RESEARCH NEWS

February 200512

Carbon nanotube (CNT)-based polymer

composites could offer high electrical

conductivity and advantageous

mechanical properties. Working toward

this goal, scientists at the University of

Cambridge and Imperial College in the

UK and Hamburg Institute of

Technology in Germany have dispersed

multiwalled nanotubes in an epoxy

system based on a bisphenol-A resin

and an amine hardener (Martin et al.,

Polymer (2005) 46, 877).

A shear-intensive mechanical stirring

process provides excellent dispersion

of the CNTs in the epoxy component.

Applying ac and dc electric fields

during nanocomposite curing results in

the formation of aligned, conductive

nanotube networks. The networks were

evaluated by in situ optical microscopy

and current density measurements for

different electric field strengths and

nanotube weight fractions.

Electric-field-induced forces acting on

the CNTs, which have a negative

surface charge after processing in the

epoxy, dominate nanotube

agglomeration. Networks formed in

ac fields are more uniform and better

aligned those obtained in dc fields.

Increased field strength improves

uniformity and alignment of the

networks as well as enhancing bulk

conductivity of the composite material.

However, the maximum specific

composite conductivity that can be

achieved using this approach is still

low compared to the conductivity value

of multiwalled CNTs alone. This

indicates the presence of polymer

barriers that prevent direct contact

between individual nanotubes.

The researchers believe their approach

shows promise for creating conductive

nanotube-polymer composites with

anisotropic electrical properties. John K. Borchardt

Electric-fieldaligned CNTcompositesCOMPOSITES

Polymers containing photocrosslinkable groups arebeing developed for macro- and microlithography,liquid crystal displays, nonlinear optical materials,photocurable coatings, and energy exchangematerials. Ultraviolet (UV) irradiation crosslinks thepolymers selectively in the illuminated regions, butrapid curing in the radiated areas is essential.While some industrial applications have alreadybeen commercialized, more rapid curing rates aredesirable. Polymers with chalcone in the backboneor side chain undergo rapid crosslinking uponirradiation with UV light. However, the preparationand evaluation of photocrosslinkable acrylamidepolymers bearing chalcone as a pendant group hasnot been reported.Now, researchers at Anna University in Chennai,India have synthesized acrylamide monomers thatcould provide significantly more rapid curing ratesthan currently achievable (Selvam and Nanjundan,Reactive Funct. Polym. (2005) 62, 179). Themonomers, 4-acrylamidophenyl-2’,3’-benzostyrylketone (APBSK) and 4-acrylamidophenyl-4’-N,N’-dimethylstyryl ketone (APDSK), have a free-radicalpolymerizable group and a photocrosslinkable

functional group. Polymerization is performed at70°C using benzoyl peroxide as the free radicalinitiator and methyl ethyl ketone as the solvent. Thermogravimetric analysis in air of the resultingpolymers indicates that they have sufficient thermaland thermo-oxidative stability to be used as negativephotoresists, with poly(APBSK) being more stablethan poly(APDSK). Photosensitivity was investigatedin various solvents in the presence and absence oftriplet photosensitizers. Rapid photocrosslinking ofpoly(APBSK) appears to be caused by the presenceof an electron donating group (NCH3). The bulkynaphthyl group present in poly(APDSK) appears tosterically hinder the approach of the pendant groupsfor photocoupling, reducing photocrosslinking rates.In the absence of triplet sensitizers, this reactionrate depends on the solvent, concentration, andsubstituent in the pendant chalcone moiety.Because the pendant chalcone unit possesses highphotosensitivity, even in the absence of a tripletphotosensitizer, poly(APBSK) and poly(APDSK) wouldbe useful as negative photoresists for variousapplications.John K. Borchardt

Rapid crosslinking in acrylamide polymersPOLYMERS

Light-processable photonic crystalsOPTICAL MATERIALS

Photonic crystals can be used to control theemission of light and could enable a newgeneration of optical devices of reduced size.Self-assembly of monodisperse spheres intoordered three-dimensional opal structures isone way of making large-area photoniccrystal films of controlled thickness. Using a surfactant-free emulsion

polymerization, Birger Lange and coworkersat the University of Mainz in Germany,Cornell University, and Georgia Institute ofTechnology have synthesized monodispersecolloids made from poly(t-butylmethacrylate)(Lange et al., Chem. Mater. (2004) 16 (25),5286). A photoacid generator (top) and anorganic dye (bottom) are incorporated intothe polymer beads, and opal photoniccrystals are crystallized from the polymersolution. Optical defects can be introducedinto this large-scale periodic structure usingultraviolet (UV) irradiation and a lithographicmask, followed by baking and treatment withaqueous base. The UV irradiation promotesacid-catalyzed ester cleavage of t-butylmethacrylate units on the polymerbackbone, resulting in crystal defects thatconfine light in localized modes. Thistechnique could permit the design of photoniccrystal waveguides, resonant cavities, filters,and other components for integrated opticalcircuits. John K. Borchardt

Photoacid generator (top) and dye sensitizer (bottom) used for

processing of opal photonic crystals.