RESEARCH NEWS

March 2005 21

Flying the flag for novel microbeads

Researchers at the National Institute of

Advanced Industrial Science and

Technology (AIST) and The University of

Tokyo in Japan have observed room-

temperature ferroelectricity in a low-

molecular-weight organic compound

formed by cocrystallization of nonpolar

conjugated molecules [Horiuchi et al.,

Nat. Mater. (2005) doi:

10.1038/nmat1298]. Conventional

organic ferroelectrics are made from

polar molecules, and are scarce for

low-molecular-weight materials.

The highly directional nature of the

strong intermolecular hydrogen bond

between the hydrogen-acceptor (A)

base molecule, phenazine (Phz), and the

hydrogen-donor (D) dibasic acid,

chloranilic or bromanilic acid (H2xa),

results in a supramolecular assembly.

Previously, with various other

π-conjugated amines, these H2xa acids

have provided cocrystals or proton-

transferred salts that show a variety of

hydrogen-bonded networks. Now, in

these Phz-H2xa cocrystals, proton-

donating or proton-accepting groups

reside symmetrically on both sides of

the respective molecules to form an

infinite supramolecular chain.

In the high-temperature phase, a

structural change without any atomic

disorder suggests that the ferroelectric

phase transition is not of the

orientational order-disorder type

characteristic of polar molecules. It is a

displacive type of transition (where

oppositely charged species are

displaced), which does not need a polar

crystal of asymmetric molecules.

Room-temperature dielectric constants

are large, especially along the b-axis

(>100 for chloranilic acid), and highly

anisotropic (values for the c-axis are

70-90 times smaller). The spontaneous

polarization is also large and uniaxial

along the b-axis. The Curie constant (C)

is 20-30 times as high as the

ferroelectric transition temperature

(Tc), in contrast to order-disorder type

transitions where C is approximately

equal to Tc. Such properties in solid

organic ferroelectrics could prove

important for the development of all-

organic electronic and photonic devices.

Mark Telford

Ferroelectricity from nonpolar organics

A simple method for preparing multicomponent polymermicrobeads with arbitrary internal structures has beendeveloped by researchers at Northwestern University[Fialkowski et al., Nat. Mater. (2005) 4 (1) 93]. Spherical particles are essential to many technologies andindustrial products, from packed beds to optics and sensing.However, techniques for preparing microbeads have largelybeen limited to compositions with spherical internalsymmetry, that is uniform, hollow, or core-shell spheres. “Wehave developed a very general method of making beads of anysymmetry,” says Bartosz A. Grzybowski.Small droplets of hydrophobic, liquid prepolymers are printedonto a thin layer of a water-soluble gel on a glass slide. Thedroplets spread out on the gel surface and coalesce intocomposite patches. The glass slide is then immersed in a saltsolution that has the same density as the prepolymers.

The solution underetches the gel beneath the prepolymers,liberating the patch. Since the solution is isodense with theprepolymers, in the absence of gravitational forces thebuoyant patch is free to fold up into a sphere driven by itsneed to minimize interfacial energy. Finally, the microspheresare solidified by curing, thermally or by ultraviolet irradiation.The prepolymers are chosen so that they are relativelyhydrophobic, have low interfacial energy, and can be cured.Density differences between prepolymers of 10% can betolerated.The internal structures of the polymeric spheres depend onlyon the positions and size of the initial prepolymer droplets.This has allowed Grzybowski and colleagues to fabricatespheres with the topologies of various national flags (shown).As well as printing droplets side-by-side to create the flags,droplets can be printed on top of each other to give core-shellor pie-chart-like particles. By mixing curable and noncurable prepolymers, containerparticles can be generated that have a solid core and fourliquid-filled capsules. This capability could be useful forcontrolled release and drug delivery. If the outer walls of thecapsules can be dissolved, the contained liquid could bereleased to the environment.The Northwestern group also hopes to develop theirtechnology to produce asymmetric beads with sticky patchesthat could assemble into colloidal crystals withpreprogrammed space groups. The microspheres produced todate are a few hundred micrometers in size because theresearchers are limited to dispensing the prepolymer dropletsusing a microsyringe. However, an ink-jet printer capable ofdelivering viscous drops would allow spheres of colloidaldimensions to be fabricated. The team is now actively lookingfor partners to develop this technology.Jonathan Wood

ELECTRONIC MATERIALS

National flags in polymeric microspheres. (Courtesy of Bartosz A. Grzybowski.)

POLYMERS