, and Bayreuth University, Germany, have found that adding nanoparticles to diblock copolymers can redirect their self-assembly. The scientists believe the technique could have applications in chemical sensing, separation, catalysis, high-density data storage and photonic materials.
"These studies show a situation of self-directed self-oriented self-assembly," Tom Russell of the University of Massachusetts, Amherst told nanotechweb.org. "The system does everything by itself!"
Russell and colleagues added either cadmium selenide or ferritin nanoparticles to diblock copolymers. Ferritin is a hollow protein molecule inside which mammals, plants and bacteria store iron. This created organic-inorganic or organic-bioparticle hybrid materials.
To make the first of these materials, the researchers spin-coated toluene solutions of polystyrene-block-poly(2-vinylpyridine) copolymer and tri-n-octylphosphine oxide-(TOPO)-covered cadmium selenide nanoparticles onto a silicon wafer. Next, they annealed the resulting 150-600 nm-thick films to enable them to reach their equilibrium morphology. They also carried out the same procedure without adding nanoparticles, so that they could compare the structures.
In the absence of nanoparticles, the film formed an array of cylindrical microdomains of poly(2-vinylpyridine) (P2VP) in a polystyrene matrix. The microdomains were oriented parallel to the substrate: the scientists say this is because the P2VP interacts preferentially with the substrate and the polystyrene has a lower surface energy.
With cadmium-selenide nanoparticles present, on the other hand, the cylindrical P2VP microdomains oriented normal to the substrate in a hexagonal array. The nanoparticles arranged themselves at the interface of the P2VP domains with the air. The scientists believe that this reduced the surface energy of the domains and induced their change in orientation.
"By adding nanoparticles to a block copolymer and by tailoring the ligands attached to the nanoparticle, the self-assembly of the block copolymer can force the nanoparticles to the surface and substrate interface where the nanoparticles will assemble, mediate interactions and cause the microdomains to orient normal to the surface," explained Russell.
The researchers also looked at the effect of adding nanoparticles of poly(ethylene glycol) (PEG)-tagged ferritin to a lamella-forming diblock copolymer of P2VP and poly(ethylene oxide) (PEO).
Without nanoparticles, the polymer phase-separated into lamellae oriented parallel to the surface, with the crystalline PEO located at the surface. But when ferritin nanoparticles were present, they moved to the PEO phase where they suppressed PEO crystallization and caused the lamellae to align themselves normal to the surface.
According to the scientists, this process has the advantage of not requiring external fields to manipulate the orientation of the domains. In addition, the ability to genetically and chemically alter the surface properties of bionanoparticles such as ferritin and to incorporate different inorganic materials into the cores means that it should be possible to develop copolymer-bioparticle hybrid systems with unique properties.
"If we can place a bias to the lateral ordering of the copolymer arrays, it will be possible to have a system that will self-assemble into a highly-ordered, highly-aligned array of nanoscopic elements where the exact positioning of each element is known," said Russell. "This is precisely what is necessary to produce addressable media that will allow access to each element of the array, fully utilizing the ultra-high density afforded by the copolymer array. Potential applications include addressable high-density magnetic media, ultra-high resolution field-effect devices for displays, and high-resolution sensors."
The researchers reported their work in Nature.
About the author
Liz Kalaugher is editor of nanotechweb.org.