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Controlling nanoparticle formation
RESEARCH NEWS
Controlling nanoparticle formation NANOTECHNOLOGY The sensitive relationship between nanoparticle properties and their size and morphology means that exact control of the particle structure is required to obtain nanomaterials with specific properties. The lack of this control results in polydisperse nanoparticles with heterogeneous properties. Now, researchers at Georgia Institute of Technology, the University of Florida, Drexel University, and Weizmann Institute of Science in Israel have used synthetic polymeric matrices to guide the formation of stable, monodisperse iron oxide nanoparticles [Tannenbaum et al., Macromolecules (2005), doi: 10.1021/ma048317x]. Unlike nanoparticles synthesized in small-molecule environments, particles formed in a polymeric matrix are stabilized against flocculation by an adsorbed polymer layer. This results in a uniform nanocomposite of welldispersed metal nanoparticles. Polymer-particle interactions determine the particle size, size distribution, and morphology. In strongly interacting polymer media, small (10-20 nm) pyramidal γ-Fe2O3 particles are formed. Larger (40-60 nm) spherical particles are formed in weakly interacting polymeric media. In all cases, the polydispersity was low compared with nanoparticles obtained in small-molecule media. Because particle size is independent of the polymer chain length, the matrix polymer molecular weight can be based on just the processing requirements without affecting the inorganic particle properties. The synthetic method can readily be extended to a variety of inorganic nanoparticles. John K. Borchardt
Shape-memory polymers see the light POLYMERS
The first plastics that can be reformed into a temporary, preprogrammed shape by illumination with ultraviolet (UV) light have been made by researchers at the GKSS Research Center and RWTH Aachen in Germany, and Massachusetts Institute of Technology [Lendlein et al., Nature (2005) 434, 879]. When exposed to UV light of a different wavelength, the materials switch back to their original shape. Such materials could have applications in minimally invasive surgery. For example, a physician could insert a plastic string into the body through a tiny incision. When activated by light from an inserted fiber-optic probe, the shape of the string would change to a corkscrew-shaped stent to hold blood vessels open. More everyday applications include paper clips that relax when not needed and staples that open when desired. Andreas Lendlein and coworkers explain that the key in obtaining a shape-memory effect is grafting photosensitive groups as ‘molecular switches’ onto a polymer network. When the polymer film is mechanically stretched and illuminated by >260 nm wavelength UV light, the photosensitive groups crosslink and lock the polymer into a new shape that is
Light-induced, shape-memory effect in a polymer: (a) original shape, (b) temporarily fixed form, and (c) and (d) recovered shape with increasing UV exposure time. (Courtesy of Sabine Benner, GKSS Research Center.)
maintained when the stress is released. The temporary shape is very stable for long times, even when heated to 50°C. Exposure to light of <260 nm at ambient temperatures cleaves the new crosslink bonds, allowing the material to spring back to its original shape. In addition to elongated films, other temporary shapes can be produced. For example, a spiral or corkscrew can be created by exposing only one side of the stretched sample to light. Crosslinks are only formed on the irradiated side of the polymer, while the other side remains flexible. When the external force is released, one side contracts much more than the other to give the arch or corkscrew geometry. John K. Borchardt
A coat for all surfaces POLYMERS Currently, there is no general method for controlling the interfacial or surface properties of materials. Such control would allow the development of surface-responsive materials for a wide variety of applications. Now, researchers at IBM Almaden Research Center in California and the University of Massachusetts, Amherst have developed a simple, versatile method to modify solid surfaces based on an ultrathin, crosslinkable random copolymer film [Ryu et al., Science (2005) 308, 236]. Surface characteristics can be tuned by making use of random copolymers. But grafting procedures, in which the chain end of a random copolymer diffuses to the surface and undergoes a reaction to anchor the polymer, tend to be slow and inefficient. Instead, Craig J. Hawker and coworkers used random copolymers containing a crosslinking group within the polymer backbone. The crosslinking reaction produced an insoluble, ultrathin random copolymer film. These films are more robust than anchored random copolymer chains.
Random copolymers of styrene and methyl methacrylate were used containing 2 wt.% benzocyclobutene incorporated along the backbone. After spin coating on a surface, the benzocyclobutene groups can be thermally crosslinked to produce a random copolymer network. The thickness of the film is determined by the copolymer solution concentration. Adjusting the ratio of styrene and methyl methacrylate in the copolymer changes the strength of the interfacial interactions. Ultrathin films were prepared on a variety of substrates: metals, metal oxides, semiconductors, and polymers. The insoluble, crosslinked films are resistant to removal and can be further processed. By removing the requirement of chemical attachment of the film to the underlying substrate, the ultrathin films can be deposited on most surfaces. Despite the absence of chemical bonding, adhesive failure of the films is not observed. John K Borchardt
June 2005
15
Controlling nanoparticle formation NANOTECHNOLOGY The sensitive relationship between nanoparticle properties and their size and morphology means that exact control of the particle structure is required to obtain nanomaterials with specific properties. The lack of this control results in polydisperse nanoparticles with heterogeneous properties. Now, researchers at Georgia Institute of Technology, the University of Florida, Drexel University, and Weizmann Institute of Science in Israel have used synthetic polymeric matrices to guide the formation of stable, monodisperse iron oxide nanoparticles [Tannenbaum et al., Macromolecules (2005), doi: 10.1021/ma048317x]. Unlike nanoparticles synthesized in small-molecule environments, particles formed in a polymeric matrix are stabilized against flocculation by an adsorbed polymer layer. This results in a uniform nanocomposite of welldispersed metal nanoparticles. Polymer-particle interactions determine the particle size, size distribution, and morphology. In strongly interacting polymer media, small (10-20 nm) pyramidal γ-Fe2O3 particles are formed. Larger (40-60 nm) spherical particles are formed in weakly interacting polymeric media. In all cases, the polydispersity was low compared with nanoparticles obtained in small-molecule media. Because particle size is independent of the polymer chain length, the matrix polymer molecular weight can be based on just the processing requirements without affecting the inorganic particle properties. The synthetic method can readily be extended to a variety of inorganic nanoparticles. John K. Borchardt
Shape-memory polymers see the light POLYMERS
The first plastics that can be reformed into a temporary, preprogrammed shape by illumination with ultraviolet (UV) light have been made by researchers at the GKSS Research Center and RWTH Aachen in Germany, and Massachusetts Institute of Technology [Lendlein et al., Nature (2005) 434, 879]. When exposed to UV light of a different wavelength, the materials switch back to their original shape. Such materials could have applications in minimally invasive surgery. For example, a physician could insert a plastic string into the body through a tiny incision. When activated by light from an inserted fiber-optic probe, the shape of the string would change to a corkscrew-shaped stent to hold blood vessels open. More everyday applications include paper clips that relax when not needed and staples that open when desired. Andreas Lendlein and coworkers explain that the key in obtaining a shape-memory effect is grafting photosensitive groups as ‘molecular switches’ onto a polymer network. When the polymer film is mechanically stretched and illuminated by >260 nm wavelength UV light, the photosensitive groups crosslink and lock the polymer into a new shape that is
Light-induced, shape-memory effect in a polymer: (a) original shape, (b) temporarily fixed form, and (c) and (d) recovered shape with increasing UV exposure time. (Courtesy of Sabine Benner, GKSS Research Center.)
maintained when the stress is released. The temporary shape is very stable for long times, even when heated to 50°C. Exposure to light of <260 nm at ambient temperatures cleaves the new crosslink bonds, allowing the material to spring back to its original shape. In addition to elongated films, other temporary shapes can be produced. For example, a spiral or corkscrew can be created by exposing only one side of the stretched sample to light. Crosslinks are only formed on the irradiated side of the polymer, while the other side remains flexible. When the external force is released, one side contracts much more than the other to give the arch or corkscrew geometry. John K. Borchardt
A coat for all surfaces POLYMERS Currently, there is no general method for controlling the interfacial or surface properties of materials. Such control would allow the development of surface-responsive materials for a wide variety of applications. Now, researchers at IBM Almaden Research Center in California and the University of Massachusetts, Amherst have developed a simple, versatile method to modify solid surfaces based on an ultrathin, crosslinkable random copolymer film [Ryu et al., Science (2005) 308, 236]. Surface characteristics can be tuned by making use of random copolymers. But grafting procedures, in which the chain end of a random copolymer diffuses to the surface and undergoes a reaction to anchor the polymer, tend to be slow and inefficient. Instead, Craig J. Hawker and coworkers used random copolymers containing a crosslinking group within the polymer backbone. The crosslinking reaction produced an insoluble, ultrathin random copolymer film. These films are more robust than anchored random copolymer chains.
Random copolymers of styrene and methyl methacrylate were used containing 2 wt.% benzocyclobutene incorporated along the backbone. After spin coating on a surface, the benzocyclobutene groups can be thermally crosslinked to produce a random copolymer network. The thickness of the film is determined by the copolymer solution concentration. Adjusting the ratio of styrene and methyl methacrylate in the copolymer changes the strength of the interfacial interactions. Ultrathin films were prepared on a variety of substrates: metals, metal oxides, semiconductors, and polymers. The insoluble, crosslinked films are resistant to removal and can be further processed. By removing the requirement of chemical attachment of the film to the underlying substrate, the ultrathin films can be deposited on most surfaces. Despite the absence of chemical bonding, adhesive failure of the films is not observed. John K Borchardt
June 2005
15