RESEARCH NEWS

May 200412

Thermoset epoxy resins are very useful

because they combine high strength

and stiffness with excellent

dimensional, thermal, and

environmental stability. However, these

highly cross-linked thermosetting

polymers are inherently brittle, which

increases with cross-link density. To

reduce this problem, two types of

impact modifiers are used to produce

multiphase epoxy blends with an

improved toughness/stiffness balance:

flexibilizers and toughening agents.

German researchers have tailored a

new class of reactive liquid rubbers,

reactive core/shell-type hyperbranched

blockcopolyethers, as flexibilizers and

toughening agents for anhydride-cured

epoxy resins [Fröhlich et al., Polymer

(2004) 45, 2155]. Their versatile,

one-pot synthesis of hyperbranched

blockcopolyethers enables some

control over reactivity and polarity,

which is key to improved blend

performance with epoxy resins.

The hyperbranched copolyether liquid

rubbers have an onion-like molecular

architecture with an inner polyglycidol

block. This block controls the degree of

branching and the resulting end-group

functionality at the surface of these

nanoscale molecular particles. Addition

of glycidol increases the number of

hydroxy end groups from 6 to 88 mole

per mole of polymer. Polarity can be

adjusted by grafting propylene oxide

onto the hyperbranched core and by

post-polymerization esterification with

stearate and 4-hydroxy benzoate.

Although the molecular weight is high,

the control of polarity provides good

solubility of the uncured epoxy resin.

Reactivity can also be modified. The

researchers are able to control the

phase separation and vary the

mechanical properties from highly

flexible to stiff and tough. John K. Borchardt

Tougher epoxythermosets POLYMERS

By encapsulating a wide range of molecules andmodifying the surface characteristics ofnanoparticles, polymer hollow nanospheres havemany potential applications including nanostructuredcomposites, homogeneous catalysis, drug delivery,and dye encapsulation. Such hollow nanospheresare produced by depositing a shell material onto acore, which is then removed. Previous syntheticroutes have used different polymers as the coreand shell. Now Jyongsik Jang and coworkers atSouth Korea’s Seoul National University report thefabrication of hollow nanospheres using core andshell nanomaterials composed of the same polymer,polypyrrole [Jang et al., Chem. Commun. (2004) 7,794].The researchers used cationic surfactants, whichform micelles in a nanoreactor, to synthesize thehollow nanospheres. Pyrrole monomer and copper(II) chloride are added sequentially to produce linear,alcohol-soluble polypyrrole nanoparticles containedwithin the micelles. Additional monomer and iron (III)chloride produce a cross-linked, alcohol-insoluble

polypyrrole shell on the nanoparticles. Addingexcess methanol etches out the linear polypyrrolecore and removes the surfactants, residualoxidants, and unreacted monomer to leave cross-linked polypyrrole hollow nanospheres behind.The average diameter of the soluble polypyrrolenanoparticles is 23 nm, while the spherical linearpolypyrrole/cross-linked polypyrrole core/shellnanoparticles have a mean diameter of 33 nm.After etching away the core, the pore size of thehollow spheres is similar to the original corediameter. This indicates that the soluble polypyrrolecore acts as the template for the deposition of theinsoluble polypyrrole. The average wall thickness ofthe hollow spheres is 5 nm. Using the same polymer could greatly simplifymanufacturing processes. In addition, the ability tocontrol the nanosphere size would greatly increasethe versatility of these structures. The surfactantused in the emulsion polymerization provides ameans of doing just this. John K. Borchardt

Nanocapsules are the same at the core POLYMERS

Green light for displaysPOLYMERS

Mixing the three primary colors red, blue,and green can produce any color or shade.One barrier to the development of flat panelplastic television and computer displays isthe need for electrically conducting polymersthat produce all three primary colors. Redand blue electrochromic polythiophenes,which change color when their redox state isaltered by an electric voltage, are alreadyknown. To produce red or blue, a polymerneeds to have only one absorption band.However, to produce green, it must havebands in both the red and blue spectral

regions. If the molecule is converted from aneutral to an oxidized state by an appliedvoltage, it must become transparent. To dothis, both absorption bands must disappearin the same voltage range. This makes thedesign of chemical structures exhibiting thenecessary light absorption for green muchmore difficult. Therefore, no greenelectrochromic polymer has been available.To overcome this problem, Fred Wudl’sresearch group at the University ofCalifornia, Los Angeles constructed apolythiophene polymer whose backbonecontains two independent color-producingelectron systems, one for each of the tworequired absorption bands [Sonmez et al.,Angew. Chem. Int. Ed. (2004) 43, 1498].The polymer, poly(2,3-di(thien-3-yl)-5,7(thien-2-yl)thieno[3,4-b]pyrazine) or poly(DDTP),exhibits high green color saturation, fastswitching between the green andtransparent states, and excellent chemicalstability. These promising properties bringpolymer electrochromics closer to use inplastic displays and other polymer-basedoptoelectronic components.John K. Borchardt

Structure of poly(DDTP), a green electrochromic polymer.