RESEARCH NEWS

June 2005 15

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 well-

dispersed 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

ControllingnanoparticleformationNANOTECHNOLOGY

Currently, there is no general method for controllingthe interfacial or surface properties of materials.Such control would allow the development ofsurface-responsive materials for a wide variety ofapplications. Now, researchers at IBM AlmadenResearch Center in California and the University ofMassachusetts, Amherst have developed a simple,versatile method to modify solid surfaces based onan ultrathin, crosslinkable random copolymer film[Ryu et al., Science (2005) 308, 236].Surface characteristics can be tuned by making useof random copolymers. But grafting procedures, inwhich the chain end of a random copolymer diffusesto the surface and undergoes a reaction to anchorthe polymer, tend to be slow and inefficient. Instead,Craig J. Hawker and coworkers used randomcopolymers containing a crosslinking group withinthe polymer backbone. The crosslinking reactionproduced an insoluble, ultrathin random copolymerfilm. These films are more robust than anchoredrandom copolymer chains.

Random copolymers of styrene and methylmethacrylate were used containing 2 wt.%benzocyclobutene incorporated along the backbone.After spin coating on a surface, thebenzocyclobutene groups can be thermallycrosslinked to produce a random copolymernetwork. The thickness of the film is determined bythe copolymer solution concentration. Adjusting theratio of styrene and methyl methacrylate in thecopolymer changes the strength of the interfacialinteractions.Ultrathin films were prepared on a variety ofsubstrates: metals, metal oxides, semiconductors,and polymers. The insoluble, crosslinked films areresistant to removal and can be further processed.By removing the requirement of chemicalattachment of the film to the underlying substrate,the ultrathin films can be deposited on mostsurfaces. Despite the absence of chemical bonding,adhesive failure of the films is not observed. John K Borchardt

A coat for all surfacesPOLYMERS

Shape-memory polymers see the lightPOLYMERS

The first plastics that can be reformed into atemporary, preprogrammed shape byillumination with ultraviolet (UV) light havebeen made by researchers at the GKSSResearch Center and RWTH Aachen inGermany, and Massachusetts Institute ofTechnology [Lendlein et al., Nature (2005)434, 879]. When exposed to UV light of adifferent wavelength, the materials switchback to their original shape. Such materials could have applications inminimally invasive surgery. For example, aphysician could insert a plastic string intothe body through a tiny incision. Whenactivated by light from an inserted fiber-opticprobe, the shape of the string would changeto a corkscrew-shaped stent to hold bloodvessels open. More everyday applicationsinclude paper clips that relax when notneeded and staples that open when desired.Andreas Lendlein and coworkers explain thatthe key in obtaining a shape-memory effect isgrafting photosensitive groups as ‘molecularswitches’ onto a polymer network. When thepolymer film is mechanically stretched andilluminated by >260 nm wavelength UV light,the photosensitive groups crosslink and lockthe polymer into a new shape that is

maintained when the stress is released. Thetemporary shape is very stable for longtimes, even when heated to 50°C. Exposureto light of <260 nm at ambient temperaturescleaves the new crosslink bonds, allowing thematerial to spring back to its original shape.In addition to elongated films, othertemporary shapes can be produced. Forexample, a spiral or corkscrew can becreated by exposing only one side of thestretched sample to light. Crosslinks areonly formed on the irradiated side of thepolymer, while the other side remainsflexible. When the external force is released,one side contracts much more than theother to give the arch or corkscrewgeometry.John K. Borchardt

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.)